JPH0517860A - Hot-dip galvanizing method - Google Patents

Abstract

PURPOSE:A To prevent hot-dip galvanizing crack occurring at the time of hot-dip galvanizing without deteriorating the adhesiveness of zinc film. CONSTITUTION:At the time of subjecting a material to be plated to hot-dip galvanizing, flux treatment is done by using an aqueous solution of flux containing one or more metals among K, Ca, Na, Cu, Ni, Ti, Mo, Co, Ce, La, Y, and Zr or ordinary flux treatment is made and then the material to be plated is coated with one or more kinds among the above metals.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for hot dip galvanizing large welded structures such as power transmission towers and bridges, and fastening members such as high-strength bolts, and more particularly to such structures and fastening members during hot dip galvanizing. The present invention relates to a hot dip galvanizing method in which hot dip galvanizing cracks do not occur in the material to be plated.

[0002]

2. Description of the Related Art From the viewpoint of rust prevention of steel materials, means for plating a metal having excellent corrosion resistance such as zinc is widely used. There are two methods for plating zinc on steel, one is by electroplating and the other is by hot dipping.In the case of large-scale welded structures with complicated shapes such as power transmission towers and bridges, the latter method is mainly used. The hot dip plating method is adopted. Further, even in the case of high-strength bolts, in the electroplating method, there is a concern that hydrogen will infiltrate into the steel and delayed fracture due to the infiltration of hydrogen.

The JIS standard H9124 defines a "hot dip galvanizing work standard" for such large structures, and hot dip galvanizing requires pretreatment for degreasing and removal of oxides on a material to be plated. It is carried out by immersing it in a molten zinc bath after application and flux treatment. The flux treatment is carried out for the purpose of preventing rust from regenerating on the plated material after the pretreatment and facilitating the Fe-Zn alloy reaction in the molten zinc bath. A flux aqueous solution of ammonium chloride mixture, zinc ammonium chloride, ammonium chloride or the like is used.

In this hot dip galvanizing process, when a material to be plated such as a welded structure is dipped in a hot dip zinc bath at a high temperature (about 450 ° C.) to be plated, the heat affected zone (hereinafter referred to as “HAZ”) is mainly used. ), Intergranular cracks, so-called hot-dip galvanizing cracks may occur. This crack is
The infiltration of liquid zinc into the grain boundaries of coarse-grained HAZ and the residual stress of welds and the thermal stress generated when immersed in a molten zinc bath are superimposed, and generally tend to occur more in high strength materials. If the HAZ hardness exceeds 260 to 270 Hv, the susceptibility to galvanizing cracks becomes extremely high.

With respect to the above hot dip galvanizing cracks, research has been widely conducted from the viewpoint of stress and the material which cause the cracks, and as a preventive method, at present, removal of residual stress in the welded portion, Various methods have been devised such as the method of dipping in a molten zinc bath with less thermal stress and the shape of the structure. In terms of material, for example, as described in Japanese Patent Publication No. 2-5814, 60 kgf / mm ² class in which the contents of C, Mn, Si, Nb, V and Al in steel are adjusted to an appropriate range. A high-strength, low-alloy steel having excellent galvanizing crack resistance has been proposed. Furthermore, research has been conducted from the aspect of impurities contained in the Fe—Zn intermetallic compound layer formed during hot dip galvanizing and the hot dip zinc bath (Journal of the High Temperature Society of Japan).
Vol. 13, No. 4, July 1987, pp. 143-152). And
It is shown in this document that the impurities Cd and Sn contained in the molten zinc bath accelerate the infiltration of zinc into the grain boundaries, and that the susceptibility to galvanizing cracks is increased by the Cd and Sn in the molten zinc bath. The effect of Cd and Sn is that Fe-Zn formed during hot dip galvanizing.
It is considered to be related to the formation of the intermetallic compound layer.
That is, to put this consideration simply, C in the molten zinc bath is
Fe in which d and Sn are formed at the interface between steel and molten zinc bath
Since the growth of the —Zn intermetallic compound layer is suppressed, the penetration of molten zinc into the steel grain boundaries is accelerated, and as a result, the hot dip galvanizing crack susceptibility is increased.

[0006]

As described above, various studies have been conducted on the cause of hot-dip galvanizing cracking and its prevention method from the viewpoint of material and design, and for steel materials excellent in galvanizing cracking resistance. Has been developed up to a strength of 60 kgf / mm ² class as disclosed in Japanese Patent Publication No. 2-5814. However, in recent years, welded structures have tended to become larger and larger, and further, steel materials having a strength of 65 kgf / mm ² class or higher are desired in order to reduce the amount of steel materials used and reduce costs. As described above, the higher the strength of the material, the higher the susceptibility to galvanizing cracks, so that hot dip galvanizing cracks tend to occur.

