KR20130003685A - Weld metal joint having excellent low temperature toughness - Google Patents

Abstract

PURPOSE: A welded joint having excellent cryogenic toughness is provided to maintain an austenitic phase which has excellent cryogenic toughness at a cryogenic temperature and to control an alloying constituent to prevent hot cracking during welding, thereby obtaining excellent impact cryogenic toughness. CONSTITUTION: A welded joint having excellent cryogenic toughness includes 0.02 to 0.75 wt% of C, 0.3 to 0.7 wt% of Si, 10 to 35 wt% of Mn, less than 15 wt% of Cr, less than 25 wt% of Ni, less than 1.45 wt% of Mo, 0.009 to 2.5 wt% of Al, less than 0.02 wt% of S, less than 0.02 wt% of P, remnant Fe, and other inevitable impurities. The welded joint further includes 0.30 to 0.75 wt% of N. The sum of C and N contents is 0.30 to 0.75 wt% in the welded joint. The welded joint satisfies following: Nieq(nickel equivalent)+0.34*Creq(chrome equivalent)≥28.37. [Reference numerals] (AA) Ni equivalent(30C+30N+Ni+0.5Mn); (BB) Cr equivalent(Cr+Mo+1.5Si);

Description

Welding joint with excellent cryogenic toughness {WELD METAL JOINT HAVING EXCELLENT LOW TEMPERATURE TOUGHNESS}

The present invention is a submerged arc welding (SUBMERGED ARC WELDING, SAW) or flux cored arc welding (FCAW) for cryogenic high manganese steel used in offshore structures, energy, shipbuilding, construction, bridges and pressure vessels The present invention relates to a welded joint having excellent cryogenic toughness.

In recent years, the demand for LNG has exploded, and the demand for cryogenic LNG transportation and storage facilities and storage tanks has greatly increased. In addition, the FPSO (Floating Production Storage and Offloading) market, a facility that combines such transportation equipment and storage tanks, is rapidly increasing.

In addition, in the case of LNG carriers, the number of vessels using LNG as a fuel with low carbon dioxide emission is increasing. The use of cryogenic materials is increasing due to the enlargement of such facilities of liquefied natural gas. Tanks that transport or store LNG must be constructed to withstand shock even at temperatures below -162 ° C, which is essentially LNG. Materials with high impact toughness at cryogenic temperatures are used. Typical materials used are 9% Ni steel, stainless steel (hereinafter referred to as STS), and aluminum.

However, since 9% Ni steel and STS304 steel contain a lot of expensive nickel, the cost is large. In addition, aluminum has a problem that a thick plate should be used due to low tensile strength.

Due to these problems, new steel materials are being developed to replace 9% Ni steel, which is a cryogenic steel, and high manganese steel is emerging as an alternative. Cryogenic high manganese steel is a steel with high content of manganese to replace 9% Ni steel and STS 304 steel used for LNG storage tank, and it forms stable austenite structure even at cryogenic temperature without additional heat treatment. It is known to have the property of no deterioration of toughness in the welding heat affected zone (HAZ).

In addition, cryogenic LNG FPSO and LNG fuel transport material is currently used 9% Ni steel, STS and aluminum. However, 9% Ni steel must use an expensive Ni-based welding material (Inconel 625 material: 50 wt% or more of Ni, 20 wt% or more of Cr), and the problem of low yield strength of the welded part is a problem. In addition, in the case of aluminum is poor weldability, STS is expensive, there is a problem such as thermal strain and cryogenic guarantee. Therefore, there is a demand for the development of a cryogenic high Mn-based welding material capable of securing economical and excellent weldability.

As described above, in order to secure the stability of the welded structure at -162 ° C or lower, which is a cryogenic region, it is essential to secure a welded joint that exhibits impact toughness of 27 J or more. Therefore, the development of such a welded joint is required.

The present invention is to provide a submerged and fluxcored arc welded joint having excellent cryogenic impact toughness by maintaining an austenite phase having excellent toughness at cryogenic temperature and controlling an alloy component for preventing high temperature cracking during welding. .

One side of the present invention is a weight%, carbon (C): 0.02-0.75%, silicon (Si): 0.3-0.7%, manganese (Mn): 10-35%, chromium (Cr): 15% or less, nickel (Ni): 25% or less, molybdenum (Mo): 1.45% or less, aluminum (Al): 0.009-2.5%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% or less, balance Fe and other unavoidable It is possible to provide a welded joint having excellent cryogenic toughness including impurities.

