JP2854650B2 - MIG wire for high tensile steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a 70 kg and 80 kg class high strength steel MIG wire capable of obtaining good fracture toughness and Charpy impact characteristics even after heat treatment after welding. is there.

(Related Art and Problems to be Solved) In recent years, the introduction of elasto-plastic fracture mechanics to ensure the safety of steel structures, particularly high-pressure tanks, pressure vessels, penstocks, etc. It is especially noted as a trend in the design concept requiring high fracture toughness values.

Typical required characteristic values include a fracture surface transition temperature (vTrs) in a Charpy impact test and a fracture toughness value (K ₁ c) at a design temperature, which are related to a brittle fracture initiation temperature.

Conventionally, these structures, particularly high-pressure tanks and pressure vessels, are often made of low-strength materials because it is difficult to secure the toughness of the welded portion. The demand for the use of tensile steel is increasing.

To this end, it is essential to develop a welding material capable of coping with this high-tensile steel, but some proposals have been made conventionally. For example, in Japanese Patent Publication No. 53-27216, 70 km, 8 km
As MIG wire for 0 kg class high tension steel, high Si, the Ti-based wire Ar-CO ₂ mixed gas (CO ₂ mixing ratio: 5 to 75%) that is welded using have been proposed. However, since it has not been studied from the viewpoint of fracture toughness, it cannot be said that there is no problem in the safety of the steel structure.

The present invention has been made in order to meet the above-mentioned demand, and is intended for 70 kg and 80 kg class high-strength steels capable of obtaining good fracture toughness and Charpy impact properties even after heat treatment after welding.
It is intended to provide a MIG wire.

(Means for Solving the Problems) In order to achieve the above object, the present inventor focused on a singularity of a high-strength weld metal and a new viewpoint (particularly, fracture toughness value K ₁ c), For 70kg and 80kg class high strength steels with good fracture toughness after heat treatment
We conducted intensive research on MIG wires.

As a result, by adjusting to a specific chemical component and regulating the shielding gas composition, it is possible to keep the amount of oxygen that inhibits hardenability small, to reduce the amount of dissolved N harmful to toughness, and to remove excess Ti harmful to toughness. It can be kept small, especially that Mn, Ni and Ti can refine the crystal grains, Cu, Mn and Ni can be stabilized at an appropriate amount to stabilize the grain boundaries, and tempering embrittlement elements Sb, As and Sn It has been found that a high strength weld metal having a stable fracture toughness value can be obtained by the restriction, and the present invention has been made here.

That is, the present invention relates to a 70 kg, 80 kg class high tensile steel
-CO ₂ mixed gas (CO ₂ mixing ratio: 15-25%) a MIG wire for welding using, C: 0.03~0.09%, Si: 0.30~0.55
%, Mn: 1.00 to 1.80%, P: 0.010% or less, S: 0.010% or less, Cu: 0.07 to 0.30%, Ni: 2.40 to 3.00%, Mo: 0.20 to 0.5
0%, Cr: 0.20 to 0.90%, Ti: 0.005 to 0.050%, Al: 0.003
~ 0.020%, B: 0.0005% or less, N: 0.0080% or less, Sb: 0.00
5% or less, As: 0.005% or less, Sn: 0.005% or less, provided that Ti /
An MIG wire for high-strength steel, comprising a ratio of N: 1.0 or more, and the balance: Fe and inevitable impurities.

 Hereinafter, the present invention is described in more detail.

(Operation) First, the reasons for limiting the chemical components in the present invention will be described.

C: 0.03 to 0.09% C is generally known as an element for securing strength, but according to the study of the present inventors, C also acts as a deoxidizing agent, and if too low, the amount of oxygen increases and quenching occurs. The toughness and toughness were found to decrease (see FIG. 1), so at least 0.03% is required. However, if too much, high carbon martensite is formed and K ₁ c deteriorates. The upper limit is set to 0.09%.

Si: 0.30 to 0.55% Si is an indispensable element for maintaining workability, and at least 0.30% is necessary. However, if the content is too large, embrittlement occurs in the crystal grains and K ₁ c deteriorates.
%.

Mn: 1.00 to 1.80% Mn acts as a deoxidizing agent together with the refinement of the crystal grains,
%, The toughness is degraded with the coarsening of crystal grains and an increase in the amount of oxygen. However, if the content is too large, primary grain boundaries develop and cause grain boundary destruction, and K ₁ c is significantly deteriorated. Therefore, the upper limit is set to 1.80%.