On the other hand, according to the above-mentioned document, if Cd and Sn contained as impurities in the molten zinc bath increase the susceptibility to galvanizing cracks, the molten zinc bath contains Cd and Sn.
If high-purity zinc with a low content of is used, reduction of hot-dip galvanizing cracks can be expected. However, the effect is not so clear, and since pure zinc is more expensive than distilled zinc that is usually used for hot dip galvanizing, it causes an increase in manufacturing cost.

Although expensive pure zinc can be obtained by an electrolytic method and can be industrially used, at present, inexpensive zinc refined by a distillation method is used in a molten zinc bath in consideration of manufacturing cost. ing. Zinc refined by the distillation method generally contains 0.05 to 0.1% of Cd and Sn, respectively.

An object of the present invention is to solve the above-mentioned problems, and more specifically, it is restricted by the type of molten zinc bath, the type of steel material, the welding method, the dipping method in the molten zinc bath, etc. It is to provide a hot dip galvanizing method that can prevent hot dip galvanizing cracks. In particular, this invention is effective for hot-dip galvanizing steel having a high susceptibility to galvanizing cracks and having a strength of 55 kgf / mm ² class, and also for high-strength steel material having a strength of 65 kgf / mm ² or higher. An object is to provide a plating method.

[0010]

[Means for Solving the Problems] When hot dip galvanizing a steel material, F of the Γ layer, δ ₁ layer and ζ layer is formed at the steel material-hot dip zinc bath interface.
An e-Zn intermetallic compound layer is formed. Hot dip galvanizing cracks are related to the morphology of these Fe-Zn intermetallic compound layers in the galvanized layer, and impurities Cd and Sn contained in the hot dip zinc bath suppress the growth of the Fe-Zn intermetallic compound layer. , Promoting hot dip galvanization cracking, and Fe
The δ ₁ layer in the —Zn intermetallic compound layer has the function of preventing the infiltration of liquid zinc into the grain boundaries, as reported in the above literature.

This inventor has also found that the purity of the molten zinc bath is 99.99%.
Pure zinc and distilled zinc containing 0.08% Cd and 0.05% Sn were used, the bath temperatures of the pure zinc bath and the distilled zinc bath were each kept at 450 ° C., and the same composition as in the examples described later having a weld The above steel material was immersed for 10 minutes for hot dip galvanizing, and the relationship between the morphology of the Fe-Zn intermetallic compound layer and hot dip galvanizing cracks was investigated. As a result, the following was found.

In the steel material plated with the distilled zinc bath containing Cd and Sn, cracks are observed in the hardened HAZ of the weld.

No cracks are observed in the steel material plated with the pure zinc bath. However, the zinc film formed in the pure zinc bath has poor adhesion and is easily peeled off.

The cause of poor adhesion of the zinc film formed in the pure zinc bath is due to cracks generated in the thickly grown Fe—Zn intermetallic compound layer.

If hot dip galvanizing is performed so that the Fe—Zn intermetallic compound layer grows thick, the zinc plating crack resistance is improved. However, if the intermetallic compound layer grows too thick, the adhesion of the zinc coating will deteriorate, so it is necessary to grow the intermetallic compound layer to an appropriate thickness.

Therefore, the present inventor has studied a hot dip galvanizing method capable of growing an intermetallic compound layer having an appropriate thickness, from a pretreatment for plating which has not been studied in the past. As a result, K, Ca, Na, Cu, Ni, Ti, Mo, C is applied to at least the welded portion when the plated material is subjected to the flux treatment or after the usual flux treatment.
When hot dip galvanizing is performed after coating at least one metal selected from o, Ce, La, Y and Zr, the Fe-Zn intermetallic compound layer grows to an appropriate thickness during hot dip galvanizing, Without losing the adhesion of the film,
It has been found that the zinc plating crack resistance is improved.

The gist of the present invention is the following hot dip galvanizing method.

(1) In a hot dip galvanizing method in which a material to be plated is subjected to flux treatment and then immersed in a hot dip zinc bath, K, Ca, Na, Cu, Ni, Ti, Mo, Co,
A hot dip galvanizing method, which comprises performing a flux treatment using an aqueous flux solution containing at least one metal selected from Ce, La, Y and Zr.

