Description

FLUX CORED ARC WELD METAL JOINT HAVING SUPERIOR

CTOD IN LOW TEMPERATURE AND STEEL MEMBER

HAVING THE WELD METAL JOINT

Technical Field

[1] The present invention relates to a weld metal joint subjected to flux cored arc welding (FCAW) used for welded structures, such as ships, buildings, bridges, marine structures, steel pipes, and line pipes, and a steel member having the weld metal joint, and more particularly, to a flux cored arc weld (FCAW) metal joint having excellent CTOD properties at low temperatures and a steel member having the FCAW metal joint. Background Art

[2] Recently, marine structures have been increasingly constructed and used in cold areas due to a continuing increase in oil prices. Steel members being used require high strength and CTOD properties at low temperatures.

[3] The CTOD properties of the weld joints of marine structures are some of the most important factors in ensuring the stability of marine structures.

[4] In general, flux cored arc welding that is performed on marine structures uses a heat input within the heat input range of approximately 7 to 25kJ/cm.

[5] In general, a weld metal, which is formed during welding, produces a coarse columnar structure during solidification, and coarse grain boundary ferrite and wid- manstatten ferrite are formed along austenite grain boundaries within the coarse grains. That is, the weld metal joint is the portion where CTOD properties deteriorate the most in the weld zone.

[6] Therefore, in order to ensure the stability of the weld structure, the fine structure of the weld metal joint is controlled to ensure the CTOD properties of the weld metal joint. To this end, a technique that specifies components of the welding material has been proposed. An example of the technique is disclosed in Japanese Patent Laid-Open Publication No. hei 11-170085. However, this technique is silent on the control of fine structure and grain size, and it is hard to obtain sufficient toughness of the weld metal joint with the welding material described therein.

[7] Further, Japanese Patent Laid-Open Publication No. 2005-171300 discloses a composition including, by weight%, at most 0.07% C, at most 0.3% Si, 1.0 to 2.0% Mn, at most 0.02% P, at most 0.1% S, 0.04 to 0.1% sol.Al, 0.0020 to 0.01% N, 0.005 to 0.02% Ti, and 0.0005 to 0.005% B, in which ARM, which is defined as ARM=197-1457C-1140sol.Al+11850N-316(Pcm-C), is in the range of 40 to 80.

However, since the specified ARM does not include a limit on the oxygen content in the weld metal joint, it is difficult to ensure the impact toughness of a weld metal joint subjected to high heat-input SAW.

[8] Furthermore, Japanese Patent Laid-Open Publication No. hei 10-180488 ensures appropriate impact toughness by including, by weight%, 0.5 to 3.0% slag formers, 0.04 to 0.2% C, at most 0.1% Si, 1.2 to 3.5% Mn, 0.05 to 0.3% Mg, 0.5 to 4.0% Ni, 0.05 to 1.0% Mo, 0.002 to 0.015% B, but is silent on oxygen and nitrogen contents. Therefore, it is difficult to ensure CTOD properties of the metal weld. Disclosure of Invention Technical Problem

[9] An aspect of the present invention is to provide a flux cored arc weld metal joint having high-strength properties by accelerating the transformation of acicular ferrite within grains using Ti oxides and soluble B, and at the same time, excellent CTOD properties at low temperatures and a steel member having the metal weld joint. Technical Solution

[10] Hereinafter, the present invention will be described.

[11] According to an aspect of the invention, the invention provides a flux cored arc weld metal joint having excellent CTOD properties at low temperatures the FCAW metal joint including, by weight: 0.01 to 0.2% C, 0.1 to 0.5% Si, 1.0 to 3.0% Mn, 0.5 to 3.0% Ni, 0.01 to 0.1% Ti, 0.0010 to 0.01% B, 0.005 to 0.05% Al, 0.003 to 0.006% N, at most 0.03% P, at most 0.03% S, 0.03 to 0.07% O, the FCAW metal joint satisfying a relation of 0.7<Ti/O<1.3, 6<Ti/N<12, 7<O/B<12, and 1.2<(Ti+4B)/O<1.9, and the balance of Fe and unavoidable impurities, the FCAW metal joint including a mi- crostructure having acicular ferrite of 85% or more and at least one of the balance of bainite, grain boundary ferrite, and polygonal ferrite.

