JP7704660B2 - Shielded metal arc welding rod, shielded metal arc welding method, and method for manufacturing welded joint - Google Patents

Description

The present invention relates to a covered metal arc welding rod (hereinafter also simply referred to as a "welding rod") used for welding high tensile steel, a weld metal, a covered metal arc welding method, and a method for manufacturing a welded joint.

Welded products such as marine structures and tanks used in the drilling and production of oil, gas, etc., require larger equipment and operation in cold regions, and the steel plates and welding materials used in their manufacture are required to have high strength and excellent low-temperature toughness. In addition to these characteristics, post-weld heat treatment (PWHT) may be performed to further improve the quality of welded products. By performing PWHT, for example, residual stress in the weld can be removed, which has the effect of suppressing cracks in the weld.

However, when this PWHT is performed, there is a risk of a decrease in toughness due to embrittlement of the welded part caused by precipitation hardening, temper embrittlement, etc. In particular, in the welded part of high tensile steel, embrittlement during PWHT is significant due to the influence of various elements added to improve strength, and the higher the strength, the more likely it is that PWHT cannot be applied.
Therefore, there is a need to develop a welding material for high tensile steel that has excellent mechanical properties even after PWHT.

For example, Patent Document 1 proposes a covered metal arc welding rod that can provide a weld metal excellent in low-temperature toughness and fracture toughness after stress relief annealing when welding high-tensile steel, for example, ^high -tensile steel of 590 N/mm 2 class or higher.
Furthermore, Patent Document 2 proposes a low-hydrogen covered metal arc welding electrode for 590 MPa-class high-tensile steel, which has good welding workability and excellent strength and toughness at low temperatures of the weld metal as-welded (AW) and after PWHT.

Japanese Patent Application Publication No. 8-257791 JP 2017-64740 A

However, the above-mentioned Patent Documents 1 and 2 consider only one type of PWHT condition, and it is not necessarily true that excellent toughness can be obtained under other PWHT conditions.
In addition, it can be said that a larger tolerance in the PWHT conditions makes it possible to apply the PWHT to a variety of welded products, simplifies construction management, and further improves the quality stability of the welds.
Furthermore, since neither Patent Document 1 nor Patent Document 2 takes hot cracking into consideration, there is an even greater demand for a welding rod capable of obtaining a weld metal having good hot cracking resistance.

On the other hand, it has been known that PWHT may reduce toughness depending on the conditions, and that the PWHT conditions are strictly determined by the type of material, thickness, welded joints, construction conditions, etc.
Therefore, it is very difficult to provide a tolerance for the PWHT conditions. In particular, the higher the strength, the more additive elements are contained, making it more likely that embrittlement phenomena such as temper embrittlement will occur, and therefore the PWHT conditions become more stringent.

The present invention has been made in consideration of these problems, and aims to provide a covered metal arc welding rod for high tensile steel, which is capable of obtaining a weld with excellent low-temperature toughness and suppressed hot cracking while maintaining the desired strength not only in the as-welded state (hereinafter also referred to as "as-welded") but also under a wide range of PWHT conditions, a covered metal arc welding method, and a method for manufacturing a welded joint. It is also an object of the present invention to provide a weld metal that is excellent in low-temperature toughness and suppressed hot cracking while maintaining the desired strength, even under a wide range of PWHT conditions.

The above object of the present invention is achieved by the following configuration [1] relating to a covered electrode.

[1] A covered metal arc welding rod having a core wire and a coating material that covers the core wire,
The coating agent contains, based on the total mass of the coating agent,
_CO2 : 16% by mass or more and 27% by mass or less,
F: 4% by mass or more and 10% by mass or less,
Si: 3% by mass or more and 11% by mass or less,
Ni: 7.5% by mass or more and 13.3% by mass or less,
Fe: 1% by mass or more and 11% by mass or less,
Mo: 0.3% by mass or more and 1.0% by mass or less,
Cr: 0.15% by mass or more and 1.20% by mass or less,
Mg: 1.5% by mass or more and 4.5% by mass or less,
Mn: 1.5% by mass or more and 4.0% by mass or less;
The content of the Ni in the coating material is represented as [Ni] in mass% based on the total mass of the coating material;
The content of Mn in the coating material is expressed as [Mn] in mass% based on the total mass of the coating material,
When the content of Si in the coating agent is expressed as [Si] in mass% relative to the total mass of the coating agent,
A shielded metal arc welding rod, characterized in that a value calculated by [Ni]/([Si]+[Mn]) is 0.85 or more and 1.45 or less.

Preferred embodiments of the present invention relating to the covered electrode include the following [2] to [4].

[2] The coating agent further comprises, based on the total mass of the coating agent,
CaO: 20% by mass or more and 40% by mass or less,
BaO: 2% by mass or more and 6% by mass or less,
The covered metal arc welding rod according to [1], characterized in that it contains

[3] The coating agent further comprises, based on the total mass of the coating agent,
Total amount of Na, K and Li: 0.3 mass% or more and 4.0 mass% or less,
Ti: 0.5% by mass or more and 4.0% by mass or less;
Al: 1.5% by mass or less,
The covered metal arc welding rod according to [1] or [2], characterized in that Zr: 0.8 mass% or less.

[4] The coating agent contains, based on the total mass of the coating agent,
Nb: 0.03% by mass or less,
The covered metal arc welding rod according to any one of [1] to [3], characterized in that V is restricted to 0.03 mass% or less.

The above object of the present invention is achieved by the following configuration [5] relating to the weld metal.

[5] A weld metal obtained by performing shielded metal arc welding using a shielded metal arc welding electrode according to any one of [1] to [4].

The above object of the present invention is achieved by the following configuration [6] relating to the shielded metal arc welding method.

[6] A method of shielded metal arc welding, characterized by performing shielded metal arc welding using a shielded metal arc welding rod according to any one of [1] to [4].

The above object of the present invention is achieved by the following configuration [7] relating to the manufacturing method of a welded joint.

[7] A method for manufacturing a welded joint, characterized in that the base material is high-tensile steel and the welded joint is welded using a covered metal arc welding rod according to any one of [1] to [4].

According to the present invention, it is possible to provide a covered metal arc welding rod for high tensile steel, which can obtain a weld that has excellent low-temperature toughness and can suppress hot cracking while maintaining a desired strength not only in as-welded but also in a wide range of PWHT conditions.
Moreover, according to the present invention, it is possible to provide a weld metal that has excellent low-temperature toughness and is capable of suppressing hot cracking while maintaining a desired strength even under a wide range of PWHT conditions.
Furthermore, according to the present invention, it is possible to provide a shielded metal arc welding method and a manufacturing method for a welded joint, which can obtain a weld that has excellent low-temperature toughness and is capable of suppressing hot cracking while maintaining a desired strength even under a wide range of PWHT conditions.

The inventors have investigated the mechanism of toughness reduction caused by PWHT in high-strength welds, and have conducted extensive research to obtain welds with excellent low-temperature toughness even when PWHT is performed under a wide range of conditions. As a result, they have discovered the following and completed the present invention. Note that the above weld refers to the weld metal and heat-affected zone (HAZ), but in this specification, we will refer to the weld metal hereafter.

First, a conventional mechanism of toughness degradation in high strength steel will be described.
Conventionally, the following two factors have been considered as the main causes of the decrease in toughness of the weld metal after PWHT.

(Cause 1) When the weld metal contains a large amount of Cr, Mo, etc., these elements combine with C to form carbides which precipitate, hardening the weld metal.
(Cause 2) Embrittlement phenomena such as temper embrittlement occur due to slow cooling from the PWHT temperature.

It has been conventionally believed that the above-mentioned (Cause 1) and (Cause 2) mainly cause a decrease in grain boundary strength of the weld metal, resulting in a decrease in toughness.
Therefore, in the past, in order to ensure excellent toughness while maintaining the desired strength after PWHT, the following measures have been taken.

(Countermeasure 1) Suppress the precipitation and growth of carbides at prior austenite grain boundaries.
(Countermeasure 2) Suppress the segregation of impurity elements at prior austenite grain boundaries.

However, depending on the PWHT conditions, simply implementing (Measure 1) or (Measure 2) above may not be sufficient. Generally, PWHT conditions consist of the elements of holding temperature and holding time, and can be organized using the Larson-Miller parameters (hereinafter referred to as "LMP"), which use these elements as parameters.

Therefore, in this specification, covered electrodes that can produce weld metal with excellent low-temperature toughness while maintaining the desired strength under the three conditions a to c shown below are considered to have a wide range of PWHT conditions.

(Condition a) As-welded.
(Condition b) A PWHT condition with high LMP, which is a condition at a temperature of 620° C. for 8 hours (hereinafter referred to as “high LMP condition”).
(Condition c) A PWHT condition with low LMP, which is a condition at a temperature of 580° C. for 2 hours (hereinafter referred to as “low LMP condition”).

It is known that in the as-welded condition (Condition a) and in the high LMP condition (Condition b), good low-temperature toughness can be obtained by adding an appropriate amount of Ni to the welding rod in addition to the above-mentioned (Measures 1) and (Measures 2).
On the other hand, under the low LMP condition of (Condition c), good low temperature toughness cannot be obtained simply by adding an appropriate amount of Ni. The present inventors have found a mechanism by which low temperature toughness decreases when PWHT is performed under the low LMP condition of (Condition c), and have also found a measure by which a weld metal with excellent low temperature toughness can be obtained while maintaining the target strength even under a wide range of PWHT conditions. The mechanism by which low temperature toughness decreases under the low LMP condition of (Condition c) and the measure will be described below.

When the weld metal contains a large amount of Ni, a segregation zone containing Ni (hereinafter referred to as "Ni segregation zone"), i.e., a region where Ni is concentrated, is formed on the weld metal structure. This Ni segregation zone does not affect the low temperature toughness in the as-welded condition (condition a) or under the high LMP condition (condition b).
However, under the low LMP condition of (Condition c), the C content in the Ni segregation zone becomes high, and the formation of island martensite and the formation and coarsening of carbides in the Ni segregation zone are promoted. In particular, the formation of island martensite is remarkable, and due to the influence of these products, brittle fracture is likely to occur in the Ni segregation zone, resulting in a decrease in low temperature toughness.

Based on these findings, the inventors have discovered that, in addition to (Measure 1) and (Measure 2), implementing (Measure 3) below is an effective measure for ensuring a weld metal with excellent low-temperature toughness while maintaining the desired strength after PWHT.

(Countermeasure 1) Suppress the precipitation and growth of carbides at prior austenite grain boundaries.
(Countermeasure 2) Suppress the segregation of impurity elements at prior austenite grain boundaries.
(Measure 3) Suppress the formation of island martensite in the Ni segregation zone.

The inventors have discovered that by appropriately controlling the parameters calculated from the chemical composition of the covered electrode and the contents of Ni, Si, and Mn, it is possible to achieve the above (Measures 1) to (Measures 3) and suppress hot cracking. In other words, by using a welding electrode that realizes (Measures 1) to (Measures 3), it is possible to obtain a weld metal with excellent low-temperature toughness while maintaining the desired strength, not only in as-welded but also after PWHT under a wide range of conditions.

The following provides a detailed description of the embodiment of the present invention (hereinafter, referred to as the "present embodiment"). Note that the present invention is not limited to the embodiment described below, and can be modified as desired without departing from the gist of the present invention.

[1. Shielded metal arc welding electrodes]
The covered metal arc welding rod according to this embodiment is formed by covering a steel core wire (hereinafter, also simply referred to as a "core wire") with a coating material.

