JP5172970B2 - Flux cored arc weld metal part excellent in low temperature CTOD characteristics and steel member having this weld metal part - Google Patents

Description

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

  Recently, offshore structures are still being constructed and used in cold regions due to continuous rises in oil prices and the like, and the steel materials used are required to have high strength and low temperature CTOD characteristics.

  In order to ensure the stability of such an offshore structure, the weld zone CTOD characteristic of the offshore structure is one of the most important factors.

  Generally, in the case of flux cored welding of offshore structures, the heat input range uses a heat input amount of approximately 7-25 kJ / cm.

  In general, a weld metal (weld metal) formed during welding forms a coarse columnar crystal structure while solidifying, and coarse grain boundary ferrite and Widmanstatten ferrite are formed in the coarse crystal grain by an austenite grain boundary. Is done. That is, the weld metal part is a part where the CTOD characteristic is most deteriorated in the weld part.

  Therefore, in order to ensure the stability of the welded structure, it is necessary to control the microstructure of the weld metal part to ensure the CTOD characteristic of the weld metal part. As a means for solving this problem, a technique that defines the components of the welding material has been proposed. An example of such a technique is disclosed in Japanese Patent Publication No. 11-170085. However, this technique is unclear about the control of the microstructure and particle size of the weld metal, and it is difficult to obtain sufficient toughness of the weld metal part with the welding material described in the above-mentioned document.

  In Japanese Patent Publication No. 2005-171300, C: 0.07% or less, Si: 0.3% or less, Mn: 1.0 to 2.0%, P: 0.02% by weight. Hereinafter, S is 0.1% or less, sol. Al: 0.04 to 0.1%, N: 0.0020 to 0.01%, Ti: 0.005 to 0.02%, and B: 0.005 to 0.005%, ARM = 197 -1457C-1140sol. A composition is disclosed wherein the ARM defined by Al + 11850N-316 (Pcm-C) is 40-80. However, the specified ARM has no limitation on the oxygen content in the welded part, and therefore it is difficult to ensure the impact toughness of the SAW high heat input welded metal part.

  In Japanese Patent Publication No. Heisei 10-180488, slag generator: 0.5 to 3.0%, C: 0.04 to 0.2%, Si ≦ 0.1%, Mn:% by weight. 1.2-3.5%, Mg: 0.05-0.3%, Ni: 0.5-4.0%, Mo: 0.05-1.0%, and B: 0.002-0 Good impact toughness is ensured by including .015%, but there is no description of the oxygen and nitrogen contents in the weld metal. For this reason, it is difficult to ensure the CTOD characteristics of the weld metal part.

  The present invention uses TiO oxide and solute B to promote acicular ferrite transformation in the grains, and has high strength physical properties and at the same time excellent flux low temperature CTOD characteristics, and this welding It aims at providing the steel member which has a metal part.

  Hereinafter, the present invention will be described.

  In one aspect of the present invention, the present invention is a flux cored arc weld metal part having excellent low temperature CTOD characteristics, wherein the FCAW weld metal part is C: 0.01-0.2% by weight. , 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, and O: 0.03 -0.07%, the FCAW weld metal part is 0.7 ≦ Ti / O ≦ 1.3, 6 ≦ Ti / N ≦ 12, 7 ≦ O / B ≦ 12, 1.2 ≦ (Ti + 4B) /O≦1.9, the balance Fe and other inevitable impurities are included, and the FCAW weld metal part is 85% or more acicular ferrite The present invention provides a flux cored arc weld metal part having excellent low temperature CTOD characteristics, including a fine structure characterized in that it includes one or more of (acicular ferrite) and the balance bainite, grain boundary ferrite and polygonal ferrite. .

  The weld metal part is Nb: 0.0001 to 0.1%, V: 0.005 to 0.1%, Cu: 0.01 to 2.0%, Cr: 0.05 to 1.0%, One or more selected from the group consisting of Mo: 0.05 to 1.0%, W: 0.05 to 0.5%, and Zr: 0.005 to 0.5%, and / or Ca: One or more elements selected from the group consisting of 0.0005 to 0.005% and REM: 0.005 to 0.05% can be further included.

In the weld metal part, it is preferable that 0.01 to 0.1 μm (micrometer) of TiO oxide is distributed in an amount of 1.0 × 10 ⁷ pieces / mm ³ or more.

  Moreover, this invention relates to the steel member which has said weld metal part.

  The present invention promotes acicular ferrite transformation in the weld metal part by using TiO oxide and solute B, and has high strength physical properties and at the same time excellent flux low temperature CTOD characteristics. And the steel member which has this weld metal part can be provided.

