JP2022072315A - Steel pipe for liquefied gas storage - Google Patents

Abstract

To provide a steel pipe suitable for a storage container used for shipping transportation of a liquefied natural gas.SOLUTION: A steel pipe for liquefied gas storage is produced by molding the base material of a steel sheet and by butt welding. The base material has the prescribed components and the conformation thereof includes bainite of 30% or more in area ratio, ferrite of 20 to 60% and (bainite+ferrite) of 90% or more in sum total area ratio, the tensile strength of the base material in the L and C directions is 570 to 760 MPa; the yield stress thereof in the same directions is 520 to 635 MPa; and the yield ratio thereof is 90% or less. Further, the absorption energy, vE-51 at -51°C of the base material by Charpy impact test is 100 J or more; the vE-51 at -51°C of a steel pipe weld metal part by Charpy impact test is 81 J or more; the vE-51 at -51°C of the heat affected zone of the steel pipe by Charpy impact test is 41 J or more; and the CTOD values at -45°C of the base material 1, weld metal 2 and the above heat affected zone are all 0.18 mm or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to steel pipes, and more particularly to steel pipes suitable for liquefied gas storage containers for ships.

A line pipe made of steel pipe is used as a long-distance transportation method for crude oil and natural gas. Steel pipes for line pipes are generally manufactured by forming a steel plate and seam welding the butt joints of the steel plates from both inside and outside in the longitudinal direction. Seam welding is usually completed by temporarily welding a part of the groove by gas shielded arc welding and then welding one layer at a time from the inner and outer surfaces of the steel pipe by submerged arc welding. Temporary welding is completely eliminated by subsequent submerged arc welding.

Examples of steel pipes manufactured in this way include UOE steel pipes and JCOE steel pipes. Welded joints of line pipes are required to have high toughness from the viewpoint of improving transportation efficiency by cooling the mining area and increasing the pressure.

Patent Document 1 discloses that a welded steel pipe of API standard X65 to X70 class has a fine acylular ferrite structure obtained by transforming a weld metal with a large number of TiOs as nuclei to achieve both high strength and excellent toughness. There is.

Japanese Unexamined Patent Publication No. 2013-49895

In recent years, containers using a plurality of steel pipes have come to be used as storage containers for liquefying natural gas and transporting it by ship. In such a steel pipe, a high level of balance between strength and toughness is required as compared with a line pipe or the like in consideration of vibration during transportation. Further, the steel plate used as the base material of the steel pipe is required to have high strength in both the L direction and the C direction.

In view of the above circumstances, it is an object of the present invention to obtain a steel pipe having an excellent balance between strength and toughness.

The present inventors have diligently studied a method for obtaining a steel sheet having an excellent balance between strength and toughness. As a result, it was found that by optimizing the components of the base metal and weld metal, and further optimizing the manufacturing conditions, it is possible to obtain a steel pipe with a better balance between strength and toughness than before.

The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] A steel pipe for a marine liquefied gas storage container formed by molding and butt-welding a steel plate, wherein the base material of the steel pipe having a plate thickness of 6 to 20 mm is C: 0.01 in mass%. ~ 0.10%, Si: 0.03 ~ 0.50%, Mn: 0.5 ~ 2.0%, P: 0.015% or less, S: 0.010% or less, Al: 0.001 ~ 0.050%, Ti: 0.005 to 0.030%, N: 0.002 to 0.006%, O: 0.005% or less, Cu: 0 to 0.5%, Ni: 0 to 0. 6%, Cr: 0 to 0.5%, Mo: 0 to 0.4%, V: 0 to 0.06%, Nb: 0 to 0.06%, balance: Fe and impurities, and Cu , Ni, Cr, Mo, V, and Nb, the content of one or more elements selected from the group is more than 0%.
The structure of the base metal contains a baynite having an area ratio of 30% or more, and a ferrite of 20 to 60% and ferrite and baynite having a total area ratio of 90% or more, in the L direction and the C direction of the base metal. The tensile strength is 570 to 760 MPa, the yield stress of the base metal in the L and C directions is 520 to 635 MPa, the yield ratio of the base metal is 90% or less, and the shearpy impact of the base metal at −51 ° C. Test absorption energy vE _-51 is 100J or more, and the charpy impact test absorption energy vE _-51 of the weld heat affected part of the steel pipe at -51 ° C is 41J or more, and the charpy impact test absorption of the weld metal of the steel pipe at -51 ° C. A steel pipe for storing liquefied gas, characterized in that the energy vE _-51 is 81 J or more, and the CTOD value of the base metal of the steel pipe, the weld heat affected portion, and the weld metal at −45 ° C. is 0.18 mm or more.

[2] A group in which the chemical composition of the base material is B: 0 to 0.002%, Mg: 0 to 0.01%, and Ca: 0 to 0.03% instead of a part of the Fe. The steel pipe for storing liquefied gas according to the above [1], which contains one kind or two or more kinds selected from the above.

