KR20220001007A - Zinc alloy plated steel sheet with excellent bending processability and its manufacturing method - Google Patents

Abstract

Disclosed are a zinc alloy-plated steel sheet and a manufacturing method thereof. The zinc alloy-plated steel sheet comprises a matrix steel sheet and a zinc alloy-plated layer. The zinc alloy-plated layer includes a Zn-Al-Mg-based intermetallic compound and a Zn single-phase structure as a microstructure. The preference orientation degree (f) of the Zn single-phase structure expressed by the following relation formula 1 is 50% or higher. Relation formula 1 is f(%)=(Ibasal/Itotal) × 100, wherein total represents a value resulting from integrating all Zn single-phase diffraction peaks when a Cu-Kα source is used to measure an X-ray diffraction pattern to a 2theta of 10-100°, and Ibasal represents a value resulting from integrating Zn single-phase diffraction peaks associated with a basal plane.

Description

Zinc alloy plated steel sheet with excellent bending processability and its manufacturing method

The present invention relates to a zinc alloy plated steel sheet having excellent bending workability and a method for manufacturing the same.

The zinc plating method, which suppresses corrosion of iron through cathodic corrosion, has excellent corrosion resistance performance and economical efficiency, and is widely used to manufacture steel materials with high corrosion resistance. In particular, hot-dip galvanized steel sheet, which forms a plating layer by immersing steel in molten zinc, has a simpler manufacturing process and lower product price than electro-galvanized steel sheet, so it is widely used in automobiles, home appliances and building materials. Its demand is increasing. Zinc-coated hot-dip galvanized steel sheet has the characteristic of sacrificial corrosion protection, in which zinc, which has a lower oxidation-reduction potential than iron, corrodes first when exposed to a corrosive environment, thereby suppressing corrosion of the steel sheet. As this is oxidized, a dense corrosion product is formed on the surface of the steel sheet to block the steel material from the oxidizing atmosphere, thereby improving the corrosion resistance of the steel sheet. However, the increase in air pollution and the deterioration of the corrosive environment according to industrial advancement are increasing, and due to strict regulations on resource and energy saving, the need for the development of steel materials having better corrosion resistance than conventional galvanized steel sheets is increasing. As part of this, various researches have been made on a zinc alloy-based plated steel sheet manufacturing technology that improves the corrosion resistance of steel by adding elements such as aluminum (Al) and magnesium (Mg) to the galvanizing bath. As a representative zinc alloy-based plating material, research on the Zn-Al-Mg-based zinc-alloy-plated steel sheet manufacturing technology in which Mg is additionally added to the Zn-Al plating composition is being actively researched. However, such a Zn-Al-Mg-based zinc alloy plated steel sheet has a disadvantage in that bending workability is poor. That is, the zinc alloy plated steel sheet contains a large amount of Zn-Al-Mg-based intermetallic compounds formed by thermodynamic interaction of Zn, Al and Mg in the plating layer. It causes cracks, which has a disadvantage in that bending workability is deteriorated.

One of the several objects of the present invention is to provide a zinc alloy plated steel sheet excellent in bending workability and a method for manufacturing the same. The subject of the present invention is not limited to the above. Additional problems of the present invention are described in the overall content of the specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the specification of the present invention.

In one aspect of the present invention, in a zinc alloy plated steel sheet including a base steel sheet and a zinc alloy plated layer, the zinc alloy plated layer includes a Zn single-phase structure and a Zn-Al-Mg-based intermetallic compound as a microstructure, and the Zn To provide a zinc alloy plated steel sheet having a preferential orientation (f) of 50% or more, which is expressed by the following relational formula (1) of a single-phase structure. [Relational Expression 1] f(%)=(Ibasal/Itotal) × 100 (Here, Itotal is the diffraction of all Zn single phases when the X-ray diffraction pattern is measured from 2theta 10° to 100° using a Cu-Kα source. means a value obtained by integrating the peak, and Ibasal means a value obtained by integrating the diffraction peak of a single Zn phase related to the basal plane) Another aspect of the present invention is the step of preparing a zinc alloy plating bath containing Mg and Al , immersing a base steel sheet in the zinc alloy plating bath, performing plating to obtain a zinc alloy plated steel sheet, gas wiping the zinc alloy plated steel sheet to adjust the plating adhesion amount, zinc alloy plating with the plating adhesion amount adjusted Zinc alloy plating in which droplets of water or aqueous solution are sprayed on the steel sheet to cool, and then air-cooled, wherein, during the droplet spraying, the droplet spraying start temperature is 405 to 425°C, and the droplet spraying end temperature is 380 to 400°C A method for manufacturing a steel sheet is provided.

