KR100957982B1 - Steel for Welding Structure having Welded Joint with Superior CTOD Properties in Weld Heat Affected Zone - Google Patents

Steel for Welding Structure having Welded Joint with Superior CTOD Properties in Weld Heat Affected Zone Download PDF

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KR100957982B1
KR100957982B1 KR1020070136659A KR20070136659A KR100957982B1 KR 100957982 B1 KR100957982 B1 KR 100957982B1 KR 1020070136659 A KR1020070136659 A KR 1020070136659A KR 20070136659 A KR20070136659 A KR 20070136659A KR 100957982 B1 KR100957982 B1 KR 100957982B1
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South Korea
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welded
steel
structural steel
ferrite
less
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KR1020070136659A
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Korean (ko)
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KR20090068868A (en
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박영환
정홍철
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주식회사 포스코
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Priority to KR1020070136659A priority Critical patent/KR100957982B1/en
Priority to CN200880122473.1A priority patent/CN101910437B/en
Priority to PCT/KR2008/007604 priority patent/WO2009082156A1/en
Priority to JP2010540569A priority patent/JP5303571B2/en
Publication of KR20090068868A publication Critical patent/KR20090068868A/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium

Abstract

The present invention is in weight%, C: 0.01 to 0.2%, Si: 0.1 to 0.5%, Mn: 1.0 to 3.0%, Ti: 0.01 to 0.1%, Ni: 0.5 to 3.0%, B: 0.0010-0.01%, N : 0.003-0.006%, P: 0.030% or less, Al: 0.005-0.05%, S: 0.030% or less, O: 0.05% or less, and other unavoidable impurities and remainder Fe is provided. . In addition, the relationship between Ti, O, N, and B includes Ti / O: 0.2 to 0.5, Ti / N: 2 to 5, O / B: 5 to 10, and (Ti + 4B) / O: 0.7 to 1.5. do. The microstructure of the welded structural steel of the present invention has a needle fraction ferrite of 85% or more as a tissue fraction, and uniformly dispersed TiO oxide in the structure at intervals of 0.5 μm or less, the particle diameter thereof is 0.01 to 0.1 μm, and the number is 1.0 × 10 7 per 1 mm 3 . It includes the above welded joint.

According to the present invention, by using TiO oxide and soluble B in high heat input welding, by using TiO oxide and soluble B, it is possible to provide a welded structural steel that can promote the acicular ferrite transformation in the weld metal part and secure the CTOD of the excellent weld joint. Can be.

Welded structural steel, needle ferrite, CTOD, high heat input welding, SAW

Description

Steel for Welding Structure having Welded Joint with Superior CTOD Properties in Weld Heat Affected Zone}

The present invention relates to a welded joint having excellent low temperature CTOD characteristics used in welded structures, and more particularly, to fine TiO oxide in the welded joint during SAW welding made of ships, buildings, bridges, offshore structures, steel pipes, line pipes, and the like. The present invention relates to a welded structural steel that can improve the CTOD characteristics of high heat input welded joints such as SAW welded joints by dispersing and suppressing grain boundary ferrite.

In recent years, the construction of offshore structures has become more extreme due to the continuous rise in international oil prices and diversification of construction technology. Among these offshore structures, the material of the structures, especially those built in cold regions, requires high strength and low temperature CTOD characteristics. However, high efficiency welding is inevitable in order to fabricate a structure in a given air by welding general high strength thick steel, and in order to weld the thickened steel in accordance with this trend, high heat welding methods have appeared, especially among the most widely used methods. The welding technique used is the submerged arc welding technique (SAW).

In general, the submerged arc welding method has a large welding amount, which reduces the number of welding passes. Therefore, the submerged arc welding method is advantageous in terms of productivity in comparison with general GMAW welding. In the case of the submerged welding which is currently used, the heat input range uses a heat input amount corresponding to approximately 25-45 kJ / cm.

