KR20090070147A - Flux cored arc weld metal joint having superior ctod in low temperature - Google Patents

Abstract

A flux cored arc weld metal part with a superior low temperature CTOD property is provided to secure the high intensity property by accelerating the acicular ferrite transformation at the welding metal part using TiO oxide and soluble B. A flux cored arc weld metal part with a superior low temperature CTOD property comprises followings. Carbon is added in order to secure the intensity and the welding hardening of the weld metal. Manganese is added in order to improve deoxidation action and intensity of the steel. Titanium is added in order to form minute TiN precipitate and the minute titanium oxide. Nickel is added in order to improve the intensity and toughness of base material.

Description

Flux Cored Arc Weld Metal Joint Having Superior CTOD in Low Temperature

The present invention relates to a weld metal part when performing flux cored arc welding (FCAW) used in welding structures such as ships, buildings, bridges, offshore structures, steel pipes, and line pipes. Preferably, the present invention relates to a flux cored arc welding metal part having excellent low temperature CTOD characteristics.

Recently, offshore structures are being built and used in more cold conditions due to the continuous rise in oil prices, and the steel materials used are required to have high strength and low temperature CTOD characteristics.

In order to secure the stability of these offshore structures, the CTOD characteristics of the welded part of the offshore structures are of paramount importance.

In general, in the case of flux-cored welding of offshore structures, the heat input range is about 7-25 kJ / cm.

In general, the weld metal (Weld Metal) formed during welding solidifies coarse columnar structure, and coarse grain boundary ferrite and Widmanstatten ferrite are formed along the austenite grain boundary in the coarse grain, so that the weld metal portion is welded. It is the site where the CTOD characteristics deteriorate most.

Therefore, in order to secure the stability of the welded structure, it is necessary to control the microstructure of the weld metal part to secure CTOD characteristics of the weld metal part. As a means to solve this problem, there is a technique that defines the components of the welding material, for example, Japanese Patent Laid-Open No. Hei 11-170085, but the microstructure of the weld metal, particle size, etc. are controlled. It is difficult to obtain sufficient weld metal part toughness with this welding material.

In 2005-171300, C: 0.07% or less, Si: 0.3% or less, Mn: 1.0 to 2.0%, P: 0.02% or less, S is 0.1% or less, sol.Al: 0.04 to 0.1%, N: 0.0020 to ARM is defined as ARM = 197-1457C-1140sol.Al + 11850N-316 (Pcm-C) in the composition consisting of 0.01%, Ti: 0.005 ~ 0.02%, B: 0.005 ~ 0.005% However, the specified ARM has no restriction on the oxygen content in the weld, so it is difficult to secure impact toughness of the SAW high heat input weld metal.

In addition, Japanese Patent Laid-Open No. 10-180488 describes slag generating agents: 0.5 to 3.0%, C: 0.04 to 0.2%, Si≤0.1%, Mn: 1.2 to 3.5%, Mg: 0.05 to 0.3%, and Ni: It has good impact toughness, including 0.5 ~ 4.0%, Mo: 0.05 ~ 1.0%, and B: 0.002 ~ 0.015%. However, since there is no description of oxygen and nitrogen content in the weld metal, it is possible to secure CTOD characteristics of the weld metal part. it's difficult.

An object of the present invention is to provide a flux cored arc welding metal part having high strength properties and excellent low temperature CTOD properties by promoting intragranular acicular ferrite transformation using Ti oxide and soluble B.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, C: 0.01-0.2%, Si: 0.1-0.5%, Mn: 1.0-3.0%, Ni: 0.5-3.0%, Ti: 0.01-0.1%, B: 0.0010-0.01%, Al by weight% : 0.005-0.05%, N: 0.003-0.006%, P: 0.03% or less, S: 0.03% or less, O: 0.03-0.07%, 0.7≤Ti / O≤1.3, 6≤Ti / N≤12, 7≤ O / B≤12, 1.2≤ (Ti + 4B) /O≤1.9, and are composed of the remaining Fe and other unavoidable impurities, the microstructure of which is more than 85% acicular ferrite and the remaining bainite, grain boundary The present invention relates to a flux cored arc welded metal part having excellent low-temperature CTOD characteristics, including one or two or more of ferrite and polygonal ferrite.

