KR102043520B1 - Welded joint having excellent ctod properties - Google Patents

Abstract

The present invention relates to welds applied to containers, shipbuilding, and the like, and more particularly, to welds having excellent CTOD (Crack Tip Opening Displasement) characteristics of welded metals and a method for manufacturing the same.

Description

WELDED JOINT HAVING EXCELLENT CTOD PROPERTIES}

The present invention relates to welded parts applied to containers, shipbuilding, and the like, and more particularly, to welded parts having excellent CTOD (Crack Tip Opening Displasement) characteristics of welded metals and a method of manufacturing the same.

In recent years, the application of high strength, ultra-thick shipbuilding steel has increased with the increase of container ships. In the case of large vessels using such ultra-thick steels, there is always a risk of brittle crack propagation that can lead to hull failure in a short time, even if small defects occur inside the steel or especially the weld due to external stress during operation or continuous fatigue environment.

Therefore, in order to prevent the risk of brittle crack propagation in advance, in large ships including the Norwegian and German ships (DNVGL), the welding heat affected zone (CGHAZ) and CTOD (Crack Tip Opening Displasement) characteristics are required for weld metal.

On the other hand, for the welding of thick plates, the shipbuilding industry uses submerged arc welding (SAW) with excellent welding efficiency and flux cored arc welding (FCAW) with excellent welding workability at various angles. ) Is mainly applied. In addition, multipass welding is mainly applied for welding ultra-thick steels. The multi-layer welding has an advantage of low weld heat input for each pass, which is superior in strength and toughness of the weld metal part or the heat affected part compared to the high heat input welding method.

For example, Patent Document 1 discloses a method of using multilayer welding as a welding method for structural steel materials containing Cr: 1.5 to 3.5%, Mo: 0.5 to 1.5%, and V: 0.15 to 0.5% by mass.

However, inside the weld metal obtained by such multi-layer welding, changes in the weld metal microstructure due to various heat cycles may occur, and thus, defects such as deterioration of toughness or reheat cracking may occur. Accordingly, deterioration of the CTOD characteristics in the welded portion has been a problem.

Korean Unexamined Patent Publication No. 10-2014-0142226

One aspect of the present invention is to provide a welded portion having excellent CTOD characteristics of the welded portion during multi-layer welding.

The problem to be solved by the present invention is not limited to the above-mentioned problem, another task that is not mentioned will be clearly understood by those skilled in the art from the following description.

One side of the present invention is a welding unit including a welding layer in which a plurality of welding metal is laminated;

The weld metal includes a heat affected zone and an unheated zone,

It relates to a welded portion having excellent CTOD characteristics in which the ratio of the heat-affected portion included on the notch line S of the welded portion in the length of the welded portion thickness L is 30% or more.

According to the present invention, excellent CTOD characteristics can be secured in the welded portion formed by multilayer welding of the ultra thick steel.

1 is a schematic diagram schematically showing a welded section according to an embodiment of the present invention.
2 is a photograph of a welding layer made of multiple welding metals in a welding part according to an embodiment of the present invention.
Figure 3 is a photograph observing the weld metal microstructure of Example 6 of the present invention.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

The present inventors conducted a study to secure the CTOD (Crack Tip Opening Displasement) characteristics of the welded part in the welding process of the ultra-thick steel, the welding part forms a welding layer made of multiple welding metals, the welding in the welding layer It is recognized that the CTOD properties are affected by the formation position of the metal, which leads to the present invention. Hereinafter, the present invention will be described with reference to FIGS. 1 and 2.

In general, when welding a shipyard that uses a lot of ultra-thick steel, V-grooves (V-grooves), V-shaped grooves or K-grooves are applied. FIG. 1 schematically shows a weld including a weld layer in which multiple weld metals are stacked in the V-groove, and the notch position and notch line S of the CTOD evaluation in the weld.

The welding part is formed by stacking multiple welding metals, which may be referred to as a welding layer. The welding layer forms a welding layer in such a manner that one welding metal is formed while the preceding bead is formed, and a trailing bead is stacked on the welding metal formed by the preceding bead. In the lamination process, the weld metal formed by the preceding bead is heated again when the trailing bead is stacked to generate a reheated portion in the weld metal, and the reheated portion is called a heat affected zone. The portion of the weld metal that is not reheated is called unheated zone. Referring to FIG. 2, in the laminated weld metal, a bright part is an unheated part and a dark part is a heat part.

The microstructure of the heat affected zone is different from the existing weld metal (or unheated affected zone of the weld metal). The heat affected zone is grown by reheating, the high-strength microstructure, such as martensite or bainite is transformed into a soft tissue, such as ferrite due to a relatively slow cooling rate to decrease the hardness and increase the toughness.

On the other hand, in the CTOD evaluation of the weld, the microstructure plays an important role on the notch line S because the notch is processed at the center of the weld. Accordingly, in the present invention, the heat affected part having excellent toughness is positioned on the notch line S, thereby ensuring a weld part having excellent CTOD characteristics.