However, in such high heat input welding, coarse columnar structure can be easily formed due to solidification of weld metal, and coarse grain boundary ferrite and Widmanstatten ferrite can be formed along the austenite grain boundary in coarse grains. have. That is, the welded joint may be said to be the site where the impact toughness deteriorates most in the welded portion. Therefore, in order to secure the stability of the weld structure formed by the high heat input welding, it is necessary to control the microstructure of the weld metal part to secure the CTOD (Crack Tip Opening Displacement) characteristics of the weld metal part.

As a means to solve this problem, conventional techniques for defining the alloying components of the welding material or improving the impact toughness by including a slag generating agent, but these techniques are to control the microstructure, particle size, etc. of the weld metal In addition, there is still a problem that it is difficult to secure the impact toughness of the welded joint during high heat input welding such as SAW welding because it does not control the oxygen or nitrogen content in the weld metal.

Therefore, there is a need to develop a welded joint having a component and a microstructure and a welded structural steel material having such a welded joint to solve the above problems and to improve the CTOD characteristics of the welded joint in a high heat input welding such as SAW welding. .

The present invention is, in weight%, C: 0.01 to 0.2%, Si: 0.1 to 0.5%, Mn: 1.0 to 3.0%, Ti: 0.01 to 0.1%, Ni: 0.5 to 3.0%, B: 0.0010-0.01%, N: 0.003-0.006%, P: 0.030% or less, Al: 0.005-0.05%, S: 0.030% or less, O: 0.05% or less, other unavoidable impurities and residual Fe, and include Ti, O, N, and B Ti / O: 0.2 to 0.5, Ti / N: 2 to 5, O / B: 5 to 10, and (Ti + 4B) / O: 0.7 to 1.5. To provide a welded structural steel. The weld joint is Cu: 0.1 ~ 2.0%, Nb: 0.0001 ~ 0.1%, V: 0.005 ~ 0.1%, Cr: 0.05 ~ 1.0%, Mo: 0.05 ~ 1.0%, W: 0.05 ~ 0.5%, and Zr: It may further include one or two or more components selected from the group consisting of 0.005 to 0.5%, Ca: 0.0005 to 0.05%, REM: 0.005 to 0.05% or Ca: 0.0005 to 0.05% and REM: 0.005 to 0.05 It may further comprise a%.

Further, the welded joint microstructure of the welded structural steel has a needle-like ferrite of 85% or more as a tissue fraction, and the remainder includes a welded joint which is polygonal ferrite or other grain boundary ferrite structure. In addition, the weld joint is uniformly dispersed TiO oxide in the structure at intervals of 0.5㎛ or less, the particle size of the TiO oxide is 0.01 ~ 0.1㎛, the number of particles is 1.0x10 7 or more per 1 mm 3 .

The welded structural steel having a welded joint according to the present invention has excellent strength and CTOD properties at the same time, and thus can exhibit excellent stability even when used in cold weather.

Hereinafter, the present invention will be described in detail.

 

The present inventors have studied the type and size of oxides on acicular ferrite known to be effective for CTOD of weld metal, and the amount of grain boundary ferrite and acicular ferrite changes depending on the presence of TiO and soluble B. It was found that the CTOD value of the weld metal part changes.

The welded structural steel of the present invention completed on the basis of these studies is large,

1. Composition using TiO oxide for high heat input weld metal such as SAW welding;

2. The structure of oxide distribution is 1.0 X10 <7> / mm <3> or more and the structure which controls the size of oxide to 0.01-0.1 micrometer; And

3. The structure to improve the toughness of the weld by securing TiO and soluble B in the welded joint to promote needle ferrite transformation to ensure the needle ferrite more than 85%;

Is done.

 1. TiO oxide management

If the ratio of Ti / O and O / B is properly maintained in the weld metal, the number of TiO oxides can be properly distributed to prevent coarsening of the austenite grains during the solidification of the weld metal and to promote needle ferrite transformation from the TiO oxide. Will be. In addition, when the TiO oxide is properly distributed in the austenite grains, as the temperature decreases in the austenite, it may act as a heterogeneous nucleus site of the acicular ferrite, and thus the acicular ferrite may be preferentially formed over the grain boundary ferrite formed at the grain boundaries. Due to the generation of a large amount of needle-like ferrite in the present invention it can significantly improve the CTOD characteristics of the weld metal.

