WO2020111775A1 - Galvanized steel sheet having excellent plating adhesion and corrosion resistance, and manufacturing method therefor - Google Patents

Abstract

The objective of one aspect of the present invention is to provide: a galvanized steel sheet having excellent plating adhesion, having a plated layer with improved friction characteristics by means of a predetermined level of Fe elution, and having excellent corrosion resistance; and a manufacturing method therefor.

Description

Galvanized steel sheet with excellent plating adhesion and corrosion resistance and method for manufacturing the same

The present invention relates to a galvanized steel sheet, and more particularly, to a galvanized steel sheet excellent in plating adhesion and corrosion resistance and a method for manufacturing the same.

In the existing hot-dip galvanized steel sheet, there is an Fe-Al inhibition layer formed between the base steel sheet and the plating layer (interface), and the suppressing layer secures plating adhesion at the interface between the base steel sheet and the plating layer, and at the same time It is known to suppress the diffusion of Fe due to a concentration gradient into the plating layer.

When the suppression layer is not continuously formed at the interface between the base material and the plating layer, Fe in the base material elutes into the plating layer, and there is a problem that the plating layer is peeled off as the plating adhesion decreases. Therefore, in order to secure a plating adhesion of a certain level or higher, the suppression layer must be continuously generated.

On the other hand, when Fe in the base material diffuses into the plating layer, it is possible to obtain an effect on friction characteristics such as improvement in roughness and surface roughness of the plating layer.

However, when the suppression layer is continuously formed at the interface between the base material and the plating layer, diffusion of Fe is suppressed, and there are many limitations in using the effect on the friction characteristics.

Accordingly, there is a need to develop a technique for a method to simultaneously obtain the effect of Fe diffusion while maintaining the adhesion to plating.

[Advanced technical literature]

[Patent Document]

(Patent Document 1) Korean Patent Publication No. 10-2015-0074882

One aspect of the present invention is to provide a galvanized steel sheet having excellent adhesion to plating as well as excellent frictional properties and excellent corrosion resistance by plating with a certain level of Fe elution.

The subject of this invention is not limited to the above-mentioned content. Anyone having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding additional problems of the present invention from the contents throughout the present specification.

One aspect of the present invention includes a steel plate and a galvanized layer formed on at least one surface of the steel plate, wherein the zinc plated layer is weight% in aluminum (Al): 5.1-35.0%, magnesium (Mg): 4.0-25.0%. It provides a zinc-plated steel sheet having excellent plating adhesion and corrosion resistance, including the remaining zinc (Zn) and other unavoidable impurities, and an Al-Fe suppression layer having a thickness of 0.01 to 15 µm at the interface between the substrate and the zinc plating layer.

In another aspect of the present invention, preparing a zinc plating bath containing aluminum (Al): 5.1-35.0%, magnesium (Mg): 4.0-25.0%, residual zinc (Zn) and other unavoidable impurities by weight%. ; Preparing a galvanized steel sheet by immersing the steel sheet in the zinc plating bath and performing plating; And cooling the galvanized steel sheet,

It provides a method for producing a zinc plated steel sheet having excellent plating adhesion and corrosion resistance, characterized in that the temperature of the zinc plating bath is greater than 555°C to less than 600°C, and the inlet temperature of the steel sheet is greater than 565°C to less than 600°C. .

Advantageous Effects of Invention According to the present invention, it is possible to provide a galvanized steel sheet having excellent plating adhesion even when Fe elution occurs from the base iron into the plating layer.

In addition, the galvanized steel sheet of the present invention not only has excellent corrosion resistance, but also has an effect of inducing an improvement in workability as the frictional properties of the plated layer are improved.

1 is a view showing a cross-sectional view of Inventive Example 1 and Comparative Example 5 according to an embodiment of the present invention.

Figure 2 shows the results of the sear-bending test (sealer-bending test) of Inventive Example 1 and Comparative Example 5 according to an embodiment of the present invention (where, the scale bar (scale bar) is 2mm).

Figure 3 shows a photograph of the shape of the crack in the outer region after the bending test of Inventive Example 1 and Comparative Example 5 according to an embodiment of the present invention.

Figure 4 shows the results of 3D roughness scan of the surfaces of Inventive Example 1 and Comparative Example 5 according to an embodiment of the present invention.

