JP2998642B2 - Manufacturing method of galvannealed steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hot-dip galvanizing, which is excellent in adhesion of a plating film to a base steel sheet, and is particularly suitable for use on a steel sheet for an automobile or a coating applied on a plating. And a method for producing the same.

[0002]

2. Description of the Related Art In recent years, zinc-based plated steel sheets have been used in large quantities in the industrial fields of home appliances, building materials, and automobiles, but are particularly excellent in rust prevention function, performance after painting, economic efficiency, and the like. As such, alloyed hot-dip galvanized steel sheets are widely used.

[0003] In an alloyed hot-dip galvanized steel sheet, the average Fe concentration of an intermetallic compound film having a surface plating film of Fe-Zn is generally 7 to 18% by weight. The alloying is
This is because, when a coating is further applied on the plating film, the adhesion of the coating is superior to that of ordinary hot-dip galvanizing, and the corrosion resistance after coating is remarkably improved. Normally, this galvannealed steel sheet is continuously hot-dip galvanized, and then heated and held in a heat treatment furnace for alloying at an atmosphere temperature of 500 to 600 ° C. for 3 to 30 s to form a zinc-plated base material. To produce an Fe-Zn alloy.

[0004] When such an alloyed hot-dip galvanized steel sheet is used for automobiles, problems are powdering and low-temperature chipping.

[0005] Powdering is a phenomenon in which a plating film peels off in a powdery form in a region that undergoes compression deformation during press working of a steel sheet. Not only does the corrosion resistance of that part deteriorate, but also the peeled powder becomes a mold ( (Die) and causes surface defects of the molded product. The corrosion resistance of the galvanized steel sheet is better as the thickness of the plating film, that is, the weight per unit area is larger, and the hot-dip galvanizing method can easily increase the weight per unit area more economically than the electroplating method. However, when the alloying treatment is performed, the powdering resistance becomes inferior as the basis weight increases,
The weight per unit area is limited in alloyed hot-dip galvanizing. Further, the powdering resistance has been improved by controlling the Al concentration in the plating bath, limiting the degree of alloying, and controlling the structure and composition of the alloy layer by selecting the alloying temperature and time. .

[0006] Low-temperature chipping is performed, for example, when a plated steel sheet is used on the outer surface of an automobile and the surface is painted. This is a phenomenon of peeling off from the film, and is a problem particularly in automobiles used in cold regions. Both the low-temperature chipping and the powdering are related to the adhesion of the plating film, and it was considered that by improving the powdering resistance, the low-temperature chipping resistance was also improved. However, even if the powdering resistance is improved, the low-temperature chipping resistance is not always improved.

[0007] When applied to steel sheets for automobiles, press formability is required. In addition to this, high strength is also important in terms of safety and weight reduction. Alloying elements added to the base material to increase the strength may cause a deterioration in powdering resistance and low-temperature chipping resistance, and it is necessary to consider this point.

Conventional measures for improving powdering resistance include, as described above, the limitation of the basis weight of the plating film, the limitation of the degree of alloying,
The main focus has been on improving the properties of the coating, such as changing the structure of the alloy layer. On the other hand, chipping is mainly caused by peeling at the interface between the film and the base material, and it is necessary to improve the adhesion at the interface.

As an example in which attention is paid to a boundary layer between a plating film and a base material, and an attempt is made to improve the adhesion, Japanese Patent Application Laid-Open
Japanese Patent Application Publication No. 2-97653 discloses an invention of a steel sheet having a structure in which zinc diffuses and penetrates into crystal grain boundaries on the surface of a base material in contact with an alloy plating film. In this method, the Al concentration in the hot-dip galvanizing bath is set to be considerably higher than usual, and plating is performed, and alloying treatment is performed at a higher temperature side. However, Al in the hot-dip galvanizing bath is generally added to suppress the formation of an alloy with Fe at the boundary in contact with the base material of the plating film, and if this concentration is increased, higher temperatures and higher temperatures are required for the alloying treatment. It is estimated that it will take a long time. In addition, it is effective for powdering resistance, but its effect is not clear for low-temperature chipping resistance.

[0010] Further, assuming that the composition of steel affects the adhesion of the plating film, JP-A-6-81009 discloses that P is 0.007%.
The invention in which the adhesion is improved is limited to the following. Furthermore, it was reported that the addition of a small amount of Si was effective in improving the adhesion of some steels (W. van Koesveld, et al .: GALVAT)
ECH`95 CONFERENCE PROCEEDINGS, p.343-355).
However, the content of these alloy elements affects the strength and workability of the base material steel, so that there is a limit in selecting an effective range.

As described above, it is difficult to say that an alloyed hot-dip galvanized steel sheet having sufficiently excellent both powdering resistance and low-temperature chipping resistance has been obtained.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a galvannealed steel sheet having both good powdering resistance and low-temperature chipping resistance. It is an object of the present invention to provide a manufacturing method for improving the interface between a film and a base material to improve the adhesion of the film.