As one of the various effects of the present invention, the zinc alloy plated steel sheet according to an embodiment of the present invention has not only excellent corrosion resistance, but also excellent bending workability. In addition, as one of the various effects of the present invention, the zinc alloy plated steel sheet according to an embodiment of the present invention has an advantage in that the surface appearance is excellent. In addition, as one of the various effects of the present invention, the zinc alloy plated steel sheet according to an embodiment of the present invention has an excellent scratch resistance.

Hereinafter, a zinc alloy plated steel sheet excellent in bending workability, which is an aspect of the present invention, will be described in detail. A zinc alloy plated steel sheet as an aspect of the present invention includes a base steel sheet and a zinc alloy plated layer. In the present invention, the type of the base steel sheet is not particularly limited, and for example, it may be a hot-rolled steel sheet or a cold-rolled steel sheet used as a base for a conventional zinc alloy plated steel sheet. However, in the case of a hot-rolled steel sheet, it has a large amount of oxide scale on its surface, and there is a problem that such oxide scale lowers plating adhesion and deteriorates plating quality. more preferably. On the other hand, the zinc alloy plating layer may be formed on one side or both sides of the steel sheet. The zinc alloy plating layer may include, by weight, Al: 0.5 to 3%, Mg: 0.5 to 3%, the balance Zn and unavoidable impurities. Mg reacts with Zn and Al in the zinc alloy plating layer to form a Zn-Al-Mg-based intermetallic compound, and thus plays a very important role in improving the corrosion resistance of the plated steel sheet. If the content is too low, the microstructure of the plating layer Since a sufficient amount of the Zn-Al-Mg-based intermetallic compound cannot be secured, there is a fear that the effect of improving the corrosion resistance may not be sufficient. Accordingly, the Mg in the zinc alloy plating layer may be included in 0.5 wt% or more, preferably in 1.0 wt% or more. However, when the content is excessive, the effect of improving corrosion resistance is not only saturated, but there is a fear that Mg oxide-related dross is formed in the plating bath, thereby deteriorating plating properties. In addition, there is a possibility that an excessively large amount of a Zn-Al-Mg-based intermetallic compound having a high hardness in the microstructure of the plating layer is formed, thereby deteriorating the bendability. Therefore, the Mg in the zinc alloy plating layer may be included in 3 wt% or less, preferably 2.9 wt% or less. The Al suppresses the formation of Mg oxide dross and reacts with Zn and Mg in the plating layer to form a Zn-Al-Mg-based intermetallic compound, which is an element that plays a very major role in improving the corrosion resistance of the plated steel sheet. If this is too low, the ability to inhibit Mg dross formation is insufficient and a sufficient amount of Zn-Al-Mg-based intermetallic compound cannot be secured in the microstructure of the plating layer, so there is a risk that the effect of improving corrosion resistance may not be sufficient. Al may be included in 0.5 wt% or more, preferably 0.6 wt% or more. However, if the content is excessive, the effect of improving corrosion resistance is not only saturated, but there is a risk that the plating bath temperature rises to adversely affect the durability of the plating apparatus. Moreover, there is a fear that an excessively large amount of a Zn-Al-Mg-based intermetallic compound having high hardness in the microstructure of the plating layer is formed, thereby deteriorating the bendability. Accordingly, the Al in the zinc alloy plating layer may be included in 3 wt% or less, preferably in 2.6 wt% or less. According to one example, the content of Mg and Al contained in the zinc alloy plating layer may satisfy Relational Equation 1 below. When [Mg]/[Al] is 1.0 or less, scratch resistance may deteriorate, whereas when [Mg]/[Al] exceeds 4.0, a large amount of Mg-based dross is generated in the hot-dip plating bath and workability This may deteriorate. [Relational Expression 1] 1.0 < [Mg]/[Al] ≤ 4.0 (here, [Mg] and [Al] each mean the weight % of the corresponding element) The zinc alloy plated