JP2015006693A - Flux-provided wire excellent in fatigue strength and low temperature cracking resistance of weld joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flux-provided wire which can notably improve the welding efficiency by improving fatigue characteristics of welds of a high-strength steel of a tensile strength of higher than 490 MPa and substantially reducing preheating work for suppression of low temperature cracking.SOLUTION: A flux-provided wire comprises, by mass% to the total mass of the wire, 3.1-6.5% of one or more of CaF, BaF, SrF, MgFand LiF, 0.4-1.4% of one or more of Ti oxide, Si oxide, Mg oxide, Al oxide and Ca oxide and one or more of 0.03-0.10% C, 0.3-1.0% Si, 1.8-3.0% Mn, 0.004-0.030% Bi, 0.2-1.4% Ni and 0.1.0.8% Cr, has an NC, defined by equation [Ni]/2+[Cr], of 0.1-1.2% and meets condition 100×[Bi]≥NC.

Description

  The present invention relates to a flux-cored wire used when welding high-strength steel, and is particularly excellent in fatigue strength of a welded portion, and further eliminates the need for preheating work to prevent cold cracking or preheating. The present invention relates to a flux-cored wire that can significantly reduce work.

In recent years, it has been desired to increase the strength of steel materials used for the purpose of reducing the weight of vehicle bodies and structures in construction machines such as excavators and cranes, and building structures such as bridges and buildings.
However, despite the increasing demand for use of high-strength steel, most of the steel materials used are general-purpose steel having a tensile strength of about 490 MPa.

  This is because, even when high strength steel is used, the fatigue strength at the welded portion is almost the same as that of general-purpose steel. Further, when a structure is manufactured by welding, tack welding is performed in order to ensure the correct dimensional position of the member before the main welding. In fillet welding with a short welding length, such as tack welding, the amount of heat input is small and quenching is likely to cause cold cracking. In particular, high-strength steel has high weld cracking susceptibility, so that preheating work is required to prevent low-temperature cracking, which causes a problem that welding efficiency is lowered and welding construction cost is increased. Therefore, high-strength steel has been rarely used so far in structures that are designed for fatigue.

  Therefore, in order for high-strength steel exceeding 490 MPa class to be widely applied to structures with fatigue design, the fatigue life of the welded portion is improved and preheating work during welding is unnecessary or preheating work. There is a strong demand for a welding wire that can significantly reduce the above.

As flux-cored wires that improve the fatigue strength of welds of high-strength steel, for example, the wires shown in Patent Documents 1 and 2 have been proposed.
Patent Document 1 proposes a wire that improves the wettability of molten metal by adding Si, S, etc., improves the shape of the toe portion of the weld bead, and improves the fatigue strength. However, the addition of Si and S has a problem of lowering the weld crack resistance of the weld metal, but the investigation for it has not been sufficiently performed.

  In Patent Document 2, the weld metal is transformed in a low temperature region by adding a large amount of Ni to the flux-cored wire, and a compressive residual stress is generated in the weld using the volume expansion during the transformation. Wires that improve the fatigue strength of steel have been proposed. However, when the weld metal is transformed in a low temperature range, the weld metal is hardened, and thus there is a problem of reducing the low temperature crack resistance. Furthermore, since Ni is an expensive alloy element, there is a problem that the cost of the welding material increases remarkably when a large amount of this high Ni welding wire is applied to the welded portion of the structure.

Moreover, as a flux cored wire which improves the low temperature cracking property of the weld part of high strength steel, the wire shown by patent document 3, for example is proposed.
In Patent Document 3, with regard to a flux-cored wire for 490 to 780 MPa class high-strength steel, the amount of V is optimized, and cold cracking resistance is improved by occluding V with diffusible hydrogen. Has proposed a wire with a temperature of 50 ° C. or lower. In this wire, fluoride is added as a slag agent, but the effect of fluoride on cold cracking resistance has not been particularly studied, and the fatigue characteristics of fillet welds of high-strength steel are also not examined. Nothing has been considered.

In addition, for example, Patent Documents 4 to 6 propose wires in which fluoride is added to a flux as a slag agent and used for fillet welding of a primer coated steel sheet to obtain excellent welding workability and primer resistance. ing.
In Patent Documents 4 to 6, the purpose of fluoride is to reduce the viscosity of molten slag during fillet welding of primer-coated steel sheets, thereby facilitating the release of primer pyrolysis gas and suppressing the generation of pits and gas grooves. It is used for the purpose of obtaining weld metal with high toughness by reducing the oxygen content of the weld metal, but the effect on the fatigue properties of fluoride has not been studied at all, and the effect on the cold cracking resistance has not been studied. The effects of chemicals have not been fully examined.

  Although Patent Document 5 describes that metal fluoride reduces diffusible hydrogen, the percentage of metal oxide in the wire is high, and no quantitative analysis has been performed on the reduction effect. In addition, since it is essential for this wire to contain moisture, it is expected that although the wire contains metal fluoride, the amount of diffusible hydrogen increases and it cannot have severe cold cracking resistance. Is done.