[0013]

Means for Solving the Problems The present inventors have conducted various studies on manufacturing conditions in order to improve the adhesion of an alloyed plating film of an alloyed hot-dip galvanized steel sheet. The adhesion is, specifically, the powdering resistance and the low-temperature chipping resistance as described above, but the powdering resistance can be dealt with to some extent by controlling the amount of plating and the degree of alloying. However, improvement of low-temperature chipping resistance is not necessarily effective only by improvement of the plating film, and it is considered necessary to improve the interface state between the plating film and the steel sheet by controlling the amount of alloying elements in the base steel sheet. However, a change in the amount of alloying elements in the steel sheet affects the strength and workability of the steel, and sometimes the degree of alloying of the plating film, so that the content in the optimum range cannot always be adopted.

Therefore, as a result of various attempts to improve the low-temperature chipping resistance by a method other than by controlling the alloying elements of the steel, the surface of the steel sheet was reduced to 1 g / m ² per side before hot-dip plating.
It has been found that the above-mentioned grinding is effective, and that further low-temperature chipping resistance can be further improved by applying an appropriate tension during alloying after the grinding.
The alloyed hot-dip galvanized steel sheet having improved low-temperature chipping resistance as described above also has improved powdering resistance. Simply grinding before plating or applying tension during alloying does not provide much noticeable improvement, but the combination of both significantly improves adhesion.

Here, a method for evaluating low-temperature chipping resistance is as follows. First, after a sheet piece cut out of a galvanized steel sheet subjected to alloying treatment is subjected to a base treatment with an immersion type phosphating solution, then a base coat, a middle coat and a top coat with a cationic electrodeposition paint, the total coating thickness is about Prepare a test piece with a 3-coat coating of 100 μm. The coated test specimen is cooled to -20 ° C and a gravel stone with a diameter of
Collision is made at a speed of 150 km / h, and the peeling diameter of the coating film at each collision point is measured. This peeling usually occurs at the interface between the steel sheet and the plating film, and it is considered important to improve the film bonding force at the interface.

The evaluation of powdering resistance is performed by deep-drawing a blank sampled from a steel plate into a cylindrical cup with a press,
The plated film on the processed cup wall is peeled off with an adhesive tape, and the amount of peeling is examined. In this case, it is presumed that peeling due to deterioration of the workability of the film is mainly caused. However, if the adhesion at the boundary surface is improved, the amount of peeling is considered to be reduced.

The addition of tension during the grinding and alloying treatment is as follows:
The reason for improving the adhesion of the plating film is not clear, but is presumed to have the following effects. First, grinding can remove foreign substances that remain on the surface of the base steel sheet and hinder contact between the plating film and the steel, such as residual scale during hot rolling, and have the effect of cleaning the surface. It is expected to contribute to the improvement of performance. However,
The effect of the cleaning is not considered to be so great because the foreign substances on the surface have already been removed by the pickling in the previous step.

It is considered that the influence of the large strain introduced to the very surface of the steel sheet by the grinding is greater than that.
The strain due to this grinding is concentrated on the extreme surface layer portion, and the strain at that portion is much larger than the strain due to rolling or cutting. In the strained part of the extreme surface layer introduced before hot-dip plating, the recrystallization and grain growth progress more rapidly than the steel inside by the heating for reduction, and the crystal grain structure greatly differs from the entire steel sheet inside Becomes

If tension is applied during the reheating and alloying after the adhesion of the molten zinc, the extremely superficial portion of the steel has a different grain structure from that of the entire lower steel, so that the deformation due to the tension is caused. And the local distribution of strain distribution becomes large. It is presumed that this promotes the uneven penetration of zinc on the steel surface, increases the anchoring effect, and significantly improves the adhesion.

The powdering resistance is as follows:
It can be improved by controlling the concentration. In this case, it is found that the combination of the grinding before plating and the application of tension during alloying and the control of the Fe concentration in the range of 8 to 18% can be further improved. Was.

It is also known that increasing the content of Si in the base steel sheet improves the adhesion of the alloyed film. However, when the Si-added steel sheet is ground and alloyed before plating. By applying the tension and controlling the Fe concentration for alloying, the low-temperature chipping resistance and the powdering resistance can be further improved. Since the addition of Si improves not only the strength of the base steel sheet but also the adhesion of the plating film, it is particularly effective for producing a galvannealed steel sheet having high strength and excellent adhesion to the film. This is presumably because the content of Si enhances the change in the recrystallized structure of the surface layer due to the above-mentioned grinding and the effect of the tension during the alloying treatment.