Preferably, the weld joint further includes 0.30-0.75 wt% of nitrogen (N).

The weld joint is preferably a sum of the content of carbon (C) and nitrogen (N) is 0.30-0.75% by weight.

The weld joint preferably satisfies nickel equivalent (Ni _eq ) + 0.34 * chromium equivalent (Cr _eq ) ≥ 28.37.

Preferably, the welded joint has a chromium equivalent (Cr _eq ) of 14 wt% or less.

When the weld joint portion has a chromium equivalent (Cr _eq ) of 1 wt% or less, the nickel equivalent (Ni _eq ) is preferably 28 wt% or more.

The impact toughness (−196 ° C.) of the weld joint is preferably 27 J or more.

According to one aspect of the present invention, it is possible to provide a submerged and fluxcored arc welded joint having excellent impact toughness even at cryogenic temperatures.

1 is a cross-sectional view of a welded joint of an embodiment of the present invention.
Figure 2 is a microstructure photograph of a welded joint of one embodiment of the present invention.
Figure 3 is a graph showing the correlation between the chromium equivalent and the nickel equivalent of one embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

The inventors have focused on the fact that, through research and experiment, the range of alloying elements of the welded joint having excellent impact toughness even at cryogenic temperatures can be set for the welded joint generated during the submerged arc welding or the flux cored arc welding. Thus, the present invention was completed based on the results.

In the present invention, it is possible to provide a submerged or flux cored arc welded joint that can maintain the austenite phase by optimally combining the alloying components of the welding wire and prevent hot cracking during welding.

The welded joint of the present invention includes the following composition.

Carbon (C): 0.02-0.75 wt%

Carbon is the most powerful element existing as an austenite stabilizing element capable of securing the strength of the weld metal and securing the cryogenic impact toughness of the weld metal, and is an essential element in the present invention. If the content of carbon is less than 0.02% by weight, since the content of the austenite stabilizing element is low, the cryogenic impact toughness may be lowered. However, if the carbon content exceeds 0.75% by weight, carbon dioxide gas may be generated during welding, which may cause defects in the weld joint, and carbides such as MC, M ₂₃ C ₆ , and the like may be combined with alloying elements such as manganese and chromium. There is a problem in that impact toughness is lowered at low temperatures. Therefore, the content of carbon is preferably limited to 0.02-0.75% by weight.

Silicon (Si): 0.3-0.7 wt%

If the content of silicon is less than 0.3% by weight, the deoxidation effect in the weld metal is insufficient and the fluidity of the weld metal may be reduced. On the other hand, when the content of silicon exceeds 0.7% by weight, it causes segregation in the weld metal, thereby deteriorating low-temperature impact toughness and adversely affecting weld cracking sensitivity. Therefore, the content of silicon is preferably limited to 0.3-0.7% by weight.

Manganese (Mn): 10-35 wt%

Manganese is the main element for producing austenite, which is a low temperature stable phase, which is an element that must be added in the present invention and is a very inexpensive element compared with nickel. If the content of manganese is less than 10% by weight, sufficient austenite is not produced, resulting in very low toughness at cryogenic temperatures. On the other hand, when the content of manganese exceeds 35% by weight, segregation is excessively generated, high temperature cracks are caused, and harmful fumes may be generated. Therefore, the content of manganese is preferably limited to 10-35% by weight.

Chromium (Cr): 15 wt% or less (excluding 0)

Chromium has the advantage of lowering the content of austenite stabilizing elements through a constant amount of chromium as a ferrite stabilizing element. However, when the content of chromium is more than 15% by weight, chromium-based carbides are excessively generated, resulting in low cryogenic toughness. Therefore, the content of chromium is preferably limited to 15% by weight or less.

Nickel (Ni): 25 wt% or less (excluding 0)

Nickel is an austenite stabilizing element which is an essential element to be added in the present invention. However, since the price of nickel is high, it is preferable to include 25% by weight or less in consideration of the manufacturing cost. Therefore, the content of nickel is preferably limited to 25% by weight or less.