P, S: 0.010% or less, P and S are impurities, and it is desirable that they are as small as possible.

Cu: 0.07 to 0.30% Cu is effective for stabilizing the crystal grain boundary, and vTrs does not greatly change, but has an effect of improving K ₁ c. 0.07 for that
% Or more is required. However, if the amount is too large, solidification cracks in the weld are likely to occur, so the upper limit is set to 0.30%.

Ni: 2.40 to 3.00% Ni has crystal grain refinement ability and is a toughness stabilizing element.
For that purpose, 2.40% or more is required. However, if it is too large, the primary grain boundaries will develop and vTrs will not change significantly, but K ₁ c will deteriorate significantly, so the upper limit is set to 3.00%.
And

Mo: 0.20 to 0.50% Mo, like Cr, is effective in improving the strength, and requires 0.20% or more. However, in order to prevent the deterioration of toughness due to precipitation hardening, the upper limit is set to 0.50%.

Cr: 0.20 to 0.90% Cr is an element effective for improving the strength, and 0.20% or more is required. However, if the amount is too large, the precipitation hardening due to the heat treatment becomes severe, and the toughness is deteriorated. Therefore, the upper limit is set to 0.90%.

Ti: 0.005 to 0.050% Ti reacts with N to form TiN, which has the effect of reducing solid solution N and improving toughness (see FIG. 2). It also acts as a deoxidizing agent. If it is too small, the amount of oxygen increases, and solute N
Increases and the toughness decreases (see FIG. 1). for that reason,
At least 0.005% is required. However, if it is too large, solid solution Ti and Ti carbide will be generated, and vTrs
Does not change much, but K ₁ c is significantly deteriorated, so the upper limit is set to 0.050%.

Al: 0.003 to .020% Al is acts as a deoxidizing agent in a small amount, since the Al ₂ O ₃ too much, deoxidation product is such that many remain in the weld metal, deoxidizing effect disappears. The limit is 0.020%, but the range in which deoxidation can be expected is preferably 0.003 to 0.020% (see FIG. 1).

B: 0.0005% or less B is an element that improves quenching properties and is effective in securing strength. However, if too large, the microstructure becomes lath-like or needle-like and K ₁ c deteriorates. I do.

N: 0.0080% or less N is a harmful element in terms of toughness, and a smaller amount is desirable. If the content is too large, the strength becomes too high due to an increase in internal strain due to solid solution N, and the toughness is deteriorated (FIG. 2).
% Or less.

Sb, As, Sn: 0.005% or less Sb, As, Sn are elements that show temper embrittlement, and have no significant effect on vTrs, but significantly increase K ₁ c in a sharp notch fracture toughness test. Deteriorates,
Each shall be 0.005% or less.

Ti / N ratio: 1.0 or more Ti / N is a ratio necessary for suppressing solid solution N. If this ratio is less than 1.0, the solute N increases and K ₁ c deteriorates.

 Next, the shielding gas will be described.

Shielding gas: Ar + (15-25%) CO _{2 As the} shielding gas, an Ar-CO ₂ mixed gas is used. In order to fix the oxygen content of the weld metal and maintain the workability, the mixing ratio should be adjusted. Need to regulate. That is, when the CO ₂ mixture ratio is more than 25%, the oxygen amount exceeds 0.03% even if the wire component is appropriately adjusted, and the performance is deteriorated. On the other hand, if the CO ₂ mixture ratio is less than 15%, the stability of the arc deteriorates, and the blowhole resistance deteriorates. Therefore, the CO ₂ mixture ratio is in the range of 15 to 25%.

It is needless to say that other MIG welding conditions are not particularly limited, and the base material can be various kinds of 70 kg and 80 kg high strength steel.

 Next, examples of the present invention will be described.

(Example) A 1.2 mmφ wire having the chemical components shown in Table 1 was prepared.

As a base steel plate, an X groove was formed on a 70 kg high-strength steel plate having a thickness of 50 mm, and MIG welding was performed with a pulse power supply at 130 A (DC) with a heat input of 30 KJ / cm. The shielding gas is Ar + 2
0% CO ₂ was used.

After welding, tensile test specimen, Charpy impact test specimen (2mmV notch) and fracture toughness test specimen (ASTM E813)
2TCT specimen) was collected and tested to determine its mechanical properties. The oxygen content of the weld metal was also examined. Table 2 shows the results.

