KR20230103175A - Welded joint having high strength and superior toughness at ultra-low temperature and Liquefied gas storage tank having the same - Google Patents

Abstract

The high-strength welded joint with excellent cryogenic toughness according to the present invention, in weight%, carbon (C): 0.3 to 0.5%, silicon (Si): 0.3 to 0.6%, manganese (Mn): 4 to 12%, chromium ( Cr): 9 to 13%, Nickel (Ni): 9 to 15%, Molybdenum (Mo): 1.0 to 5.0%, Sulfur (S): 0.02% or less, Phosphorus (P): 0.02% or less, Titanium (Ti) : 0.02 to 0.1%, nitrogen (N): 0.05 to 0.15%, and the balance may include iron (Fe).

Description

Welded joint having high strength and superior toughness at ultra-low temperature and liquefied gas storage tank having the same}

The present invention relates to a high-strength welded joint with excellent cryogenic impact toughness and a liquefied gas storage tank including the same.

Due to the recent explosive increase in LNG demand, the demand for transportation facilities and storage tanks for transporting and storing cryogenic LNG is explosively increasing. A tank for transporting or storing LNG must inevitably be made of a structure and material that can sufficiently withstand impact at a temperature of -162 ° C or lower, which is the LNG temperature.

As the base material of the LNG storage tank, materials with high impact toughness at cryogenic temperatures are used, and typical materials with high impact toughness at cryogenic temperatures are Al, stainless steel (hereinafter referred to as STS), and 9% Ni steel.

However, in the case of Al, there is a problem that a thick thick plate must be used due to low tensile strength and poor weldability, and in the case of STS, there are problems such as high price and high thermal strain.

In addition, in the case of 9% Ni steel, it can be used as a material for cryogenic use by itself, but the welding material of 9% Ni steel (Inconel 625 material: containing more than 50% by weight of Ni and more than 20% by weight of Cr) is expensive, and the yield strength of the welded part There is a problem that is low. Due to these problems, development of a welding material that can be used for 9% Ni steel, which is a steel for cryogenic use, is being promoted.

On the other hand, as a material that can be used as a base material for an LNG storage tank, the development of high manganese steel having high impact toughness at cryogenic temperatures is being promoted. High manganese steel for cryogenic use is a steel to which high manganese content is added to replace 9% Ni steel used for LNG storage tanks, STS 304 steel, etc. It is known to have the characteristic of no deterioration in toughness in the Heat Affected Zone (HAZ) during welding.

However, since the room temperature yield strength of high manganese steel is 400 to 800 MPa, when a welding material having a room temperature yield strength of 360 MPa or less is applied, there is a problem in that the thickness of the steel plate must be thickened to increase the strength of the welded joint.

In order to solve the above problem without increasing the thickness of the steel sheet, a welding material having a yield strength at room temperature of 400 MPa or more is required. As a material with a yield strength at room temperature of 400 MPa or more, a material with high Ni and Cr content (containing more than 50% by weight of Ni and more than 20% by weight of Cr) was previously used, but since Ni is expensive, it was not suitable as a material for welded joints. .

In addition, since materials with high Ni and Cr contents cannot be welded with high current or high heat input due to cracking and dilution problems, a single pass welding method in which welding is completed by passing through the weld improvement surface once cannot be applied.

Most of these materials are used in Flux Cored Arc Welding (FCAW) or SUBMERGED ARC WELDING (SAW). Heat given) is difficult to exceed 30 kJ / cm, and there is a limit that welding grooves (improvement, grooves of members to be joined by welding) cannot be filled with a single pass, and this inevitably reduces welding efficiency.

Therefore, as an austenite stabilizing element, there is a demand for the development of a high-manganese welded joint for cryogenic use that is inexpensive compared to Ni and capable of single-pass welding.

The present invention was created to solve the problems of the prior art as described above, and an object of the present invention is to weld with a high heat input of 30 kJ / cm or more by electro gas welding (EGW), welding in a single pass It is to provide a welded joint that is.

The tasks of the present invention are not limited to the tasks mentioned above, and other tasks not mentioned will be clearly understood by those skilled in the art from the description below.

