JP2009248175A - Tig welding method of high-strength steel using flux-containing wire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a TIG welding method of high-strength steel by using a flux-containing wire, the method capable of advantageously solving the problem of a decrease of tensile strength and toughness of a welding metal due to unevenness of a component in the welding metal. <P>SOLUTION: The flux wire has a steel-made outer skin having a cross sectional thickness of 0.30 to 1.0 mm; contains 0.04 to 0.4% (by mass% based on the whole mass of the wire) of C, 0.2 to 2.0% of Si, 0.3 to 2.0% of Mn and 0.002 to 0.05% of Al, and further contains one or two or more of 0.1 to 12% of Ni, 0.01 to 4.0% of Cr, 0.1 to 4.0% of Mo, 0.1 to 4.0% of W and 0.01 to 1.5% of Cu, the remaining components being iron and an inevitable impurity; and satisfies that a carbon equivalent is 0.40 to 1.5%, and that a difference of the carbon equivalent between the whole wire and the steel-made outer skin is not less than 0.10%. The TIG welding is conducted by using the flux and inputting welding heat of 1.70 to 4.0 kJ/cm×g per g of welding wire. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to TIG welding of high-strength steel using a flux-cored wire in which a flux containing at least a metal or an alloy is filled inside the steel outer shell, and in particular, a steel outer shell melted by an arc of TIG welding Flux-cored wire that can ensure the strength and toughness of the weld metal stably by obtaining a weld metal with uniform components, with no component unevenness in the weld metal caused by insufficient mixing of the internal flux. The present invention relates to a TIG welding method for high-strength steel using steel.

  In recent years, as the demands for larger and lighter structures such as construction machinery and industrial machinery have increased, the strength of steel plates used has increased, and recently, high strength steels with a tensile strength TS of 780 MPa are also available. It has come to be used generally, and the demand for higher strength is also increasing.

  In addition, steel structures in which such high strength steel is used often require low temperature toughness as well as strength. When producing a welded joint using steel with a tensile strength of 780 MPa or high strength steel with a tensile strength higher than that, the toughness of the welded joint decreases, and a high strength and high toughness weld metal is required. It will be difficult to obtain. In order to obtain high strength and high toughness welded joints, alloy elements are added in the welding wire composition design, and the strength of the weld metal is ensured by bainite or martensite by improving the hardenability. In addition, with regard to toughness, low temperature toughness is ensured by adding Ni that is effective for maintaining toughness as well as strength. Moreover, in the welding of high-strength steel, there is a concern about the occurrence of low-temperature cracking due to hydrogen contained in the weld metal. TIG welding that can reduce the hydrogen and oxygen contained in the weld metal by using Ar gas as the shield gas for gas shielded arc welding to suppress the low temperature cracking of the weld metal and obtain a weld joint with high strength and toughness Is preferred.

Recently, a high-strength wire technology development has yielded a weld metal that has a tensile strength of 950 MPa or more and a good absorbed energy vE- ₄₀ in a 2 mmV notch Charpy impact test at -40 ° C. A wire is realized (see Patent Document 1).

  However, such a solid wire for high-strength steel has high material strength and is easily broken by adding work hardening in the wire drawing process of solid wire manufacturing. The rate is limited small, the number of wire drawing is increased, and the wire drawing speed is slowed down. Moreover, since the alloy element which improves hardenability is contained in order to ensure strength, softening due to annealing is small, so the number of annealings during the wire drawing process is increased. For these reasons, the solid wire for high-strength steel has problems of low productivity and high manufacturing cost.

Flux-cored wire uses mild steel or a steel steel with a strength level equivalent to that of mild steel, and contains an alloying element that enhances hardenability from the flux contained in the steel shell. Even if it is a welding wire, manufacturability is not different from the welding wire for general mild steel. However, the flux-cored wire for high-strength steel is only realized for MIG welding and MAG welding (Ar + CO ₂ welding or CO ₂ welding).

  Conventionally, in MAG welding, a flux-cored wire has been used from the viewpoint of low spattering properties and low slag properties. On the other hand, in TIG welding, since spatter and slag generally do not occur, an example of using a flux-cored wire for the purpose of improving welding workability has been reported. A wire was never used (for example, refer to Patent Documents 2 and 3). This is because the amount of oxygen in the weld metal increases during welding due to the oxide on the powder particle surface of the metal or alloy in the internal flux of the flux-cored wire or the moisture adsorbed on the particle surface during the flux granulation process. This is because the toughness and ductility of the metal are reduced.

  On the other hand, in order to solve the problems of productivity and manufacturing cost of solid wire for high-strength steel, TIG welding using a flux-cored wire is desired.

  In examining the TIG welding of high-strength steel using a flux-cored wire, the present inventors mixed the components of the steel outer shell melted by the arc and the components of the metal or alloy of the molten flux inside. In the case of TIG welding, the agitation of the molten pool is large and the agitation of the molten pool is small, and the time until solidification is short. We found the phenomenon of uneven component in the weld metal due to the steel outer component remaining in the part. In the flux-cored wire for high-strength steel, an alloy component that enhances hardenability is added from the flux, but the alloy component is not mixed in the uneven component portion, so the hardenability is low compared to the surroundings, and bainite Or, it does not become martensite and has low strength. When such a component uneven part exists, in the tensile test, the strength decreases according to the area ratio of the portion with low strength, and the strength predicted from the component of the weld metal cannot be secured. Also, with regard to toughness, since there is a difference in strength at the boundary between the part where the alloy elements are mixed and the component unevenness part, stress tends to concentrate, and it becomes the starting point of fracture in the Charpy impact test, resulting in deterioration of toughness. Cause.