<1-1. Coverage rate>
The coverage can be set to any value as long as the content of each element in the coating falls within the range of the present invention. The coverage can be calculated by the formula: {[Coating]/([Coating]+[Core])}×100, where the mass (g) of the coating in the total mass of the welding rod is [Core], and the mass (g) of the core wire in the total mass of the welding rod is [Core]. In this embodiment, the coverage calculated by the above formula is preferably 25% by mass or more and 40% by mass or less.

<1-2. Coating agent>
The chemical composition of the coating material of the covered electrode according to the present embodiment and the reasons for limiting the numerical values of the contents thereof will be described in more detail below. The content in the present embodiment means mass% with respect to the total mass of the coating material unless otherwise specified.
In addition, unless otherwise specified, each of the elements shown below may be contained in the coating agent in the form of a metal, in the form of a compound, or in the form of both a metal and a compound. That is, regardless of the form in which each of the elements is contained in the coating agent, it is specified by the converted value of the element alone. For example, in the case of Si, the Si content refers to the sum of the Si converted values of metal Si and Si compounds. Metal Si includes both simple Si and Si alloys.

( _CO2 : 16-27% by mass)
In this embodiment, the carbonate content in the coating agent is defined as the _CO2 content. Carbonates decompose into _CO2 and oxides during welding, and have the effect of preventing oxidation and nitridation of the weld metal. If the _CO2 content in the coating agent is less than 16 mass%, sufficient gas is not generated during welding, which leads to nitridation and oxidation of the weld metal and deterioration of low-temperature toughness. Therefore, the _CO2 content in the coating agent is set to 16 mass% or more, preferably 17 mass% or more, and more preferably 18 mass% or more, based on the total mass of the coating agent.
On the other hand, if the _CO2 content in the coating agent exceeds 27% by mass, the _CO2 source carbonate is contained in a large amount, so that the fluidity of the molten slag increases, making it difficult to form a uniform slag with good encapsulation properties, and the slag removability deteriorates. Therefore, the _CO2 content in the coating agent is set to 27% by mass or less, preferably 24% by mass or less, and more preferably 21% by mass or less, based on the total mass of the coating agent.
In addition, examples of the _CO2 source in _the coating agent include carbonates such as _CaCO3 , _BaCO3 , MgCO3, _MnCO3 , _{FeCO3 , Na2CO3} , _{and K2CO3} .

(F: 4% by mass or more and 10% by mass or less)
Metal fluorides such as _CaF2 , _MgF2 , and _AlF3 have the effect of lowering the melting point of the molten slag, improving the slag encapsulation, and improving the bead appearance. In addition, F reacts with hydrogen during welding and can reduce the hydrogen partial pressure in the weld metal, so it also has the effect of reducing the hydrogen content of the weld metal. If the F content in the coating is less than 4 mass%, the above-mentioned effect cannot be fully obtained. Therefore, the F content in the coating is set to 4 mass% or more, preferably 5 mass% or more, and more preferably 6 mass% or more, based on the total mass of the coating.
On the other hand, if the F content in the coating agent exceeds 10 mass%, the arc becomes unstable and the amount of spatter generation increases. Therefore, the F content in the coating agent is set to 10 mass% or less, preferably 9 mass% or less, and more preferably 8 mass% or less, based on the total mass of the coating agent.

(Si: 3% by mass or more and 11% by mass or less)
Metallic Si and Si contained in alloys such as ferrosilicon have the effect of increasing the viscosity of the molten metal, adjusting the fluidity, and improving the bead appearance and bead shape.
Oxides such as _SiO2 act as slag forming agents. In addition, oxides such as _SiO2 have the effect of increasing the viscosity of the molten slag to improve its fluidity and improve the bead appearance and shape. As described above, the metal Si, Si alloy, and Si oxide in the coating agent each have various effects, so in this embodiment, the total Si content contained in the metal Si, Si alloy, and Si compound in the coating agent is specified.

If the Si content in the coating material is less than 3 mass%, the viscosity of the molten metal and slag decreases, making it difficult to form a bead during welding in the vertical position, and a good bead cannot be obtained. Therefore, the Si content in the coating material is set to 3 mass% or more, preferably 4 mass% or more, and more preferably 5 mass% or more, based on the total mass of the coating material.
On the other hand, if the Si content in the coating exceeds 11 mass%, hard island martensite is formed in the Ni-containing segregation zone, which promotes temper embrittlement and reduces low-temperature toughness. Therefore, the Si content in the coating is set to 11 mass% or less, preferably 9 mass% or less, and more preferably 8 mass% or less, based on the total mass of the coating.
Examples of the Si source in the coating agent include oxides of Si such as _SiO2 , silicates of Si, metal Si, alloys such as ferrosilicon, and adhesives such as water glass.

(Ni: 7.5% by mass or more and 13.3% by mass or less)
Ni is a component that has the effect of improving the strength and low-temperature toughness of the weld metal by strengthening the matrix. If the Ni content in the coating material is less than 7.5 mass%, the desired tensile strength and low-temperature toughness cannot be obtained. Therefore, the Ni content in the coating material is set to 7.5 mass% or more, preferably 8.0 mass% or more, more preferably 8.5 mass% or more, and even more preferably 9.0 mass% or more, based on the total mass of the coating material.
On the other hand, if the Ni content in the coating exceeds 13.3 mass%, low melting point impurity elements are concentrated in the segregation zone, increasing the risk of hot cracking. Therefore, the Ni content relative to the total mass of the coating is set to 13.3 mass% or less, preferably 12.0 mass% or less, and more preferably 11.0 mass% or less.

