JP2014133258A - Welded metal superior in hydrogen embrittlement resistance sensitivity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded metal that is superior in hydrogen embrittlement resistance sensitivity even when it has a high strength of a tensile strength being over 780 MPa.SOLUTION: A welded metal has a predetermined chemical composition, in which retained austenite particles are present in the number of 2500/mmor more, a volume fraction of the retained austenite particles is 4.3% or more, and the ratio of content between Cr and Mn in the welded metal, [Cr]/[Mn], is 0.20 or more.

Description

  The present invention relates to a weld metal used in a welded structure and capable of reducing the sensitivity to hydrogen embrittlement. Specifically, when evaluating the hydrogen embrittlement susceptibility using the SSRT (Slow Strain Rate Technique) method, even when a test is performed using a large test piece that tends to contain more systematic weakened parts, The present invention relates to a weld metal that can be excellent in embrittlement sensitivity.

  When welding high-strength steel, it is necessary to strictly control the preheating / interpass temperature from the viewpoint of preventing cold cracking of the weld metal part, which causes a reduction in construction efficiency. In recent years, steel materials used for welded structures have been increasingly strengthened, and there is an increasing demand for higher strength in weld metals (for example, HT780: High Tens 780 MPa class).

  When such an increase in strength is promoted, the cold cracking resistance tends to decrease. Therefore, it is necessary to achieve both high strength and low temperature crack resistance. In particular, in submerged arc welding, the amount of heat input during welding is large, and the welding construction efficiency is excellent. Therefore, a technique for ensuring low temperature crack resistance in a weld metal formed by this welding method is required.

  The low temperature cracks as described above are presumed to be caused by diffusible hydrogen segregating at the grain boundaries and decreasing the grain boundary strength (hereinafter referred to as “hydrogen embrittlement”). Therefore, in order to improve the cold cracking resistance, it is important to reduce diffusible hydrogen and reduce the hydrogen embrittlement susceptibility of the weld metal, and various techniques have been proposed from these viewpoints.

  For example, Patent Document 1 discloses a technique for preventing cold cracking by dispersing Mo carbide (carbide containing Mo) having a high hydrogen trapping capability in a weld metal. However, in this technology, in order to disperse Mo carbide, after welding steel materials, after performing submerged arc welding from the inner surface side, a special welding technique is used to control the maximum heating temperature of the weld metal obtained on the inner surface side. It must be adopted and cannot be applied to general welding of steel materials.

  Patent Document 2 proposes a technique for preventing cold cracking by managing the cooling time during welding. This technology requires strict construction management according to chemical components, and has a problem that the work load is high.

  Patent Document 3 proposes a technique for preventing cold cracking by setting the retained austenite fraction for trapping diffusible hydrogen to 1% or more in the weld metal. However, this technique is premised on double-sided one-pass seam welding in steel pipes, and cannot be applied to general welding of steel materials.

  Patent Document 4 proposes a technique for improving cold cracking resistance by reducing the amount of diffusible hydrogen and appropriately controlling the strength and chemical component composition. However, even in this technique, since a satisfactory strength level is affected by the components, the number of application points is limited in actual construction.

  Patent Documents 5 and 6 disclose a special welding method called laser-arc hybrid welding. This method has a merit that a weld metal with excellent crack resistance can be obtained while obtaining a construction efficiency similar to that of a large heat input submerged arc welding with a low heat input, but there is a problem that it cannot be applied to general arc welding. .

  All the technologies proposed so far have improved hydrogen embrittlement resistance as a means of improving cold cracking resistance. However, in actual welding work, the amount of hydrogen in the weld metal may increase due to various factors. In such a case, hydrogen embrittlement becomes a problem regardless of cold cracking resistance. Therefore, it is necessary to make the improvement of hydrogen embrittlement susceptibility a direct problem to be solved without relating to the existence of overcoming the cold cracking resistance.

  In the patent document 7, the present inventors have developed a technique for improving the resistance to hydrogen embrittlement of an HT780 MPa class weld metal by controlling the form of retained austenite. However, the welding method assumed in this technique is gas shielded arc welding mainly using a flux cored wire (FCW). For example, there is still room for improvement with respect to hydrogen embrittlement susceptibility when using other welding methods that are frequently used in implementations such as submerged arc welding. Moreover, in the technique of patent document 7, the comparatively narrow area | region in a weld metal is evaluated. In an actual weld metal, the structure greatly varies depending on the observation position. Therefore, in order to evaluate the hydrogen embrittlement susceptibility with higher accuracy, a method capable of evaluating a relatively wide region in the weld metal is required.