[3] The components of the weld metal of the steel pipe are C: 0.03 to 0.10%, Si: 0.03 to 0.50%, Mn: 0.5 to 2.0%, P: 0.015. % Or less, S: 0.010% or less, Al: 0.001 to 0.030%, Ti: 0.005 to 0.040%, N: 0.002 to 0.006%, O: 0.015 to 0.050%, Cu: 0 to 0.6%, Ni: 0 to 0.5%, Cr: 0 to 0.5%, Mo: 0 to 0.4%, V: 0 to 0.06%, Nb: 0 to 0.06%, balance: Fe and impurities, and the content of one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, V, and Nb. More than 0%, 0 ≦ α'≦ 50… (1), 0.3 ≦ Al / O ≦ 0.8… (2), 0.30 ≦ Ceq ≦ 0.50… (3), 0.5 ≦ Pcm ≦ 2.0 ... (4) is satisfied, and the liquefaction of the above [1] or [2] is characterized in that the charpy impact test absorption energy vE _-51 of the weld metal at −51 ° C. is 54 J or more. Steel pipe for gas storage. Here, α', Ceq, and Pcm are α'= (1.5 × (O−0.89Al) +3.4 × N—Ti) × 1000… (5), Ceq = C + Mn / 6 + (Cr + Mo + V, respectively). ) / 5+ (Ni + Cu) / 15 ... (6), Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ... (7). The element symbols (5), (6), and (7) indicate the content (mass%) of each element.

[4] A group in which the chemical composition of the weld metal is B: 0 to 0.035%, Mg: 0 to 0.01%, and Ca: 0 to 0.005% instead of a part of Fe. The steel pipe for storing liquefied gas according to the above [3], which contains one kind or two or more kinds selected from the above.

[5] The steel pipe for storing liquefied gas according to any one of the above [1] to [4], wherein the old γ particle size of the weld heat affected zone is 120 μm or less and the EBSD particle size is 100 μm or less.

[6] The structure of the weld metal contains, in terms of area ratio, acicular ferrite: 80% or more, grain boundary ferrite: 10% or less, island-like martensite: 3% or less, and an EBSD particle size of 10 μm or less. The steel pipe for storing liquefied gas according to any one of the above [1] to [5].

[7] The above-mentioned [1] to [6], wherein the bainite fraction of the structure of the base metal is 30% or more in area ratio, and the ferrite fraction is 20 to 60% in area ratio. Steel pipe for storing liquefied gas in any of.

[8] The steel pipe for storing liquefied gas according to any one of the above [1] to [7], wherein the difference between the maximum value and the minimum value of the inner diameters of the body portion and the end portion of the steel pipe is 20 mm or less.

According to the present invention, it is possible to obtain a steel pipe having strength and toughness suitable for a liquefied gas storage container for ships.

It is a figure explaining the notch position of a CTOD test.

Hereinafter, embodiments of the present invention will be described in detail.

First, the components of the base material will be described. In the following, "%" for the component shall represent "mass%".

C: 0.01-0.10%
C is effective for improving the strength of steel and contains 0.01% or more. If the amount of C is too large, the low-temperature toughness of the base metal and HAZ deteriorates due to excessive strength, and further the weldability deteriorates. Therefore, the amount of C is set to 0.10% or less. It is preferably 0.03 to 0.07%.

Si: 0.03 to 0.50%
Si is an element necessary for deoxidation and contains 0.03% or more. If the amount of Si is large, island-like martensite is likely to be formed and the low temperature toughness is significantly deteriorated. Therefore, the amount of Si is set to 0.50% or less. It is preferably less than 0.35%.

Mn: 0.5-2.0%
Mn acts as an element for improving hardenability, and is contained in an amount of 0.5% or more in order to obtain the effect. If the amount of Mn is large, the hardenability of the steel increases, and the HAZ toughness and weldability deteriorate. Further, the amount of Mn is set to 2.0% or less because it promotes the central segregation of the continuously cast steel pieces and the low temperature toughness of the base metal deteriorates. It is preferably 1.0 to 1.8%.

P: 0.015% or less S: 0.010% or less P and S are both impurities and elements that deteriorate the toughness of the joint. The content thereof is preferably as low as possible, with P being 0.015% or less and S being 0.01% or less. Preferably, P is 0.008% or less. Preferably, S is 0.003% or less.

Al: 0.001 to 0.050%
Al is an element contained in a steel material as a deoxidizing material. Al further combines with N to form AlN, which suppresses the coarsening of crystal grains in the hardened portion of the steel material. If the Al content is too low, this effect cannot be obtained, so 0.001% or more is contained. If the Al content is too high, the coarse Al ₂ O ₃ becomes the fracture starting point and the toughness of the base metal decreases. Therefore, the Al content is set to 0.050% or less. It is preferably 0.020 to 0.040%.