In the weld metal part, Nb: 0.0001 to 0.1%, V: 0.005 to 0.1%, Cu: 0.01 to 2.0%, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%, W: 0.05 to 0.5%, Zr: 0.005 One or two or more selected from the group of ˜0.5% and / or one or two selected from the group of Ca: 0.0005 to 0.005% and REM: 0.005 to 0.05%.

In the weld metal part, 0.01 to 0.1 μm of TiO oxide is preferably distributed at 1.0 × 10 ⁷ / mm ³ or more.

The present invention can provide a flux cored arc welding metal part having high strength properties and excellent low temperature CTOD properties by promoting the acicular ferrite transformation in the weld metal part by using TiO oxide and soluble B.

Hereinafter, the present invention will be described in detail.

In order to develop a weld metal part having high strength properties and excellent CTOD in FCAW welding having a welding heat input of 7 to 30 kJ / cm, the present inventors have described the type of oxide that affects acicular ferrite known to be effective for CTOD of a weld metal part. As a result of the investigation on the size and the size, it was found that the amount of grain boundary ferrite and acicular ferrite of the weld metal portion changes according to TiO and soluble B, and the CTOD value of the weld metal portion changes accordingly.

Based on these studies, in the present invention,

[1] With the use of TiO oxides in FCAW weld metals,

[2] Oxide number 1.0 X10 ⁷ / mm ³ or more in the weld metal part of 0.01 ~ 0.1㎛ size to improve the toughness by transforming the needle-like ferrite more than 85%.

[3] It is possible to promote needle ferrite transformation by securing TiO and soluble B.

These [1] [2] [3] are demonstrated in more detail.

[1] TiO oxide management

When the ratio of Ti / O and O / B is properly maintained in the weld metal, the present inventors properly distribute the appropriate number of TiO oxides to prevent coarsening of austenite grains in the solidification process of the weld metal, and to prevent acicular ferrite transformation from TiO oxide. It was found to be accelerated.

If the TiO oxide is properly distributed in the austenite grains, as the temperature decreases in the austenite grains, the TiO oxides in the austenite grains act as heterogeneous nucleation sites of the acicular ferrite transformation, and thus preferentially over the grain boundary ferrites formed at the grain boundaries. It turns out that ferrite can be formed. This significantly improves the CTOD characteristics of the weld metal part.

For this purpose, it is important to distribute TiO oxide finely and uniformly. In addition, the inventors examined the size, amount, and distribution of TiO oxides according to the ratios of Ti / O and O / B. As a result, when Ti / O is 0.2 to 0.5 and the ratio of O / B is 5 to 10, the thickness is 0.01-0.1 μm. It was confirmed that the TiO oxide of the size is obtained at 1.0x10 ⁷ / mm ³ or more.

[2] microstructure, welded metal

The facts of the present invention revealed that when Ti / O is 0.2 to 0.5 and O / B is 5 to 10, the size and amount of TiO oxide and the distribution of Ti / O and O / B are examined. It was confirmed that the TiO oxide having a size of 0.01-0.1 μm was obtained at 1.0 × 10 ⁷ / mm ³ or more.

If the TiO oxide is properly distributed in the weld metal, it is possible to secure the composition ratio of the acicular ferrite in the weld metal part by promoting needle transformation of the acicular ferrite in the crystal grain preferentially rather than the grain boundary during the cooling process of the weld metal part.

[3] soluble boron role in welded metal parts

In fact, in the study of the present invention, boron, which is separately dissolved in the oxide uniformly dispersed in the weld metal part, diffuses into the grain boundary and lowers the energy of the grain boundary, thereby suppressing the grain boundary ferrite transformation at the grain boundary. It plays a role in promoting needle ferrite transformation in the grain. In this way, the grain boundary ferrite transformation is suppressed at the grain boundaries, and the needle grain ferrite transformation is promoted in the grains, thereby contributing to the improvement of the CTOD of the weld metal part.

Hereinafter, the present invention will be described in detail the components of the weld metal portion.

[ingredient]

It is preferable to make content of carbon (C) into 0.01 to 0.2%.

Carbon (C) is an essential element in order to secure the strength of the weld metal and to secure weld hardenability. However, when the carbon content exceeds 0.2%, weldability is greatly decreased, and low-temperature cracking is likely to occur in the weld metal part, and the thermal shock toughness is greatly reduced.

The content of silicon (Si) is preferably limited to 0.1 ~ 0.5%.

If the silicon content is less than 0.1%, the deoxidation effect in the weld metal is insufficient and the fluidity of the weld metal is insufficient. If the content of the silicon is more than 0.5%, the low temperature impact toughness is promoted by promoting the transformation of MA constituent in the weld metal. It is not preferable because it lowers the pressure and affects the weld cracking susceptibility.

The content of manganese (Mn) is preferably limited to 1.0 ~ 3.0%.

Mn, along with the effective effect of improving the deoxidation and strength in steel, precipitates in the form of MnS around the TiO oxide to play a role in promoting the formation of acicular ferrite, which is advantageous for improving the toughness of the weld metal part by the Ti complex oxide.

Such Mn forms a solid solution to form a solid solution in the matrix structure to strengthen the matrix to secure the strength and toughness, for this purpose it is preferably contained 1.0% or more. However, if it exceeds 3.0%, it is not preferable because it generates low temperature metamorphic tissue.

The content of titanium (Ti) is preferably limited to 0.01 ~ 0.1%.

Ti is indispensable in the present invention because it combines with O to form a fine Ti oxide as well as to form a fine TiN precipitate. In order to obtain such a fine TiO oxide and TiN composite precipitate effect, it is preferable to add Ti or more than 0.01%, but when it exceeds 0.1%, coarse TiO oxide and coarse TiN precipitate are formed, which is not preferable.

The content of nickel (Ni) is preferably limited to 0.5 ~ 3.0%.

Ni is an effective element which improves the strength and toughness of the matrix by solid solution strengthening. In order to obtain such an effect, it is preferable to contain Ni content of 0.5% or more, but when it exceeds 3.0%, it is not preferable because it greatly increases the hardenability and there is a possibility of hot cracking.

The content of boron (boron, B) is preferably limited to 0.0010-0.01%.

B is an element that improves the quenching property, and segregation at grain boundaries is required at least 0.0010% to suppress grain boundary ferrite transformation, but when it exceeds 0.01%, the effect is saturated and weld hardenability is greatly increased to promote martensite transformation. It is not preferable because it causes low temperature crack generation and toughness. Therefore, the B content is limited to 0.0010% to 0.01%.

The content of nitrogen (N) is preferably limited to 0.003-0.006%.

N is an indispensable element for forming TiN precipitates and the like, and increases the amount of fine TiN precipitates. In particular, since the TiN precipitate size and precipitate spacing, precipitate distribution, complex precipitation frequency with oxide, high temperature stability of the precipitate itself, etc. have a significant influence, the content is preferably set to 0.003% or more.

However, if the nitrogen content exceeds 0.006%, the effect is saturated, and the toughness may be reduced due to the increase in the amount of solid solution nitrogen present in the weld metal.

The content of phosphorus (P) is preferably limited to 0.030% or less.

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

The content of aluminum (Al) is preferably limited to 0.005-0.05%.