For this purpose, it is important to distribute the TiO oxide finely and uniformly. As a result of the present inventors, when optimizing the ratio of Ti / O and O / B, the size, amount and distribution of TiO oxide pursued in the present invention can be obtained. I could see that. In the present invention, Ti / O and O / B are limited to Ti / O: 0.2 to 0.5 and O / B: 5 to 10, respectively. In this case, the TiO oxide having a size of 0.01-0.1 μm is 1.0 × 10 7 / mm 3 What was obtained above was confirmed.

2. Microstructure of welded joint

When a large amount of the TiO oxide obtained as described above is properly distributed in the welded joint, acicular ferrite transformation is promoted in the crystal grains in preference to the grain boundaries during the cooling process of the welded metal portion. Therefore, the present invention is characterized by forming a needle-like ferrite 85% or more in the welded joint by securing a large amount of the needle-like ferrite.

3. Role of Soluble Boron in Weld Joints

According to the research of the present inventors, apart from the oxide uniformly dispersed in the welded joint, the solid solution boron present in the welded joint diffuses to the grain boundaries, lowering the energy of the grain boundaries and suppressing grain boundary ferrite formation at the grain boundaries. Play a role. Such suppression of grain boundary ferrite can promote needle-like ferrite transformation in the grain, thus contributing to the improvement of CTOD of the welded joint.

 Hereinafter, the present invention will be described in more detail through the component system of the welded structural steel (hereinafter, referred to as% by weight).

C: 0.01 ~ 0.2%

C is added 0.01% or more to secure the strength of the weld metal and to secure the weld hardenability. However, if the content of C exceeds 0.2%, the weldability may be greatly reduced, low temperature cracks may occur in the welded joint, and the heat input impact toughness may be greatly reduced, so the range is limited to 0.01 to 0.2%.

Si: 0.1-0.5%

Si is an element added for the deoxidation effect, when the content of Si is less than 0.1%, the deoxidation effect in the weld metal is insufficient. In addition, too small Si may lower the fluidity of the weld metal, which is disadvantageous. On the other hand, if the content of Si exceeds 0.5%, it can promote the transformation of the MA constituent structure in the weld metal, so that the low-temperature impact toughness can be drastically lowered and the adverse effect on the weld crack susceptibility is caused. It is limited to 0.1 to 0.5%.

Mn: 1.0-3.0%

Mn is an alloying element that improves deoxidation and strength. In particular, in the present invention, Mn is precipitated in the form of MnS around the TiO oxide to promote the formation of acicular ferrite, which is advantageous for improving the toughness of the weld metal part. Therefore, in the present invention, Mn is added by 1.0% or more, but if the content is too high, it can lead to the formation of low temperature metamorphic tissue, so the upper limit is limited to 3.0%.

Ti: 0.01 ~ 0.1%

Ti is combined with O and can form fine Ti oxide and fine TiN precipitate, which is a very important element in the present invention. In order to obtain such a fine TiO oxide and TiN composite precipitate effect, Ti should be added at least 0.01%. However, when excessively excessive amount is added, coarse TiO oxide and coarse TiN precipitate may be formed, and thus the upper limit is 0.1%. It is limited to.

Ni: 0.5 ~ 3.0%

Ni is more than 0.5% because it is an effective element that enhances the strength and toughness of the matrix through the solid solution strengthening effect. However, when the content of Ni exceeds 3.0%, the hardenability is greatly increased and high temperature cracking may occur, so the upper limit is limited to 3.0%.

B: 0.0010-0.01%

B is an element that improves quenchability and segregates at grain boundaries and requires 0.0010% or more to suppress grain boundary ferrite transformation. However, when the amount of B exceeds 0.01%, the effect is saturated, the weld hardenability is greatly increased, and the upper limit is limited to 0.01% because it may promote the martensitic transformation and lower the welding low temperature crack and toughness. .

N: 0.003-0.006%

N is an element that forms precipitates, such as TiN, and increases the amount of fine TiN precipitates, especially because it has a significant effect on the TiN precipitate size, precipitate interval, precipitate distribution, the frequency of complex precipitation with oxide, the high temperature stability of the precipitate itself. , And the content is set to 0.003% or more. However, if the content of N exceeds 0.006%, the effect is saturated, and the toughness may appear due to an increase in the amount of solid solution N present in the weld metal, so the content of N is limited to 0.003 to 0.006%.

P: 0.030% or less

Since P is an impurity element that promotes high temperature cracking during welding, it is preferable to control P as low as possible. In order to improve toughness and reduce cracking, it is recommended to manage it to 0.03% or less.

Al: 0.005-0.05%

Al serves as a deoxidizer to reduce the amount of oxygen in the weld metal. In addition, it combines with the solid solution nitrogen to form a fine AlN precipitate, so for this effect is added 0.005% or more. However, if excessively excessive amount is added, coarse Al 2 O 3 may be formed and the formation of TiO oxide necessary for toughness improvement may be rather hindered, so the upper limit is limited to 0.05%.

S: 0.030% or less

S is an element necessary for MnS formation, so it is controlled to 0.03% or less for the precipitation of MnS composite precipitates. If the upper limit of S exceeds 0.030%, low melting point compounds such as FeS can be formed, which is not preferable because the possibility of high temperature cracking increases.

O: 0.05% or less

O is an element that forms Ti oxide by reacting with Ti during solidification of the weld metal, and this Ti oxide serves to promote the transformation of acicular ferrite in the weld metal. However, if the content of O is too large, coarse Ti oxide and other oxides such as FeO are generated, which adversely affects the weld metal part, so the content of O is controlled to 0.05% or less.

Ti / O: 0.2-0.5

If the Ti / O value is less than 0.2, the number of TiO oxides required for austenite grain growth inhibition and acicular ferrite transformation in the weld metal becomes insufficient. In particular, since the Ti ratio contained in the TiO oxide is low, the needle ferrite nucleation role can be lost, and the needle ferrite phase fraction effective for improving the toughness of the weld heat affected zone is lowered. On the other hand, when the Ti / O value exceeds 0.5, the effect of inhibiting the growth of austenite grains in the weld metal is saturated, and the proportion of the alloying components contained in the oxide is rather small, thus functioning as a nucleation site of the acicular ferrite. Ti / O ratio is controlled at 0.2 ~ 0.5 level as it may be lost.

Ti / N: 2 ~ 5

When the Ti / N ratio is less than 2, the amount of TiN precipitates formed in the TiO product decreases, making it difficult to promote needle ferrite transformation, which is effective for improving toughness. On the other hand, when the value exceeds 5, the effect is saturated and the amount of solid solution N increases. Since toughness may fall, the value of Ti / N is limited to 2-5.

O / B: 5 ~ 10

If the value of O / B is less than 5, the amount of solid solution B which diffuses into the austenite grain boundary during the post-weld cooling process and suppresses grain boundary ferrite transformation is insufficient, whereas when the value of O / B exceeds 10, The effect is saturated and the amount of solid solution nitrogen increases, which may lower the toughness of the weld heat affected zone. Therefore, the value of O / B is controlled from 5 to 10.

(Ti + 4B) / O: 0.7-1.5

In the present invention, when the value of (Ti + 4B) / O is less than 0.7, the amount of solid solution N increases, which is not effective for improving the toughness of the weld metal part, whereas when the value of (Ti + 4B) / O is less than 0.7, the number of precipitates, such as TiN and BN, is insufficient. I can't.

In the present invention, one or two or more of Nb, V, Cu, Mo, Cr, W, and Zr are further added to the steel formed as described above.

Cu: 0.1 ~ 2.0%

Cu is an element that is dissolved in the base and is useful for improving strength and toughness through a solid solution strengthening effect. To this end, the Cu content may be included in an amount of 0.1% or more. However, when the content is more than 2.0%, the hardness of the weld metal may be increased to reduce toughness and the high temperature crack may be promoted in the weld metal. It is limited to 0.1-2.0%.

In addition, in the case of complex addition of Cu and Ni, the total of these is adjusted to 3.5% or less. The reason is that when the sum of the added amounts of Cu and Ni exceeds 3.5%, the hardenability becomes too large, which is not good for toughness and weldability.