  In recent years, the application of the HT780 class is also expanding in weld metals used for offshore structures. These weld metals are required to have excellent resistance to hydrogen embrittlement at a strength of 780 MPa so that they can be used in cold regions.

Japanese Patent Laying-Open No. 2005-40816 JP 2003-33876 A JP 2002-115032 A JP 11-147196 A JP 2007-260715 A JP 2007-260716 A JP 2012-176434 A

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a weld metal excellent in resistance to hydrogen embrittlement even if the tensile strength is high strength exceeding 780 MPa.

With the weld metal excellent in hydrogen embrittlement susceptibility according to the present invention that was able to solve the above problems,
C: 0.02 to 0.12% (meaning “mass%”; the same applies to the chemical composition)
Si: 0.18 to 2.00%,
Mn: 0.90 to 2.5%,
Ni: 1.0 to 3.5%
Cr: 0.3 to 2.0%,
Al: 0.030% or less (excluding 0%),
N: 0.015% or less (excluding 0%) and O: 0.050% or less (excluding 0%), respectively,
The balance consists of iron and inevitable impurities,
Containing 2,500 particles / mm ² or more of retained austenite particles having an equivalent circle diameter of 0.15 μm or more, and a volume fraction of the retained austenite phase is 4.3% or more of the entire structure;
The main point is that the ratio [Cr] / [Mn] of the Cr and Mn content is 0.20 or more.

  The size of the retained austenite particles to be used in the measurement of the number density was set to be equal to or greater than the measurement limit, and the equivalent circle diameter was set to be 0.15 μm or more. In addition, the equivalent circle diameter is a diameter assuming a circle having an equal area by paying attention to the size of residual austenite particles observed on the observation surface of the optical microscope.

  In the weld metal of the present invention, as other elements, (a) Mo: 0.95% or less (not including 0%), Ti: less than 0.040% (not including 0%), V: 0 .1% or more selected from the group consisting of 60% or less (not including 0%) and Cu: 1.0% or less (not including 0%), (b) Zr: 0.10% or less (0% (C) B: 0.0050% or less (not including 0%) and the like are also preferable, and the characteristics of the weld metal are further improved depending on the type of element to be included.

  In a preferred embodiment of the present invention, it is formed by submerged arc welding.

  According to the present invention, the number density and volume fraction of the retained austenite particles are appropriately controlled together with the chemical component composition, so that a weld metal having excellent resistance to hydrogen embrittlement can be realized even at a high strength exceeding 780 MPa. .

It is a schematic explanatory drawing which shows the groove shape when producing a weld metal. It is explanatory drawing which shows the shape of a test piece when a tension test is done. It is explanatory drawing which shows the shape of the large sized test piece when measuring hydrogen storage amount by SSRT method.

  The inventors have improved the hydrogen embrittlement susceptibility measured by the SSRT test by controlling the retained austenite form and oxide form in the invention of Patent Document 7 (hereinafter referred to as the prior invention), for example. ing.

  However, in the prior application invention, the assumed welding method is mainly gas shielded arc welding using FCW, and the heat input during welding is limited to 2.5 kJ / mm or less. It has been shown in the prior invention that when the heat input of welding exceeds 2.5 kJ / mm, the predetermined retained austenite form cannot be obtained, and the predetermined characteristics cannot be satisfied by the SSRT test.

  Even in high-efficiency welding methods such as submerged arc welding, where there are many actual construction examples, a weld metal having excellent hydrogen embrittlement resistance in a large SSRT test is required. In highly efficient submerged arc welding, the welding heat input is often 2.0 kJ / mm or more (preferably 2.5 kJ / mm or more). A means for realizing a weld metal that exhibits excellent resistance to hydrogen embrittlement when evaluated in a large-scale SSRT test, even with a weld metal obtained under such welding conditions with a large heat input, was studied. As a result, the following knowledge was obtained.

  When the welding heat input becomes large, the cooling rate at the time of welding decreases, so that the decomposition of retained austenite during the cooling is promoted. Moreover, since the prior austenite structure becomes coarse, it is generally disadvantageous for hydrogen embrittlement sensitivity. On the other hand, the present inventors appropriately control the chemical composition of the welding material, and at the same time, the ratio of Cr and Mn content in the weld metal [Cr] / [Mn] (that is, Cr content [ Cr] and Mn content [Mn] ratio) and Ti content were suppressed to less than 0.040% (including 0%). When such control is performed, it has been found that stable retained austenite is ensured in a predetermined form, and excellent resistance to hydrogen embrittlement can be obtained in a large SSRT test even when the welding heat input is relatively large. It was.

