JPH10176254A - Production of hot dip galvanized steel plate excellent in plating adhesion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot-dip galvanized steel plate excellent in plating adhesion by applying, prior to plating, heating treatment to the surface of a steel plate under specific conditions, and developing cracking in an oxide film, at the time of applying hot-dip galvanizing to a steel plate on which an oxide film remains. SOLUTION: Before hot-dip galvanizing is applied to a steel plate (e.g. hot rolled steel plate) on which an oxide film remains, a part of the oxide film is reduced by holding for >=20sec in a reducing atmosphere of >=500 deg.C. Subsequently, prior to plating treatment, the surface of the steel plate is heated to >=500 deg.C while regulating the average heating rate between room temp. to 500 deg.C to >=10 deg.C/sec, or the steel plate is heated and the surface of the steel plate is cooled, before immersion into a plating bath, by >=50 deg.C at >=10 deg.C/sec average cooling rate, or the temp. of the surface of the steel plate is regulated so that it is higher by >=50 deg.C than the temp. of the plating bath. By this method, cracks are developed in the oxide film and plating metal is allowed to penetrate into the oxide film, and the hot-dip galvanized steel plate having superior adhesion of plating can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of hot-dip galvanizing a steel sheet (for example, a hot-rolled steel sheet) in which an oxide film remains.

[0002]

2. Description of the Related Art Conventionally, a hot-dip galvanized steel sheet which has been subjected to hot-dip galvanizing (for example, Zn plating, Zn-5% Al alloy plating, Zn-55% Al alloy plating) has been used in automobiles because of its excellent corrosion resistance. It is used in many fields such as buildings. Among them, hot-dip galvanized steel sheets using hot-rolled steel sheets as base sheets are less expensive than those using cold-rolled steel sheets as base sheets, so relatively thick hot-rolled steel sheets of 1.0 mm or more are mainly used for construction and civil engineering applications. Used in

Meanwhile, an oxide film is formed on the surface of a hot-rolled steel sheet during the manufacturing process. Some oxide films have a thickness of as much as 10 μm or more. When hot-dip steel sheet is hot-dip galvanized, this oxide film may cause non-plating or poor plating adhesion. Before coating, remove the oxide film by pickling and polishing beforehand. Has been done.

However, the pickling process is changed to another method, specifically a reduction method, due to the problem of foreign matter sticking to the steel sheet surface and the problem of the pickling cost that may occur when the pickling line is passed. Various studies have been made on whether the oxide film can be easily removed. For example, Japanese Patent Application Laid-Open No. 54-147125 discloses a method in which a hot rolled steel sheet having a considerably reduced oxide film adhesion amount is annealed in a reducing gas to remove the oxide film and then subjected to plating. JP-A-6-145937 discloses a hot-rolled steel sheet having an oxide film layer of 5 to 10 μm in a hydrogen atmosphere of 2 to 100%.
A method of performing a hot dip galvanizing after performing a reduction treatment at 750 ° C. is disclosed. Japanese Patent Application Laid-Open No. 6-279967 discloses a method for defining the relationship between the treatment time, the temperature, the hydrogen concentration and the thickness of the oxide film layer with respect to the reduction conditions of the hot-rolled steel sheet in which the oxide film remains.

All of these techniques are directed in the same direction as the pickling method in that the oxide film is removed, and an attempt is made to remove the oxide film as much as possible by defining the reduction conditions. Things.

[0006]

However, according to the present inventors' detailed examination on hot-dip galvanized steel sheets, when any oxide film remains, even if a small amount of oxide film remains, the above-mentioned reduction conditions before plating must be adjusted. It has been clarified that the adhesion of hot-dip galvanized plating is not sufficient for the reasons described below, and various problems remain in practical use.

[0007] In the case where the hot-rolled steel sheet is reduced under the conditions described in the above-mentioned publications, the surface of the oxide film layer is reduced to metallic iron, but the oxide film layer remains below it. Many. When a steel sheet in such a state is immersed in a hot-dip bath, the surface of the steel sheet is made of metallic iron, so that the plating has good wettability and no non-plating occurs, but is hard and brittle under the plating layer. Since the oxide film layer remains, plating adhesion becomes poor. For this reason, in order to improve the plating adhesion, the reduction conditions are strengthened, and all or most of the oxide film is reduced to metallic iron, and the hard residual oxide film under the plating layer is eliminated, or to the extent that no adverse effect is caused. It was necessary to reduce as much as possible. It was particularly preferred to remove all of the oxide film.