Al is a necessary element because it reduces the amount of oxygen in the weld metal as a deoxidizer. In addition, in order to form fine AlN precipitates in combination with solid solution nitrogen, the Al content is preferably 0.005% or more. However, if it exceeds 0.05%, coarse Al ₂ O ₃ is formed, which hinders formation of TiO oxide necessary for toughness improvement.

The content of sulfur (S) is preferably limited to 0.030% or less.

S is an element necessary for MnS formation. In order to precipitate the composite precipitate of MnS, it is preferable to be 0.03% or less. If more than this is present, it is not preferable because it may cause a high temperature crack by forming a low melting point compound such as FeS.

The content of oxygen (O) is preferably limited to 0.03-0.07% or less.

O is an element that forms Ti oxide by reacting with Ti during the solidification process of the weld metal, and Ti oxide promotes the transformation of acicular ferrite in the weld metal. If the O content is less than 0.03%, the Ti oxide is not properly distributed in the weld metal part. If the O content is more than 0.07%, coarse Ti oxide and other oxides such as FeO are formed, which is not preferable because it affects the weld metal part.

It is preferable to make ratio of Ti / O into 0.7-1.3.

If the Ti / O ratio is less than 0.7, the number of TiO oxides required for austenite grain growth inhibition and acicular ferrite transformation in the weld metal is insufficient, and the Ti ratio contained in the TiO oxide becomes small, thus losing its function as acicular ferrite nucleation site. As a result, the acicular ferrite phase fraction effective for improving the toughness of the weld heat affected zone is lowered. When the ratio of Ti / O exceeds 1.3, the austenite grain restraining effect in the weld metal is saturated, and the proportion of the alloying components contained in the oxide is rather small, thus losing the function of nucleation sites of the needle-like ferrite.

It is preferable to make ratio of Ti / N into 6-12.

In the present invention, when the Ti / N ratio is less than 6, the amount of TiN precipitates formed in the TiO oxide is reduced, which is not preferable because it adversely affects the needle ferrite transformation, which is effective for improving toughness. It is not preferable because the amount of nitrogen increases and the impact toughness is lowered.

It is preferable to make ratio of O / B into 7-12.

In the present invention, when the O / B ratio is less than 7, the amount of solid solution B that diffuses into the austenite grain boundary during the post-welding cooling process to suppress grain boundary ferrite transformation is insufficient, and when the O / B ratio exceeds 12, the effect is saturated. In addition, the amount of nitrogen dissolved in the solution increases the toughness of the weld heat affected zone.

It is preferable to make ratio of (Ti + 4B) / O into 1.2-1.9.

In the present invention, when the ratio of (Ti + 4B) / O is less than 1.2, it is not preferable because the amount of solid solution is not effective to improve the toughness of the weld metal part, and when it exceeds 1.9, the number of TiN and BN precipitates is insufficient.

In the present invention, in order to further improve the mechanical properties, one or two or more selected from the group of Nb, V, Cu, Mo, Cr, W and Zr may be further added to the steel formed as described above.

The content of copper (Cu) is preferably limited to 0.1 ~ 2.0%.

Cu is an element that is effective to secure strength and toughness due to solid solution at the base. To this end, the Cu content should be contained 0.1% or more, but when it exceeds 2.0% is not preferable because it increases the hardenability in the weld metal portion to reduce toughness and promote high temperature cracking in the weld metal.

In addition, when adding Cu and Ni compositely, it is preferable to make these sum total less than 3.5%. The reason is that less than 3.5% of the hardenability increases, which adversely affects the toughness and weldability.

The content of Nb is preferably limited to 0.0001-0.1%.

Nb is an essential element for improving the hardenability, and in particular, it is necessary to obtain bainite structure because it has an effect of lowering the Ar ₃ temperature and widening the bainite formation range even in a low cooling rate range.

In order to expect the effect of improving strength, 0.0001% or more is required. However, if the content exceeds 0.1%, it is not preferable because it promotes the formation of phase martensite in the weld metal part during welding, which adversely affects the toughness of the weld metal part.

The content of V is preferably limited to 0.005-0.1%.

