JPH05331607A - Galvannealed steel sheet excellent in powdering resistance and aptitude for external surface and its production - Google Patents

Abstract

PURPOSE:To produce a galvannealed steel sheet used for the purposes, e.g. of automobile outer sheet and showing superior coating suitability and powdering resistance. CONSTITUTION:A two-layer plating layer consisting of a galvannealing layer, on the lower layer side, composed essentially of delta1 phase and a plating layer, on the upper layer side, composed of Fe or ferrous alloy is provided to one side of a steel sheet, to be the coated surface of an outer sheet, etc., and also a galvannealing layer where zeta-phase is formed at least in a surface layer is provided to the other side of the steel sheet, to be the compression side at the time of press forming, etc., by which superior coating suitability and superior powdering resistance can be provided to one side and the other side of the steel sheet, respectively. This steel sheet can be produced by making respective alloying temps. of both sides of the steel sheet in the alloying treatment furnace differ from each other by allowing a temp. difference to occur between both sides of the steel sheet before the introduction into the alloying treatment furnace by means of roll cooling, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and outer surface suitability suitable for automobile outer plate applications and the like, and a method for producing the same.

[0002]

2. Description of the Related Art A two-layer alloy hot-dip galvanized steel sheet having an Fe-based plating layer on top of an alloyed hot-dip galvanized layer has excellent post-painting corrosion resistance and paint compatibility, so it is used as an anticorrosion steel sheet for automobile exterior panels. The demand for this has increased in recent years, and particularly recently, there is a tendency for the zinc plating film to have a thicker basis weight in order to ensure corrosion resistance. By the way, when this two-layer alloy hot-dip galvanized steel sheet is used for an automobile outer panel, the outer surface of the outer panel, which is the coated surface, has excellent finish finish, and when it is processed, the outer panel is mainly the compression side. Excellent powdering resistance is required on the inner surface side.

As a conventional alloyed hot-dip galvanized steel sheet for outer panels of this type, a double-layer alloy hot-dip galvanized steel sheet having excellent powdering resistance is obtained by mainly using an alloy phase of ζ phase. Then, a two-layer alloyed hot-dip galvanized steel sheet is known in which excellent paintability is obtained by making the alloy phase mainly a δ ₁ phase.

[0004]

However, these alloyed hot-dip galvanized steel sheets each have merits and demerits as materials for outer panels of automobiles, and it is hard to say that all of them have sufficient characteristics. That is, in order to improve the powdering resistance, it is necessary to prevent the generation of the Γ phase, and the alloying heat cycle in this case is generally a low temperature cycle of 495 ° C. or lower. As a result, like the former two-layer galvannealed steel sheet described above, the alloyed plating layer has a columnar ζ-phase-based crystal morphology. This two-layer galvannealed steel sheet is excellent in powdering resistance on the inner surface side of the outer plate (usually the surface side that becomes the compression side by processing), where the film peeling easily occurs during processing, but as described above, Since the ζ phase is a columnar crystal, the upper layer plating (F
e-type plating), the covering property of the upper layer plating is poor,
There is a problem that good paintability cannot be obtained.

On the other hand, in the latter two-layer galvannealed steel sheet having an alloy phase mainly composed of δ ₁ phase, the crystal form of the alloyed plating layer is a smooth lump crystal in order to improve the coating property of the upper layer plating. Although the δ ₁ phase is used, it is usually necessary to set the alloying temperature to higher than 495 ° C. in order to obtain the δ ₁ phase. In addition to the δ ₁ phase, the Γ phase is easily generated,
There is a problem that the powdering resistance is deteriorated.

As described above, the conventional two-layer alloy hot-dip galvanized steel sheet has a problem in either the coating property or the powdering resistance, and is sufficient as a material for an automobile outer panel or the like which requires both properties. It is hard to say.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of such a conventional problem. In applications such as automobile outer plates, the main characteristic required on the outer surface of the outer plate is paintability. Focusing on the fact that powdering resistance is a characteristic that is generally required on the inner surface of the outer plate, which is the compression side during processing, a plating film of different alloy phases is formed on both surfaces of the steel sheet according to the required characteristics. It was made. That is, the present invention is an alloyed hot-dip galvanized steel sheet characterized by the following configurations and a method for producing the same.

(1) On one surface of a steel sheet, there is provided a two-layer plating consisting of a lower-side alloyed hot-dip galvanized layer mainly composed of a δ ₁ phase and an upper-side plated layer made of Fe or Fe-based alloy, A galvannealed steel sheet excellent in powdering resistance and outer surface suitability, which has, on the other surface of the steel sheet, a galvannealed layer in which at least a ζ phase is formed in the surface layer.