JP 2002-361480 A JP 2007-296535 A JP-A-8-257785 JP 2001-205484 A Japanese Patent Laid-Open No. 9-239587 Japanese Patent Laid-Open No. 3-180298

  In view of the problems of the background art described above, the present invention improves the fatigue characteristics of welds of high-strength steels with a tensile strength exceeding 490 MPa, so that high-strength steels can be applied to structures that are designed for fatigue. It is an object of the present invention to provide a flux-cored wire that can be reduced in weight and does not require preheating work for suppressing low-temperature cracking or can significantly improve welding work efficiency by significantly reducing the preheating work.

The inventors of the present invention have made studies focusing on fluorides in the fillet welds of high-strength steel having a tensile strength exceeding 490 MPa and flux-cored wires used for tack welding.
As described above, in order to enable the wire added with fluoride to be used for welding high-strength steel with a tensile strength exceeding 490 MPa, the influence of fluoride on the fatigue properties and cold crack resistance of weld metal. Need to be considered further.

Therefore, first, various studies were made on the addition of alloy elements in order to improve fatigue strength in the slag component composition mainly composed of fluoride. As a result, it has been found that the fatigue characteristics of the welded portion of high-strength steel can be improved by optimally adding alloy elements such as Bi, Ni, and Cr.
It has also been found that the diffusible hydrogen of the weld metal can be significantly reduced by appropriately containing a metal oxide added together with fluoride as a flux.

This makes it possible to obtain excellent fatigue characteristics even in a welded portion of high-strength steel having a tensile strength of over 490 MPa, and to eliminate or simplify preheating performed for suppressing low-temperature cracking. The present inventors have found the composition and further studied based on the knowledge to arrive at the present invention.
The gist of the present invention thus made is as follows.

(1) A flux-cored wire for gas shield arc welding in which a flux is filled inside a steel outer sheath,
During the wire, wire percentage by weight relative to the total _weight, CaF _{2, BaF} 2, SrF _2, MgF 2, 3.1-6.5% as the total amount alpha 1, two or more of LiF, Ti oxide 1 type or 2 types or more among products, Si oxides, Mg oxides, Al oxides, Ca oxides, containing 0.4 to 1.4% as a total amount β,
And the ratio of the content of the CaF _{2 to} the total amount α is 0.90 or more, the content of the Ca oxide is less than 0.2%, and the total amount α relative to the total amount β So that the ratio is 3.0 or more and 15.0 or less,
As an alloy component, in mass% with respect to the total mass of the wire,
C: 0.03-0.10%,
Si: 0.3 to 1.0%,
Mn: 1.8-3.0%,
P: 0.02% or less,
S: 0.02% or less,
Al: 0.003 to 0.05%,
Bi: 0.004 to 0.030%
In addition,
Ni: 0.2-1.4%
Cr: 0.1 to 0.8%
1 or 2 or more of them, the balance consists of Fe and inevitable impurities, NC defined by the following formula (a) is 0.1 to 1.2%, and the following ( b) A flux-cored wire characterized by satisfying the relationship of the formula:
NC = [Ni] / 2 + [Cr] (a)
100 × [Bi] ≧ NC (b)
However, the element with [] represents the content (% by mass) of each element.

(2) The flux-cored wire is further in mass% with respect to the total mass of the wire,
Cu: 0.1 to 0.5%,
Mo: 0.1 to 0.8%,
V: 0.01-0.04%,
Ti: 0.01 to 0.3%,
Nb: 0.01 to 0.1%,
B: 0.0003 to 0.01%
1 type or 2 types or more of these are contained, The flux cored wire as described in said (1) characterized by the above-mentioned.

(3) The flux-cored wire according to (1) or (2), wherein the steel outer shell has no slit-like gap.
(4) The flux-cored wire according to (1) or (2), wherein the steel outer shell has a slit-like gap.
(5) The flux cored wire according to any one of (1) to (4), wherein perfluoropolyether oil is applied to a surface of the steel outer shell.

  According to the present invention, in a flux-cored wire used for fillet welding and tack welding of high-strength steel with a tensile strength of over 490 MPa, preheating work is performed for excellent fatigue characteristics of the weld and for suppressing low-temperature cracking. Can be provided, or a flux-cored wire that can significantly reduce the preheating work can be provided.

It is a figure which shows the relationship between NC and fatigue strength. It is a figure which shows the relationship between CaO content and the amount of diffusible hydrogen. It is a figure which shows the collection position of the test piece in an Example (JISZ3111 (2005)). It is a figure which shows the cross welded joint fatigue test body used for the fatigue test. It is a figure which shows an example of the cut cross section of a flux cored wire.

Conventionally, fluoride has been said to have the effect of reducing the viscosity of molten slag in fillet welding of primer-coated steel sheets, thereby facilitating the release of primer pyrolysis gas and suppressing the generation of pits and gas grooves. The inventors of the present invention have made a detailed examination on the effect of the welded portion on the fatigue characteristics and the effect on hydrogen in the weld metal by experimentally producing a flux-cored wire.
That is, the flux-cored wire contains CaF ₂ , a metal oxide, and further contains Bi to improve the wettability between the molten metal and the base material, improve the fatigue characteristics, and further, Ni, It is a flux-cored wire in which fatigue characteristics are ensured by containing an appropriate amount of Cr, and the content of CaF ₂ , Bi and alloy elements is changed at various ratios, and using this wire, a high-strength steel exceeding 490 MPa Fillet welding and tack welding were performed.

