JPH03197660A - Method for hot dip galvanizing welded steel structure - Google Patents

Abstract

PURPOSE:To prevent the cracking of the zinc of a hot dip galvanized welded steel structure due to embrittlement by regulating each of the Pb, Cd and Sn contents of molten zinc to a specified value or below when a welded steel structure is hot dip galvanized with the molten zinc. CONSTITUTION:A welded steel structure is hot dip galvanized with molten zinc contg. <=0.1wt.% Pb, <=0.02wt.% Cd and <=0.005wt.% Sn. The cracking of the zinc of the hot dip galvanized welded steel structure due to embrittlement is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a method for hot-dip galvanizing welded steel structures that prevents the occurrence of molten metal embrittlement cracking due to molten zinc.

Conventional technology When galvanizing a welded steel structure, intergranular cracking may occur in the coarse grain region of the weld heat affected zone.

This phenomenon occurs during immersion in a hot-dip zinc bath during the galvanizing process, and is considered to be a type of embrittlement of solid metal due to liquid metal. )It is called.

For example, power transmission towers are often located in mountainous areas, making it difficult to paint them after assembly on site, and steel pipes for towers are hot-dip galvanized after the members are assembled by welding.

However, when applying this hot-dip galvanizing, plating cracking may occur.

The above-mentioned plating cracks tend to occur more often as the strength of the steel pipe material is higher, the plate thickness is greater, and the hardness of the weld heat affected zone is greater. For example, if the carbon equivalent of the steel pipe for a steel tower is Ceq≧0.40% or the maximum hardness of the weld heat affected zone is Hv≧260, plating cracking may occur.

In recent years, large power transmission steel pipe towers have become increasingly larger and stronger, and there is a desire for a better method for preventing plating cracking.

As a preventive measure, it has been proposed to adjust the composition of steel materials from the viewpoint of plating cracking (for example, JP-A-58-8
Publication No. 4959).

Problems to be Solved by the Invention As mentioned above, even if the composition of the steel material is adjusted as a measure to prevent plating cracking in welded steel structures, plating cracking is also caused by other causes such as plate thickness and hardness of the weld heat affected zone. As a result, sufficient effects have not yet been achieved.

Means for Solving the Problems In order to achieve the above objects, the hot-dip galvanizing method for welded steel structures of the present invention uses zinc that suppresses the inclusion of harmful impurities that cause molten metal embrittlement cracking due to molten zinc. The purpose is to apply hot-dip galvanizing.

When Pb and Cd are included as harmful impurities,
Hot-dip galvanizing is performed using zinc in which the Pb content is suppressed to 0.1% by weight or less and the Cd content is suppressed to 0.02% by weight or less.

In addition, if Sn is included in addition to Pb and Cd as harmful impurities, hot-dip galvanizing should be performed using zinc suppressed to 0.1% by weight or less of Pb, 0.02% by weight or less of Cd, and 0.005% by weight or less of Sn. It is.

Function The zinc used in the hot-dip galvanizing method of the present invention suppresses the content of harmful impurities such as Pb and Cd, which cause plating cracking, to an extremely small amount that is harmless. The occurrence of cracks can be completely prevented.

In this invention, the content of harmful impurities contained in zinc is limited for the following reasons.

As seen in the components of distilled zinc bullion shown in Table 1,
Ordinary zinc ingots contain about 1% by weight of PB, about 0.1% by weight of Cd, and in some cases about 0.01% by weight of S.
n is mixed in. However, the melting points of these impurities are 327.5°C for Pb, 320.9°C for Cd, and 2°C for Sn.
The temperature is 31.97°C, which is nearly 100°C lower than the melting point of Zn, 419.6°C.

In this way, since the melting points of Zn and impurities are different, 450
During hot-dip galvanizing at temperatures on the order of degrees Celsius, impurity low-melting metals diffuse into the grain boundaries of the steel, resulting in cracking.

As a result of experiments, this phenomenon has been confirmed as shown in Table 1 for the ingots used in the present invention, PB is 0.1% by weight or less, Cd is 0.0%
It was found that this problem could be prevented by limiting the amount of Sn to 2% by weight or less and the content of Sn to 0.005% by weight or less.