More specifically, the welded portion of the present invention preferably has a ratio of the heat-affected portion included on the notch line S of the welded portion in the length of the welded portion thickness L of 30% or more. If the ratio of the heat affected zone is less than 30%, it is difficult to secure the toughness on the notched line, and thus it is difficult to secure sufficient CTOD characteristics.

On the other hand, the heat affected zone is preferably located in the center of the welded portion of the notched wire (S). The center means a position of 1/4 to 3/4 of the weld thickness. The ratio of the heat affected part included on the notch line S at the center of the weld part is preferably 70% or more to secure sufficient CTOD characteristics in the weld part.

On the other hand, in general high strength weld metal, it is inevitable to add various alloying elements to improve strength, and in the case of some high strength weld metal, carbon (C) and silicon included in welding wire, flux component during multilayer welding Elements such as (Si) and manganese (Mn) may form an MA constituent structure that adversely affects impact toughness by reheating, thereby deteriorating impact toughness or CTOD characteristics.

In order to prevent this, the welded part of the present invention is a weight%, carbon (C): 0.01 to 0.06%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.0 to 2.0%, nickel (Ni): 1.0 ~ 3.0%, molybdenum (Mo): 0.001-0.5%, copper (Cu): 0.01-0.7%, chromium (Cr): 0.01-0.5%, titanium (Ti): 0.01-0.1%, phosphorus (P): 0.02 % Or less, sulfur (S): 0.01% or less, oxygen (O): 0.03 to 0.05%, preferably containing Fe and unavoidable impurities. Hereinafter, each component is briefly described.

Carbon (C): 0.01 to 0.06% by weight (hereinafter,%)

The C is an element necessary for securing the strength of the weld metal and at the same time securing the hardenability, and in order to obtain the above-mentioned effect, the C needs to contain 0.01% or more of C. However, if the content of C exceeds 0.06%, there is a problem that low-temperature cracking easily occurs at the weld, and the impact toughness of the weld is greatly lowered. Therefore, it is preferable not to exceed 0.06%.

Silicon (Si): 0.05-0.50%

The Si is an element added for the deoxidation effect, when the content is less than 0.05%, the deoxidation effect in the weld metal is insufficient and the induction of the weld metal is reduced. On the other hand, if the content exceeds 0.50%, the deformation of the in-phase martensite (MA) in the weld metal may be reduced, thereby reducing the impact toughness and affecting the weld cracking susceptibility.

Manganese (Mn): 1.0-2.0%

The Mn is an essential element for improving deoxidation and strength, and precipitates in the form of MnS around the TiO oxide to promote the formation of acicular ferrite, which is advantageous for improving the toughness of the Ti composite oxide. In addition, Mn forms a substituted solid solution in the matrix to strengthen the matrix to solidify the matrix to secure strength and toughness, but in order to obtain such an effect, it is preferable to include 1.0% or more. However, if it exceeds 2.0%, there is a problem that the toughness is lowered by creating low-temperature metamorphic tissue, it is preferable not to exceed 2.0%.

Nickel (Ni): 1.0 ~ 3.0%

Ni is an essential element for improving the strength and toughness of the matrix by solid solution strengthening. In order to acquire the said effect, it is preferable to contain Ni 1.0% or more. On the other hand, excessively more than 3.0% is not preferable because it greatly increases the hardenability or there is a possibility of hot cracking.

Molybdenum (Mo): 0.001-0.5%

The Mo is an element that improves the known strength, but for this purpose it is necessary to contain 0.001% or more, but when it exceeds 0.5%, the effect is saturated and the weld hardenability is greatly increased to promote the transformation of martensite, thereby improving the toughness. There is a problem of deterioration.

Copper (Cu): 0.01-0.7%

The Cu is an element that is dissolved in a matrix and is advantageous in securing strength and toughness due to the solid solution strengthening effect, and for this effect, it is preferable to add 0.01% or more of Cu. However, when the content is more than 0.7%, the hardness is increased by decreasing the toughness in the welded part, which is not preferable.

Chromium (Cr): 0.01-0.5%

The Cr is dissolved in the base and is essential for improving the hardenability, improving the strength, and is an advantageous element for securing strength and toughness. For this purpose, it is preferable to include 0.01% or more, but if the content exceeds 0.5% may increase the hardenability in the weld portion may inhibit the toughness.

Titanium (Ti): 0.01 ~ 0.1%

The Ti not only combines with oxygen to form fine Ti oxide, but also forms fine TiN nitride to promote formation of acicular ferrite, thereby improving strength and toughness. To this end, it is preferable to contain 0.01% or more, but too much coarse oxide or precipitate is formed, so that the toughness is lowered, it is preferable not to exceed 0.1%.

Phosphorus (P): 0.02% or less, Sulfur (S): 0.01% or less

P and S are impurities that promote high temperature cracking and are preferably managed as low as possible.

Oxygen (O): 0.03 to 0.05%

O is an element that reacts with Ti to form Ti oxide during solidification of the weld, and Ti oxide promotes acicular ferrite transformation in the weld. However, if the O content is less than 0.03%, Ti oxide is not properly distributed in the welded portion, and if it exceeds 0.05%, coarse Ti oxide or the like is generated, which is not preferable because it affects the impact toughness of the welded portion.