Ti: 0.005 to 0.030%
Ti forms fine TiN in steel, and a simple substance thereof or a composite inclusion with Mg (MgAl ₂ O ₄ ) oxide acts as pinning particles. As a result, the coarsening of the austenite grains of HAZ is suppressed, the microstructure becomes finer, and the low temperature toughness is improved. In order to obtain this effect, Ti is contained in an amount of 0.005% or more. When the amount of Ti is large, the Ti oxide is aggregated and coarsened, and the toughness is deteriorated. Therefore, the amount of Ti is set to 0.030% or less. It is preferably 0.010 to 0.020%.

N: 0.002 to 0.006%
N is an element that combines with Ti to form TiN, and contains 0.002% or more. If the amount of N is large, the solid solution N that does not bind to Ti lowers the toughness, so the amount of N is set to 0.006% or less. It is preferably 0.003 to 0.005%.

O: 0.005% or less O is an element that forms pinning particles. However, if O is contained, the cleanliness of the steel is lowered, so a smaller amount is preferable, and the content is 0.005% or less. It is preferably 0.003% or less.

The following Cu, Ni, Cr, Mo, V, and Nb do not need to contain all the elements in the base material, but in order to improve the strength of the base material, one or more of them is contained in excess of 0%. Let me.

Cu: 0-0.5%
Cu is an element that can improve the strength of the base metal. As the amount of Cu increases, the effect saturates. The Cu content may be 0. The suitable amount of Cu for obtaining the effect is 0.1 to 0.5%.

Ni: 0-0.6%
Ni is an element that can improve the strength of the base metal without lowering the toughness. As the amount of Ni increases, the effect saturates. The Ni content may be 0. The suitable amount of Ni for obtaining the effect is 0.1 to 0.6%.

Cr: 0-0.5%
Cr is an element that can improve the strength of the base metal. As the amount of Cr increases, the effect saturates. The Cr content may be 0. The suitable amount of Cr for obtaining the effect is 0.1 to 0.5%.

Mo: 0-0.4%
Mo is an element that can improve the strength of the base metal. When the amount of Mo is large, the effect is saturated and the toughness is further lowered. The Mo content may be 0. The suitable amount of Mo for obtaining the effect is 0 to 0.4%.

V: 0 to 0.06%
Nb is an element that improves the strength of the base metal. When the amount of V is large, the yield ratio may increase due to precipitation hardening. The content of V may be 0. A suitable amount of V for obtaining the effect is 0.01 to 0.06%.

Nb: 0 to 0.06%
Nb is an element that improves the strength of the base metal. When the amount of Nb is large, island-like martensite is likely to be formed, and the toughness is lowered. The content of Nb may be 0. A suitable amount of Nb for obtaining the effect is 0.01 to 0.04%.

The rest other than those described above are Fe and impurities. Impurities are components contained in raw materials or mixed in during the manufacturing process, and are components not intentionally contained in steel.

The base material may contain the following elements in place of a part of the above Fe. The content of the elements described below is not essential, and the content in the base metal may be zero.

B: 0 to 0.002%
B is an element effective for improving the hardenability of the base material and suppressing the formation of grain boundary ferrite. As the amount of B increases, the effect saturates. The content of B may be 0. A suitable amount of B for obtaining the effect is 0.001 to 0.002%.

Mg: 0-0.01%
Mg is an element that forms inclusions such as Mg Al ₂ O ₄ and Mg S. MgAl ₂ O ₄ precipitates on TiN. These inclusions act as pinning particles, suppress the coarsening of austenite particles in HAZ, refine the microstructure, and improve low temperature toughness. As the amount of Mg increases, the effect saturates. The Mg content may be zero.

Ca: 0 to 0.03%
Ca is an element that controls the morphology of sulfide-based inclusions and improves low temperature toughness. Further, phosphide and sulfide are formed to substantially reduce the concentration of P and S and improve the sulfide stress cracking resistance. When the amount of Ca is large, CaO-CaS becomes large clusters and inclusions, which may adversely affect toughness. The Ca content may be 0. A suitable amount of Ca for obtaining the effect is 0.01 to 0.03%.

Next, the structure of the base material will be described.

The structure of the base material of the present invention contains bainite in an area ratio of 30% or more, and ferrite of 20 to 60% and a total area ratio of ferrite and bainite of 90% or more. Such a structure can be obtained by forming a slab having the above-mentioned components into a steel sheet by the manufacturing method described later. The remaining structures other than ferrite and bainite are bainite, island martensite, cementite and pearlite.

With such a structure, the balance between strength and toughness is good. Specifically, the tensile strength of the base metal in the L and C directions is 570 to 760 MPa, the yield stress of the base metal in the L and C directions is 520 to 635 MPa, and the yield ratio of the base metal is 90% or less. Charpy impact test absorption energy vE _-51 at −51 ° C. of the base metal can be 100 J or more.