Molybdenum (Mo): 1.45 wt% or less (excluding 0)

Molybdenum is an element that can improve the strength of a matrix. However, when the content of molybdenum exceeds 1.45% by weight, the molybdenum carbide is excessively generated, which has the disadvantage of lowering the cryogenic toughness. Therefore, the content of molybdenum is preferably limited to 1.45% by weight or less.

Aluminum (Al): 0.009-2.5 wt%

Aluminum reduces the strength, but also increases the stacking fault energy (SFE) to ensure toughness at low temperatures, and in order to exhibit such effects in the present invention, it is preferably included at least 0.009% by weight. However, when the content of aluminum exceeds 2.5% by weight, an aluminum-based oxide is excessively generated in the weld part, thereby deteriorating the cryogenic impact toughness. Therefore, the content of aluminum is preferably limited to 0.009-2.5% by weight.

Sulfur (S): 0.02 wt% or less

Sulfur is an element that precipitates the MnS composite precipitate, but when the content exceeds 0.02% by weight, it is not preferable because sulfur may form a low melting point compound such as FeS to cause high temperature cracking. Therefore, the content of sulfur is preferably limited to 0.02% by weight or less.

Phosphorus (P): 0.02 wt% or less

Phosphorus is an element influencing low-temperature toughness, so that a phosphorus compound embrittled at the grain boundary is produced. Therefore, the upper limit is preferably limited to 0.03% by weight.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

In addition, the welded joint of the present invention may further improve the effect of the present invention when additionally adding nitrogen (N) described below.

Nitrogen (N): 0.30-0.75 wt%

Nitrogen is an element exhibiting the same properties as carbon, and in the present invention, it is preferable to contain 0.30% by weight or more in order to exhibit such an effect. On the other hand, when the content of nitrogen exceeds 0.75% by weight, an excessive amount of nitride is generated, which impairs the impact toughness. Therefore, the content of nitrogen is preferably limited to 0.30-0.75% by weight.

It is preferable that the sum of content of carbon (C) and nitrogen (N) is 0.30 to 0.75 weight%. Since carbon and nitrogen similarly act as austenite stabilizing elements, it is preferable to include 0.30% by weight or more in the present invention. However, since the formation of carbides and nitrides is an easy element, the present invention is preferably contained at 0.75% by weight or less.

In addition, nickel equivalent (Ni _eq ) may be represented by 30C + 30N + 0.5Mn + Ni, chromium equivalent (Cr _eq ) may be represented by Cr + Mo + 1.5Si. The welded joint of the present invention preferably satisfies nickel equivalent (Ni _eq ) + 0.34 * chromium equivalent (Cr _eq ) ≥ 28.37. When lower than 28.37, the welded joint may exhibit an impact toughness of less than 27J at -196 ° C.

It is preferable to limit the chromium equivalent mentioned above to 14 weight% or less. Elements such as chromium and molybdenum are easily made of carbides and nitrides, so when the chromium equivalent exceeds 14% by weight, the contents of the carbon and the nitrides may be increased, thereby reducing the cryogenic impact toughness.

In addition, when the chromium equivalent (Cr _eq ) in the welded joint is less than 1% by weight, the nickel equivalent (Ni _eq ) is preferably secured to 28% by weight or more. If it is less than 28% by weight, impact toughness is shown to be less than 27J at cryogenic temperature of -196 ° C.

The welded joint, which is one side of the present invention, may secure high impact toughness of 27 J or more as a result of measuring impact toughness at -196 ° C.

Hereinafter, embodiments of the present invention will be described in detail. The following examples are provided for the understanding of the present invention, and the present invention is not limited thereto.

(Example 1)

Flux cored arc welding was performed at a 1.7 KJ / mm heat input with 100% CO ₂ protective gas. The test specimens for evaluating the mechanical properties of the welded welded joints were collected at the center of the welded joint, and the tensile test specimens were used in KS standard (KS B 0801) No. 4 test specimens. ) At 10 mm / mim. The impact test piece was made of CVN (Charphy V Notch) according to KS (KS B 0809) No. 3 test piece, the impact test was carried out at -196 degrees and the measured values are shown in Table 1 below. In addition, by analyzing the components of the welded joint, the additive elements, nickel equivalents and chromium content are shown in Table 1 below, and the presence or absence of welds are also shown together.