(Leaving space below) Examples Example 1 Five types of steels whose chemical compositions are shown in Table 2 were selected from among the steels used for steel pipe materials for steel towers, etc., and a welded steel structure was made using these steels. A restrained joint test was conducted to investigate plating cracking when hot-dip galvanizing was applied.

That is, as shown in Fig. 1, the opposing intersection lines of the test plates (1) assembled in a cross shape were restrained welded (2), and the other opposing intersection lines were welded using steel types 1 to 5 in Table 2. Test welding using
).

Each of the test pieces made as described above was tested according to the practice of this invention (0.05% by weight of Pb, 0.01% by weight of Cd, 0.0% by weight of Fe.
008% by weight, the remainder consisting essentially of Zn), galvanization was performed in a normal hot-dip plating process. For comparison, one type of distilled zinc shown in Table 1 was used to perform zinc plating in the same manner as a comparative example.

The galvanizing process is a commonly used method,
After degreasing, pickling, and flux treatment with a solution containing ZnC15 and NH4CI in a molar ratio of 1:2, the sample was immersed in a molten zinc bath at a bath temperature of 455°C for 10 minutes. The occurrence of cracks at the bead toe of test welding (3) was then investigated. The results are shown in Table 3 along with the mechanical properties of the steel and the maximum hardness (Hv) of the weld heat affected zone.

(Left below) Steel types 1 to 5 above are steel materials that cause plating cracking when conventional hot-dip galvanizing is applied. , plating cracking occurred in all others.

However, when this invention was implemented, there was no plating cracking at all.

Example 2 A restrained test piece was made in the same manner as in Example 1 using steel types A and B having the chemical composition, mechanical properties, and maximum hardness of the weld heat-affected zone shown in Table 4. The presence or absence of plating cracking was investigated by changing the chemical composition. The results are shown in Table 5 along with the chemical components of zinc.

(Margin below) In Table 5, sample No. Both A-1 and B-1 are conventional methods using distilled zinc ingots shown in Table 1, and sample No. A-2 and B-2 are Pb and Cd in zinc
This is a comparative example in which the content exceeds the limited amount of this invention. And sample no.

A-3 to A-5 and B-3 to B-5 are examples of this invention. This test result shows that if hot-dip plating is performed using zinc with a limited content of PB and Cd according to the present invention, no plating cracking will occur.

Example 3 A restrained test piece was made in the same manner as in Example 1 using steel types A and B having the chemical composition shown in Table 4 and the highest hardness of the weld heat affected zone, and Pb was detected as a harmful impurity that causes plating cracking. , Hot-dip plating was performed by changing the chemical composition of zinc containing Sn in addition to Cd, and the presence or absence of plating cracking was investigated. The results are shown in Table 6 along with the chemical composition of zinc.

(Left below) Each sample in Table 5 is the same as the sample classification in Example 2. In this case as well, the test results show that if hot-dip galvanizing is performed using zinc in which the content of harmful impurities Pb, Cd, and Sn is limited according to the present invention, no plating cracking will occur.

Effects of the Invention This invention provides the following advantages in hot-dip galvanizing of welded steel structures:
Since harmful impurities that cause plating cracking contained in the zinc used are suppressed to an extremely small amount that does not cause plating cracking, the occurrence of plating cracking in hot-dip galvanizing can be completely eliminated.

[Brief explanation of drawings]

Figure 1 is a front view (Figure a) showing a restrained joint test piece for testing plating cracking in a hot-dip galvanized weld heat-affected zone.
and a side view (figure b). 1...Test plate 2...Restricted welding 3...
Test welding (a) Figure 1

Claims (1)

[Scope of Claims] 1. A method for hot-dip galvanizing a welded steel structure using zinc containing 0.1% by weight or less of Pb, 0.02% by weight or less of Cd, and the remainder substantially consisting of Zn. 2 Pb 0.1% by weight or less, Cd 0.02% by weight or less,
Contains 0.005% by weight or less of Sn, the remainder being substantially Zn
A method for hot-dip galvanizing welded steel structures using zinc consisting of: