KR20090062598A - High strength steel plate for high heat input welding having welded joint with superior impact toughness in weld heat affected zone - Google Patents

Abstract

A high strength steel plate for high heat input welding is provided to control the composition and micro-structure of molten steel in high heat input welding efficiently, thereby improving tensile strength, elongation ratio, hardness and surface property of the steel plate. A high strength steel plate for high heat input welding is composed of C: 0.01~0.2 weight%, Si: 0.1~0.5 weight%, Mn: 1.0~3.0 weight%, Ti: 0.01~0.1 weight%, Ni: 0.5~3.0 weight%, B: 0.0003~0.01 weight%, Mo: 0.05~1.0 weight%, N: 0.004~0.008 weight%, P: 0.030 weight% or less, Al: 0.005~0.05 weight%, S: 0.030 weight% or less, O: 0.01~0.03 weight%, and the remnant Fe and inevitable impurities, wherein Ti, O, N, and B satisfy the relations, Ti/O=1.3~3.0, Ti/N=7~12, N/B=0.8~1.5, and (Ti+4B)/N=11~16, and Mn and S satisfy the relation, Mn/S= 220~400.

Description

High strength Steel Plate for High Heat Input Welding having Welded Joint with Superior Impact Toughness in Weld Heat Affected Zone}

The present invention relates to a welded structural steel including welded joints whose physical properties are stable even when high heat input SAW welding is used for welding structures such as ships, buildings, bridges, offshore structures, steel pipes, and line pipes. Preferably, the fine structure of the welded structural steel including a welded joint having improved impact toughness of the high heat input SAW welded joint by promoting dispersal of fine ferrite by finely dispersing fine TiO and TiO- (Ti, B) N-MnS composite oxides. It is about.

In recent years, with the trend of larger ships and higher building structures, structures are becoming larger, and steels used therein are being gradually replaced with high-strength steels and thick steels. Therefore, when welding such a high strength thick steel material by the conventional welding method, it is often difficult to manufacture the structure within a given period of time, it is becoming increasingly inevitable to weld high-performance.

In this case, the most widely used welding technique for welding thick steel is the submerged arc welding technique, which is more productive than conventional GMAW welding because the welding volume is reduced due to the large amount of welding. It is much more advantageous.

However, since the submerged arc welding method is a high heat input welding method in which the amount of heat applied to the weld heat affected zone is very large, generally the welded joint of the weld metal can form coarse columnar tissue as the tissue solidifies. In the coarse grains, coarse grained ferrites, Widmanstatten ferrites, and the like may be formed along the austenite grain boundaries. Therefore, the welded joint is the site where the impact toughness is most degraded in the welded structure, and is a part where the risk of cracking and fracture can always occur.

Therefore, in order to secure the stability of the welded structure, it is necessary to secure the impact toughness of the welded joint by controlling the microstructure of the welded joint, and many studies have been made in the related art. In particular, various techniques for defining the components of the welding material have appeared, but it is difficult to obtain sufficient weld joint toughness only by limiting the component system because the microstructure, particle size, etc. of the weld metal are not particularly controlled.

In addition, although a technique for improving welded joint properties has been recently shown, the ARM defined by ARM = 197-1457C-1140sol.Al, + 11850N-316 (Pcm-C) is adjusted to 40 to 80. The existing ARM has a problem that it is difficult to secure impact toughness of the SAW high heat input weld joint because there is no oxygen content limitation in the weld joint.

Accordingly, the present inventors have solved the above problems and provide a welded structural steel including a welded joint having excellent tensile strength, elongation, hardness and surface properties by efficiently controlling the component system and microstructure of the welded structural steel during high heat input welding. I would like to.

In the present invention, 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: 0.0003-0.01%, Mo : 0.05 to 1.0%, N: 0.004-0.008%, P: 0.030% or less, Al: 0.005-0.05%, S: 0.030% or less, O: 0.03% or less, and other unavoidable impurities and balance Fe To provide a welded structural steel, and if necessary, between Ti, O, N, B, Mn and S, Ti / O: 1.3 ~ 3.0, Ti / N: 7 ~ 12, N / B: 0.8 ~ 1.5 and (Ti + 4B) / N: Provides welded structural steels that satisfy the relationship of 11 to 16.