(Fe: 1% by mass or more and 11% by mass or less)
Fe is a component that affects the deposition efficiency and welding workability. If the Fe content in the coating is less than 1 mass%, the welding efficiency decreases, the arc flutters, and the welding workability decreases. Therefore, the Fe content in the coating is set to 1 mass% or more, preferably 3 mass% or more, and more preferably 5 mass% or more, based on the total mass of the coating.
On the other hand, if the Fe content in the coating material exceeds 11 mass%, the shielding effect decreases and the welding workability decreases. Therefore, the Fe content in the coating material is set to 11 mass% or less, preferably 9 mass% or less, and more preferably 8 mass% or less, based on the total mass of the coating material.

(Mo: 0.3% by mass or more and 1.0% by mass or less)
Mo is a component that improves the strength of the weld metal and is effective in suppressing temper embrittlement. Mo carbides precipitate within the grains of the weld metal, thereby suppressing the precipitation of cementite at grain boundaries and suppressing a decrease in low-temperature toughness after PWHT.
If the Mo content in the coating material is less than 0.3 mass%, it is not possible to suppress the precipitation of cementite at grain boundaries by PWHT, and it is not possible to obtain the desired low-temperature toughness. Therefore, the Mo content in the coating material is set to 0.3 mass% or more, preferably 0.5 mass% or more, and more preferably 0.6 mass% or more, based on the total mass of the coating material.
On the other hand, if the Mo content exceeds 1.0 mass%, the low-temperature toughness in AW decreases, and in PWHT, _MoC2 is excessively precipitated in the grains of the weld metal, decreasing the low-temperature toughness. Therefore, the Mo content in the coating material is set to 1.0 mass% or less, preferably 0.9 mass% or less, and more preferably 0.8 mass% or less, based on the total mass of the coating material.

(Cr: 0.15% by mass or more and 1.20% by mass or less)
Cr is a component that improves the strength of the weld metal and has the effect of suppressing the coarse structure that precipitates at the grain boundaries. If the Cr content in the coating material is less than 0.15 mass%, the coarse structure that precipitates at the grain boundaries cannot be suppressed, and the desired tensile strength and low-temperature toughness after PWHT cannot be obtained. Therefore, the Cr content in the coating material is set to 0.15 mass% or more, preferably 0.30 mass% or more, and more preferably 0.40 mass% or more, based on the total mass of the coating material.
On the other hand, Cr is a component that promotes the precipitation and growth of coarse grain boundary carbides by PWHT, thereby reducing low-temperature toughness. If the Cr content in the coating exceeds 1.20 mass%, the low-temperature toughness after PWHT decreases. Therefore, the Cr content in the coating is set to 1.20 mass% or less, preferably 1.10 mass% or less, more preferably 1.00 mass% or less, and even more preferably 0.80 mass% or less, based on the total mass of the coating.

(Mg: 1.5% by mass or more and 4.5% by mass or less)
Mg is a component that reduces the amount of oxides in the weld metal through its deoxidizing effect and improves low-temperature toughness. If the Mg content in the coating material is less than 1.5 mass%, the desired deoxidizing effect cannot be obtained. Therefore, the Mg content in the coating material is set to 1.5 mass% or more, preferably 2.0 mass% or more, and more preferably 2.2 mass% or more, based on the total mass of the coating material.
On the other hand, Mg has the effect of reducing the arc force during welding. If the Mg content in the coating exceeds 4.5 mass%, the arc becomes unstable and the bead shape becomes poor. Therefore, the Mg content in the coating is set to 4.5 mass% or less, preferably 3.8 mass% or less, and more preferably 3.0 mass% or less, based on the total mass of the coating.

(Mn: 1.5% by mass or more and 4.0% by mass or less)
Mn is a component that has the effect of improving the strength of the weld metal. If the Mn content in the coating material is less than 1.5 mass%, the desired strength cannot be obtained. Therefore, the Mn content in the coating material is set to 1.5 mass% or more, preferably 2.0 mass% or more, and more preferably 2.2 mass% or more, based on the total mass of the coating material.
On the other hand, Mn, in particular, promotes temper embrittlement by forming hard island martensite in the Ni-containing segregation zone, thereby reducing low-temperature toughness and is also a component that causes hot cracking. If the Mn content in the coating exceeds 4.0 mass%, not only does the low-temperature toughness after PWHT decrease, but the concern about hot cracking increases. Therefore, the Mn content in the coating is set to 4.0 mass% or less, preferably 3.5 mass% or less, and more preferably 3.0 mass% or less, based on the total mass of the coating.
_{The Mn source in the coating agent may be an oxide such as MnO, MnO2, Mn3O4, or Mn2O3 , a sulfide} of Mn, a carbonate of Mn, metallic Mn, or an alloy such as ferromanganese.

(Value calculated by [Ni]/([Si]+[Mn]): 0.85 or more and 1.45 or less)
As described above, by appropriately controlling the value of this parameter calculated from the contents of Ni, Si, and Mn, it is possible to suppress the generation of island martensite in the Ni segregation band, which is the above-mentioned (Measure 3), while maintaining a target strength, and to obtain a weld metal that has excellent low-temperature toughness and is suppressed from hot cracking.
If the value obtained by [Ni]/([Mn]+[Si]) is less than 0.85, island martensite is likely to be generated in the Ni segregation band, and the low-temperature toughness after PWHT is reduced. Therefore, the value calculated by [Ni]/([Mn]+[Si]) is set to 0.85 or more, preferably 0.90 or more, more preferably 0.95 or more, and even more preferably 1.00 or more.
On the other hand, if the value obtained by [Ni]/([Mn]+[Si]) exceeds 1.45, the Ni content becomes high, and thus the risk of hot cracking increases. Therefore, the value calculated by [Ni]/([Mn]+[Si]) is set to 1.45 or less, preferably 1.30 or less, more preferably 1.20 or less, even more preferably 1.10 or less, and particularly preferably 1.07 or less.

In the above formula, [Ni] is the Ni content in the coating agent expressed as a mass % relative to the total mass of the coating agent, [Mn] is the Mn content in the coating agent expressed as a mass % relative to the total mass of the coating agent, and [Si] is the Si content in the coating agent expressed as a mass % relative to the total mass of the coating agent.

