JP2836380B2 - Alloyed hot-dip galvanized steel sheet excellent in powdering resistance and flaking resistance and method for producing the same - Google Patents

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 an alloyed hot-dip galvanized steel sheet which is excellent in powdering resistance and flaking resistance and is suitable for automotive materials and the like.

[0002]

2. Description of the Related Art The demand for alloyed hot-dip galvanized steel sheets as rust-resistant steel sheets for automobiles has been increasing in recent years because of their excellent post-paint corrosion resistance and paint compatibility. Particularly recently, zinc has been used to ensure corrosion resistance. The plating film tends to be thicker. By the way, when this alloyed hot-dip galvanized steel sheet is used for applications such as automobiles, it has excellent plating peeling resistance (powdering resistance) and press formability (flaking resistance).
Is required.

Conventionally, as this type of galvannealed steel sheet, an alloyed hot-dip galvanized steel sheet whose alloy phase is mainly composed of a ζ phase to obtain excellent powdering resistance and an alloy phase of δ There is known an alloyed hot-dip galvanized steel sheet in which excellent flaking resistance is obtained by mainly using _one phase.

[0004]

However, these alloyed hot-dip galvanized steel sheets have advantages and disadvantages as materials for automobiles and the like, and it is hard to say that all of them have sufficient properties. That is, in order to improve the powdering resistance, it is necessary to prevent the formation of the Γ phase. In this case, the alloying heat cycle is generally a low temperature cycle of 495 ° C. or less. As a result, as in the former alloyed hot-dip galvanized steel sheet, the alloyed plating layer has a columnar shape.
It is a phase-based crystal form. Although this alloyed hot-dip galvanized steel sheet has excellent powdering resistance, the ζ phase of columnar crystals has a large coefficient of friction due to its large surface irregularities, and therefore has poor flaking resistance.

On the other hand, the galvannealed steel sheet having an alloy phase which was latter [delta] ₁ main phase, smooth crystalline forms of alloying the plating layer for improving the resistance to flaking resistance by lowering the friction coefficient of the surface If it is obtained by the [delta] ₁ phase is bulk crystal, usually, this in order to obtain a [delta] ₁ phase must be alloyed temperatures above 495 ° C., which was carried out alloying treatment at such a high temperature In addition, there is a problem that a Γ phase is easily generated in addition to a target δ ₁ phase, and powdering resistance is deteriorated. As described above, the conventional galvannealed steel sheet has a problem in either the powdering resistance or the flaking resistance, and cannot be said to be sufficient as a material for an automobile or the like which requires both properties.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems. In the case where a galvannealed steel sheet is processed in applications such as automotive materials, the powdering resistance is particularly low. Focusing on the steel plate surface on the compression side that is strongly required, while focusing on the steel plate surface on the tensile side where the anti-flaking property is particularly required,
A plating film of a different alloy phase corresponding to each required characteristic is formed on both surfaces of the steel sheet . That is, the present invention is a method for producing a galvannealed steel sheet having the following configuration.

[0007]

(1) After hot-dip galvanizing a steel sheet and before introducing the steel sheet into the alloying treatment furnace, a temperature difference is generated on both surfaces of the steel sheet, so that the alloying treatment temperature (alloy) in the alloying treatment furnace is increased. The maximum steel sheet temperature in the heat treatment cycle, the same applies hereinafter) to 495 ° C. or more for one side of the steel sheet and 495 ° C. or less for the other side of the steel sheet,
On one side of the steel sheet, an alloyed hot-dip galvanized layer mainly composed of δ ₁ phase is formed, and on the other side of the steel sheet, an alloyed hot-dip galvanized layer having at least a surface layer formed with a ζ phase is formed. A method for producing an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and flaking resistance.

(2) In the manufacturing method of the above (1) ,
An alloyed hot-dip galvanized steel sheet with excellent powdering and flaking resistance, characterized in that only one side of the steel sheet is cooled after the steel sheet is hot-dip galvanized and before the steel sheet is introduced into the alloying treatment furnace. Manufacturing method.

(3) In the manufacturing method of the above (1) or (2) , the alloying treatment is performed in a high-frequency induction heating type alloying furnace, which is excellent in powdering resistance and flaking resistance. Manufacturing method of galvannealed steel sheet.

[0011]

The alloyed hot-dip galvanized steel sheet obtained by the production method of the present invention is used as a steel sheet surface having an alloyed hot-dip galvanized layer mainly composed of δ ₁ phase, which is mainly a tensile side during press working. Can be Since the alloyed hot-dip galvanized layer on the steel sheet surface which is mainly on the tension side during processing is a smooth alloy phase mainly composed of δ ₁ phase, it has excellent flaking resistance. On the other hand, since the alloyed hot-dip galvanized layer mainly on the compression side during processing is an alloy phase having a ζ phase, excellent powdering resistance is obtained. In the film structure of the alloyed plating layer having the ζ phase, the ζ phase is formed on the surface layer, and the δ ₁ phase exists below the ζ phase.

