JPH0874020A - Production of galvannealed steel sheet - Google Patents

Abstract

PURPOSE: To provide a method for producing a galvannealed steel sheet in which the alloying rate is promoted, having a uniform plating layer small in ruggedness and having a beautiful surface appearance without impairing its plating peeling resistance, as for the method for producing a galvannealed steel sheet, particularly, a high strength steel sheet contg. high strength elements (such as P and Si). CONSTITUTION: At the time of producing a galvannealed steel sheet in such a manner that, after galvanizing, the coating weight of plating is regulated, and alloying treatment under heating is executed, the galvanizing is executed so as to regulate the concn. of Al on the inlet side of a galvanizing bath for the steel sheet to <=0.1% and the concn. of Al on the outlet side of the galvanizing bath for the steel sheet to >=0.12%. Thus, the high strength galvannealed steel sheet can be produced with high productivity at a low cost without deteriorating its plating peeling resistance, and the galvannealed steel sheet having a more beautiful surface appearance can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvannealed steel sheet, and particularly to a high strength element (for example, P,
In a high strength steel sheet containing Si), a hot-dip galvanized steel sheet with a beautiful surface appearance that promotes the alloying rate without impairing the plating peel resistance and has a uniform plating layer with few irregularities is provided. The present invention relates to a manufacturing method.

[0002]

2. Description of the Related Art Generally, galvannealed steel sheets are F
It has a Zn-Fe alloy composition containing 6 to 18% of e and is excellent in coating corrosion resistance, press formability, and weldability, and thus is a material used in a wide range of fields such as automobiles, home appliances, and building materials. The manufacturing method is as follows: normal cold-rolled steel sheet or hot-rolled steel sheet is heated in an oxidizing or non-oxidizing atmosphere to incinerate dirt such as oil adhering to the surface, and then the surface is cleaned in a reducing atmosphere. After that, hot dip galvanizing is performed, and then heat treatment is performed to diffuse Fe from the steel sheet into the plating layer to alloy the Fe. Since the Zn-Fe alloying reaction is controlled by the diffusion rate of Fe, the reaction rate varies depending on the steel type. In particular, the high-strength steel sheet containing Si or P, which has been used as a high-strength steel sheet, has a slow alloying reaction, so that the line speed is inevitably reduced and the production is seriously impaired.

Further, the galvannealed steel sheet is used for the outer panels of home appliances, building materials and automobiles, and since the beauty of the surface and the sharpness after painting are important, the unevenness of the plating layer and the unevenness of alloying are reduced. There is a strong demand for an alloyed hot-dip galvanized steel sheet that has a more beautiful surface appearance. For steel types such as P-added steels that have a slow alloying reaction, the temperature of the alloying furnace is usually raised or the line speed is changed, but this has the disadvantage of lowering operating efficiency and causing quality variations. Further, it is known that the alloying reaction is accelerated by lowering the concentration of Al added to a usual hot-dip galvanizing bath, but the resistance to peeling of plating is deteriorated, and when it is used in an automobile body or the like. During press molding, a plating peeling phenomenon called powdering or flaking easily occurs.

[0004]

Therefore, as a method for accelerating the alloying reaction, F was applied to the surface of the steel sheet before hot dip galvanizing.
Disclosed is to perform e-based electroplating (Japanese Patent Laid-Open No. 59-23858).
However, the cost is high due to electroplating, which is industrially disadvantageous. Further, as a method for producing a high strength galvannealed steel sheet containing P, there is disclosed in
In Japanese Patent Laid-Open No. 5-148603, a method of applying a sulfur compound aqueous solution having a concentration of 0.1% or more to the surface of a steel sheet before hot dip galvanizing is proposed. However, this method is effective for a steel plate which has been strengthened by adding only P, but when other alloying elements such as Si are contained in a large amount, the effect of promoting alloying is not always sufficient. Moreover, since the sulfur compound aqueous solution is only applied, the applied film is easily peeled off and the effect is unstable.