[12] The FCAW metal joint may further include one or more elements selected from the group consisting of 0.0001 to 0.1% Nb, 0.005 to 0.1% V, 0.01 to 2.0% Cu, 0.05 to 1.0% Cr, 0.05 to 1.0% Mo, 0.05 to 0.5% W, and 0.005 to 0.5% Zr or/and one or more elements selected from the group consisting of 0.0005 to 0.005% Ca, and 0.005 to 0.05% REM.

[13] TiO oxides having a size in the range of 0.01 to 0.1/M (micrometers) may be distributed in the FCAW metal joint at 1.0 x 10 ⁷ /mm ³ or more.

[14] According to another aspect of the invention, the invention provides a steel member having the FCAW metal joint.

Advantageous Effects

[15] There can be provided a flux cored arc weld metal joint that has a high-strength property by accelerating the transformation of acicular ferrite in a weld metal joint

using TiO oxides and soluble B and at the same time, excellent CTOD properties at low temperatures and a steel member having the weld metal joint. Best Mode for Carrying out the Invention

[16] The present invention will now be described in detail.

[17] The present inventors have conducted research into the kind and size of oxides that affect acicular ferrite that has been known to be effective in ensuring excellent CTOD properties in weld metal joints for the development of a weld metal joint having properties such as high strength and excellent CTOD properties when flux cored arc welding (FCAW) is performed at a welding heat input of 7 to 30kJ/cm. As a result of this research, they have found out that the amounts of grain boundary ferrite and the amount of acicular ferrite in the weld metal joint are changed by TiO and soluble B and the CTOD value of the weld metal joint is correspondingly changed.

[18] Based on this research, in the present invention,

[19] [1] TiO oxides are used in a FCAW metal,

[20] [2] Acicular ferrite is transformed at 85% or more in the FCAW metal joint including oxides having a size of 0.01 to 0.1 μm (micrometers), the number of which is 1.0 X 10 ⁷ / mm ³ or more, to thereby improve toughness, and

[21] [3] The transformation of the acicular ferrite is accelerated by ensuring TiO and soluble B.

[22] [ 1 ] [2] [3] will now be described in detail.

[23] [1] Management of TiO oxide

[24] The present inventors have found out that if the ratio between Ti/O and O/B in the weld metal joint is appropriately maintained, a suitable number of TiO oxides are appropriately distributed to prevent the coarsening of austenite grains during the solidification of the weld metal, and accelerate the transformation of acicular ferrite in the TiO oxides.

[25] They have also found out that when the TiO oxides are appropriately distributed in the austenite grains, as temperatures decrease in the austenite, the TiO oxides in the austenite grains act as heterogeneous nucleation sites for the transformation of the acicular ferrite, thereby forming ferrite within grains before grain boundary ferrite that is formed at the grain boundaries. Accordingly, the CTOD properties of the weld metal joint can be remarkably improved.

[26] To this end, it is important to finely and uniformly distribute the TiO oxides. Furthermore, while researching the size, quantity, and distribution of the TiO oxides according to the ratios of Ti/O and O/B, the inventors have found out that when the ratio of Ti/O is in the range of 0.2 to 0.5, and the ratio of O/B is in the range of 5 to 10, TiO oxides having a size within the range of 0.01 to Oλμm (micrometers) are obtained

at 1.0 x lOVmm ³ or more.

[27] [2] Microstructure of weld metal joint

[28] As a result of the research on the size, quantity, and distribution of TiO oxides according to the ratios of Ti/O and O/B according to the present invention, it has been confirmed that when the ratio of Ti/O is in the range of 0.2 to 0.5, and the ratio of O/B is in the ratio of 5 to 10, TiO oxides having a size of 0.01-0. lμm (micrometer) are formed at l.OxlOVmm ³ or more.