V is an element that promotes ferrite transformation by forming VN precipitates, but it requires 0.005% or more, but when it exceeds 0.1%, it forms a hardened phase such as carbide in the weld metal, which adversely affects the toughness of the weld metal. Not desirable

It is preferable to make chromium (Cr) into 0.05 to 1.0%.

Cr increases the hardenability and also improves the strength. If the content is less than 0.05%, the strength cannot be obtained and if the content exceeds 1.0%, the toughness of the weld metal portion is degraded.

Molybdenum (Mo) is preferably made 0.05 to 1.0%.

Mo is an element that increases the hardenability and improves the strength at the same time, the content is 0.05% or more to secure the strength, but in order to suppress the hardening of the weld metal and the occurrence of welding low temperature cracks, the upper limit is 1.0% as in Cr. .

The content of W is preferably limited to 0.05-0.5%.

W is an effective element for improving high temperature strength and strengthening precipitation. However, less than 0.05% is not preferable because the strength increase effect is weak, and at 0.5% or more, it is not preferable because it adversely affects the weld metal part toughness.

The content of Zr is preferably limited to 0.005-0.5%.

Since Zr is effective in increasing the strength, it is preferable to add 0.005% or more, and when it exceeds 0.5%, it is not preferable because it adversely affects the toughness of the weld metal part.

In the present invention, one or two of Ca and REM may be further added to suppress grain growth of the old austenite.

Ca and REM are preferred elements because they stabilize the arc during welding and form oxides in the weld metal portion. In addition, it suppresses austenite grain growth during cooling and promotes ferrite transformation in the mouth, thereby improving the toughness of the weld metal part. To this end, it is preferable to add more than 0.0005% of calcium (Ca) and more than 0.005% of REM. However, when Ca is 0.05% and REM is more than 0.05%, a large oxide may be formed to adversely affect toughness. As REM, 1 type, or 2 or more types, such as Ce, La, Y, and Hf, may be used, and any of the above effects can be obtained.

[Microstructure of Welding Metal Part]

In the present invention, the microstructure of the weld metal formed after FCAW welding is acicular ferrite, and its phase fraction is preferably 85% or more. The reason is that needle-like ferrite tissue is a tissue that can simultaneously obtain high strength and low temperature CTOD.

The remainder includes one or two or more of bainite, grain boundary ferrite and polygonal ferrite.

When ferrite and bainite structures are mixed, it is advantageous for CTOD, but the strength of weld metal part is low, and when the microstructure is martensite and bainite structure, the strength of weld metal part is high but mechanical properties such as CTOD of welding metal part are high. It is not preferable because it decreases and the low temperature crack susceptibility increases.

[oxide]

Oxides present in the weld metal part have a great influence on the microstructure transformation of the weld metal part after welding. That is, the type, size, and number of oxides to be distributed are greatly affected.

In particular, in the FCAW weld metal part, grains are coarsened during solidification and coarse grain boundary ferrite, Widmanstatten ferrite, and bainite are formed from the grain boundary, thereby deteriorating the properties of the weld metal part.

In order to prevent this, it is important to uniformly disperse the TiO oxide in the weld metal at intervals of 0.5 mu m or less.

In addition, it is preferable to limit the particle diameter and the critical number of the TiO oxide to 0.01 to 0.1 mu m and 1.0 x 10 ⁷ or more per 1 mm ³ , respectively. The reason is that if it is less than 0.01㎛, it does not play a role of promoting the transformation of acicular ferrite in the FCAW welded metal part, and if it exceeds 0.1㎛, the effect of pinning on grains of austenite grains decreases and This is because the same behavior as the non-metallic inclusions on the metal adversely affects the CTOD characteristics of the weld metal.

In the present invention can also be produced by other welding process (Process) other than FCAW. In this case, if the cooling rate of the weld metal part is high, a high heat input welding process having a high cooling rate is preferable because the oxide is finely dispersed and the structure is fine.