The high-strength welded joint with excellent cryogenic toughness according to the present invention, in weight%, carbon (C): 0.3 to 0.5%, silicon (Si): 0.3 to 0.6%, manganese (Mn): 4 to 12%, chromium ( Cr): 9 to 13%, Nickel (Ni): 9 to 15%, Molybdenum (Mo): 1.0 to 5.0%, Sulfur (S): 0.02% or less, Phosphorus (P): 0.02% or less, Titanium (Ti) : 0.02 to 0.1%, nitrogen (N): 0.05 to 0.15%, and the balance iron (Fe).

Specifically, the high-strength welded joint having excellent cryogenic toughness satisfies a relational expression of 0.3 ≤ carbon (C) + nitrogen (N) ≤ 0.6.

Specifically, the high-strength welded joint having excellent cryogenic toughness is welded under a welding condition of a heat input of 30 kJ/cm or more.

Specifically, the high-strength welded joint having excellent cryogenic toughness is welded by electro gas welding (EGW).

Specifically, the high-strength welded joint having excellent cryogenic toughness is welded to a base material of high manganese steel or 9% Ni steel.

The liquefied gas storage tank according to the present invention includes a high-strength welded joint having excellent cryogenic toughness.

The high-strength welded joint having excellent cryogenic toughness according to the present invention can be welded with high heat input and thus can be welded in a single pass, thereby reducing the time and cost required for welding.

The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

1 is a table showing the heat input, yield strength, tensile strength, elongation and impact toughness of the welded joint according to the alloy composition of the welded joint of the present invention.
Figure 2 is a graph showing yield strength, tensile strength and impact toughness according to the total weight ratio of carbon and nitrogen of the welded joint of the present invention.

Hereinafter, the technical configuration according to the present invention will be described in more detail with reference to various drawings and embodiments.

The present invention relates to a welded joint obtained by welding high-strength high-manganese steel for cryogenic use and 9% Ni steel, wherein the welded joint contains, by weight, C: 0.3 to 0.5%, Si: 0.3 to 0.6%, Mn: 4 to 12%, Cr: 9 to 13%, Ni: 9 to 15%, Mo: 1.0 to 5.0%, sulfur (S): 0.02% or less, P: 0.02% or less, Ti: 0.02 to 0.1%, N: 0.05 to 0.15% balance Fe and other unavoidable impurities.

Hereinafter, the characteristics of each alloying element and the critical significance of the composition range will be briefly described.

Carbon (C): 0.3 to 0.5% by weight

Carbon is an austenite stabilizing element capable of securing strength of welded joints and cryogenic impact toughness of welded joints, and is the strongest existing element, and is an essential element in the present invention. It is sufficient to limit the lower limit of the carbon content to 0.3% by weight. However, if the carbon content exceeds 0.5% by weight, carbon dioxide gas may be generated during welding, which may cause defects in the welded joint, and it may combine with alloy elements such as manganese and chromium to create carbides such as MC and M23C6 at low temperatures. There is a problem in that the impact toughness is lowered. Therefore, the content of carbon is preferably limited to 0.3 to 0.5% by weight.

Silicon (Si): 0.3 to 0.6% by weight

Silicon is an element added for the deoxidation effect in the weld joint and the spreadability of the weld bead. If the content of silicon is insufficient (less than 0.3% by weight), the fluidity of the welded joint may be reduced. On the other hand, if the content of silicon exceeds 0.6% by weight, segregation in the welded joint may occur, resulting in low-temperature impact toughness. There is a problem in that this degrades and adversely affects the weld crack susceptibility. Therefore, in the present invention, it is preferable to limit the content of the silicon to 0.3 to 0.6% by weight.

Manganese (Mn): 4 to 12% by weight

Manganese is a main element for generating austenite, which is a low-temperature stable phase, and as an austenite stabilizing element, it is a component that serves to improve slag fluidity to improve bead shape and maintain appropriate strength and toughness of a welded joint. In addition, manganese is an element that must be added indispensably in the present invention and is a very inexpensive element compared to nickel. If the content of manganese is less than 4% by weight, sufficient austenite is not formed, resulting in very low toughness at cryogenic temperatures. On the other hand, when the content of manganese exceeds 12% by weight, a rapid decrease in toughness may occur. Therefore, the content of manganese is preferably limited to 4 to 12% by weight.