Based on the above findings, the optimal range of the grinding amount
Limit of the appropriate value of the tension during alloying, and effective S
The content range and the like of the i amount were clarified, and the production method of the present invention was completed. That is, the gist of the present invention is as follows.

(1) The surface of the steel sheet before plating is ground in an amount of 1 to 8 g / m ² per side, then heated and subjected to reduction annealing to perform hot-dip galvanizing, followed by 2.0 to 7.0 kgf / mm ² . A method for producing an alloyed hot-dip galvanized steel sheet, comprising performing an alloying treatment while applying tension.

(2) As a base material steel plate for plating, 0.05% of Si is used.
(1) characterized in that a steel containing up to 0.5% is used.
Of manufacturing galvannealed steel sheet.

The present invention can be carried out, for example, in a usual production line for galvannealed steel sheets. The configuration of the general production line is as follows. First, the steel sheet of the material coming out of the coil of the pay-off reel passes through a surface cleaning device such as electrolytic degreasing and is guided to an annealing furnace. Annealing furnace is divided into reducing zone and the heating zone, first steel sheet is rapidly heated in a heating zone atmosphere or non-oxidizing or weakly oxidizing atmosphere, then the reduction of high-temperature H ₂ -N ₂ atmosphere containing a large amount of hydrogen The iron oxide on the surface is reduced in the band. If the base steel sheet is cold rolled,
Here, annealing recrystallization is performed, and strength and workability can be adjusted to required levels by selecting a temperature and a time based on the chemical composition of the steel.

A steel sheet which has been reduced or reduction-annealed at a high temperature (although plating can be performed only by reduction, but reduction annealing is performed when the steel sheet is annealed as it is rolled), is subjected to H ₂ -N ₂
After cooling in an atmosphere and adjusting the temperature, it is immersed in a molten zinc bath and plated. When the steel sheet is lifted out of the bath, gas wiping is performed by blowing a gas such as air or nitrogen to squeeze out zinc by the pressure and control the basis weight.
In the case of alloying, immediately after the zinc is deposited, the steel sheet is reheated in a heating zone called an alloying furnace to react the iron of the base steel sheet with the deposited zinc. Alloying is controlled while monitoring the progress with an alloying degree meter.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION In implementing the present invention, the form and range of conditions and the reason for setting them will be described below.
At that time, the chemical compositions of the steel and the plating layer are all by weight.

The grinding before plating may be performed in another line before plating in the production line, but is performed before or after the surface cleaning process in the production line, and before the reduction or reduction annealing process. There is no need for additional steps. Grinding is
Any method can be used as long as it can continuously grind the surface of the steel sheet by a predetermined amount, such as a wire brush or a brush containing abrasive grains.

The surface grinding amount is 1 to 8 g on one side as the amount of steel.
/ M ² . This is because if the grinding amount is less than 1 g / m ² , the effect of improving the adhesion by grinding is insufficient, and the adhesion tends to be uneven depending on the surface location. While 8
Even if grinding exceeds g / m ² , the effect is saturated, resulting in increased costs such as tool wear, increased power consumption, and increased iron powder due to grinding.

The steel sheet whose surface has been cleaned and ground is heated and reduced or reduction-annealed. Reduction or reduction annealing is
Perform in a reducing atmosphere containing hydrogen at a temperature in the range of 600 to 900 ° C. In this case, if the temperature is lower than 600 ° C, there is a danger that non-plating may occur due to insufficient reduction, and if the temperature is higher than 900 ° C, the press workability of the annealed steel sheet is deteriorated. Usually, the temperature is maintained for 10 to 100 seconds within the above temperature range.

The zinc basis weight of the plating is generally determined on one side of the steel sheet.
20-70 g / m ² is chosen. Is this less than 20 g / m ^2, corrosion resistance not only insufficient, current is difficult to stably manufactured by gas wiping, it exceeds 70 g / m ^2, the steel sheet alloying This is because the powdering resistance deteriorates.

The alloying treatment after plating is performed at 480 to 600 ° C., and in the method of the present invention, a tension of 2.0 to 7.0 kgf / mm ² is applied during the alloying treatment. This is because the surface grinding before plating and the application of this tension greatly improve low-temperature chipping resistance and powdering resistance. The tension may be applied, for example, by a bridle roll after the alloying furnace. The tension during alloying is 2.0 kgf / mm ²
If it is less than 7.0 kgf / mm ² , the effect will not only be saturated, but the equipment for applying the tension will be large, and the cost will increase.

For alloying, it is desirable that the Fe concentration of the coating be 8 to 18%. This is because if the Fe concentration is less than 8%, the paintability and weldability are insufficient, and if it exceeds 18%, the powdering resistance deteriorates.

In the above-mentioned method for producing a galvannealed steel sheet, the method of improving the low-temperature chipping resistance and the powdering resistance, in particular, is most characterized by surface grinding before plating and alloying. This is the addition of tension during processing, and other conditions may be applied as long as the combination of the two is within the range defined by the present invention.