  The present invention will be described in detail below.

  In order to develop a weld metal part having high strength physical properties and capable of securing excellent CTOD in FCAW welding with a welding heat input of 7 to 30 kJ / cm, the present inventors have developed a CTOD of the weld metal part. The type and size of oxides on acicular ferrite, which was known to be effective, were investigated. As a result, it was found that the amount of grain boundary ferrite and needle-like ferrite in the weld metal part was changed by TiO and solute B, thereby changing the CTOD value of the weld metal part.

Based on this research, in the present invention,
[1] TiO oxide is used for FCAW weld metal,
[2] Toughness by transforming acicular ferrite by 85% or more in a weld metal part having a particle size of 0.01 to 0.1 μm (micrometer) and an oxide of 1.0 × 10 ⁷ pieces / mm ³ or more And [3] acicular ferrite transformation is promoted by securing TiO and solute B.

  These [1], [2], and [3] will be described more specifically.

[1] Management of TiO Oxide By properly maintaining the ratio of Ti / O, O / B in the weld metal, the present inventors appropriately distributed a suitable number of TiO oxides, It has been found that the austenite grains are prevented from coarsening during the solidification process, and the TiO oxide promotes acicular ferrite transformation.

  The inventors have also found that when the TiO oxide is properly distributed within the austenite grain, the TiO oxide in the austenite grain plays a role in the heterogeneous nucleation site of the acicular ferrite transformation as the temperature decreases in the austenite grain. It has been found that ferrite can be formed in crystal grains preferentially over grain boundary ferrite formed in crystal grain boundaries. Thereby, the CTOD characteristic of a weld metal part can be remarkably improved.

For this purpose, it is important to distribute the TiO oxide finely and uniformly. Moreover, as a result of investigating the size, amount, and distribution of the TiO oxide according to the ratio of Ti / O and O / B, the present inventors have found that the ratio of Ti / O is 0.2 to 0.5 and the ratio of O / B. It was found that 0.01 to 0.1 μm (micrometer) size of TiO oxide was obtained at 1.0 × 10 ⁷ pieces / mm ³ or more.

[2] Microstructure of weld metal part From the result of investigating the size, amount, and distribution of TiO oxide according to the ratio of Ti / O and O / B according to the present invention, Ti / O is 0.2 to 0.5. When O / B is 5 to 10, it was confirmed that 0.01 × 0.1 7 (micrometer) size TiO oxide was obtained at 1.0 × 10 ⁷ pieces / mm ³ or more. .

  If such TiO oxide is properly distributed in the weld metal, the acicular ferrite transformation is promoted in the crystal grains prior to the grain boundaries in the cooling process of the weld metal parts, and the acicular ferrite in the weld metal parts. 85% or more can be ensured.

[3] Role of solute boron in the weld metal part Boron, which is a solid solution separately from the oxide uniformly dispersed in the weld metal part, which is a fact clarified in the study of the present invention, is a grain boundary. To lower the energy of the grain boundaries. Therefore, the dispersion of the solid solution boron suppresses the grain boundary ferrite transformation at the crystal grain boundary, and promotes the acicular ferrite transformation within the crystal grain. Thus, solute boron suppresses the grain boundary ferrite transformation at the crystal grain boundary, promotes the acicular ferrite transformation within the crystal grain, and contributes to the improvement of the CTOD characteristics of the weld metal part.

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

[component]
The content of carbon (C) is in the range of 0.01 to 0.2%.
Carbon (C) is an essential element for ensuring the strength and weld curability of the weld metal. However, if the carbon content exceeds 0.2%, the weldability is greatly reduced, low temperature cracks are likely to occur in the weld metal part, and the large heat input impact toughness is significantly reduced.

The content of silicon (Si) is in the range of 0.1 to 0.5%.
When 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 lowered. When the silicon content exceeds 0.5%, the transformation of island martensite (MA constituent) in the weld metal is promoted to lower the low temperature impact toughness and affect the weld crack sensitivity.

The content of manganese (Mn) is in the range of 1.0 to 3.0%.
Mn improves deoxidation and strength in steel, and precipitates in the MnS form around TiO oxide, promoting the formation of acicular ferrite that is advantageous for improving the toughness of weld metal in the Ti composite oxide. To play a role.

  Such Mn secures strength and toughness by forming a substitutional solid solution in the matrix and strengthening the solid solution. For this purpose, the Mn content is preferably 1.0% or more. However, when the Mn content exceeds 3.0%, an undesirable low temperature transformation structure is formed.

