JPH0242312B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a low-hydrogen coated arc welding rod, and particularly to a low-hydrogen coated arc welding rod from which weld metal with good toughness can be obtained. [Prior art] There is a particularly strong demand for ensuring safety for large structures such as LPG tanks, pressure vessels, and offshore structures for cold regions.
It is required to have good brittle fracture occurrence characteristics. In addition to the conventionally widely used Shapey impact value, the required characteristic values include (1) the fracture surface transition temperature (vTrS) in the Shapey impact test, which is related to the temperature at which brittle fracture occurs, and (2) COD (Crack Opening
A representative example is the COD value at the design temperature, which is based on the concept of displacement (opening displacement at the tip of a crack). Large structures such as those mentioned above can be efficiently welded using automatic or semi-automatic welding machines, but in relation to the required characteristics above, low hydrogen welding containing alloys such as Mn, Ni, B, etc. There are still many parts where welding with a small heat input (for example, about 20 kj/cm) using a rod remains, and the following problems have been pointed out. [Problems to be Solved by the Invention] In other words, in the prior art as described above, there are problems such as unstable toughness of the welded part and poor welding efficiency due to small heat input. There is a strong demand for the development of a welding rod with high toughness and high efficiency. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a coated arc welding rod that can obtain high toughness even under large heat input, thereby improving welding efficiency. [Means for solving the problems] The low hydrogen-based coated arc welding rod of the present invention has N:
The outer periphery of the steel core wire is 0.005% (weight%) or less, contains 8 to 28% of metal carbonate in terms of _CO2 , 2 to 12% of metal fluoride in terms of _F2 , and further contains at least a deoxidizer and Coating with a coating compounded with a slag forming agent, and coverage ratio A = coating weight/rod weight x 100: 20 to 45% [P content of welding rod] = [P content in core wire] x 100−
A/100 + [P content in coating material] x A/100≦0.01% [Mn content in welding rod] = [Mn content in core wire] x 100-
A/100 + [Mn content in coating material] x A/100≦1.9% [Ni content in welding rod] = [Ni content in core wire] x 100-
A/100+[Ni in coating material]×A/100≦4% [Ni+Mn content of welding rod]=1.2 to 4.3%. [Function] The present inventors have found that even in high heat input welding, the fracture surface transition temperature (vTrS) is below -60℃, and the fracture surface transition temperature (vTrS) is 0.5 at -60℃.
We investigated various aspects of coated arc welding rods that can obtain COD values of mm or higher, from both the steel core wire and the coating material. As a result, the N in the weld metal was kept to 0.005% or less, the coating rate was 20 to 45%, the CO ₂ in the coating was 8% or less, and the N in the weld metal was kept to 0.01% or less, preventing the coarsening of ferrite grains. The P of the entire welding rod should be
To prevent embrittlement due to segregation of P in the final solidified part of each welding pass during high heat input welding by suppressing it to 0.01% or less; To prevent coarsening of ferrite grains by keeping Ni + Mn in the welding rod to 1.2% or more; Preventing the precipitation of bainitic acicular ferrite by setting the Mn content of the entire welding rod to 1.9% or less, setting the Ni content of the entire welding rod to 4% or less, and setting Ni+Mn to 4.3% or less.
By preventing the occurrence of grain boundary fracture due to the development of primary grain boundaries during high heat input welding, weld metal with no weak parts and consisting of uniform fine ferrite can be obtained, resulting in good vTrS and COD values. The present inventors have discovered that the following can be obtained, and have completed the present invention. That is, the present invention has been achieved by focusing on the specificity of high heat input welding and based on a new perspective, and by completing the welding rod according to the present invention, highly efficient high heat input welding can be performed. Even if implemented, the safety of large structures will be sufficiently ensured and will greatly contribute to the development of welding technology and industry. Various constituent elements of the present invention will be individually explained below. <Coating agent> The coating agent of the present invention has a metal carbonate content _of 8
~28%, containing 2-12% metal fluoride in terms of _F2 ,
Furthermore, at least a deoxidizing agent and a slag forming agent are blended. Metal carbonates refer to CaCO ₃ , MgCO ₃ , BaCO _{3 ,} etc., and are decomposed in an arc to generate CO ₂ to protect molten metal from the atmosphere and prevent air from entering. If the CO ₂ equivalent value of the metal carbonate is less than 8%, the amount of gas generated is insufficient, so the amount of N penetrating into the weld metal increases (see Figure 1), and the toughness decreases (see Figure 2). On the other hand, the CO ₂ equivalent value of metal carbonate is 28