 

Nb: 0.0001-0.1%

Nb is an element to improve the hardenability, and in particular, it has an effect of lowering the Ar 3 temperature and broadening the formation range of acicular ferrite tissue even at a low cooling rate, thereby helping to obtain acicular ferrite structure efficiently. Therefore, in order to expect such an effect of improving strength, 0.0001% or more may be added. However, when the Nb content exceeds 0.1%, the formation of phase martensite in the weld metal part may be accelerated, thereby reducing the toughness of the weld metal part. The content of Nb is limited to 0.0001 to 0.1% range.

V: 0.005 ~ 0.1%

V is an element that promotes ferrite transformation by forming VN precipitates, and it is preferable to add it to 0.005% or more.However, if it exceeds 0.1%, V forms a hard phase such as carbide in the weld metal, which is not good for the toughness of the weld metal. Limited to 0.005 ~ 0.1%.

Cr: 0.05-1.0%

Cr improves the hardenability and strength, but if the content is less than 0.05%, such an effect is insignificant, whereas if it exceeds 1.0%, the content is limited since it may cause toughness of the weld metal part.

Mo: 0.05-1.0%

Mo is an element that improves quenchability and strength, and its content is preferably 0.05% or more for securing strength, but the upper limit thereof is limited to 1.0% in order to suppress hardening of the weld metal portion and generation of low temperature welding cracks.

W: 0.05-0.5%

W can be added at 0.05% or more since it acts as an effective element for enhancing high temperature strength and strengthening precipitation. However, if the content exceeds 0.5%, the content of the weld metal part is not good, so the content is limited to 0.05 to 0.5%.

Zr: 0.005 ~ 0.5%

Zr can be added more than 0.005% because it is effective in increasing the strength, but if the content exceeds 0.5% Zr content is limited to 0.005 ~ 0.5% because it adversely affects the toughness of the weld metal.

In addition, in the present invention, Ca and / or REM may be additionally added to suppress grain growth of guustenite.

Ca and REM function as elements capable of stabilizing the arc during welding and forming oxides in the weld metal part. In addition, by suppressing the growth of austenite grains during the cooling process, and promotes intra-ferrite ferrite transformation to improve the toughness of the weld metal part. To this end, Ca may be added 0.0005% or more, REM is 0.005% or more, but when Ca is 0.05%, REM is more than 0.05% may form a large oxide to adversely affect the toughness, so the content You need to control it. Specifically, Ce, La, Y, Hf, or the like can be used as the element that can be used as the REM.

Hereinafter, the microstructure and oxide constituting the welded structural steel of the present invention will be described in detail.

Primary tissue: Over 85% of acicular ferrite tissue

In the present invention, the microstructure of the weld metal formed after SAW welding is composed of acicular ferrite, and its phase fraction is 85% or more. The needle-like ferrite structure, like ferrite + bainite structure, is advantageous for CTOD but has low weld metal part strength, but the martensite + bainite mixed structure shows a high strength of the weld metal part but does not have good mechanical properties such as CTOD of the weld metal part. Unlike tissues that are highly susceptible to cold cracking, they can function as tissues that can simultaneously acquire both high strength and low temperature CTOD. And the remainder of the structure except the needle ferrite may be composed of a polygonal ferrite and a small amount of grain boundary ferrite.

Oxides: TiO oxides are uniformly dispersed at intervals of 0.5 μm or less, and the particle diameter and the critical number thereof are 0.01 to 0.1 μm and 1.0 × 10 7 or more per 1 mm 3

In general, the type, size, number, etc. of oxides present in the weld metal part have a great influence on the microstructure transformation of the weld metal part after welding. In particular, in the case of SAW weld metal parts, grains are coarsened during the solidification process, and coarse grain boundary ferrite, Widmanstatten ferrite, and bainite may be formed from the grain boundary. In order to prevent this, in the present invention, TiO oxide is uniformly dispersed in the weld metal at intervals of 0.5 μm or less, and the particle size and the critical number of TiO oxides are limited to 0.01 × 0.1 μm and 1.0 × 10 7 or more per 1 mm 3 . If the particle size of the oxide is smaller than 0.01 μm, it does not play a role in promoting the transformation of acicular ferrite in the SAW weld metal part, whereas if it exceeds 0.1 μm, the pinning effect on the austenite grain is reduced. The same behavior as coarse nonmetallic inclusions may adversely affect the CTOD characteristics of the weld metal.