Further, the welded structural steel of the present invention is Cu: 0.01 to 2.0%, Nb: 0.0001 to 0.1%, V: 0.005 to 0.1%, Cr: 0.05 to 1.0%, W: 0.05 to 0.5%, and Zr: 0.005 to 0.5%. It may additionally include one or two or more components selected from the group consisting of, and further comprises 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% can do.

Further, the microstructure of the welded joint of the welded structural steel of the present invention has a needle fraction ferrite of 85% or more as a tissue fraction, and the remainder is ferrite which is inevitably precipitated at polygonal ferrite or other grain boundaries, and TiO and TiO- (Ti , B) N-MnS composite oxide is preferably uniformly dispersed at intervals of 0.5㎛ or less.

Further, in the steel of the present invention, the TiO and TiO- (Ti, B) N-MnS composite oxides in the weld joint may have a particle diameter of 0.01 μm to 0.1 μm. In addition, the number of TiO and TiO- (Ti, B) N-MnS composite oxide particles in the welded joint of the present invention is 1.0x107 or more per mm ³ .

The welded structural steel produced by the present invention may have excellent tensile strength, elongation, hardness, and surface properties, and thus can be used with high stability.

The present inventors have thoroughly examined the type and size of oxides on needle-like ferrites known to be effective in the toughness of welded joints. It was found that the amount of acicular ferrite in the joint changes and that the toughness of the high heat input weld joint varies with the amount of acicular ferrite.

Based on these studies, the present inventors

(1) presents a technique using TiO and TiO- (Ti, B) N-MnS composite oxides for SAW weld metals;

(2) Limit the number of oxides (1.0 X10 ⁷ / mm ³ or more) and size (0.01 ~ 0.1㎛) of welded joints respectively, and improve the toughness by transforming and maintaining acicular ferrite to 85% or more,

(3) A technique for promoting needle-like ferrite transformation has been proposed by securing TiO and TiO- (Ti, B) N-MnS complex oxides and soluble B.

Hereinafter, the present invention will be described in detail.

1.Management of TiO and TiO- (Ti, B) N-MnS Composite Precipitates

If the ratio of Ti / O, Ti / N, B / N, and Mn / S is properly maintained in the weld metal, the number of TiO oxides and the composite oxides of TiO- (Ti, B) N-MnS is properly distributed, and the In the coagulation process, coarsening of austenite grains can be prevented and needle ferrite transformation can be promoted. If the TiO oxide and TiO- (Ti, B) N-MnS composite oxides are properly distributed in the austenite grains, the acicular ferrite transformation is preferred to the grain boundary ferrites, which act as heterogeneous nucleation sites in austenite as the temperature decreases. Because it happens. Therefore, these results can significantly improve the toughness of the welded joint.

For this purpose, it is important to distribute the TiO oxide and the composite oxide of TiO- (Ti, B) N-MnS finely and uniformly. In addition, the size, content and distribution of TiO oxides and TiO- (Ti, B) N-MnS composite oxides according to the ratio of Ti / O, Ti / N, B / N and Mn / S should be optimized. In Ti / O: 1.3-3.0, Ti / N: 7-12, N / B: 0.8-1.5, (Ti + 4B) / N: 11-16, and Mn / S: 220-400. In this case, the composite oxide of TiO oxide and TiO- (Ti, B) N-MnS having a size of 0.01-0.1 μm is formed at 1.0 × 10 ⁷ / mm ³ or more to secure a more stable fine oxide.

2. Role of Soluble Boron (Employment B) in Weld Joints

The inventors found that, apart from the oxides uniformly dispersed in the weld seam, the solid solution B diffuses into the grain boundary and reduces the energy of the grain boundary, thereby serving to suppress grain boundary ferrite transformation at the grain boundary. In addition, B has been found to play a role in promoting acicular ferrite transformation by diffusing into the oxide to form a B-deplted zone around the oxide to inhibit hardening around the complex oxide.

3. Microstructure of welded joint

With the use of the complex oxide and the solid solution B described above, there is a need to limit the type and fraction of the microstructures shown in the present invention. It is important to properly distribute TiO oxides and TiO- (Ti, B) N-MnS composite oxides in the weld metal to promote acicular ferrite transformation in the grains rather than grain boundaries during cooling of the welded joint. In this case, the composition ratio of the needle-like ferrite of the welded joint is ensured more than 85%, the remainder may be composed of polygonal ferrite and ferrite structure inevitably precipitated at the grain boundary.

