JP7698178B2 - Manufacturing method of welded joint and flux-cored cut wire for filling groove - Google Patents

Description

This invention relates to a method for manufacturing a welded joint in which a cut wire is filled into a groove between base materials and then welded, and to a flux-cored cut wire for filling the groove used in this method.

In recent years, in the field of manufacturing large steel structures such as ships, steel frames, and bridges, the strength of steel plates has been increasing in order to achieve both weight reduction and high performance. In addition, from the perspective of improving construction efficiency, submerged arc welding (SAW), which is mainly used for plate joint welding, requires welding materials that can handle high heat input welding such as one-sided welding and double-sided single layer welding.

In general, the strength of the weld metal must be equal to or greater than the strength of the base material. Therefore, as the strength of steel plates increases, the strength of the weld metal must also increase. However, when welding high-strength steel plates with a tensile strength of 780 MPa or more, cold cracking at the weld becomes a problem. The probability of cold cracking is particularly high in the first layer of multi-layer welding of extremely thick high-strength steel plates. Therefore, in order to prevent cold cracking, it is necessary to preheat the weld, but in the case of extremely thick steel plates, the preheating process itself imposes a great deal of strain.

Since this cold cracking is affected by diffusible hydrogen in the weld metal, a known method is to add fluoride to the flux and welding wire used in submerged arc welding, which causes hydrogen ions (H ⁺ ) to combine with fluoride ions (F ^{- )} and be expelled from the arc, thereby reducing the hydrogen concentration absorbed into the molten pool.

For example, Patent Document 1 describes a welding wire used for submerged arc welding, which contains a flux-cored wire having a melting point of 1000° C. or less and containing 0.1% by mass or more and less than 10% by mass of a powdered fluorine compound in the flux components filled in the _wire . Patent Document 2 describes submerged arc welding of 780 MPa high tensile steel together with a specified wire using a flux containing, in addition to SiO ₂ and the like, 9 to 21% CaF ₂ , 2 to 10% of one or both of MgF _{2 and} MnF 2, and 0.02 to 1.0% Li fluoride in terms of Li.

However, the above-mentioned Patent Document 1 is aimed at welding steel pipes such as high-strength pipelines for transporting crude oil, and the thickness of the steel plate used in the examples is also 25 mm. In other words, when attempting to perform multi-layer welding with a flux-cored wire as in Patent Document 1 for extremely thick high-strength steel plate with a thickness of 50 mm or more, problems with the working environment due to the generation of fumes and spatter become prominent. Moreover, using such a flux-cored wire for all passes leads to high costs.

In addition, in the above Patent Document 2, an example is shown in which 780 MPa steel with a plate thickness of 80 mm is welded in one layer in two passes, but as is clear from the description in the specification, preheating at a preheat temperature of 100°C is required (see paragraph 0044). Furthermore, when submerged arc welding is performed using the flux-cored wire of Patent Document 1 or the flux of Patent Document 2, the fluorides are dissociated by the arc, making the arc unstable and resulting in a poor bead shape.

On the other hand, when manufacturing joints with large heat input in submerged arc welding, in order to achieve high welding efficiency and control of penetration depth, cut wire, which is made by cutting a thin steel wire into small pieces of a specified length, is filled into the groove. Patent Document 3 describes a cut wire in which the cut wire is covered with a Cu cladding layer and the surface is coated with oil.

In Patent Document 3, the reason for providing a Cu coating layer is to take into consideration the arc start performance due to its high electrical conductivity, and the reason for applying an appropriate amount of oil to the wire surface is to improve rust resistance without impairing arc start performance and to prevent a decrease in low-temperature cracking (see paragraphs 0013 and 0021). In other words, the oil amount control of the cut wire in Patent Document 3 can suppress the increase in diffusible hydrogen, but does not reduce it. Therefore, when the strength of steel plates and weld metals increases, it is difficult to reliably prevent the problem of low-temperature cracking. Note that up until now, cut solid wires have mainly been used as cut wires to fill the groove as described above.

JP 2013-123711 A Japanese Patent Application Publication No. 5-212583 Japanese Patent Application Publication No. 9-94690

As the strength of steel plates and weld metals continues to increase, the inventors have conducted extensive research into a method that can prevent cold cracking while reducing the burden of preheating, even in the case of extremely thick high-strength steel plates, and minimize problems in the working environment caused by fumes and spatter. As a result, they discovered that the above problems can be solved by filling the groove with a cut wire that contains flux containing a specified fluoride and welding it, and thus completed the present invention.

Therefore, the object of the present invention is to provide a method for manufacturing a welded joint that can prevent low-temperature cracking while reducing the burden of preheating work, and can also produce a welded joint that is advantageous in terms of the working environment and cost.

Another object of the present invention is to provide a flux-cored cut wire for use in the manufacturing method of such welded joints.

