JP6829090B2 - Shielded metal arc welding rod - Google Patents

Description

The present invention relates to a shielded metal arc welding rod.

Structures such as boilers and heat exchangers in thermal power generation facilities are required to have characteristics such as heat resistance and pressure resistance, and the steam temperature and vapor pressure of thermal power generation have been increasing more and more in recent years from the viewpoint of improving thermal efficiency. For example, the steam temperature in ultra-supercritical coal-fired power generation is about 500 to 600 ° C. Since the structure is held at high temperature and high pressure for a long time, stress is applied and a creep phenomenon occurs in which strain increases with the passage of time.

As the material of the structure, heat-resistant steel containing a relatively large amount of Cr is used because it has characteristics such as heat resistance and pressure resistance. Further, the above-mentioned material is required to have excellent creep rupture characteristics that do not break even when exposed to high temperature and high pressure for a long time, and is also required to have excellent toughness.

The above structure is generally constructed by arc welding high Cr steel as a material, and even a weld metal formed by welding high Cr steel has excellent creep breaking characteristics and toughness. Is required. Further, the weld metal formed by arc welding is usually subjected to post-weld heat treatment (PWHT) in order to remove residual stress.

By the way, since Cr has an action of stabilizing ferrite, when high Cr steel is welded, δ ferrite may be generated at the time of welding and may remain in the weld metal after welding is completed. δ-ferrite is a coarse structure observed in the weld metal as it is welded before the post-weld heat treatment, and does not disappear even after the post-weld heat treatment, which adversely affects the creep break characteristics and toughness of the weld metal after the post-weld heat treatment. Is known to exert.

The creep rupture characteristics and toughness of a weld metal are generally evaluated using a test piece taken from a specific part of the weld metal, so if the part where the test piece was taken does not happen to contain δ ferrite, Good properties are shown. However, in the weld metal actually constructed, if δ ferrite is generated even in a part, there is a risk of breakage or breakage. Therefore, for safety, the formation of δ ferrite is suppressed in the entire area of the weld metal. Need to be done.

For example, Patent Documents 1 to 3 are known as a technique for suppressing the formation of δ ferrite during welding. Patent Documents 1 to 3 relate to welding materials used at the time of welding.

Patent Document 1 describes that Ni is an effective element for improving toughness, but on the other hand, it promotes aggregation of carbides and oxides and lowers creep strength at high temperature for a long time. .. Then, in this document, the formation of δ ferrite is suppressed by adding Co, Cu, or one of them instead of Ni, which is effective for improving toughness, in the steel core wire or the coating agent. It is described to improve creep strength while ensuring toughness.

Patent Document 2 describes high temperature creep strength, toughness, and high temperature creep strength by adding appropriate amounts of C, Si, Mn, Cr, Ni, Co, Cu, Mo, W, V, Nb, and N to the welding wire. Crack resistance can be ensured, and by adding the ferrite-forming elements of Cr, W, and Mo and the elements that suppress the ferrite formation of Ni and Co in an appropriate content relationship, δ-ferrite in the weld metal It is described that the formation can be suppressed and the creep strength and toughness can be further improved, and that the transformation to the σ phase after holding at a high temperature is suppressed by suppressing the amount of Mo.

According to Patent Document 3, the creep strength of the weld metal is improved as the amount of MX (carbonitride) precipitates increases, and the toughness largely depends on the amount of δ-ferrite precipitates and the Ae1 transformation point. Have been described.

Further, in Patent Document 4, by adding an appropriate amount of C, Si, Mn, Cu, Ni, Co, Cr, Mo, V, Nb, W and N to the weld wire, the high temperature creep strength of the welded portion is obtained. It is described that a welding wire for high Cr ferrite-based heat-resistant steel having excellent both toughness and toughness can be provided.

Japanese Unexamined Patent Publication No. 7-268562 Japanese Unexamined Patent Publication No. 8-187592 Japanese Unexamined Patent Publication No. 11-17807 Japanese Unexamined Patent Publication No. 2004-42116

The above-mentioned Patent Documents 1 to 4 describe a welding material used at the time of welding and the formation of a welding metal using the welding material. However, it is unclear whether the formation of δ ferrite is suppressed in the entire region of the weld metal.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a shielded metal arc welding rod capable of obtaining a weld metal having excellent creep rupture characteristics and toughness.