[0012] In the production method of the present invention, after hot-dip galvanized steel sheets, by generating a temperature difference of the steel sheet both sides before the steel sheet is introduced into the alloying treatment furnace, said alloying soluble
Manufactures galvanized steel sheets. If alloying hot-dip galvanizing, in order to be prevented the generation of Γ phase ζ phase is formed, it is necessary to alloying treatment at 495 ° C. below the temperature while the alloy phase [delta] ₁ In order to obtain a smooth bulk crystal mainly composed of a phase, it is necessary to perform alloying treatment at a temperature higher than 495 ° C. FIG. 1 shows the relationship between the alloying treatment temperature and the crystal morphology of the plated layer after the alloying treatment.

In the present invention, the temperature of the alloying treatment in the alloying treatment furnace is reduced by causing a temperature difference on both sides of the steel sheet before the hot-dip galvanized steel sheet is introduced into the alloying treatment furnace. Is more than 495 ° C. and the other side of the steel sheet is 495 ° C. or less, whereby a smooth alloyed hot-dip galvanized layer mainly composed of δ ₁ phase is formed on one side of the steel sheet. On the other side of the steel sheet, an alloyed hot-dip galvanized layer having a ζ phase formed on at least the surface layer is formed.

In order to generate a temperature difference between both sides of the hot-dip galvanized steel sheet, one side of the steel sheet may be heated or cooled, but a relatively easy method is to cool one side of the galvanized steel sheet by cooling means. . For example, by bringing a cooling roll such as a water-cooled roll into contact with one side of a plated steel sheet, only one side of the steel sheet can be cooled. In this cooling, it is preferable that the surface of the steel sheet is reduced by the heat of fusion of the molten zinc to solidify the plating layer, and the temperature of the plated surface cooled in this manner is hardly increased by the latent heat of solidification in the alloying treatment. The alloying treatment temperature decreases.

FIG. 2 schematically shows the method of the present invention using a cooling roll as a cooling means, wherein 1 is a molten zinc pot,
2 is a gas wiping nozzle, 3 is an alloying furnace, and S is a steel plate to be plated. A cooling roll 4 (water-cooled roll) is provided between the gas wiping nozzle 2 and the alloying furnace 3. The plated steel sheet S that has exited the hot-dip zinc pot 1 is cooled on one side by a water-cooled roll 4 before being introduced into the alloying treatment furnace, and thereafter is introduced into the alloying treatment furnace and alloyed. FIG. 3 shows an example of a heat pattern on each surface of the plated steel sheet from the position of the gas wiping nozzle 2 to the cooling zone of the alloying furnace 3. In addition, as a method of generating a temperature difference on both sides of the steel sheet, a method of cooling or heating both sides of the steel sheet, making a difference in the degree of heating or cooling of both sides of the steel sheet at that time, heating one side of the steel sheet, and heating another side of the steel sheet Any method, such as a method of cooling the water, can be adopted.

The alloying treatment can be carried out in a gas heating type alloying furnace, but in order to properly obtain the desired alloy phase on each surface of the steel sheet, a high frequency induction heating type alloying furnace is used. Preferably, an alloying treatment is performed. This is for the following reasons. First, by using a high-frequency induction heating method in the alloying treatment, the steel sheet itself can be directly heated, and the interface in contact with the plating film is heated most. The Fe-Zn reaction occurs uniformly in a short time and independently of the position on the steel sheet surface, and therefore, the desired alloy phase can be appropriately generated without causing partial overalloy on the steel sheet. .

Second, in the case of the gas heating method, since the emissivity continues to change along with the alloying of the coating in the gas furnace, the amount of heat given to the steel sheet also changes, so that the steel sheet can be heated to the target sheet temperature. Have difficulty. On the other hand, in the high-frequency induction heating, since the steel sheet itself is directly heated in a short time, the steel sheet itself can be easily heated to a target sheet temperature, and the target alloy phase can be appropriately generated from this aspect. .