That is, the gist of the present invention is as follows.
(1) A method for manufacturing a welded joint in which a cut wire is filled into a groove provided between base materials and welded,
A manufacturing method for a welded joint, comprising a flux-cored cut wire welding process for filling at least a portion of the groove with a flux-cored cut wire having a steel outer sheath and a flux containing fluoride filled inside the steel outer sheath, and welding the same.
(2) The method for _producing a welded joint according to claim (1), wherein the fluoride is one or more selected from the group consisting _{of CaF2} , _MgF2 , LiF, NaF, _K2ZrF6 , _K2SiF6 , and _{Na3AlF6 .}
(3) The flux-cored cut wire has a mass ratio relative to the total mass of the flux-cored cut wire of:
The fluoride is contained in an amount of 0.01% by mass or more in terms of F, and the contents of the chemical components consisting of C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, Al, B, and Bi are C: 0.120% or less, Si: 2.00% or less, Mn: 3.50% or less, Cu: 5.00% or less, Ni: 5.00 or less, Cr: 5.00% or less, Mo: 5.00% or less, Nb: 0.50% or less, V: 0.50% or less, Ti: 0.50% or less, Al: 1.70% or less, B: 0.020% or less, and Bi: 0.030% or less,
The method for manufacturing a welded joint according to (1) or (2), wherein the contents of impurity elements consisting of P and S are P: 0.030% or less and S: 0.020% or less.
(4) A method for manufacturing a welded joint according to any one of (1) to (3), wherein the flux-cored cut wire welding process comprises flux-cored cut wire filling one-pass welding in which the flux-cored cut wire is filled in the groove and welding is performed in one pass.
(5) A method for manufacturing a welded joint described in any one of (1) to (3), wherein the flux-cored cut wire welding process comprises a single-layer welding in which the flux-cored cut wire is filled in the first layer in the groove and welded, or a multi-layer welding in which the flux-cored cut wire is filled in the first layer and second and subsequent layers in the groove and welded, and is performed at least until the temperature of the base material reaches a temperature equivalent to preheating.
(6) A method for manufacturing a welded joint according to any one of (1) to (5), wherein the welding means is submerged arc welding or gas shielded arc welding.
(7) A method for manufacturing a welded joint according to any one of (1) to (6), wherein the base material is made of a high-strength steel material having a tensile strength of 780 MPa or more.
(8) A method for manufacturing a welded joint according to any one of (1) to (7), wherein the base material is made of an extremely thick steel material having a thickness of 50 mm or more.
(9) A cut wire for groove filling, which is used to fill a groove provided between base materials when manufacturing a welded joint, characterized in that it has a steel outer sheath and a flux containing fluoride that is filled inside the steel outer sheath.
(10) The flux-cored cut wire for _{groove filling} according to (9), wherein the fluoride is one or more selected from the group consisting of _CaF2 , _MgF2 , LiF, NaF, _K2ZrF6 , _K2SiF6 , and _{Na3AlF6 .}
(11) A mass ratio of the flux-cored cut wire to a total mass of the wire,
The fluoride is contained in an amount of 0.01% by mass or more in terms of F, and the contents of the chemical components consisting of C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, Al, B, and Bi are C: 0.120% or less, Si: 2.00% or less, Mn: 3.50% or less, Cu: 5.00% or less, Ni: 5.00 or less, Cr: 5.00% or less, Mo: 5.00% or less, Nb: 0.50% or less, V: 0.50% or less, Ti: 0.50% or less, Al: 1.70% or less, B: 0.020% or less, and Bi: 0.030% or less,
The flux-cored cut wire for groove filling according to (9) or (10), wherein the contents of impurity elements consisting of P and S are P: 0.030% or less and S: 0.020% or less.
(12) The flux-cored cut wire for groove filling according to any one of (9) to (11), wherein the base material is made of a high-strength steel material having a tensile strength of 780 MPa or more.
(13) The flux-cored cut wire for groove filling according to any one of (9) to (12), wherein the base material is made of an extra-thick steel material having a thickness of 50 mm or more.

According to the present invention, even when welding extremely thick high-strength steel plates, it is possible to reduce the burden of preheating work and prevent cold cracking, and it is also possible to manufacture welded joints cost-effectively by minimizing problems in the work environment caused by fumes and spatter.

FIG. 1 is an explanatory diagram showing an example of a flux-cored cut wire welding process according to the present invention. FIG. 2 is an explanatory diagram showing another example of the flux-cored cut wire welding process according to the present invention. FIG. 3 is an explanatory diagram showing the groove shape used in the evaluation of cold cracking resistance in the examples and the state of filling with a flux-cored cut wire. FIG. 4 is an explanatory diagram showing the groove shape used in the evaluation of cold cracking resistance in the initial layer of the groove in the examples and the state of filling with a flux-cored cut wire.

The present invention uses a flux-cored cut wire that has a steel sheath and flux filled inside the steel sheath, and includes a flux-cored cut wire welding process in which the flux-cored cut wire is filled into at least a portion of a groove between base materials and welded to produce a welded joint. Below, we will explain the flux-cored cut wire according to the present invention and the method for manufacturing a welded joint using the same.

[Flux-cored cut wire]
First, for flux-cored cut wire, a flux containing fluoride is used. Fluoride reduces the amount of diffusible hydrogen in the weld metal, significantly improving the cold cracking resistance of the weld metal. This is presumably because when flux-cored cut wire is welded, the flux melts and the fluorine ( ^F- ) in the flux combines with hydrogen (H ⁺ ) to become hydrogen fluoride (HF), which is then released outside the weld metal.

The fluorides are preferably one or more selected from the group consisting of _CaF2 , _MgF2 , _LiF , NaF, _K2ZrF6 , _K2SiF6 , and _Na3AlF6 . These compounds produce Ca, Mg, Li, Na, K, Zr, Si, and _{Al which} are ionized and bond with oxygen to reduce the amount of oxygen in the weld metal, acting as deoxidizing elements. This is advantageous in terms of improving the toughness and elongation of the weld metal.

As described above, from the viewpoint of sufficiently reducing the amount of diffusible hydrogen in the weld metal, the content of the fluoride is preferably 0.01% or more in mass ratio to the total mass of the flux-cored cut wire, preferably 0.03% or more, more preferably 0.05% or more, and even more preferably 0.10% or more, if the amount of fluoride contained in the fluoride is specified in terms of F conversion value, which is expressed as mass % relative to the total mass of the flux-cored cut wire, from the viewpoint of sufficiently reducing the amount of diffusible hydrogen in the weld metal. This content represents the total amount of fluoride contained in the flux. In other words, as long as the total mass ratio of fluoride contained in the flux is 0.01% or more in F conversion value, the lower limit of the content of each fluoride is not particularly limited. In addition, since this F-converted value indicates the amount of fluorine (F) contained in the fluoride in mass % relative to the total mass of the flux-cored wire, when the type of the fluoride is the above-mentioned preferred example of fluoride, the F-converted value can be calculated by the following formula A.
0.487×CaF ₂ +0.610×MgF ₂ +0.732×LiF+0.452×NaF+0.402×K ₂ ZrF ₆ +0.517×K ₂ SiF ₆ +0.543×Na ₃ AlF ₆ ...Formula A
Here, the chemical formula of the fluoride in formula A indicates the mass% of the corresponding fluoride with respect to the total mass of the flux-cored wire. The coefficient of the chemical formula of each fluoride is calculated from the chemical formula weight of each fluoride. If the types of fluoride are as above, the content is preferably 0.10% or more in mass ratio with respect to the total mass of the flux-cored cut wire, preferably 0.5% or more, more preferably 1.0% or more, and even more preferably 2.0% or more.