The present invention provides a high-strength alloyed hot-dip galvanized steel sheet which can achieve a sufficiently high alloying rate even in the above-mentioned high strength steel sheet, does not impair the peeling resistance to plating, and has a more beautiful surface appearance. It is intended to provide a method of manufacturing at low cost.

[0006]

In the production of galvannealed steel sheet, a slight amount of Al is usually added to the hot-dip galvanizing bath. When the steel sheet is dipped in the hot dip galvanizing bath, it reacts with Al in the galvanizing bath and promptly reacts with F
An e-Al alloy layer is formed. Since this Fe-Al alloy layer delays the start of the alloying reaction until it is heated to an appropriate temperature in an alloying furnace, it is possible to obtain a plating layer having a δ phase as a main component and good resistance to stripping of plating. .

The present inventors have studied in detail the reason why alloying is delayed in a high-strength steel sheet containing P and / or Si. As a result, when an Fe-Al alloy layer is formed in a hot dip galvanizing bath. In addition, P and / or Si contained in the steel diffuses together with Fe into the Fe-Al alloy layer to form a solid solution to strengthen the Fe-Al alloy layer and significantly delay the decomposition thereof, so that the alloying reaction is slowed as a whole. I found out that. Furthermore, in a low Al concentration bath, Fe--Zn
It has been found that the alloy layer grows rapidly and uniformly, and by subsequently immersing it in a bath having a relatively high Al concentration, it is possible to obtain a uniform alloy layer mainly composed of the δ phase with good resistance to delamination and less unevenness. It was

The present invention is based on such knowledge, and when a galvannealed steel sheet is manufactured by hot galvanizing after hot dip galvanizing, the zinc content of the galvannealed steel sheet is controlled. Al concentration on the plating bath entrance side is 0.1
% Or less, the Al concentration on the zinc plating bath exit side of the steel sheet is 0.12
%, The hot-dip galvanizing is performed, and a method for producing an alloyed hot-dip galvanized steel sheet.

[0009]

The present invention will be described in detail below. In the present invention, after the steel sheet is annealed in an annealing furnace, it is first immersed in a hot dip galvanizing bath with the Al concentration on the hot-dip galvanizing bath entrance side of the steel sheet being 0.1% or less. %, A thick Fe—Al alloy layer is formed, which hinders the alloying reaction and reduces the effect. In addition, Fe-Al
When the alloy layer becomes thick, the thickness of the Fe-Al alloy layer tends to become non-uniform, and the unevenness of the Fe-Zn alloy layer becomes severe, so that the plating appearance is impaired. A on the side of the plating bath
The lower limit of the l concentration is not particularly limited as long as it does not contain Al, but 0.02% or more is preferable in consideration of operability problems such as generation of dross.

Next, the reason for setting the Al concentration of the molten zinc bath on the exit side of the steel sheet to 0.12% or more is that if it is less than 0.12%, the ζ phase and / or the Γ phase are likely to grow in the subsequent alloying process. As a result, the resistance to peeling of plating during press working deteriorates. 0.1% Al concentration on the molten zinc bath side of the steel sheet
Hereinafter, by setting the Al concentration on the outlet side to 0.12% or more, a thin and uniform Fe-Al alloy layer is formed on the inlet side,
At the same time as accelerating the initiation of alloying, the Al concentration of the plated layer after it has been removed from the hot dip galvanizing bath is increased. When the Al concentration is high, the ζ phase and the Γ phase are unstable, and a stable δ phase is easily formed, so that the plating peeling resistance is not impaired.

As a method for controlling the Al concentrations on the inlet side and the outlet side, for example, the plating bath may be modified as shown in FIGS. FIG. 1 shows that a zinc pot is divided by a partition plate 9 into an inlet bath 5 and an outlet bath 6, and a steel plate 1 is guided from the inlet bath 5 to the outlet bath 6 without contact with the atmosphere. The folding roll 7 may be provided in the box 8.
If the atmosphere in the seal box 8 is a non-oxidizing atmosphere (including a reducing atmosphere), the surface of the plating layer formed in the inlet bath is not oxidized, which is preferable in terms of plating adhesion and plating appearance. In addition, adjustment of the Al concentration of the entrance and exit baths,
It may be performed by a usual method.