Third, in the gas heating method, the heating of each part of the steel sheet tends to be non-uniform due to temperature unevenness, and insufficient alloying or over-alloying is likely to occur partially. Particularly, in a gas furnace, a high temperature gas rises due to a chimney effect. Therefore, when an attempt is made to ensure a necessary plate temperature, partial overalloying is likely to occur.
On the other hand, in the case of high-frequency induction heating, since the steel sheet itself is directly heated, each part of the steel sheet can be uniformly heated, and a target alloy phase can be uniformly generated. Fourth, since the ambient temperature is high in and just above the gas furnace, it is practically impossible to measure the sheet temperature during operation, and the sheet temperature controllability is poor. On the other hand, the high-frequency induction heating furnace can strictly control the sheet temperature.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a continuous line as shown in FIG.
After hot-dip galvanizing a cold-rolled steel sheet manufactured from killed steel and Ti-added IF steel, one side of the steel sheet is cooled with a water-cooled roll before being introduced into the alloying furnace, and then alloyed by gas heating or high-frequency induction heating By processing, an alloyed hot-dip galvanized steel sheet was manufactured. As comparative examples, an alloyed hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which were alloyed without being cooled by a water-cooled roll and both surfaces of which were cooled by a water-cooled roll and then alloyed were produced.
Tables 1 and 2 show the crystal morphology and properties of the plating film on each side of the obtained alloyed hot-dip galvanized steel sheet, together with the production conditions such as the use conditions of the water-cooled roll and the alloying treatment conditions.

In the present embodiment, the sheet temperatures on the inlet side and the outlet side of the alloying furnace are the surface temperatures of the steel sheet measured by a radiation thermometer. The methods for measuring the phase of the product and the tests and evaluation methods for each property are as follows.

○ Amount of phase in product film: X obtained film
Line diffraction, taking the peak intensity Iζ ( ₄₂₁ ) of d = 1.900 for the ζ phase and the peak intensity Iδ ₁ ( ₂₄₉ ) of d = 1.990 for the δ ₁ phase, The amount of ζ phase in the film was represented by the strength ratio. In addition,
Ibg is a background, and if Z / D is 20 or less, substantially no ζ phase exists. Z / D = (Iζ ( ₄₂₁ ) −Ibg) / (Iδ ₁ ( ₂₄₉ ) −Ibg) × 100

○ Powdering resistance: 1 g / rust-proof oil (Knoxlast 530F manufactured by Parker Kosan Co., Ltd.) was added to the test piece.
After applying m ² , a bead pull-out test was performed with a bead radius R: 0.5 mm, a pressing load P: 500 kg, and a pressing depth h: 4 mm. After the tape was peeled, the peel amount was calculated from the weight change before and after molding. The numerical values in the table are average values of a plurality of measured values (5 × 5 = 25).

The maximum deviation in the sheet width direction of powdering resistance:
The above-mentioned powdering resistance was measured at 5 points in the coil length direction and 5 points in the coil width direction (both edges, 1/4 position and the center portion) where the operating conditions were stable, and the maximum and minimum values were measured. The difference was taken.

Friction coefficient: 1 g / m ² of rust-preventive oil (Knoxlast 530F, manufactured by Parker Kosan Co., Ltd.) was applied to the test piece, and an indenter made of tool steel SKD11 was pressed with a load of 400 kg to apply a pressure of 1 m / m ^2. The drawing was performed at a drawing speed of min, and the ratio between the drawing load and the pressing load was defined as the friction coefficient. The numerical values in the table represent a plurality of measured values (5 × 5 = 25).
).

The maximum deviation of the coefficient of friction in the sheet width direction: The coefficient of friction was measured at the same location as the powdering resistance, and the difference between the maximum value and the minimum value was determined.

[0026]

[Table 1]

[0027]

[Table 2]

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an alloying treatment temperature and a crystal morphology of a plating layer after an alloying treatment.

FIG. 2 is an explanatory view showing an outline of the method of the present invention.

FIG. 3 is an explanatory view showing an example of a heat pattern on each side of a steel sheet in the method of the present invention.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (3)

(57) [Claims] (1) After hot-dip galvanizing a steel sheet, a temperature difference is generated on both sides of the steel sheet before the steel sheet is introduced into the alloying processing furnace, so that the alloying processing temperature in the alloying processing furnace is reduced. Of 495 ° C or less on one side of the steel sheet, and 495 ° C or less on the other side of the steel sheet. An alloyed hot-dip galvanized layer mainly composed of δ ₁ phase is formed on one side of the steel sheet. A method for producing an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and flaking resistance, characterized in that an alloyed hot-dip galvanized layer having at least a surface layer formed on its surface is formed. 2. The powdering resistance and flake resistance according to claim 1 , wherein after the steel sheet is hot dip galvanized, only one side of the steel sheet is cooled before the steel sheet is introduced into the alloying treatment furnace. Manufacturing method of alloyed hot-dip galvanized steel sheet with excellent kingness. 3. The galvannealed steel sheet having excellent powdering resistance and flaking resistance according to claim 1 or 2 , wherein the alloying treatment is performed in an alloying furnace of a high frequency induction heating system. Manufacturing method.