layer has a Zn single-phase structure and a Zn -Al-Mg-based intermetallic compound may be included. In the present invention, the type of the Zn-Al-Mg-based intermetallic compound is not particularly limited, but for example, Zn/Al/MgZn2 ternary eutectic structure, Zn/MgZn2 binary eutectic structure, Zn-Al binary eutectic structure It may be at least one selected from the group consisting of a tissue and a MgZn2 single-phase tissue. The present inventors have studied in depth to improve the bendability of the zinc alloy plated steel sheet. As a result, the Zn single phase structure having a hexagonal close packing (HCP) among the microstructures of the zinc alloy plated layer is grown in an orientation. , it has been found that the slip is easy and the ductility is increased, which can significantly reduce the occurrence of cracks during bending. In order to obtain such an effect in the present invention, it is preferable to control the preferred orientation degree expressed by the following relation 1 of the Zn single-phase structure to 50% or more, and more preferably to control it to 60% or more. [Relational Expression 1] f(%)=(Ibasal/Itotal) × 100 (Here, Itotal is the diffraction of all Zn single phases when the X-ray diffraction pattern is measured from 2theta 10° to 100° using a Cu-Kα source. It means a value obtained by integrating the peak, and Ibasal means a value obtained by integrating the diffraction peak of the Zn single phase related to the basal plane) In addition, the present inventors have developed a method for refining the size of the coarsely formed Zn single phase structure in the zinc alloy plating layer. It was also found that it helps to reduce the occurrence of cracks during bending. In order to obtain the desired effect in the present invention, it is preferable to control the average particle diameter of the Zn single-phase structure to 15 μm or less, more preferably to control it to 12 μm or less, and even more preferably to control it to 10 μm or less. Here, the average particle diameter of the single-phase Zn structure means the average equivalent circular diameter of the single-phase Zn structure detected by observing the cross section in the thickness direction of the plating layer. The zinc alloy plated steel sheet according to the present invention has not only excellent corrosion resistance, but also very excellent bending workability. According to one example, the surface appearance of the zinc alloy plated steel sheet according to the present invention may also be very excellent, and more specifically, the number of black spots per unit area on the surface of the zinc alloy plated steel sheet is 0.1 pieces/cm2 or less. can In order to obtain such an effect in the present invention, it is preferable that the area fraction of the Zn single-phase structure observed on the surface of the zinc alloy plated layer is 40% or less (excluding 0%). That is, by maximizing the fraction of the Zn-Al-Mg-based intermetallic compound observed on the surface of the zinc alloy plating layer, the surface appearance can be maximized. According to an example, the zinc alloy plated steel sheet according to the present invention may also have very excellent scratch resistance. As a result of the research of the present inventors, when the area fraction of the layered Zn/MgZn2 binary eutectic structure and the Zn/Al/MgZn2 ternary eutectic structure observed on the surface of the zinc alloy plating layer is maximized, the scratch resistance can be significantly improved. have. In order to obtain the desired effect in the present invention, the sum of the area fractions of the Zn/MgZn2 binary eutectic structure and the Zn/Al/MgZn2 ternary eutectic structure observed on the surface of the zinc alloy plating layer is 50% or more (excluding 100%) and the area fraction of the MgZn2 single-phase structure is preferably 10% or less (including 0%). MgZn2 single-phase structure has high hardness, which causes cracks during processing. Therefore, it is desirable to reduce the area fraction as much as possible. The zinc alloy plated steel sheet of the present invention described above can be manufactured by various methods, and the manufacturing method is not particularly limited. However, as a preferred example, when the molten zinc alloy plating layer is solidified by spraying droplets on its surface to cool, and then air-cooling, the preferential