As a result, in a specific content range of CaF ₂ , Bi and alloy elements, the fatigue characteristics of the welded portion of high-strength steel are improved, and furthermore, the amount of diffusible hydrogen in the weld metal is greatly reduced, so that The present inventors have found that the preheating work necessary to suppress the low temperature cracking is unnecessary or can be significantly reduced even in such a severe part to the low temperature cracking.

The present invention has been made as a result of the above studies, and the reasons for limiting the technical requirements and preferred aspects of the flux-cored wire of the present invention will be sequentially described below.
First, the reason for limitation of the steel outer shell and the alloy component, metal deoxidation component, and content of each component included in the flux constituting the flux-cored wire of the present invention will be described.
In the following description, “%” means “% by mass” unless otherwise specified, and the content of each component is the sum of the mass% of each component in the steel outer sheath and flux with respect to the total mass of the wire. It shall mean the component content.

(C: 0.03-0.10%)
C is an element that improves the strength, and must be contained according to the strength level of the steel sheet to be welded. In order to ensure the strength of the welded portion of the high strength steel having a tensile strength exceeding 490 MPa, it is necessary to contain 0.03% or more of C.
The higher the C content in the welding wire, the more the C content in the weld metal increases, which increases the strength of the weld metal. However, if the amount is too large, the weld will be excessively hardened against the base metal and the toughness will be lowered. Rise. Therefore, in order to ensure toughness and cold cracking resistance, the upper limit of the C content is 0.10%. Moreover, in order to ensure toughness stably, it is good also considering the upper limit of C as 0.085%, 0.08%, or 0.075%.

(Si: 0.3-1.0%)
Si is effective in improving fatigue characteristics by increasing the wettability of the molten metal. Further, Si is a deoxidizing element, and reduces the amount of O in the weld metal to increase the cleanliness. In order to obtain these effects, a content of 0.3% or more is necessary. However, in a high-strength steel weld, if it exceeds 1.0%, the toughness deteriorates, so this is the upper limit. Moreover, in order to ensure the toughness of a weld metal stably, the upper limit of Si is good also as 0.8%, 0.7%, or 0.6%.

(Mn: 1.8-3.0%)
Mn is an element necessary for ensuring the hardenability of the weld metal and increasing the strength. In order to exhibit the effect reliably, it is necessary to make it contain 1.8% or more. On the other hand, if the content exceeds 3.0%, the hardenability becomes excessive and the cold cracking resistance and toughness deteriorate, so this is the upper limit. Further, in order to further suppress deterioration of cold cracking resistance and toughness, the upper limit of Mn may be 2.8%, 2.6%, 2.4%.

(P: 0.02% or less)
P is an impurity element, and it is necessary to reduce it as much as possible in order to deteriorate weld crack resistance and toughness, but the P content is 0.02% or less as a range in which these adverse effects can be tolerated.
(S: 0.02% or less)
S is an impurity element, but may be used for the purpose of improving fatigue strength by improving the wettability of the molten metal and smoothing the shape of the weld bead toe. However, if S is excessively present in the weld metal, the weld crack resistance and toughness are remarkably deteriorated, so it is preferable to reduce as much as possible. The S content is 0.02% or less as a range in which the adverse effects on weld crack resistance and toughness can be tolerated.

(Al: 0.003-0.05%)
Al is a deoxidizing element and, like Si, is effective in reducing O in the weld metal and improving cleanliness, and is contained in an amount of 0.003% or more in order to exhibit the effects. On the other hand, if the content exceeds 0.05%, nitrides and oxides are formed and the toughness of the weld metal is inhibited, so this is the upper limit. Further, in order to sufficiently obtain the effect of improving the toughness of the weld metal, the lower limit of Al may be 0.004% or 0.005%, and in order to suppress the formation of coarse oxides, It may be 0.04%, 0.03%, or 0.025%.

(Bi: 0.004 to 0.030%)
Bi is a molten slag mainly composed of fluoride, which optimizes the viscosity of the molten slag and suppresses the generation of strong oxide scale that tends to occur at the weld toe, thereby smoothing the shape of the weld toe. It is effective in improving the fatigue characteristics of the weld. In order to acquire the effect, it is necessary to contain 0.004% or more of Bi. On the other hand, if the content exceeds 0.030%, hot cracking occurs in the weld metal, so this is the upper limit.

The flux-cored wire of the present invention further needs to contain one or more of Ni and Cr.
(Ni: 0.2-1.4%)
Ni is an element that enhances the weather resistance of the weld metal part, and can maintain the shape of the weld toe part even over time and suppress deterioration of fatigue characteristics. Ni improves strength by improving hardenability, and further improves toughness by solid solution toughening. In order to obtain these effects, Ni needs to be contained by 0.2% or more.
On the other hand, if the Ni content exceeds 1.4%, the hardenability becomes excessive and the weld metal is hardened.