In order to achieve the above object, the present inventors have investigated means for suppressing the formation of δ-ferrite during welding and improving the creep breaking characteristics of the weld metal at a high level. As a result, the alloy composition of the shielded metal arc welding rod It has been found that the object can be achieved by appropriately controlling the above, and the present invention has been completed.

That is, the present invention
A shielded metal arc welding rod in which a coating agent is coated on the outer circumference of a steel core wire.
In one or both of the coating agent and the steel core wire, in mass% as an alloy component, per the total mass of the shielded metal arc welding rod.
C: 0.05 to 0.20%,
Si: 0.3-1.0%,
Mn: 0.1-1.1%,
Ni: 0% or more, 0.5% or less,
Cr: 3-12%,
Mo: 0.05-0.55%,
V: 0.02 to 0.50%,
Nb: 0.01-0.09%,
Co: 0.55 to 2.20%,
W: 0.4 to 2.0%,
N: 0.01 to 0.10%,
B: Contains 0.001 to 0.100%,
The balance relates to a shielded metal arc welding rod consisting of a gas generator, a slag forming agent, an arc stabilizer, iron and unavoidable impurities.

The shielded metal arc welding rod further contains Ti: more than 0% and 0.03% or less in mass% as an alloy component per total mass of the shielded metal arc welding rod in one or both of the coating agent and the steel core wire. It may be contained.

Further, the shielded metal arc welding rod has Cu: more than 0% and 0.25% or less in mass% as an alloy component per total mass of the shielded metal arc welding rod in one or both of the coating agent and the steel core wire. May be further contained.

Further, the shielded metal arc welding rod has Al: more than 0% and 1.5% or less as an alloy component per total mass of the shielded metal arc welding rod in one or both of the coating agent and the steel core wire. May be further contained.

According to the shielded metal arc welding rod of the present invention, a weld metal having excellent creep rupture characteristics and toughness can be obtained.

FIG. 1 is an atomic map of C and Cr atoms in a test piece of a certain weld metal (note that each point represents an atom). FIG. 2 is an analysis of a columnar shape perpendicular to the grain interface and penetrating the grain interface, which is set when measuring the surface density ( _BGB ) of segregation B existing at the matrix grain boundary for a certain weld metal test piece. It is a figure which shows the area. FIG. 3 is a diagram showing a profile when the atomic concentration of the columnar analysis region shown in FIG. 2 is measured along the analysis direction. FIG. 4 is a schematic view showing the sampling position of the test piece used for evaluating the creep rupture characteristics in the embodiment of the present invention. FIG. 5 is a schematic view showing a sampling position of a test piece used for evaluation of toughness in an example of the present invention.

Hereinafter, embodiments for carrying out the present invention will be described in detail. The present invention is not limited to the embodiments described below. Further, in the present specification, the percentage based on mass (mass%) is synonymous with the percentage based on weight (% by weight).

The shielded metal arc welding rod of the present invention is a shielded metal arc welding rod in which a coating agent is coated on the outer periphery of the steel core wire, and the shielded metal arc welding rod is contained in one or both of the coating agent and the steel core wire. C: 0.05 to 0.20%, Si: 0.3 to 1.0%, Mn: 0.1 to 1.1%, Ni: 0% or more in terms of mass% as an alloy component per total mass. , 0.5% or less, Cr: 3 to 12%, Mo: 0.05 to 0.55%, V: 0.02 to 0.50%, Nb: 0.01 to 0.09%, Co: 0 .55-2.20%, W: 0.4-2.0%, N: 0.01-0.10%, B: 0.001-0.100%, the balance is a gas generator, A shielded metal arc welding rod composed of a slag forming agent, an arc stabilizer, iron and unavoidable impurities.

B stabilizes the M ₂₃ C ₆ particles under the creep rupture test by dissolving in carbide particles generally represented by M ₂₃ C ₆ (hereinafter, also simply referred to as “M ₂₃ C ₆ particles”), and these particles. Is said to improve creep rupture characteristics by inhibiting dislocation movement.
Therefore, the present inventors investigated the stabilization mechanism of M ₂₃ C ₆ particles by B in order to make the best use of this effect.

In high temperature environments, such as creep rupture test, M ₂₃ C ₆ particles, rapidly total particle number decreases by Ostwald growth. Here, Ostwald growth is a phenomenon in which particles having a small particle size disappear when heat-treated, while particles having a large particle size continue to grow. The present inventors have found that the Ostwald growth of M ₂₃ C ₆ particles is rate-determined by the diffusion of B.
That _is, in the course of Ostwald growth of _M 23 _{C 6 particles,} to coarsening of _M 23 _{C 6} particles, it is necessary to have B from the surrounding matrix is supplied. At this time, by reducing the B present at the matrix grain boundaries or in the grains to a certain value or less, the Ostwald growth of the M ₂₃ C ₆ particles is suppressed.
As a result, the rate of decrease of M ₂₃ C ₆ particles at high temperature is reduced, and the creep rupture characteristics of the weld metal can be improved.

In the present invention, based on the above findings, an alloy of a shielded metal arc welding rod is used so that B can be present in an appropriate form in the welding metal obtained by welding and in order to suppress the formation of δ ferrite during welding. It was decided to specify the amount of ingredients as follows. These alloy components may be added to one of the steel core wire and the coating agent, or may be added to both. Further, the amount of each alloy component (%) in the shielded metal arc welding rod below is the content (mass%) per the total mass of the shielded metal arc welding rod.

(C: 0.05 to 0.20% by mass)
C is an element that forms carbides and contributes to improving the creep rupture properties of the weld metal. In order to exert such an effect, the amount of C is set to 0.05% or more in the present invention. The amount of C is preferably 0.07% or more, more preferably 0.09% or more. However, if C is excessively contained, the carbide may become too coarse and the toughness of the weld metal may decrease. Therefore, in the present invention, the amount of C is 0.20% or less. The amount of C is preferably 0.18% or less, more preferably 0.16% or less. When C is added from a coating agent, it can be added by being contained in another metal raw material.

(Si: 0.3 to 1.0% by mass)
Si functions as a deoxidizer and improves the strength and toughness of the weld metal. Further, Si is an element that contributes to the improvement of workability during welding, and if the amount of Si is less than 0.3%, the welding workability deteriorates. Therefore, in the present invention, the amount of Si is 0.3% or more. The amount of Si is preferably 0.4% or more, more preferably 0.5% or more. However, if Si is excessively contained, the strength of the weld metal is excessively increased, resulting in deterioration of toughness. Therefore, in the present invention, the amount of Si is 1.0% or less. The amount of Si is preferably 0.8% or less, more preferably 0.7% or less. When Si is added from a coating agent, it can be added by Fe-Si or the like.

(Mn: 0.1 to 1.1% by mass)
Mn functions as a deoxidizer to improve the strength and toughness of the weld metal. Further, Mn is an element having an action of suppressing the formation of δ ferrite during welding. If the amount of Mn is less than 0.1%, δ ferrite is likely to be generated during welding, and the generated δ ferrite does not disappear even after heat treatment after welding. Therefore, if δ ferrite is generated during welding, δ ferrite is generated. It can adversely affect the creep rupture properties and toughness of weld metal. Therefore, in the present invention, the amount of Mn is 0.1% or more. The amount of Mn is preferably 0.2% or more, more preferably 0.3% or more. However, if Mn is excessively contained, the Ostwald growth of the carbide particles is excessively promoted, and the creep rupture characteristics of the weld metal deteriorate. Therefore, in the present invention, the amount of Mn is set to 1.1% or less. The amount of Mn is preferably 0.8% or less, more preferably 0.7% or less. When Mn is added from a coating agent, it can be added by Fe-Mn or the like.

(Ni: 0% by mass or more, 0.5% by mass or less)
Ni is an element that promotes the diffusion of alloying elements in the matrix. However, if Ni is added excessively, the diffusion of B is promoted, the Ostwald growth of the M ₂₃ C ₆ particles is likely to proceed, and the creep rupture characteristics of the weld metal are deteriorated. Therefore, in the present invention, the amount of Ni is 0.5% or less. The amount of Ni is preferably 0.4% or less, more preferably 0.3% or less. When Ni is added from a coating agent, it can be added by Fe-Ni or the like.

(Cr: 3 to 12% by mass)
Cr is an element that forms M ₂₃ C ₆ particles and improves the creep rupture characteristics of the weld metal when a metal element such as Cr, Fe, or Mo is expressed as M. In order to exert such an effect, the amount of Cr is set to 3% or more in the present invention. The amount of Cr is preferably 4% or more, more preferably 5% or more. However, if Cr is excessively contained, δ ferrite is likely to be generated during welding, and the generated δ ferrite does not disappear even after heat treatment after welding. Therefore, when δ ferrite is generated during welding, δ ferrite is a weld metal. It can adversely affect the improvement of creep rupture properties and toughness. Therefore, in the present invention, the amount of Cr is set to 12% or less. The amount of Cr is preferably 11% or less, more preferably 10% or less. When Cr is added from a coating agent, it can be added by Fe-Cr or the like.

(Mo: 0.05 to 0.55% by mass)
Mo is an element that improves the creep rupture characteristics of weld metals by strengthening solid solution. In order to exert such an effect, the amount of Mo is set to 0.05% or more in the present invention. The amount of Mo is preferably 0.10% or more, more preferably 0.15% or more. However, if Mo is excessively contained, δ ferrite is likely to be generated during welding, and the generated δ ferrite does not disappear even after heat treatment after welding. Therefore, when δ ferrite is generated during welding, δ ferrite is a weld metal. It can adversely affect the improvement of creep rupture properties and toughness. Therefore, in the present invention, the amount of Mo is 0.55% or less. The amount of Mo is preferably 0.50% or less, more preferably 0.45% or less. When Mo is added from a coating agent, it can be added by Fe-Mo or the like.

(V: 0.02 to 0.50% by mass)
V is an element that forms MX (carbonitride) and improves the creep rupture characteristics of the weld metal. Further, by fixing N as MX, it has the effect of reducing the amount of B to generate a _BN, by increasing the amount of B to blend into _M 23 _{C 6 particles,} inhibit Ostwald ripening of the _M 23 _{C 6} particles. In order to exert such an effect, the amount of V is set to 0.02% or more in the present invention. The amount of V is preferably 0.05% or more, more preferably 0.10% or more. However, if V is excessively contained, δ ferrite is formed during welding. It also causes Ostwald growth of MX (carbonitride) at high temperatures. As a result, the creep rupture properties and toughness of the weld metal deteriorate. Therefore, in the present invention, the amount of V is 0.50% or less. The amount of V is preferably 0.40% or less, more preferably 0.30% or less. When V is added from a coating agent, it can be added by being contained in another metal raw material.

(Nb: 0.01 to 0.09% by mass)
Nb is an element that forms MX (carbonitride) and improves the creep rupture characteristics of the weld metal. In order to exert such an effect, the amount of Nb is set to 0.010% or more in the present invention. The amount of Nb is preferably 0.015% or more, more preferably 0.020% or more. However, if Nb is excessively contained, the strength of the weld metal is excessively increased and the toughness is deteriorated. Therefore, in the present invention, the amount of Nb is 0.090% or less. The amount of Nb is preferably 0.075% or less, more preferably 0070% or less. When Nb is added from a coating agent, it can be added by being contained in another metal raw material.

(Co: 0.55 to 2.20% by mass)
Co is an element that suppresses the formation of δ ferrite during welding and improves the creep rupture characteristics and toughness of the weld metal. In order to exert such an effect, the amount of Co is set to 0.55% or more in the present invention. The amount of Co is preferably 0.65% or more, more preferably 0.75% or more. However, if Co is excessively contained, the strength of the weld metal is excessively increased and the toughness is deteriorated. Therefore, in the present invention, the amount of Co is 2.20% or less. The amount of Co is preferably 2.00% or less, more preferably 1.80% or less. When Co is added from a coating agent, it can be added by Fe-Co or the like.

(W: 0.4 to 2.0% by mass)
Like Mo, W improves the creep rupture characteristics of the weld metal as a solid solution strengthening element, and also precipitates at the grain boundaries as the Laves phase at high temperatures, hindering B diffusion at the grain boundaries, so that M ₂₃ C ₆ It is expected to have the effect of suppressing the Ostwald growth of particles. In order to exert such an effect, the W amount is set to 0.4% or more in the present invention. The amount of W is preferably 0.6% or more, more preferably 0.8% or more. However, if W is excessively contained, δ ferrite is likely to be generated during welding, and the generated δ ferrite does not disappear even after heat treatment after welding. Therefore, when δ ferrite is generated during welding, δ ferrite is a weld metal. It can adversely affect the improvement of creep rupture properties and toughness. Therefore, in the present invention, the amount of W is set to 2.0% or less. The amount of W is preferably 1.8% or less, more preferably 1.6% or less. When W is added from the coating agent, it can be added by Fe-W or the like.

(N: 0.01 to 0.10% by mass)
Like Nb, N is an element that forms MX (carbonitride) and improves the creep rupture characteristics of the weld metal. In order to exert such an effect, the amount of N is set to 0.010% or more in the present invention. The amount of N is preferably 0.015% or more, more preferably 0.020% or more. However, if N is excessively contained, the strength of the weld metal is excessively increased and the toughness is deteriorated. Therefore, in the present invention, the amount of N is 0.10% or less. The amount of N is preferably 0.09% or less, more preferably 0.08% or less. When N is added from a coating agent, it can be added by being contained in another metal raw material.

(B: 0.001 to 0.100% by mass)
_B, by blend into _M 23 _{C 6 particles,} to suppress Ostwald ripening of the _M 23 _{C 6} particles is an element to improve creep rupture properties of the weld metal. In order to exert such an effect, the amount of B is set to 0.001% or more in the present invention. The amount of B is preferably 0.003% or more, more preferably 0.005% or more, still more preferably 0.011% or more. However, if B is added excessively, the strength of the weld metal increases excessively, and a predetermined toughness cannot be secured. Therefore, in the present invention, the amount of B is 0.100% or less. The amount of B is preferably 0.090% or less, more preferably 0.080% or less. When B is added from a coating agent, it can be added by being contained in another metal raw material.

Further, the shielded metal arc welding rod of the present invention may contain at least one selected from the group consisting of the following (a) to (c) as other elements in addition to the above alloying elements.
(A) Ti: More than 0%, 0.03% or less.
(B) Cu: More than 0%, 0.25% or less.
(C) Al: More than 0%, 1.5% or less.

Ti is not an essential element, but it is an element that forms MX (carbonitride) and contributes to the improvement of creep rupture characteristics of weld metal. In order to effectively exert such an effect, the amount of Ti is preferably 0.001% or more, more preferably 0.002% or more, still more preferably 0.003% or more. However, if Ti is excessively contained, the strength of the weld metal may be excessively increased and the toughness may be deteriorated. Therefore, in the present invention, the amount of Ti is preferably 0.03% or less. The amount of Ti is more preferably 0.025% or less, still more preferably 0.020% or less.

Cu is not an essential element, but it is an element that has the effect of suppressing the formation of δ ferrite during welding. In order to effectively exert such an effect, the amount of Cu is preferably 0.01% or more, more preferably 0.02% or more, still more preferably 0.025% or more. However, if Cu is excessively contained, the formation of a structure in which ferrite grows in a strip shape (sometimes called a ferrite band) is promoted, and the creep rupture characteristics and toughness of the weld metal may deteriorate. Therefore, in the present invention, the amount of Cu is preferably 0.25% or less. The amount of Cu is more preferably 0.20% or less, still more preferably 0.15% or less.

Al is not an essential element, but it is an element that acts as an antacid. In order to effectively exert such an effect, the amount of Al is preferably 0.01% or more, more preferably 0.02% or more, still more preferably 0.025% or more. However, if Al is excessively contained, coarse oxides are generated, which may become a starting point of brittle fracture and reduce the toughness of the weld metal. Therefore, in the present invention, the amount of Al is preferably 1.5% or less. The amount of Al is preferably 1.3% or less, more preferably 1.1% or less.

The rest of the shielded metal arc welding rod of the present invention is preferably a gas generating agent, a slag forming agent, an arc stabilizer, iron and unavoidable impurities. Further, the shielded metal arc welding rod of the present invention preferably contains a metal carbonate, a metal fluoride, SiO ₂ and an alkali metal oxide as a gas generator, a slag forming agent and an arc stabilizer.
As the metal carbonate, for example, CaCO ₃ or the like can be added. As the metal fluoride, for example, CaF ₂ or the like can be added. As the alkali metal oxide, for example, Na ₂ O, K ₂ O and the like can be added. However, metal carbonates, metal fluorides and alkali metal oxides are not limited to these examples, and can be usually added to the coating agent of the shielded metal arc welding rod as a gas generating agent, a slag forming agent or an arc stabilizer. If it is, it can be applied without exception as long as the effect of the present invention is not impaired.

The content of the metal carbonate per the total mass of the shielded metal arc welding rod of the present invention is not particularly limited, but is, for example, 5 to 22% by mass.
Further, the content of the metal fluoride per the total mass of the shielded metal arc welding rod of the present invention is not particularly limited, but is, for example, 2 to 12% by mass.
Further, the content of SiO ₂ per the total mass of the shielded metal arc welding rod of the present invention is not particularly limited, but is, for example, 5 to 12% by mass.
Further, the content of the alkali metal oxide per the total mass of the shielded metal arc welding rod of the present invention is not particularly limited, but is, for example, 4 to 12% by mass.

The ratio of the coating agent to the shielded metal arc welding rod of the present invention is not particularly limited, but is, for example, 20 to 40% by mass with respect to the total mass of the shielded metal arc welding rod.

Further, the shielded metal arc welding rod of the present invention can be manufactured, for example, as follows.
First, the coating agent is coated and coated around the steel core wire with a binder such as sodium silicate and water glass typified by potassium silicate by a normal welding rod coating machine. Then, in order to remove water, it is fired at, for example, 400 to 550 ° C.

According to the shielded metal arc welding rod of the present invention, a weld metal having excellent creep rupture characteristics and toughness can be obtained. Here, it is preferable that the weld metal satisfies the following requirements.

(Concentration of solid solution B present in the matrix grain of weld metal: 0.0008% by mass or less)
By controlling the solid solution B concentration present in the matrix grains of the weld metal (B _M) below 0.0008 wt%, the supply of contributing B is suppressed in coarsening of M ₂₃ C ₆ particles at high temperatures , M ₂₃ C ₆ particles are preferable because the ostwald growth is suppressed and the creep rupture time is lengthened. B _M is more preferably 0.0005 mass% or less, more preferably not more than 0.0003 mass%.

(Area density of segregation B existing at the matrix grain boundaries of the weld metal (B _GB ): 3.0 pieces / nm ² or less)
Supply of B that contributes to the coarsening of M ₂₃ C ₆ particles at high temperature by controlling the surface density (B _GB ) of segregation B existing at the matrix grain boundaries of the weld metal to 3.0 pieces / nm ² or less. There is inhibited, since it is Ostwald growth inhibition of M ₂₃ C ₆ particles, it is prolonged creep rupture time, preferably. B _GB is more preferably 2.0 pieces / nm ² or less, still more preferably 1.0 pieces / nm ² or less.

(Amount of O in weld metal: 0.080% or less)
O is an element that forms an oxide, and if O is excessively contained, the oxide becomes coarse and becomes a starting point of brittle fracture and the toughness decreases. Therefore, the amount of O in the weld metal is 0.080% or less. It is preferable to have. The amount of O is more preferably 0.075% or less, still more preferably 0.070% or less. The lower the amount of O, the better the toughness, but usually about 0.030 to 0.060% is contained.

Further, the weld metal preferably satisfies the relationship of the following formula (1).
([V] × [B] / [N]) × 100 ≧ 0.42 (1)
(In the formula, [V], [B] and [N] represent the contents of V, B and N in the weld metal, respectively.)

The reason is as follows.
That, B inhibits Ostwald ripening of the particles in a way to push dissolved in M ₂₃ C ₆ particles, when the B to generate a BN increases, the amount of B Komu dissolved in M ₂₃ C ₆ particles can not be sufficiently secured. Further, N is first formed as a carbonitride containing V, and the remaining N forms BN. Therefore, in order to secure the amount of B that dissolves in the M ₂₃ C ₆ particles, it is necessary to add a predetermined amount of B and then sufficiently precipitate the carbonitride containing V.
From the above viewpoint, in the weld metal, X = ([V] × [B] / [N]) × 100 is preferably 0.42 or more. When X is less than 0.42, the amount of BN produced increases and the amount of B dissolved in the M ₂₃ C ₆ particles decreases, and as a result, the Ostwald growth of the particles cannot be sufficiently suppressed. X is more preferably 0.44 or more, still more preferably 0.45 or more.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples, and the present invention is modified to the extent that it can be adapted to the gist of the present invention. All of which are within the technical scope of the invention.

Satisfy the composition shown in Table 1, using the base material balance of iron and inevitable impurities, also using a covered electrode to satisfy the compositions shown in Table 2, dissolved Te in welding conditions below Welded metal was prepared and various characteristics were evaluated .

<Welding conditions>
Welding method: Shielded metal arc welding (SMAW)
Base material plate thickness: 20 mm
Groove angle: 20 ° (V-shaped)
Route spacing: 16mm
Welding posture: Downward Welding rod diameter: φ4.0 mm
Heat input conditions: Approximately 2.2 kJ / mm (150A-24V, 8-12 cm / min)
Laminating method: 1 layer 2 passes Preheating / inter-pass temperature: 200-400 ° C

(Presence or absence of δ ferrite)
From the obtained weld metal as it is welded, a test piece is collected so that a cross section perpendicular to the welding direction can be observed, the cross section of the test piece is corroded by a ferric chloride etching solution, and the metal is subjected to a magnification of 400 times by an optical microscope. The tissue was observed. When δ-ferrite was not observed in all cross sections, it was judged as “no δ-ferrite” and evaluated as acceptable. When even one δ ferrite was observed in the entire cross section, it was judged as “with δ ferrite” and evaluated as rejected. The judgment results are shown in Table 3 below.

Next, the weld metal obtained by welding was subjected to post-welding heat treatment (PWHT) under the conditions of a holding temperature of 700 to 800 ° C. and a holding time of 2 to 10 hours to evaluate the characteristics of the weld metal. The results are shown in Table 3 below.

(Concentration of solid solution B existing in matrix grains and surface density of segregation B existing at matrix grain boundaries)
A test piece is collected from the weld metal after welding and heat treatment so that a surface perpendicular to the welding direction can be observed, the cross section of the test piece is corroded by a ferric chloride etching solution, and the metal structure is obtained by an optical microscope at a magnification of 400 times. Observed. Using a focused ion beam device (Helios-600 manufactured by FEI), a needle-shaped test piece containing a grain boundary and a needle-shaped test piece not containing a grain boundary (only in the grain) are prepared, and a three-dimensional atom probe is produced. The measurement was carried out by (LEAP 3000HR manufactured by CAMECA) under the conditions of a measurement temperature of 50 K and a pulse fraction of 15%. The obtained data was analyzed by the analysis software IVAS. More specifically, the measurements and analyzes were carried out as follows.

The solid solute B concentration present in the matrix grains (B _M), was used measurement data that does not include the grain boundary (grain inside only) specimens. That is, the region where carbon is uniformly distributed was determined to be in the matrix grain, and the B concentration was determined after removing the background noise from the detected total number of atoms. The concentration measured by the atom probe was a few percent of atoms, but it was converted to mass% during the evaluation. The results are shown in Table 3 below.

For the areal density ( _BGB ) of segregation B existing at the matrix grain boundaries, the measurement data of the test piece including the grain boundaries was used. With reference to FIGS. 1 to 3, a method for measuring the areal density ( _BGB ) of segregation B existing at the matrix grain boundary for a test piece of a certain weld metal will be described. First, from the atomic map as shown in FIG. 1, a planar region having a high atomic concentration of alloying elements such as C and Cr was determined to be a grain interface. Then, as shown in FIG. 2, a columnar analysis region having a bottom radius of 5 nm (hereinafter, also simply referred to as “cylinder”) is set so as to penetrate the grain interface perpendicularly to the grain interface, and the column is formed. The atomic concentration contained in the region of 0.5 nm in length in the long axis direction was measured from one end of the above, and the profile shown in FIG. 3 was created. Subsequently, the surface density of segregation B was calculated by dividing the number of B atoms contained in the region having a high concentration of alloying elements such as C and Cr by the bottom area (area of a circle) of the cylinder. The results are shown in Table 3 below.