JP 2006-110581 A JP 61-154793 A Japanese Patent Laid-Open No. 11-171592

  Therefore, in view of the problems of the background art described above, the present invention provides component unevenness in the weld metal that tends to occur when TIG welding is performed on high-strength steel using a flux-cored wire, and consequently the tensile strength and toughness of the weld metal. An object of the present invention is to provide a TIG welding method for high-strength steel using a flux-cored wire, which can advantageously solve the problem of characteristic deterioration such as the above.

  The present invention solves the above technical problems, and the requirements of the invention are as follows.

  (1) In the TIG welding method for high-strength steel using a flux-cored wire in which a flux containing at least a metal or an alloy is filled inside the steel outer shell, the cross-sectional thickness of the steel outer shell is 0.30 to 0.30. It is 1.0 mm, and is mass% with respect to the total mass of the wire, C: 0.04 to 0.4%, Si: 0.2 to 2.0%, Mn: 0.3 to 2.0%, P: 0 0.02% or less, S: 0.01% or less, Al: 0.002 to 0.05%, Ni: 0.1 to 12%, Cr: 0.01 to 4.0%, Mo: Contains one or more of 0.1 to 4.0%, W: 0.1 to 4.0%, Cu: 0.01 to 1.5%, the balance being iron and inevitable impurities Further, the carbon equivalent (Ceq) represented by the following (formula 1) satisfies 0.40 to 1.5%, and Using a flux-cored wire in which the difference between the carbon equivalent of the whole component and the carbon equivalent of the steel outer shell component satisfies 0.10% or more, welding per 1 g of welding wire represented by the following (formula 2) A TIG welding method for high-strength steel using a flux-cored wire, characterized in that TIG welding is performed in a heat amount range of 1.70 to 4.0 kJ / cm · g.

Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Mo] / 4 + [Cr] / 5 + [W] / 8 + [Cu] / 40 + [Ti] / 30 + [Nb] / 3+ [V] / 5 + [Ta] / 8 + [Co] / 40 + 5 × [B] (Formula 1)
However, the element with [] represents the content (% by mass) of each element.

    [Welding heat input per 1 g of wire (kJ / cm · g)] = [Welding current (A)] × [Welding voltage (V)] × 60 / [Welding speed (cm / min)] / [Welding amount (g / Min)] (Equation 2)

  (2) The flux-cored wire is further in mass% with respect to the total mass of the wire, Ti: 0.005-0.3%, Nb: 0.005-0.1%, V: 0.005-0.5. %, Ta: 0.005 to 0.5%, Co: 0.01 to 6%, and B: 0.001 to 0.015%. A TIG welding method for high-strength steel using the flux-cored wire described in (1) above.

  (3) The flux-cored wire further contains one or two of Ca: 0.0002 to 0.01% and REM: 0.0002 to 0.01% by mass% with respect to the total mass of the wire. A TIG welding method for high-strength steel using the flux-cored wire according to (1) or (2) above.

  (4) The TIG welding method for high-strength steel using the flux-cored wire according to any one of (1) to (3), wherein the flux-cored wire is a hot wire.

  According to the present invention, the uneven component in the weld metal that is likely to occur when the flux-cored wire is applied to TIG welding is advantageously eliminated by defining the wire and the welding conditions, and a uniform component weld metal is obtained. Stable tensile strength and toughness can be secured. As a result, the flux-cored wire can be applied to TIG welding of high-strength steel as an alternative to solid wire for high-strength steel, which has problems in productivity and manufacturing cost. .

  Embodiments of the present invention will be described below.

  In general, weld metal is basically a solid structure and cannot be controlled by a fine graining process such as hot rolling or a tempering process for the entire structure like steel sheets, so the strength of the weld metal is increased. As demands on the metal alloy become higher, the hardenability is improved by adding alloy elements to the welding wire, and the strength is ensured by making the weld metal structure bainite or martensite. Moreover, since low temperature toughness is also required at the same time as strength, low temperature toughness is ensured by adding Ni effective for maintaining toughness as well as strength to the welding wire.

  When a solid wire having such an alloy composition is manufactured, the strength of the material is increased due to the alloying elements contained in the material, and work hardening in the wire drawing process is also added, so that the wire is easily disconnected. For this reason, a device has been devised to limit the single area reduction rate of wire drawing, increase the number of wire drawing, and further reduce the speed of wire drawing. Moreover, since the alloy element which improves hardenability is contained in order to ensure strength, softening due to annealing is small, so the number of annealings during the wire drawing process is increased. For these reasons, the solid wire for high-strength steel has poor productivity and increased manufacturing costs.

  Therefore, in view of problems in manufacturing a solid wire for high-strength steel, the present inventors adopted a flux-cored wire, and without increasing the hardenability element in the steel outer shell, the hardenability element is TIG welding method for high-strength steel using flux-cored wire on the premise of the same productivity and manufacturing cost as flux-cored wire for mild steel by containing as a metal or alloy in the flux filled in the outer skin Was examined.

Since the tensile strength that begins to affect productivity and manufacturing cost in solid wire production is about 700 MPa, the absorbed energy vE ₋ by the 2 mmV notch Charpy impact test at −40 ° C. with a tensile strength TS of 700 MPa or more. That ₄₀ can ensure 27J or more is the target mechanical property of the flux-cored wire used in the TIG welding method of the present invention.