(2) In the method for producing a galvannealed steel sheet according to the above (1), a temperature difference is caused on both surfaces of the steel sheet after the hot dip galvanizing of the steel sheet and before the steel sheet is introduced into an alloying furnace. As a result, the alloying treatment temperature in the alloying treatment furnace (the maximum steel sheet temperature in the alloying heat cycle, the same applies below) is set to more than 495 ° C. on one side of the steel sheet and 495 on the other side of the steel sheet. ℃ or less, to form an alloyed hot dip galvanized layer mainly composed of δ ₁ phase on one surface of the steel sheet, and to form an alloyed hot dip galvanized layer having ζ phase on at least the surface layer on the other surface of the steel sheet is allowed, after cooling, [delta] ₁ main phase to powdering resistance and an outer surface suitable for characterized by applying the upper layer plating made of Fe or Fe-based alloy galvannealed layer steel sheet surface side formed a Excellent production method of the galvannealed steel sheet to.

(3) On one surface of the steel sheet, there is provided a two-layer plating consisting of a lower-layer alloyed hot-dip galvanized layer mainly composed of a δ ₁ phase and an upper-layer plated layer made of Fe or Fe-based alloy, On the other surface of the steel sheet, there is a powdering resistance having two-layer plating including a lower-layer alloyed hot-dip galvanized layer having at least a ζ phase formed on the surface layer and an upper-layer plated layer made of Fe or Fe-based alloy, and Alloyed hot-dip galvanized steel sheet with excellent external suitability.

(4) In the method for producing an alloyed hot-dip galvanized steel sheet according to the above (3), after the hot-dip galvanizing of the steel sheet, a temperature difference is caused on both sides of the steel sheet before the steel sheet is introduced into an alloying treatment furnace. As a result, the alloying treatment temperature in the alloying treatment furnace is set to more than 495 ° C. on one side of the steel sheet and 495 ° C. or less on the other side of the steel sheet, and the δ ₁ phase is formed on _one side of the steel sheet. An alloyed hot-dip galvanized layer is formed as a main component, and an alloyed hot-dip galvanized layer having a ζ phase formed on at least the surface layer is formed on the other surface of the steel sheet, and after cooling, Fe or Fe-based alloy is formed on both surfaces of the steel sheet. A method for producing an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and outer surface suitability, which is characterized by performing an upper layer plating comprising

(5) In the manufacturing method of the above (2) or (4), after the hot dip galvanizing of the steel sheet, only one side of the steel sheet is cooled before being introduced into the alloying furnace. For producing a galvannealed steel sheet excellent in powdering resistance and suitability for outer surface.

(6) The above (2), (4) or (5)
The method for producing a galvannealed steel sheet having excellent powdering resistance and outer surface suitability, which is characterized in that the alloying treatment is performed in a high frequency induction heating type alloying furnace.

[0014]

In the alloyed hot-dip galvanized steel sheet of the present invention, the steel sheet surface side having the two-layer plating consisting of the lower layer plating mainly consisting of δ ₁ phase and the Fe-based upper layer plating is used as the outer surface side (painted surface) of the outer plate or the like. Be done. Since the alloyed hot-dip galvanized layer on the surface of the steel sheet, which is the outer surface of the outer plate, is a smooth alloy phase mainly composed of the δ ₁ phase, it has excellent coatability of the upper Fe-based plating and has a good coating finish.

On the other hand, when the alloyed hot-dip galvanized steel sheet is used as a vehicle outer panel or the like, it is the inner surface of the outer panel which is usually the compression side in press working that the powdering resistance becomes a big problem. In the present invention, since the steel plate surface which is the inner surface side of the outer plate is the alloyed hot-dip galvanized layer having the ζ phase, excellent powdering resistance can be obtained. In the film structure of the alloyed plating layer having this ζ phase, the ζ phase is formed in the surface layer and the δ ₁ phase exists in the layer below it. In addition, an upper layer plating made of Fe or a Fe-based alloy is formed on the surface on which the alloyed plating layer having the ζ phase is formed, if necessary.

The alloyed hot-dip galvanized steel sheet of the present invention can be manufactured by hot-dip galvanizing the steel sheet and then producing a temperature difference between both surfaces of the steel sheet before the steel sheet is introduced into the alloying treatment furnace. 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 form a smooth massive crystal mainly composed of a phase, it is necessary to perform an 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 controlled by making a temperature difference between both surfaces of the steel sheet before the hot dip galvanized steel sheet is introduced into the alloying treatment furnace. Is higher than 495 ° C. and the other side of the steel plate is 495 ° C. or lower, whereby a smooth galvannealed layer mainly composed of δ ₁ phase is formed on one surface of the steel plate, Further, on the other surface of the steel sheet, an alloyed hot-dip galvanized layer having a ζ phase formed on at least the surface layer is formed.

To cause a temperature difference between both surfaces 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 coated steel sheet by a cooling means. .. For example, by bringing a cooling roll such as a water cooling roll into contact with one side of the plated steel sheet, only one side of the steel sheet can be cooled. In this cooling, it is preferable to lower the steel plate surface by the amount of heat of fusion of molten zinc to solidify the plating layer, and the plating surface cooled in this way is difficult to raise the temperature due to the solidification latent heat in the alloying treatment, The alloying treatment temperature becomes low.

FIG. 2 shows the outline of the method of the present invention in which a cooling roll is used as a cooling means. 1 is a molten zinc pot,
2 is a gas wiping nozzle, 3 is an alloying treatment furnace, S is a steel plate to be plated, and a cooling roll 4 (water cooling roll) is provided between the gas wiping nozzle 2 and the alloying treatment furnace 3. The plated steel sheet S exiting the molten zinc pot 1 is cooled on one side by the water-cooling roll 4 before being introduced into the alloying treatment furnace, and then introduced into the alloying treatment furnace for alloying treatment. 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 treatment furnace 3. In addition, as a method of causing a temperature difference between both sides of the steel sheet, cooling or heating both sides of the steel sheet, at that time, a method of making a difference in heating or cooling of both sides of the steel sheet, heating one side of the steel sheet, other one side Any method such as a method of cooling the can be adopted.

The alloying treatment can be carried out in a gas heating type alloying treatment 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 treatment furnace is used. It is preferable to perform alloying treatment. This is for the following reasons. First, by using the high frequency induction heating method in the alloying process, the steel sheet itself can be directly heated, and the interface in contact with the plating film is heated most, so that the interface in the interface is different from the gas heating method. The Fe-Zn reaction occurs uniformly in a short time and regardless of the position on the steel sheet surface, and therefore, the desired alloy phase can be appropriately generated without causing partial overalloying on the steel sheet. ..

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

Thirdly, in the gas heating method, heating of each part of the steel sheet is likely to be nonuniform due to temperature unevenness, and partial alloying or overalloying is likely to occur partially. In particular, in a gas furnace, a high temperature gas rises due to a chimney effect, so that partial overalloying is likely to occur when it is attempted to secure the necessary plate temperature.
On the other hand, in the case of high frequency induction heating, since the steel plate itself is directly heated, each part of the steel plate can be uniformly heated, and a desired alloy phase can be uniformly generated. Fourthly, since the atmospheric temperature is high inside the gas furnace and directly above the gas furnace, it is practically impossible to measure the plate temperature during operation, and the plate temperature controllability is poor. On the other hand, the high frequency induction heating furnace allows strict control of the plate temperature.

The plated steel sheet which has been subjected to the alloying treatment as described above is subjected to the upper layer plating of Fe or Fe-based alloy on at least the steel sheet surface having the alloyed hot-dip galvanized layer mainly containing the δ ₁ phase. It Further, the steel sheet surface on which the alloyed hot-dip galvanized layer having the ζ phase on the other surface is formed is subjected to an upper layer plating made of Fe or Fe-based alloy, if necessary. When the alloying treatment is performed by high frequency induction heating as described above, the plating surface is not oxidized,
The upper layer plating can be properly adhered to the alloyed plating layer, and also from this aspect, the upper layer plating with good coverage can be applied with a smaller amount of adhesion than in the case of alloying treatment by gas heating.

[0024]

EXAMPLE In a continuous line as shown in FIG.
After hot-dip galvanizing a cold-rolled steel sheet manufactured from killed steel and IF-added IF steel, one side of the steel sheet is cooled with a water-cooled roll before being introduced into an alloying furnace, and then alloyed by gas heating or high-frequency induction heating. After the treatment, a two-layer alloy hot-dip galvanized steel sheet was produced by applying an upper layer plating. In addition, as a comparative example, alloying treatment was performed without cooling with a water-cooled roll, the same two-layer galvannealed steel sheet with the same upper layer plating and both sides of the steel sheet were cooled with water-cooled rolls, and then alloyed and melted. A galvanized steel sheet was manufactured. Table 1 and Table 2 show the crystal morphology and the characteristics of the plating film on each surface of the obtained two-layer galvannealed steel sheet together with the production conditions such as the use conditions of the water cooling roll and the alloying treatment conditions.

In the present embodiment, the plate temperature on the inlet side and the outlet side of the alloying furnace is the surface temperature of the steel sheet measured by a radiation thermometer. Further, the method for measuring the ζ phase of the product and the test and evaluation method for each property are as follows.