Welded structural steel such as the present invention may be sufficiently applied by other welding methods other than the SAW welding method. Particularly, in the present invention, a high heat input welding process having a high cooling speed is preferable because the oxide is finely dispersed and the structure is fine when the cooling speed of the welding metal part is high. For the same reason, it is also advantageous to steel cooling and Cu-backing methods in order to improve the cooling rate of the weld. However, even if the well-known techniques are applied to the present invention, it is natural that they are interpreted to be substantially within the technical scope of the present invention as a simple change of the present invention.

Hereinafter, the present invention will be described in detail by way of examples.

EXAMPLE

The weld metal part having the composition as shown in Table 1 was manufactured by SAW welding by applying a welding heat input of 30 to 45 kJ / cm or more, and the composition ratio between the alloy elements of the weld metal part to show the effect of the present invention is shown in Table 2. Indicated.

As described above, the test pieces for evaluating the mechanical properties of the welded metal part were taken from the center part of the welded metal part, and the tensile test piece used KS standard (KS B 0801) No. 4 test piece. ) At 10 mm / mim. CTOD specimens were prepared in accordance with BS7448-1 and fatigue cracks were located in the center of the SAW weld metal.

The size, number, and spacing of oxides, which have a significant effect on the CTOD of the weld metal, were measured by the point counting method using an image analyzer and an electron microscope. At this time, the test surface was evaluated based on 100 mm 2 . In addition, the CTOD evaluation of the SAW weld metal part was processed into a CTOD test piece after SAW welding, and evaluated through a CTOD tester at -10 ° C.

Chemical composition (% by weight) C Si Mn P S Ni Mo Ti B (ppm) N (ppm) Cu Al Cr Nb V Ca REM O (ppm) Inventive Steel 1 0.06 0.19 1.54 0.010 0.005 1.54 0.14 0.017 50 45 - 0.01 - - - - - 300 Inventive Steel 2 0.07 0.32 1.50 0.012 0.005 1.44 0.15 0.016 45 54 - 0.005 - - - - - 340 Invention Steel 3 0.08 0.25 1.48 0.011 0.004 1.65 0.15 0.013 52 53 0.05 0.04 - - - - - 380 Inventive Steel 4 0.08 0.22 1.48 0.008 0.005 1.54 0.12 0.012 50 50 - 0.03 - - - - - 380 Inventive Steel 5 0.07 0.16 1.60 0.011 0.004 1.50 0.10 0.015 45 50 - 0.01 - - - - - 350 Inventive Steel 6 0.07 0.14 1.50 0.09 0.005 1.65 0.12 0.010 42 45 - 0.02 - 0.1 - - - 380 Inventive Steel 7 0.10 0.25 1.48 0.011 0.005 1.45 0.15 0.018 45 55 0.04 0.02 - - - - - 360 Inventive Steel 8 0.11 0.35 1.52 0.012 0.006 1.55 0.18 0.014 46 40 - 0.01 - - 0.01 - - 340 Inventive Steel 9 0.09 0.28 1.50 0.010 0.005 1.48 0.20 0.014 40 52 - 0.01 0.1 - - 0.001 - 350 Inventive Steel 10 0.07 0.18 1.55 0.009 0.006 1.50 0.25 0.016 43 50 - 0.01 - - - - 0.005 360 Comparative Steel 1 0.03 0.06 1.25 0.011 0.006 2.60 0.19 0.01 29 92 0.02 0.005 - - - - - 150 Comparative Steel 2 0.05 0.13 1.93 0.011 0.004 1.71 0.20 0.025 69 110 0.04 0.001 - - - - - 120 Comparative Steel 3 0.06 0.06 1.25 0.010 0.007 1.61 0.010 0.014 21 74 - 0.007 - - - - - 150 Comparative Steel 4 0.04 0.19 2.0 0.008 0.004 1.75 0.15 0.02 105 56 0.02 - - - - - - 300 Comparative Steel 5 0.06 0.28 1.56 0.013 0.008 1.46 0.14 0.058 58 71 0.012 - - - - - - 170 Comparative Steel 6 0.06 0.26 1.53 0.012 0.007 1.50 0.16 0.057 52 140 0.03 0.012 - - - - - 240 Comparative Steel 7 0.05 0.22 1.58 0.015 0.008 1.51 0.12 0.04 41 270 0.03 0.01 - - - - - 260 Comparative Steel 8 0.07 0.14 1.56 0.011 0.006 1.52 0.11 0.024 42 180 0.32 0.03 - - 0.013 - - 200 Comparative Steel 9 0.06 0.37 1.74 0.015 0.010 1.44 0.17 0.081 11 100 0.03 0.02 - - - - - 140 Comparative Steel 10 0.05 0.26 1.66 0.009 0.004 0.05 0.15 0.042 45 130 - 0.006 - - - - - 250 Comparative Steel 11 0.06 0.23 1.72 0.008 0.004 1.30 0.14 0.03 52 230 0.05 0.01 - - - - - 290

Alloy element composition ratio for showing the effect of the present invention Ti / O Ti / N O / B (Ti + 4B) / O Inventive Steel 1 0.5 3.8 6.0 1.2 Inventive Steel 2 0.5 3.0 7.6 1.0 Invention Steel 3 0.3 2.4 7.3 0.9 Inventive Steel 4 0.3 2.6 7.6 0.8 Inventive Steel 5 0.4 3.0 7.8 0.9 Inventive Steel 6 0.3 2.2 9.0 0.7 Inventive Steel 7 0.5 3.3 8.0 1.0 Inventive Steel 8 0.4 3.5 7.4 1.0 Inventive Steel 9 0.4 2.7 8.8 0.9 Inventive Steel 10 0.4 3.2 8.4 0.9 Comparative Steel 1 0.7 1.1 5.2 1.4 Comparative Steel 2 2.0 2.3 1.7 4.4 Comparative Steel 3 0.9 1.9 7.1 1.5 Comparative Steel 4 0.7 3.6 2.9 2.1 Comparative Steel 5 3.4 8.2 2.9 4.8 Comparative Steel 6 2.4 4.1 4.6 3.2 Comparative Steel 7 1.5 1.5 6.3 2.2 Comparative Steel 8 1.2 1.3 4.8 2.0 Comparative Steel 9 5.8 8.1 12.7 6.1 Comparative Steel 10 1.6 3.2 5.6 2.4 Comparative Steel 11 1.0 1.3 5.6 1.8

division Welding process and heat input TiO oxide Acicular ferrite fraction of weld metal (%) Mechanical Properties of Welded Metal Parts Welding process Welding heat input (kJ / cm) Number (pcs / mm 3 ) Average size (㎛) Tensile Strength (MPa) CTOD (mm, -10 ℃) Inventive Steel 1 SAW 30 3.4 × 10 8 0.016 89 640 1.2 Inventive Steel 2 SAW 35 4.6 × 10 8 0.017 89 630 1.3 Invention Steel 3 SAW 32 3.7 × 10 8 0.012 87 660 1.1 Inventive Steel 4 SAW 40 4.6 × 10 8 0.016 88 640 0.9 Inventive Steel 5 SAW 38 6.4 × 10 8 0.018 87 650 1.2 Inventive Steel 6 SAW 39 5.2 × 10 8 0.025 89 630 1.3 Inventive Steel 7 SAW 42 3.6 × 10 8 0.013 90 640 1.4 Inventive Steel 8 SAW 430 4.3 × 10 8 0.026 91 660 1.0 Inventive Steel 9 SAW 430 5.6 × 10 8 0.024 88 665 1.1 Inventive Steel 10 SAW 45 5.3 × 10 8 0.014 85 620 1.1 Comparative Steel 1 SAW 120 3.0 × 10 6 0.045 46 650 0.2 Comparative Steel 2 SAW 120 4.3 × 10 6 0.051 52 640 0.3 Comparative Steel 3 SAW 120 2.5 × 10 6 0.054 44 650 0.3 Comparative Steel 4 SAW 120 3.0 × 10 6 0.064 45 660 0.3 Comparative Steel 5 SAW 120 2.5 × 10 5 0.037 37 650 0.2 Comparative Steel 6 SAW 130 2.5 × 10 6 0.056 42 680 0.1 Comparative Steel 7 SAW 130 3.0 × 10 6 0.043 44 665 0.3 Comparative Steel 8 SAW 230 4.1 × 10 5 0.046 52 610 0.2 Comparative Steel 9 SAW 130 2.8 × 10 5 0.041 59 610 0.4 Comparative Steel 10 SAW 230 3.4 × 10 5 0.046 52 620 0.3 Conventional Steel 11 SAW 130 2.6 × 10 6 0.043 41 625 0.3