In the covered metal arc welding electrode according to the present embodiment, the above-mentioned essential components are contained in the coating material within a predetermined content range, so that it is possible to obtain a weld metal that has excellent low-temperature toughness and suppressed hot cracking while maintaining a desired strength not only in AW but also in a wide range of PWHT conditions.
The coating of the covered electrode according to the present embodiment preferably contains _CaCO3 and _BaCO3 as carbonates serving as a _CO2 source. This is because CaO, which is a decomposition product of _CaCO3 , and BaO, which is a decomposition product of _BaCO3 , have the effect of forming good slag and obtaining an excellent bead shape. The preferred contents of CaO and BaO are described below.

(CaO: 20% by mass or more and 40% by mass or less)
CaO is a slag-forming compound, which enables the formation of uniform and well-encapsulated slag and improves the slag removability. CaO also has the effect of ensuring the insulation of the coating. If the CaO content in the coating is 20% by mass or more, sufficient slag is formed and beads with a good shape can be obtained. Therefore, the CaO content in the coating is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 28% by mass, based on the total mass of the coating.
In addition, when the CaO content in the coating agent is 40% by mass or less, the fluidity of the molten slag can be maintained good, so that a uniform slag with good encapsulation can be formed, and a good bead shape can be obtained. In addition, the arc can be prevented from becoming strong, and the amount of spatter generated can be appropriately adjusted. Therefore, the CaO content in the coating agent is preferably 40% by mass or less, more preferably 38% by mass or less, and even more preferably 36% by mass or less, based on the total mass of the coating agent.

The CaO source in the coating material may be CaO, a carbonate of Ca which is thermally decomposed during welding to produce CaO in the coating material, or a silicate of Ca.
In this specification, the CaO content is a value obtained by converting all Ca contained in the coating agent into CaO.

(BaO: 2% by mass or more and 6% by mass or less)
BaO is a main slag-forming compound and plays a role in adjusting the basicity of the slag. When the BaO content in the coating material is 2 mass% or more, it is possible to prevent the oxygen content in the weld metal from increasing, and to suppress the deterioration of low-temperature toughness. Therefore, the BaO content in the coating material is preferably 2 mass% or more, more preferably 3 mass% or more, based on the total mass of the coating material.
In addition, if the BaO content in the coating agent is 6% by mass or less, the fluidity of the molten slag can be kept good and a good bead can be formed. Therefore, the BaO content in the coating agent is preferably 6% by mass or less, more preferably 5% by mass or less, based on the total mass of the coating agent.

The source of BaO in the coating material may be BaO, a carbonate of Ba which is thermally decomposed during welding to produce BaO in the coating material, or a silicate of Ba.
In this specification, the BaO content is a value obtained by converting all Ba contained in the coating agent into BaO.

In the covered metal arc welding electrode according to this embodiment, the coating may further contain Na, K, Li, Ti, Al and Zr in the ranges shown below in order to improve the arc stability, the mechanical properties of the weld metal, the bead shape, etc. Furthermore, when Nb and V are contained in the coating, it is preferable to restrict the contents of these elements in the ranges shown below. The contents of each component that may be contained in the coating of the covered metal arc welding electrode according to this embodiment and the reasons for the limitations will be further explained below.

(Total amount of Na, K and Li: 0.3 mass% or more and 4.0 mass% or less)
Na, K and Li are components that have the effect of stabilizing the arc.
When the total amount of Na, K and Li in the coating is 0.3 mass% or more and 4.0 mass% or less, the arc stabilization effect can be sufficiently obtained. Therefore, the total amount of Na, K and Li in the coating is preferably 0.3 mass% or more, more preferably 0.8 mass% or more, and even more preferably 1.5 mass% or more, based on the total mass of the coating. Also, the total amount of Na, K and Li in the coating is preferably 4.0 mass% or less, more preferably 3.5 mass% or less, and even more preferably 3.0 mass% or less, based on the total mass of the coating.
The Na, K and Li in the coating agent are contained in oxides such as Na ₂ O, K ₂ O and Li ₂ O, metallic Na, metallic K, metallic Li, Na alloys, K alloys, Li alloys, adhesives such as water glass, etc.

(Ti: 0.5% by mass or more and 4.0% by mass or less)
Metallic Ti and Ti contained in alloys are deoxidizing elements that have the effect of improving the strength of weld metal. After acting as a deoxidizing agent, they are present in the weld metal as oxides, which have the effect of refining crystal grains. Oxides such as _TiO2 act as slag formers, and have the effect of improving the fluidity of slag.
When the Ti content in the coating agent is 0.5 mass% or more and 4.0 mass% or less, the deoxidizing effect, the crystal grain refining effect, and the effect of improving the fluidity of the slag can be sufficiently obtained. Therefore, the Ti content in the coating agent is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, and even more preferably 1.3 mass% or more, based on the total mass of the coating agent. Moreover, the Ti content in the coating agent is preferably 4.0 mass% or less, more preferably 3.0 mass% or less, and even more preferably 2.5 mass% or less, based on the total mass of the coating agent.
In addition, examples of the Ti source in the coating agent include metal Ti, alloys such as ferrotitanium, and compounds such as _TiO2 .

(Al: 1.5% by mass or less)
Metallic Al and Al contained in the alloy act as a deoxidizing element. Al oxides such as Al ₂ O ₃ act as slag formers. Furthermore, when Al ₂ O ₃ or the like is contained in the coating agent, the viscosity of the molten slag is increased, the fluidity is improved, and the bead appearance and bead shape can be improved. In this embodiment, Al may be contained in the coating agent as necessary in order to improve the bead appearance and bead shape, and the Al content in the coating agent is preferably 0.02 mass% or more with respect to the total mass of the coating agent.
On the other hand, if the Al content in the coating agent is 1.5 mass% or less based on the total mass of the coating agent, the viscosity of the molten slag can be properly adjusted, the fluidity can be controlled, and the bead appearance and shape can be maintained good. Therefore, the Al content in the coating agent is preferably 1.5 mass% or less based on the total mass of the coating agent, more preferably 1.0 mass% or less, and even more preferably 0.7 mass% or less.
The Al source in the coating agent may be an oxide of Al such as Al ₂ O3, metallic Al, or an alloy such as aluminum magnesium.

