KR20200138704A - Anti corrosive coated steel having good resistance against liquid metal embrittlement and coating adhesion - Google Patents

Abstract

The present invention relates to a high anti-corrosive coated steel sheet with excellent resistance against welding liquefaction brittleness and coating adhesion. According to one aspect of the present invention, the high anti-corrosive coated steel sheet comprises: a basic steel sheet; and a coated layer sequentially containing an Fe-Al alloy layer and a MgZn_2 layer from an interface between the basic steel sheet and the coated layer.

Description

High corrosion resistance plated steel sheet with excellent resistance to welding liquefaction embrittlement and plating adhesion {ANTI CORROSIVE COATED STEEL HAVING GOOD RESISTANCE AGAINST LIQUID METAL EMBRITTLEMENT AND COATING ADHESION}

The present invention relates to a highly corrosion-resistant plated steel sheet having excellent resistance to welding liquefaction embrittlement and plating adhesion.

Recently, as interest in weight reduction of automobiles and passenger safety increases, interest in high-strength steel sheets for automobiles having a strength of 900 MPa or more is increasing. In order to satisfy these needs, high-strength steel sheets containing large amounts of elements such as C, Si, Mn, Al, Ti, and Cr have recently been developed.

In addition, such a high-strength steel sheet is often required to have high corrosion resistance at the same time, and many plated steel sheets in which the surface of the steel sheet is plated have been proposed. The aluminum-based plated steel sheet occupies a large proportion of the plated steel sheet, and in the case of the aluminum plated steel sheet, the problem that the sacrificial corrosion resistance of the plating layer is weaker than that of the galvanized steel sheet has been pointed out.

In order to solve this problem, steel materials having improved corrosion resistance by adding Zn or Mg to the aluminum plating layer have been proposed in Patent Documents 1 and 2, and the like. Specifically, Patent Literature 1 discloses alloy plating consisting of 25 to 85 mass% of Al, 0.5% or more and 10% or less of the Al content and Zn for the balance, and Patent Literature 2 discloses Al: 25 A technique in which corrosion resistance is improved by performing an alloy plating consisting of ~75%, Mg: 0.1-10, Si: 1~7.5, Cr: 0.05-5%, and the balance Zn is disclosed.

However, when plating is performed with the plating composition disclosed in the above patent documents, dross is generated in the plating bath, and in order to prevent this, the temperature of the plating bath must be maintained at a high temperature, and as a result, a problem of material deterioration of the steel sheet may occur. In addition, when a large amount of Zn is included in the steel sheet, a problem of welding liquefaction embrittlement may occur that causes cracks due to the penetration of liquefied metal into the grain boundaries during welding, and when Mg is included as in Patent Document 2, the plating adhesion decreases. Can occur.

Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-264188 Patent Document 2: Korean Patent Application Publication No. 2011-0088573

According to one aspect of the present invention, a plated steel sheet having sufficient corrosion resistance and excellent resistance to welding liquefaction embrittlement and plating adhesion can be provided.

The subject of the present invention is not limited to the above. Anyone of ordinary skill in the art to which the present invention pertains will not have any difficulty in grasping the additional subject of the present invention from the overall contents of the present specification.

The highly corrosion-resistant plated steel sheet according to one aspect of the present invention may be formed of a plated layer including an Fe-Al alloy layer and a MgZn ₂ layer sequentially from an interface between the holding steel plate and the holding steel plate.

In addition, a method of manufacturing a highly corrosion-resistant plated steel sheet according to another aspect of the present invention includes: preparing a holding steel sheet; Heating the holding steel sheet to 700 ~ 1050 °C; Obtaining an aluminum plated steel plate by immersing the steel plate in an aluminum-based plating bath maintained at 530 to 750°C; Adjusting the plating amount of the obtained aluminum-based plated steel sheet; Cooling the steel sheet with a controlled plating amount to 350° C. at a cooling rate of 7 to 25° C./sec; And it may include the step of cooling from 350 ℃ to 80 ℃ at a cooling rate of 5 ~ 15 ℃ / sec.

As described above, according to the present invention, by appropriately controlling the interface between the plating layer and the holding steel sheet and the alloy phase formed in the plating layer, it is possible to improve welding liquefaction brittle resistance and plating adhesion while maintaining excellent corrosion resistance.