 From Table 2, it is considered as follows.

All of the inventive wires A1 to A8 have good workability, and furthermore, all the vTrs are -50 ° C or less, and the fracture toughness value at -20 ° C (K ₁ c) is also 500 kgf / mm ^{3 / Two} or more good results were shown.

On the other hand, B9 to B28 are the case of the comparative example wire, first,
Comparative Example Wire B9 exhibited low toughness due to an increase in the amount of oxygen in the weld metal because C was too low at 0.01%. In B10, C
Was too high as 0.12%, so that K ₁ c showed a low value.

For B11, the workability was poor because the Si content was too low at 0.23%, and the subsequent test was stopped. B12 exhibited a low K ₁ c value because the content of Si was too high at 0.72%.

B13 has an increased oxygen content due to a low Mn of 0.87%,
The crystal grains became coarse and showed low toughness. B14 has Mn of 1.96
%, The grain boundary fracture occurred and K ₁ c showed a low value.

B15 exhibited a low K ₁ c value because Cu was as small as 0.01%. For B16, the test was stopped because solidification cracking occurred during welding due to too much Cu, which was 0.39%.

B17 has low toughness because Ni is too low at 2.12%, and B18
Is caused by grain boundary fracture because Ni is too large at 3.28%, and K ₁ c
Showed a low value.

B19 has low strength due to too small amounts of Cr and Mo,
It is not suitable for 0-80kg class high tensile steel. B20 is
Because Cr is too large at 1.10%, B21 also has Mo at 0.61%
, The precipitation hardening phenomenon became severe and the toughness was reduced.

B22 contained 0.002% Ti and 0.001% Al or less, each of which was too small to increase the amount of oxygen, increase the amount of solute N and increase the crystal grain size, and exhibited low toughness. B23 has a Ti of 0.068
For% and more, but vTrs was good, K ₁ c showed low values.

B24 is, K ₁ c showed low values the ratio of Ti / N is caused an increase of solute N in smaller and 0.8.

B25 exhibited low toughness due to an increase in the amount of oxygen because Al was too large at 0.037%.

In B26, since N was too large as 0.0097%, solute N was increased and toughness was deteriorated.

B27 produced acicular martensite because B was as large as 0.0009%, and showed a low value of K ₁ c.

B28 is an example containing a large amount of Sb, As, and Sn, respectively.
Although s was relatively good, K ₁ c was significantly degraded.

In addition, all the above experimental examples are the results of performing a heat treatment at 580 ° C. for 4 hours after welding, and then performing various mechanical tests.In the as-welded state, the wires of the present invention show good results. Was confirmed.

(Effects of the Invention) As described above in detail, according to the present invention, not only as-welded but also after heat treatment after welding, particularly good fracture toughness is obtained and the Charpy impact characteristics are also good. It is suitable for MIG of 70 kg and 80 kg high strength steels, which can sufficiently secure the safety of steel structures and has a remarkable effect of greatly contributing to industrial development.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amounts of C, Ti, and Al and the amount of oxygen in the weld metal, and FIG. 2 is a diagram showing the relationship between the amount of N in the wire and the amount of solute N in the weld metal.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-196286 (JP, A) JP-A-60-158895 (JP, A) JP-A-49-39536 (JP, A) 57953 (JP, B2) JP-B-39-20326 (JP, B1) "Modern Welding Technology Dai-kei, Vol. 11," published Jan. 23, 1980, Sanpo Publishing Co., p. 122 “Welding after heat treatment embrittlement” (58) Fields investigated (Int. Cl. ⁶ , DB name) B23K 35/30 320

Claims (1)

(57) [Claims] 1. A MIG wire for welding 70 kg, 80 kg class high strength steel using an Ar-CO ₂ mixed gas (CO ₂ mixture ratio: 15 to 25%). ), C: 0.03-0.09%, Si: 0.30-0.55%, Mn: 1.00-1.80%, P: 0.010% or less, S: 0.010% or less, Cu: 0.07-0.30%, Ni: 2.40-3.00%, Mo : 0.20 to 0.50%, Cr: 0.20 to 0.90%, Ti: 0.005 to 0.050%, Al: 0.003 to 0.020%, B: 0.0005% or less, N: 0.0080% or less, Sb: 0.005% or less, As: 0.005% or less , Sn: 0.005% or less, provided that the ratio of Ti / N is 1.0 or more, and the balance is composed of Fe and unavoidable impurities.