Here, in the flux-cored cut wire of the present invention, since the flux is filled inside the steel sheath, the amount of spatter during welding does not increase even if a large amount of fluoride is contained. Therefore, from the viewpoint of reducing the amount of diffusible hydrogen, the upper limit of the mass percentage of fluoride is not particularly limited. However, considering the presence of the steel sheath and the fact that it fills with flux, it can be said that the upper limit of the mass percentage of fluoride is essentially 71.80% in F equivalent. Moreover, this upper limit may be 60% in F equivalent, 55% in F equivalent, or 50% in F equivalent.

In addition, the flux-cored cut wire of the present invention may contain, in addition to fluorides, alloy components for controlling the chemical composition and carbon equivalent (Ceq) of the weld metal (these chemical components and alloy components are simply referred to as "chemical components"). In this case, for example, metal powder or alloy powder melts in the same manner as the steel sheath during welding. Therefore, chemical components other than fluorides may be included in the flux in the form of metal powder or alloy powder, in the form of the steel sheath, or as plating on the outer surface of the steel sheath, all of which have the same effect. Note that, unlike the case of flux-cored wires that are not cut wires, these chemical components other than fluorides do not need to be in the form of oxides or carbonates, and can be included in the form of metal powder or alloy powder as described above, since arc stability and all-position weldability are not required.

Specifically, the flux-cored cut wire of the present invention contains 0.01% or more fluoride in terms of F, as a mass ratio relative to the total mass of the flux-cored cut wire, and contains one or more chemical components selected from the group consisting of C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, Al, B, and Bi as optional components, and the contents of these chemical components are C: 0.120% or less, Si: 2.00% or less. In addition, the contents of the impurity elements P and S are preferably P: 0.030% or less and S: 0.020% or less. In the present invention, the content of the impurity elements P and S is preferably P: 0.030% or less and S: 0.020% or less, respectively, by including the above-mentioned fluorides, the amount of diffusible hydrogen can be reduced without including these optional chemical components, so the lower limit of the content of each of these optional chemical components is 0%.

Among these, in the optional chemical components, C in "C: 0.120% or less" is an important element for ensuring the yield strength and tensile strength of the weld metal by solid solution strengthening. However, if the C content in the flux-cored cut wire exceeds 0.120%, the C content in the weld metal becomes excessive, and the yield strength and tensile strength of the weld metal increase excessively, decreasing the toughness of the weld metal. In order to stably ensure all of the toughness, yield strength, and tensile strength of the weld metal, it is preferable to set the upper limit of this C content to 0.10%. On the other hand, the lower limit of the C content is 0%, but if necessary, the lower limit of the C content may be 0.010%, 0.020%, 0.030%, 0.040%, or 0.050%. Similarly, the upper limit of the C content may be 0.100%, 0.090%, 0.080%, or 0.070%.

The Si in "Si: 2.00% or less" is a deoxidizing element that reduces the amount of oxygen in the weld metal and increases the cleanliness of the weld metal. If the Si content in the flux-cored cut wire exceeds 2.00%, Si deteriorates the toughness of the weld metal. To stably ensure the toughness of the weld metal, the upper limit of the Si content may be 1.90%, 1.80%, 1.70%, or 1.50%. On the other hand, the lower limit of the Si content is 0%, but if necessary, the lower limit of the Si content may be 0.01%, 0.02%, 0.03%, or 0.04%.

The Mn in "Mn: 3.50% or less" is an element necessary to ensure the hardenability of the weld metal and increase the strength of the weld metal, but it may be 0%. To increase the strength of the weld metal, the lower limit of the Mn content in the flux-cored cut wire may be 0.50%, 0.75%, or 1.0%. On the other hand, if the Mn content exceeds 3.50%, the susceptibility of the weld metal to grain boundary embrittlement increases and the toughness of the weld metal deteriorates. Therefore, although the upper limit of the Mn content is 3.50%, the upper limit of the Mn content may be 3.00%, 2.50%, or 2.00%.

The Cu in "Cu: 5.00% or less" has the effect of improving the strength and toughness of the weld metal, but may be 0%. Cu may be included in the plating on the surface of the steel sheath of the flux-cored cut wire, or may be included in the flux as a single element or as an alloy. This Cu content is the total amount of Cu contained in the steel sheath or flux and Cu contained in the surface plating. If this Cu content exceeds 5.00%, the toughness of the weld metal decreases. The upper limit of the Cu content is preferably 4.00%, 3.00%, or 2.00%. On the other hand, the lower limit of the Cu content is preferably 0.05%, 0.1%, or 0.15%.

Since Ni in "Ni: 5.00% or less" is not an essential component, the lower limit of the Ni content in the flux-cored cut wire is 0%. Furthermore, if the Ni content exceeds 5.00%, solidification cracking is more likely to occur. The upper limit of the Ni content is preferably 4.00%, 3.00%, or 2.00%. On the other hand, the lower limit of the Ni content is preferably 0.05%, 0.1%, or 0.15%.

Since Cr in "Cr: 5.00% or less" is not an essential component, the lower limit of the Cr content in flux-cored cut wire is 0%. On the other hand, Cr increases the hardenability of the weld metal, so it is an effective element for improving the strength of the weld metal. To fully obtain this effect, it is preferable that the Cr content be 0.10% or more. Furthermore, if the Cr content exceeds 5.00%, the weld metal will be excessively hardened, deteriorating the toughness of the weld metal. The upper limit of the Cr content is preferably 4.00%, 3.00%, 2.00%, or 1.00%.

