JP2018122329A - Coated arc welding electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coated arc welding electrode from which a weld metal having excellent creep rupture characteristic and toughness can be obtained.SOLUTION: A coated arc welding electrode in which a coating material coating an outer periphery of a steel core wire. One or both of the coating material and the steel core wire contains, as alloy components, C: 0.05 to 0.20%, Si: 0.3 to 1.0%, Mn: 0.1 to 1.1%, Ni: 0% or more and 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 to 2.20%, W: 0.4 to 2.0%, N: 0.01 to 0.10%, and B: 0.001 to 0.100% in mass% per a total mass of the coated arc welding electrode, and the balance comprises a gas generating agent, a slag forming agent, an arc stabilizer, an iron and inevitable impurities.SELECTED DRAWING: None

Description

  The present invention relates to a coated 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 in recent years from the viewpoint of improving thermal efficiency. For example, the vapor temperature in ultra super critical coal-fired power generation is about 500-600 ° C. Since the structure is held at a high temperature and a high pressure for a long time, a stress is applied, and a creep phenomenon occurs in which the 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 in order to have characteristics such as heat resistance and pressure resistance. Further, the material is required to have excellent creep rupture characteristics that do not break even when exposed to high temperatures and high pressures 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 the material, and it has excellent creep rupture characteristics and toughness even in weld metal formed by welding high Cr steel. Is required. In addition, a 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 completion of welding. δ-ferrite is a coarse structure observed in as-welded weld metal before heat treatment after welding, and does not disappear even after heat treatment after welding, adversely affecting the creep rupture properties and toughness of the weld metal after heat treatment after welding. Is known to affect.

  Since the creep rupture properties and toughness of weld metal are generally evaluated using test specimens taken from specific parts of the weld metal, if δ ferrite happens to be included in the part from which the test specimen was taken, Good properties are shown. However, in actual weld metal, if any part of δ ferrite is generated, there is a risk of breakage or breakage. For safety reasons, the formation of δ ferrite is suppressed in all areas of the weld metal. Need to be done.

  For example, Patent Documents 1 to 3 are known as techniques for suppressing the formation of δ ferrite during welding. Patent Documents 1 to 3 all relate to a welding material used during welding.

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

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

  In Patent Document 3, the creep strength of weld metal is improved with an increase in the amount of precipitates of MX (carbonitride), and toughness is largely dependent on the amount of precipitation of δ ferrite and the Ae1 transformation point. Have been described.

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

JP 7-268562 A Japanese Patent Application Laid-Open No. 8-187592 Japanese Patent Laid-Open No. 11-170087 JP 2004-42116 A

  Patent Documents 1 to 4 describe a welding material used at the time of welding and forming a weld metal using the welding material. However, it is unknown whether the formation of δ ferrite is suppressed in the entire area of the weld metal.

  The present invention has been made paying attention to the above-described circumstances, and an object thereof is to provide a coated 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 studied a means capable of achieving a high level of both suppression of δ ferrite formation during welding and improvement of creep rupture properties of the weld metal. The present inventors have found that the object can be achieved by appropriately controlling the above, and have completed the present invention.

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

  In the above-mentioned coated arc welding rod, in one or both of the coating material and the steel core wire, Ti is more than 0% and not more than 0.03% by mass as an alloy component per total mass of the coated arc welding rod. You may contain.

  Further, the above-mentioned coated arc welding rod is, in one or both of the coating material and the steel core wire, as an alloy component per mass of the coated arc welding rod in mass%, Cu: more than 0%, 0.25% or less May further be contained.

  In addition, the above-mentioned coated arc welding rod, in one or both of the coating material and the steel core wire, is mass% as an alloy component per total mass of the coated arc welding rod, Al: more than 0%, 1.5% or less May further be contained.

  According to the coated 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 atoms and Cr atoms in a test piece of a weld metal (where each point represents an atom). FIG. 2 shows a columnar analysis perpendicular to the grain interface and penetrating through the grain interface, which is set when measuring the surface density (B _GB ) of segregation B existing at the matrix grain boundary for a weld metal specimen. It is a figure which shows an area | region. FIG. 3 is a diagram showing a profile when the atomic concentration is measured along the analysis direction in the cylindrical analysis region shown in FIG. FIG. 4 is a schematic diagram showing a sampling position of a test piece used for evaluation of creep rupture characteristics in an example of the present invention. FIG. 5 is a schematic diagram showing the sampling position of the test piece used for toughness evaluation in the examples of the present invention.