Figure 5 shows a photograph taken the surface after the salt spray test of Inventive Example 1 and Comparative Example 5 according to an embodiment of the present invention.

The present inventors have studied in depth how to improve the friction characteristics and corrosion resistance of the plating layer while maintaining the plating adhesion of the galvanized steel sheet.

As a result, from optimizing the alloy composition and plating conditions in the plating bath, Fe diffused from the iron in the plating layer (unlike the existing Fe-Al alloy layer is formed at the interface between the steel plate (base iron) and the plating layer) It has been confirmed that a galvanized steel sheet having desired properties can be provided by constructing a plating layer to have this concentration gradient, and the present invention has been completed.

Hereinafter, the present invention will be described in detail.

A galvanized steel sheet excellent in plating adhesion and corrosion resistance according to an aspect of the present invention may include a steel sheet and a zinc plated layer formed on at least one surface of the steel sheet.

In the present invention, the type of the steel sheet is not particularly limited, and may be, for example, a Fe-based steel sheet used as a substrate of a conventional galvanized steel sheet, that is, a hot rolled steel sheet or a cold rolled steel sheet. However, in the case of a hot-rolled steel sheet, it has a large amount of oxidation scale on its surface, and this oxidation scale has a problem of degrading plating adhesion and deteriorating plating quality. It is more preferable. For example, carbon steel, ultra-low carbon steel, and high manganese steel used as materials for automobiles may be mentioned.

Meanwhile, the zinc plated layer may be formed on one side or both sides of the steel sheet.

The zinc plating layer may include aluminum (Al): 5.1 to 35.0%, magnesium (Mg): 4.0 to 25.0%, residual zinc (Zn) and other unavoidable impurities in weight percent, which may include Al, Mg, It can be formed from a plating bath containing the balance Zn and other inevitable impurities.

Mg in the galvanized layer is an element that plays a very important role in improving the corrosion resistance of the plating layer, and Mg contained in the plating layer suppresses the growth of zinc oxide-based corrosion products with little effect of improving corrosion resistance in harsh corrosion environments, and is dense and improves corrosion resistance. The highly effective zinc hydroxide-based corrosion product is stabilized on the surface of the plating layer.

If the content of Mg is less than 4.0%, the effect of improving corrosion resistance due to the production of a Zn-Mg-based compound cannot be sufficiently obtained, whereas when the content exceeds 25.0%, the effect of improving corrosion resistance is saturated, while the Mg oxidative dross is a plating bath. There is a problem that is excessively generated in the bath.

Therefore, in the present invention, it is preferable that Mg is contained in the zinc plating layer at 4.0 to 25.0%. The Mg may be more advantageously contained at 5.1% or more, and the Mg may be more advantageously contained at 9.0% or less.

Al in the galvanized layer is added for the purpose of reducing the dross generated by the Mg oxidation reaction in the molten zinc alloy plating bath to which Mg is added, and Al is combined with Zn and Mg to improve the corrosion resistance of the plated steel sheet. It is also advantageous.

When the Al content is less than 5.1%, the effect of preventing oxidation of the surface layer portion of the plating bath by adding Mg is insufficient, and an effect of improving corrosion resistance cannot be sufficiently obtained. On the other hand, when the content exceeds 35.0%, the Fe dissolution amount of the steel sheet immersed in the plating bath increases rapidly to form a Fe alloy-based dross, and furthermore, a Zn/Al binary vacancy phase in the plating layer is formed to form the cross-section and the coating section It lowers the effect of improving the corrosion resistance of Mg.

Therefore, in the present invention, it is preferable to include Al in the galvanized layer at 5.1 to 35.0%, and more advantageously it may be included at 11 to 15%.

As described above, in the present invention, the zinc plating layer may be referred to as a Zn-Al-Mg-based alloy plating layer by containing a certain amount of Al, Mg and Zn and inevitable impurities, and a thickness of 20 to 40 μm, preferably It may have a thickness of 20 to 35㎛.

The galvanized steel sheet of the present invention may include an Al-Fe suppression layer having a thickness of 0.01 to 15 µm at the interface between the base steel sheet and the zinc plating layer, and the Al-Fe suppression layer is preferably a FeAl ₃ alloy phase.