Chromium (Cr): 9 to 13% by weight

Chromium is a ferrite stabilizing element, and the content of the austenite stabilizing element can be lowered through a certain amount of chromium, and is a component that serves to improve the strength of a welded joint. However, if the amount of chromium is less than 9% by weight, the effect is ineffective, and if the amount exceeds 13% by weight, chromium carbide is excessively formed, resulting in a decrease in toughness. Therefore, the content of chromium is preferably limited to 9 to 13% by weight.

Nickel (Ni): 9 to 15% by weight

Nickel is an austenite stabilizing element, and is a component added as an austenite stabilizing element. When nickel is added, the low temperature impact toughness increases very rapidly. However, nickel is not only an element that lowers the strength but also an element that increases the price of a welding material, so it is more preferable to keep it below 15% by weight.

Molybdenum (Mo): 1.0 to 5.0% by weight

Molybdenum is an element that can improve the strength and impact toughness of deposited metal. In addition, it can play a role in suppressing the occurrence of high-temperature cracks by narrowing the solid-liquid coexistence section during construction in austenitic welding materials. However, when the content of molybdenum exceeds 5.0% by weight, molybdenum carbide is excessively generated, thereby reducing cryogenic toughness. Therefore, the content of molybdenum is preferably limited to 1.0 to 5.0% by weight.

Sulfur (S): 0.02% by weight or less

Sulfur is an element that precipitates MnS composite precipitates, but when its content exceeds 0.02% by weight, it is not preferable because it can cause high-temperature cracking by forming low-melting compounds such as FeS. Therefore, it is preferable to limit the sulfur content to 0.02% by weight or less.

Phosphorus (P): 0.02% by weight or less

Phosphorus is an element that affects low-temperature toughness and forms embrittled phosphorus compounds at grain boundaries, so it is preferable to set the upper limit to 0.02% by weight.

Titanium (Ti): 0.02 to 0.1% by weight

Titanium enters the welded joint in the form of oxide or nitride. However, these oxides or nitrides (or carbonitrides) exist in the crystal grains, and act as nucleation sites during solidification at high temperatures to play a role in making the grains of austenite small. In addition, titanium oxides and nitrides (or carbonitrides) play a role in improving strength within the structure. Even if only 0.02% by weight of titanium is added, the strength improvement effect is shown. On the other hand, when a large amount of titanium is contained, impact toughness is lowered. When titanium exceeds 0.1% by weight, the effect of improving strength is large, but may act as a cause of lowering low-temperature toughness. Considering this, in the present invention, it is preferable to limit the content of titanium to the range of 0.02 to 0.1% by weight.

Nitrogen (N): 0.05 to 0.15% by weight

Nitrogen is an element that exhibits the same properties as carbon, and is an element that makes nitride together with titanium. When nitrogen is contained in an amount of 0.05% by weight or more, strength can be improved along with chromium and titanium, so it is preferably included in an amount of 0.05% by weight or more. On the other hand, when the nitrogen content exceeds 0.15% by weight, pores are likely to occur in the welded joint, and the cryogenic impact toughness is reduced by increasing the amount of nitride produced along with titanium, so it is recommended to set the upper limit to 0.15% by weight. desirable.

On the other hand, the above-mentioned alloy component range is a basic component system that can be preferably applied to the welded joint of the present invention, and therefore, better physical properties can be imparted to the welded joint additionally by the addition of the alloy elements described below.

The sum of at least one selected from tungsten (W), niobium (Nb) and vanadium (V): 5% by weight or less

Tungsten (W), niobium (Nb), or vanadium (V) are elements that increase room temperature strength and can be selectively contained in the present invention. These elements combine with carbon in the weld joint to generate carbides (or carbonitrides), and exhibit the effect of improving room temperature tensile strength by this formed phase. However, when the content exceeds 5% by weight, cracks are easily generated, and in addition, it acts as a cause of lowering the cryogenic impact toughness. Therefore, in the present invention, it is more preferable to limit the sum of one or more of tungsten (W), niobium (Nb), and vanadium (V) to 5% by weight or less.