In the use of storage containers for liquefying and transporting natural gas, from the viewpoint of ensuring the safety of the tank, the fracture resistance under severe fracture conditions where the initial cracks are sharper, specifically in the low temperature range. It is required to have excellent CTOD characteristics, and in particular, the CTOD value of the welded portion needs to be sufficiently high. In the steel pipe of the present invention, the CTOD value at −45 ° C. is 0.18 mm or more.

The following, preferred components of the weld metal, will be described.

C: 0.03 to 0.10%
C is an element necessary for ensuring the strength of steel, and must contain 0.03% or more. If the amount of C is large, high-temperature cracking in the weld is likely to occur in the weld seam portion, so the upper limit is set to 0.10%. C is preferably 0.05% or more and 0.065% or less.

Si: 0.03 to 0.50%
Si must be contained in an amount of 0.03% or more in order to prevent blow holes and to have a structure mainly composed of acicular ferrite. If the amount of Si is large, island-like martensite is likely to be formed and the low temperature toughness is significantly deteriorated. Therefore, the upper limit is set to 0.50%. Si is preferably 0.15% or more and 0.25% or less.

Mn: 0.5-2.0%
Mn acts as an element for improving hardenability. The content of 0.5% or more is required to make the weld metal a structure mainly composed of acicular ferrite. If the amount of Mn is large, coarse MnS is formed and becomes the starting point of fracture, so the upper limit is set to 2.0%. Mn is preferably 1.2% or more and 1.5% or less.

P: 0.015% or less (including 0%)
S: 0.010% or less (including 0%)
Both P and S are impurities and are elements that deteriorate the toughness of the joint. P is limited to 0.015% or less, and S is limited to 0.010% or less. It is preferable that these contents are as low as possible. Preferably, P is 0.008% or less. Preferably, S is 0.003% or less.

Al: 0.001 to 0.030%
Al acts as a deoxidizing element and is necessary for controlling the amount of oxygen to disperse Ti oxide, which is effective as an acylular ferrite nucleation site. Considering the dilution of the base metal, the content of 0.001% or more is required. If the amount of Al exceeds 0.030%, the formation of oxides is inhibited and toughness cannot be ensured, so the upper limit is 0.030%. It is preferably 0.010% or more and 0.015% or less.

Ti: 0.005 to 0.040%
Ti reacts with oxygen in the weld metal to form the Ti oxide, which is the core of acicular ferrite. In order to finely disperse a large amount of this oxide in the weld metal, a content of 0.005% or more is required. When the amount of Ti becomes excessive, the Ti oxide aggregates and coarsens, the ability to generate nuclei of acicular ferrite decreases, and the Ti oxide becomes the starting point of fracture and the toughness cannot be ensured, so the upper limit is 0. It is set to 040%. It is preferably 0.009% or more and 0.015% or less.

N: 0.002 to 0.006%
Since N is an effective element for adjusting the amount of Ti effective for forming an acylular ferrite structure, it is necessary to contain N in 0.002% or more. However, if it exceeds 0.006%, the solid solution N remaining without reacting with Ti significantly lowers the toughness, so the upper limit thereof is preferably 0.006%. It is preferably 0.003% or more and 0.004% or less.

O: 0.015 to 0.050%
O is an element necessary for forming an oxide which is the core of acicular ferrite. Therefore, the content of 0.015% or more is required. If the amount of O exceeds 0.050%, the toughness decreases due to overformation, aggregation and coarsening of oxides, so the upper limit is set to 0.050%. It is preferably 0.020% or more and 0.030% or less.

The following Cu, Ni, Cr, Mo, V, and Nb do not need to contain all the elements in the weld metal, but in order to improve the strength of the weld metal, any one or more of them are contained in excess of 0%. Let me.

Cu: 0 to 0.60%
Cu is an element that can improve the strength of the weld metal. The Cu content may be 0. If it exceeds 0.6%, the effect will be saturated, so the upper limit is set to 0.6%.

Ni: 0-0.5%
Ni is an element that can improve the strength of the weld metal without lowering the toughness. The Ni content may be 0. If it exceeds 0.5%, the effect will be saturated, so the upper limit is 0.5%.

Cr: 0-0.5%
Cr is an element that can improve the strength of the weld metal. The Cr content may be 0. If it exceeds 0.5%, the effect will be saturated, so the upper limit is 0.5%.

Mo: 0-0.4%
Mo is an element that can improve the strength of the weld metal. The Mo content may be 0. If it exceeds 0.4%, the effect will be saturated, so the upper limit is set to 0.4%.

V: 0 to 0.06%
V is an element that can improve the strength of the weld metal. The content of V may be 0. If it exceeds 0.06%, the effect will be saturated, so the upper limit is 0.06%.