Specifically, the Al-Fe-based suppression layer is interposed between the base iron and the plating layer, and may serve to impart adhesion between the base iron and the alloy plating layer. Particularly, when the galvanized steel sheet is processed, the Al-Fe-based suppression layer prevents peeling of the plating layer, thereby improving the workability.

Meanwhile, the Al-Fe suppression layer preferably has a thickness variation of 0.01 to 3 μm. When the thickness variation of the Al-Fe suppressing layer exceeds 3 µm, the suppressing layer is formed discontinuously, so that the effect of plating adhesion by the suppressing layer cannot be sufficiently obtained. It is most preferable that the Al-Fe suppression layer has a thickness variation of 0, but the present invention limits the lower limit of the thickness variation to 0.01 µm in consideration of this, as Fe is diffused from the base iron into the plating layer as described later. Can be.

The zinc plated layer of the present invention includes a first region having a Fe content of 40 to 95% and a second region having a Fe content of 0.01% or more and less than 40%, and the Fe is diffused (eluted) from the iron into the plating layer. It mainly includes.

The first region and the second region are formed on the Al-Fe suppression layer, of which the first region is mainly adjacent to the iron in the zinc plating layer, and the second region is mainly the zinc plating layer. It exists adjacent to the surface. Accordingly, the second region may be formed in an area fraction of 0.01 to 40% in the surface layer portion of the zinc plated layer.

Referring to FIG. 1, in the drawing showing an example of the invention, a portion that is continuously darkened at the interface between the base iron and the plating layer is the first region, and the remaining region up to the surface layer of the plating layer except for this is the second region.

In the present invention, the first region and the second region in the galvanized layer may be formed over the entire thickness of the plating layer, but the size, shape, and fraction (the fraction occupied by the cross-section or surface of the plating layer) are not particularly limited. According to the plating conditions suggested in the invention, it is revealed that the regions according to the Fe content will be formed as described above.

As described above, the zinc-plated layer of the present invention, despite the diffusion of Fe from the base iron into the plating layer, the Al-Fe suppression layer is continuously formed at the interface between the base iron and the plating layer, as well as excellent plating adhesion, Fe diffusion By (elution), the surface properties of the plated layer are improved to have a surface roughness (Ra) of 3 to 4 μm.

That is, by having a high surface roughness (Ra) compared to the existing zinc plated material, the friction and lubrication properties are improved, which is advantageous in improving the workability during subsequent processing.

Hereinafter, a method of manufacturing a galvanized steel sheet having excellent plating adhesion and corrosion resistance according to another aspect of the present invention will be described in detail.

The zinc-plated steel sheet of the present invention comprises the steps of preparing a zinc plating bath together with the steel sheet; Preparing a galvanized steel sheet by immersing the steel sheet in the zinc plating bath and performing plating; And cooling the galvanized steel sheet.

At this time, the zinc plated layer may be formed on one or both sides of the steel sheet.

The steel sheet is as mentioned above, the zinc plating bath is aluminum (Al): 5.1 to 35.0%, magnesium (Mg): 4.0 to 25.0%, the balance by weight to obtain the zinc plating layer intended in the present invention. It is preferred to include zinc (Zn) and other unavoidable impurities. More advantageously, the Al may be included in an amount of 11 to 15%. In addition, more advantageously, Mg may be included at 5.1% or more, and may be included at 9.0% or less.

In the present invention, plating is performed by immersing the steel sheet in a zinc plating bath having the above-described alloy composition, the temperature of the zinc plating bath is greater than 555°C to less than 600°C, and the inlet temperature of the steel sheet is greater than 565°C~ It is characterized by being less than 600°C.

In general, in the case of manufacturing a galvanized steel sheet, the temperature of the plating bath is controlled to a temperature not higher than the melting point and not more than 500°C, whereas in the present invention, the intended plating layer can be formed by controlling the temperature of the galvanizing bath relatively high. It is.

Specifically, the present invention provides sufficient thermal energy to form the Al-Fe suppression layer at the interface between the base iron and the plating layer while dipping the steel plate in the zinc plating bath while simultaneously allowing the Fe in the iron to diffuse into the plating layer. It is necessary to do this, and this cannot be achieved when the temperature of the galvanizing bath is controlled to 555°C or less, or when the inlet temperature of the steel sheet is controlled to 565°C or less. That is, the Al-Fe suppressing layer is discontinuously formed, or the diffusion of Fe into the plating layer does not sufficiently occur, so that a zinc plated layer having the intended properties cannot be obtained.

On the other hand, if the temperature of the zinc plating bath is 600°C or higher, the possibility that the inside of the steel sheet and the plating bath is eroded will lead to a reduction in the life of the equipment. In addition, if the temperature of the zinc plating bath is too high, or if the inlet temperature of the steel sheet is 600° C. or more, there is a problem in that the surface of the plating material becomes defective due to an increase in Fe alloy dross.

More preferably, the inlet temperature of the steel sheet can be controlled to 5 to 20°C higher than the zinc plating bath temperature.

The present invention can be carried out with a plating adhesion amount of 130 to 180 g/m ² in plating according to the above, from which a galvanized layer having a thickness of 20 to 40 μm can be obtained.

According to the above, the plating material obtained after completing the plating in the zinc plating bath may be cooled, and in the present invention, cooling may be performed stepwise in order to obtain a zinc plated layer having the above-described first and second regions.

Specifically, the cooling may include a first cooling step of cooling at a cooling rate of 0.01 to 5°C/s to 230 to 270°C and a second cooling step at a cooling rate of 0.05 to 20°C/s to room temperature.

In the present invention, the solid-liquid phase is properly formed while sufficiently solidifying the single phase through the first cooling process, and then the cooling rate during the second cooling is relatively increased compared to the first cooling to completely solidify.

In the present invention, a method of cooling including moisture during cooling is excluded, and gas can be preferably injected.

At this time, gas is sprayed on both the front and back surfaces of the plating material, and a desired cooling rate can be secured through pressure control of the gas. For example, an inert gas such as nitrogen or argon may be used as the gas.

On the other hand, prior to performing the cooling may further include the step of gas wiping the plating material is formed a plating layer. The gas wiping is a process for adjusting the plating adhesion amount, and the method is not particularly limited.

At this time, air or nitrogen may be used as a gas to be used, and nitrogen is more preferable. This is because when using air, Mg oxidation occurs preferentially on the surface of the plating layer, which may cause surface defects in the plating layer.

By completing the above-described series of processes, while including an Al-Fe suppression layer continuously formed at the interface between the base iron and the plating layer, a zinc plating layer including the above-described first and second regions in the plating layer is formed on one aspect of the present invention. The galvanized steel sheet can be obtained.

The galvanized steel sheet of the present invention not only has excellent plating adhesion and corrosion resistance, but also has an effect of improving workability during subsequent processing as the frictional properties of the surface of the plating layer are improved.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and reasonably inferred therefrom.

(Example)

Prepare a cold rolled steel sheet (0.0016%C-0.081%Mn-0.002%Si-0.0091%P-0.0043%S-0.036%Sol.Al) having a thickness of 1.0mm, a width of 110mm, and a length of 200mm as a test piece for plating. Each galvanized steel sheet was manufactured by plating under the conditions of Table 1 below. At this time, the cooling after plating was performed by spraying nitrogen on the front and back surfaces of the steel sheet, the first cooling was terminated at 250°C, and the second cooling was performed to room temperature.

division	Plating bath composition (% by weight)		Plating condition				Plating layer thickness (㎛)
	Al	Mg	Plating bath temperature (℃)	Inlet temperature (℃)	1st cooling rate (℃/s)	2nd cooling rate (℃/s)
Inventive Example 1	24	9	557	567	2.9	10	32
Inventive Example 2	24	4	557	567	2.9	10	28
Inventive Example 3	24	5.1	557	567	2.9	10	29
Comparative Example 1	24	4.5	545	560	2.9	10	34
Comparative Example 2	24	5.5	550	565	2.9	10	36
Comparative Example 3	24	3	557	567	2.9	10	25
Comparative Example 4	24	3.8	557	567	2.9	10	27
Comparative Example 5	24	3	555	565	2.9	10	29
Comparative Example 6	24	3.8	555	565	2.9	10	28

(The remainder of the plating bath composition in Table 1 is Zn and inevitable impurities.)