In "Mo: 5.00% or less," Mo is not an essential component, so the lower limit of the Mo content in flux-cored cut wire is 0%. On the other hand, Mo has the effect of improving the hardenability of the weld metal, so it is an effective element for increasing the strength of the weld metal. To obtain this effect, it is preferable that the Mo content be 0.01% or more. However, if the Mo content exceeds 5.00%, the toughness of the weld metal may deteriorate, so the Mo content is set to 5.00% or less. The upper limit of the Mo content is preferably 4.00%, 3.00%, 2.00%, 1.00%, or 0.60%.

In "Nb: 0.50% or less," Nb is not an essential component, so the lower limit of the Nb content in flux-cored cut wire is 0%. On the other hand, Nb forms fine carbides in the weld metal, and these fine carbides cause precipitation strengthening in the weld metal, so Nb improves the tensile strength of the weld metal. In order to fully obtain this effect, it is preferable that the Nb content be 0.005% or more. However, it is not preferable for the Nb content to exceed 0.50%, because Nb forms coarse precipitates in the weld metal and deteriorates the toughness of the weld metal. The upper limit of the Nb content is preferably 0.40%, 0.30%, 0.20%, or 0.10%.

In "V: 0.500% or less," V is not an essential component, so the lower limit of the V content in the flux-cored cut wire is 0%. On the other hand, V improves the hardenability of the weld metal, so it is an effective element for increasing the strength of the weld metal. To fully obtain this effect, it is preferable that the V content be 0.010% or more. If the V content exceeds 0.500%, the amount of V carbide precipitated in the weld metal becomes excessive, causing the weld metal to harden excessively and deteriorating the toughness of the weld metal. The upper limit of the V content is preferably 0.400%, 0.300%, 0.200%, or 0.100%.

In "Ti: 0.50% or less", Ti is not an essential component, so the lower limit of the Ti content in the flux-cored cut wire is 0%. On the other hand, Ti is a deoxidizing element and has the effect of reducing the amount of oxygen in the weld metal. In addition, Ti contained in the flux-cored cut wire remains slightly in the weld metal and fixes the solid solution N, so it has the effect of mitigating the adverse effect of the solid solution N on the toughness of the weld metal. Therefore, the flux-cored cut wire may contain 0.01% or more of Ti. However, if the Ti content exceeds 0.50%, there is a risk of deterioration of the toughness due to the formation of excessive precipitates in the weld metal. Here, when Ti is contained in the flux-cored cut wire, it is considered to contain ferrotitanium (an alloy of iron and titanium) in the flux. The upper limit of the Ti content is preferably 0.40%, 0.30%, 0.20%, or 0.10%.

The Al in "Al: 1.70% or less" is a deoxidizing element, and like Si, it reduces the amount of oxygen in the weld metal and has the effect of improving the cleanliness of the weld metal. If the Al content in the flux-cored cut wire exceeds 1.70%, Al forms nitrides and oxides, etc., reducing the toughness of the weld metal. Therefore, the upper limit of the Al content in the flux-cored cut wire is set to 1.70%. This upper limit is preferably 1.60%, 1.50%, 1.40%, or 1.30%. The lower limit of the Al content is preferably 0.005%, 0.010%, 0.050%, 0.100%, 0.150%, or 0.200%.

Since B in "B: 0.020% or less" is not an essential component, the lower limit of the B content in the flux-cored cut wire is 0%. On the other hand, B combines with solute N in the weld metal to form BN, and therefore has the effect of reducing the adverse effect of solute N on the toughness of the weld metal. In addition, B also has the effect of improving the strength of the weld metal by increasing the hardenability of the weld metal. Therefore, the flux-cored cut wire may contain 0.0005% or more of B. However, if the B content exceeds 0.020%, the B in the weld metal becomes excessive, forming coarse BN and B compounds such as _Fe23 (C,B) ₆ , which deteriorates the toughness of the weld metal, and this is not preferable. The upper limit of the B content is preferably 0.015%, 0.010%, 0.005%, 0.003%, or 0.001%.

In "Bi: 0.030% or less," Bi is not an essential component, so the lower limit of the Bi content in the flux-cored cut wire is 0%. On the other hand, Bi is an element that improves the removability of slag. To fully obtain this effect, it is preferable that the Bi content be 0.005% or more, 0.010% or more, or 0.012% or more. On the other hand, if the Bi content exceeds 0.030%, solidification cracks are more likely to occur in the weld metal, so the upper limit of the Bi content is 0.030%. This upper limit of the Bi content is preferably 0.025%, 0.020%, or 0.010%.

In addition, P in the "P: 0.030% or less" impurity element reduces the toughness of the weld metal, so the P content in the flux-cored cut wire must be reduced as much as possible. Therefore, the lower limit of the P content is 0%. Furthermore, if the P content is 0.030% or less, the adverse effect of P on toughness falls within an acceptable range. To prevent solidification cracking of the weld metal, the P content is more preferably 0.020% or less, 0.015% or less, or 0.010% or less.

In addition, S, which is present as an impurity element in "S: 0.020% or less," deteriorates both the toughness and ductility of the weld metal if present in excess in the weld metal, so it is desirable to reduce the S content in the flux-cored cut wire as much as possible. Therefore, the lower limit of this S content is 0%. Furthermore, if the S content is 0.020% or less, the adverse effects of S on the toughness and ductility of the weld metal are within an acceptable range. More preferably, it is 0.010% or less, 0.008% or less, 0.006% or less, or 0.005% or less.