[29] When the TiO oxides are appropriately distributed in the weld metal, the transformation of the acicular ferrite within the grains is accelerated before the grain boundaries in the process of cooling the weld metal joint, such that the acicular ferrite can make up 85% or more of the weld metal joint.

[30]

[31] [3] Role of soluble boron in weld metal joint

[32] According to the research according to this invention, separate from the oxides uniformly distributed in the weld metal joint, boron, which is solid-dissolved, is diffused throughout the grain boundaries in order to lower the energy of the grain boundaries. Thus, the diffusion of soluble boron prevents the transformation of the grain boundary ferrite to thereby accelerate the transformation of the acicular ferrite within the grains. That is, the soluble boron suppresses the transformation of the grain boundary ferrite at the grain boundaries and accelerates the transformation of the acicular ferrite with the grains, which contributes to the improvement of the CTOD properties of the weld metal joint.

[33]

[34] Hereinafter, the composition of the weld metal joint will be described in detail.

[35] [COMPOSITION]

[36] Carbon (C) content is in the range of 0.01 to 0.2%.

[37] Carbon (C) is an essential element to ensure the strength of weld metal and weld hardenability. However, when the carbon (C) content exceeds 0.2%, weldability is significantly reduced, low-temperature cracking is likely to occur in the weld metal joint, and impact toughness under high heat input is significantly reduced.

[38]

[39] Silicon (Si) content is in the range of 0.1 to 0.5%.

[40] When the silicon content is less than 0.1%, the effect of deoxidation is insufficient, and the fluidity of the weld metal is reduced. When the silicon content exceeds 0.5%, the transformation of M-A constituents in the weld metal is accelerated to reduce the impact toughness and affect weld cracking susceptibility.

[41]

[42] Manganese (Mn) content is in the range of 1.0 to 3.0%.

[43] Mn enhances deoxidization and strength, and is precipitated into MnS around the

TiO oxides such that Ti complex oxides accelerate the formation of acicular ferrite for the improvement of the toughness of weld metal joint.

[44] The Mn forms a substitutional solid solution in a matrix, thereby causing solid- solution strengthening of the matrix to ensure strength and toughness. To this end, the Mn content is preferably 1.0% or more. However, when the Mn content exceeds 3.0%, a low-temperature transformed structure is undesirably formed.

[45]

[46] Titanium (Ti) content is in the range of 0.01 to 0.1%.

[47] Ti is bonded to O to form fine Ti oxides and fine TiN precipitates. Thus, Ti is an essential element. In order to form the fine TiO oxides and the TiN complex precipitates, the Ti content is preferably 0.01% or more. However, when the Ti content exceeds 0.1%, coarse TiO oxides and coarse TiN precipitates are undesirably formed.

[48]

[49] Nickel (Ni) content is in the range of 0.5 to 3.0%.

[50] Ni is an element that improves the strength and toughness of the matrix by solid- solution strengthening. In order to obtain such effects, the Ni content is preferably 0.5% or more. However, when the Ni content exceeds 3.0%, hardenability is significantly increased, and high-temperature cracking may occur.

[51]

[52] Boron (B) content is in the range of 0.0010 to 0.01%.

[53] B is an element that increases hardenability. The B content needs to be 0.0010% or more such that the B is segregated along the grain boundaries to suppress the transformation of the ground boundary ferrite. However, when the B content exceeds 0.01%, an increase in the weld hardenability may undesirably occur to accelerate the transformation of martensite, cause low-temperature cracking, and reduce toughness. Therefore, the B content is in the range of 0.0010 to 0.01%.

[54]

[55] Nitrogen (N) content is in the range of 0.003 to 0.006%.

[56] N is an essential element to form TiN precipitates and increases the amount of fine

TiN precipitates. N has a significant effect on the size, space, and density of TiN precipitates, the frequency of the TiN precipitates to form complex precipitates with oxides, and the high-temperature stability of the precipitates. Therefore, the N content is preferably 0.003% or more.