  The greatest difference between the present invention and the prior invention is the Ti content in the weld metal. In the prior invention, the Ti content in the weld metal is 0.040 to 0.15%, and the microstructure of the Ti oxide starting point is developed to ensure the number density of residual austenite particles and to prevent hydrogen embrittlement. To improve chemical sensitivity. However, in welding with a large amount of heat input such as submerged arc welding, the cooling rate during welding decreases, so the bainite (grain boundary bainite) structure from the prior austenite grain boundaries is the main component, and the microstructure of the Ti oxide origin is Not enough. Moreover, Ti itself is a ferrite forming element and has an adverse effect on the stabilization of retained austenite.

  Therefore, in the present invention, it is basically assumed that the weld metal does not contain Ti, or even if Ti is contained if necessary, the content is less than 0.040% to stabilize the retained austenite. On the other hand, for grain boundary bainite structure refinement, the ratio [Cr] / [Mn] of Cr and Mn content in the weld metal is set to 0.20 or more to disperse many residual austenite particles. Succeeded.

  However, the resistance to hydrogen embrittlement when the heat input is large cannot be ensured only by dispersing the amount of retained austenite and the number of retained austenite particles as in the prior invention. This is because, as described above, when the heat input is large, the prior austenite structure becomes coarse and adversely affects the resistance to hydrogen embrittlement (by controlling the ratio [Cr] / [Mn] It is the bainite structure in the former austenite grains).

  On the other hand, by making the Ti content in the weld metal less than 0.040%, individual retained austenite particles are stabilized, and even when the heat input is large, excellent resistance to hydrogen embrittlement is obtained. It will be obtained. That is, residual austenite is thought to contribute to the improvement of hydrogen embrittlement susceptibility by trapping hydrogen inside, but some residual austenite undergoes martensitic transformation due to tension during the SSRT test, resulting in hydrogen trapping. The effect is lost. Reducing the Ti content stabilizes the retained austenite and suppresses martensitic transformation during the SSRT test, which is considered to improve the resistance to hydrogen embrittlement.

  In addition, since Nb which is a ferrite forming element has a disadvantageous action from the viewpoint of stabilization of retained austenite, it is controlled to an impurity level (<0.01%) in the present invention and is not actively added.

  In the present specification, “high strength” means that the tensile strength TS exceeds 780 MPa, preferably about 800 to 980 MPa.

  In this specification, “excellent in resistance to hydrogen embrittlement” means that when the resistance to hydrogen embrittlement is evaluated by the method described in Examples described later, the elongation at break when using a large test piece is 2.0. It means something that satisfies over%.

  Hereinafter, the constituent requirements of the present invention will be described in detail.

As described above, the weld metal of the present invention includes C: 0.02 to 0.12%, Si: 0.18 to 2.00%, Mn: 0.90 to 2.5%, Ni: 1.0 to 3.5%, Cr: 0.3-2.0%, Al: 0.030% or less (not including 0%), N: 0.015% or less (not including 0%), and O: 0 0.050% or less (excluding 0%), respectively, the balance is made of iron and inevitable impurities, and the residual austenite particles containing 2500 particles / mm ² or more having an equivalent circle diameter of 0.15 μm or more are contained. The volume fraction of the phase is 4.3% or more with respect to the entire structure, and the ratio [Cr] / [Mn] of the content of Cr and Mn is 0.20 or more.

  First, the retained austenite characterizing the weld metal of the present invention will be described.

As described above, in the present invention, the residual austenite particles present in the weld metal are controlled to 2500 particles / mm ² or more, and the volume fraction of the retained austenite phase (ratio to the entire structure) is controlled to 4.3% or more. According to the present invention, since the retained austenite particles are dispersed at an appropriate number density, a weld metal excellent in resistance to hydrogen embrittlement can be obtained.

  In the present invention, the above-mentioned requirements are defined for retained austenite present in the raw material portion among weld metals. Residual austenite in the weld metal decomposes due to the influence of the post-pass during welding, so the amount of retained austenite tends to vary depending on the measurement location, particularly in the reheated part, while the original part of the final pass This is because the amount of retained austenite is easily evaluated accurately without being affected by the heat of the back pass.

  Since retained austenite serves as a trapping site for diffusible hydrogen, it has already been reported that it has a diffusible hydrogen reducing action and contributes to an improvement in resistance to hydrogen embrittlement resistance. However, until now, only the amount of retained austenite (ratio in the whole structure) is prescribed, and no attention has been paid to its dispersion state (number density). However, according to the study results of the present inventors, it is clear that no matter how much the amount of retained austenite is controlled, the desired hydrogen embrittlement resistance cannot be obtained unless the dispersion state is appropriately controlled. (For example, see Experiment Nos. 39 and 43 in Table 7 of Examples described later).