For the same reason, steel cooling and Cu-backing methods are also advantageous 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 through examples.

EXAMPLE

The weld metal parts having the composition shown in Table 1 and Table 2 were prepared by FCAW by applying a welding heat input of 7 to 30 kJ / cm or more.

The specimen was taken from the center of the welded metal part welded as described above, and subjected to tensile test and CTOD test. The results are shown in Table 3 below.

The test piece of the tensile test was used KS standard (KS B 0801) No. 4 test piece and the tensile test was carried out at a cross head speed (10 mm / mim).

The CTOD test piece was manufactured according to the BS7448-1 standard and the fatigue crack was located at the center of the SAW weld metal part.

The size, number, and spacing of oxides, which have a significant effect on the CTOD of the weld metal, were measured by a point counting method using an image analyzer and an electron microscope, and the results are shown in Table 3 below.

At this time, the test surface was evaluated based on 100 mm ² .

The CTOD evaluation of the FCAW weld metal part was processed by CTOD test piece after FCAW welding and evaluated by CTOD tester at -10 ° C.

Psalm No. 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 0.034 50 45 - 0.001 - - - - - 380 Inventive Steel 2 0.07 0.32 1.50 0.012 0.005 1.44 0.15 0.036 45 54 - 0.005 - - - - - 440 Invention Steel 3 0.08 0.25 1.48 0.011 0.004 1.65 0.15 0.038 52 53 0.05 0.004 - - - - - 480 Inventive Steel 4 0.08 0.22 1.48 0.008 0.005 1.54 0.12 0.032 50 50 - 0.003 - - - - - 450 Inventive Steel 5 0.07 0.16 1.60 0.011 0.004 1.50 0.10 0.035 45 50 - 0.001 - - - - - 450 Inventive Steel 6 0.07 0.14 1.50 0.09 0.005 1.65 0.12 0.040 42 45 - 0.002 - 0.1 - - - 480 Inventive Steel 7 0.10 0.25 1.48 0.011 0.005 1.45 0.15 0.038 45 55 0.04 0.002 - - - - - 460 Inventive Steel 8 0.11 0.35 1.52 0.012 0.006 1.55 0.18 0.044 46 40 - 0.001 - - 0.001 - - 440 Inventive Steel 9 0.09 0.28 1.50 0.010 0.005 1.48 0.20 0.046 45 52 - 0.001 0.1 - - 0.001 - 500 Inventive Steel 10 0.07 0.18 1.55 0.009 0.006 1.50 0.25 0.042 43 50 - 0.001 - - - - 0.005 320 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 1`0 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

Psalm No. Composition ratio of Ti, O, N, B Ti / O Ti / N O / B (Ti + 4B) / O Inventive Steel 1 0.9 7.6 7.6 1.4 Inventive Steel 2 0.8 6.7 9.8 1.2 Invention Steel 3 0.8 7.2 9.2 1.2 Inventive Steel 4 0.7 6.4 9.0 1.2 Inventive Steel 5 0.8 7.0 10.0 1.2 Inventive Steel 6 0.8 8.9 11.4 1.2 Inventive Steel 7 1.0 6.9 10.2 1.2 Inventive Steel 8 0.9 11.0 9.6 1.4 Inventive Steel 9 0..9 8.8 11.1 1.3 Inventive Steel 10 1.3 8.4 7.4 1.9 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