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

  The coated arc welding rod of the present invention is a coated arc welding rod formed by coating a coating material on the outer periphery of a steel core wire, and the coated arc welding rod is provided in one or both of the coating material and the steel core wire. As an alloy component per mass, in mass%, C: 0.05 to 0.20%, Si: 0.3 to 1.0%, Mn: 0.1 to 1.1%, Ni: 0% or more 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 to 2.20%, W: 0.4 to 2.0%, N: 0.01 to 0.10%, B: 0.001 to 0.100%, the balance being a gas generating agent, A coated arc welding rod comprising a slag forming agent, an arc stabilizer, iron and inevitable impurities.

B is generally dissolved in carbide particles represented by M ₂₃ C ₆ (hereinafter, also simply referred to as “M ₂₃ C ₆ particles”) to stabilize the M ₂₃ C ₆ particles during the creep rupture test. Is said to improve creep rupture properties by inhibiting dislocation migration.
Therefore, the present inventors examined the stabilization mechanism of M ₂₃ C ₆ particles by B in order to make the most of this effect.

Under a high temperature environment such as a creep rupture test, the total particle number of M ₂₃ C ₆ particles is rapidly reduced by Ostwald growth. Here, Ostwald growth is a phenomenon in which small particles disappear while large particles continue to grow when heat-treated. The inventors have found that Ostwald growth of M ₂₃ C ₆ particles is rate limited by B diffusion.
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, the Ostwald growth of M ₂₃ C ₆ grains is suppressed by reducing B existing in the matrix grain boundaries or grains within a certain value.
As a result, the decrease rate of the M ₂₃ C ₆ particles at a high temperature decreases, and the creep rupture characteristics of the weld metal can be improved.

  In the present invention, based on the above knowledge, in order to allow B to exist in a suitable form in the weld metal obtained by welding, and to suppress the formation of δ ferrite during welding, the alloy of the coated arc welding rod The amount of ingredients was determined as follows. In addition, these alloy components may be added to one of the steel core wire and the coating agent, or may be added to both. Moreover, the amount (%) of each alloy component in the coated arc welding rod in the following is the content (% by mass) per the total mass of the coated arc welding rod.

(C: 0.05-0.20 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, in the present invention, the C content is 0.05% or more. The amount of C is preferably 0.07% or more, more preferably 0.09% or more. However, when C is contained excessively, carbides may become too coarse and the toughness of the weld metal may be lowered. Therefore, in the present invention, the C content is 0.20% or less. The amount of C is preferably 0.18% or less, more preferably 0.16% or less. In addition, when adding C from a coating agent, it can be made to contain and add to another metal raw material.

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

(Mn: 0.1 to 1.1% by mass)
Mn functions as a deoxidizer and improves the strength and toughness of the weld metal. Mn is an element that has 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. The creep rupture properties and toughness of weld metal may be adversely affected. 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 contained excessively, the Ostwald growth of the carbide particles is promoted too much and the creep rupture characteristics of the weld metal deteriorate. Therefore, in the present invention, the Mn content is 1.1% or less. The amount of Mn is preferably 0.8% or less, more preferably 0.7% or less. In addition, Mn can be added by Fe-Mn etc., when adding from a coating agent.

(Ni: 0% by mass or more and 0.5% by mass or less)
Ni is an element that promotes diffusion of alloy elements in the matrix. However, when Ni is added excessively, the diffusion of B is promoted, the Ostwald growth of M ₂₃ C ₆ particles is facilitated, and the creep rupture properties of the weld metal are deteriorated. Therefore, in the present invention, the Ni content is 0.5% or less. The amount of Ni is preferably 0.4% or less, more preferably 0.3% or less. In addition, Ni can be added by Fe-Ni etc., when adding from a coating agent.

(Cr: 3-12% by mass)
Cr is an element that forms M ₂₃ C ₆ particles and improves the creep rupture properties 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 Cr content 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 contained excessively, δ ferrite is likely to be generated during welding, and the generated δ ferrite does not disappear even after heat treatment after welding. It may adversely affect the improvement of creep rupture properties and toughness. Therefore, in the present invention, the Cr content is 12% or less. The amount of Cr is preferably 11% or less, more preferably 10% or less. In addition, when adding from a coating agent, Cr can be added by Fe-Cr etc.