That is, in order to obtain a weld metal with excellent resistance to hydrogen embrittlement, the amount of retained austenite serving as a diffusible hydrogen trap site is secured, and the number of retained austenite particles is increased by making the matrix structure finer ( Specifically, it has been found that the dispersion effect of 2500 pieces / mm ² or more maximizes the trapping effect of diffusible hydrogen and significantly improves the resistance to hydrogen embrittlement. For example, the experiment Nos. 39 and experiment no. 43 is an example in which the volume fraction of retained austenite particles is 4.3% or more as defined in the present invention, and a predetermined amount of retained austenite exists. However, since it does not have a predetermined number density (the dispersion state is not appropriate), the resistance to hydrogen embrittlement when using a large-sized test piece is lowered.

From the viewpoint of improving hydrogen embrittlement susceptibility, the number density of retained austenite particles is preferably as large as possible, preferably 3000 / mm ² or more, and more preferably 3300 / mm ² or more. The upper limit is not particularly limited from the viewpoint of improving the resistance to hydrogen embrittlement resistance, but may be, for example, 7500 pieces / mm ² or less.

  Further, from the viewpoint of improving hydrogen embrittlement resistance, the volume fraction of the retained austenite phase in the entire structure is preferably as high as possible, preferably 4.7% or more, and more preferably 5.0% or more. The upper limit is not particularly limited from the viewpoint of improving hydrogen embrittlement susceptibility, but considering the fact that the yield stress decreases if it exists in excess, for example, 10% or less, preferably 9% or less, more preferably It may be 8% or less.

  The present invention is characterized by controlling the amount of retained austenite phase (volume fraction) and the number density of retained austenite particles among the structures constituting the weld metal, and the structure other than retained austenite is limited in any way. However, any structure that is usually included in the weld metal may be used. Specifically, bainite is included as a main structure (a structure having a volume fraction of 50% or more, preferably 70% or more, more preferably 90% or more with respect to the entire structure), and in addition, grain boundary ferrite, martensite, and the like. May be included. The bainite, intergranular ferrite and martensite are all types of “ferrite phase”, and the fraction of the retained austenite phase measured by the method (Example) described later is the retained austenite, bainite, grain boundary. It is a ratio to the total of ferrite and martensite. Moreover, the amount of bainite is calculated | required as an approximate area fraction by structure | tissue observation using an optical microscope.

  Next, the chemical component composition in the weld metal of the present invention will be described.

[C: 0.02 to 0.12%]
C is an element indispensable for securing the strength of the weld metal, and in order to exert such effects, the lower limit of the C content is set to 0.02% or more. Preferably it is 0.04% or more, More preferably, it is 0.05% or more. However, if the C content exceeds 0.12%, the strength increases excessively and the hydrogen embrittlement susceptibility increases (that is, the hydrogen embrittlement susceptibility deteriorates), so the upper limit is 0.12% or less. And The upper limit with preferable C content is 0.10% or less, More preferably, it is 0.08% or less.

[Si: 0.18 to 2.00%]
Si exists in a solid solution state, thereby delaying carbide formation and stabilizing retained austenite. If the Si content is less than 0.18%, the predetermined retained austenite cannot be ensured, and the above-described effects are not effectively exhibited. Therefore, the lower limit of the Si content is set to 0.18% or more. Preferably it is 0.30% or more, more preferably 0.35% or more. On the other hand, if the Si content is excessive, the hydrogen embrittlement susceptibility increases due to a significant increase in strength, so the upper limit is regulated to 2.00% or less. Preferably it is 1.5% or less, More preferably, it is 1.0% or less.

[Mn: 0.90 to 2.5%]
Mn is an element necessary for ensuring the strength of the weld metal, and in order to exert such effects, the lower limit of the Mn content is set to 0.90% or more. Preferably it is 1.2% or more, More preferably, it is 1.4% or more. However, if the Mn content exceeds 2.5%, the hydrogen embrittlement susceptibility increases due to a significant increase in strength, so the upper limit is made 2.5% or less. Preferably it is 2.2% or less, More preferably, it is 2.0% or less.

[Ni: 1.0 to 3.5%]
Ni is an element necessary for ensuring the strength of the weld metal, and in order to exert such effects, the lower limit of the Ni content is set to 1.0% or more. Preferably it is 1.2% or more, More preferably, it is 1.5% or more. However, if the Ni content exceeds 3.5% and becomes excessive, the hydrogen embrittlement susceptibility increases due to an excessive increase in strength, so the upper limit is made 3.5% or less. Preferably it is 3.0% or less, More preferably, it is 2.8% or less.