As described above, in the present invention, the impact toughness of the welded joint can be further improved by suppressing the formation of grain boundary ferrite at the grain boundaries and promoting the formation of acicular ferrite in the grains by the efficient use of fine oxide and solid solution B.

Hereinafter, the component system of the welded structural steel in which the welded joint desired in the present invention can be formed will be described in detail.

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 exceeds 0.2%, weldability may be greatly reduced, low temperature cracking may occur at the welded joint, and thermal shock toughness may also be reduced, so the content of C is limited to 0.01 to 0.2%.

Si: 0.1 ~ 0.5%

Si is added 0.1% or more as an element having a deoxidation effect, if the content is less than 0.1% does not exhibit a sufficient deoxidation effect and the fluidity of the weld metal may be reduced. On the other hand, when the amount of Si added is more than 0.5%, the transformation of in-phase martensite in the weld metal may be promoted, which may lower the low-temperature impact toughness and adversely affect the weld cracking sensitivity, so the Si content is 0.1 to 0.5. It is limited to%.

Mn: 1.0-3.0%

Mn is formed in the form of MnS precipitates around the TiO oxides, with an effective effect of improving the deoxidation and strength in the steel, thereby promoting the formation of acicular ferrite, which is advantageous for the improvement of the welded joint toughness. In addition, since Mn forms a solid solution to form a solid solution in the matrix structure, the matrix can be strengthened by solid solution to secure strength and toughness, so that 1.0% or more is added. However, if the content exceeds 3.0% Mn content is limited to 1.0 ~ 3.0% because low temperature metamorphic tissue can be produced.

Ti: 0.01 ~ 0.1%

Ti is combined with O to form a fine Ti oxide and is necessary for the formation of fine TiN precipitates, which is a very important element in the present invention and should be added at least 0.01%. However, when the content of Ti is excessive, coarse TiO oxides and coarse TiN precipitates may be formed, and the upper limit thereof is limited to 0.1%.

Ni: 0.5 ~ 3.0%

Ni is added to 0.5% or more as an effective element to improve the strength and toughness of the matrix by solid solution strengthening. However, if the content is excessive, the hardenability is greatly increased and the possibility of high temperature cracking is high, so the upper limit is limited to 3.0%.

B: 0.0003-0.01%

In the present invention, B is an element that improves the quenching property, so it is added to 0.0003% or more because it segregates at grain boundaries and plays an important role in suppressing grain boundary ferrite transformation, but when more than necessary, the effect is saturated and the weld hardenability is greatly increased. The upper limit is limited to 0.01% because martensite transformation can be promoted, which can cause welding low temperature crack generation and toughness.

N: 0.004-0.008%

N is an element necessary for forming TiN precipitates and the like, and is an element for increasing the amount of fine TiN precipitates. In particular, since the TiN precipitates have a significant influence on the size of the precipitate, the interval between the precipitates, the distribution thereof, the frequency of complex precipitation with the oxide, and the high temperature stability of the precipitates themselves, the content is set at 0.004% or more. However, if the content of N exceeds 0.008%, the effect is saturated, and the upper limit is limited to 0.008% because the amount of solid solution N present in the weld metal may increase to reduce toughness.

P: 0.030% or less

Since P is an impurity element that promotes high temperature cracking during welding, it is desirable to manage P as low as possible. In particular, in order to improve toughness and reduce cracking, it is better to manage at 0.03% or less.

Al: 0.005-0.05%

Al is a deoxidizer and is an element necessary to reduce the amount of oxygen in the weld metal. In addition, the content is added to 0.005% or more because it is necessary to combine with solid solution nitrogen to form a fine AlN precipitate. However, if the content is excessive, coarse Al ₂ O ₃ is formed, which may interfere with the formation of TiO oxide necessary for toughness improvement, so the upper limit thereof is limited to 0.05%.

Mo: 0.05-1.0%

Mo, like Cr, is an element that increases the hardenability and at the same time improves the strength. If Mo is added, the effect can be obtained by adding 0.05% or more. Limited to 1.0%.

S: 0.030% or less

S is preferably made 0.030% or less in order to form MnS composite precipitates. However, when present in excess of 0.030%, it is necessary to limit the content because it may cause a high temperature crack by forming a low melting point compound such as FeS.