[57] However, when the N content exceeds 0.006%, it is hard to expect any further effects, and toughness may be reduced due to an increase in the amount of soluble nitrogen in the weld metal.

[59] P (P) content is 0.030% or less.

[60]

[61] P is an impurity element that promotes high-temperature cracking during welding.

Therefore, the less the P content, the more desirable. In order to increase the toughness and reduce cracks, the P content is preferably 0.03% or less.

[62]

[63] Aluminum (Al) content is in the range of 0.005 to 0.05%.

[64] Al serves as a deoxidizer and is a necessary element to reduce the amount of oxygen in the weld metal. Furthermore, the Al content is 0.005% or more so that Al is bonded with the soluble N to form fine AlN precipitates. However, when the Al content exceeds 0.05%, coarse Al ₂ O ₃ may be formed to prevent the formation of TiO oxides necessary to increase toughness. Therefore, the Al content is preferably 0.05% or less.

[65]

[66] Sulfur (S) content is limited to 0.030% or less.

[67] S is an element necessary to form MnS. The content of S is 0.03% or less to obtain complex precipitates of MnS. When the S content exceeds 0.03%, a low-melting compound, such as FeS, may be formed to cause high-temperature cracking.

[68]

[69] Oxygen (O) content is in the range of 0.03 to 0.07% .

[70] O is an element that reacts to Ti to form Ti oxides while the weld metal joint is solidified. The Ti oxides accelerate the transformation of the acicular ferrite in the weld metal joint. When the O content is less than 0.03%, it may be impossible to appropriately distribute the Ti oxides in the weld metal joint. When the O content exceeds 0.07%, coarse Ti oxides and oxides such as other FeO are formed to affect the weld metal joint.

[71]

[72] The ratio of Ti/O is in the range of 0.7 to 1.3.

[73] When the ratio of Ti/O is less than 0.7, TiO oxides in the weld metal required for the control of the growth of austenite grains and the transformation of the acicular ferrite are insufficient. Further, the ratio of Ti contained in the TiO oxides is reduced, the TiO oxides may not serve as a nucleation site for the acicular ferrite, and thus the phase fraction of the acicular ferrite effective in improving the toughness of the heat-affected zone is reduced. When the ratio of Ti/O exceeds 1.3, the growth of the austenite grains in the weld metal may not be prevented any longer. Rather, the ratio of the alloy components contained in the oxides is reduced, and thus the oxides do not serve as a nucleation site for the acicular ferrite.

[74]

[75] The ratio of Ti/N is in the range of 6 to 12.

[76] In the invention, when the ratio of Ti/N is less than 6, the amount of TiN precipitates formed in the TiO oxides is reduced and thus has an adverse effect on the effective transformation of the acicular ferrite. When the ratio of Ti/N exceeds 12, further effects cannot be expected and the amount of the soluble nitrogen increases, thereby undesirably lowering the impact toughness.

[77]

[78] The ratio of O/B is in the range of 7 to 12.

[79] In this invention, when the ratio of O/B is less than 7, the amount of soluble B that is diffused across the austenite grain boundaries to prevent the transformation of the grain boundary ferrite during the cooling process after welding is insufficient. When the ratio of O/B exceeds 12, further effects cannot be expected, and the amount of soluble N is increased to undesirably lower the toughness of the heat-affected zone.

[80]

[81] The ratio of (Ti+4B)/O is in range of 1.2 to 1.9.

[82] In this embodiment, when the ratio of (Ti+4B)/O is less than 1.2, the amount of soluble N increases to be ineffective in increasing the toughness of the weld metal joint. When the ratio of (Ti+4B)/O exceeds 1.9, the number of TiN and BN precipitates is inefficient.

[83]

[84] In this invention, in order to improve mechanical properties, one or more elements selected from the group consisting of Nb, V, Cu, Mo, Cr, W, and Zr may be further added to the steel having the above-described composition.

[85]

[86] Copper (Cu) content is in the range of 0.1 to 2.0%.

[87] Cu is an element that is solid-dissolved in a matrix to ensure strength and toughness by solid- solution strengthening. To this end, the Cu content needs to be 0.1% or more. However, when the Cu content exceeds 2.0%, the hardenability of the weld metal joint is increased to reduce the toughness and cause high-temperature cracking in the weld metal.

[88]

[89] Further, when Cu and Ni are added together, the total content thereof is less than

3.5%. When the Cu and Ni content exceeds 3.5%, hardenability increases to have adverse effects on the toughness and weldability.

[90]

[91] Nb content is in the range of 0.0001 to 0.1%.

[92] Nb is an essential element to improve hardenability. Since Nb lowers the Ar ₃ temperature and extends the range in which bainite is generated despite a low cooling rate, Nb needs to be used to obtain a bainite structure.

[93] The Nb content needs to be 0.0001% or more to increase strength. However, when the Nb content exceeds 0.1%, the formation of the M-A constituent is accelerated in the weld metal joint during welding to have adverse effects on the toughness of the weld metal joint.

[94]

[95] V content is in the range of 0.005 to 0.1 %.

[96] V is an element that forms VN precipitates to accelerate ferrite transformation. The V content needs to be 0.005% or more. However, when the V content exceeds 0.1%, a hard phase, such as carbide, may be formed in the weld metal joint to have adverse effects on the toughness of the weld metal joint.

[97]

[98] Chromium (Cr) is in the range of 0.05 to 1.0% .

[99] Cr increases hardenability and strength. When the Cr content is less than 0.05%, it may be impossible to obtain desired strength. When the Cr content exceeds 1.0%, toughness degradation of the weld metal joint is caused.

[100]

[101] Molybdenum (Mo) is in the range of 0.05 to 1.0%.

[102] Mo is also an element that increases hardenability and strength. The Mo content needs to be 0.05% or more to obtain desirable strength. In order to harden the weld metal joint and prevent undesirable low-temperature welding cracking, the Mo content is 1.0% or less, like Cr.

[103]

[104] W content is in the range of 0.05 to 0.5%.

[105] W is an element that increases high-temperature strength and is effective in precipitation hardening. However, when the W content is less than 0.05%, strength is only slightly increased. When the W content exceeds 0.5%, it has an adverse effect on the toughness of the weld metal joint.

[106]

[107] Zr content is in the range of 0.005 to 0.5%.

[108] Since Zr is effective in increasing strength, Zr content is preferably 0.005% or more. When the Zr content exceeds 0.5%, it has an adverse effect on the toughness of the weld metal joint.

[109]

[110] In this invention, one or both of Ca and REM may be further added in order to suppress the growth of prior austenite grains.

[I l l]

[112] Ca and REM are desirable elements that are used to stabilize the arc during welding and form oxides in the weld metal joint. Furthermore, in the cooling process, Ca and

REM suppress the growth of the austenite grains and promote the ferrite transformation within the grains, thereby increasing the toughness of the weld metal joint. To this end, it is preferable that the calcium (Ca) content is 0.0005% or more, and the REM content is 0.005% or more. However, when the Ca content exceeds 0.005%, and the REM exceeds 0.05%, large-sized oxides are formed, thereby degrading toughness. In terms of REM, one or more of Ce, La, Y, and Hf may be used to obtain the above- described effects.

[113]

[114] [Microstructure of weld metal joint]

[115] In this invention, the microstructure of the weld metal joint formed after FCAW may contain acicular ferrite and have a phase fraction of 85% or more. Acicular ferrite structures allow high strength and low-temperature CTOD at the same time.

[116] The microstructure includes one or more of bainite, grain boundary ferrite, and polygonal ferrite.

[117] When the microstructure includes a mixture of ferrite and bainite structures, higher CTOD values can be obtained, but the strength of the weld metal joint becomes low. When the microstructure includes a mixture of martensite and bainite structures, the strength of the weld metal joint becomes high, but the mechanical properties of the weld metal joint, such as CTOD properties, are degraded and low-temperature cracking susceptibility increases.