[Cr: 0.3-2.0%]
Cr is an element that contributes to fine dispersion of the retained austenite particles by refining the grain boundary bainite structure. In order to exert such effects, the lower limit of the Cr content is set to 0.3% or more. Preferably it is 0.4% or more, More preferably, it is 0.5% or more. However, if the Cr content exceeds 2.0% and becomes excessive, the hydrogen embrittlement susceptibility increases due to an excessive increase in strength, so the upper limit is made 2.0% or less. Preferably it is 1.8% or less, More preferably, it is 1.5% or less.

[Al: 0.030% or less (excluding 0%)]
Al is added as a deoxidizing element. If added in excess, the formation of AlN causes an excessive increase in strength and deteriorates the resistance to hydrogen embrittlement, so the upper limit is made 0.030% or less. Preferably it is 0.025% or less, More preferably, it is 0.020% or less.

[N: 0.015% or less (excluding 0%)]
N is one of the elements inevitably mixed in, and it is difficult to make it 0% industrially. N is effective for improving the strength of the weld metal. However, when it is contained excessively, the hydrogen embrittlement susceptibility increases due to an excessive increase in strength. Therefore, the upper limit of the N content is 0.015% or less. Preferably it is 0.010% or less, More preferably, it is 0.006% or less.

[O: 0.050% or less (excluding 0%)]
O is an element inevitably contained in the weld metal, and it is difficult to make it 0% industrially. If the O content exceeds 0.050%, Si oxide is formed, and the amount of retained austenite cannot be ensured due to a decrease in solute Si, so the upper limit is made 0.050% or less. Preferably it is 0.045% or less, More preferably, it is 0.040% or less.

  The basic components contained in the weld metal of the present invention are as described above, with the balance being iron and inevitable impurities. Inevitable impurities include elements (for example, P and S) that are brought in depending on the status of raw materials, materials, manufacturing equipment, and the like. However, in general, impurities segregate at the grain boundaries to lower the grain boundary strength and promote low temperature cracking. For example, P: 0.02% or less (excluding 0%), S: 0.025% or less ( It is preferable to suppress each of them.

  Although the basic components in the weld metal of the present invention are as described above, as other elements, (a) Mo: 0.95% or less (not including 0%), Ti: less than 0.040% (0%) 1) or more selected from the group consisting of V: 0.60% or less (not including 0%) and Cu: 1.0% or less (not including 0%), (b) Zr: 0 .10% or less (not including 0%), (c) B: 0.0050% or less (not including 0%), etc. may be included, and the characteristics of the weld metal depend on the type of element to be included. Further improvement. The elements belonging to (a), (b) and (c) may be contained alone or in appropriate combination.

[Mo: 0.95% or less (not including 0%), Ti: less than 0.040% (not including 0%), V: 0.60% or less (not including 0%), and Cu: 1. One or more selected from the group consisting of 0% or less (excluding 0%)]
Mo, Ti, V, and Cu are useful as elements for improving the strength of the weld metal, and may be contained alone or in combination of two or more. Among these elements, Mo is an element effective for securing the strength. However, if excessively contained, the strength is excessively increased, and the resistance to hydrogen embrittlement deteriorates. Therefore, the upper limit is preferably 0.95% or less. More preferably, it is 0.85% or less, More preferably, it is 0.50% or less. The Mo content for obtaining the effect of improving the strength is preferably 0.05% or more, more preferably 0.20% or more.

  Ti is effective in improving strength, but has the effect of destabilizing retained austenite. Therefore, when the Ti content is excessive, the retained austenite becomes martensite due to stress-induced transformation during a large SSRT test. It is impossible to secure a high hydrogen embrittlement resistance. From such a viewpoint, the Ti content is preferably less than 0.040%. More preferably, it is 0.035% or less, More preferably, it is 0.030% or less. The Ti content for obtaining the effect of improving the strength is preferably 0.010% or more, more preferably 0.015% or more.

  V and Cu are useful as elements for improving the strength of the weld metal, and the preferable lower limit for exhibiting such an effect is 0.02% or more for V and 0.05% or more for Cu. However, if the content of these elements is excessive, the hydrogen embrittlement susceptibility increases due to an excessive increase in strength. Therefore, with respect to the upper limit of the amount of each element, V is 0.60% or less (more preferably 0.50% or less, more preferably 0.40% or less), and Cu is 1.0% or less (more preferably 0.5%). % Or less, more preferably 0.2% or less).