O: 0.01% to 0.03%

O is an element that forms Ti oxide by reacting with Ti during solidification of the weld seam. Ti oxide can promote the transformation of acicular ferrite in the weld metal, but coarse Ti when O content exceeds 0.03% Oxides and other oxides such as FeO can be produced, thus limiting their upper limit. In addition, it is difficult to control to less than 0.01%, and the lower limit is 0.01% because the formation of an oxide advantageous to the present invention is limited.

Ti / O: 1.3 to 3.0

If the Ti / O ratio is less than 1.3, the number of TiO oxides required for the inhibition of austenite grain growth and acicular ferrite transformation in the weld metal is insufficient, and the Ti fraction contained in the TiO oxide is reduced to lose the function as acicular ferrite nucleation sites. As a result, the acicular ferrite phase fraction effective for improving the toughness of the weld heat affected zone can be lowered. On the other hand, when the ratio of Ti / O exceeds 3.0, the effect of inhibiting the growth of austenite grains in the weld metal is saturated, and the proportion of Mn and the like contained in the oxide is rather small, so that the function of the needle-like ferrite as a nucleation site is reduced. Since it may be lost, the ratio of Ti / O in the present invention is controlled to 1.3 to 3.0.

Ti / N: 7-12

When Ti / N is less than 7, the amount of TiN precipitates formed in the TiO oxide is reduced, which is not good for acicular ferrite transformation effective for toughness improvement. On the other hand, when the ratio exceeds 12, the effect is saturated, and the amount of solid solution is increased, so that the impact toughness is lowered. Therefore, the ratio of Ti / N is limited to 7-12.

N / B: 0.8-1.5

If N / B is less than 0.8, the amount of diffusion of solid solution B into the austenite grain boundary during cooling after welding is insufficient, and thus the level of inhibition of grain boundary ferrite transformation is low, whereas when N / B exceeds 1.5, the effect is saturated and the amount of nitrogen dissolved This increase may reduce the toughness of the weld heat affected zone, so N / B is limited to 0.8 to 1.5.

(Ti + 4B) / N: 11-16

If (Ti + 4B) / N is less than 11, the amount of solid solution N increases, which is not effective for improving the toughness of the welded joint, and if it exceeds 16, it is not sufficient to generate precipitates such as TiN and BN. Restrict to

Mn / S: 220 ~ 400

If Mn / S exceeds 400, the strength may increase sharply and cracks may occur or low-temperature toughness may be degraded. On the other hand, the ratio is limited to 220-400 because high temperature cracks may occur below 220.

In the present invention, while forming a steel sheet based on the above-described component system, in order to further improve the mechanical properties, one or more selected from the group of Nb, V, Cu, Cr, W and Zr is further added.

Cu: 0.01 ~ 2.0%

Cu is added to 0.01% or more as an effective element that can be dissolved in the base and cause a solid solution effect to secure strength and toughness. However, excessive content thereof is not preferable because it increases the hardenability of the welded joint to reduce toughness and promotes high temperature cracking in the weld metal.

In addition, in the case of complex addition of Cu and Ni, the total thereof is limited to 3.5% or less. This is because when the sum of both components exceeds 3.5%, the hardenability becomes too large in the case, which adversely affects toughness and weldability.

Nb: 0.0001-0.1%

Nb is an element capable of improving quenchability, in particular Ar ₃ It has the effect of widening the bainite formation even at low temperature and low cooling rate, which helps to obtain the bainite structure stably. Therefore, in order to expect such an effect, it is necessary to add more than 0.0001%, but if the content exceeds 0.1%, it promotes the formation of phase martensite in the welded joint during welding, which adversely affects the toughness of the welded joint. The upper limit is limited to 0.1%.

V: 0.005 ~ 0.1%

V is an element that promotes ferrite transformation by forming VN precipitates, and may be added more than 0.005%. However, when the content is excessive, V forms a hard phase such as carbide on the weld joint, which adversely affects the toughness of the weld joint. Therefore, the upper limit is limited to 0.1%.

Cr: 0.05-1.0%

Cr is an element that increases the hardenability and improves the strength, and may be added at least 0.05%. However, if the content is excessive, the weld seam toughness is deteriorated, so the upper limit is limited to 1.0%.

W: 0.05-0.5%

W is an element that improves high temperature strength and is effective for strengthening precipitation, so that 0.05% or more is added. However, exceeding 0.5% limits the amount because it adversely affects the weld seam toughness.

Zr: 0.005 ~ 0.5%

Zr may add 0.005% or more because it is effective in increasing the strength, but if it exceeds 0.5%, the upper limit thereof is limited to 0.5% because it is not good for the weld joint toughness.