[0008] However, as described above, in the ordinary hot-rolled steel sheet, the thickness of the oxide film is as large as 10 μm or more, and it is difficult to control the reduction conditions to reduce all or most of the oxide film. In addition, the time required for reduction becomes longer, which makes it difficult not only to adjust the actual production line, but also to reduce costs.

The present invention has been made in view of the above-mentioned circumstances, and has an object to provide a hot-rolled steel sheet or the like in which an oxide film remains to obtain a good plating adhesion when hot-dip zinc-based plating is performed. To provide a method for producing a hot-dip galvanized steel sheet.

[0010]

The method for producing a hot-dip galvanized steel sheet having good plating adhesion according to the present invention, which achieves the above object, comprises applying a hot-dip galvanized coating to a steel sheet having an oxide film remaining. A step of reducing a part of the oxide film in a reducing atmosphere before performing, and a step of generating cracks in the oxide film before or / and while performing the hot-dip galvanizing plating This is a method for producing a hot-dip galvanized steel sheet containing: Specifically, as a method of generating cracks, prior to performing hot-dip galvanizing,
The average heating rate from room temperature to 500 ° C is 10
° C / sec or higher, and heating to 500 ° C or higher. Similarly, prior to hot-dip galvanizing, heating the steel sheet,
A method of cooling the steel sheet surface to 50 ° C. or more at an average cooling rate of 10 ° C./sec or more before immersion in a hot-dip galvanizing bath;
A method in which the temperature of the surface of the steel sheet is made 50 ° C. or more higher than the plating bath temperature and immersed in the plating bath is exemplified.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted a detailed study on a hot-dip galvanized steel sheet and found that the quality of the plating adhesion depends on the form of the oxide film layer remaining under the plating layer. Was. That is, before or / and while hot-dip galvanizing is performed,
The inventors have found that a crack is generated in the remaining oxide film and that the plating metal is infiltrated into the crack, whereby the plating adhesion is significantly improved, and the present invention has been accomplished.

Although the details of the reason why the plating adhesion is improved are unknown, cracks are present in the oxide film layer remaining under the plating layer, and when the plating metal infiltrates there, a so-called "anchor" effect is exhibited. It is considered that the plating adhesion is improved. In addition, since the oxide film layer is finely cut by the cracks and changes into an aggregate shape of a large number of mutually independent fragments, when the steel plate is deformed, the fragments are individually subjected to a load and each of them is subjected to a corresponding displacement. It is also considered that the effect of reducing the shear stress applied between the base steel sheet and the oxide film layer by the treatment is one of the factors for improving the adhesion.

[0013] Such cracks can be specifically generated by the following various methods. Prior to hot-dip galvanizing, a method of heating the surface of a steel sheet to 500 ° C. or more at an average heating rate from room temperature to 500 ° C. of 10 ° C./sec or more. Prior to hot-dip galvanizing, heat the steel sheet,
Before immersion in a hot dip galvanizing bath, the surface of the steel
A method of cooling at 50 ° C. or more at an average cooling rate of at least 2 seconds. A method in which the temperature of the steel sheet surface is set to a temperature higher than the plating bath temperature by 50 ° C. or more and immersed in the plating bath. Furthermore, it is also effective to employ a combination of these methods. For example, do the above, do the next, do the above, do the next, do the above, do the next, do the above, do the next, do the next, etc. .

The above-described crack generation method will be described in detail. The oxide film formed in the hot rolling process is very dense because the cooling rate after winding is slow. It is presumed that when this is rapidly heated or cooled, thermal stress is generated due to the difference in thermal expansion coefficient between the base steel sheet and the oxide film layer, and cracks are generated in the hard and brittle oxide film. In order to generate a crack sufficient to exert a practical plating adhesion improving effect, a corresponding thermal stress is required. Therefore, at the time of heating, at least the surface of the steel sheet is heated at an average temperature from room temperature to 500 ° C. It is necessary to heat at a rate of 10 ° C./sec or higher to 500 ° C. or higher. If the heating rate is lower than this, cracks may not be generated sufficiently. The average heating rate from room temperature to 500 ° C is 20
C./sec or more forms more preferable cracks.