The flux-cored cut wire of the present invention may or may not contain the chemical components described above, but in addition to these chemical components, the fluorides, and impurity elements, it also contains Fe and impurities. Among these, Fe may be included in addition to the steel outer sheath, and iron powder. That is, in order to adjust the flux filling rate in the flux-cored cut wire or to improve the welding efficiency, iron powder may be included in the flux as necessary. The content of this iron powder is not particularly limited, but since oxygen attached to the surface layer of the iron powder may increase the oxygen content of the weld metal and reduce toughness, it is preferable that the mass ratio of the total mass of the flux-cored cut wire is less than 90.0%, preferably less than 80.0%. The upper limit of the iron powder content may be limited to 8.0%, 6.0%, 4.0%, 2.0%, or 1.0%. Of course, iron powder is not essential in the flux-cored cut wire of the present invention, so the lower limit of the iron powder content is 0%.

In addition, impurities are components that are mixed in during the industrial production of flux-cored cut wire, either from the raw materials or due to various factors in the manufacturing process. These are acceptable to the extent that they do not adversely affect the flux-cored cut wire of the present invention.

Furthermore, the flux-cored cut wire of the present invention may contain oxides and carbonates of metal elements other than the above-mentioned fluorides, chemical components, and impurity elements, to the extent that the characteristics are not impaired. In this case, these oxides and carbonates are not included in the above-mentioned fluorides, chemical components, impurity elements, and Fe and impurities. However, this does not mean that these oxides and carbonates are excluded from the content. In other words, the flux-cored cut wire of the present invention has a chemical composition that contains the above-mentioned fluorides at a predetermined ratio in F conversion value, the contents of the above-mentioned chemical components and impurity elements are each within a predetermined range, and the balance is Fe and impurities, or Fe, impurities, and their oxides and carbonates. Preferably, it has a chemical composition that contains the above-mentioned fluorides at a predetermined ratio in F conversion value, the contents of the above-mentioned chemical components and impurity elements are each within a predetermined range, and the balance is Fe and impurities.

Furthermore, as long as the above-mentioned conditions are met, the steel sheath of the flux-cored cut wire according to the present invention is not particularly limited. For example, when the steel sheath is made of a mild steel sheath, the chemical composition of the sheath can be C: 0.1% or less, Si: 0.10% or less, Mn: 3.00% or less, P: 0.030% or less, S: 0.020% or less, Al: 0.1% or less, and N: 0.030% or less, with the remainder being iron and impurities.

The flux-cored cut wire of the present invention can be obtained by cutting the wire-shaped steel sheath filled with flux into small pieces to a specified length. There are no particular restrictions on the shape of the cut flux-cored cut wire, and the thin-diameter steel wire can be cut into small pieces to a specified length to produce a shape similar to that of a general cut wire with a circular cross section. However, taking into account the fact that it is filled with flux, the diameter is preferably φ1.0 to φ3.0 mm. Incidentally, the diameter of a conventional cut wire is about φ1.0 to φ2.0 mm. On the other hand, the length of the flux-cored cut wire is preferably 0.5 to 3.5 mm. For a general cut wire, the length is equivalent to 0.5 to 2.0 times the wire diameter.

The flux filling rate is not particularly limited as long as the above-mentioned conditions are satisfied. For example, the lower limit of the flux filling rate may be 10% or 12% in terms of mass ratio to the total mass of the flux-cored cut wire. The upper limit of the flux filling rate may be 80% or 90%.

Here, in order to reduce the amount of diffusible hydrogen in the weld metal, it is preferable that the amount of hydrogen contained in the flux-cored cut wire is 12 ppm or less relative to the total mass of the flux-cored cut wire. This amount of hydrogen enters during the manufacture of the flux-cored cut wire, and may increase due to moisture entering the wire during storage. Therefore, if the period between the manufacture of the cut wire and its use is long, it is desirable to store the wire while preventing moisture from entering.

The procedure for producing the flux-cored cut wire of the present invention is not particularly limited, but the following example can be given as a method for producing flux-filled wire before cutting.
First, a method for producing a seamless flux-cored cut wire in which the seam of the steel sheath is welded and there are no slit-like gaps includes the steps of preparing the flux so that the fluoride and chemical components are within a predetermined range, forming the steel strip using a forming roll while feeding the steel strip in the longitudinal direction to obtain a U-shaped open tube, supplying the flux into the open tube through the opening of the open tube, butt-welding the opposing edges of the opening of the open tube to obtain a seamless tube, drawing the seamless tube to obtain a flux-cored wire having a predetermined wire diameter, and annealing the flux-cored wire during or after the drawing step. The wire is then cut to a predetermined length to obtain the flux-cored cut wire.

Here, the butt welding is performed by electric resistance welding, laser welding, TIG welding, or the like. In addition, annealing is performed during the wire drawing process or after completion of the wire drawing process to remove moisture from the wire. Preferably, the annealing temperature is 650°C or higher and the annealing time is 4 hours or longer to reduce the H content in the wire to 12 ppm or less. However, in order to prevent deterioration of the flux, the annealing temperature is 900°C or lower. Note that instead of butt welding, wire without slit-shaped gaps can also be obtained by brazing the gaps in the steel sheath.

It is also possible to obtain a flux-cored cut wire with slit-shaped gaps without welding the seams of the steel sheath. In that case, the method is the same as the method for manufacturing a wire having a seamless shape, except that instead of the process of butt-welding the ends of an open tube to obtain a seamless tube, a process of forming an open tube and butting the ends of the open tube to obtain a tube with slit-shaped gaps is included. The method for manufacturing a wire with slit-shaped gaps may further include a process of crimping the butted ends of the open tube. In the method for manufacturing a wire with slit-shaped gaps, a tube is drawn while having slit-shaped gaps.

As described above, the flux-cored cut wire according to the present invention may be a wire cut from a seamless shape in which the seam of the steel sheath is welded and there is no slit-shaped gap, or a wire cut from a wire having a slit-shaped gap without welding the seam of the steel sheath. However, it is preferable that the flux-cored cut wire is a wire cut from a steel sheath having no slit-shaped gap. H (hydrogen) that penetrates the welded part during welding diffuses into the weld metal and the welded material, accumulates at the stress concentration part, and causes low-temperature cracking. There are various sources of H, but if the cleanliness of the welded part and the welding conditions are strictly controlled, the moisture (H ₂ O) contained in the flux-cored cut wire can be a source of H, and the amount of this moisture may affect the amount of diffusible hydrogen in the welded joint. Therefore, a wire cut from a seamless shape without slit-shaped gaps is preferable, but if the wire has slit-shaped gaps, the flux-cored cut wire may be stored, for example, in a vacuum package or in a container that can maintain a dry state.