(Zr: 0.8% by mass or less)
Metallic Zr and Zr contained in alloys act as deoxidizing elements. Zr oxides such as _ZrO2 act as slag formers. Furthermore, when _ZrO2 is contained in the coating agent, it is possible to obtain the effect of improving the conformability of the bead and forming a flat bead. In this embodiment, Zr may be contained in the coating agent as necessary in order to improve the conformability and bead shape of the bead, and the Zr content in the coating agent is preferably 0.01 mass% or more with respect to the total mass of the coating agent.
On the other hand, if the Zr content in the coating agent is 0.8 mass% or less, it is possible to improve the conformity of the bead and obtain a flat bead shape while maintaining good slag removability. Therefore, the Zr content in the coating agent is preferably 0.8 mass% or less, more preferably 0.6 mass% or less, and even more preferably 0.3 mass% or less, based on the total mass of the coating material.
The Zr source in the coating agent may be metallic Zr, Zr contained in an alloy, or Zr oxide such as _ZrO2 .

(Nb: 0.03% by mass or less)
Nb is an ingredient that has the effect of improving the strength of the weld metal, but it is also an ingredient that reduces low-temperature toughness by precipitating carbides by PWHT, so in this embodiment, it is preferable to restrict the Nb content in the coating material to a predetermined value or less, and it may be 0 mass%. If the Nb content in the coating material is 0.03 mass% or less, it is possible to suppress the reduction in low-temperature toughness after PWHT. Therefore, the Nb content in the coating material is preferably 0.03 mass% or less with respect to the total mass of the coating material.

(V: 0.03% by mass or less)
V is a component that has the effect of improving the strength of the weld metal, but it is also a component that promotes the precipitation and growth of carbides and reduces low-temperature toughness, so in this embodiment, it is preferable to regulate the V content in the coating material to a predetermined value or less, and it may be 0 mass%. If the V content in the coating material is 0.03 mass% or less, it is possible to suppress the decrease in low-temperature toughness after PWHT. Therefore, the V content in the coating material is preferably 0.03 mass% or less with respect to the total mass of the coating material.

(B: 0.10% by mass or less)
B is a component that has the effect of improving the toughness of the weld metal by segregating at prior austenite grain boundaries and suppressing pro-eutectoid ferrite, but is also a component that may cause high-temperature cracking and SR cracking. In this embodiment, the lower limit of the B content in the coating material is not particularly specified, and may be 0 mass%.
When the B content in the coating material is 0.10 mass% or less, the occurrence of hot cracking and SR cracking in the weld metal can be suppressed. Therefore, the B content in the coating material is preferably 0.10 mass% or less with respect to the total mass of the coating material.

(Cu: 0.6% by mass or less)
Cu is a component that has the effect of refining the structure of the weld metal and improving low-temperature toughness while maintaining strength, but depending on the Cu content in the coating material, it may promote the formation of precipitates and reduce low-temperature toughness. In this embodiment, Cu may be contained in the coating material as necessary, but the lower limit of the Cu content in the coating material is not particularly specified and may be 0 mass%.
When the Cu content is 0.6 mass% or less, the formation of precipitates is not promoted and the decrease in low-temperature toughness can be suppressed. Therefore, the Cu content in the coating material is preferably 0.6 mass% or less with respect to the total mass of the coating material.

(remainder)
In this embodiment, another component that may be contained in the coating agent is C. When the C content in the coating agent is 0.30 mass% or less, the generation of carbides can be suppressed. Therefore, the C content in the coating agent is preferably 0.30 mass% or less with respect to the total mass of the coating agent.

In addition, in the shielded metal arc welding according to this embodiment, the total content of the essential components _CO2 , F, Si, Ni, Fe, Mo, Cr, Mg and Mn contained in the coating material is preferably 40 mass% or more, more preferably 45 mass% or more, and even more preferably 50 mass% or more, relative to the total mass of the coating material.
Furthermore, when CaO and BaO are further contained, the total content of these is preferably 85 mass % or more, more preferably 87 mass % or more, and even more preferably 90 mass % or more, based on the total mass of the coating agent.
The coating agent further contains at least one of Na, K, and Li, as well as Ti, and regardless of whether or not it contains Al and Zr, the total content of these elements is preferably 90 mass% or more, more preferably 93 mass% or more, and even more preferably 95 mass% or more, based on the total mass of the coating agent.

(impurities)
Other elements that may be contained in the coating agent include unavoidable impurities such as P, S, Sn, Sb, As, Pb, and N. From the viewpoint of ensuring welding quality such as hot cracking resistance, it is preferable to restrict the unavoidable contents of P, S, Sn, Sb, As, Pb, and N to 0.5 mass% or less, respectively, relative to the total mass of the coating agent. In addition, it is preferable to restrict the total value of impurities to 3 mass% or less, relative to the total mass of the coating agent.

<1-3. Core wire>
Next, the components contained in the core wire of the covered electrode according to the present embodiment and the preferred contents thereof will be described below.
In the present embodiment, the core wire may be, for example, an iron-based core wire or a steel core wire mainly composed of Fe. The steel core wire may be, for example, a steel core wire made of mild steel, high-tensile steel, or low-alloy steel.

