JPH11123589A - Low hydrogen type covered electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low hydrogen type covered electrode, which is superior in cold crack resistance in shielded metal arc welding of high tensile steel having a tensile strength of 880 MPa or above. SOLUTION: This low hydrogen type covered electrode is such that a steel cored wire having not more than 0.03 wt.% C is covered with a coating containing an arc stabilizer, slag forming agent, deoxidizer, alloying agent and a binder, other than 0.1-0.3 wt.% B, 30-60 wt.% metallic carbonate, and 10-30 wt.% metallic fluoride.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-hydrogen coated arc welding rod for high-tensile steel, and particularly to a tensile strength of 880 MP.
a The present invention relates to a low-hydrogen coated arc welding rod exhibiting excellent low-temperature cracking resistance when welding the above steel materials.

[0002]

2. Description of the Related Art With the increase in the size of welded steel structures, the thickness of steel materials used in the structures has increased, and the strength has been changing to higher strength. It has been known that low-temperature cracking is apt to occur in a welded part when covering arc welding of this high-tensile steel material, and measures for low-temperature cracking are required in welding. There are mainly three causes of low-temperature cracking: the amount of diffusible hydrogen in the weld, the hardness, and the degree of constraint. Above all, the effect of diffusible hydrogen is considered to be the largest, and therefore, a method of reducing the amount of diffusible hydrogen in a welded portion to prevent low-temperature cracking is generally applied. Of these, the most commonly used is preheating or afterheating of the weld. However, this method increases the degree of high tension and requires a higher preheating temperature or a post-heating temperature, so that the welding work environment is deteriorated and welding work costs are increased. A solution is needed.

Under such circumstances, in coated arc welding of high-tensile steel having a tensile strength of 880 MPa or less, since low-temperature cracking occurs in the heat affected zone, the chemical composition of the steel sheet, the amount of diffusible hydrogen, and the degree of restraint are limited. A low-temperature cracking susceptibility index based on the low-temperature cracking susceptibility index was proposed, and a high-strength steel sheet excellent in low-temperature cracking resistance and having a reduced low-temperature cracking susceptibility index was developed. In other words, by applying the technology of controlled rolling and controlled cooling, it is possible to produce a steel sheet having a low carbon equivalent and a low cold cracking susceptibility index despite its high strength, which is generated in the weld heat affected zone. The technology for preventing cold cracking has been greatly improved. Accordingly, it can be said that the preheating temperature has been reduced, and the deterioration of the welding work environment due to the preheating and the like, and the increase in welding construction costs have been greatly improved.

[0004]

However, even with the above-described method for preventing the low-temperature cracking by lowering the carbon equivalent of the steel sheet, low-temperature cracking occurs when a high-tensile steel sheet having a tensile strength of 880 MPa or more is formed, as described above. It cannot be suppressed. The reason is that as the strength of the steel sheet increases, the location of low temperature cracking shifts from the weld heat affected zone to the weld metal,
This is because in a high-tensile steel having a tensile strength of 880 MPa or more, most of the low-temperature cracks occur in the weld metal. Therefore, in order to improve low-temperature cracking of high-tensile steel having a tensile strength of 880 MPa or more, it is necessary to improve a welding rod rather than a steel plate. Then, for example, a low hydrogen-based coated arc welding rod that can significantly reduce the preheating temperature during welding is required.

By the way, much research has been conducted on low-hydrogen-based coated arc welding rods for high-tensile steel sheets, but most of them are aimed at improving the toughness of the weld metal. . This is because generally, the higher the strength of a weld metal, the lower its toughness tends to be. As a conventional method for improving this toughness, for example, Japanese Patent Publication No.
Disclosed is a technique for improving fracture toughness by using a low hydrogen coated arc welding rod coated with a coating containing a metal carbonate, a metal fluoride, and Mg around a steel core wire whose Ni% is regulated. . Japanese Patent Publication No. 8-25059 discloses a technique for improving the toughness of a weld metal by limiting the average particle size of metal Mg added to a coating agent for a low hydrogen-based coated arc welding rod. . However, the fact is that even when welding is performed using such a low hydrogen-based coated arc welding rod, the low-temperature cracking resistance is still insufficient.

Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art, and further provides low-temperature cracking resistance in coated arc welding of high-tensile steel having a tensile strength of 880 MPa or more. An object of the present invention is to provide an excellent low hydrogen-based coated arc welding rod.

[0007]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have developed low-temperature cracks generated in a weld metal in coated arc welding of a high-tensile steel material having a tensile strength of 880 MPa or more, particularly, in the production of welded joints. The composition of the coating agent which affects the low temperature cracks generated in the first layer welding at the time was examined. As a result, it has been found that it is extremely effective to contain an appropriate amount of B in the coating agent in order to improve the low-temperature cracking resistance during the production of a welded joint, leading to the completion of the present invention. Was.

[0008] That is, the gist of the present invention is that a coating agent containing B: 0.1 to 0.3% by weight is coated around a steel core wire containing C: 0.03% by weight or less. It is a hydrogen-coated arc welding rod, and C: 0.03% by weight
B: 0.1 to 0.3% by weight, metal carbonate: 30 to 60% by weight, metal fluoride: 10 to 30% by weight, an arc stabilizer, a slag forming agent, A low-hydrogen coated arc welding rod characterized by being coated with a coating agent containing an acid agent, an alloying agent and a fixing agent.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the gist of the present invention will be described below. First, the steel core wire will be described. In order to secure the toughness of the deposited metal and obtain excellent low-temperature cracking resistance, the C content must be sufficiently reduced. For this purpose, the C content of the core wire must be reduced to 0.03% by weight or less. It is necessary to reduce it. In particular, in the first layer welding at the time of producing a welded joint, the welded portion is rapidly cooled, so that the weld metal has a structure mainly composed of martensite, and its hardness depends on C%. If the C of the core wire exceeds 0.03% by weight, the C% of the weld metal increases, the hardness of the weld metal increases, and it becomes impossible to prevent low-temperature cracking of the weld metal. As chemical components in the steel core wire other than C, from the viewpoint of improving the mechanical properties of the welded portion, P: 0.01% by weight or less, S: 0.01% by weight or less, N: 0.005% by weight or less, O:
It is desirable to limit the range to 0.001 to 0.01% by weight.

Next, regarding the coating agent, adding B to the coating agent is one of the important points of the present invention. The addition of B in the coating significantly reduces cold cracking in the weld metal. The reason why such an effect can be obtained is as follows.
It is considered that B present in the weld metal crystal grain boundaries lowers the grain boundary energy and suppresses the generation and propagation of low-temperature cracks at the grain boundaries. If the amount of B is less than 0.1% by weight, the above effect is not exhibited. If the amount of B exceeds 0.3% by weight, hardening due to an increase in the hardenability of the weld metal becomes remarkable, and on the contrary, low-temperature cracking tends to occur. Therefore, the amount of B in the coating agent is 0.
It should be in the range of 1 to 0.3% by weight. The addition form of B is
Although it is not particularly limited, it is preferable that it is uniformly mixed in a coating agent such as ferroboron.

The metal carbonate in the coating material decomposes in the arc, generates CO ₂ gas, shuts off the molten metal from the atmosphere, reduces the partial pressure of hydrogen and nitrogen in the arc atmosphere, It has the effect of generating slag. In order to exert such an effect, it is desirable to contain the coating agent in the range of 30 to 60% by weight based on the total amount of the coating agent. If the content is less than 30% by weight, the shielding gas becomes insufficient and the toughness and crack resistance are deteriorated, while if it exceeds 60% by weight, the arc becomes unstable and the bead shape becomes unstable. . Note that specific examples of the metal carbonate include calcium carbonate, barium carbonate, manganese carbonate, and magnesium carbonate.