Amount of ζ phase in the product coating: The obtained coating is X
And ray diffraction, for ζ phase takes a peak intensity of d = 1.900 Iζ _(421), also the peak intensity i? ₁ of d = 1.990 for [delta] ₁ phase _(249), respectively, the peak indicated by the following formula The strength ratio represents the amount of ζ phase in the film. 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 / corrosion-preventing oil (Knox Thrust 530F manufactured by Parker Kosan Co., Ltd.) was applied to the test piece.
After applying m ² , a bead pull-out test was conducted with a bead radius R: 0.5 mm, a pressing load P: 500 kg, and a pressing depth h: 4 mm. After tape peeling, peeling was performed from the weight change before and after molding. The amount was calculated. The numerical values in the table are average values of a plurality of measured values (5 × 5 = 25 pieces).

○ Maximum deviation of powdering resistance in the plate width direction:
At locations where the operating conditions were stable, the above 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 center part), and the maximum and minimum values were measured. I took the difference.

Coating compatibility: After forming a phosphate film by dipping on the surface of each of the material of the present invention and the comparative material, cationic type electrodeposition coating was applied under the following conditions. Voltage: 300 V Bath temperature: 26.5 ° C. Specimen area / Anode area: 1/1 Coating thickness: 20 μm Baking temperature: 170 ° C. Baking time: 20 minutes Specimen electrodeposited as described above The crater-like defects generated in the coating film of No. 1 were visually examined and evaluated by the following. ○: 100 or less crater-like defects ×: More than 100 crater-like defects

[0030]

[Table 1]

[0031]

[Table 2]

[Brief description of drawings]

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

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

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

Claims (6)

[Claims] 1. A two-layer plating comprising a lower-side alloyed hot-dip galvanized layer mainly composed of δ ₁ phase and an upper-side plated layer made of Fe or Fe-based alloy, on one surface of a steel sheet,
A galvannealed steel sheet excellent in powdering resistance and outer surface suitability, which has, on the other surface of the steel sheet, a galvannealed layer in which at least a ζ phase is formed in the surface layer. 2. An alloying treatment temperature in an alloying treatment furnace is controlled by producing a temperature difference between both surfaces of the steel sheet after hot dip galvanizing the steel sheet and before introducing the steel sheet into the alloying treatment furnace. On one side of the steel sheet is higher than 495 ° C. and on the other side of the steel sheet is 495 ° C. or less, and an alloyed hot dip galvanized layer mainly consisting of δ ₁ phase is formed on one side of the steel sheet and On the surface, to form an alloyed hot dip galvanized layer in which at least the ζ phase is formed in the surface layer,
After cooling, the steel plate surface side on which the alloyed hot-dip galvanized layer mainly composed of the δ ₁ phase is subjected to upper layer plating made of Fe or Fe-based alloy, is excellent in powdering resistance and outer surface suitability. A method for manufacturing a galvannealed steel sheet. 3. A two-layer plating comprising, on one surface of a steel sheet, a lower-layer alloyed hot-dip galvanized layer mainly composed of a δ ₁ phase and an upper-layer plated layer made of Fe or Fe-based alloy,
On the other surface of the steel sheet, there is a powdering resistance having two-layer plating including a lower-layer alloyed hot-dip galvanized layer having at least a ζ phase formed on the surface layer and an upper-layer plated layer made of Fe or Fe-based alloy, and Alloyed hot-dip galvanized steel sheet with excellent external suitability. 4. After hot-dip galvanizing a steel sheet, a temperature difference is generated between both surfaces of the steel sheet before the steel sheet is introduced into the alloying treatment furnace, so that the alloying treatment temperature in the alloying treatment furnace is controlled. On one side of the steel sheet is higher than 495 ° C. and on the other side of the steel sheet is 495 ° C. or less, and an alloyed hot dip galvanized layer mainly consisting of δ ₁ phase is formed on one side of the steel sheet and On the surface, to form an alloyed hot dip galvanized layer in which at least the ζ phase is formed in the surface layer,
After cooling, an upper layer plating made of Fe or an Fe-based alloy is applied to both surfaces of the steel sheet, and a method for producing an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and outer surface suitability. 5. The powdering resistance according to claim 2 or 4, wherein after hot dip galvanizing the steel sheet, only one side of the steel sheet is cooled before the steel sheet is introduced into an alloying furnace. A method for producing an alloyed hot-dip galvanized steel sheet having excellent outer surface suitability. 6. The alloyed hot dip galvanized steel sheet excellent in powdering resistance and outer surface suitability according to claim 2, 4 or 5, wherein the alloying treatment is performed in a high frequency induction heating type alloying furnace. Manufacturing method.