1 is a SEM photograph showing a cross section of an aluminum-based hot-dip plated steel sheet manufactured according to Inventive Example 3, and the left side shows the results observed at low magnification and the right side shows the results observed at high magnification.
2 is a cross-sectional photograph of a steel sheet manufactured according to Inventive Example 1.
3 is a cross-sectional photograph of a steel sheet manufactured according to Comparative Example 4-3.
4 is a cross-sectional photograph of a steel sheet manufactured according to Comparative Example 5-4.
5 is a photograph showing the results of a plating adhesion test on the steel sheets manufactured according to the inventive examples and comparative examples of the present invention.

Hereinafter, the present invention will be described in detail.

In the case of controlling the alloy layer formed by adding an alloy component such as Zn or Mg in the plating layer of a plated steel sheet, especially an aluminum-based plated steel sheet, the inventors of the present invention improve the sacrificial corrosion resistance and suppress the propagation of corrosion while forming a welding liquid. It was discovered that not only the resistance to brittleness could be improved, but also plating adhesion could be improved in some cases, and the present invention was reached.

Liquid metal embrittlement (LME for short) refers to a phenomenon in which the plating layer is melted at a high temperature, and the metal of the molten plating layer penetrates into the grain boundaries of the holding steel sheet and generates cracks.

The inventors of the present invention have found that welding liquefaction embrittlement can be improved if the plated layer of the plated steel sheet has a composition including Zn, Mg, Si, etc. and the conditions of each layer included in the plated layer are properly controlled.

That is, the plating layer of the present invention may have several distinct layers. Among them, the portion closest to the base steel sheet, that is, at the interface between the plating layer and the base steel sheet, consists of an alloy phase of Fe and Al (Fe-Al alloy phase). Can have layers. Such an Fe-Al alloy layer is not necessarily limited thereto, but may mainly have a Fe ₂ Al ₅ or FeAl ₃ phase. When the Fe-Al alloy layer is present at the interface between the holding steel sheet and the plating layer, even if the Zn of the plating layer is melted due to high temperature during welding, it is effective in preventing the occurrence of welding liquefaction embrittlement because it can block contact with the molten metal and the steel sheet. In the present invention, the Fe-Al alloy does not necessarily include only Fe and Al, and does not exclude other elements. Therefore, even if some of the additional elements included in the plating bath or the holding steel sheet are included, all cases in which the total content of Fe and Al is 70% or more can be regarded as an Fe-Al alloy. In one embodiment of the present invention, for this purpose, the thickness of the Fe-Al alloy phase may be set to 1 μm or more. Conversely, even when the thickness of the Fe-Al alloy phase is excessively thick, the plating adhesion may decrease due to the characteristics of the hard Fe-Al phase. Therefore, in one embodiment of the present invention, the thickness of the Fe-Al alloy layer may be limited to 8 μm or less.

However, in order to secure resistance to welding liquefaction embrittlement, it is not enough to form an Fe-Al alloy layer.

Therefore, in one embodiment of the present invention, a MgZn ₂ layer is formed directly on the Fe-Al alloy layer. As can be seen in FIG. 1, the Fe-Al phase may have an irregular shape (especially, on the opposite side of the surface in contact with the steel sheet), and the MgZn ₂ phase is formed on the Fe-Al phase by contacting the Fe-Al phase.

When the MgZn ₂ alloy phase is present in the plating layer, the ratio of the Zn alone phase in the plating layer is reduced, and the Fe-Al alloy layer prevents welding liquefaction embrittlement because even if the plating layer is liquefied during welding, the contact between Zn and the holding steel sheet is prevented.

In one embodiment of the present invention, for this effect, the average thickness of the MgZn ₂ phase may be 0.5 μm or more, and most preferably 1 μm or more. In one embodiment of the present invention, the thickness of the MgZn ₂ layer is measured in a direction perpendicular to the interface with the alloy layer (Fe-Al layer) in contact with it, and 3㎛ intervals based on the interface length of the steel plate and the Fe-Al phase. Measured 10 times as an average is set as the average thickness of one time, and the thickness for each time obtained by measuring a total of 5 times for each location is averaged as the final thickness. However, if the thickness is too thick, the workability of the plating layer may be deteriorated. Therefore, the average thickness of the layer including the MgZn ₂ phase may be set to 3.5 μm or less, and in some cases, it may be set to 2.5 μm or less.