Psalm No. Welding method and heat input TiO oxide Needle Ferrite Fraction of Welding Metal Part (%) Mechanical Properties of Welded Metal Parts welding method Welding heat input (kJ / cm) Number (pcs / mm ³ ) Average size (㎛) Tensile Strength (MPa) CTOD (mm, -10 ℃) Inventive Steel 1 FCAW 20 3.3 X ^{10 8} 0.016 89 640 1.1 Inventive Steel 2 FCAW 25 4.6 X ^{10 8} 0.017 89 630 1.0 Invention Steel 3 FCAW 22 3.7 X ^{10 8} 0.012 87 660 0.8 Inventive Steel 4 FCAW 20 4.6 X ^{10 8} 0.016 88 640 0.9 Inventive Steel 5 FCAW 18 6.4 X ^{10 8} 0.018 87 650 1.2 Inventive Steel 6 FCAW 19 5.3 X ^{10 8} 0.025 89 630 1.1 Inventive Steel 7 FCAW 22 3.6X10 ⁸ 0.013 90 640 1.3 Inventive Steel 8 FCAW 30 3.3 X ^{10 8} 0.026 91 660 1.0 Inventive Steel 9 FCAW 30 5.6 X ^{10 8} 0.024 88 665 1.2 Inventive Steel 10 FCAW 25 5.3 X ^{10 8} 0.014 85 620 1.2 Comparative Steel 1 FCAW 20 3.0 X 10 ⁶ 0.045 46 650 0.2 Comparative Steel 2 FCAW 20 4.3X10 ⁶ 0.051 52 640 0.3 Comparative Steel 3 FCAW 20 2.5 X 10 ⁶ 0.054 44 650 0.3 Comparative Steel 4 FCAW 20 3.0 X 10 ⁶ 0.064 45 660 0.3 Comparative Steel 5 FCAW 20 2.5 X 10 ⁵ 0.037 37 650 0.2 Comparative Steel 6 FCAW 30 2.5 X 10 ⁶ 0.056 42 680 0.1 Comparative Steel 7 FCAW 30 3.0 X 10 ⁶ 0.043 44 665 0.3 Comparative Steel 8 FCAW 20 4.1 X 10 ⁵ 0.046 52 610 0.2 Comparative Steel 9 FCAW 30 2.8X10 ⁵ 0.041 59 610 0.4 Comparative Steel 10 FCAW 20 3.4 X 10 ⁵ 0.046 52 620 0.3 Comparative Steel 11 FCAW 25 2.6 X 10 ⁶ 0.043 41 625 0.3

As shown in Table 3, the number of TiO oxides produced by the present invention has a range of ³ × ^{10 8} / mm ³ or more, whereas in the case of comparative steel 4.3 × 10 ⁶ / mm ³ or less It can be seen that the inventive steel compared to the comparative steel has a fairly uniform and fine composite precipitate size and the number thereof is also significantly increased.

On the other hand, in the case of the microstructure of the present invention, both the acicular ferrite phase fraction is composed of a high fraction of 85% or more.

Therefore, in FCAW 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 welded metal parts compared to the comparative steel.

Claims (3)

By weight% C: 0.01-0.2%, Si: 0.1-0.5%, Mn: 1.0-3.0%, Ni: 0.5-3.0%, Ti: 0.01-0.1%, B: 0.0010-0.01%, Al: 0.005-0.05 %, N: 0.003-0.006%, P: 0.03% or less, S: 0.03% or less, O: 0.03-0.07%, 0.7≤Ti / O≤1.3, 6≤Ti / N≤12, 7≤O / B≤ 12, 1.2? Flux cored arc welding metal part having excellent low temperature CTOD characteristics, characterized by including one or two or more of them The method of claim 1, wherein Nb: 0.0001 to 0.1%, V: 0.005 to 0.1%, Cu: 0.01% to 2.0%, Cr: 0.05% to 1.0%, Mo: 0.05% to 1.0%, W: 0.05% to 0.5%, Zr: 0.005% to 0.5%, Ca: 0.0005% to 0.005%, and REM: 0.005% to 0.05% Flux cored arc welding metal part having excellent low temperature CTOD characteristics, characterized by further containing one or two selected from the group. The flux cored arc welding metal part having excellent low temperature CTOD characteristics according to claim 1 or 2, wherein the weld metal part has a TiO oxide of 0.01 to 0.1 μm of 1.0 × 10 ⁷ / mm ³ or more.