[Zr: 0.10% or less (excluding 0%)]
Zr is a strong deoxidizing element and has the effect of promoting the increase in retained austenite due to the increase in solid solution Si. A preferable lower limit for effectively exhibiting such an effect is 0.010% or more. However, when the Zr content is excessive, the intragranular transformation of the oxide starting point decreases, and the hydrogen embrittlement susceptibility increases due to the coarsening of the structure. Therefore, the upper limit of the Zr content is preferably suppressed to 0.10% or less (more preferably 0.050% or less).

[B: 0.0050% or less (excluding 0%)]
B is an element that contributes to improving the strength by suppressing the formation of ferrite from the prior austenite grain boundaries. In order to effectively exhibit such an action, the lower limit of the B content is preferably set to 0.0010% or more. However, when the B content is excessive, the strength is remarkably increased and the hydrogen embrittlement sensitivity is increased. Therefore, the upper limit is preferably suppressed to 0.0050% or less (more preferably 0.0030% or less).

[Ratio [Cr] / [Mn]: 0.20 or more]
By controlling the Cr / Mn content ratio [Cr] / [Mn] to 0.20 or more, the bainite structure from the prior austenite grain boundaries is refined, and high-density dispersion of residual austenite particles becomes possible. . Preferably it is 0.25 or more, more preferably 0.40 or more.

  Next, a method for producing the weld metal of the present invention will be described.

Although the welding method for producing the weld metal of this invention is not limited, Submerged arc welding (SAW) with a large heat input and favorable construction efficiency is preferable. In SAW, preferable conditions [flux component, wire component, welding condition] for obtaining a weld metal satisfying a predetermined retained austenite form are as follows (a) to (d). Of course, in order to make the chemical composition of the weld metal appropriate, it is necessary to appropriately adjust the chemical composition of the welding wire to be used. Further, the flux contains MgO, Al ₂ O ₃ and metal fluoride in addition to SiO ₂ .

(A) SiO ₂ concentration in the flux: 8 to 18%
(B) Metal Si concentration in the flux: 1-4%
(C) Si concentration in the wire: 0.10 to 1.60%
(D) Heat input during welding: 2.0 to 3.0 kJ / mm

  The requirements (a) to (c) are set in order to secure an amount of solute Si effective to secure a predetermined amount of retained austenite. If the lower limit value is not reached, a predetermined retained austenite form cannot be obtained. If the upper limit is exceeded, even if a welding wire having an appropriate chemical composition is used, the Si concentration in the weld metal exceeds the specified range, and the hydrogen embrittlement susceptibility deteriorates due to excessive strength. .

The more preferable lower limit of the SiO ₂ concentration in the flux is 9% or more (more preferably 10% or more), and the more preferable upper limit is 15% or less (more preferably 14% or less). The more preferable lower limit of the metal Si concentration in the flux is 1.2% or more (more preferably 1.5% or more), and the more preferable upper limit is 3.5% or less (more preferably 3.0% or less). . A preferable upper limit of the Si concentration in the wire is 1.2% or less, and a preferable lower limit is 0.12% or more.

(D) Heat input during welding: 2.0 kJ / mm or more, 3.0 kJ / mm or less If the heat input during welding exceeds 3.0 kJ / mm, the cooling rate during welding decreases, and the residual austenite decomposes. Is promoted. As a result, desired residual austenite particles (number density and volume fraction) cannot be obtained. More preferably, it is 2.8 kJ / mm or less. The smaller the amount of heat input, the better. However, from the viewpoint of construction efficiency, it is preferably 2.0 kJ / mm or more. More preferably, it is 2.5 kJ / mm or more.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and can be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all included in the technical scope of the present invention.

Example 1
A combination of the flux (F1 to F9) having the chemical composition shown in Table 1 below and the wires (W1 to W52) having the chemical composition shown in Tables 2 and 3 below are used to weld metal under the following (A) welding conditions. Produced. In the columns of Tables 2 and 3, “-” means no addition (not contained).

(A) Welding conditions Welding method: Submerged arc welding (SAW)
Wire diameter: 4.0mmφ
Welding base material: 80kg thick steel plate (plate thickness: 32mm)
Groove shape: V-shaped groove with a groove angle of 30 °, using a backing material with a root interval of 13 mm (see Fig. 1)
Polarity: DCEP (DC reverse polarity)
Heat input conditions (current-voltage-speed):
(A) 500A-29V-40cpm (2.2kJ / mm)
(I) 550A-30V-40cpm (2.5kJ / mm)
(U) 550A-30V-36cpm (2.8kJ / mm)
(D) 580A-32V-36 cpm (3.1 kJ / mm)
Lamination method: 9 layers 19 passes Preheating-pass temperature: 140-160 ° C

  The chemical component composition of the obtained weld metal is shown in the following Tables 4 and 5 together with the fluxes used (Table 1) and the welding wires (Tables 2 and 3). Various performances (tensile strength, number density of retained austenite particles, volume fraction of retained austenite particles, sensitivity to hydrogen embrittlement) of each weld metal produced are as follows (1), (2), (3), (4 ) And evaluated.