  In TIG welding, a welding wire is inserted into an arc generated between a tungsten electrode and a base material, and the wire is melted and welded by the heat of the arc. When flux-cored wire is applied to TIG welding, the steel outer shell component melted by the heat of the arc and the alloy element component of the inner flux are mixed by diffusion, but in TIG welding the molten pool is less agitated In addition, since the time until solidification is short, the steel outer shell component and the internal flux alloy element component are not sufficiently mixed, and the phenomenon of uneven component in the weld metal occurs due to the steel outer shell component remaining. . FIG. 1 is a cross-section of a welded joint, and a portion 1 that remains as an outer skin component without being mixed with a component can be seen in a lower portion in one pass. Since the component unevenness part is not mixed with the alloy component, the hardenability is low as compared with the surroundings, and the strength is low without becoming bainite or martensite. When such a component uneven part exists, in the tensile test, the strength decreases according to the area ratio of the portion with low strength, and the strength predicted from the component of the weld metal cannot be secured. In addition, as for toughness, because there is a difference in strength at the boundary between the component unevenness part and the part containing baking, the stress tends to concentrate, causing fracture toughness in the Charpy impact test and causing toughness deterioration. I found out that

  The present invention eliminates the unevenness of components in the weld metal that affects the tensile strength and toughness when TIG welding is performed on a steel core or a flux-cored wire containing a component within a predetermined range in the flux. The goal of ensuring stable tensile strength and toughness by obtaining a weld metal is the result of repeated experimental research. As a result, the steel sheath thickness of the cross-section of the flux-cored wire and the welding heat input per gram of welding wire are within the specified range. It was found that a weld metal having no component unevenness can be obtained.

  The details of the wire and welding conditions that can eliminate the component unevenness in the weld metal of the present invention will be described.

  On the premise that welding is performed under the welding conditions satisfying the welding heat input conditions per 1 g of the welding wire, the steel outer thickness of the cross section of the flux-cored wire may be 0.30 to 1.0 mm, more preferably 0. It is good to set it as 40-0.80 mm. If the wire skin thickness is less than 0.30 mm, wire breakage may occur in the wire drawing process, and wire productivity deteriorates. Also, if the thickness exceeds 1.0 mm, the steel outer shell becomes thicker, so the time required for the components of the metal or alloy of the internal flux and the steel outer shell components to be uniformly mixed by diffusion becomes longer. May solidify to cause unevenness in the weld metal. Here, the skin thickness is a value at the final wire diameter, and the skin thickness before or during drawing is not relevant.

Assuming that welding is performed with a wire that satisfies the above conditions, the welding heat input per 1 g of welding wire expressed by the following (formula 2) is in the range of 1.70 to 4.0 kJ / cm · g. More preferably, it is good to set it as 1.8-2.8 kJ / cm * g. If the welding heat input per 1 g of welding wire is less than 1.70 kJ / cm · g, there is almost no agitation of the molten pool of TIG welding and component unevenness occurs in the weld metal. On the other hand, if it exceeds 4.0 kJ / cm · g, the heat input to the entire welded joint becomes high, so that the toughness of the weld metal or the weld heat affected zone deteriorates. Equation (2) is obtained by dividing the amount of heat input, which is an index of arc heat energy input per unit weld length generally used in the welding field, by the amount of welding. In TIG welding, the amount of heat input and the amount of welding can be controlled separately. Since the heat input and the amount of welding influence the temperature of the molten pool or the time until solidification, the formula (2) is necessary to uniformly mix the components of the steel sheath and the flux inside the wire. Give the right conditions.
[Welding heat input per 1 g of wire (kJ / cm · g)] = [Welding current (A)] × [Welding voltage (V)] × 60 / [Welding speed (cm / min)] / [Welding amount (g / Min)] (Equation 2)

  In addition, when a hot wire is used in the present invention, it takes longer to solidify the molten welding wire than normal TIG welding, which is effective in eliminating component unevenness. In the present invention, the hot wire itself is not particularly limited, and for example, a known technique disclosed in Japanese Patent Laid-Open No. 2003-320454 may be used.

  Next, details of the flux-cored wire used in the present invention will be described.

  The material of the steel sheath of the flux-cored wire is usually the same strength as mild steel, and the hardenability elements required to increase the strength are contained as a metal or alloy in the flux. The productivity and manufacturing cost of the cored wire are made equal to those of ordinary mild steel wires.

  Even if the flux inside the flux-cored wire contains a bulking agent such as a slag forming agent, a deoxidizing agent, and Fe powder in addition to those prescribed in the present invention, the effect is not changed.

  In addition, the effect of the present invention does not change even if the steel sheath of the flux-cored wire is manufactured by either a seamless pipe or caulking.

  Next, the reason for limiting the component composition of the flux-cored wire will be described.

  First, the steel outer shell constituting the flux-cored wire, the components contained in the flux, and the reasons for limiting the content thereof will be described.

In addition, the content of each component shown below includes the component content in the steel sheath and the component content in the flux measured by component analysis of the steel sheath and the flux, and the filling rate of the flux (total wire Based on (mass% of total mass of flux with respect to mass)), it can be obtained by the following (formula 3).
Content of component i in wire (% by mass) = Content of component i in steel outer sheath (% by mass) × (1−filling rate) + Content of component i in flux (% by weight) × Filling rate ... (Formula 3)
In addition, the said flux total mass means the total amount of the component content in the flux containing the slag formation agent and the arc stabilizer other than the element added as a metal or an alloy. Moreover, the component content in a flux means the ratio of the element amount in the metal and alloy which contribute to a weld metal composition with respect to the flux total mass.