Nb: 0 to 0.06%
Nb is an element effective for allowing solid solution B, which is effective for improving strength and suppressing grain boundary ferrite, to exist. The content of Nb may be 0. If the amount of Nb exceeds 0.06%, island-shaped martensite is likely to be formed and the toughness is lowered, so the upper limit is set to 0.06%. Desirably, it is 0.02%.

The balance of the weld metal is Fe and impurities. Impurities are components that are mixed in from welding wires, flux, steel plates, ambient atmosphere, etc. in the process of welding, and are components that are not intentionally contained.

The weld metal may contain the following elements in place of a part of the above Fe. The content of the elements described below is not essential, and the content in the weld metal may be zero.

B: 0 to 0.035%
B in a solid solution state promotes the formation of acicular ferrite by suppressing the formation of grain boundary ferrite in the weld metal. The content of B may be 0, but in order to obtain this effect, the content of B is preferably 0.0001% or more. If the amount of B exceeds 0.035, the strength becomes too high and the toughness decreases, so the upper limit is set to 0.035%. The B addition to the weld metal can be added from any of the plate base material, flux, or wire. For example, when the base material is B-free steel, the flux contained in B oxide may be used. B is preferably 0.0005% or more and 0.010% or less.

Mg: 0-0.01%
Mg forms MgS or MgAl ₂ O ₄ and acts as pinning particles. The Mg content may be zero. In order to suppress the growth of austenite grains in the weld metal, the content is preferably 0.001% or more. If it exceeds 0.010%, the effect will be saturated, so the upper limit is 0.010%. It is preferably 0.0015% or more and 0.0025% or less.

Ca: 0 to 0.005%
Ca is an element effective for improving ductility and microstructuring by morphological control. The Ca content may be 0. If the amount of Ca is large, coarsening of sulfides and oxides occurs, and ductility and toughness deteriorate. Therefore, the upper limit is set to 0.005%.

It is preferable that the components of the weld metal in the present embodiment further satisfy the relationships described below.

0 ≤ α'≤ 50
As for the component composition of the weld metal of the welded joint, α'expressed by the following formula is preferably 0 to 50.

α'= (1.5 x (% O-0.89% Al) + 3.4 x% N-% Ti) x 1000

α'is a parameter showing an effective acylular ferrite forming ability based on the stoichiometric ratios of Al, O and Ti, N. By controlling α'in the range of 0 to 50, the ability to generate acylular ferrite nuclei is improved.

When α'is less than 0, either the amount of Al or Ti is excessive, or the amount of N or O is too small, so that the ability to generate acylical ferrite nuclei is significantly reduced. When α'is more than 50, either the amount of Al or Ti is too small, or the amount of N or O is too large, so that the ability to generate acylical ferrite nuclei is significantly reduced.

Al / O: 0.3 to 0.8
Al / O is a ratio of the amount of Al to the amount of O, and is an index showing the oxygen potential after the completion of aluminum deoxidation. By controlling Al / O to 0.3 to 0.8, the amount of acicular ferrite produced can be improved.

When the Al / O ratio is less than 0.3, the amount of O is excessive, and the dissolved oxygen that does not form the Ti oxide lowers the cleanliness of the steel, so that the toughness is lowered. On the other hand, when Al / O exceeds 0.8, the amount of Al becomes excessive, the amount of O bonded to Ti decreases, the amount of Ti oxide serving as an acylular ferrite nucleus decreases, and the toughness decreases. Therefore, Al / O is preferably 0.3 to 0.8.

Ceq: 0.35 to 0.50%
The composition of the weld metal is preferably 0.35 to 0.50% for Ceq represented by the following formula.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5+ (Ni + Cu) / 15

Ceq is the sum of the curing ability of each alloying element converted into the amount of C for the curing ability due to the influence of welding heat of the base metal. It is preferable to control the Ceq to 0.35 to 0.50% in order for the weld metal to achieve the desired tensile strength.

Pcm: 0.5-2.0%

The composition of the weld metal preferably has a Pcm of 0.5 to 2.0% represented by the following formula. % X in the formula means the content (mass%) of the element X in the weld metal (same in the following description). In addition, elements that are not added to the weld metal are calculated as zero (same as described below).

Pcm =% C +% Si / 30 + (% Mn +% Cu +% Cr) / 20 +% Ni / 60
+% Mo / 15 +% V / 10 + 5% B

In order to secure the tensile strength of the weld metal, it is preferable to set the Pcm value to 0.5% or more by containing various alloying elements in a well-balanced manner. The Pcm value is preferably 2.0% or less because excessive inclusion of alloying elements leads to an increase in cost.

Next, a preferable metal structure of the weld metal will be described.