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

(V: 0.02-0.50 mass%)
V is an element that forms MX (carbonitride) to improve the creep rupture properties 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 V amount is set to 0.02% or more in the present invention. The amount of V is preferably 0.05% or more, and more preferably 0.10% or more. However, if V is contained excessively, δ ferrite is generated during welding. Further, it causes Ostwald growth of MX (carbonitride) at a high temperature. As a result, the creep rupture characteristics and toughness of the weld metal deteriorate. Therefore, in the present invention, the V amount is 0.50% or less. The amount of V is preferably 0.40% or less, more preferably 0.30% or less. V can be added to other metal raw materials when added from a coating agent.

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

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

(W: 0.4-2.0 mass%)
W, like Mo, improves the creep rupture properties of the weld metal as a solid solution strengthening element, and also precipitates at the grain boundary as a Laves phase at a high temperature to prevent B diffusion at the grain boundary, thereby preventing M ₂₃ C _6. The effect of suppressing the Ostwald growth of particles is expected. In order to exert such effects, 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 contained excessively, δ ferrite is likely to be generated during welding, and the generated δ ferrite does not disappear even if heat treatment is performed after welding. It may adversely affect the improvement of creep rupture properties and toughness. Therefore, in the present invention, the W amount is set to 2.0% or less. The amount of W is preferably 1.8% or less, more preferably 1.6% or less. In addition, W can be added by Fe-W etc., when adding from a coating agent.

(N: 0.01-0.10% by mass)
N, like Nb, is an element that forms MX (carbonitride) to improve the creep rupture properties of the weld metal. In order to exert such an effect, in the present invention, the N amount is set to 0.010% or more. The N amount is preferably 0.015% or more, more preferably 0.020% or more. However, when N is contained excessively, the strength of the weld metal is excessively increased and the toughness is deteriorated. Therefore, in the present invention, the N amount is 0.10% or less. The N amount is preferably 0.09% or less, more preferably 0.08% or less. In addition, when adding N from a coating agent, it can be made to contain and add to another metal raw material.

(B: 0.001 to 0.100 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 B amount 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, and still more preferably 0.011% or more. However, when B is added excessively, the strength of the weld metal is excessively increased, and a predetermined toughness cannot be ensured. Therefore, in the present invention, the B amount is 0.100% or less. The amount of B is preferably 0.090% or less, more preferably 0.080% or less. In addition, when adding B from a coating agent, it can be made to contain and add to another metal raw material.

Moreover, the coated arc welding rod of this invention may contain at least 1 sort (s) chosen from the group which consists of following (a)-(c) as another element in addition to the said alloy element.
(A) Ti: more than 0% and 0.03% or less.
(B) Cu: more than 0% and 0.25% or less.
(C) Al: more than 0% and 1.5% or less.

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

  Cu is not an essential element, but is an element having an action of suppressing the formation of δ ferrite during welding. In order to effectively exhibit such an effect, the amount of Cu is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.025% or more. However, when Cu is excessively contained, the formation of a long and narrow ferrite structure (sometimes called a ferrite band) is promoted, and the creep rupture characteristics and toughness of the weld metal may be deteriorated. Therefore, in this invention, it is preferable that the amount of Cu shall be 0.25% or less. The amount of Cu is more preferably 0.20% or less, and still more preferably 0.15% or less.

  Al is not an essential element, but is an element that acts as a deoxidizer. In order to exhibit such an effect effectively, the amount of Al is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.025% or more. However, if Al is contained excessively, a coarse oxide is generated, which may become a starting point of brittle fracture, and the toughness of the weld metal may be lowered. Therefore, in the present invention, the Al content is preferably 1.5% or less. The amount of Al is preferably 1.3% or less, more preferably 1.1% or less.

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

The content of the metal carbonate per total mass of the coated 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 total mass of the coated 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 total mass of the coated arc welding rod of the present invention is not particularly limited, but is, for example, 5 to 12% by mass.
Moreover, although content of the alkali metal oxide per total mass of the covering arc welding rod of this invention is not specifically limited, For example, it is 4-12 mass%.

  The ratio of the coating agent in the coated 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 coated arc welding rod.

The coated arc welding rod of the present invention can be manufactured, for example, as follows.
First, the coating material is coated with a normal welding rod coating machine around a steel core wire with a binder such as water glass represented by sodium silicate and potassium silicate. Then, in order to remove a water | moisture content, it bakes at 400-550 degreeC, for example.