orientation and average particle diameter as described above can be obtained. In this case, the droplet spraying may be electrifying spraying so that the droplets are attached by electrostatic attraction with the zinc alloy plated steel sheet. Such electrification not only helps to form droplets finely and uniformly, but also reduces the amount of ejection after the ejected droplets collide with the surface of the zinc alloy plated steel sheet, thereby rapidly cooling the molten zinc alloy plated layer. This is advantageous for growth and miniaturization of the (0001) orientation of single-phase Zn structures. The droplet may be an aqueous solution of phosphate, and the aqueous solution of phosphate is effective for oriented growth and miniaturization of a Zn single-phase structure by rapidly cooling a molten zinc alloy plated layer by an endothermic reaction. For example, ammonium hydrogen phosphate ((NH4)2HPO4) aqueous solution, sodium ammonium hydrogen phosphate (NaNH4HPO4) aqueous solution, primary zinc phosphate (Zn(H2PO4)2) aqueous solution, calcium phosphate (Ca3(PO4)2) aqueous solution, etc. are mentioned. can In addition, the concentration of the phosphate aqueous solution may be 1 to 3% by weight. If the concentration of the phosphate aqueous solution is less than 1% by weight, there is a fear that the effect may not be sufficient. may cause In addition, during the droplet injection, the droplet injection start temperature may be 405 ~ 425 ℃, more preferably 410 ~ 420 ℃. At this time, the droplet injection start temperature means the surface temperature of the zinc alloy plated steel sheet at the time of starting the droplet injection. If the droplet injection start temperature is less than 405°C, solidification of the Zn single phase has already started and there is a risk of causing black spots on the surface of the galvanized steel sheet. On the other hand, if it exceeds 425°C, the endothermic reaction by droplet injection is effective Otherwise, it may be difficult to secure the desired organization. In addition, during the droplet injection, the droplet injection end temperature may be 380 ~ 400 ℃, more preferably 390 ~ 400 ℃. In this case, the droplet injection termination temperature means the surface temperature of the zinc alloy plated steel sheet at the time of termination of the droplet injection. If the droplet injection end temperature exceeds 400℃, the endothermic reaction by the droplet injection is not effective, so it may be difficult to secure the desired tissue. On the other hand, if it is less than 380℃, the Zn/MgZn2 binary process and During the initiation of solidification of the /MgZn2 three-way phase, the Mg2Zn11 phase is induced by supercooling, and there is a fear that the preferential orientation of the Zn single-phase structure may be lowered due to the occurrence of a large number of black spots. In addition, during the droplet injection, the difference between the droplet injection start temperature and the droplet injection end temperature may be 15° C. or more. If the difference in temperature is less than 15°C, the endothermic reaction by droplet injection is not effective, so it may be difficult to secure the desired tissue. In addition, when the droplet is sprayed, the spraying amount of the droplet may be 50-100 g/m2. If the injection amount is less than 50 g/m 2 , there is a fear that the effect may be insufficient. On the other hand, if it exceeds 100 g/m 2 , the effect is saturated, which is not preferable. Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only for exemplifying and specifying the present invention, and not for limiting the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

Claims (1)

In the zinc alloy plated steel sheet comprising a base steel sheet and a zinc alloy plated layer, the zinc alloy plated layer includes a Zn single-phase structure and a Zn-Al-Mg-based intermetallic compound as a microstructure, A zinc alloy plated steel sheet having an expressed preferential orientation (f) of 50% or more. [Relational Expression 1] f(%)=(Ibasal/Itotal) × 100 (Here, Itotal is the diffraction of all Zn single phases when the X-ray diffraction pattern is measured from 2theta 10° to 100° using a Cu-Kα source. It means the value obtained by integrating the peak, and Ibasal means the value obtained by integrating the diffraction peak of the Zn single phase related to the basal plane)