%, the viscosity of the slag increases, the bead appearance deteriorates, and vertical welding with large heat input becomes difficult. The metal fluoride refers to CaF ₂ , NaF, AlF ₃ , BaF _{2 ,} etc., and is used to adjust the viscosity of the slag and maintain workability. If the metal fluoride content is less than 2% in terms of F ₂ , the viscosity of the slag is too high and the bead shape deteriorates, making vertical welding difficult with large heat input. On the other hand, if the metal fluoride content exceeds 12% in terms of _F2 , the arc becomes unstable, and the arc becomes short-circuited, making welding difficult, especially in upward welding in the low current range. The mixing ratio of the metal fluoride was expressed in terms of F ₂ only for convenience of analysis and the like. As deoxidizers, single metals such as Al and Mg, Fe-
One or more types selected from iron alloys such as Ti, Fe-Si, Fe-Al, Fe-Si-B, etc., or alloys such as Si-Mn, Ca-Si, Al-Mg, etc. may be used as necessary. %. In addition, as a slag forming agent, TiO ₂ , SiO ₂ ,
MgO, Al ₂ O ₃ , ZrO ₂ and the like can be appropriately added mainly for the purpose of coating the molten metal and adjusting the viscosity. It goes without saying that alloy components, iron powder, etc. can be added as necessary. The above-mentioned coating material is kneaded with a binder such as potassium silicate or sodium silicate and coated on the outer periphery of the steel core wire. <Steel core wire> The steel core wire used in the present invention is not subject to any particular restrictions on alloy composition, but there are particularly severe restrictions on N, which is generally considered an impurity element, and N: 0.005%. It is necessary that the following is true. In high heat input welding using a low hydrogen welding rod, it is difficult to completely prevent N from entering the atmosphere even if the coating contains a large amount of metal carbonate. As a result, it was confirmed that there was a nearly perfect proportional relationship between the N in the steel core wire and the N in the weld metal, as shown in Figure 3. Therefore, in order to suppress the N content in the weld metal to 0.01% or less, it is necessary to reduce the N content in the core wire to 0.005% or less. On the other hand, if the N content in the core wire exceeds 0.005%, the ferrite grains become coarse as shown in FIG. 2, and the toughness of the weld metal is significantly reduced. Carbon steel core wires are generally used as the core wires used in the present invention, but if necessary, deoxidizing elements such as Si, Mn, Ti, and Al, and elements such as Ni, Cr, Mo, and Cu may be added. It may be. <Coverage rate> Coverage rate A in the present invention is expressed by the following formula,
The coverage A must be within the range of 20-45%. Coverage rate A (%) = coating weight / rod weight x 100 In other words, if the coverage rate A is less than 20%, even if the coating material contains a large amount of metal carbonate, the protective tube will not be formed sufficiently and the shield will fail. 4th
As shown in the figure, N in the weld metal increases, and as a result, the toughness decreases as shown in Figure 2 above. the other 45
If it exceeds %, the arc length becomes long in the high current range, causing arc breakage and shielding failure, and N in the weld metal increases and toughness decreases. <P in the welding rod> [P content in the welding rod] must be 0.01% or less. In other words, if the [P content of the welding rod] exceeds 0.01%, P segregates in the final solidification part of each welding pass during high heat input welding, resulting in COD as shown in Figure 5.
The value drops significantly. Regarding the P content, we initially focused only on the core wire, but it turned out that a large amount of P was also mixed in the coating material, and the P content of the welding rod as a whole was
It was discovered that the content of When using industrial coating material raw materials, the P content in the coating material is around 0.01% even when it is as low as possible. It is desirable that the P content be 0.008% or less. <Mn, Ni in the welding rod> In the present invention, [Mn content in the welding rod]: ≦1.9%, [Ni content in the welding rod]: ≦4%, and [(Mn+Ni) content in the welding rod]: It is necessary to satisfy each condition of 1.2 to 4.3%. Ni can be added in an appropriate amount depending on the amount of Mn, but Ni+
If the Mn content is less than 1.2%, the ferrite grains of the weld metal will become coarse and the toughness will decrease. If the Ni + Mn content exceeds 4.3% or the Ni content exceeds 4%, the primary grain boundaries of the weld metal become brittle during high heat input welding, causing intergranular fracture and decreasing toughness ( 6
(see figure). On the other hand, if the Mn content exceeds 1.9%, the weld metal will be composed of bainitic ferrite grains, which will harden and reduce the COD value. The preferable content of Mn is about 0.3 to 1.9%. Mn and Ni can be added from the steel core wire, the coating material, or both, and metal Mn, Fe-Mn, Si-Mn, etc. can generally be used as the Mn source in the coating material. In addition, when the slag basicity is high like the welding rod according to the present invention (it is a so-called low hydrogen coating material based on metal carbonate or metal fluoride, and is usually basic), MnO or MnO ₂ Since manganese oxides such as oxides can be reduced to Mn during welding, they can also be used as Mn sources. In this case, the total amount of Mn may be calculated by including Mn added separately. [Example] Core wire No.C1~ with various chemical components as shown in Table 1
C7 was prepared, and various welding rods as shown in Table 2 were made. Welding rods Nos. E1 to E7 in Table 2 are examples that satisfy all of the requirements defined by the present invention, and welding rods Nos. G1 to G12 are comparative examples that lack any of the requirements. Welding was performed using each welding rod shown in Table 2.