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

Ca and / or REM are elements that can stabilize the arc during welding and form oxides at the weld seam. In addition, austenite grain growth is suppressed in the cooling process and the ferrite transformation in the mouth is promoted to improve the toughness of the welded joint. Because of this effect, it is preferable to add at least 0.0005% of Ca and at least 0.005% of REM. However, when Ca is 0.05% and REM is more than 0.05%, toughness may be reduced by forming large oxides. The REM may be used one kind or two or more kinds of components such as Ce, La, Y, Hf.

Hereinafter, the microstructure constituting the steel sheet of the present invention will be described in detail.

In the present invention, the microstructure of the welded joint formed after the high heat input welding is acicular ferrite, and its phase fraction should be 85% or more because the acicular ferrite structure is a structure that can simultaneously obtain high strength and high toughness. When the ferrite and bainite structures are mixed, the impact toughness is good, but the weld seam strength is low. When the microstructure is the martensite and bainite structure, the weld seam is high, but the mechanical properties such as toughness of the weld seam are high. It is not preferable because of poor properties and increased cold cracking susceptibility. Therefore, the tissue of the present invention is composed of acicular ferrite as the main structure, and the remainder is composed of polygonal ferrite and ferrite tissue that inevitably precipitates at grain boundaries.

In addition, the oxide present in the welded joint has a great influence on the microstructure transformation of the welded joint after welding. That is, the formation and properties of the tissue are greatly influenced by the type, size and number of oxides to be distributed. In particular, in the case of high heat input weld joints, the cooling speed of the weld joint is slowed, so that the grains are coarsened, and coarse grain boundary ferrite, Widmanstatten ferrite, bainite, etc. are formed from the grain boundary, which may lower the properties of the weld joint. Therefore, in order to prevent this, it is important to uniformly disperse the TiO and TiO- (Ti, B) N-MnS composite oxides at intervals of 0.5 μm or less in the weld metal.

Furthermore, the particle diameter of TiO and TiO- (Ti, B) N-MnS composite oxides is limited to 0.01 to 0.1 μm, but when the particle size is less than 0.01 μm, the role of promoting the transformation of acicular ferrite in the high heat input joint is insufficient. In addition, when the thickness exceeds 0.1 μm, the effect of pinning on the austenite grains is reduced and the same behavior as the coarse nonmetallic inclusions adversely affects the mechanical properties of the high heat input joints. .

In addition, in order to obtain such effects sufficiently, the number of oxides should be sufficient, and the critical number of TiO and TiO- (Ti, B) N-MnS composite oxides should be 1.0 × ^{10 7} or more per mm ³ .

Hereinafter, a welded structural steel including a welded joint formed according to the present invention will be described in detail.

The high heat input welded joint provided in accordance with the present invention comprises at least 85% acicular ferrite tissue. In addition, TiO and TiO- (Ti, B) N-MnS composite oxides have a size of 0.01 to 0.1 µm and more than 1.0 × 10 ⁷ per mm ³ , and the intervals are finely distributed to 0.5 µm or less.

Welded structural steel including such weld seams is not limited to high heat input SAW but may be used in other high heat input welding processes. In this case, if the cooling rate of the high heat input welding joint is high, the high heat input welding process having a high cooling rate may be used because the oxide is finely dispersed and the structure becomes fine. For the same reason, cooling and Cu-backing of steel materials may also be used to improve the cooling rate of welded joints. However, even if the known techniques are applied to the present invention, this is merely a change of the present invention and may be interpreted as being substantially included in the technical scope of the present invention.

The present invention will be described in more detail with reference to the following examples.

Welded structural steel having the composition as shown in Table 1 was manufactured by SAW applying a large input heat welding heat input of 100kJ / cm or more. Table 2 shows the values of Ti / O, Ti / N, N / B, Mn / S, and (Ti + 4B) / N in the welded joint, which may exhibit the effects of the present invention.