  In the following description, “%” means “% by mass” unless otherwise specified. In addition, the content of each component element in the following flux-cored wire is expressed as a ratio (mass%) to the whole wire, and means the total amount of components contained in either or both of the steel outer sheath and the flux.

[C: 0.04 to 0.4%]
C is an element essential for increasing the tensile strength of the weld metal, and is hardly adopted for solid wires because it adversely affects the manufacturability and productivity. In order to ensure, C is contained 0.04% or more. However, since excessive inclusion of C significantly deteriorates the toughness of the weld metal, the upper limit in the welding wire is set to 0.4% in order to ensure the toughness of the weld metal. Therefore, in the present invention, the C content in the welding wire is 0.04 to 0.4%. In addition, at 0.09% or more where the difficulty of manufacturing with a solid wire becomes remarkable, the effect of forming a cored wire becomes remarkable, which is preferable.

[Si: 0.2-2.0%]
Si is a deoxidizing element, and in order to reduce the amount of O in the weld metal and increase the cleanliness, the Si content in the welding wire needs to be 0.2% or more. On the other hand, if the Si content in the welding wire exceeds 2.0% and becomes excessive, a coarse oxide is generated and the toughness of the weld metal is significantly deteriorated. For this reason, in this invention, Si content in a welding wire shall be 0.2 to 2.0%.

[Mn: 0.3 to 2.0%]
Mn is an element that ensures the hardenability of the weld metal and enhances the strength, and is also effective for improving the toughness by refining the structure. It is necessary to contain. On the other hand, if the Mn content in the welding wire exceeds 2.0%, residual austenite is excessively generated in the weld metal, so that the susceptibility to intergranular embrittlement increases and the weld metal deteriorates toughness and weld crack resistance. The possibility of is increased.
For this reason, in this invention, Mn content in a welding wire shall be 0.3-2.0%.

[P: 0.02% or less]
P is an impurity element and needs to be reduced as much as possible in order to inhibit toughness. However, if the content in the welding wire is 0.02% or less, adverse effects on toughness can be tolerated. Therefore, in the present invention, P in the welding wire is acceptable. The content is 0.02% or less.

[S: 0.01% or less]
S is also an impurity element, and if it is excessively present in the weld metal, it deteriorates both toughness and ductility, so it is preferable to reduce it as much as possible.

  If the content in the welded wire is 0.01% or less, adverse effects on toughness and ductility can be tolerated. Therefore, in the present invention, the S content in the welding wire is set to 0.01% or less.

[Al: 0.002 to 0.05%]
Al is a deoxidizing element and, like Si, is effective in reducing the amount of oxygen in the weld metal and improving cleanliness. In order to exert the effect, it is necessary to contain 0.002% or more in the welding wire. On the other hand, if the content exceeds 0.05% in the welding wire, a coarse oxide is formed in the weld metal, and this coarse oxide significantly deteriorates the toughness, which is not preferable. Therefore, in this invention, Al content in a welding wire shall be 0.002-0.05% or less.

  In the present invention, in order to ensure the target tensile strength and toughness of the weld metal, in addition to the above components, the welding wire further includes one of Ni, Cr, Mo, W, and Cu, Or it is a case where 2 or more types are contained in the following predetermined ranges.

[Ni: 0.1 to 12%]
Ni is the only element that can stably improve toughness regardless of the other components and structure of the weld metal by solid solution toughening. In particular, it is an element necessary to ensure toughness with a high-strength weld metal. Yes, it is necessary to contain 0.1% or more.

A higher Ni content is advantageous in improving toughness, but if the content in the welding wire exceeds 12%, the effect of improving toughness is saturated. Therefore, in the present invention, the Ni content in the welding wire is limited to 0.1 to 12%.
In addition, in order for the effect of Ni to contribute to an improvement in toughness reliably, 1.0 to 10.0% is more preferable. Furthermore, in order to ensure toughness at low temperature, 6.0 to 10.0% is more preferable.

[Cr: 0.01-4.0%]
Cr is an element effective for increasing the strength by enhancing the hardenability. Therefore, when it contains in a welding wire, 0.01% or more is required. On the other hand, if the content exceeds 4.0%, bainite and martensite are hardened unevenly and the toughness is remarkably deteriorated. Therefore, in the present invention, the upper limit of the content in the welding wire is 4.0. %. In order to surely achieve high strength through hardenability, it is preferably 0.5% or more. Moreover, since the contribution to the strengthening through the hardenability of Cr gradually decreases when it exceeds 2.5%, it is preferable to make it 2.5% or less.

[Mo: 0.1 to 4.0%]
Mo is a hardenability improving element for increasing the tensile strength TS of the weld metal. Moreover, strength and toughness can be ensured by increasing tempering resistance. In order to exert these effects, it is necessary to contain 0.1% or more of Mo in the wire.
On the other hand, if Mo is contained in the welding wire in an amount exceeding 4.0%, coarse precipitates are generated in the weld metal and the toughness of the weld metal is deteriorated. For this reason, in this invention, Mo content in a welding wire shall be 0.1-4.0%. In order to surely exert the effect of tempering resistance, it is preferably set to 0.3 or more. Moreover, since the effect of tempering resistance is gradually reduced when it exceeds 2.0%, it is preferable to make it 2.0% or less.

[W: 0.1-4.0%]
W, like Mo, is a hardenability improving element. Moreover, it is effective for securing strength by forming fine carbides and strengthening the precipitation. In order to exert these effects, at least 0.1% is necessary even in consideration of combined effects with other elements having similar effects. On the other hand, if it exceeds 4.0% and is contained in the welding wire, the toughness deteriorates remarkably.

  For this reason, in this invention, content when making W contain in a welding wire shall be 0.1-4.0%. In order to surely achieve high strength by precipitation strengthening, it is preferably 0.5% or more. Moreover, since the contribution to the strengthening by precipitation strengthening of W gradually decreases when it exceeds 1.5%, it is preferable to be 1.5% or less.

[Cu: 0.01 to 1.5%]
Cu is an element effective for improving the strength, and in order to sufficiently obtain the effect of improving the strength of the weld metal, the content of Cu contained in the wire, and in the case where Cu is plated on the surface, The total content of Cu to be contained and Cu to be plated needs to be 0.01% or more.

  On the other hand, if the Cu content in the wire exceeds 1.5%, the case where the wire surface is plated or the case where it is contained in the wire is not preferable because the toughness of the weld metal deteriorates.

  Therefore, in this invention, it is preferable that Cu content in a wire shall be 0.01 to 1.5%.

In the present invention, in order to ensure the target tensile strength TS and toughness, when the above components are added within the specified ranges of the respective contents, the weld metal represented by the following (formula 1) is further added. This is a case where the carbon equivalent (Ceq) indicating the quenching hardness becomes the content of each component in the wire within a predetermined range.
Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Mo] / 4 + [Cr] / 5 + [W] / 8 + [Cu] / 40 + [Ti] / 30 + [Nb] / 3+ [V] / 5 + [Ta] / 8 + [Co] / 40 + 5 × [B] (Formula 1)
However, the element with [] represents the content (% by mass) of each element.

  In order to ensure the target tensile strength, the contents of C, Mn, Si, Ni, Mo, Cr, W, Cu, Ti, Nb, V, Ta, Co, and B contained in the welding wire Therefore, it is necessary to limit the carbon equivalent Ceq, which is an index of the quenching hardness obtained in the above (Equation 1), to 0.40% or more. When the carbon equivalent Ceq is less than 0.40%, the quenching hardness is insufficient and the target tensile strength TS cannot satisfy 700 MPa. The larger the carbon equivalent, the higher the quenching hardness. However, if the carbon equivalent exceeds 1.5%, the toughness of the weld metal deteriorates, which is not preferable. For the above reasons, in the present invention, the carbon equivalent (Ceq) of the welding wire is limited to 0.40 to 1.5%.

  Furthermore, in order for the difference in strength between the uneven portion of the weld metal and its peripheral portion to affect the tensile strength and toughness, the difference between the carbon equivalent of the component of the entire welding wire and the carbon equivalent of the component of the steel sheath This is a case where the absolute value is 0.10% or more. If it is less than 0.10%, the difference in hardness between the component unevenness portion and its peripheral portion is small, and the influence on the tensile strength or toughness is small.

  The above is the reason for limiting the basic component elements of the flux-cored wire used in the TIG welding of the present invention and the contents of impurities to be restricted. The present invention further provides for the adjustment of specific mechanical properties of the weld metal, optionally in the wire and further with one or more of Ti, Nb, V, Ta, Co and B, or When two or more kinds are contained in the welding wire in the following content range, the tensile strength or toughness is affected.

[Ti: 0.005 to 0.3%]
Ti is effective as a deoxidizing element in the weld metal, and can fix the solid solution N in the weld metal as a nitride to alleviate the adverse effect on the toughness of the solid solution N. In this case, there is also an effect of refining superheated austenite grains in the reheat region of the weld metal. In order to exhibit the effect of improving the toughness of the weld metal by the action of Ti, it is necessary to contain 0.005% or more of Ti in the welding wire. On the other hand, if the Ti content in the welding wire exceeds 0.3% and excessive, there is a great possibility that the formation of coarse oxides in the weld metal and the toughness deterioration due to excessive precipitation of TiN will remarkably occur. It becomes. For this reason, in this invention, Ti content in a welding wire shall be 0.005-0.3%.

[Nb: 0.005 to 0.1%]
Nb is also a ferrite stabilizing element and is effective in reducing retained austenite, and is effective in securing strength by forming fine carbides and strengthening precipitation. In order to exert these effects, it is necessary to make the Nb content in the wire 0.005% or more in consideration of a composite effect with other elements having similar effects. On the other hand, if the Nb content in the wire exceeds 0.1%, it is not preferable because it is excessively contained in the weld metal and coarse precipitates are formed to deteriorate the toughness.
Therefore, in the present invention, the content when Nb is contained in the welding wire is 0.005 to 0.1%.

[V: 0.005 to 0.5%]
V forms fine carbides and is effective in securing strength by precipitation strengthening. In order to exhibit this effect, at least 0.005% is necessary even in consideration of the combined effect with other elements having similar effects. On the other hand, if it exceeds 0.5%, it is not preferable because it is excessively contained in the weld metal and coarse precipitates are formed to deteriorate the toughness.
Therefore, in the present invention, the content when V is contained in the welding wire is 0.005 to 0.5%.