In this embodiment, other components in the core wire are not particularly limited, but in addition to Fe, C, Si, Mn, P, S, N, Cu, etc. may be contained. With respect to the total mass of the core wire, it is preferable that the C content in the core wire is 0.13 mass% or less, the Si content is 0.3 mass% or less (including 0 mass%), the Mn content is 0.2 mass% to 1.0 mass%, the P content is 0.040 mass% or less (including 0 mass%), the S content is 0.035 mass% or less (including 0 mass%), and the Cu content is 0.2 mass% or less (including 0 mass%).
The core wire may further contain Nb, V, Cr, Ni, Mo, Ti, Al, and B. Of these components, the content of Nb and V is preferably 0.02 mass% or less, respectively. The total content of Cr, Ni, Mo, Ti, and Al is preferably 4.0 mass% or less. Furthermore, the content of B is preferably 0.02 mass% or less.

In this embodiment, the outer diameter of the core wire is not particularly limited, but is preferably, for example, 2.6 mm or more and 5.0 mm or less.

[2. Manufacturing method of covered metal arc welding electrode]
The covered metal arc welding rod according to this embodiment can be manufactured by blending raw materials for the coating agent so that the coating agent has the above-mentioned component composition, kneading the mixture together with a specified adhesive, applying the mixture to the surface of a specified core wire such that the mass of the coating agent is in the range of 25 mass % to 40 mass % relative to the total mass of the covered metal arc welding rod, and firing the mixture at 450° C. to 550° C. for about 1 hour.
In addition, when manufacturing the covered metal arc welding rod according to the present embodiment, the type of core wire, the type of adhesive, the method of forming the coating, and the like are not particularly limited, and normal specifications and conditions for manufacturing a covered metal arc welding rod can be used.

[3. Welding metal]
The weld metal according to this embodiment is obtained by performing shielded metal arc welding using the shielded metal arc welding rod according to this embodiment described above in [1. Shielded metal arc welding rod].
In addition, in the weld metal according to the present embodiment, there are no particular limitations on the conditions other than the use of the covered metal arc welding electrode according to the present embodiment, and the type of base metal can be appropriately selected according to the required characteristics.

[4. Shielded Metal Arc Welding Method]
The covered metal arc welding method according to the present embodiment is a method of welding using the covered metal arc welding electrode according to the present embodiment described above in [1. Covered metal arc welding electrode].
In the covered metal arc welding method according to the present embodiment, various welding conditions are not particularly limited except for using the covered metal arc welding electrode according to the present embodiment, and general conditions in a welding method using a covered metal arc welding electrode can be used for the type of base metal, welding voltage, welding current, welding position, etc.

[5. Manufacturing method of welded joint]
The method for producing a welded joint according to the present embodiment is a method for producing a welded joint by using a high tensile steel as a weld base material and the covered metal arc welding rod according to the present embodiment described above in [1. Covered metal arc welding rod] by covered metal arc welding.
In the method for manufacturing a welded joint, the welding conditions are not particularly limited except that high tensile steel is used as the weld base material and that the covered metal arc welding electrode according to the present embodiment is used for covered metal arc welding. General conditions for a welding method using a covered metal arc welding electrode can be used for the welding voltage, welding current, welding position, etc.
Any high-tensile steel may be used as the base material, but it is preferable that the steel be of the 720 MPa class or higher. Examples of such steel include P690Q, P690QH, P690QL1, and P690QL2 of EN 10028-6:2017, KD620, KD690, KE620, and KE690 specified by Nippon Kaiji Kyokai (NK), and VL690 specified by DNV.

Below, we will explain the invention example and comparative example of the covered metal arc welding rod according to this embodiment.

[Shielded metal arc welding]
(Preparation of covered electrodes)
Using a welding rod coating machine, the surface of a steel core wire having a diameter of 4.0 mm was coated with coating materials having various component compositions, and then baked for about 1 hour at 450 to 550° C. to produce each of the covered electrodes of the invention and the comparative example. The coating rate was set to be in the range of 25 mass % to 40 mass % with respect to the total mass of the covered electrode.

(Shielded Metal Arc Welding)
Next, using the obtained covered metal arc welding rod, covered metal arc welding was performed on steel plates having the plate thickness and chemical composition shown in Table 2 below under the welding conditions shown in Table 1 below, to produce welded joints.

[Evaluation of mechanical properties]
(Preparation of test specimens)
The mechanical properties of the weld metal were evaluated in accordance with "Tensile and impact test method for weld metal" specified in JIS Z 3111:2005, by taking tensile test pieces (A2) and impact test pieces (V-notch test pieces) from the center of the plate thickness direction of the weld metal. In examining a wide range of PWHT conditions, a method is often used in which the change in mechanical properties when PWHT (post-weld heat treatment) is performed at various holding temperatures and holding times is organized using the Larson-Miller parameter (hereinafter abbreviated as LMP). In this example, the PWHT conditions with low LMP were 580°C for 2 hours, and the conditions with high LMP were 620°C for 8 hours, and the evaluation was performed.

(Tensile test)
Tensile tests were performed at room temperature (about 20±2°C) on as-welded test pieces and test pieces that had been subjected to PWHT at a temperature of 580°C for 2 hours and at a temperature of 620°C for 8 hours, and the tensile performance was evaluated by measuring the yield stress and tensile strength.
In the present invention, the strength was judged to be good when the as-welded tensile strength (TS) was 780 MPa or more, and the strength was judged to be good when the tensile strength (TS) was 750 MPa or more after PWHT at a temperature of 580°C for 2 hours and at a temperature of 620°C for 8 hours.

(Impact test)
The impact test was performed on as-welded test pieces and test pieces that had been subjected to PWHT at 580°C for 2 hours and 620°C for 8 hours. The test temperatures were -40°C and -60°C, and the Charpy absorbed energy (vE-40°C, vE-60°C) was measured three times at each test temperature, and the toughness was evaluated using the minimum of the three Charpy absorbed energies. In the present invention, the toughness was determined to be good when the minimum absorbed energy values at -40°C and -60°C after as-welded and PWHT were 100J or more and 80J or more, respectively.
Those which were good in both strength and toughness both as-welded and after PWHT were rated as pass, and those other than that were rated as fail.