(2) In the hot dip galvanizing method in which the material to be plated is subjected to flux treatment and then immersed in a hot dip zinc bath, K, Ca, Na, Cu, Ni, Ti is applied to at least the welded portion of the material to be plated after the flux treatment. , Mo, Co,
A hot dip galvanizing method, which comprises coating at least one metal selected from Ce, La, Y and Zr.

The material to be plated is a steel plate, shaped steel, steel pipe,
They are welded structures such as transmission towers and bridges made by welding and assembling materials such as flat steel and bar steel, and fastening members such as high-strength bolts that are processed from these materials.

[0021]

As described above, hot dip galvanizing cracks mainly occur in the HAZ. This is caused by the infiltration of liquid zinc into the grain boundaries of the coarse-grained HAZ and the residual stress of the welded portion and the thermal stress generated when immersed in the molten zinc bath. Therefore, if at least the liquid zinc can be prevented from entering the grain boundaries of the HAZ, the occurrence of hot dip galvanizing cracks can be reduced.

The above K, Ca, Na, Cu, Ni, T
The metals i, Mo, Co, Ce, La, Y and Zr are
When they both dissolve in zinc, they raise the melting point of zinc,
It has a function of promoting the growth of the e-Zn intermetallic compound layer. Before plating a material to be plated with a zinc bath containing Cd and Sn, one or more of these metals are coated on the entire surface including the welded portion or only on the welded portion,
When hot-dip galvanizing, a δ ₁ layer, which has the effect of preventing liquid zinc from entering the grain boundaries, grows thick in the portions coated with these metals during hot-dip galvanizing. Moreover,
The Fe-Zn intermetallic compound layer is not cracked and does not grow thick until the adhesion of the zinc coating is impaired. This is K, Ca, Na, Cu, Ni, Ti, Mo, Co,
Ce, La, Y and Zr have the action of promoting the growth of the Fe-Zn intermetallic compound layer, and Cd and S in the zinc bath.
This is because the effect of n suppressing the growth of the Fe-Zn intermetallic compound layer is offset.

In the present invention, the above K, Ca, Na and C are used.
u, Ni, Ti, Mo, Co, Ce, La, Y and Z
The metal of r may be subjected to a flux treatment with a flux aqueous solution containing one or more of these metals for coating, or may be subjected to a usual flux treatment with a flux aqueous solution containing no such metals, and then one of these metals may be coated. You may coat the above.

K, Ca, Na, Cu, Ni, Ti, M
When coating with a flux aqueous solution containing one or more of O, Co, Ce, La, Y and Zr, chlorides of these metals are mixed with zinc chloride / ammonium chloride mixture, zinc ammonium chloride, ammonium chloride. It is better to dissolve in a flux aqueous solution such as. Flux treatment, for example, when the material to be plated is a welded structure such as a steel tower or bridge, and the flux aqueous solution in which the chlorides of these metals are dissolved is a 20 to 30% zinc ammonium chloride aqueous solution, the liquid temperature It may be carried out under conditions of 30 to 90 ° C. and immersion time of 3 to 5 minutes. In the coating that also serves as the flux treatment, a flux layer containing one or more of these metals is formed on the entire surface including the welded portion.

After the usual flux treatment, K, C
a, Na, Cu, Ni, Ti, Mo, Co, Ce, L
When coating one or more metals of a, Y and Zr, the coating can be performed by the following method.

(A) K, Ca, Na, Cu, Ni, T
1 of i, Mo, Co, Ce, La, Y and Zr
An aqueous solution containing one or more metals is applied or sprayed.

(B) K, Ca, Na, Cu, Ni, T
One or more powders of metal powders of i, Mo, Co, Ce, La, Y and Zr, oxides and chlorides of these metals, which are stable in the atmosphere and easy to handle, are applied or sprayed.

(C) K, Ca, Na, Cu, Ni, T
A metal powder of i, Mo, Co, Ce, La, Y, and Zr, and one or more powders of oxides and chlorides of these metals which are stable in the air and easy to handle are suspended. Apply or spray an aqueous solution.

In the case of coating by the above methods (a) to (c), the entire surface including the welded portion may be coated, but since the hot dip galvanizing cracks often occur in the welded portion, the coating is performed. Only the welded part may be used.

In the coating method (a), in order to prepare an aqueous solution containing the above-mentioned metal, a water-soluble one of chloride, oxide, sulfate and nitrate of these metals may be dissolved. . The method (a) of coating with an aqueous solution containing metal ions and the method of applying a flux treatment with an aqueous solution of flux containing metal ions are carried out by uniformly leaving each metal ion at a predetermined portion of the material to be plated. Is a suitable method.