[Ta: 0.005 to 0.5%]
Ta also forms fine carbides and is effective in securing strength by precipitation strengthening. In order to exert these effects, a minimum of 0.005% is necessary even in consideration of combined effects with other elements having similar effects. On the other hand, if it exceeds 0.5%, it is not preferable because it is excessively contained in the weld metal and coarse precipitates are formed to deteriorate the toughness. Therefore, in this invention, content in the case of containing Ta in a welding wire shall be 0.005-0.5%.

[Co: 0.01 to 6%]
Co is an element effective for adjusting the strength of the weld metal and suppressing the formation of retained austenite in the weld metal by suppressing the extremely lowering of the transformation points of the bainite and martensite of the weld metal. is there. In order to exert these effects reliably, it is necessary to contain 0.01% or more in the welding wire. On the other hand, if the content exceeds 6% in the wire, these effects are saturated and the manufacturing cost of the wire becomes excessive. Therefore, in the present invention, when Co is contained in the welding wire, the content range is preferably 0.01 to 6%.

[B: 0.001 to 0.015%]
B is an element that enhances the hardenability and contributes to the improvement of the strength of the weld metal, and also has the effect of improving the toughness of the weld metal by forming BN in combination with the solid solution N in the weld metal. In order to exert these effects reliably, the B content in the welding wire needs to be 0.001% or more. On the other hand, when the B content in the welding wire exceeds 0.015%, the B in the weld metal becomes excessive and forms coarse B compounds such as BN and Fe ₂₃ (C, B) ₆ to reverse the toughness. It is not preferable because it deteriorates. Therefore, in the present invention, when B is contained in the welding wire, it is preferable to limit the B content to 0.001 to 0.015%.

[One or more of Ca and REM: 0.0002 to 0.01%]
In the present invention, in addition to the above components, for the purpose of adjusting the ductility and toughness of the weld metal, one or more of Ca and REM may be used within the following ranges as necessary. It can be contained.

  Both Ca and REM are effective in improving ductility and toughness by changing the structure of sulfides and reducing the size of sulfides and oxides in the weld metal. In order to fully exhibit these effects, it is preferable that the contents of Ca and REM are both 0.0002% or more. On the other hand, if Ca and REM are excessively contained in excess of 0.01% in the wire, sulfide and oxide are coarsened, ductility and toughness are deteriorated, and weld bead shape is deteriorated. There is also the possibility of deterioration of weldability. For this reason, it is preferable that the upper limit of the content of Ca and REM in the wire is 0.01%.

  The effects of the present invention will be described in more detail with reference to examples. TIG welding was performed using a flux-cored welding wire having the chemical composition shown in Table 1, and the weld metal characteristics and the presence / absence of component unevenness in the weld metal were investigated.

  The flux-cored wire uses the steel outer skin shown in Table 2, and the required amount of metal or alloy in the flux and a bulking agent such as Fe powder for the purpose of adjusting the filling rate so as to have the chemical composition shown in Table 1. After the addition, seam welding was performed at the joint while processing the cross section into a circle, and a slit-free wire (hereinafter referred to as a seamless wire) and a machine-fastened wire by caulking were produced. Thereafter, the wire was cold drawn to a final wire diameter of 1.2 mm to 2.8 mm. Annealing is performed for the purpose of softening the work-hardened wire during the cold working.

  Since the tensile strength TS that begins to affect the productivity and manufacturing cost in solid wire production is about 700 MPa, the absorbed energy vE by the 2 mm V notch Charpy impact test at −40 ° C. with a tensile strength TS of 700 MPa or more. It was made into the target mechanical characteristic of the flux cored wire used for the TIG welding method of this invention that -40 can ensure 27J or more.

  FIG. 1 is a schematic view of a non-uniform portion of a steel shell component mainly remaining due to insufficient mixing of weld metal components, which is likely to occur in the TIG welding method of high-strength steel using a flux-cored wire, which is an object of the present invention. FIG. 2 is a diagram schematically showing the groove shape of the welded joint used in the example and the sampling procedure for a 2 mm V notch Charpy impact test piece. The steel plate used was SM490B material and was subjected to buttering on the groove portion of the steel plate 2 and the surface of the backing material 3 with the same welding wire used in the test in order to reduce the influence of dilution of the welding wire component as much as possible. As shown in FIGS. 1 and 2, the buttered steel plate is assembled with a groove angle of 45 ° and a V groove with a root gap of 12 mm, multi-layer welding is performed by TIG welding with a shield gas of 100% Ar, and a welded joint is obtained. The strength and toughness of the weld metal were evaluated by a round bar tensile test and a 2 mm V notch Charpy impact test. FIG. 1 shows an uneven component 1.

  As shown in FIG. 2, a 2 mm V notch Charpy impact test piece 5 and a round bar tensile test piece 6 were sampled from the weld metal 4 of the welded joint welded with the backing material 3 and examined for mechanical properties. Tensile strength is measured from the center of the thickness of the steel plate 2 and the center of the weld metal width of a round bar tensile test piece 6 having a parallel part diameter of 6 mm and a parallel part length of 32 mm. The samples were taken in parallel and measured by performing a tensile test at room temperature. Some weld metals did not produce a clear yield point or yield elongation. In that case, 0.2% proof stress (0.2% PS) was adopted as the yield stress YP.

  As shown in FIG. 2, the toughness is 10 mm × 10 mm square from the position where the center of the test piece is at the thickness ¼ position 7 of the steel plate 2 and the notch is at the center in the width direction of the weld metal 4. A test piece 5 having a length of 55 mm was collected and evaluated by an average absorbed energy vE-40 (average of three measured values) at −40 ° C. in a 2 mmV notch Charpy impact test.

  Determination of the presence or absence of component unevenness was performed as follows. Several macro test pieces with a weld bead cross section were cut out, subjected to mirror polishing and nital corrosion, and the presence or absence of component unevenness was determined by observation with an optical microscope. Since the component unevenness portion is a component of the steel outer shell, almost no alloy element is contained, and the structure and the manner of corrosion are clearly different as compared with the surroundings, so that the determination is easy.

  Table 3 shows the TIG welding conditions of welding wire numbers Y1A to Y7A in Table 1, and Table 4 shows the mechanical test results of welded joints produced under the welding conditions. Since the thickness of the steel outer shell of Y6A was thin, breakage occurred in the wire drawing process.

  The joint symbol Y1A-1 using the wire number Y1A satisfies the present invention, has no component unevenness, and has stable mechanical characteristics. Since the joint symbol Y1A-2 has insufficient welding heat input per 1 g of wire, the steel outer sheath and the internal flux components are not mixed and remain in the steel outer sheath, so that the components in the weld metal Unevenness was confirmed. Tensile test specimens were collected from the welded joint of Y1A-2 and tested. As a result, the tensile strength was more varied than the joint Y1A-1 using the same wire number Y1A. When the cross section of the test piece Y1A-2.1 after the tensile test where the tensile strength was low was investigated, there was clearly a component unevenness that did not contain the alloy component, and the tensile strength according to the area ratio of the component uneven portion Became lower.

  When the joint symbol Y1A-2 was subjected to a Charpy test, the value of vE-40 in the Charpy test piece Y1A-2.3 was lower than the other values. As a result of investigating the starting point of the fracture, the starting point of the fracture was the boundary part with the component unevenness, and the stress was concentrated due to the strength difference between the peripheral part of the component uneven part and the fracture occurred, so the vE-40 became low. .

  In the joint symbol Y1A-3, the welding heat input per 1 g of the wire is excessive, and the heat input applied to the entire joint is excessive, so that the toughness is significantly deteriorated and is lower than the target value of vE-40.

  The joint symbol Y1A-4 used a hot wire, had no component unevenness, and obtained stable mechanical characteristics.

  The joint symbol Y2A-1 using the wire number Y2A satisfies the present invention, has no component unevenness, and has stable mechanical characteristics. The joint symbol Y2A-2 has a low amount of welding heat input per gram of wire, resulting in component unevenness and large variations in mechanical properties. In Y2A-3, the amount of welding heat input per 1 g of wire was excessive, and the heat input applied to the entire joint was excessive, so that the toughness was greatly deteriorated and fell below the target value of vE-40. Y2A-4 used a hot wire, and stable mechanical properties were obtained.

  The joint symbol Y3A-1 using the wire number Y3A satisfies the present invention, has no component unevenness, and has stable mechanical properties. Since the joint symbol Y3A-2 has a low amount of welding heat input per gram of wire, the component unevenness occurs, and the variation in mechanical characteristics is also large. In Y3A-3, the amount of welding heat input per 1 g of wire was excessive, and the heat input applied to the entire joint was excessive, so that the toughness was greatly deteriorated and fell below the target value of vE-40.

  The joint symbol Y4A-1 using the wire number Y4A satisfies the present invention, has no component unevenness, and has stable mechanical properties. The joint symbol Y4A-2 has a low amount of welding heat input per gram of wire, so that component unevenness occurs and the mechanical characteristics vary greatly. In Y4A-3, the amount of welding heat input per 1 g of wire was excessive, and the heat input applied to the entire joint was excessive, so that the toughness was significantly deteriorated and fell below the target value of vE-40.

  The joint symbols Y5A-1 and Y5A-2 using the wire number Y5A satisfy the welding heat input requirements per gram of wire, but because the thickness of the steel outer sheath of the wire is as large as 1.03 mm, the outer sheath of the wire The component and the alloy component of the internal flux were not sufficiently mixed by diffusion, and component unevenness occurred. For this reason, the variation in mechanical characteristics has increased. In Y5A-3, the amount of welding heat input per 1 g of wire was excessive, and the heat input applied to the entire joint was excessive, so that the toughness was greatly deteriorated and fell below the target value of vE-40.

  The joint symbol Y7A-1 using the wire number Y7A satisfies the present invention, has no component unevenness, and has stable mechanical characteristics. In the joint symbol Y7A-2, although the component unevenness was confirmed in the weld metal, the difference between the carbon equivalent of the component of the whole wire of the wire symbol Y7A and the carbon equivalent of the component of the constituent sheath is 0.10% or less. The difference in strength between the component unevenness part and its periphery is small, and the tensile strength and toughness are not affected.

  Table 5 shows welding conditions and mechanical test results for producing welded joints using the welding wires in Table 1. The joint symbols Y1B-1 to Y26B-1 using the welding wires Y1B to Y26B are 700 MPa of tensile strength TS that starts to affect productivity and manufacturing cost in the production of solid wires in the TIG welding method for flux cored wires of the present invention. In addition, the absorbed energy vE-40 by the 2 mmV notch Charpy impact test at −40 ° C. is 27 J or more.

  On the other hand, the joint symbols Y1C-1 to Y12C-1 using the welding wires Y1C to Y12C are not suitable for the composition of the flux-cored wire used in the TIG welding method of the present invention. Or toughness is not enough.