The chemical composition of the core wire is shown in Table 3 below, and the chemical composition of the coating material is shown in Tables 4 and 5 below. The evaluation results of the mechanical properties are shown in Table 6 below. The remainder of the components of the core wire shown in Table 3 below is Fe and impurities. The remainder of the components of the coating material shown in Tables 4 and 5 below is impurities.
In addition, in Table 4 below, [Ni] is the Ni content in the coating agent expressed as a mass % relative to the total mass of the coating agent, [Mn] is the Mn content in the coating agent expressed as a mass % relative to the total mass of the coating agent, and [Si] is the Si content in the coating agent expressed as a mass % relative to the total mass of the coating agent.

In addition, in the content descriptions in Table 5 below, those marked with "-" indicate that the content was below the quantification limit. Furthermore, those for which the mechanical properties were not evaluated when PWHT was performed for 2 hours at a temperature of 580°C are marked with "-" in the evaluation results column in Table 6.

As shown in Tables 4 to 6, in the invention examples Nos. 1 to 7 in which the contents of each component in the coating agent were within the ranges specified in the present invention, the as-welded tensile strength (TS) was the target 780 MPa or more, and the tensile strength (TS) after two different types of PWHT was the target 750 MPa or more, and weld metals with excellent strength were obtained. In addition, the absorbed energy at -40°C was 100 J or more, and the absorbed energy at -60°C was 80 J or more, and excellent low-temperature toughness was obtained. From this, it was possible to obtain weld metals that had excellent low-temperature toughness and suppressed the occurrence of hot cracking while maintaining the target strength not only in the as-welded state but also in a wide range of PWHT conditions.
In addition, in Examples 1 to 7, it was possible to manufacture welded joints having welds with excellent low-temperature toughness and suppressed occurrence of hot cracking while maintaining the target strength under a wide range of PWHT conditions, not just as-welded.

On the other hand, in Comparative Example No. 1, the Cr content in the coating agent was below the lower limit of the range specified in the present invention, so the tensile strength after as-welded and long-term PWHT was reduced, and the toughness at -60°C after short-term PWHT was also reduced. In Comparative Examples No. 2 and 5, the value obtained by the formula [Ni]/([Mn]+[Si]) was below the lower limit of the range specified in the present invention, so the low-temperature toughness after short-term PWHT and long-term PWHT was reduced. In Comparative Example No. 3, the Mo content in the coating agent exceeded the upper limit of the range specified in the present invention, and the value obtained by the formula [Ni]/([Mn]+[Si]) was below the lower limit of the range specified in the present invention, so the low-temperature toughness after as-welded, short-term PWHT, and long-term PWHT was reduced.

In Comparative Example No. 4, since the Mo content in the coating exceeded the upper limit of the range specified in the present invention and the Cr content in the coating was less than the lower limit of the range specified in the present invention, the strength did not decrease, but since the value obtained by the formula [Ni]/([Mn]+[Si]) was less than the lower limit of the range specified in the present invention, the low-temperature toughness after the short-time PWHT and the long-time PWHT decreased.
In Comparative Examples 6 to 9, the Ni content in the coating agent was less than the lower limit of the range defined in the present invention, and the value obtained by the formula [Ni]/([Mn]+[Si]) was less than the lower limit of the range defined in the present invention, so that at least one of the As-welded strength, the toughness at -60°C, and the toughness at -40°C, and the strength after long-term PWHT, the toughness at -60°C, and the toughness at -40°C decreased.

In Comparative Example No. 10, the value obtained by the formula [Ni]/([Mn]+[Si]) exceeded the upper limit of the range specified in the present invention, raising concerns about hot cracking. In Comparative Example No. 11, the Ni content in the coating agent and the value obtained by the formula [Ni]/([Mn]+[Si]) both exceeded the upper limit of the range specified in the present invention, raising concerns about hot cracking, just like Comparative Example No. 10.

Claims (3)

A covered metal arc welding rod having a core wire and a coating material that covers the core wire,
The coating agent contains, based on the total mass of the coating agent,
_CO2 : 16% by mass or more and 27% by mass or less,
F: 4% by mass or more and 10% by mass or less,
Si: 3% by mass or more and 11% by mass or less,
Ni: 7.5% by mass or more and 13.3% by mass or less,
Fe: 1% by mass or more and 11% by mass or less,
Mo: 0.3% by mass or more and 1.0% by mass or less,
Cr: 0.15% by mass or more and 1.20% by mass or less,
Mg: 1.5% by mass or more and 4.5% by mass or less,
Mn: 1.5% by mass or more and 4.0% by mass or less,
CaO: 20% by mass or more and 40% by mass or less,
BaO: 2% by mass or more and 6% by mass or less,
Total amount of Na, K and Li: 0.3 mass% or more and 4.0 mass% or less,
Ti: 0.5% by mass or more and 4.0% by mass or less;
Al: 1.5% by mass or less,
Zr: 0.8% by mass or less,
Nb: 0.03% by mass or less,
V: 0.03% by mass or less,
C: 0.30% by mass or less;
The content of the Ni in the coating material is represented as [Ni] in mass% based on the total mass of the coating material;
The content of Mn in the coating material is expressed as [Mn] in mass% based on the total mass of the coating material,
When the content of Si in the coating agent is expressed as [Si] in mass% relative to the total mass of the coating agent,
A shielded metal arc welding rod, characterized in that a value calculated by [Ni]/([Si]+[Mn]) is 0.85 or more and 1.45 or less. A method for shielded metal arc welding, characterized in that shielded metal arc welding is performed using the shielded metal arc welding rod according to claim 1. A method for manufacturing a welded joint, characterized by using a high tensile steel as a base material and performing shielded metal arc welding using the shielded metal arc welding rod described in claim 1.