The content of titanium (Ti) is in the range of 0.01 to 0.1%.
Ti combines with O to form fine TiO oxide and fine TiN precipitates. For this reason, Ti is an indispensable element. In order to obtain the effect of such fine TiO oxide and TiN composite precipitates, the Ti content is preferably 0.01% or more. However, when the Ti content exceeds 0.1%, undesirable coarse TiO oxides and coarse TiN precipitates are formed.

The content of nickel (Ni) is in the range of 0.5 to 3.0%.
Ni is an effective element that improves the strength and toughness of the matrix by solid solution strengthening. In order to obtain such an effect, the Ni content is preferably 0.5% or more. However, if the Ni content exceeds 3.0%, the hardenability is greatly increased and high temperature cracks are generated.

The boron content is in the range of 0.0010-0.01%.
B is an element that improves hardenability. In order to suppress segregation at the grain boundaries and suppress the grain boundary ferrite transformation, the B content needs to be 0.0010% or more. However, if the content of B exceeds 0.01% or more, the weld hardenability is greatly increased, the martensitic transformation is promoted, and the occurrence of weld cold cracks and toughness are reduced. Therefore, the content of B is in the range of 0.0010 to 0.01%.

The content of nitrogen (N) is in the range of 0.003-0.006%.
N is an essential element for forming TiN precipitates and the like, and increases the amount of fine TiN precipitates. N significantly affects the size of TiN precipitates and the interval between the precipitates, the distribution of precipitates, the frequency of complex precipitation with oxides, the high-temperature stability of the precipitates themselves, and the like. For this reason, the content of N is preferably 0.003% or more.

  However, if the nitrogen content exceeds 0.006%, no further effect can be expected, and the toughness can be reduced by increasing the amount of solid solution nitrogen present in the weld metal.

The content of phosphorus (P) is 0.030% or less.
P is an impure element that promotes high temperature cracks during welding. For this reason, the one where the content of P is low is preferable. In order to improve toughness and reduce cracks, the P content is preferably 0.03% or less.

The content of aluminum (Al) is in the range of 0.005-0.05%.
Al plays a role as a deoxidizer and is an element necessary for reducing the amount of oxygen in the weld metal. Moreover, in order to combine with solid solution nitrogen and form a fine AlN precipitate, the content of Al is made 0.005% or more. However, if the Al content exceeds 0.05%, coarse Al ₂ O ₃ is formed, which hinders the formation of TiO oxide necessary for improving toughness. For this reason, the Al content should be 0.05% or less.

The sulfur (S) content is limited to 0.030% or less.
S is an element necessary for forming MnS. In order to precipitate the composite precipitate of MnS, the S content is set to 0.03% or less. When the content of S is 0.03% or more, a low melting point compound such as FeS can be formed to induce high temperature cracking.

The content of oxygen (O) is in the range of 0.03-0.07%.
O is an element that reacts with Ti in the solidification process of the weld metal part to form Ti oxide. Ti oxide promotes the transformation of acicular ferrite in the weld metal. If the O content is less than 0.03%, the Ti oxide cannot be properly distributed in the weld metal part. When the content of O exceeds 0.07%, coarse TiO oxide and other oxides such as FeO are generated and affect the weld metal part.

The ratio of Ti / O is in the range of 0.7 to 1.3.
When the Ti / O ratio is less than 0.7, the TiO oxide required for suppressing the growth of austenite crystal grains and acicular ferrite transformation is insufficient in the weld metal. Furthermore, the Ti ratio contained in the TiO oxide decreases, the TiO oxide loses its function as an acicular ferrite nucleation site, and the fraction of the acicular ferrite phase effective in improving the toughness of the weld heat affected zone decreases. Is done. When the ratio of Ti / O exceeds 1.3, the growth of austenite crystal grains in the weld metal cannot be further suppressed. Rather, the ratio of the alloy component contained in the oxide is reduced, and the oxide loses its function as a nucleation site of acicular ferrite.

The ratio of Ti / N is in the range of 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 adversely affects the acicular ferrite transformation effective for improving toughness. When the Ti / N ratio exceeds 12, no further effect can be expected, the amount of dissolved nitrogen increases, and the impact toughness decreases.

The ratio of O / B is in the range of 7-12.
In the present invention, if the O / B ratio is less than 7, the amount of the solid solution B that diffuses into the austenite grain boundary during the cooling process after welding and suppresses the grain boundary ferrite transformation becomes insufficient. When the O / B ratio exceeds 12, no further effect can be expected, the amount of dissolved nitrogen increases, and the toughness of the weld heat affected zone decreases.