On the other hand, the composition of the C, Si, Mn, Ni, Cr, Mo and the like is preferably Ceq (carbon equivalent) of the following relation 1 is 0.2 to 0.55%.

[Relationship 1]

Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14

(C, Si, Mn, Ni, Cr, Mo, V means the composition (wt%) of each component)

If a weak phase such as phase martensite (MA) is formed in the weld metal, it may adversely affect the impact toughness of the weld. If the Ceq value of the weld exceeds 0.55%, the MA structure may be excessively formed. On the other hand, if the Ceq value is less than 0.2%, there is a problem that the strength of the weld is too low, the Ceq is preferably 0.2 to 0.55%.

Hereinafter, an example of the method of manufacturing the weld part of this invention is demonstrated.

The method for producing a welded part having excellent CTOD characteristics of the present invention is not particularly limited, and a method in which the heat affected part of the weld metal can be positioned on the notch line S described above is sufficient. As an example, a method of forming a large amount of heat affected zones of a weld metal by stacking a central portion at a weld portion first in a multilayer lamination welding, and as another example, in a process of laminating weld metal as shown briefly in FIG. Is stacked vertically so that the heat affected zone of the weld metal is straight in the thickness direction, and the heat affected zone at this time is located on the notched line S.

Hereinafter, the Example of this invention is described in detail. The following examples are only for the understanding of the present invention, but not for limiting the present invention.

(Example)

A yield strength of 450 MPa grade steel having a thickness of Table 1 was prepared as a base material, and multi-layer welding was performed by submerged arc welding (SAW) welding to prepare a welded portion having a weld layer. In addition, the thickness L of a weld part shall be the same as a base material thickness.

In the weld, the ratio of the heat affected zone among the weld metals present on the CTOD notch line was measured and the values are shown together in Table 1 below. The measurement of the said ratio measured the length of the heat-affected part located on the CTOD notch line S with respect to the whole test piece thickness (thickness of weld part, L) with respect to the weld cross section, and calculated | required the ratio.

In addition, the CTOD at -10 ° C of each welded part was measured and the results are shown in Table 1 together. The CTOD measurement was processed to the notch at the test temperature of -10 ℃, and tested according to the commonly used BS7448 standard, the results are shown in Table 1.

division Thickness (mm) % Of heat affected zone present on notch line -10 ℃ CTOD evaluation (mm) Inventive Example 1 100 60 0.55 Inventive Example 2 100 57 0.37 Inventive Example 3 100 35 0.29 Inventive Example 4 100 35 0.27 Inventive Example 5 95 31 0.24 Inventive Example 6 100 100 0.45 Comparative Example 1 100 20 0.21 Comparative Example 2 100 11 0.07 Comparative Example 3 95 7 0.09

As shown in the results of Table 1, when the ratio of the heat affected zone of the weld metal present on the CTOD notch line to the total thickness is 30% or more, the CTOD value at -10 ° C is 0.2 mm or more required by the general classification. It can be seen that it shows an excellent value.

FIG. 3 shows a microstructure of an MA etched in a welded part of Inventive Example 6. FIG. In particular, the hardness of the portion where the heat affected zone is present has a relatively low value, and at this time, it can be seen that there are no phases that adversely affect toughness such as MA as a result of the microstructure analysis. In FIG. 4, the area where the MA is present should appear in a bright color (white), but it can be seen in FIG. 4 that it is hardly observed.

Claims (6)

A welding part including a welding layer in which multiple welding metals are stacked;
The weld metal includes a heat affected zone and an unheated zone,
In the length of the weld portion (L), the ratio of the heat affected portion included on the notch line (S) of the weld portion is 30% or more,
The welding part in weight%, carbon (C): 0.01 ~ 0.06%, silicon (Si): 0.05 ~ 0.50%, manganese (Mn): 1.0 ~ 2.0%, nickel (Ni): 1.0 ~ 3.0%, molybdenum (Mo ): 0.001-0.5%, copper (Cu): 0.01-0.7%, chromium (Cr): 0.01-0.5%, titanium (Ti): 0.01-0.1%, phosphorus (P): 0.02% or less, sulfur (S) : 0.01% or less, oxygen (O): 0.03 to 0.05%, the rest of which is excellent in CTOD characteristics including Fe and unavoidable impurities.
The method according to claim 1,
In the center of the welded portion, the ratio of the heat affected portion included on the notched wire S is 70% or more,
The center portion is a welded portion having excellent CTOD characteristics of 1/4 to 3/4 of the weld portion thickness (L).
delete delete The method according to claim 1,
The composition of the C, Si, Mn, Ni, Cr, Mo, etc. is a welded part excellent in CTOD characteristics of Ceq (carbon equivalent) of the following relation 1 is 0.2 ~ 0.55%.
[Relationship 1]
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
(C, Si, Mn, Ni, Cr, Mo, V means the composition (wt%) of each component)
The method according to claim 1,
The welded part is a welded part having excellent CTOD characteristics of 0.2 mm or more at -10 ° C.

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