[118]

[119] [Oxide]

[120] Oxides existing in the weld metal joint have a great effect on the transformation of the microstructure of the weld metal joint subjected to welding. That is, the transformation of the microstructure is significantly affected by the kind, size, and number of oxides distributed in the weld metal joint.

[121] In particular, in the case of the FCAW metal joint, grains are coarsened in a solidification process, and structures, such as coarse grain boundary ferrite, Widmanstatten ferrite, and bainite, are formed along grain boundaries, thereby deteriorating the properties of the weld metal joint.

[122] In order to prevent this, it is important to uniformly disperse TiO oxides in the weld metal joint at regular intervals of 0.5/M (micrometers) or less.

[123] Further, the particle diameter of the TiO oxides is in the range of 0.01 to 0. lμm

(micrometers) and the critical number of the TiO oxides is 1.0 x 10 ⁷ per lmm ³ or more. When the particle diameter is less than 0.01/M (micrometers), the TiO oxides in the FCAW weld metal joint cannot promote the transformation of the acicular ferrite. Further, when the particle diameter exceeds 0.1 μm (micrometers), the pinning (inhibition of the grain growth) of the austenite grains is reduced, and the TiO oxides

act as coarse non-metallic inclusions, which have adverse affects on the CTOD properties of the weld metal joint. [124] Furthermore, in this invention, a steel member having the above-described weld metal joint is provided. [125] In this invention, a weld metal joint can be manufactured using another welding process in addition to FCAW. When the cooling rate of the weld metal joint is high, oxides are finely dispersed, and a microstructure is obtained. Therefore, a high heat input welding process with a high cooling rate is desirable. [126] Further, for the same reason, a steel cooling method and a Cu-backing method can also be used to increase the cooling speed of the weld metal joint. [127] However, even though the above-described known techniques are applied to this invention, these are merely modifications of the invention and thus substantially fall within the scope of the invention.

Mode for the Invention [128] Hereinafter, the invention will be described in detail with reference to embodiments of the invention. [129] [Embodiment] [130] Weld metal joints having compositions, shown in Tables 1 and 2, were manufactured by FCAW at a welding heat input in the range of 7 to 30kJ/cm or more. [131] Test pieces were cut from the central part of the weld metal joints and subjected to a tensile test and a CTOD test, and results are shown in Table 3. [132] A No. 4 test piece according to KS B 0801 was used as the test piece for the tensile test, and the tensile test was performed at a cross head speed of lOmm/min. [133] The CTOD test piece was manufactured according to BS7448-1 specifications, and the fatigue crack is located at the center of the SAW weld metal joint. [134] The size, number and space of oxides that have a significant effect on the CTOD properties of the weld metal joints were measured according to a point counting method using an image analyzer and an electron microscope. Results are shown in

Table 3.

[135] A surface to be tested was evaluated on the basis of a size of 100mm ² . [136] The CTOD of the FCAW metal joint was evaluated using CTOD test equipment at a temperature of -1O ⁰ C by processing the FCAW metal joint, obtained by a FCAW process, into a CTOD test piece. [137] Table 1

[Table 1]

[138] [139] Table 2

[Table 2] [Table ]

[140] [141] Table 3

[Table 3]

[142] As shown in Table 3, the weld metal joint manufactured according to this invention has TiO oxides, the number of which is 3 X 10 ⁸ /mm ³ or more, while comparative steel has TiO oxides, the number of which is 4.3 X 10 ⁶ /mm ³ or less. When compared to the comparative steel, the inventive steel has more uniform and fine composite precipitates, the number of which is significantly increased.

[143] Meanwhile, the microstructure of the inventive steel includes acicular ferrite having a high phase fracture of 85% or more.

[144] Therefore, according to the FCAW process, the inventive steel includes polygonal ferrite and acicular ferrite within grains. Here, the acicular ferrite has a phase fracture of 85% or more, and when compared to the comparative steel, the inventive steel has excellent CTOD properties of the weld metal joint.