(Cr: 0.1-0.8%)
Cr, like Ni, is an element that enhances the weather resistance of the weld metal part, and can maintain the shape of the weld toe part over time and suppress deterioration in fatigue characteristics. Moreover, Cr increases strength by improving hardenability. In order to obtain these effects, it is necessary to contain 0.1% or more of Cr. On the other hand, if the content exceeds 0.8%, the hardenability becomes excessive and the weld metal hardens, so this is the upper limit. In order to suppress excess hardenability, the upper limit of Cr may be 0.7% or 0.5%.

  The flux-cored wire of the present invention is a kind of Cu, Mo, V, Ti, Nb, and B depending on the strength level and toughness of the steel sheet to be welded in addition to the above basic components as an alloy component or a metal deoxidation component. Or 2 or more types can be contained.

(Cu: 0.1-0.5%)
Cu can be added to the surface of the outer surface of the wire and to the flux as a simple substance or an alloy, thereby improving the strength and toughness of the weld metal. In order to sufficiently obtain these effects, it is preferable to contain 0.1% or more. On the other hand, if the content exceeds 0.5%, the toughness decreases. Therefore, the content when Cu is contained is set to 0.1 to 0.5%.
In addition, about content of Cu, in addition to the part contained in outer skin itself or a flux, when the copper surface is plated on the wire surface, the part is also included.

(Mo: 0.1-0.8%)
Mo is an element effective for increasing the strength by enhancing the hardenability. In order to acquire the effect, it is good to make it contain 0.1% or more. On the other hand, if the content exceeds 0.8%, the weld metal is hardened and the toughness is deteriorated. Therefore, the content when Mo is included is set to 0.1 to 0.8%. In order to suppress hardening of the weld metal and suppress deterioration of toughness, the upper limit of Mo may be set to 0.7%, 0.6%, or 0.5%.

(V: 0.01-0.04%)
V is an element effective for increasing the strength by enhancing the hardenability. In order to acquire the effect, it is good to make it contain 0.01% or more. On the other hand, when it is contained excessively exceeding 0.04%, the carbide precipitates and the weld metal is hardened and deteriorates toughness. Therefore, the content when V is contained is 0.01 to 0.00. 04%.

(Ti: 0.01-0.3%)
Ti is an element effective for increasing the strength by increasing the hardenability. Moreover, like Al, it is effective as a deoxidizing element and has the effect of reducing the amount of O in the weld metal. It is also effective for fixing solute N and mitigating the adverse effect on toughness. In order to exhibit these effects, it is preferable to contain 0.01% or more. However, when the content in the welding wire exceeds 0.3% and becomes excessive, the weld metal is hardened and deteriorates toughness. Therefore, when Ti is contained, the content is 0.01 to 0.3%. And In order to suppress hardening of the weld metal and suppress deterioration of toughness, the upper limit of Ti may be 0.2%, 0.12%, or 0.08%.

(Nb: 0.01 to 0.1%)
Nb forms fine carbides and is effective in securing tensile strength by precipitation strengthening. In order to obtain these effects, the content is preferably 0.01% or more even in consideration of combined effects with other elements having similar effects. On the other hand, if it exceeds 0.1%, it is not preferable because it is excessively contained in the weld metal and coarse precipitates are formed to deteriorate toughness. For this reason, content in the case of containing Nb shall be 0.01 to 0.1%. Further, in order to further suppress toughness deterioration due to Nb, the upper limit of Nb may be set to 0.05%, 0.04%, or 0.03%.

(B: 0.0003 to 0.01%)
When B is contained in an appropriate amount in the weld metal, it has the effect of reducing the adverse effect on the toughness of the solid solution N by combining with the solid solution N and contributing to the improvement of the strength by increasing the hardenability. There is also an effect. In order to obtain these effects, the B content in the welding wire needs to be 0.0003% or more. On the other hand, when the content exceeds 0.01%, B in the weld metal becomes excessive, and coarse toughness is adversely deteriorated by forming B compounds such as coarse BN and Fe ₂₃ (C, B) ₆ . It is not preferable. Therefore, the content when B is contained is set to 0.0003 to 0.01%. Further, in order to further suppress toughness deterioration due to B, the upper limit of B may be 0.008%, 0.006%, or 0.004%.

The flux-cored wire of the present invention contains each element as described above as an alloy component or a metal deoxidation component. In order to ensure the fatigue strength of the welded portion, 1/2 defined by the following formula (a): The total NC of Ni and Cr content of double (when Ni or Cr is not included, it is calculated as 0%) is 0.1 to 1.2%, and NC is 100 times Bi and the following It is necessary to adjust so as to satisfy the relationship of the expression (b).
NC = [Ni] / 2 + [Cr] (a)
100 × [Bi] ≧ NC (b)
However, the elements with [] indicate the content (% by mass) of each element.

Normally, steel structures are used after being painted after welding to prevent aging deterioration due to rust, etc., but at the weld toe, the coating tends to peel off due to discontinuities between the base metal and the weld metal, Corrosion is likely to occur. When corrosion occurs, it becomes a stress concentration part and deteriorates fatigue characteristics.
In order to prevent this, it is effective to increase the weather resistance of the weld metal by adding Ni or Cr. In order to improve the weather resistance, 0.1% or more is added as NC. If NC exceeds 1.2%, the hardenability of the weld metal becomes excessive and the strength becomes too high, so this is the upper limit.