In addition, since the MgZn ₂ phase must exist in a large area in contact with the Fe-Al alloy phase in a large area, the ratio of the MgZn ₂ phase to the Fe-Al alloy phase in the plating layer is 90% or more. good. The ratio may be determined as the ratio of the area occupied by the MgZn ₂ phase relative to the total area when a layer formed above MgZn ₂ (eg, an Al alloy layer) is removed and observed. Since MgZn ₂ formed in such a large area was confirmed to be formed directly on the Fe-Al alloy phase, in the present invention, even if it is not specifically confirmed whether another phase is included between the MgZn ₂ and the Fe-Al alloy phase, It can be considered that MgZn ₂ is formed in direct contact with the Fe-Al alloy phase.

An Al alloy layer may be formed on the above-described MgZn ₂ layer (meaning the far side from the holding steel sheet). In the present invention, the Al alloy layer is mainly derived from the components of the plating bath, and refers to the upper layer excluding the Fe-Al alloy layer formed by the alloying reaction of Al with the holding steel sheet or the MgZn ₂ layer formed thereon. This is the main component and refers to an alloy layer having a composition containing elements derived from the plating bath. However, it is not excluded that the Al alloy layer may further contain a small amount of elements derived from the holding steel sheet that can be diffused from the holding steel sheet.

Accordingly, the plated layer of the plated steel sheet of the present invention may include an Fe-Al alloy layer, an MgZn ₂ layer, and an Al alloy layer sequentially from the interface with the holding steel sheet.

In addition, in general, when the aluminum-based plated steel sheet contains Mg and Si, the Mg ₂ Si phase is formed. The Mg ₂ Si phase serves to improve the corrosion resistance of the plating layer, but it is not only hard, but also weakens the adhesion to the steel sheet. When forming at the interface between the plating layer and the plating layer, it is necessary to control it so that it can be formed on the top of the plating layer because it causes a decrease in plating adhesion.

Therefore, in another embodiment of the present invention, the ratio of the Mg ₂ Si phase in the plating layer may be set to 10% or less based on the area. In this case, the ratio of the Mg ₂ Si phase in the plating layer may be determined as the ratio of the area occupied by the Mg ₂ Si phase relative to the entire area of the plating layer when the plated steel sheet is cut and the cut surface is observed. If the area ratio of the Mg ₂ Si phase is excessively high, the ratio of Mg ₂ Si formed at the interface with the steel sheet may increase or the brittleness of the entire plating layer may be increased to cause peeling of the plating layer, so the area ratio of the Mg ₂ Si phase is 10%. It is limited to the following. In addition, in some cases, the area ratio of the Mg ₂ Si phase may be set to 8% or less. In the present invention, the lower limit of the ratio of the Mg ₂ Si phase for securing the plating adhesion is not particularly limited, but since the Mg ₂ Si phase may contribute to the improvement of the corrosion resistance of the plating layer, when additionally considering the corrosion resistance, in one embodiment of the present invention, the above The ratio of the Mg ₂ Si phase may be set to 3% or more based on the area, and in some cases, it may be set to 5% or more.

In addition, the Mg ₂ Si phase is advantageously dispersed and present in the upper layer of the plating layer (meaning the upper layer excluding the Fe-Al alloy layer and the MgZn ₂ phase formed directly thereon). For this purpose, the Mg ₂ Si phase has an average size. It is preferable that the (average of the length of the major axis of each particle) is 6 μm or less (excluding 0 μm). In terms of preventing peeling of the plating layer, the smaller the average size is, the more advantageous the lower limit of the size is not specifically set. However, since Mg ₂ Si having a size greater than or equal to a certain level may contribute to improvement of corrosion resistance, an average size of the Mg ₂ Si phase may be set to 3 μm or more in consideration of this in one embodiment of the present invention. For the purpose of the present invention, if the Mg ₂ Si particles are not separated by other phases and are continuous, they can be treated as one particle and the average size can be measured. As described above, since Mg ₂ Si exists in a dispersed form rather than formed as a separate layer, it is not indicated as a specially divided layer in the present invention.