Next, FIG. 2 shows the zinc pot 4 of the snout 3.
The inside of the zinc pot 4 of the snout 3 is made to be convex and the tip end thereof is opened in a slit shape to be used as the entrance bath 5 through which the steel plate 1 can pass. The Al concentration of is adjusted to be low, and the zinc pot 4 outside the inlet bath 5 is used as the outlet bath 6 to adjust the Al concentration to be high. In order to suppress the uniformization of Al concentration through the opening,
An appropriate amount of zinc source (eg, metallic zinc) may be supplied to the entrance bath 5 in the snout. Further, the Al concentration in the outlet bath 6 is 0.
If it is 12% or more, the effect of the present invention is not impaired even if the density is not uniform and the density varies.

There is no particular limitation on the annealing treatment that is usually performed prior to immersion in the plating bath. For example, after preheating a steel sheet in a weakly oxidizing atmosphere or a non-oxidizing atmosphere, the steel sheet contains 3 to 15% hydrogen. 600 to 8 in nitrogen
It can be annealed by heating to 50 ° C. Alternatively, after cleaning the steel plate with an alkaline solution or the like, hydrogen 3 to 15%
Preheated in nitrogen containing, and 600 ~ in the same atmosphere.
It can also be annealed by heating to 850 ° C. It goes without saying that it is desirable to immerse the annealed steel sheet in the plating bath without exposing it to the atmosphere.

As for the temperature of the steel sheet when immersed in the plating bath and the temperature of the plating bath, the conditions conventionally applied may be applied. For example, conditions such as a steel plate temperature of 430 to 480 ° C and a plating bath temperature of 440 to 480 ° C can be used. Further, it is not necessary to make a difference between the temperatures of the inlet bath and the outlet bath, but the effect of the present invention is not affected even if the temperature is changed within the condition range conventionally applied. After being immersed in the first and second molten zinc baths, the thickness of the zinc adhered to the surface of the steel sheet is adjusted by a usual method, and the steel sheet is heated to alloy the plated zinc with the steel sheet. The temperature for alloying is preferably 460 ° C. or higher, and more preferably 480 ° C. or higher for reliable alloying in a short time. The heating method for alloying is not particularly limited, and any conventionally used method such as direct heating with a combustion gas flame, induction heating, or direct current heating can be used. According to this method, in addition to Al, Pb, Sb, Si, Fe, Sn, and M are added to the zinc plating bath.
Even a bath containing or mixed with g, Mn, Ni, Cr, Co, Ca, Li, Ti, or a rare earth element does not cause any trouble.

The steel sheet targeted by the present method is a steel sheet containing a strengthening element (for example, the steel composition is P ≧ 0.01).
% And / or Si ≧ 0.1%), but even if the present invention is applied to a general material that inevitably contains a strengthening element in the steel sheet, there is no problem and the strength is increased. Compared to the steel sheet containing the element, the effect of improving the alloying rate is small, but the effect of obtaining a hot-dip galvanized steel sheet with a beautiful surface appearance is the same as the steel sheet containing the strengthening element. .

As a matter of course, it is of course possible to perform various electroplating on the alloyed hot-dip galvanized steel sheet obtained by the method of the present invention for the purpose of improving paintability, weldability and the like. It does not depart from the invention of the present application. Further, on the alloyed hot-dip galvanized steel sheet obtained by the method of the present invention,
It is of course possible to add various kinds of treatments, for example, chromate treatment, phosphate treatment, treatment for improving phosphate treatment, lubricity improving treatment, weldability improving treatment, resin coating treatment. Even if the above is applied, it does not depart from the scope of the present invention, and various kinds of processing can be applied according to the characteristics required additionally.