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 0.057 56 52 - 0.01 - - - - - 200 Inventive Steel 2 0.07 0.32 1.50 0.012 0.005 1.44 0.15 0.046 45 54 - 0.005 - - - - - 240 Invention Steel 3 0.08 0.25 1.48 0.011 0.004 1.65 0.15 0.063 52 53 0.05 0.04 - - - - - 280 Inventive Steel 4 0.08 0.22 1.48 0.008 0.005 1.54 0.12 0.040 50 50 - 0.03 - - - - - 280 Inventive Steel 5 0.07 0.16 1.60 0.011 0.004 1.50 0.10 0.045 45 50 - 0.01 - - - - - 250 Inventive Steel 6 0.07 0.14 1.50 0.09 0.005 1.65 0.12 0.050 42 54 - 0.02 - 0.1 - - - 280 Inventive Steel 7 0.10 0.25 1.48 0.011 0.005 1.45 0.15 0.048 45 55 0.04 0.02 - - - - - 260 Inventive Steel 8 0.11 0.35 1.52 0.012 0.006 1.55 0.18 0.060 46 65 - 0.01 - - 0.01 - - 240 Inventive Steel 9 0.09 0.28 1.50 0.010 0.005 1.48 0.20 0.044 40 52 - 0.01 0.1 - - 0.001 - 250 Inventive Steel 10 0.07 0.18 1.55 0.009 0.006 1.50 0.25 0.046 43 55 - 0.01 - - - - 0.005 260 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 - - - - - 350 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 - - - - - 320 Comparative Steel 3 0.06 0.06 1.25 0.010 0.007 1.61 0.010 0.014 21 74 - 0.007 - - - - - 350 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 11 160 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

Composition ratio between alloy elements Ti / O Ti / N N / B Mn / S (Ti + 4B) / N Inventive Steel 1 2.9 11.0 0.9 308 15.3 Inventive Steel 2 1.9 8.5 1.2 300 11.9 Invention Steel 3 2.3 11.9 1.0 370 15.8 Inventive Steel 4 1.4 8.0 1.0 296 12.0 Inventive Steel 5 1.8 9.0 1.1 400 12.6 Inventive Steel 6 1.8 9.3 1.1 300 12.4 Inventive Steel 7 1.8 8.7 1.3 296 12.0 Inventive Steel 8 2.5 9.2 1.4 253 12.1 Inventive Steel 9 1.8 8.5 1.4 300 11.5 Inventive Steel 10 1.8 8.4 1.3 258 11.5 Comparative Steel 1 0.3 1.1 3.2 208 2.3 Comparative Steel 2 0.8 2.3 1.6 483 4.8 Comparative Steel 3 0.4 1.9 3.5 178 3.0 Comparative Steel 4 0.7 3.6 0.5 500 11.1 Comparative Steel 5 3.4 8.2 1.2 195 11.4 Comparative Steel 6 2.4 4.1 2.7 218 5.6 Comparative Steel 7 1.5 1.5 6.6 198 2.1 Comparative Steel 8 1.2 1.3 4.3 260 2.3 Comparative Steel 9 5.8 8.1 9.1 174 8.5 Comparative Steel 10 1.6 3.2 2.9 415 4.6 Comparative Steel 11 1.0 4.4 430 2.2

Test pieces for evaluating the mechanical properties of the welded joints welded as described above were taken from the center of the welded joint. Tensile test piece was used KS standard (KS B 0801) No. 4 test piece and the tensile test was tested at cross head speed (10mm / mim). The impact test piece was prepared according to KS (KS B 0809) No. 3 test piece.

The size, number, and spacing of oxides and composite oxides, which have a significant effect on the toughness of welded joints, were measured by the point counting method using an image analyzer and electron microscope. At this time, the test surface was evaluated based on 100 mm ² . The impact toughness of the high heat input welded joints was evaluated by the Charpy impact test at -20 ° C, processed into impact specimens.

As described above, the physical properties of the weld joint were evaluated, and the results are shown in Table 3 below.