In addition, a graph showing a correlation between nickel equivalents and chromium equivalents is shown in FIG. 1. In addition, the weld seam picture and the microstructure picture for the invention example 1 are shown in Figs.

division Nieq Creq C Si Mn Cr Ni Mo Al S N Welding Impact toughness Comparative Example 1 20.4 1.2 0.23 0.702 26.99 0.02 0.01 0.09 0.009 0.007 - ○ 15 Comparative Example 2 26.2 2.5 0.62 0.705 15.15 0.027 0.016 1.37 0.017 0.009 - ○ 22 Comparative Example 3 24.9 4.3 0.55 0.331 16.72 3.84 0.012 <0.010 0.01 0.004 - ○ 11 Comparative Example 4 14.9 4.8 0.21 0.465 17.25 4.12 0.013 <0.010 0.024 0.007 - ○ 8 Comparative Example 5 34.2 One 0.72 0.676 18.07 0.015 3.58 <0.010 0.015 0.023 - flaw - Comparative Example 6 39.1 6.2 0.82 0.623 21.35 5.25 3.82 <0.010 0.023 0.003 - flaw - Comparative Example 7 28.2 17.3 0.63 0.523 18.57 16.5 0.01 <0.010 0.012 0.008 - ○ 23 Comparative Example 8 32.0 0.9 0.68 0.612 18.42 0.012 2.41 1.54 0.012 0.009 - ○ 24 Comparative Example 9 38.2 1.0 0.52 0.651 17.25 0.013 4.42 <0.010 0.011 0.011 0.32 ○ 25 Inventory 1 43.1 0.7 0.54 0.471 11.2 0.016 21.34 <0.010 0.009 0.01 - ○ 107 Inventive Example 2 34.2 One 0.72 0.676 18.07 0.015 3.58 <0.010 0.015 0.009 - ○ 44 Inventory 3 32.3 0.6 0.51 0.421 18.08 0.014 7.98 <0.010 0.013 0.013 - ○ 65 Honorable 4 28.9 0.9 0.39 0.58 18.13 0.021 8.12 <0.010 0.021 0.008 - ○ 42 Inventory 5 34.1 0.9 0.71 0.591 17.34 0.025 4.12 <0.010 1.02 0.008 - ○ 52 Inventory 6 32 0.8 0.62 0.523 18.04 0.012 4.41 <0.010 1.98 0.009 - ○ 61 Honorable 7 33.9 0.8 0.63 0.512 21.87 0.013 4.11 <0.010 0.013 0.008 - ○ 63 Inventive Example 8 37.3 0.5 0.67 0.351 26.52 0.011 3.98 <0.010 0.011 0.005 - ○ 72 Proposition 9 41 0.7 0.68 0.431 33.24 0.011 3.98 <0.010 0.011 0.006 - ○ 98 Inventory 10 29.8 2.2 0.62 0.523 18.12 0.015 2.12 1.45 0.015 0.008 - ○ 44 Exhibit 11 25.69 7.1 0.58 0.521 16.56 6.32 0.01 <0.010 0.011 0.004 - ○ 45 Inventory 12 24 13.1 0.52 0.431 16.78 12.41 0.011 <0.010 0.013 0.004 - ○ 65 Inventory 13 32.0 0.9 0.42 0.572 18.12 0.024 3.41 <0.010 0.011 0.011 0.23 ○ 52 Inventive Example 14 29.0 0.9 0.31 0.562 18.42 0.064 6.31 <0.010 0.008 0.013 0.14 ○ 32

(However, the unit of the elements listed in Table 1 is the weight percent, the impact toughness unit is J)

Comparative Examples 1 to 4 do not satisfy the above-described relationship between nickel equivalents and chromium content. In Comparative Example 5, a high sulfur content caused a defect during welding. In Comparative Example 6, the carbon content was high, and a large amount of carbon dioxide was generated during welding, and defects occurred during welding. In Comparative Example 7, the cryogenic toughness was lowered due to the high chromium content due to overproduction of the chromium carbide. In Comparative Example 8, since the content of molybdenum was high, molybdenum carbide was excessively produced, and thus cryogenic toughness was lowered. On the contrary, Inventive Examples 1 to 14 satisfying the content of the alloying element controlled by the present invention showed excellent impact toughness at -196 ° C.