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

(Solid solution B concentration present in matrix particles of weld metal: 0.0008 mass% or less)
By controlling the solid solution B concentration (B _M ) present in the matrix grains of the weld metal to 0.0008 mass% or less, the supply of B contributing to the coarsening of M ₂₃ C ₆ particles at high temperatures is suppressed. The Ostwald growth of M ₂₃ C ₆ particles is suppressed, and the creep rupture time is prolonged, which is preferable. B _M is more preferably 0.0005% by mass or less, and still more preferably 0.0003% by mass or less.

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

(O content in weld metal: 0.080% or less)
O is an element that forms an oxide. If O is contained excessively, the oxide becomes coarse, and the toughness is lowered by becoming the starting point of brittle fracture. Therefore, the amount of O in the weld metal is 0.080% or less. Preferably there is. The amount of O is more preferably 0.075% or less, and still more preferably 0.070% or less. The lower the amount of O, the more improved the toughness, but usually 0.030 to 0.060% is contained.

Moreover, it is preferable that the said weld metal satisfies the relationship of 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 is, B dissolves in the M ₂₃ C ₆ particles to suppress the Ostwald growth of the particles. However, when the amount of B generated as BN increases, the amount of B dissolved in the M ₂₃ C ₆ particles cannot be secured sufficiently. N is first generated as a carbonitride containing V, and the remaining N forms BN. Therefore, in order to ensure the amount of B dissolved in the M ₂₃ C ₆ particles, it is necessary to add a predetermined amount of B and to sufficiently precipitate 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 B dissolved in the M ₂₃ C ₆ particles decreases. As a result, the Ostwald growth of the particles cannot be sufficiently suppressed. X is more preferably 0.44 or more, and further preferably 0.45 or more.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples, and may be implemented with modifications within a range that can be adapted to the gist of the present invention. All of which are within the scope of the present invention.

Using the base material consisting of iron and inevitable impurities, the balance satisfying the component composition shown in Table 1 below, and using the coated arc welding rod satisfying the component composition shown in Table 2 below, under the welding conditions described below, A weld metal satisfying the component composition shown in Table 3 with the balance being iron and inevitable impurities was produced, and various properties were evaluated.
Moreover, "-" in each component amount in Table 3 represents that the content is an impurity level.

<Welding conditions>
Welding method: Covered arc welding (SMAW)
Base material plate thickness: 20 mm
Groove angle: 20 ° (V-shaped)
Route interval: 16mm
Welding posture: downward Welding rod diameter: φ4.0mm
Heat input condition: about 2.2 kJ / mm (150A-24V, 8-12 cm / min)
Lamination method: 1 layer, 2 passes Preheating, temperature between passes: 200-400 ° C

(With or without δ ferrite)
A test piece was taken from the obtained weld metal as-welded so that a cross section perpendicular to the welding direction could be observed. The tissue was observed. When no δ ferrite was observed in the entire cross section, it was judged as “no δ ferrite” and evaluated as acceptable. A case where even one δ ferrite was observed in the entire cross section was judged as “having δ ferrite” and evaluated as rejected. The determination results are shown in Table 4 below.

  Next, the weld metal obtained by welding was subjected to post-weld heat treatment (PWHT) under the conditions of holding temperature: 700 to 800 ° C. and holding time: 2 to 10 h, and the characteristics of the weld metal were evaluated. The results are shown in Table 4 below.

(Solution B concentration present in matrix grains and surface density of segregation B present in matrix grain boundaries)
A specimen was taken from the weld metal after the heat treatment after welding so that a surface perpendicular to the welding direction could be observed, the cross section of the specimen was corroded with ferric chloride etching solution, and the metal structure was magnified 400 times with an optical microscope. Observed. Using a focused ion beam apparatus (Helios-600 manufactured by FEI), a needle-like test piece including a grain boundary and a needle-like test piece not containing a grain boundary (only within a grain) are produced, and a three-dimensional atom probe The measurement was performed under the conditions of a measurement temperature of 50K and a pulse fraction of 15% using (LEAP 3000HR manufactured by CAMECA). The obtained data was analyzed by analysis software IVAS. More specifically, measurement and analysis were performed as follows.

For the solid solution B concentration (B _M ) existing in the matrix grain, the measurement data of the test piece not containing the grain boundary (only in the grain) was used. That is, a region where carbon was uniformly distributed was determined to be within the matrix grain, and after removing background noise from the total number of atoms detected, the B concentration was determined. The concentration measured with an atom probe is several percent of atoms, but was converted to mass percent for evaluation. The results are shown in Table 4 below.