Yttrium (Y) and/or rare earth metal (REM): 0.1% by weight or less

Yttrium (Y) and/or rare earth metal (REM) are elements that are selectively contained in the present invention, and are formed as oxides at high temperatures and act as nucleation sites during solidification at high temperatures, thereby reducing grain size of austenite. will do This serves to improve strength. However, if it exceeds 0.1% by weight, the content should be controlled to 0.1% by weight or less because it plays a role in generating defects in the joint during welding. Therefore, in the present invention, it is more preferable to contain yttrium (Y) and/or rare earth metal (REM) at 0.1% by weight or less.

Boron (B): 0.001 to 0.01% by weight

Boron is an element that is selectively contained in the present invention and exhibits a property of being segregated at grain boundaries. The segregated boron serves to improve the strength of the grain boundary, which can show the effect of improving the strength. These effects are sufficiently exhibited even when only 0.001% by weight of boron is added. However, when 0.01% by weight or more is added, the effect of improving strength is large, but it acts as a cause of lowering low-temperature toughness. Therefore, in the present invention, it is preferable to limit the boron content to 0.001 to 0.01% by weight.

In addition to the above composition, the remainder includes Fe and unavoidable impurities. However, this does not preclude the addition of other components.

In addition, in order to ensure proper mechanical properties (yield/tensile strength and impact toughness) of the welded joint, it is more preferable to follow the following relational expression.

Carbon (C) + Nitrogen (N): 0.3 to 0.6% by weight

Carbon and nitrogen are elements capable of securing the strength and cryogenic impact toughness of a welded joint, as described above for the effect of each individual element. When the sum of the carbon and nitrogen contents is within a certain range, the strength and low-temperature impact toughness of the welded joint can be optimized. If the combined content of carbon and nitrogen is less than 0.3% by weight, it is difficult to secure sufficient strength, and if the combined content of carbon and nitrogen exceeds 0.6% by weight, impact toughness at low temperatures may be reduced due to a large amount of precipitates.

Meanwhile, the weld joint of the present invention can be applied to various high manganese steels requiring high strength and low temperature toughness at cryogenic temperatures, and is not limited to a specific weld base material composition.

Hereinafter, the present invention will be described in detail through preferred embodiments. However, it should be noted that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example 1)

Table 1 is a table showing the composition of the steel sheath in the flux-filled wire for electrogas arc welding according to this embodiment, and may be 50 to 80% by weight compared to the welded product of the sheath.

division C
(wt%) Si (wt%) Mn (wt%) P
(wt%) S
(wt%) Ni (wt%) Cr (wt%) Mo (wt%) Fe (wt%) Content 1 0.010 0.4 1.0 0.010 0.003 10 18.5 - balance content 2 0.010 0.4 1.0 0.010 0.003 12 18.5 2.5 balance

At this time, the composition of the flux filling wire for electrogas arc welding used is, by weight%, C: 0.3 to 0.5%, Si: 0.3 to 0.6%, Mn: 4 to 12%, Cr: 9 to 13%, Ni: 9 to 15%, Mo: 1.0 to 5.0%, S: 0.02% or less, P: 0.02% or less, Ti: 0.02 to 0.1%, N: 0.05 to 0.15%, the balance including Fe and other unavoidable impurities, and, if necessary, tungsten ( W), at least one of niobium (Nb) and vanadium (V): 5% or less, and yttrium (Y) and/or rare earth metal (REM) 1% or less.

1 is a table showing the heat input, yield strength (Y.S), tensile strength (TS), elongation (EL) and impact toughness of the welded joint according to the alloy composition of the welded joint of the present invention.

Using the welding wire, welding was performed using cryogenic high manganese steel having a basic composition of high manganese steel and 9% Ni steel as a welding base material. The welding at this time was performed under the conditions of a DC current of about 200 to 300 A, a voltage of 25 to 40 V, and a welding speed of 8 to 20 CPM. And welding was performed under a heat input condition of about 30 kJ/cm or more.

Referring to Figure 1, Comparative Examples 1 to 5 and Inventive Examples 1 to 5 are experiments on the welded joint according to the present invention on a base material of 9% Ni steel, and Comparative Examples 6 to 8 and Inventive Example 6 are high manganese steel The welded joint according to the present invention was tested on the base material.

In order to secure the stability of the welded structure in the cryogenic region below -196 ℃, it is essential to secure a welded joint that exhibits an impact toughness of 27J or more. Accordingly, in order to evaluate the impact toughness of the welded joint according to the present invention, a Charpy impact test (-196 ° C) was performed, and the results are shown in FIG.