The ratio of (Ti + 4B) / O is in the range of 1.2 to 1.9.
In the present invention, when the ratio of (Ti + 4B) / O is less than 1.2, the amount of solute nitrogen increases, which is not effective in improving the toughness of the weld metal part. When the ratio of (Ti + 4B) / O 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 more elements selected from the group consisting of Nb, V, Cu, Mo, Cr, W and Zr are added to the steel having the composition as described above. Can be further added.

The content of copper (Cu) is in the range of 0.1 to 2.0%.
Cu is an element effective for securing strength and toughness by the effect of solid solution strengthening by being dissolved in the matrix. For this purpose, the Cu content needs to be 0.1% or more. However, when the Cu content exceeds 2.0%, the hardenability is increased at the weld metal portion to reduce the toughness, and the hot crack is promoted at the weld metal.

  Further, when Cu and Ni are added in combination, the total of these is made less than 3.5%. When the content of Cu and Ni exceeds 3.5%, the hardenability is increased, which adversely affects toughness and weldability.

The content of Nb is in the range of 0.0001-0.1%.
Nb is an essential element for improving hardenability. In particular, there is an effect that the Ar ₃ temperature is lowered and the bainite generation range is expanded even in a range where the cooling rate is low, and Nb is necessary for obtaining a bainite structure.

  In order to expect the effect of improving the strength, an Nb content of 0.0001% or more is necessary. However, if the Nb content exceeds 0.1%, the formation of island martensite is promoted in the weld metal part during welding, which adversely affects the toughness of the weld metal part.

The content of V is in the range of 0.005-0.1%.
V is an element that promotes ferrite transformation by forming VN precipitates. The V content should be 0.005% or more. However, if the content of V exceeds 0.1%, a hardened phase such as carbide is formed in the weld metal part, which adversely affects the toughness of the weld metal part.

Chromium (Cr) is in the range of 0.05 to 1.0%.
Cr improves hardenability and strength. If the Cr content is less than 0.05%, strength cannot be obtained. When the Cr content exceeds 1.0%, the toughness of the weld metal part is deteriorated.

Molybdenum (Mo) is in the range of 0.05 to 1.0%.
Mo is an element that increases hardenability and at the same time improves strength. The Mo content is 0.05% or more to ensure strength. In order to suppress the hardening of the weld metal part and the occurrence of welding low temperature cracks, the Mo content is set to 1.0% or less as in the case of Cr.

The content of W is in the range of 0.05-0.5%.
W is an element that is effective not only for improving high-temperature strength but also for precipitation strengthening. However, if the W content is less than 0.05%, the effect of increasing the strength is weak. If the W content is 0.5% or more, the toughness of the weld metal part is adversely affected.

The content of Zr is in the range of 0.005-0.5%.
Zr is effective in increasing strength, so 0.005% or more is preferably added. When the content of Zr exceeds 0.5%, the toughness of the weld metal part is adversely affected.

  Moreover, in this invention, in order to suppress the crystal grain growth of primary austenite, one or both of Ca and REM can be further added.

  Ca and REM are preferable elements for stabilizing the arc during welding and forming an oxide in the weld metal part. Further, Ca and REM suppress the growth of austenite crystal grains in the cooling process, promote the ferrite transformation in the grains, and improve the toughness of the weld metal part. For this reason, it is preferable to add 0.0005% or more of calcium (Ca) and 0.005% or more of REM. However, when Ca exceeds 0.005% and REM exceeds 0.05%, a large oxide may be formed to adversely affect toughness. As REM, one or more kinds of Ce, La, Y and Hf may be used, and any of the above effects can be obtained.

[Microstructure of weld metal part]
In the present invention, the microstructure of the weld metal part formed after FCAW welding is acicular ferrite and has a phase fraction of 85% or more. The acicular ferrite structure can simultaneously obtain high strength and low temperature CTOD.

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

  When ferrite and a bainite structure are mixed, it is advantageous for CTOD, but the strength of the weld metal part is lowered. Further, when the microstructure is a martensite and bainite mixed structure, the strength of the weld metal part is high, but the mechanical properties such as CTOD of the weld metal part are lowered, and the low temperature crack sensitivity is increased.

[Oxide]
The oxide present in the weld metal part has a great influence on the transformation of the microstructure of the weld metal part after welding. That is, the transformation of the microstructure is greatly influenced by the kind, size and number of oxides distributed.