In an actual production line, deviation from the average heating rate should occur inside the steel sheet, but it is rather advantageous from the viewpoint of increasing the thermal stress load. Therefore, the temperatures and heating / cooling rates shown as the preferred working conditions of the present invention are all to be understood as average values, and even if these conditions are not partially satisfied, the technical scope of the present invention is thereby reduced. Does not deviate.

The heating method is not particularly limited. However, in the radiant tube heating usually used in the non-oxidizing furnace, the capacity tends to be insufficient in order to rapidly heat a relatively thick hot-rolled steel sheet. -Adoption of electric heating etc. is desired.

On the other hand, at the time of cooling, it is necessary to cool the surface of the steel sheet at 50 ° C. or more at an average cooling rate of at least 10 ° C./sec. If the cooling rate is less than 10 ° C./sec, or if the cooling temperature is less than 50 ° C., cracks may not be sufficiently generated. The cooling rate is desirably 30 ° C./sec or more, and more effective when it is 100 ° C./sec or more. As a cooling method, it is desirable to employ gas jet cooling or roll cooling. With these methods, a cooling rate of 10 ° C./second or more can be easily achieved. In particular, if a roll cooling device is used, a cooling rate of 100 ° C./sec or more can be achieved, which is extremely advantageous from the viewpoint of plating adhesion. Further, a sufficient cooling rate can be obtained by immersing the steel sheet in a plating bath with the temperature of the steel sheet surface being 50 ° C. or higher than the plating bath temperature.

Incidentally, in addition to the above-described method, a mechanical method can be used as a method for generating a crack in the oxide film layer. For example, a method of subjecting a hot-rolled steel sheet to skin pass rolling or a method of applying a tension leveler is used. It is also considered that applying a strain to the surface layer when unwinding the hot-rolled coil is effective for a hot-rolled steel sheet having a large thickness. By using such a mechanical method together, plating adhesion can be further improved.

In practicing the present invention, a step of reducing a part of the oxide film is required in addition to the step of generating a crack in the oxide film. In this case, the reduction conditions are sufficient to reduce only the surface of the oxide film layer, thereby achieving a significant improvement in non-plating. That is, when only the above-described conventional reduction method is employed, it is necessary to reduce all or most of the oxide film layer.
In addition, when the reduction method is used in combination, the two problems of non-plating and plating adhesion are more certain if the reduction has progressed at least to the extent that the surface portion is reduced even if a part of the oxide film layer remains. To be improved. Therefore, when used together, the reduction conditions need not be strengthened as in the past. Further, the reduction step may be different from the step of causing cracks, but it is efficient if performed simultaneously in actual production. Therefore, it is desirable to perform a part or all of the various crack generation steps described above in a reducing atmosphere. In the method of generating a crack by rapid heating or rapid cooling prior to plating, a method of producing a crack in a reducing atmosphere during the heating or in a high heat state before the rapid cooling is used.
If the soaking time is at least 30 seconds, preferably at least 30 seconds, the effect of reduction can be sufficiently obtained. It is more effective if the soaking temperature is 500 ° C. or higher.

The type of steel, such as a hot-rolled steel sheet, to be used as an original sheet and the production method are not particularly limited, but it is desirable that the thickness of the oxide film layer formed is small. The thickness of the oxide film layer varies depending on the hot rolling finishing temperature, winding temperature, cooling conditions after finishing, etc., so that the finishing temperature and winding temperature can be reduced and the finishing It is desirable to make the cooling film layer as thin as possible by increasing the cooling rate as much as possible. Also, by lowering the winding temperature, the ratio of brittle hematite (Fe ₂ O ₃ ) in the oxide film layer is reduced, and the oxide film layer mainly composed of magnetite (Fe ₃ O ₄ ) having large deformability is also obtained. Can be expected. Further, as described above, skin pass rolling and leveling may be performed. After the occurrence of cracks, hot-dip galvanizing is performed, but there is no particular limitation on the plating conditions such as components in the bath. After plating, alloying treatment may be performed.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples do not limit the present invention, and all changes and implementations without departing from the spirit of the present invention will be described. Within the technical scope of