division Heat input TiO and TiO + MnS + (Ti-B) N Welded joint needle ferrite fraction (%) Welded Joint Mechanical Properties Welding heat input (kJ / cm) Number (pcs / mm ³ ) Average size (㎛) Tensile Strength (MPa) vE _{-20 ℃} (J) Inventive Steel 1 120 3.4 X ^{10 8} 0.016 89 640 197 Inventive Steel 2 150 4.6 X ^{10 8} 0.017 89 650 223 Invention Steel 3 120 3.7 X ^{10 8} 0.012 87 680 210 Inventive Steel 4 100 4.6 X ^{10 8} 0.016 88 660 235 Inventive Steel 5 180 6.4 X ^{10 8} 0.018 87 650 220 Inventive Steel 6 190 5.2 X ^{10 8} 0.025 89 630 220 Inventive Steel 7 220 3.6X10 ⁸ 0.013 90 640 198 Inventive Steel 8 130 4.3X10 ⁸ 0.026 91 660 188 Inventive Steel 9 130 5.6 X ^{10 8} 0.024 88 665 241 Inventive Steel 10 110 5.3 X ^{10 8} 0.014 85 620 209 Comparative Steel 1 120 3.0 X 10 ⁶ 0.045 46 650 44.7 Comparative Steel 2 120 4.3X10 ⁶ 0.051 52 640 64.4 Comparative Steel 3 120 2.5 X 10 ⁶ 0.054 44 650 58.8 Comparative Steel 4 120 3.0 X 10 ⁶ 0.064 45 660 46.2 Comparative Steel 5 120 2.5 X 10 ⁵ 0.037 37 650 57.04 Comparative Steel 6 130 2.5 X 10 ⁶ 0.056 42 680 47.0 Comparative Steel 7 130 3.0 X 10 ⁶ 0.043 44 665 56.4 Comparative Steel 8 230 4.1 X 10 ⁵ 0.046 52 610 46.9 Comparative Steel 9 130 2.8X10 ⁵ 0.041 59 610 58.2 Comparative Steel 10 230 3.4 X 10 ⁵ 0.046 52 620 56.7 Comparative Steel 11 130 2.6 X 10 ⁶ 0.043 41 625 57.6

As shown in Table 3, the high heat input welded structural steel manufactured by the present invention has a number of TiO and TiO- (Ti, B) N-MnS composite oxides in the range of ² × ^{10 8} / mm ³ or more, The comparative steel shows a range of 4.3 X10 ⁶ / mm ³ or less, and thus, the inventive steel has a fairly uniform and fine complex precipitate size and a significant number thereof compared to the comparative steel.

In addition, the microstructure of the inventive steels consists of acicular ferrite, which is inevitably precipitated in polygonal ferrite and other grain boundaries, and the acicular ferrite phase fraction is higher than 85%. It can be seen that the weld seam impact toughness is shown.

According to the embodiment of the present invention, the welded structural steel including the welded joint manufactured by the present invention can be seen to have more excellent tensile strength, elongation, hardness and surface properties.

Claims (10)

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: 0.0003-0.01%, Mo: 0.05- 1.0%, N: 0.004% to 0.008%, P: 0.030% or less, Al: 0.005% to 0.05%, S: 0.030% or less, O: 0.01% to 0.03%, other inevitable impurities and a weld comprising a balance Fe Structural steel. The method according to claim 1, wherein Ti, O, N, and B are Ti / O: 1.3 to 3.0, Ti / N: 7 to 12, N / B: 0.8 to 1.5, and (Ti + 4B) / N: 11 to Welded structural steel, characterized in that the relationship of 16 holds. The weld structural steel according to claim 1, wherein a relationship of Mn / S: 220 to 400 is established between the Mn and S. According to claim 1 or 2, wherein the welded structural steel is Cu: 0.01 to 2.0%, Nb: 0.0001 to 0.1%, V: 0.005 to 0.1%, Cr: 0.05 to 1.0%, W: 0.05 to 0.5%, Zr : Welded structural steel, further comprising one or two or more components selected from the group consisting of 0.005 to 0.5%. The welded structural steel according to claim 4, wherein when Cu and Ni are added to the welded structural steel, the sum thereof is 3.5% or less. According to claim 1 or 2, wherein the welded structural steel is characterized in that it further 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. The welded joint steel according to claim 1 or 2, wherein the welded joint microstructure of the welded structural steel has a needle-like ferrite of 85% or more in a tissue fraction, and the remainder is polygonal ferrite or other grain boundary ferrite tissue. The weld joint according to claim 1 or 2, wherein the weld joint of the welded structural steel is uniformly dispersed in the structure with TiO and TiO- (Ti, B) N-MnS composite oxides at intervals of 0.5 µm or less. Structural steel. The weld structural steel according to claim 8, wherein the TiO and TiO- (Ti, B) N-MnS composite oxides have a particle diameter of 0.01 to 0.1 mu m. The method of claim 8, wherein the structural steel welding, characterized in that the welded construction of welded Steel TiO and TiO- in the vagina (Ti, B) N-MnS composite oxide particle number is 1.0x107 or more per 1mm ^3.