[0017]

EXAMPLES Hot-rolled steel sheets and cold-rolled steel sheets whose components are shown in Table 1 were used as test materials. Here, the plate thickness is 1.6 mm (hot rolled steel plate), 0.
8 mm (cold-rolled steel sheet) and the time required for alloying completion, surface appearance and plating peel resistance when hot-dip galvanizing in a plating bath with Al concentration shown in Tables 2 and 3 are shown in Tables 2 and 3. Indicate. At the time of plating, the annealing atmosphere was a mixed gas of 5% hydrogen and 95% nitrogen, and the annealing temperature was 80.
The temperature was 0 ° C., the annealing time was 75 seconds, and after dipping in a molten zinc bath at a bath temperature of 450 ° C., the amount of zinc deposited was 60 with a gas wiper.
g / m ² . The alloying was performed with induction heating type heating equipment, and the heating temperature was 490 ° C. The apparatus shown in FIG. 1 was used as the plating bath.

[0018]

[Table 1]

Tables 2 and 3 show examples and comparative examples. In Tables 2 and 3, the alloying time is evaluated as ◎.
Indicates that the time until the completion of alloying was 15 seconds or less, ○ indicates that the time until the completion of alloying was 25 seconds or less, and × indicates the time until the completion of alloying exceeded 25 seconds. It means that As for the evaluation of the plating appearance, the more uneven the plating layer is, the worse the appearance is. Therefore, the roughness curve is measured using a roughness meter, and the number of peaks per 1 cm (threshold value 0-2 μm) is 60 or less. ◎, 60
Those exceeding 100 and less than 100 were evaluated as ◯, and those exceeding 100 were evaluated as x. Furthermore, the case where there is a portion where the plating layer is not attached is regarded as non-plating. Further, as an evaluation of the plating peeling resistance, an alloyed hot-dip galvanized steel sheet having cellophane tape stuck on the compression side in advance was bent by 60 degrees V,
After bending back, the cellophane tape was peeled off, and those with almost no peeling of the plating layer were marked with ⊚, those almost peeled off were marked with ×, and the middle thereof was marked with ◯.

[0020]

[Table 2]

[0021]

[Table 3]

As is apparent from Tables 2 and 3, No. No. plated according to the method of the present invention was used. Nos. 1 to 38 had a high alloying rate, did not cause non-plating, and were excellent in surface appearance and resistance to peeling of plating. On the other hand, No. No. plated with a normal zinc plating bath having no difference in Al concentration between the steel plate inlet side and the steel plate outlet side of the zinc plating bath. No. 39 has poor plating peel resistance,
No. No. 40 requires a long time for alloying. In addition, in the case where the Al concentration on the steel plate inlet side and the steel plate outlet side of the zinc bath was not appropriate, any of the alloying rate, surface appearance, and plating peeling resistance was poor. From the comparative examples in Table 3, it is clear that if the Al concentrations on the inlet side and the outlet side were not appropriate, all of the alloying rate, the plating surface appearance, and the plating peeling resistance could not be satisfied.

[0023]

As described above, the present invention makes it possible to produce a high-strength hot-dip galvanized steel sheet with good productivity and at low cost without impairing the peeling resistance to plating, and It is possible to provide an alloyed hot-dip galvanized steel sheet having a more beautiful surface appearance, and therefore, it is extremely important to contribute to the development of the industry.

[Brief description of drawings]

FIG. 1 is a view showing an example of a hot dip galvanizing bath provided with a partition plate,

FIG. 2 is a diagram showing an example of a hot dip galvanizing bath using a snout portion as an entrance bath.

[Explanation of symbols]

 1 Steel plate 2 Sink roll 3 Snout 4 Zinc pot 5 Inlet bath 6 Outlet bath 7 Folding roll 8 Seal box 9 Partition plate

Claims (1)

[Claims] 1. When producing an alloyed hot-dip galvanized steel sheet by controlling the coating amount after hot-dip galvanizing and performing a heat alloying treatment, the Al concentration on the steel sheet entrance side of the galvanizing bath is 0.1% or less, A method for producing an alloyed hot-dip galvanized steel sheet, which comprises performing hot-dip galvanizing with the Al concentration on the steel sheet outlet side of the galvanizing bath being 0.12% or more.