In the production method of the present invention, the chemical composition of the base steel sheet can exhibit a sufficient effect within the range of ordinary use without being particularly determined, but by adding Si and regulating the content thereof. In addition, the adhesion of the film can be further improved. Its Si content is 0.05-0.5%.
This is because if the content is less than 0.05%, the effect of the addition is not exhibited, while if the content exceeds 0.5%, non-plating is liable to occur during immersion in a zinc bath after reduction.

[0036]

EXAMPLE A steel having the chemical composition shown in Table 1 was melted in a laboratory, and hot-rolled and cold-rolled to obtain a 0.8 mm thick base steel sheet. A piece of 80 mm wide and 200 mm long was cut out from these steel sheets, and the surface was ground using a brush roll tester manufactured by Hotani Co., Ltd. Brush is mainly 8S-500
-AH brush was used, and 12S-100-3H brush was used for finishing. The grinding amount was confirmed by weighing the plate piece weight before and after grinding.

[0037]

[Table 1]

Next, a hot-dip galvanized steel sheet test specimen was prepared using a hot-dip galvanizing simulator (manufactured by Resca). After grinding, degrease and clean the plate in a nitrogen atmosphere
The temperature is raised to 550 ° C at 15 ° C / s, and then reduced annealing is performed at 800 ° C in a 10% hydrogen-nitrogen atmosphere with a dew point of -60 ° C or less.
Temperature up to 15 ℃ / s and hold for about 20s, then 440 ～ 520 ℃
After cooling to room temperature, hot-dip plating was performed. The plating conditions are as follows: bath temperature 460 ° C, bath Al concentration 0.08-0.12%
And the immersion time was in the range of 1 to 5 s.

The test piece after plating was heated to 480 to 600 ° C. by direct energization while applying tension with a hydraulic cylinder to perform alloying, thereby obtaining a galvannealed steel sheet test piece. The degree of alloying was determined by dissolving the film with a 6% aqueous HCl solution containing an inhibitor (Abit: 0.01%, Asahi Chemical Co.) and chemically analyzing the Fe concentration.

Table 2 shows the results of analysis of the surface grinding amount of these test pieces before plating, the basis weight of plating, the added tension during alloying treatment, and the Fe concentration of the alloyed film.

[0041]

[Table 2]

The low-temperature chipping resistance was evaluated as follows. 70mm width, 150mm length from the specimen after alloying
After a base piece treatment with an immersion type phosphating solution (Nippon Parkerizing Co., Ltd .: PBL-8080), the amount of cationic electrodeposition paint (Nippon Paint Co., Ltd .: U-80) attached is about 20 μm. Undercoat, 35-40 μm of middle coat (Kansai Paint Co., Ltd .: TP-37), and 35-40 μm of top coat (Nippon Paint Co., Ltd .: Neo 63), total film thickness 100 μm
Was applied in three coats. These painted test specimens are
℃, and 10 gravel stones with a diameter of 4 to 6 mm were adjusted to an air pressure of 2.0 km / cm ² by a gravelo tester at a speed of 100 to 150
Collisions were made at a speed of km / h, and the peeling diameter of the coating at each collision point was measured to determine the average peeling diameter.

The powdering resistance is determined by stamping a 60 mm diameter disk from a steel plate after alloying, pressing a cylindrical cup with a die having a punch diameter of 30 mm and a die shoulder R of 3 mm, and adhering on the outer surface of the cup wall. Peeling with a tape was performed, and the total peeling weight of the plating was measured.

Table 2 also shows the results of the above tests. From these results, when the amount of surface grinding before plating and the tension during alloying treatment are within the range defined by the present invention,
It is understood that a steel sheet having excellent low-temperature chipping resistance and powdering resistance can be obtained. Also, test number 1
From comparison of 8, 34 and 41 with those in which the tension during other surface grinding and alloying is within the range specified in the present invention, the steel having a Si content of 0.05% or more has a better low-temperature chipping resistance. It is clear that there is.

[0045]

According to the method of the present invention, an alloyed hot-dip galvanized steel sheet having excellent low-temperature chipping resistance and powdering resistance, which is particularly required for an outer panel of an automobile, can be easily obtained. Obtainable.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] (1) The surface of a steel sheet before plating is ground in an amount of 1 to 8 g / m ² as a steel amount, then heated and reduced or reduction-annealed to perform hot-dip galvanizing, followed by 2.0 to 7.0 kgf / mm.
^2. A method for producing an alloyed hot-dip galvanized steel sheet, wherein the alloying treatment is performed while applying a tension of ² . 2. The base material steel sheet for plating contains 0.05 to 0.05% of Si.
The method for producing a galvannealed steel sheet according to claim 1, wherein a steel containing 0.5% is used.