【table】

【table】

【table】

【table】

[Table] The diameter of each welding rod is 5 mm, and an X groove is formed on a 32 mm thick aluminum killed steel plate.
Welding was carried out under conditions of large heat input of 200 A (A/C) and 60 to 70 KJ/cm. Siopy impact test pieces (2 mm V notch) and COD test pieces (fatigue notch) were taken from the obtained weld metal, and a toughness test was conducted on each test piece. The results are shown in Table 3.

【table】

[Table] As is clear from the results in Table 3, when welding was performed using welding rods No. E1 to E7, which are examples of the present invention, workability was good in all cases.
The vTrS of the obtained weld metals is all below -60℃, -60
It showed good toughness with a COD value of 0.8 mm or more at °C. On the other hand, welding rods No. G1 to G12 lack any of the requirements stipulated by the present invention, and therefore the following problems are pointed out. (1) When using welding rod No. G1, N: 0.007
%, the ferrite grains of the weld metal became coarse and showed low toughness. (2) When welding rod No. G2 is used, the P content in the welding rod is as high as 0.013%, so the COD value is low with significant segregation of P. (3) When using welding rods No. G3 and G4, the coverage ratio A is outside the range specified by the present invention, so
Workability was poor, N content increased, and toughness was low. (4) When welding rod No. G5 was used, the CO ₂ equivalent value was as low as 6.6%, so the N content increased and showed low toughness. (5) When welding rod No. G6 is used, the CO ₂ equivalent value is too high at 30.8%, and when welding rod No. G7 is used, the metal fluoride content is too low at 3%. In both cases, convex beads formed and there was no need to perform mechanical tests. (6) When welding rod No. G8 was used, the toughness was good, but the arc was unstable because it contained 31% metal fluoride. (7) When using welding rod No. G9, the welding rod
Since the Mn content was as high as 2.22%, the weld metal hardened and its ductility decreased, resulting in a low COD value. (8) When using welding rod No. G10, the welding rod
Since the Ni+Mn content was as low as 1.06%, the ferrite grains in the weld metal became coarse, resulting in poor toughness. (9) When welding rod No. G11 is used, the Ni content exceeds 4%, and the Ni + Mn content exceeds 4.30%, and when welding rod No. G12 is used,
Since the Ni+Mn content exceeded 4.3%, destruction occurred at the primary grain boundaries in all cases, and both the vTrS and COD values showed extremely poor values. [Effects of the Invention] As described above, according to the present invention, by employing the above-mentioned configuration, high toughness can be obtained even with large heat input, thereby significantly improving welding efficiency. be.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the CO ₂ content in the coating material and the N content in the weld metal, and Figure 2 shows the relationship between the N content in the core wire and the vTrS of the weld metal and the COD at -60°C.
Figure 3 is a graph showing the relationship between the N content in the core wire and the N content in the weld metal, and Figure 4 is a graph showing the relationship between the coverage ratio A and the N content in the weld metal. Figure 5 is a graph showing the relationship between P in the welding rod.
Figure 6 is a graph showing the effect of content on vTrS of weld metal and COD value at -60℃.
Ni+Mn content is vTrS of weld metal and -60℃
It is a graph showing the influence on COD value.

Claims (1)

[Claims] 1 N: 0.005% (weight%, the same applies hereinafter) or less of metal carbonate on the outer periphery of the steel core wire, 8 to 28% in terms of _CO2 ,
It is coated with a coating material containing 2 to 12% metal fluoride in terms of _F2 , and further contains at least a deoxidizing agent and a slag forming agent, and coverage ratio A = coating weight/bar weight x 100: 20 to 45 % [P content in welding rod] = [P content in core wire] x 100−
A/100 + [P content in coating material] x A/100≦0.01% [Mn content in welding rod] = [Mn content in core wire] x 100-
A/100 + [Mn content in coating material] x A/100≦1.9% [Ni content in welding rod] = [Ni content in core wire] x 100-
A low hydrogen-based coated arc welding rod, characterized in that A/100+[Ni in coating material]×A/100≦4% [Ni+Mn content of welding rod]=1.2 to 4.3%.