In the coating method (b), it is preferable that the powder is adhered to the flux-treated film while the flux-treated film is in an undried state. When the flux-treated film is dried, it becomes difficult to attach these powders. In the coating method (c), the powder may be suspended in a non-aqueous solution such as acetone before use.

When the material to be plated is subjected to the flux treatment or after the flux treatment, K, by any of the above methods,
Ca, Na, Cu, Ni, Ti, Mo, Co, Ce, L
Hot dip galvanizing cracks are reduced by coating one or more metals of a, Y and Zr and then hot dip galvanizing. However, for example, even if flux treatment is performed with a flux aqueous solution containing one or more of these metals, if the total concentration of these metals is too low, it is possible to remove these metals remaining on the surface of the plated material after the flux treatment. The amount is small, the Fe-Zn intermetallic compound layer does not grow sufficiently during hot dip galvanizing, and the effect of improving hot dip galvanizing cracks is small.
On the other hand, even if the flux aqueous solution contains more of these metals than necessary, the effect of preventing hot-dip galvanizing cracks is saturated and only the cost increases. Therefore, the desirable concentration or coating amount in each coating method is as follows.

K, Ca, Na, Cu, Ni, Ti, M
In the case of a flux aqueous solution containing one or more metals of o, Co, Ce, La, Y and Zr and an aqueous solution containing one or more of these metals, one or more of them in total is 0.1 to 3.0 mol /
1 is the desired concentration range.

Metal powders and oxides of these metals,
When chloride is directly adhered to the flux layer, it is desirable to apply one or more kinds of powders in total equivalent to 0.001 to 0.1 mol / m ² in terms of metal simple substance. Even in the case of a non-aqueous solution in which these powders are suspended, the concentration and spray amount of the suspension such that at least one kind of powder in total of 0.001 to 0.1 mol / m ² in terms of metal simple substance adheres Is desirable.

[0035]

Example 1 C: 0.09%, Si: 0.25%, Mn: 1.55
%, P: 0.020%, S: 0.002%, Nb: 0.40%, T
Steel containing i: 0.40% and Al: 0.026% is melted, cast into a slab, and hot rolled to obtain a tensile strength of 65 at a plate thickness of 15 mm.
A steel plate of kgf / mm ² grade was obtained.

A plurality of restraint joints shown in FIG. 1 were produced from this steel plate. Further, a plurality of 10 cm × 10 cm test pieces were cut out from this steel plate. K, Ca, Na, Cu, Ni, Ti, Mo, Co, C
Flux treatment was performed with a flux aqueous solution of a zinc chloride / ammonium chloride mixture containing at least one of e, La, Y and Zr. These metals were added as chlorides to the aqueous flux solution.

After flux treatment, 0.08% Cd and
It was immersed in a distilled zinc bath containing 0.05% Sn (bath temperature: 450 ° C.) for 10 minutes for plating, taken out from the bath, and checked for hot dip galvanization cracking and adhesion of zinc coating.

Regarding hot-dip galvanizing cracks, the test bead portion of the restraint joint was divided into 10 equal parts, and the cross section thereof was inspected for the presence of cracks. Adhesion of zinc coating is JIS standard H040
The hammer test specified in 1 was conducted and investigated. 10 cm x 1
Hit 30 points each with a hammer on the plated part of 0 cm test piece, and check the peeling of the zinc plating film. If the number of defects is 3 or more, that is, if the peeling of the zinc film is 3 points or more, the adhesion is poor. Those with less than the hitting point were regarded as good adhesion. The hot-dip galvanizing cracks and the adhesion of the zinc coating were examined by N number 2 under each flux treatment condition.
Table 1 shows these results. The initial flux liquid composition in the column of flux treatment in Table 1 is the composition before the addition of the above metal chloride.

[0039]

[Table 1 (1)]

[0040]

[Table 1 (2)]

In Table 1, trial number 1 is K, Ca, Na,
After flux treatment with a flux aqueous solution of a usual zinc chloride / ammonium chloride mixture containing no metal of Cu, Ni, Ti, Mo, Co, Ce, La, Y and Zr, plating is performed with the same distilled zinc bath as above. This is a conventional example. In this case, hot-dip galvanizing cracks occurred in all two of the joints investigated.