Referring to FIG. 1, denoted by □ is a comparative example, denoted by ◇ is an example of an invention, and when it satisfies the controlled alloy conditions according to the present invention, it is located above the straight line shown in FIG. 1, and nickel equivalent (Ni _eq ) + 0.34 * chromium equivalent (Cr _eq ) was confirmed to satisfy ≥ 28.37.

(Example 2)

Submerged arc welding was performed at a welding heat input of 4.2 KJ / mm. The test pieces for evaluating the impact toughness of the welded joint were taken from the center of the welded joint, and the tensile test specimen was used by KS standard (KS B 0801) No. 4 test specimen. Tested in mim. The impact test piece was manufactured by CVN (Charphy V Notch) according to KS (KS B 0809) No. 3 test piece. In addition, by analyzing the components of the welded joint, the additive elements, nickel equivalents and chromium content are shown in Table 2 below, and the presence or absence of welding was also shown together.

division Nieq Creq C Si Mn Cr Ni Mo Al S N Welding Impact toughness Comparative Example 10 15.3 0.7 0.24 0.473 16.23 0.016 0.03 <0.010 0.01 0.01 - ○ 18 Comparative Example 11 21.7 0.8 0.29 0.523 18.22 0.042 3.89 <0.010 0.012 0.007 - ○ 21 Inventive Example 15 29.3 2.3 0.61 0.53 18.01 0.013 2.01 1.43 0.015 0.007 - ○ 57 Inventive Example 16 40.8 0.8 0.52 0.521 10.21 0.021 20.12 <0.010 0.009 0.012 - ○ 112 Inventive Example 17 35.6 0.8 0.74 0.542 18.23 0.013 4.31 <0.010 0.015 0.007 - ○ 87 Inventory 18 34 0.6 0.54 0.423 18.41 0.014 8.62 <0.010 0.013 0.011 - ○ 86 Inventive Example 19 29 0.7 0.38 0.441 18.87 0.015 8.14 <0.010 0.021 0.007 - ○ 51 Inventory 20 34.4 0.9 0.72 0.572 17.41 0.023 4.1 <0.010 1.01 0.009 0.24 ○ 76 Inventory 21 32.8 0.8 0.64 0.513 18.01 0.014 4.56 <0.010 2.31 0.011 0.12 ○ 65 Inventory 22 29.8 0.9 0.21 0.573 18.2 0.045 7.21 <0.010 1.01 0.003 ○ 36 Inventive Example 23 28.2 0.9 0.23 0.565 18.52 0.042 8.42 <0.010 2.31 0.011 ○ 35

(However, the unit of the elements listed in Table 2 is the weight percent, the unit of impact toughness is J)

Comparative Examples 10 and 11 can be confirmed that the cryogenic impact toughness is low due to the lack of austenite stabilizing elements outside the range of the carbon content controlled in the present invention. In contrast, Inventive Examples 15 to 23 satisfying the content of the alloying element controlled by the present invention showed excellent impact toughness values at -196 ° C.

Claims (7)

By weight%, carbon (C): 0.02-0.75%, silicon (Si): 0.3-0.7%, manganese (Mn): 10-35%, chromium (Cr): 15% or less, nickel (Ni): 25% Or less, molybdenum (Mo): 1.45% or less, aluminum (Al): 0.009-2.5%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% or less, cryogenic toughness including residual Fe and other unavoidable impurities This excellent welded seam.
The method according to claim 1,
The weld joint is nitrogen (N): a weld joint excellent in cryogenic toughness further comprising 0.30-0.75% by weight.
The method according to claim 2,
The weld joint is a weld joint excellent in cryogenic toughness of the sum of the content of carbon (C) and nitrogen (N) is 0.30-0.75% by weight.
The method according to claim 1,
The weld joint is a nickel joint (Ni _eq ) + 0.34 * chromium equivalent (Cr _eq ) ≥ 28.37 weld joint excellent in cryogenic toughness.
The method according to claim 1,
The weld joint is a weld joint excellent in cryogenic toughness of chromium equivalent (Cr _eq ) is 14% by weight or less.
The method according to claim 1,
Wherein the welded joint is excellent in cryogenic toughness having a chromium equivalent (Cr _eq ) of less than 1% by weight, nickel equivalent (Ni _eq ) of 28% by weight or more.
The method according to claim 1,
Impact toughness (-196 ℃) of the welded joint is a weld joint excellent in cryogenic toughness of 27J or more.

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