When the components and parameters of the weld metal are set in the above range and the above-mentioned steel sheet is submerged arc welded at a welding heat input of 15 to 60 kJ / cm, the metal structure of the weld metal becomes a structure mainly composed of cyclic ferrite. Become. The steel pipe targeted by the present invention has a plate thickness of about 6 to 20 mm, and when submerged arc welding a steel plate having such a thickness is performed, the welding heat input is in the range of 15 to 60 kJ / cm. Then, the cooling rate received by the weld metal is determined, and the metal structure of the weld metal in the final pass becomes the following structure. The ratio shown below is the area ratio.

Accular ferrite: 80% or more Acicular ferrite is a needle-shaped ferrite structure with Ti oxide as the core, and the larger the ratio, the finer the fracture unit of the weld metal part. In order to obtain the effect, it is preferable that the amount of acicular ferrite is 80% or more.

Grain boundary ferrite: 10% or less Grain boundary ferrite is one of the embrittled phases, which is a starting point of fracture and a factor of reducing toughness. Therefore, the grain boundary ferrite is preferably 10% or less.

Island-shaped martensite: 3% or less One of the island-shaped martensite embrittled phases, which has a very high hardness and is a starting point of fracture and a factor of reducing toughness. Therefore, it is preferable that the island-shaped martensite is 3% or less.

EBSD particle size: 10 μm or less The EBSD (Electron Back Scatter Diffraction) particle size is a crystal particle size that serves as a guideline for the fracture unit. When the EBSD particle size is 10 μm or less, the fracture unit is fine, which is preferable in terms of ensuring toughness at low temperatures.

By using the above-mentioned components and structure, the Charpy impact test absorption energy of the weld metal at −51 ° C. can be set to 81 J or more.

In the steel pipe of the present invention, it is desirable that the old γ particle size in the vicinity of the first fusion line of the weld heat affected zone is 120 μm or less and the EBSD particle size is 100 μm or less. As a result, the fracture unit becomes fine and the toughness of the weld heat affected zone becomes good.

When a steel pipe is used as a storage container for liquefying and transporting natural gas, it is preferable that the roundness of the steel pipe is high because a plurality of steel pipes may be stacked and used. It is preferable that the difference between the maximum value and the minimum value of the inner diameter of the end portion is 20 mm or less.

Next, the method for manufacturing the steel pipe of the present invention will be described.

First, a steel material having the chemical composition of the base material described above is melted by a known method, and the refined molten steel is made into a slab by a continuous casting method. Next, the slab is heated to 1000 to 1200 ° C., and hot rolling is performed with the finishing temperature set to 750 to 850 ° C.

Subsequently, the steel sheet after finish rolling is accelerated and cooled. Accelerated cooling conditions are important in order to make the structure of the base metal the above-mentioned predetermined structure and to have a good balance between strength and toughness. Specifically, accelerated cooling is performed with the start temperature of accelerated cooling set to 740 to 800 ° C, the stop temperature of accelerated cooling set to 400 to 500 ° C, and the average cooling rate set to 10 to 100 ° C / min. As a result, the structure of the base metal becomes a mixed structure of bainite with an area ratio of 30% or more and ferrite with an area ratio of 20 to 60% and ferrite and bainite with a total area ratio of 90% or more, and a predetermined strength and toughness balance is achieved. Obtainable.

Next, the obtained steel sheet is subjected to groove processing having a predetermined shape. As the groove shape, for example, the end portion of the steel sheet can be machined into a groove shape that can be welded from both the front and back surfaces, for example, an X-shaped groove. The present invention can be manufactured by performing submerged arc welding from the outer surface side in the longitudinal direction after the submerged arc welding from the inner surface side is completed by abutting the end portions with the machined grooves.

Flux is sprayed in the groove, a steel wire for submerged arc welding is used, and joining is performed by large heat input submerged arc welding with a heat input of 15 to 50 kJ / cm. The flux and the steel wire are not particularly limited, and known ones can be used. When a steel wire is used, a known calcined flux, a molten flux, or the like can be used as the flux, and if the above-mentioned weld metal component can be obtained by this, a weld metal having excellent toughness can be obtained. Further, if necessary, flux preheating before welding may be performed.

The method of submerged arc welding is not particularly limited, and includes submerged arc welding of multiple electrodes, any known welding method can be applied, and the welding conditions are not particularly limited.

The steel material having the chemical composition shown in Table 1 was melted, and the refined molten steel was made into a slab by a continuous casting method, heated to 1150 ° C., and then hot-rolled. After that, the finishing temperature of hot rolling was set to 780 ° C., accelerated cooling was started from 760 to 795 ° C., and accelerated cooling was stopped at 400 to 500 ° C.

From the position of the obtained steel plate with a thickness of 1 / 4t, the No. 4 test piece specified by JIS Z 2241 (2011) is placed in the direction parallel to the rolling direction (L direction) and in the direction perpendicular to it (C direction). ), And the yield stress (YS) and tensile strength (TS) were measured. Further, the Charpy test piece was collected so that the longitudinal direction of the test piece was parallel to the rolling direction, and the Charpy impact test absorption energy at −51 ° C. was measured. The CTOD test was performed according to the ISO standard. The test piece was collected at B × 2B in total thickness, and the CTOD value at −45 ° C. was measured.