  Moreover, when Ni and Cr are contained, a strong oxide scale is formed at the weld toe. Since this oxide scale deteriorates the shape of the weld toe, it causes deterioration in fatigue characteristics. In order to suppress the deterioration of the weld toe shape due to the oxide scale while ensuring the weather resistance due to the addition of Ni and Cr, it is necessary to add Bi so that 100 × [Bi] ≧ NC. If NC is higher than 100 × [Bi], the generation of this oxide scale cannot be sufficiently suppressed, so that the fatigue characteristics are not improved.

An experiment in which such knowledge is obtained is shown in FIG.
FIG. 1 shows a prototype of a flux-cored wire that satisfies the requirements of the present invention except that the NC value is different. A test specimen was prepared using the wire in the same manner as in the examples described later. The relationship between the obtained fatigue strength and the NC of the wire is shown. From FIG. 1, it was found that a fatigue strength of 160 MPa or more was obtained with a flux-cored wire added so that NC was 0.1 to 1.2 and satisfying 100 × [Bi] ≧ NC. On the other hand, the fatigue strength of the wire of 100 × [Bi] <NC hardly changes even if NC is 0.1 to 1.2%.

The content of elements contained as the above alloy component or metal deoxidation component does not include the content when these elements are contained as fluoride, metal oxide, or metal carbonate.
Further, these elements are not necessarily pure substances (inevitable impurities can be contained), and there is no problem even if they are contained in the form of an alloy such as Cu-Ni. In addition, since these elements have the same effect regardless of whether they are contained in the steel skin or as a flux, they can be contained in either the steel skin or the flux.

Then, the flux component inserted in the inside of the steel outer skin of a wire is demonstrated.
(Metal fluoride mainly composed of CaF ₂ : 3.1 to 6.5%)
In the flux-cored wire of the present invention, a metal fluoride mainly composed of CaF ₂ is added to the wire in a total amount α of 3.1 to 6.5%. As the metal fluoride, in addition to CaF ₂ , one or more of BaF ₂ , SrF ₂ , MgF ₂ , and LiF can be added as necessary. In that case, the ratio ([CaF ₂ ] / α) of the CaF ₂ content [CaF ₂ ] to the total amount α of metal fluoride is set to 0.90 or more.

CaF ₂ or CaF ₂ and BaF ₂ , SrF ₂ , MgF ₂ , LiF containing one or two or more metal fluorides as described above in the wire improves the wettability of the molten slag. Accordingly, the shape of the weld toe becomes smooth, and the fatigue characteristics of the welded portion of the high strength steel having a tensile strength of 490 MPa or more can be improved. In addition, metal fluoride improves the cold cracking resistance by reducing the amount of diffusible hydrogen in the weld metal and eliminates preheating even under severe conditions such as low temperature cracking such as tack welding of high-strength steel. Alternatively, it is possible to simplify welding.

In order to obtain these effects, it is necessary to contain 3.1% or more of metal fluoride. If the metal fluoride content is less than 3.1%, a sufficient effect cannot be obtained. If it exceeds 6.5%, welding fumes are excessively generated and the shielding effect by the shielding gas is reduced. In addition, since air is entrained or entrainment occurs due to excessive generation of slag, welding workability is significantly deteriorated, which is not preferable.
In addition, from the viewpoint of securing arc stability and suppressing spatter in fillet welding, the ratio of the CaF ₂ content in the metal fluoride is preferably 0.90 or more, and if it is less than 0.90, the arc stability decreases, This is not preferable because the weld toe shape deteriorates. In order to improve fatigue characteristics and cold cracking resistance, the lower limit of the metal fluoride content may be 3.6% or 4.1%, and in order to suppress deterioration of welding workability, the upper limit is 6. It may be 2%, 5.9%, or 5.4%.

  The fact that metal fluoride acts to reduce the amount of diffusible hydrogen is known for coated arc welding rods and the like, but there has been no detailed study of reducing diffusible hydrogen with a flux-cored wire. In consideration of other flux components, weld metal mechanical properties, welding workability, etc., the present inventors have found an optimum form for reducing diffusible hydrogen. The reason why metal fluoride reduces diffusible hydrogen is that metal fluoride is decomposed by the welding arc and the generated fluorine is combined with hydrogen and dissipated into the atmosphere as HF gas, or in the weld metal as it is. This is probably because hydrogen was fixed as HF.

(Metal oxide: 0.4-1.4%)
In the flux-cored wire of the present invention, Ti oxide (TiO ₂ ), Si oxide (SiO ₂ ), Mg oxide (MgO), Al oxide (Al ₂ O ₃ ), Ca oxide ( A metal oxide composed of one or more of CaO) is added. These affect the wettability of the molten slag and are added to maintain the bead shape and the weld toe shape well. In order to obtain the appropriate effect, it is necessary to add 0.4% or more in total. However, when the content of the metal oxide exceeds 1.4%, the bead shape becomes convex, and the weld toe shape deteriorates accordingly.

The content of these metal oxides includes the total amount of TiO ₂ , SiO ₂ , MgO, Al ₂ O ₃ and CaO, as well as the total amount of metal oxides contained in binders used for flux granulation. And Moreover, in order to suppress more that a bead shape becomes convex shape by addition of a metal oxide, it is good also considering the upper limit of content of a metal oxide as 1.2% and 1.0%.

In the present invention, it is preferable not to add CaO as a metal oxide. However, CaO may be contained in the flux raw material. In that case, CaO is limited to less than 0.20% by mass% with respect to the total mass of the wire. If the content is limited to less than 0.20%, the effect of the present invention can be obtained.
Since CaO changes to CaOH when exposed to the atmosphere, it increases the diffusible hydrogen of the weld metal. An experiment in which such knowledge is obtained is shown in FIG.

FIG. 2 shows a prototype of a flux-cored wire that satisfies the requirements of the present invention except that the value of CaO is different. The wire is used for welding, and the diffusible hydrogen content of the resulting weld metal is compared with the examples described later. The relationship between the CaO content in the wire and the amount of diffusible hydrogen was measured in the same manner.
From FIG. 2, the amount of diffusible hydrogen in the weld metal increases as CaO increases, but up to 0.20%, 1.5 ml / 100 g or less is obtained. If 1.5 ml / 100 g or less, the effect of reducing the preheating work is obtained, so CaO is made less than 0.20%. That is, it is preferable to select the raw material of the flux so as to satisfy this range.

In addition to the content of each of the above metal fluoride and metal oxide, the ratio of the total content α of metal fluoride to the total content β of metal oxide expressed by mass% (α / β) Needs to satisfy 3.0 or more and 15.0 or less.
If the value of α / β is less than 3.0, the bead shape becomes convex, so that the effect of improving the fatigue characteristics of the weld cannot be obtained. If the value of α / β exceeds 15.0, the bead shape cannot be maintained well. If necessary, the lower limit of α / β may be 3.5 or 4.0, and the upper limit may be 14.0, 13.0, or 12.0.
Further, regulating the value of the ratio α / β is important for obtaining the effect of reducing diffusible hydrogen, and the effect of reducing diffusible hydrogen can be obtained within the above range.

Although the above is the reason for limitation regarding the component composition of the flux-cored wire of the present invention, the other remaining components are Fe and inevitable impurities. The Fe component includes Fe in the steel outer shell, iron powder added in the flux, and Fe in the alloy component.
In addition to the above, if necessary, an oxide or fluoride of Na, K or fluoride (Na ₂ O, NaF, K ₂ O, KF, K ₂ SiF ₆ , K ₂ ZrF ₆ ) may be further contained as an arc stabilizer. Good. Note that the oxides and fluorides exemplified here are not included in the above metal oxides and metal fluorides.

Subsequently, the form of the flux-cored wire will be described.
The flux-cored wire includes a seamless wire having no slit-like gap in the steel outer shell as shown in FIG. 5 (a), and a slit-like gap in the steel outer shell as shown in FIGS. 5 (b) and 5 (c). It can be roughly classified into a wire having a seam.
In the present invention, any cross-sectional structure can be adopted. However, in order to suppress cold cracking of the weld metal, it is preferable to use a wire (seamless wire) having no slit-like gap in the steel outer sheath.
Moreover, in order to improve the feeding property of the wire at the time of welding, a lubricant can be apply | coated to the wire surface. As the lubricant for the welding wire, various kinds of lubricants can be used, but in order to suppress the low temperature cracking of the weld metal, it is preferable to use perfluoropolyether oil (PFPE oil).

Hydrogen entering the welded portion diffuses into the weld metal and the steel material side, accumulates in the stress concentration portion, and causes cold cracking. This hydrogen source can increase moisture contained in the welding material, moisture mixed in from the atmosphere, rust and scale attached to the steel surface, etc., but under the welding where the cleanliness of the weld and the gas shield conditions are sufficiently controlled. Then, hydrogen mainly contained in water in the wire is a main factor of diffusible hydrogen in the welded joint.
For this reason, it is desirable that the steel outer shell has a slit-like gap (seamless) and suppresses the intrusion of hydrogen in the atmosphere from the steel outer shell to the flux after the wire is manufactured and used.
In the case of a pipe having a slit-like gap (seam) in the steel outer skin, moisture in the atmosphere easily enters the flux from the gap of the outer skin, and as it is, prevents entry of hydrogen sources such as moisture. Therefore, when the period until use after production is long, a hydrogen source such as hydrogen may enter from the atmosphere and increase the amount of hydrogen in the weld metal. In order to prevent this, it is desirable to take measures to prevent intrusion of the hydrogen source such as vacuum packaging the entire wire, storing it in a container that can be kept dry, or filling the gap by a method such as brazing.

The flux cored wire of the present invention configured as described above can be manufactured by a normal flux cored wire manufacturing process.
That is, first, a steel strip to be the outer skin, and a flux containing metal fluoride, an alloy component, a metal oxide, a metal carbonate and an arc stabilizer so as to have predetermined contents are prepared, and the steel strip is elongated. Formed into an open tube (U-shaped) with a forming roll while feeding in the direction to form a steel outer shell. During this forming, flux is supplied from the opening of the open tube, and the opposite edge surfaces of the opening are butt seam welded. In the slit shape, the seamless pipe obtained by welding is drawn, annealed in the middle of the drawing or after completion of the drawing process, has a desired wire diameter, and the inside of the steel outer shell is filled with flux. Get a wire with no gaps (seamless). A wire having a seam can be obtained by supplying a flux from an opening of an open pipe, then forming a pipe having a slit-like gap that does not perform seam welding, and drawing the pipe.

A cross section of a wire without slit-like gaps made by butt seam welding looks like FIG. 5 (a). If this cross section is polished and etched, welding marks are observed, but if not etched, no welding marks are observed. Therefore, it may be called seamless as described above. For example, it is described as “seamless type” in the “Introduction to Welding and Joining Techniques” (2008) Sangyo Publishing, p.111 edited by the Japan Welding Society.
FIG. 5B shows an example in which the edge surfaces are butted together, and FIG. 5C shows an example in which the edge surfaces are crimped. After the butting as shown in FIG. 5B, brazing or FIG. Even after brazing as in c), a wire having no slit-like gap can be obtained. In FIGS. 5B and 5C, the wire as it is without brazing becomes a wire having a slit-like gap.

The flux-cored wire of the present invention uses gas shielded arc welding as a welding method, and is used for fillet welding, tack welding, and the like of a high-strength steel plate having a tensile strength of 540 MPa to 690 MPa in a thick plate having a thickness of 6 mm or more. Suitable for doing.
The conditions of the shielding gas are not particularly limited, but a mixed gas of Ar and 5 to 30 vol% CO ₂ is preferable. Further, although the spatter increases, the effect of the present invention can be obtained even when 100 vol% carbon dioxide gas is used.

Next, the feasibility and effects of the present invention will be described in more detail with reference to examples.
While forming the steel strip into the open tube with the forming roll while feeding the steel strip in the longitudinal direction, supplying the flux from the opening of the open tube in the middle of the forming, and butt seam welding the opposite edge surfaces of the opening, slit shape A pipe with no gap was added, and annealing was performed in the course of wire drawing of the piped wire, and a flux-cored wire with a final wire diameter of φ1.2 mm was made as a prototype. After the trial production, a lubricant was applied to the wire surface. In addition, a part of the tube was not subjected to seam welding and had a slit-like gap, and a wire with a wire diameter of φ1.2 mm was prototyped by drawing it. In the case of a wire having a slit-like gap, the entire wire was vacuum packaged and stored in a container that can be kept dry until welding.
The component composition of the prototype flux-cored wire is shown in [Tables 1-1 and 2] and [Tables 2-1 and 2].
Note that TiO ₂ , SiO ₂ , MgO, and Al ₂ O ₃ were used for the Ti oxide, Si oxide, Mg oxide, and Al oxide, respectively.

  Using this flux-cored wire, the mechanical properties of the weld metal were evaluated in accordance with JIS Z3111 (2005). That is, the procedure is as shown in FIG. A steel plate 1 having a thickness of 20 mm is abutted at a root gap of 16 mm and a groove angle of 20 °, and a backing metal 2 is used. The first and second layers are 1 or 2 passes, and the third to final layers are 2 or 3 A test specimen was prepared by welding with a pass. The welding conditions are shown in [Tables 3-1, 2]. In addition, although SM490A was used for the steel plate 1 and the backing metal 2, two or more layers and 3 mm or more of the grooved surface of the steel plate 1 and the surface of the backing metal 2 using a flux-cored wire to be tested. Battering was performed and used.

From the prepared specimen, as shown in FIG. 3, A0 tensile test piece (round bar) (diameter = 10 mm) 5 and Charpy test piece (2 mmV notch) 4 conforming to JIS Z3111 (2005) as mechanical test pieces. Were collected and subjected to respective mechanical property tests to measure the yield strength, tensile strength and Charpy absorbed energy of the weld metal. The measurement results and evaluation results of the obtained mechanical properties are shown in [Tables 4-1 and 2].
The mechanical properties were evaluated as acceptable when the tensile strength was 570 to 780 MPa and the toughness was a Charpy impact test at −40 ° C. and the absorbed energy was 47 J or more.

Next, fatigue characteristics were evaluated for the flux-cored wires that passed both the tensile strength and the Charpy absorbed energy in the evaluation results of Table 4. The high-strength steels used in the tests were welded high-strength steel sheets WEL-TEN540 (symbol: P1), WEL-TEN590 (symbol: P2), WEL-TEN690 (symbol: P3) (trade name: Nippon Steel & Sumikin Co., Ltd.) (Made by company) having a plate thickness of 20 mm was used. [Table 5] shows the components and mechanical properties of the steel sheet.
Depending on the strength level of the steel sheet, a load non-transmission type cross welded joint specimen shown in FIG. 4 was welded using the flux-cored wires shown in Tables 1 and 2. [Tables 6-1 and 2] show the flux-cored wires, steel plates, and welding conditions used.

About the obtained cross welded joint test body, the fatigue test was implemented in the state in which the welding part was welded. The fatigue test was performed under the conditions of a stress ratio of 0.1 and a frequency of 10 Hz, and the stress range in which fracture was not caused by repetition of 2 × 10 ⁶ times or more was evaluated as fatigue strength. In this test, a fatigue strength of 160 MPa or more, which provides fatigue characteristics as excellent as those obtained by grinding the weld toe, was regarded as acceptable.

  Next, the low-temperature cracking resistance was evaluated for the flux-cored wire in which the tensile strength, Charpy absorbed energy, and fatigue strength all passed in the evaluation results of Table 4. The low temperature cracking resistance was evaluated by measuring the amount of diffusible hydrogen and y-type weld cracking test.

  The amount of diffusible hydrogen was measured by a gas chromatograph method in accordance with JIS Z 3118 (method for measuring the amount of hydrogen in steel welds) under the same welding conditions as in the mechanical property test. The results of the diffusivity measurement are shown in [Tables 4-1, 2].

The y-type weld crack test is a steel plate (symbol) of a high-strength steel sheet for welded structure WEL-TEN690 (trade name: manufactured by Nippon Steel & Sumikin Co., Ltd.) under constant atmosphere control at a temperature of 0 ° C. and a humidity of 60%. : P4) was carried out by a method based on JIS Z 3158 (y-type weld cracking test) under the welding conditions shown in [Table 3].
The obtained y-type weld crack test results are shown in [Tables 4-1 and 2]. When diffusible hydrogen is less than 1.5 ml / ml, there is no cross-section crack in all cross sections of the y-type weld crack test even at a very low temperature of 0 ° C. and no preheating (no cross-section cracks have occurred) And extremely high cold cracking resistance was proved.
The thickness of the specimen used in this test is thick, and since there is no preheating, the cooling rate is extremely fast and the binding force is high, so that the conditions are as severe as those of tack welding. Therefore, the fact that cracking did not occur without preheating in this test indicates that cold cracking can be suppressed even with tack welding and without preheating.

As shown in the test results of [Tables 4-1 and 2], the wire numbers A1 to A32 as examples of the present invention were all excellent in strength, toughness, fatigue strength, and cold cracking resistance, and passed.
On the other hand, the wire numbers B1 to B34 which are comparative examples do not satisfy the requirements defined in the present invention, and therefore cannot satisfy at least one term of strength, toughness, fatigue strength, and low temperature cracking resistance, and are rejected in the comprehensive judgment. became.

  When welding high-strength steel exceeding 490 MPa, by using the flux-cored wire of the present invention, a welded portion having excellent fatigue strength can be obtained. The weight of the structure can be reduced, and the wire of the present invention is also excellent in low temperature cracking resistance. Preheating work for suppressing low temperature cracking is not required even for high-strength steel, or the preheating work is significantly reduced. Therefore, the welding operation efficiency can be remarkably improved, and the value in the industry is extremely high.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Backing metal 3 Weld bead 4 2mmV notch Charpy impact test piece 5 Round bar tensile test piece

Claims (5)

A flux-cored wire for gas shielded arc welding in which a flux is filled inside a steel outer shell,
During the wire, wire percentage by weight relative to the total _weight, CaF _{2, BaF} 2, SrF _2, MgF 2, 3.1-6.5% as the total amount alpha 1, two or more of LiF, Ti oxide 1 type or 2 types or more among products, Si oxides, Mg oxides, Al oxides, Ca oxides, containing 0.4 to 1.4% as a total amount β,
And the ratio of the content of the CaF _{2 to} the total amount α is 0.90 or more, the content of the Ca oxide is less than 0.2%, and the total amount α relative to the total amount β So that the ratio is 3.0 or more and 15.0 or less,
As an alloy component, in mass% with respect to the total mass of the wire,
C: 0.03-0.10%,
Si: 0.3 to 1.0%,
Mn: 1.8-3.0%,
P: 0.02% or less,
S: 0.02% or less,
Al: 0.003 to 0.05%,
Bi: 0.004 to 0.030%
In addition,
Ni: 0.2-1.4%
Cr: 0.1 to 0.8%
1 or 2 or more of them, the balance consists of Fe and inevitable impurities, NC defined by the following formula (a) is 0.1 to 1.2%, and the following ( b) A flux-cored wire characterized by satisfying the relationship of the formula:
NC = [Ni] / 2 + [Cr] (a)
100 × [Bi] ≧ NC (b)
However, the element with [] represents the content (% by mass) of each element. The flux-cored wire is further mass% with respect to the total mass of the wire,
Cu: 0.1 to 0.5%,
Mo: 0.1 to 0.8%,
V: 0.01-0.04%,
Ti: 0.01 to 0.3%,
Nb: 0.01 to 0.1%,
B: 0.0003 to 0.01%
The flux-cored wire according to claim 1, wherein one or more of them are contained.   The flux cored wire according to claim 1 or 2, wherein the steel outer shell has no slit-like gap.   The flux cored wire according to claim 1, wherein the steel outer shell has a slit-like gap.   The flux cored wire according to any one of claims 1 to 4, wherein perfluoropolyether oil is applied to a surface of the steel outer shell.