(1) Evaluation of tensile strength TS From the center part of the obtained weld metal, a tensile test piece shown in FIG. 2 was collected in parallel with the welding direction, and a tensile test was performed according to JIS-Z2241. And the thing whose tensile strength TS exceeds 780 MPa was set as the pass.

(2) Measurement of Number Density of Residual Austenite Particles The final pass original part of the obtained weld metal was mirror-polished, corroded with a repeller reagent, and two views of a 1000-fold image were taken with an optical microscope. The white corrosion contrast of the retained austenite particles was analyzed by an image analysis software (“Image-Pro Plus” Media Cybernetics), and the number density of the retained austenite particles having a circle equivalent diameter of 0.15 μm or more was calculated.

(3) Measurement of volume fraction of retained austenite phase The surface of the obtained weld metal was subjected to electrolytic polishing on the final pass, and a secondary micro part X-ray diffractometer (RINT-RAPIDII) manufactured by Rigaku Corporation. ) X-ray diffraction measurement was performed. The peaks of the lattice planes of (110), (200), (211), and (220) of the ferrite phase, and the lattice planes of (111), (200), (220), and (311) of the retained austenite phase For the peaks, the volume fractions of (111), (200), (220), and (311) of the retained austenite phase are calculated based on the integrated intensity ratio of each peak, and the average value (arithmetic average) of these is obtained. This was designated as “volume fraction of retained austenite phase”.

(4) Evaluation of hydrogen embrittlement susceptibility using a large SSRT test piece From the center of the obtained weld metal, a large test piece shown in FIG. Charged.

(B) Hydrogen charging conditions Aqueous solution: A solution in which NaCl (30 g) and KSCN (1 g) are dissolved in 1 L Current density: 0.1 A / dm ²
Charge time: 100 hours

Under the condition (B), after charging the large test piece with hydrogen, zinc plating for preventing hydrogen escape was performed according to the following (C) plating condition.
(C) Plating conditions Aqueous solution: solution of ZnSO ₄ .7H ₂ O (350 g), 97% H ₂ SO ₄ (20.6 g) and Na ₂ SO ₄ (60 g) in 1 L bath temperature: 60 ° C.
Current density: 50 A / dm ²
Plating time: 3 minutes

An SSRT test was performed on the plated test piece at a crosshead speed of 3.0 × 10 ⁻² mm / min (strain rate: 6.94 × 10 ⁻⁶ / sec), and the elongation at break of the test piece was 2 Those exceeding 0.0% were evaluated as being excellent in hydrogen embrittlement resistance in large test pieces.

  These results are shown in Tables 6 and 7 below (Experiment Nos. 1 to 52).

  From these results, it can be considered as follows.

  Experiment No. 6 in Table 6 1 to 26 and Table 7 Nos. 27 to 33 are examples satisfying the requirements defined in the present invention, and a weld metal having excellent resistance to hydrogen embrittlement in a large test piece was obtained even with high strength exceeding 780 MPa. Specifically, the chemical composition of the weld metal was obtained because welding was performed under appropriate heat input conditions [(A) to (C)] using appropriate welding materials (flux, welding wire) shown in Tables 1 to 3. As a result of properly controlling the number density and volume fraction of residual austenite particles, a weld metal having desired characteristics was obtained.

  On the other hand, Experiment No. 34 to 52 are examples that do not meet any of the requirements defined in the present invention, and the desired characteristics were not obtained.

  First, experiment no. 34 is an example in which an appropriate flux F1 was used, but welding was performed under a heat input condition (d) with a large amount of heat input. As a result, the volume fraction of retained austenite particles in the weld metal decreased, and the hydrogen embrittlement susceptibility of large test pieces decreased.

Experiment No. No. 35 is an example using the flux F6 with a small amount of SiO ₂ . As a result, the volume fraction of retained austenite particles in the weld metal was reduced, and the hydrogen embrittlement susceptibility of large specimens was also reduced. Moreover, due to the welding wire used, the C content in the weld metal is low, and the tensile strength is reduced.