The welded joint according to the embodiment of the present invention has a yield strength (YS) of 360 MPa or more, a tensile strength (TS) of 640 MPa or more, and an impact toughness (CVN, Charpy V Notch) of 27 J or more even at a high heat input of 30 kJ/cm or more. (-196 ℃ or less) was able to secure the physical properties. In the following, each embodiment is described in detail.

In Comparative Example 1, the strength was insufficient and the impact toughness was satisfactory because the carbon and nitrogen contents were not within the appropriate range.

In Comparative Example 2, the carbon content was not within the appropriate range, but the strength was satisfied due to the high Cr content, but the impact toughness was insufficient because sufficient austenite was not formed due to the low Mn content.

In Comparative Example 3, the components were in an appropriate range, but the strength was satisfactory due to the high carbon + nitrogen content, but the impact toughness was insufficient.

In Comparative Example 4, nitrogen and carbon+nitrogen exceeded the appropriate range, and impact toughness was insufficient.

In Comparative Example 5, the carbon and carbon+nitrogen content exceeded the appropriate range, and the impact toughness was insufficient.

In Comparative Example 6, carbon, silicon, manganese, and nickel exceeded the dropping point range, and Mo was below the appropriate range, so the elongation was very low and the impact toughness was insufficient.

In Comparative Example 7, the strength was insufficient and the impact toughness was satisfactory because the carbon and nitrogen contents were not within the appropriate range.

In Comparative Example 8, the carbon and nitrogen contents were below the appropriate range, and Ni and Mo were increased, but the strength was difficult to satisfy.

2 is a graph showing yield strength (Y.S), tensile strength (TS) and impact toughness according to the result of FIG. 1 shown above according to the content of carbon and nitrogen.

As can be seen in FIG. 2, if the content of carbon and nitrogen combined is less than 0.3% by weight, it is difficult to increase the strength, and if it exceeds 0.6% by weight, it is difficult to secure impact toughness of 27J or more in the cryogenic region below -196 ° C.

As such, the welded joint according to the present invention can be welded with a high heat input of 30 kJ / cm or more using an electro gas welding (EGW) method, so it can be welded in a single pass.

However, it is natural that the welded joint according to the present invention may be welded in a single pass and may be welded through an additional pass.

In addition, the welded joint according to the present invention is welded using electrogas welding. Since electrogas welding has a higher welding efficiency than flux-cored wire welding for vertical up welding, the electrogas Gas welding can be used to shorten the manufacturing process time of the liquefaction storage tank and reduce the manufacturing process cost.

Of course, the present invention is not limited to the above-described embodiment, and may include a combination of the above embodiments or a combination of at least one of the above embodiments and known technology as another embodiment.

Although the present invention has been described in detail through specific examples, this is for explaining the present invention in detail, the present invention is not limited thereto, and within the technical spirit of the present invention, by those skilled in the art It will be clear that the modification or improvement is possible.

Simple variations or modifications of the present invention all fall within the scope of the present invention, and the specific protection scope of the present invention will be clarified by the appended claims.

Claims (6)

By weight, carbon (C): 0.3 to 0.5%, silicon (Si): 0.3 to 0.6%, manganese (Mn): 4 to 12%, chromium (Cr): 9 to 13%, nickel (Ni): 9 to 15%, molybdenum (Mo): 1.0 to 5.0%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% or less, titanium (Ti): 0.02 to 0.1%, nitrogen (N): 0.05 to 0.15 %, and a high-strength welded joint with excellent cryogenic toughness containing the balance iron (Fe). According to claim 1,
A high-strength welded joint with excellent cryogenic toughness that satisfies the relationship 0.3 ≤ carbon (C) + nitrogen (N) ≤ 0.6. According to claim 1,
A high-strength welded joint with excellent cryogenic toughness that is welded under welding conditions of heat input of 30 kJ/cm or more. According to claim 1,
A high-strength welded joint with excellent cryogenic toughness, which is welded by Electro Gas Welding (EGW). According to claim 1,
A high-strength welded joint with excellent cryogenic toughness that is welded to a base material of high manganese steel or 9% Ni steel. A liquefied gas storage tank comprising the high-strength welded joint having excellent cryogenic toughness according to any one of claims 1 to 5.

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