The flux-cored cut wire of the present invention may have an oil (lubricant) applied to its surface. The lubricant applied to the filler surface has the effect of suppressing rust generation during storage. Various types of lubricants (e.g. vegetable oils such as palm oil) can be used as such lubricants, but in order to suppress low-temperature cracking of the weld metal, it is preferable to use perfluoropolyether oil (PFPE oil) that does not contain H (hydrogen). Note that, if the flux-cored cut wire has a plating on its surface, the lubricant is applied to the plating surface.

[Method of manufacturing welded joints]
Next, in manufacturing a welded joint using the above-mentioned flux-cored cut wire, the present invention includes a flux-cored cut wire welding process in which at least a part of a groove provided between base materials is filled with flux-cored cut wire and welded. That is, in manufacturing a welded joint, the flux-cored cut wire according to the present invention is filled in the groove of the base materials and welded in one or more of the first pass to the final pass. When welding is performed in only one pass, the flux-cored cut wire according to the present invention is used in that one pass.

In particular, for this flux-cored cut wire welding process, it is preferable to fill at least the first layer in the groove with the flux-cored cut wire of the present invention and then perform welding. In other words, by filling the first layer in the groove, where the probability of cold cracking is high, with flux-cored cut wire, the load of the preheating work can be reliably reduced, for example by lowering the preheating temperature to 50°C or less, or the preheating work itself can be made unnecessary.

Figures 1 and 2 each show an example of the flux-cored cut wire welding process of the present invention. Of these, Figure 1 shows an example in which flux-cored cut wire 4 is filled into a part of a groove 3 provided between base materials 1 and 2, and welding is performed. First, as shown in Figure 1(a), a backing material 5 is attached to the back surface of base materials 1 and 2, and then the flux-cored cut wire 4 according to the present invention is filled into the first layer of the groove 3. Next, as shown in Figure 1(b), a welding wire 6 is placed approximately on the center of the filled flux-cored cut wire 4, and an arc is generated to perform welding.

Here, if the temperature of the base material 1 and the base material 2 reaches the preheat temperature (preheat equivalent temperature) when performing the preheating work by the welding in FIG. 1(b), for example, if it reaches 100°C, the risk of cold cracking is eliminated, so in the subsequent welding, a welded joint may be manufactured without using the flux-cored cut wire of the present invention. If the base material 1 and the base material 2 have not reached the preheat equivalent temperature, as shown in FIG. 1(c), the flux-cored cut wire 4 of the present invention is again dispersed on the weld metal 7 obtained including the flux-cored cut wire 4 filled in the first layer, and filled into the groove 3, and the welding wire 6 is placed and welding is performed. Thereafter, this is repeated until the temperature of the base material 1 and the base material 2 reaches the preheat equivalent temperature, and welding is performed without using the flux-cored cut wire of the present invention, as in the previous case.

The example of the flux-cored cut wire welding process shown in FIG. 1 is a single-layer welding process in which the first layer in the groove is filled with flux-cored cut wire and welded, or a multi-layer welding process in which the first layer and the second layer onwards in the groove are filled with flux-cored cut wire and welded, and the flux-cored cut wire is filled and welded at least until the temperature of the base metal reaches 100°C. In other words, by using the flux-cored cut wire according to the present invention for the first layer welding in the groove, the amount of diffusible hydrogen in the weld metal can be reduced. Moreover, since this flux-cored cut wire welding process also serves as a preheating operation, the preheating operation can be eliminated or the burden of the preheating operation can be significantly reduced.

As shown in FIG. 2, the flux-cored cut wire welding process according to the present invention may be a flux-cored cut wire filling one-pass welding process in which the groove is filled with flux-cored cut wire and welded in one pass. That is, as shown in FIG. 2(a), a backing material 5 is attached to the back surface of the base materials 1 and 2, and then the flux-cored cut wire 4 according to the present invention is dispersed so as to fill almost the entire groove 3, for example, about 80% of the height of the groove, in the example of FIG. 2. Next, as shown in FIG. 2(b), a welding wire 6 is placed approximately on the center of the filled flux-cored cut wire 4, and an arc is generated to weld, thereby producing a welded joint with the weld metal 7 containing the flux-cored cut wire 4, as shown in FIG. 2(c).

In the example of the flux-cored cut wire welding process shown in Figure 2, the flux-cored cut wire of the present invention is used almost entirely within the groove, which also makes preheating unnecessary or significantly reduces the burden of preheating.

In these Figures 1 and 2, an example is shown in which the groove between the base materials is a so-called V-shaped groove, but in the present invention, there is no limitation to this groove shape, and the groove shape may be, other than V-shaped, for example, I-shaped, L-shaped, K-shaped, J-shaped, X-shaped, U-shaped, H-shaped, or any other shape. In addition, the present invention may be applicable to both single-sided welding and double-sided welding. Furthermore, in the case of multi-layer welding as in the example of Figure 1, each layer may be welded in two or more passes.

The welding method (welding means) used in the present invention is not particularly limited, but in order to reliably melt the flux-cored cut wire filled in the groove, submerged arc welding or gas-shielded arc welding is preferable. However, since it may be difficult to fill the groove with the flux-cored cut wire in vertical welding or overhead welding, the welding position is preferably downward or sideways.