Next, creep rupture characteristics and toughness were evaluated as the characteristics of the weld metal that had been heat-treated after welding.

<Creep rupture characteristics>
A creep test piece with a gauge point distance of 30 mm and a φ6.0 mm in the welding line direction was collected from the central portion of the weld metal plate thickness that had been heat-treated after welding, and at 675 ° C. under the condition of 100 MPa. A creep test was performed and the time until the test piece broke was measured. In FIG. 4, T indicates the plate thickness of the base material. When the rupture time Tr exceeded 400 hours, it was judged that the creep rupture characteristics were excellent, and it was evaluated as acceptable.

<Toughness>
A Charpy impact test piece was collected perpendicular to the welding line direction from the central portion of the thickness of the weld metal that had been heat-treated after welding, and a Charpy impact test was performed. In FIG. 5, T indicates the plate thickness of the base material. For the Charpy impact test piece, a No. 4 V notch test piece specified in JIS Z3111 was taken. The Charpy impact test was carried out at 20 ° C. in the manner of JIS Z2242, and the absorbed energy was measured. The measurement was performed three times, and the average value (vE _{20 ° C.} ) of the measured absorbed energy was determined. The results are shown in Table 3 below. Welded metals with an average value (vE _{20 ° C.} ) of 41 J or more were evaluated as having excellent toughness.

Of Examples 1 to 20, Examples 1 to 13 are Examples, and Examples 14 to 20 are Comparative Examples.

In Example 14, since the amount of V in the shielded metal arc welding rod was as low as 0.01%, the obtained weld metal was inferior in creep rupture characteristics.
In Example 15, since the amount of Ni in the shielded metal arc welding rod was as high as 0.62%, the obtained weld metal was inferior in creep rupture characteristics.
In Example 16, the amount of Si in the shielded metal arc welding rod was as low as 0.22%, and the amount of Mn was as low as 0.05%, so that the obtained weld metal was inferior in toughness.
In Example 17, the amount of Si in the shielded metal arc welding rod was as high as 1.10%, the amount of Nb was as high as 0.092%, and the amount of B was as high as 0.110%, so that the obtained weld metal became tough. It was inferior.
In Example 18, since the amount of Cr in the shielded metal arc welding rod was as high as 13.67%, δ ferrite was formed in the obtained weld metal, and the creep rupture characteristics and toughness were inferior.
In Example 19, since the amount of W in the shielded metal arc welding rod was as high as 2.1% and B was not contained, δ ferrite was generated in the obtained welding metal, and the creep rupture characteristics and toughness were inferior. It was.
In Example 20, since the amount of Co in the shielded metal arc welding rod was as low as 0.49% and B was not contained, δ ferrite was formed in the obtained weld metal, and the creep rupture characteristics were inferior.

On the other hand, in Examples 1 to 13 in which the shielded metal arc welding rod satisfies the provisions of the present invention, δ ferrite was not generated in the weld metal, and the creep rupture characteristics and toughness were excellent.

Claims (2)

A shielded metal arc welding rod in which a coating agent is coated on the outer circumference of a steel core wire.
In one or both of the coating agent and the steel core wire, in mass% as an alloy component, per the total mass of the shielded metal arc welding rod.
C: 0.05 to 0.20%,
Si: 0.3-1.0%,
Mn: 0.1-1.1%,
Ni: 0% or more, 0.5% or less,
Cr: 3-12%,
Mo: 0.05-0.55%,
V: 0.02 to 0.50%,
Nb: 0.01-0.09%,
Co: 0.55 to 2.20%,
W: 0.4 to 2.0%,
N: 0.01 to 0.10%,
B: 0.001 to 0.100% ,
Cu: Over 0%, 0.25% or less,
Al: Contains more than 0% and less than 1.5% ,
A shielded metal arc welding rod with the balance consisting of a gas generator, slag forming agent, arc stabilizer, iron and unavoidable impurities. In one or both of the coating agent and the steel core wire, in mass% as an alloy component, per the total mass of the shielded metal arc welding rod.
The shielded metal arc welding rod according to claim 1, further containing Ti: more than 0% and 0.03% or less.

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