Experiment No. 36 is an example using the flux F7 having a large amount of SiO ₂ . As a result, the Si content of the weld metal was increased, the strength was remarkably increased, and the hydrogen embrittlement susceptibility of the large specimen was decreased. Further, due to the welding wire used, the Mn content in the weld metal is increased, and the tensile strength is remarkably increased.

  Experiment No. 37 is an example using the flux F8 with a small amount of metal Si. As a result, the volume fraction of retained austenite particles in the weld metal was reduced, and the hydrogen embrittlement susceptibility of large specimens was also reduced. Further, due to the welding wire used, the Mn content in the weld metal is reduced, and the tensile strength is reduced.

  Experiment No. 38 is an example using the flux F9 with a large amount of metal Si. As a result, the Si content in the weld metal increased, the strength increased significantly, and the hydrogen embrittlement susceptibility of large test pieces decreased. Further, due to the welding wire used, the Ni content in the weld metal is increased, and the tensile strength is significantly increased.

  Experiment No. No. 39 is an example in which the ratio [Cr] / [Mn] of the content of Cr and Mn in the weld metal is small. As a result, the number density of retained austenite particles in the weld metal was reduced, and the hydrogen embrittlement susceptibility of large specimens was reduced. Moreover, due to the welding wire used, the C content in the weld metal is increased, and the tensile strength is significantly increased.

  Experiment No. 40 is an example in which the Si content in the weld metal is low. As a result, the volume fraction of the retained austenite phase in the weld metal was reduced, and the hydrogen embrittlement susceptibility to large specimens was reduced. Experiment No. 41 is an example with much Si content in a weld metal. As a result, the tensile strength was remarkably increased, and the hydrogen embrittlement susceptibility of the large specimen was decreased.

  Experiment No. 42 is an example in which the ratio [Cr] / [Mn] of the content of Cr and Mn in the weld metal is small. As a result, the number density of retained austenite particles in the weld metal was reduced, and the hydrogen embrittlement susceptibility of large specimens was reduced. In addition, the O content in the weld metal is increased, the volume fraction of retained austenite particles in the weld metal is decreased, and the hydrogen embrittlement susceptibility of large test pieces is also lowered from this point. Furthermore, the Ni content in the weld metal has become scarce and the tensile strength has been reduced.

  Experiment No. No. 43 is an example in which the ratio [Cr] / [Mn] of the content of Cr and Mn in the weld metal is small. As a result, the number density of retained austenite particles in the weld metal was reduced, and the hydrogen embrittlement susceptibility of large specimens was reduced.

  Experiment No. 44 is an example with a large Cr content in the weld metal. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased. Experiment No. 45 is an example in which the Mo content in the weld metal is high. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased.

  Experiment No. 46 is an example in which the Al content in the weld metal is high. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased. Experiment No. 47 is an example in which the N content in the weld metal is large. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased.

  Experiment No. 48 is an example with a large Ti content in the weld metal. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased. No. 49 is an example with much V content in a weld metal. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased.

  Experiment No. 50 is an example in which the Cu content in the weld metal is high. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased. Experiment No. 51 is an example with much Zr content in a weld metal. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased. Experiment No. 52 is an example with much B content in a weld metal. As a result, the strength of the weld metal increased excessively, and the hydrogen embrittlement susceptibility of the large specimen decreased.

Claims (5)

C: 0.02 to 0.12% (meaning “mass%”. The same applies to the chemical composition), Si: 0.18 to 2.00%,
Mn: 0.90 to 2.5%,
Ni: 1.0 to 3.5%
Cr: 0.3 to 2.0%,
Al: 0.030% or less (excluding 0%),
N: 0.015% or less (excluding 0%) and O: 0.050% or less (excluding 0%), respectively,
The balance consists of iron and inevitable impurities,
Containing 2,500 particles / mm ² or more of retained austenite particles having an equivalent circle diameter of 0.15 μm or more, and a volume fraction of the retained austenite phase is 4.3% or more of the entire structure;
A weld metal excellent in hydrogen embrittlement resistance, characterized in that the ratio of Cr to Mn content [Cr] / [Mn] is 0.20 or more.   Furthermore, Mo: 0.95% or less (not including 0%), Ti: less than 0.040% (not including 0%), V: 0.60% or less (not including 0%), and Cu: 1 The weld metal according to claim 1, which contains at least one selected from the group consisting of 0.0% or less (not including 0%).   The weld metal according to claim 1 or 2, further comprising Zr: 0.10% or less (excluding 0%).   The weld metal according to any one of claims 1 to 3, further containing B: 0.0050% or less (excluding 0%). The weld metal according to any one of claims 1 to 4, wherein the weld metal is formed by submerged arc welding.

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