In addition, the manufacturing method of the welded joint in the present invention is not particularly limited in the type or shape of the base material, but it is most effective when cold cracking is an issue and preheating work is a burden. That is, a typical example is welding of base material made of high-strength steel with a tensile strength of 780 MPa to 1500 MPa. In addition, in the case of extremely thick steel material with a thickness of 50 mm to 250 mm, the burden of preheating work is particularly large, so it can be said that it is extremely effective to apply the present invention when the base material is an extremely thick high-strength steel material. The combination of base materials forming the groove is arbitrary, and the base materials may be the same type or different types.

In addition, while a typical method for manufacturing a welded joint produces a welded joint that includes a base steel plate (base material) and a welded joint that is composed of a weld metal and a weld heat-affected zone, the present invention uses a flux-cored cut wire that contains a specified fluoride as a groove filler, and therefore can preferably produce a welded joint with a diffusible hydrogen content in the weld metal of 1.0 ml/100 g or less. In particular, by using a flux-cored cut wire that further contains a specified chemical component, a welded joint with high strength and high toughness can be obtained.

(Example)
Next, the present invention will be described in more detail based on examples, etc. However, the following examples are not intended to limit the present invention, and any design changes that are made in accordance with the spirit described above and below are within the technical scope of the present invention.

The flux-cored cut wires of the present invention and comparative examples (sample numbers 8 to 47) were produced by the following method.
First, a steel strip was fed in the longitudinal direction and formed using a forming roll to obtain a U-shaped open tube. Flux was supplied into the open tube through the opening of the open tube, and the opposing edges of the opening of the open tube were butt-welded to obtain a seamless tube. This seamless tube was drawn to obtain a flux-cored wire without slit-shaped gaps. However, at that time, some samples were made into tubes with slit-shaped gaps without seam welding, and these were drawn to obtain wire. In this way, a flux-cored wire with a final filler diameter of φ2.0 mm was experimentally produced.
During the wire drawing process, the flux-cored wires were annealed for 4 hours or more within a temperature range of 650 to 950°C. After the prototypes were produced, a lubricant was applied to the surface of some of the wires. The produced flux-cored wires were then cut to a length of 2.0 mm to prepare samples of flux-cored cut wires. The configurations of these flux-cored cut wires are shown in Tables 1 and 2. Among these, sample number 35 corresponds to a reference example.

The units of the content of each fluoride, optional chemical component and iron powder (Fe powder), and each element contained as an impurity element disclosed in Tables 1 and 2 are the mass percentage (mass%) relative to the total mass of the flux-cored cut wire. The F equivalent value of the flux-cored cut wire disclosed in the tables indicates the amount of fluorine (F) contained in the fluorides in the flux-cored cut wire, expressed as mass% relative to the total mass of the flux-cored cut wire, and is the value calculated by the above-mentioned formula A.

The remainder of the chemical composition of the flux-cored cut wire (i.e., components other than the elements disclosed in Tables 1 and 2) is iron (including intentionally added Fe powder) and impurities. The wire structure of each flux-cored cut wire is as shown in Table 1, and unless otherwise specified in the remarks column, no oil is applied. Meanwhile, each element contained as a chemical component in the flux-cored cut wire disclosed in Table 2 is contained in the form of a steel sheath or metal powder. In addition, in the table, blanks in the table related to the content of a chemical component, compound, etc., mean that the chemical component, compound, etc. is not intentionally added. Also, fluorides and chemical components may be unavoidably mixed in.

The flux-cored cut wires of the invention and comparative examples were evaluated by the methods described below.
First, i) for evaluation of cold crack resistance, welding was performed on a tensile strength 780 MPa-class steel having a plate thickness of 50 mm under welding condition 1 in Table 3, and evaluation was performed by an H-type restraint cracking test in accordance with JIS Z3159:1993. The groove shape was a Y-shape as shown in FIG. 3, with a slit length of 300 mm. The spread thickness of the flux-cored cut wire 4 in the groove 3 was 8 mm. Furthermore, NB-250H manufactured by Nippon Steel Welding Industry Co., Ltd. was used as the welding flux, and Y-80M (wire diameter φ4.8 mm) manufactured by Nippon Steel Welding Industry Co., Ltd. was used as the welding wire.

ii) To evaluate the cold crack resistance in the first layer in the groove, welding was performed on a 780 MPa tensile strength steel plate with a thickness of 50 mm under welding condition 2 in Table 3. At that time, the groove shape was a V-shape as shown in Figure 4, and the thickness of the flux-cored cut wire 4 in the groove 3 was 2 mm. Furthermore, the welding wire used was Y-82C (wire diameter φ1.2 mm) manufactured by Nippon Steel Welding Industry Co., Ltd., and the backing material 5 was SB-41 manufactured by Nippon Steel Welding Industry Co., Ltd. Note that in both welding conditions 1 and 2 in these evaluations, the tests were performed without preheating the base materials (steel materials) 1 and 2.

iii) The amount of diffusible hydrogen in the weld metal was evaluated as follows. That is, the welding conditions for evaluating the amount of diffusible hydrogen in the weld metal were welding conditions 3 described in Table 3. The flux-cored cut wires of the invention and comparative examples were spread on the test plate and end tab to a thickness of 2 mm, and then the welding flux was spread on the test plate to a thickness of 10 mm. Here, NB-250H manufactured by Nippon Steel Welding Industry Co., Ltd. was used as the welding flux, and Y-80M (wire diameter φ4.0 mm) manufactured by Nippon Steel Welding Industry Co., Ltd. was used as the welding wire. The amount of diffusible hydrogen in the weld metal was measured by gas chromatography in accordance with JIS Z 3118:2007 (Method for measuring hydrogen amount in steel welds). The flux-cored cut wires that gave a diffusible hydrogen amount in the weld metal of 1.0 ml/100 g or less were evaluated as passing the test for diffusible hydrogen amount.