The metal fluoride lowers the melting point of the slag to improve its fluidity, and the fluorine decomposed in the arc reacts with the hydrogen in the molten metal or the molten slag to reduce the hydrogen partial pressure of the molten metal. It has the effect of producing a weld metal with good crack resistance by lowering it. In order to exert such an effect, the content of the metal fluoride is desirably in the range of 10 to 30% by weight based on the total amount of the coating agent. If the content of the metal fluoride is less than 10% by weight, the viscosity of the molten slag is insufficient and the bead shape is deteriorated, while if it exceeds 30% by weight, the stability of the arc is deteriorated. Note that specific examples of the metal fluoride include calcium fluoride, barium fluoride, and manganese fluoride.

In addition, rutile, alkali metal, iron powder and the like can be added to the coating agent as an arc stabilizer and a slag forming agent. In addition, as a deoxidizing agent, Si, Mn, Mg, T
i, Al, etc., and Si, Mh, Ni, Cr, Mo, Ti, Al, etc. can be added as an alloying agent for securing strength and toughness.

As the fixing agent, Si is used for the alkali component.
O ₂ concentration and 10 to 35 wt% of 10 to 30 wt% of lithium silicate solution, also 20 to 30 wt% aqueous solution of sodium silicate 20
60% by weight and 20-30% by weight aqueous potassium silicate solution
It is desirable to add a mixture of 20 to 60% by weight.

[0015]

EXAMPLES Hereinafter, the effects of the present invention will be specifically described with reference to examples. Various types of coated arc welding rods (core diameter: 4.0 mm) having the components shown in Table 1 and consisting of a core wire and a coating agent were produced. The production conditions were 350-55 to cover with a normal welding rod coating machine, pre-dry and then remove moisture.
Dried at 0 ° C. Also, before use in welding, 400-500
Re-dried at ° C. The diffusible hydrogen content of the weld metal in the welding rod used was in the range of 3 to 5 cc / 100 g. Using these welding rods, a low-temperature cracking resistance test was performed by a y-type welding crack test (JIS Z3158). The test conditions are strength 9
It is to be welded to a high-strength steel plate of 70 MPa, Pcm 0.27 wt%, thickness of 50 mm in a preheating temperature of 75 ° C, welding current of 170 A, welding heat input of 17 kJ / cm, and in a downward position. From observation of the cross section after welding,
If the root cracking rate is less than 20%, it is judged as good and marked with a circle, 20%
A sample having a value of 40% or more and less than 40% was evaluated as slightly good, and a sample having a value of 40% or more was evaluated as poor, and indicated with a cross. The test results are also shown in Table 1.

[0016]

[Table 1]

In the invention examples Nos. 1 to 13 satisfying the requirements of the present invention, even if the preheating temperature is as low as 75 ° C.,
All have relatively good low-temperature cracking resistance, especially No. 1 to No. 1 containing metal carbonate and metal fluoride in an appropriate range.
11, excellent cold cracking resistance was achieved. On the other hand, in Comparative Examples Nos. 14 to 22 in which at least one of the content of C in the core wire and the content of B in the coating material was out of the range of the invention, low-temperature cracking was generated, resulting in failure.

[0018]

As described above, according to the present invention,
The present invention can provide a low-hydrogen-based coated arc welding rod that can significantly improve low-temperature cracking resistance when used for coated arc welding of high-tensile steel having a tensile strength of 880 MPa or more. For this reason, the present invention is expected to greatly contribute to the development of various industrial fields involving welding of high-tensile steel.

Claims (2)

[Claims] 1. A low-hydrogen coated arc welding rod comprising a steel core wire containing C: 0.03% by weight or less and a coating agent containing B: 0.1 to 0.3% by weight. 2. A steel core wire containing C: 0.03% by weight or less, B: 0.1 to 0.3% by weight, metal carbonate: 30 to 60% by weight, metal fluoride: 10 to 30% by weight, and arc Stabilizer,
A low-hydrogen coated arc welding rod obtained by coating a coating material containing a slag forming agent, a deoxidizing agent, an alloying agent and a fixing agent.