[0022]

【Example】

(Example 1) A steel having a composition indicated by steel type A in Table 1 was formed into a slab by a conventional method, and a finishing temperature of 890 ° C and a winding temperature of 6 were used.
A hot-rolled steel sheet having a thickness of 2.3 mm at 50 ° C. was obtained. The thickness of the oxide film layer was 4 to 7 μm. This is 100 × 200 (m
m) and hot-dip galvanized (using a Zn-0.2% Al bath) with a hot-dip galvanizing apparatus shown in FIG. Prior to plating, the sample was subjected to various changes in the average heating rate from room temperature to 500 ° C. in a 5% H ₂ —N ₂ atmosphere to generate cracks in the oxide film layer and to have a soaking temperature of 500 ° C. The oxide film layer was reduced by changing the soaking time variously. Then, it was immersed in a zinc bath, and the plating adhesion and the presence or absence of an unplated portion were examined. The plating adhesion was evaluated by a four-stage peeling state of the plating layer outside the bent portion by a bending test specified in JIS G 3302. Table 2 shows the results.

As shown in Table 2, when the average heating rate is low, the plating adhesion is insufficient.
0 ° C./sec or more is required. If the soaking time is set to 20 seconds or more, occurrence of non-plating can be sufficiently suppressed.

[0024]

[Table 1]

[0025]

[Table 2]

(Example 2) Steels having the components shown in Table 1 in steel types A to C were continuously cast to obtain a slab having a mass of about 20 t.
The slab was hot-rolled under various conditions to obtain a 2.3 mm thick hot-rolled steel sheet. In addition, a continuous hot-dip galvanizing line with a direct-fired heating zone (non-oxidizing furnace) is used.
By changing the heating conditions in 9) and the soaking conditions in a reducing furnace (in a 15% H ₂ —N ₂ atmosphere), the oxide film layer was reduced and cracks were generated. After that, the steel plate
After cooling to 0 ° C. at a cooling rate of 5 to 8 ° C./sec, it was immersed in a Zn-0.2% Al bath at a bath temperature of 460 ° C., and the plating adhesion and the presence or absence of unplated portions were examined. Evaluation of plating adhesion is the same as in Example 1. Table 3 shows the experimental conditions and results.

From Table 3, it can be seen that even when the oxide film layer is relatively thick, plating adhesion is improved by setting the average heating rate to 10 ° C./sec or more. Further, by adjusting the soaking temperature and the soaking time, non-plating can be prevented.

[0028]

[Table 3]

(Embodiment 3) The winding temperature is 700.degree.
A low carbon aluminum killed steel hot rolled steel sheet was produced at 50 ° C. and 580 ° C. at three levels. The plate thickness was 1.4 mm. The thickness of the oxide film layer of these hot-rolled steel sheets was measured by microscopic observation of the cross section. Further, the ratio of hematite in the oxide film layer was measured by an X-ray diffraction method. Table 4 shows the results.
Shown in In addition, magnetite was all except for hematite in the oxide film layer.

The original plates DF shown in Table 4 were converted to 100 × 200
(Mm), and used as a plating base plate, and hot-dip plating was performed using a hot-dip plating experimental apparatus shown in FIG. Two types of plating baths were used: a zinc bath containing 0.2% Al and a zinc bath containing 5% Al.

In the plating step, first, a steel sheet is made 10% H ₂ -N
_The mixture was heated from room temperature to 780 ° C. at a heating rate of 5 ° C./sec in a _two- gas atmosphere, and then reduced at 780 ° C. for 30 seconds. Thereafter, the steel sheet was cooled by directly cooling the furnace or by changing the cooling rate by a water-cooled cooling plate simulating gas jet or roll cooling. Thereafter, a step of dipping in a plating bath at 460 ° C. and pulling up was performed. The cooling rate in the case of furnace cooling is 2 to 5 ° C./sec. Further, the coating weight was fixed at 120 g / m ² . The temperature of the steel sheet was measured with a thermocouple welded to the steel sheet surface.

The plated steel sheet was evaluated for plating adhesion by a bending test specified in JIS G 3302. That is, a plated steel plate having a thickness of 1.4 mm is turned 180 °
After bending, the outer surface of the bent portion was observed and evaluated. The results and experimental conditions are shown in Tables 5 and 6.

From Tables 5 and 6, it can be seen that Sample No. having a cooling rate of less than 10 ° C./sec. 25 to 27 and 45 have poor plating adhesion, whereas N has a cooling rate of 10 ° C./sec or more.
o. 28 to 31, 33 to 37, 46 and 47 show good plating adhesion. Even if the cooling rate is 10 ° C./second or more, the cooling temperature range of No. In the case of No. 32, the plating adhesion is not good. Also, if there is a temperature difference of 50 ° C. or more between the steel sheet temperature and the bath temperature when immersed in the plating bath,
Since it is rapidly cooled at a cooling rate of about 50 ° C./sec.
As shown in 38 and 39, the plating adhesion becomes good. still,
No. Examples 41 to 44 are examples in which rapid cooling before plating and rapid cooling during plating bath immersion are used in combination.

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

[Table 6]

[0037]

The present invention is configured as described above, and causes cracks to be generated in the oxide film remaining between the base steel sheet and the hot-dip galvanized coating layer, thereby allowing the plating metal to penetrate into the cracks. As a result, a hot-dip galvanized steel sheet having good plating adhesion can be provided. In addition, the cracks may be generated by rapid heating or rapid cooling before or during hot-dip galvanizing.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a hot-dip plating apparatus used in an embodiment of the present invention.

[Description of Signs] 1 Thermocouple 2 Sample lifting / lowering device 3 Gas inlet 4 Sample attachment / detachment door 5 Infrared heater 6 Gas jet cooler 7 Gate valve 8 Water-cooled cooling plate 9 Gas wiping 10 Heater 11 Sample 12 Hot dipping bath 13 Crucible

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takafumi Iwai 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Inside the Kobe Steel Works Kakogawa Works (72) Inventor Masaaki Miura 1 Kanazawacho, Kakogawa City, Hyogo Prefecture God Co., Ltd. Inside Koto Steel Works Kakogawa Works (72) Inventor Ichiro Tsukaya 1 Kanazawa-cho, Kakogawa City, Hyogo Prefecture Kobe Works Co., Ltd. 1 inside Kakogawa Works, Kobe Steel Co., Ltd. Sokogakogawa Works

Claims (5)

[Claims] 1. A method for producing a hot-dip galvanized steel sheet in which a hot-dip galvanized steel sheet is coated on a steel sheet in which an oxide film remains, comprising a step of reducing a part of the oxide film before performing the hot-dip galvanized plating, A method for producing a hot-dip galvanized steel sheet having good plating adhesion, comprising a step of generating cracks in an oxide film before hot-dip galvanizing or / and while hot-dip galvanizing is performed. . 2. Prior to hot-dip galvanizing, the surface of the steel sheet is heated at an average heating rate from room temperature to 500.degree.
The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the steel sheet is heated to 500 ° C or more at a rate of at least 500 ° C / sec to cause cracks in the oxide film. 3. A crack is generated in the oxide film by heating the steel sheet and cooling the surface of the steel sheet at 50 ° C. or more at an average cooling rate of 10 ° C./sec or more prior to performing hot-dip galvanizing. Or the method for producing a galvanized steel sheet according to item 2. 4. The step of partially reducing an oxide film, which is performed prior to performing hot-dip galvanizing, is a step of maintaining a soaking temperature for 20 seconds or more in a reducing atmosphere at 500 ° C. or higher.
The method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 3. 5. When performing hot-dip galvanizing, the temperature of the steel sheet surface is made 50 ° C. or more higher than the plating bath temperature, and the steel sheet is immersed in the bath to generate cracks in the oxide film. The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein plating is performed.