For the structure of the steel sheet, a sample was taken so that the vertical cross section of the steel sheet in the rolling direction could be observed, and the metal structure at 1 mm from the surface, 1/4 of the plate thickness, and the center of the plate thickness was photographed with an optical microscope at a magnification of 500 times. Next, after performing binarization processing under appropriate conditions using image analysis software, the total area of ferrite and bainite was obtained, and the total area of the photographing portion was divided to obtain the respective fractions. The results are shown in Table 2.

Subsequently, an X-shaped groove was formed on the obtained steel sheet, formed into a tubular shape, and submerged arc welding was performed in this order on the inner surface side and the outer surface side of the pipe using known wires and flux to obtain a UO steel pipe. At the time of welding, the welding speed and the like were adjusted so that the heat input was about 65 kJ / cm. Tables 3-1 to 3-3 show the components of the weld metal, Ceq, Pcm, Al / O and α'.

After welding, the structures of the weld metal part and the weld heat-affected zone were observed, and the absorbed energy at −51 ° C. was measured by the Charpy impact test.

After submerge arc welding, the area ratio (%) of the weld metal structure (acicular ferrite, total of grain boundary ferrite and island-shaped martensite), the old γ grain size of the weld heat-affected zone, and the weld metal part and the weld heat-affected zone The EBSD particle size, the tensile strength of the welded joint, the absorbed energy of the Charpy impact test of the weld metal and the weld heat affected zone, and the CTOD value of the CTOD test were measured. The results are shown in Tables 4-1 to 4-3. The AF rate, GBF rate, and MA rate in Tables 4-1 to 4-3 indicate the area ratios of acicular ferrite, grain boundary ferrite, and island martensite in the weld metal structure, respectively.

If there is a welding defect, it is "Yes", and if there is no welding defect, it is "No". The results are shown in Tables 4-1 to 4-3. Here, welding defects refer to high-temperature or low-temperature cracking, blowholes, and slag entrainment.

The absorbed energy of the Charpy impact test was measured as follows.

Charpy test pieces were collected from the center of the weld metal part 2 mm below the outer surface surface layer of the steel sheet, the weld heat affected zone 2 mm below the outer surface surface layer, and the molten line position where the weld metal was 50:50, and Charpy at −51 ° C. according to JIS Z2242. An impact test was performed and the absorbed energy was measured. For the absorbed energy, the Charpy impact test was carried out three times, and the average value was taken. It was judged that the toughness was poor when the weld metal was less than 81 J and the weld heat affected zone was less than 41 J.

The CTOD test was performed according to the ISO standard. The test piece had a total thickness of B × 2B, and was collected for evaluation of the weld metal and for evaluation of the weld heat-affected part. The test piece for evaluation of the weld heat-affected portion was tested and evaluated at −45 ° C. by making a notch at the molten wire position 12 where the weld heat-affected portion and the weld metal were 50:50. Three tests were performed for each, and those with a CTOD value of less than 0.18 mm were judged to have poor toughness. The CTOD values in Tables 4-1 to 4-3 are the maximum values of the three test results.

The area ratio of the tissue was measured as follows.

½ part of the weld bead width at the wall thickness t / 4 position is taken from the outer surface layer, and after polishing, it is subjected to nightal corrosion and repera corrosion, and the exposed structure is measured with an optical microscope in the range of 1000 μm × 1000 μm. The tissues observed in the above were measured in 10 fields, the obtained images were image-analyzed, and the average area ratio of each tissue was calculated and obtained.

For the old γ grain size of the welding heat affected part, a test piece was taken so that the position 2 mm from the surface layer of the inner and outer surfaces could be observed with an optical microscope, and polishing and corrosion to reveal the old γ grains were performed. Fifty first proximity particle sizes were measured, and the average particle size was determined.

The EBSD grain size was analyzed by 20-field EBSD analysis in the range of 500 μm × 500 μm, and the average crystal grain size when separated by a crystal orientation difference of 15 ° was used.

As shown in Tables 4-1 to 4-3, in each of the invention examples satisfying the weld joint component composition of the present invention, the charpy absorption energy at −51 ° C. is 81 J for the weld metal and 41 J or more for the weld heat affected portion. , The CTOD value at −45 ° C. was 0.18 mm or more in the weld metal and the weld heat affected portion, and the weld metal portion and the weld heat affected portion had excellent toughness.

On the other hand, in the comparative examples that do not satisfy the weld joint component composition of the present invention, the charpy absorption energy at −51 ° C. is 81 J for the weld metal, less than 41 J for the weld heat affected portion, and the CTOD value at −45 ° C. is welding. It was less than 0.18 mm in the metal and the weld heat affected part, and the toughness of the weld metal part and the weld heat affected part became low.

According to the present invention, it is possible to provide a vertical seam welded steel pipe having excellent toughness of a weld metal portion even when a thick steel plate is joined by performing large heat input welding. Therefore, the present invention has high industrial applicability.

1 Base metal 2 Weld metal 11 Weld metal center 12 Weld line position where the weld heat affected zone and weld metal are 50:50

Claims (8)

A steel pipe for a liquefied gas storage container for ships, which is made by forming a steel plate and butt welding it.
The component of the base material of the steel pipe is mass%,
C: 0.01 to 0.10%,
Si: 0.03 to 0.50%,
Mn: 0.5-2.0%,
P: 0.015% or less,
S: 0.010% or less,
Al: 0.001 to 0.050%,
Ti: 0.005 to 0.030%,
N: 0.002 to 0.006%,
O: 0.005% or less,
Cu: 0-0.5%,
Ni: 0-0.6%,
Cr: 0-0.5%,
Mo: 0-0.4%,
V: 0 to 0.06%,
Nb: 0 to 0.06%,
Remaining: Fe and impurities,
Moreover, the content of one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, V, and Nb is more than 0%.
The structure of the base metal contains bainite having an area ratio of 30% or more, and ferrite having an area ratio of 20 to 60% and ferrite and bainite having a total area ratio of 90% or more.
The tensile strength of the base metal in the L direction and the C direction is 570 to 760 MPa.
The yield stress of the base metal in the L direction and the C direction is 520 to 635 MPa.
The yield ratio of the base metal is 90% or less, and the yield ratio is 90% or less.
Charpy impact test absorption energy vE _-51 at -51 ° C of the base material is 100J or more,
Charpy impact test absorption energy vE _-51 at -51 ° C of the weld heat affected zone of the steel pipe is 41J or more,
Charpy impact test at -51 ° C of the weld heat affected zone of the steel pipe Absorption energy vE _-51 is 81 J or more When the CTOD value of the base metal, weld metal and weld heat affected zone of the steel pipe at -45 ° C is 0.18 mm or more. A steel pipe for storing liquefied gas, which is characterized by being present. The chemical composition of the base material replaces a part of the Fe.
B: 0 to 0.002%,
Mg: 0 to 0.01% and Ca: 0 to 0.03%
The steel pipe for storing liquefied gas according to claim 1, which contains one kind or two or more kinds selected from the group consisting of. The component of the weld metal of the steel pipe
C: 0.03 to 0.10%,
Si: 0.03 to 0.50%,
Mn: 0.5-2.0%,
P: 0.015% or less,
S: 0.010% or less,
Al: 0.001 to 0.030%,
Ti: 0.005 to 0.040%,
N: 0.002 to 0.006%,
O: 0.015 to 0.050%,
Cu: 0-0.6%,
Ni: 0-0.5%,
Cr: 0-0.5%,
Mo: 0-0.4%,
V: 0 to 0.06%,
Nb: 0 to 0.06%,
Remaining: Fe and impurities,
Moreover, the content of one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, V, and Nb is more than 0%.
0 ≤ α'≤ 50 ... (1)
0.3 ≤ Al / O ≤ 0.8 ... (2)
0.30 ≤ Ceq ≤ 0.50 ... (3)
0.5 ≤ Pcm ≤ 2.0 ... (4)
The filling,
The steel pipe for storing liquefied gas according to claim 1 or 2, wherein the charpy impact test absorption energy vE _-51 of the weld metal at −51 ° C. is 81 J or more.
Here, α', Ceq, and Pcm are, respectively.
α'= (1.5 x (O-0.89Al) +3.4 x N-Ti) x 1000
… (5)
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5+ (Ni + Cu) / 15 ... (6)
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B ... (7)
The element symbols in the above formulas (1), (2), (5), (6), and (7) indicate the content (mass%) of each element. The chemical composition of the weld metal replaces a part of the Fe.
B: 0 to 0.035%,
Mg: 0 to 0.01% and Ca: 0 to 0.005%
The steel pipe for storing liquefied gas according to claim 3, wherein the steel pipe contains one or more selected from the group consisting of two or more. The steel pipe for storing liquefied gas according to any one of claims 1 to 4, wherein the old γ particle size of the weld heat-affected zone is 120 μm or less, and the EBSD particle size is 100 μm or less. The structure of the weld metal is the area ratio,
6. Liquefied gas storage steel pipe. The steel pipe for storing liquefied gas according to any one of claims 1 to 6, wherein the ferrite fraction of the structure of the base metal is 20% or less in terms of area ratio. The steel pipe for storing liquefied gas according to any one of claims 1 to 7, wherein the difference between the maximum value and the minimum value of the inner diameters of the body portion and the end portion of the steel pipe is 20 mm or less.

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