As shown in Table 3, the number of TiO oxides manufactured by the present invention had a TiO oxide number of 2 × 10 8 / mm 3 or more, whereas the comparison steel showed a range of 4.3 X 10 6 / mm 3 or less. It can be seen that the number of the invention steel is significantly increased while the invention steel has a relatively uniform and fine complex precipitate size compared to the comparative steel. On the other hand, in the case of the microstructure of the present invention, the acicular ferrite phase fraction is all composed of a high fraction of 85% or more. Therefore, in the SAW welding, the present invention steel is composed of intragranular acicular ferrite and polygonal ferrite. Among them, the acicular ferrite phase ratio is more than 85% and shows excellent CTOD characteristics of the welded metal part compared to the comparative steel.

Claims (8)

By weight%, C: 0.01-0.2%, Si: 0.1-0.5%, Mn: 1.0-3.0%, Ti: 0.01-0.1%, Ni: 0.5-3.0%, B: 0.0010-0.01%, N: 0.003- 0.006%, P: 0.030% or less, Al: 0.005 to 0.05%, S: 0.030% or less (excluding 0), O: 0.05% or less (excluding 0), other unavoidable impurities and residual Fe, and the Ti Between O, N, and B, there is a weld where Ti / O: 0.2 to 0.5, Ti / N: 2 to 5, O / B: 5 to 10, and (Ti + 4B) / O: 0.7 to 1.5. A welded steel structure comprising a welded joint, wherein the microstructure of the welded joint has a needle-like ferrite of 85% or more as a tissue fraction, and the remainder includes a welded joint of polygonal ferrite and other grain boundary ferrite. According to claim 1, The welded structural steel is Cu: 0.1 ~ 2.0%, Nb: 0.0001 ~ 0.1%, V: 0.005 ~ 0.1%, Cr: 0.05 ~ 1.0%, Mo: 0.05 ~ 1.0%, W: 0.05 ~ 0.5 %, And Zr: Welded structural steel comprising a welded joint further comprising one or two or more components selected from the group consisting of 0.005 to 0.5%. 3. The welded structural steel according to claim 2, wherein when Cu and Ni are added to the welded structural steel, the sum thereof is 3.5% or less. According to claim 1, The welded structural steel is characterized in that it comprises a weld joint additionally comprises Ca: 0.0005 ~ 0.05%, REM: 0.005 ~ 0.05% or Ca: 0.0005 ~ 0.05% and REM: 0.005 ~ 0.05%. Welded structural steel. delete The welded joint of any one of claims 1 to 4, wherein the welded joint of the welded structural steel includes a welded joint in which TiO oxide is uniformly dispersed at intervals of 0.5 µm or less (excluding 0) in the structure. Welded structural steel. The weld structural steel according to claim 6, wherein the TiO oxide comprises a welded joint having a particle diameter of 0.01 to 0.1 mu m. Claim 6, wherein the welded construction Steel Number of TiO oxide particles in the welded joints is 1mm per 3 1.0x10 7 6.4x10 8 gae ~ individual welding the welding structural steel comprising the vagina on.
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