  In particular, in the case of the FCAW weld metal part, the crystal grains are coarsened during the solidification process, and coarse grain boundary ferrite, Widmanstatten ferrite, bainite, and other structures are formed from the crystal grain boundaries, thereby reducing the physical 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 μm (micrometer) or less.

The particle diameter of the TiO oxide is in the range of 0.01 to 0.1 μm (micrometer), and the critical number is 1.0 × 10 ⁷ pieces / mm ³ or more. When the particle size is less than 0.01 μm (micrometer), the TiO oxide cannot promote the transformation of acicular ferrite in the FCAW weld metal part. In addition, when the particle diameter exceeds 0.1 μm (micrometer), the effect of pinning on the austenite crystal grains is reduced, and the TiO oxide behaves like a coarse non-metallic inclusion. This adversely affects the CTOD characteristics of the weld metal part.

  Moreover, this invention provides the steel member which has said weld metal part.

  In this invention, it can manufacture also by other welding methods other than FCAW. At this time, if the cooling rate of the weld metal part is high, the oxide is finely dispersed and a fine structure is obtained. For this reason, a high heat input welding method with a high cooling rate is preferable.

  For the same reason, steel cooling and Cu-backing methods are also advantageous to improve the cooling rate of the weld.

  However, even if such a known technique is applied to the present invention, it is a matter of course that this is a simple modification of the present invention and is substantially within the scope of the technical idea of the present invention.

  Hereinafter, the present invention will be described in detail through examples.

[Example]
The weld metal part which has a component composition like the following Table 1 and Table 2 was manufactured by FCAW, applying the welding heat input of 7-30 kJ / cm or more.

  A specimen was taken from the center of the weld metal part welded as described above, and a tensile test and a CTOD test were conducted. The results are shown in Table 3 below.

  As the test piece for the tensile test, a KS standard (KS B 0801) No. 4 test piece was used, and the tensile test was performed at a cross head speed of 10 mm / min.

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

  The size and number of oxides having an important influence on the CTOD of the weld metal part, and the interval were measured by a point counting method using an image analyzer and an electron microscope. 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 with a CTOD test piece after FCAW welding and evaluated through a CTOD tester at −10 ° C.

As shown in Table 3 above, the weld metal part manufactured according to the present invention is a TiO oxide of 3 × 10 ⁸ pieces / mm ³ or more, whereas in the case of the comparative steel, 4.3 × 10 8. ^{It was 6} pieces / mm ³ or less. When compared with the comparative steel, it can be seen that the inventive steel has a very uniform and fine composite precipitate size and the number thereof is significantly increased.

  On the other hand, in the case of the microstructure of the steel of the present invention, a high acicular ferrite phase fraction of 85% or more was included.

  Therefore, during FCAW welding, the steel of the present invention contained acicular ferrite and polygonal ferrite in the grains. Among them, the acicular ferrite has a phase fraction of 85% or more, and the inventive steel has excellent weld metal part CTOD characteristics when compared with the comparative steel.

Claims (6)

A flux cored arc (FCAW) weld metal part with excellent low temperature CTOD characteristics,
The FCAW weld metal part is, 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, and O: 0.03-0.07%,
The FCAW weld metal part has a relationship of 0.7 ≦ Ti / O ≦ 1.3, 6 ≦ Ti / N ≦ 12, 7 ≦ O / B ≦ 12, 1.2 ≦ (Ti + 4B) /O≦1.9. And the balance Fe and other inevitable impurities,
The FCAW weld metal includes 85% or more of acicular ferrite and a microstructure containing at least one of bainite, grain boundary ferrite and polygonal ferrite. Department.   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: 0.05-0.5%, Zr: 0.005-0.5%, Ca: 0.0005-0.005% and REM: 0.005-0.05% The FCAW weld metal part according to claim 1, further comprising at least one selected from the group. ^3. The TiO oxide of 0.01 to 0.1 μm (micrometer) is distributed at 1.0 × 10 ⁷ pieces / mm ³ or more in the weld metal part, according to claim 1 or 2. FCAW weld metal part.   A steel member having a flux cored arc weld metal part excellent in low temperature CTOD characteristics according to claim 1.   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: 0.05-0.5%, Zr: 0.005-0.5%, Ca: 0.0005-0.005% and REM: 0.005-0.05% The steel member according to claim 4, further comprising at least one selected from the group. The TiO oxide of 0.01 to 0.1 μm (micrometer) is distributed at 1.0 × 10 ⁷ pieces / mm ³ or more in the weld metal part, according to claim 4 or 5. Steel members.