Furthermore, after i) the H-type weld cracking test and ii) the evaluation of the cold cracking resistance in the first layer, the height difference of the bead surface was measured at a pitch of 25 mm along the bead length to evaluate the bead shape. At this time, both after i) the H-type weld cracking test and after ii) the evaluation of the cold cracking resistance in the first layer, if the surface unevenness of the bead shape was 3 mm or less, it was evaluated as good (○), and if it was more than 3 mm, it was evaluated as poor (×). And, regarding the evaluation of the cold cracking resistance and bead shape in ii) the first layer, if there was "no cracking" and the bead shape was good (○), the overall judgment was passed, and if one or more were evaluated as poor (×), the overall judgment was failed.

The test results evaluated by the above-mentioned method are shown in Table 4. When welding was performed using the flux-cored cut wire of the invention example, even if the steel material was not preheated, there were no cross-sectional cracks (no cross-sectional cracks) in either the H-type weld cracking test or the evaluation of low-temperature cracking resistance in the first layer. In other words, it was confirmed that the flux-cored cut wire of the invention example has extremely high low-temperature cracking resistance. In addition, as shown in the test results in Table 4, by using the flux-cored cut wire of the invention example, the bead shape evaluation was passed, and good welding workability was demonstrated. On the other hand, when the flux-cored cut wire of the comparative example was used, there were cross-sectional cracks (cross-sectional cracks) in either the H-type weld cracking test or the evaluation of low-temperature cracking resistance in the first layer.

As described above, according to the present invention, even when welding extremely thick high-strength steel plates, it is possible to reduce the burden of preheating work and prevent cold cracking, and it is possible to weld with a good bead shape while minimizing problems in the working environment caused by fumes and spatter, and it is also possible to manufacture welded joints in a cost-effective manner.

1, 2: base material (steel material), 3: groove, 4: flux-cored cut wire, 5: backing material, 6: welding wire, 7: weld metal.

Claims (9)

A method for manufacturing a welded joint in which a cut wire is filled into a groove provided between base materials and welded,
The method includes a flux-cored cut wire welding process for filling at least a part of the groove with a flux-cored cut wire having a steel outer skin and flux filled inside the steel outer skin, and welding the steel outer skin,
The flux contains one or more fluorides selected from the group consisting of _CaF2 , _MgF2 , LiF, NaF, _K2ZrF6 , _K2SiF6 , and _Na3AlF6 , and the content _of the fluoride is 4.02% or more _and 71.80% or less in terms of F _conversion value, which represents the amount of fluorine contained in the fluoride in mass % relative to the total mass of the flux-cored cut wire, and
The flux-cored cut wire has a chemical component C content of 0.010% or more and 0.120% or less in terms of mass ratio to the total mass of the flux-cored cut wire, and the content of the chemical component C of the flux-cored cut wire is 0.010% or more and 0.120% or less in terms of mass ratio to the total mass of the flux-cored cut wire.
The contents of chemical components consisting of Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, Al, B, and Bi are Si: 2.00% or less, Mn: 3.50% or less, Cu: 5.00% or less, Ni: 5.00 or less, Cr: 5.00% or less, Mo: 5.00% or less, Nb: 0.50% or less, V: 0.50% or less, Ti: 0.50% or less, Al: 1.70% or less, B: 0.020% or less, and Bi: 0.030% or less;
The content of impurity elements consisting of P and S is P: 0.030% or less and S: 0.020% or less, and the balance is Fe.
A method for manufacturing a welded joint comprising the steps of: The method for manufacturing a welded joint according to claim 1, wherein the flux-cored cut wire welding step comprises flux-cored cut wire filling one-pass welding in which the flux-cored cut wire is filled in the groove and welding is performed in one pass. 2. The method for manufacturing a welded joint according to claim 1, wherein the flux-cored cut wire welding process comprises single-layer welding in which the flux-cored cut wire is filled in a first layer in the groove and welded, or multi-layer welding in which the flux-cored cut wire is filled in the first layer and second and subsequent layers in the groove and welded, and is performed at least until the temperature of the base material reaches a temperature equivalent to preheating. The method for manufacturing a welded joint according to any one of claims 1 to 3 , wherein the welding means is submerged arc welding or gas shielded arc welding. The method for manufacturing a welded joint according to any one of claims 1 to 4 , wherein the base material is made of a high-strength steel material having a tensile strength of 780 MPa or more. The method for manufacturing a welded joint according to any one of claims 1 to 5 , wherein the base material is made of an extra-thick steel material having a thickness of 50 mm or more. A cut wire for groove filling, which is used to fill a groove provided between base materials when manufacturing a welded joint, is made of a flux-cored cut wire having a steel sheath and flux filled inside the steel sheath,
The flux contains one or more fluorides selected from the group consisting of _CaF2 , _MgF2 , LiF, NaF, _K2ZrF6 , _K2SiF6 , and _Na3AlF6 , and the content _of the fluoride is 4.02% or more _and 71.80% or less in terms of F _conversion value, which represents the amount of fluorine contained in the fluoride in mass % relative to the total mass of the flux-cored cut wire, and
The flux-cored cut wire has a chemical component C content of 0.010% or more and 0.120% or less in terms of mass ratio to the total mass of the flux-cored cut wire, and the content of the chemical component C of the flux-cored cut wire is 0.010% or more and 0.120% or less in terms of mass ratio to the total mass of the flux-cored cut wire.
The contents of chemical components consisting of Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, Al, B, and Bi are Si: 2.00% or less, Mn: 3.50% or less, Cu: 5.00% or less, Ni: 5.00 or less, Cr: 5.00% or less, Mo: 5.00% or less, Nb: 0.50% or less, V: 0.50% or less, Ti: 0.50% or less, Al: 1.70% or less, B: 0.020% or less, and Bi: 0.030% or less;
The content of impurity elements consisting of P and S is P: 0.030% or less and S: 0.020% or less, and the balance is Fe.
A flux-cored cutting wire for groove filling. 8. The flux-cored cut wire for groove filling according to claim 7 , wherein the base material is made of a high-strength steel material having a tensile strength of 780 MPa or more. 9. The flux-cored cut wire for groove filling according to claim 7 or 8 , wherein the base material is made of an extremely thick steel material having a thickness of 50 mm or more.

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