For each galvanized steel sheet prepared according to the above, it was evaluated for plating adhesion, surface properties and corrosion resistance.

First, in the case of Inventive Examples 1-3 in which not only the composition of the plating bath but also the conditions of the manufacturing process of the plated steel sheet satisfy the scope of the present invention, it is possible to form a continuous Fe-Al suppression layer at the interface between the base iron and the plating layer, and the illuminance increases According to this, it is possible to maintain the friction/lubricating properties, so it is possible to effectively suppress the plating layer drop-off during processing, thereby providing excellent corrosion resistance.

On the other hand, although the composition of the plating bath is within the scope of the present invention, the manufacturing process conditions of the plated steel sheet are out of the scope of the present invention. -It was difficult to form an Al suppression layer. Therefore, the friction/lubricating properties of the plating layer decreased as the roughness decreased, resulting in cracks of branches in the plating layer after processing, and the corrosion resistance was low due to the dropping of the plating layer.

In addition, Comparative Example 3-4 had a low Mg content compared to the Inventive Example, so that the corrosion resistance of the surface and the cross section was poor.

In addition, in Comparative Example 6, in which the composition components of the plating bath as well as the manufacturing process conditions were outside the scope of the present invention, it was difficult to form a continuous Fe-Al suppression layer due to low thermal energy required for Fe diffusion of the steel sheet. Therefore, the friction/lubricating properties of the plating layer decreased as the roughness decreased, resulting in cracks of branches in the plating layer after processing, and the corrosion resistance was low due to the dropping of the plating layer. Furthermore, the amount of Mg-Zn vacancy phase formation was small, and the corrosion resistance of the surface of the plating layer and the cross-section was reduced.

On the other hand, in order to observe the cross section of the galvanized steel sheet, it was cut in the vertical direction (thickness direction) in the rolling direction, and then observed using a scanning electron microscope (SEM). 1 is a view showing a cross-sectional view of the plating layer of the invention example 1 and comparative example 5 in the embodiment of the present invention.

As shown in FIG. 1, it can be confirmed that Fe-Al suppressing layer was present between the base iron and the plating layer in both of Inventive Example 1 and Comparative Example 5, and Fe diffusion occurred into the plating layer. However, in the case of the comparative example 5, it is predicted that the adhesion of plating will decrease as the diffusion of Fe occurs and the Fe-Al suppressing layer is discontinuously formed.

On the other hand, in observing the cross section of the galvanized steel sheet, the cut surface is FIB-processed, and the plate is coated with platinum, gold, or carbon to protect the processed section. 1 is a part of the area coated on the flat plate is observed together.

In addition, in order to evaluate the plating adhesion of the galvanized steel sheet, a sealer-bending test was performed, and the results are shown in FIG. 2. Figure 2 shows the results of the sear-bending test (sealer-bending test) of Example 1 and Comparative Example 5 in the Examples of the present invention (here, scale bar (scale bar) is 2mm). The sealer bending experiment was evaluated to the extent that the plated layer was exposed to the mastic sealer portion of the bent portion by bending the plate portion 90 degrees after applying adhesive to the plated layer of the mastic sealer (purple) and the plate. At this time, a picture of the mastic sealer portion taken with an optical microscope is shown in FIG. 2.

As shown in FIG. 2, in the case of Inventive Example 1, peeling of the plating layer did not occur, whereas in Comparative Example 5, it was confirmed that the plating layer was peeled.

And, after the bending test of the galvanized steel sheet was observed whether or not cracks in the outer zone, the results are shown in Figure 3. Figure 3 shows a photograph of the shape of the crack of the outer region after the bending test of the invention example 1 and comparative example 5 in the embodiment of the present invention. At this time, the bending test is shown in FIG. 3 a picture taken by an electron scanning microscope after bending the plating material itself 180 degrees.

As shown in Fig. 3, inventive example 1, the shape of the crack is formed in parallel in one direction, while in comparison 5, the fracture is changed to a branch-type in the behavior formed in parallel in one direction, and Crack propagation behavior can be observed. It is expected that these results will affect the corrosion resistance of the processed area.

In addition, in order to confirm the surface characteristics of the galvanized steel sheet, the galvanized steel sheet was scanned in 3D roughness, and the results are shown in FIG. 4. Figure 4 shows the results of 3D roughness scan of the surfaces of Inventive Example 1 and Comparative Example 5 in the Examples of the present invention.

As shown in FIG. 4, it can be seen that Fe diffusion occurs throughout the plating layer, and in the case of Inventive Example 1 in which the layers were separated according to the Fe concentration gradient in the plating layer, roughness and surface roughness were significantly improved compared to Comparative Example 5.

Furthermore, FIG. 5 shows a photograph of the surface of the plated steel sheet of Inventive Example 1 and Comparative Example 5 in 1200 hours after the salt spray test for 1200 hours. At this time, 5 vol.% of NaCl brine (35° C.) was placed in the chamber, and the brine was sprayed at 1.55 ml per hour on each plating material (150×70 (mm ² ) sized specimen) to generate red rust. The corrosion resistance was evaluated.

As shown in FIG. 5, while the corrosion resistance of Example 1 was maintained for 1200 hours in a corrosive environment, it was confirmed that corrosion occurred significantly in Comparative Example 5.

Claims (10)

1. A steel plate and a zinc plated layer formed on at least one surface of the steel plate,

   The zinc plated layer contains aluminum (Al): 5.1 to 35.0%, magnesium (Mg): 4.0 to 25.0%, residual zinc (Zn) and other unavoidable impurities in weight percent.

   A galvanized steel sheet having excellent plating adhesion and corrosion resistance, comprising an Al-Fe suppression layer having a thickness of 0.01 to 15 µm at the interface between the base steel sheet and the zinc plated layer.
2. According to claim 1,

   The zinc plating layer includes a first region having a Fe content of 40 to 95% and a second region having a Fe content of 0.01% or more and less than 40%,

   The second region is a galvanized steel sheet having excellent plating adhesion and corrosion resistance, which is formed in an area fraction of 0.01 to 40% in the surface layer portion of the galvanized layer.
3. According to claim 1,

   The zinc plated layer is a galvanized steel sheet having excellent adhesion to plating and corrosion resistance, including aluminum (Al): 11-15%, magnesium (Mg): 5.1-9.0%, residual zinc (Zn) and other inevitable impurities in weight percent.
4. According to claim 1,

   The zinc plated layer is a galvanized steel sheet having a surface roughness (Ra) of 3 to 4 µm with excellent plating adhesion and corrosion resistance.
5. Preparing a zinc plating bath containing aluminum (Al): 5.1-35.0%, magnesium (Mg): 4.0-25.0%, residual zinc (Zn) and other inevitable impurities by weight;

   Preparing a galvanized steel sheet by immersing the steel sheet in the zinc plating bath and performing plating; And

   Cooling the galvanized steel sheet,

   A method of manufacturing a galvanized steel sheet having excellent plating adhesion and corrosion resistance, characterized in that the temperature of the zinc plating bath is greater than 555°C to less than 600°C, and the inlet temperature of the steel sheet is greater than 565°C to less than 600°C.
6. The method of claim 5,

   A method for manufacturing a zinc plated steel sheet having excellent plating adhesion and corrosion resistance, wherein the temperature of the steel sheet is 5 to 20°C higher than the temperature of the zinc plating bath.
7. The method of claim 5,

   The cooling is a galvanized steel sheet having excellent plating adhesion and corrosion resistance, including cooling at a cooling rate of 0.01 to 5°C/s to 230 to 270°C and cooling at a cooling rate of 0.05 to 20°C/s to room temperature. Method of manufacturing.
8. The method of claim 5,

   The cooling is performed by spraying a gas, and the method of manufacturing a galvanized steel sheet excellent in plating adhesion and corrosion resistance.
9. The method of claim 5,

   Method of manufacturing a galvanized steel sheet excellent in plating adhesion and corrosion resistance further comprising the step of gas-wiping the galvanized steel sheet before the cooling.
10. The method of claim 5,

    The zinc plating bath is aluminum by weight (11% to 15% by weight), magnesium (Mg): 5.1% to 9.0%, zinc zinc plated steel sheet with excellent adhesion to plating and corrosion resistance, including residual zinc (Zn) and other inevitable impurities. Method of manufacturing.

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