Test Nos. 2 to 20 are examples of the present invention in which the flux treatment was carried out with a flux aqueous solution of a zinc chloride / ammonium chloride mixture containing at least one of the above metals. It can be seen that the hot-dip galvanizing crack resistance is improved without impairing the adhesion of the zinc coating. In particular, those treated with a flux aqueous solution in which the concentrations of these metals were in the desired ranges showed no hot dip galvanizing cracks in any of the two joints investigated.

[0043]

Example 2 Using the steel plate obtained in Example 1, a plurality of restraint joints shown in FIG.
A plurality of 0 cm × 10 cm test pieces were cut out. After flux-treating these restraint joints and test pieces with a flux aqueous solution of a mixture of zinc chloride and ammonium chloride, K, C
a, Na, Cu, Ni, Ti, Mo, Co, Ce, L
An aqueous solution containing at least one of chlorides, sulfates and nitrates of a, Y and Zr was applied or sprayed.

Then, 0.08% Cd and 0.05% Sn
It was immersed in a distilled zinc bath (bath temperature: 450 ° C.) containing 10 minutes for plating, taken out of the bath, and examined for hot dip galvanizing cracks and adhesion of the zinc coating.

The hot-dip galvanizing cracks and the adhesion of the zinc coating were examined by N number 2 by the same test method as in Example 1.
Table 2 shows these results.

[0046]

[Table 2 (1)]

[0047]

[Table 2 (2)]

In Table 2, test No. 21 is K, Ca, Na, Cu, Ni, Ti, Mo, Co, after the flux treatment.
A conventional example of hot dip galvanizing without coating or spraying an aqueous solution containing Ce, La, Y and Zr, test Nos. 22 to 40 are hot dip galvanized by coating or spraying an aqueous solution containing one or more of these metals. It is an example of the invention. It can be seen that hot-dip galvanizing cracks are improved even if an aqueous solution containing these metals is applied or sprayed after the flux treatment. As in the case of Example 1, in particular, in the case of applying or spraying an aqueous solution in which the concentrations of these metals were in the desired ranges, hot-dip galvanizing cracks did not occur in any of the two restraint joints.

[0049]

[Embodiment 3] Using the steel plate obtained in Embodiment 1, a plurality of restraint joints shown in FIG.
A plurality of × 10 cm test pieces were cut out. After flux treatment of these restraint joints and test pieces with a flux aqueous solution of a mixture of zinc chloride and ammonium chloride, K, Ca,
Na, Cu, Ni, Ti, Mo, Co, Ce, La, Y
One or more powders of Zr and Zr metal powders, oxides or chlorides of these metals were sprayed while the flux layer was undried. Alternatively, any one or more of these powders was suspended in ethanol or acetone and applied.

Then, 0.08% Cd and 0.05% Sn
It was immersed in a distilled zinc bath (bath temperature: 450 ° C.) containing 10 minutes for plating, taken out of the bath, and examined for hot dip galvanizing cracks and adhesion of the zinc coating.

The hot-dip galvanizing cracks and the adhesion of the zinc coating were examined by N number 2 by the same test method as in Example 1.
Table 3 shows these results.

[0052]

[Table 3 (1)]

[0053]

[Table 3 (2)]

From Table 3, it can be seen that hot-dip galvanizing cracks did not occur in any of the restraint joints, and the adhesion of the zinc coating was not impaired.

[0055]

As shown in the examples, K, Ca, Na,
By flux treatment with an aqueous flux solution containing one or more metals of Cu, Ni, Ti, Mo, Co, Ce, La, Y and Zr, or by coating these metals after the usual flux treatment. It is possible to prevent hot-dip galvanizing cracks that occur during hot-dip galvanizing without impairing the adhesion of the zinc coating.

The plating method of the present invention can be applied to both low-strength steel material and high-strength steel material to be plated. In particular, 55 kgf /
It is effective for high-strength steel materials with strength of mm ² or more.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of a restraint joint test body used for investigation of hot-dip galvanizing cracks.

Claims (2)

[Claims] 1. A hot dip galvanizing method in which a material to be plated is subjected to a flux treatment and then immersed in a hot dip zinc bath, wherein K and C are used.
a, Na, Cu, Ni, Ti, Mo, Co, Ce, L
A hot dip galvanizing method comprising performing a flux treatment using an aqueous flux solution containing at least one metal selected from a, Y and Zr. 2. A hot dip galvanizing method in which a material to be plated is subjected to a flux treatment and then immersed in a hot dip zinc bath, and K and C are applied to at least a welded portion of the material to be plated after the flux treatment.
a, Na, Cu, Ni, Ti, Mo, Co, Ce, L
A hot dip galvanizing method, which comprises coating at least one metal selected from a, Y and Zr.