  In the joint symbol Y1C-1 using the wire number Y1C, the tensile strength is lower than the target value (TS ≧ 700 MPa) of the present invention because the C content of the welding wire is insufficient.

  In the joint symbol Y2C-1 using the wire number Y2C, since the C content of the welding wire is excessive, the toughness vE-40 of the weld metal is greatly deteriorated.

  In the joint symbol Y3C-1 using the wire number Y3C, the degree of deoxidation deteriorated and the toughness deteriorated because the Si content of the welding wire was insufficient.

  The joint symbol Y4C-1 using the wire number Y4C was significantly deteriorated in toughness because the Si content of the welding wire was excessive.

  The joint symbol Y5C-1 using the wire number Y5C was not hardened because the Mn content of the welding wire was insufficient, and was below the target value of tensile strength.

  In the joint symbol Y6C-1 using the wire number Y6C, since the Mn content of the welding wire is excessive, the toughness is remarkably deteriorated.

  The joint symbol Y7C-1 using the wire number Y7C was significantly deteriorated in toughness because the P content of the welding wire was excessive.

  In the joint symbol Y8C-1 using the wire number Y8C, the toughness deteriorated remarkably because the S content of the welding wire was excessive.

  In the joint symbol Y9C-1 using the wire number Y9C, the toughness deteriorated remarkably because the Al content of the welding wire was excessive.

  The joint symbol Y10C-1 using the wire number Y10C did not contain any required amounts of Ni, Cr, Mo, W, and Cu in the welding wire, so the hardenability was insufficient and the tensile strength was below the target value. .

  In the joint symbol Y11C-1 using the wire number Y11C, since the Ceq of the welding wire is too small, the strength improvement effect by quenching cannot be sufficiently obtained, and the tensile strength is lower than the target value.

  In joint symbol Y12C-1 using wire number Y12C, the Ceq of the welding wire is excessive, so that the toughness is remarkably deteriorated.

  From the above examples, according to the present invention, it is possible to eliminate the component unevenness in the weld metal when the flux-cored wire is applied to TIG welding, and to obtain a uniform weld metal, so that stable tensile strength and toughness can be obtained. Can be secured. This clearly makes it possible to apply a flux-cored wire to TIG welding as an alternative to a solid wire for high-strength steel, which has problems in productivity and manufacturing cost.

A cross section of a welded portion of a steel shell component mainly remaining due to insufficient mixing of weld metal components, which is likely to occur in the TIG welding method of high-strength steel using a flux-cored wire, which is an object of the present invention. It is a figure typically shown with a figure. It is a figure which shows typically the groove shape of the welded joint used for the Example, and the sampling procedure of a 2mmV notch Charpy impact test piece.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Component uneven part of steel outer shell component main body 2 Steel plate 3 Backing metal 4 Weld metal 5 2mmV notch Charpy impact test piece 6 Round bar tensile test piece 7 1/4 thickness position of steel plate

Claims (4)

In the TIG welding method of high-strength steel using a flux-cored wire filled with a flux containing at least a metal or an alloy inside the steel outer shell,
The cross-sectional thickness of the steel outer skin is 0.30 to 1.0 mm, and the mass% with respect to the total mass of the wire,
C: 0.04 to 0.4%,
Si: 0.2-2.0%,
Mn: 0.3 to 2.0%,
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.05%
Including,
Ni: 0.1 to 12%,
Cr: 0.01 to 4.0%
Mo: 0.1-4.0%,
W: 0.1-4.0%,
Cu: 0.01 to 1.5%
1 or 2 or more of them, the balance consists of iron and inevitable impurities, and the carbon equivalent (Ceq) shown by the following (Formula 1) satisfies 0.40 to 1.5%. In addition, using a flux-cored wire, the difference between the carbon equivalent of the component of the entire wire and the carbon equivalent of the component of the steel outer shell satisfies 0.10% or more,
A high-strength steel using a flux-cored wire, characterized in that TIG welding is performed in the range of 1.70 to 4.0 kJ / cm · g of welding heat input per 1 g of welding wire represented by the following (formula 2). TIG welding method.
Ceq = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Mo] / 4 + [Cr] / 5 + [W] / 8 + [Cu] / 40 + [Ti] / 30 + [Nb] / 3+ [V] / 5 + [Ta] / 8 + [Co] / 40 + 5 × [B] (Formula 1)
However, the element with [] represents the content (% by mass) of each element.
[Welding heat input per 1 g of wire (kJ / cm · g)] = [Welding current (A)] × [Welding voltage (V)] × 60 / [Welding speed (cm / min)] / [Welding amount (g / Min)] (Equation 2) The flux-cored wire is further mass% with respect to the total mass of the wire,
Ti: 0.005 to 0.3%,
Nb: 0.005 to 0.1%,
V: 0.005-0.5%
Ta: 0.005 to 0.5%,
Co: 0.01 to 6%
B: 0.001 to 0.015%
The TIG welding method for high-strength steel using the flux-cored wire according to claim 1, wherein one or more of them are contained. The flux-cored wire is further mass% with respect to the total mass of the wire,
Ca: 0.0002 to 0.01%,
REM: 0.0002 to 0.01%
A TIG welding method for high-strength steel using a flux-cored wire according to claim 1 or 2, characterized by containing one or two of them.   The TIG welding method for high-strength steel using the flux-cored wire according to any one of claims 1 to 3, wherein the flux-cored wire is a hot wire.

Images