For the surface density (B _GB ) 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. 1-3, the method for measuring the surface density ( _BGB ) of the segregation B which exists in the matrix grain boundary about the test piece of a certain weld metal is demonstrated. First, from the atomic map as shown in FIG. 1, a planar region having a high atomic concentration of alloy elements such as C and Cr was determined as the grain interface. Then, as shown in FIG. 2, a cylindrical analysis region (hereinafter, also simply referred to as “column”) having a bottom surface radius of 5 nm is set so as to penetrate the grain interface perpendicularly to the grain interface. The atomic concentration contained in the region of every 0.5 nm in the major axis direction was measured from one end of the substrate, and the profile shown in FIG. 3 was created. Subsequently, the surface density of the segregated B was calculated by dividing the number of B atoms contained in a region having a high alloying element concentration such as C or Cr by the bottom area (circle area) of the cylinder. The results are shown in Table 4 below.

  Next, creep rupture characteristics and toughness were evaluated as characteristics of the weld metal subjected to post-weld heat treatment.

<Creep rupture properties>
A creep test piece having a gauge distance of 30 mm and a diameter of 6.0 mm was taken in the weld line direction based on FIG. A creep test was performed, and the time until the test piece broke was measured. In FIG. 4, T represents the thickness of the base material. The case where the rupture time Tr exceeded 400 hours was judged to be excellent in creep rupture characteristics, and was evaluated as acceptable.

<Toughness>
A Charpy impact test piece perpendicular to the weld line direction was sampled from the central portion of the thickness of the weld metal subjected to heat treatment after welding, and a Charpy impact test was performed. In FIG. 5, T indicates the thickness of the base material. As the Charpy impact test piece, a No. 4 V-notch test piece specified in JIS Z3111 was collected. The Charpy impact test was performed at 20 ° C. in accordance with 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 4 below. A weld metal having an average value (vE _{20 ° C.} ) of 41 J or more was 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 V amount in the coated 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 coated arc welding rod was as high as 0.62%, the obtained weld metal was inferior in creep rupture characteristics.
In Example 16, since the Si content in the coated arc welding rod was as low as 0.22% and the Mn content was as low as 0.05%, the obtained weld metal was inferior in toughness.
In Example 17, the Si content in the coated arc welding rod was as high as 1.10%, the Nb content was as high as 0.092%, and the B content was as high as 0.110%. It was inferior.
In Example 18, since the amount of Cr in the coated arc welding rod was as high as 13.67%, the obtained weld metal produced δ ferrite and was inferior in creep rupture characteristics and toughness.
In Example 19, since the W amount in the coated arc welding rod was as high as 2.1% and did not contain B, δ ferrite was generated in the obtained weld metal, and the creep rupture characteristics and toughness were inferior. It was.
In Example 20, the Co content in the coated arc welding rod was as low as 0.49%, and B was not contained. Therefore, in the obtained weld metal, δ ferrite was generated and the creep rupture characteristics were inferior.

  On the other hand, in Examples 1 to 13 in which the coated arc welding rod satisfied 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 (4)

A coated arc welding rod formed by coating a coating on the outer periphery of a steel core wire,
In one or both of the coating agent and the steel core wire, the total mass of the coated arc welding rod as an alloy component, in mass%,
C: 0.05 to 0.20%,
Si: 0.3 to 1.0%,
Mn: 0.1 to 1.1%
Ni: 0% or more, 0.5% or less,
Cr: 3-12%,
Mo: 0.05 to 0.55%,
V: 0.02 to 0.50%,
Nb: 0.01 to 0.09%,
Co: 0.55 to 2.20%
W: 0.4-2.0%,
N: 0.01-0.10%
B: 0.001 to 0.100% is contained,
A coated arc welding rod with the balance being a gas generating agent, a slag forming agent, an arc stabilizer, iron and inevitable impurities. In one or both of the coating agent and the steel core wire, the total mass of the coated arc welding rod as an alloy component, in mass%,
The coated arc welding rod according to claim 1, further comprising Ti: more than 0% and 0.03% or less. In one or both of the coating agent and the steel core wire, the total mass of the coated arc welding rod as an alloy component, in mass%,
The coated arc welding rod according to claim 1 or 2, further comprising Cu: more than 0% and not more than 0.25%. In one or both of the coating agent and the steel core wire, the total mass of the coated arc welding rod as an alloy component, in mass%,
The covered arc welding rod according to any one of claims 1 to 3, further comprising Al: more than 0% and 1.5% or less.

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