In one embodiment of the present invention, the area ratio of the Mg ₂ Si phase in contact with the Fe-Al phase may be 2% or less (the area ratio of the Mg ₂ Si phase in contact with the Fe-Al phase relative to the area of the holding steel sheet). The more the Mg ₂ Si phase in contact with the Fe-Al phase, the less the area in which the MgZn ₂ phase is in contact with the holding steel sheet, thereby lowering the plating adhesion and resistance to welding liquid embrittlement. Therefore, the area ratio of the Mg ₂ Si phase in contact with the Fe-Al phase is 2 Limit to less than %. The ratio may be determined as the ratio of the area occupied by the MgZn ₂ phase relative to the total area when the layer formed above the Mg ₂ Si in contact with the Fe-Al phase (eg, an Al alloy layer) is removed and observed.

According to one embodiment of the present invention, the aluminum-based plating layer may include Si: 5 to 10%, Zn: 5 to 30%, and Mg: 1 to 10% by weight. Hereinafter, the reason why the composition of the plating layer is determined in this way will be briefly described. Unless otherwise specified in the present invention, the content of elements included in the plated layer and the holding steel sheet is based on weight. In addition, it should be noted that in the present invention, the aluminum-based plating layer is a concept including all of the layers as described above, and the composition of the aluminum plating layer means an average composition obtained by analyzing the composition of each layer.

Silicon (Si): 5-10%

Si has the advantage of lowering the melting point through the formation of an alloy of Al, and lowering the melting point of the aluminum ingot through the addition of Si to lower the plating bath temperature. When the plating bath temperature is lowered, it may have the advantage of reducing the amount of solid dross generated due to chemical bonding between Fe and Al eluted into the plating bath and the components of the plating bath composition. Therefore, in one embodiment of the present invention, the Si content may be set to 5% or more. However, when the Si content of the plating layer exceeds 10%, the Si content of the plating bath also increases. As a result, the plating bath temperature of the Al alloy rises again. In addition, Si is not a solid solution in the Al matrix, but forms a needle-shaped Si phase, and has the characteristic of easily forming a secondary phase such as Mg ₂ Si. When the Si content is high, the formability of the plating layer decreases and the adhesion of the plating layer decreases. Occurs and must be properly adjusted. However, if the Si content is less than 5%, the function of suppressing the formation of the Fe-Al alloy phase in the plating layer of Si weakens, and the fraction of the Fe-Al alloy phase becomes excessive, so it must be controlled. A more preferable Si content in the plating layer may be set to 6 to 9%.

Zinc (Zn): 5-30%

Zn lowers the melting point of the plating bath, and acts as a sacrificial method in which the steel sheet is oxidized or corroded more rapidly in a corrosive environment than aluminum. Since Al itself does not have sacrificial corrosion capability, it is difficult to secure sacrificial corrosion resistance when the Zn content is less than 5%. In addition, when the Zn content is low, it is not easy to form an Mg-Zn alloy phase. However, if the Zn content exceeds 30%, the LME phenomenon occurs during welding, and oxides are easily formed in the plating bath during plating, causing defects in the steel sheet after plating. Therefore, in one embodiment of the present invention, the Zn content may be set to 5 to 30%. In one embodiment of the present invention, a more preferable content of Zn may be set to 15 to 25%.

Magnesium (Mg): 1-7%

Mg also lowers the melting point of the plating bath, and as an element having a high oxygen affinity similar to Zn, it has the characteristic of a sacrificial method that oxidizes or corrodes more rapidly in a corrosive environment compared to aluminum. It plays the same role as Zn, but shows more effective sacrificial corrosion resistance than Zn with a small amount. Therefore, in order to obtain sufficient sacrificial corrosion resistance while reducing the risk of LME due to Zn, the Mg content may be set to 1% or more. Excessive Mg content may cause dross, which is a result of oxide formation in the plating bath due to high oxygen affinity. Dross causes a dent defect of the steel plate after plating. In addition, Mg has a high affinity with Si to form a secondary phase such as Mg ₂ Si to weaken the adhesion of the plating layer. In consideration of this point, in one embodiment of the present invention, the upper limit of the Mg content may be set to 7%. In one embodiment of the present invention, a more preferable Mg content may be 1.5 to 5%.

In one embodiment of the present invention, the remainder other than the above-described elements may be Al and impurities unavoidably included. In addition, the plating layer of the present invention may further include Be and Sr in an amount determined below, if necessary.

Be and Sr: 0.5% or less in total

Be and Sr are elements with strong oxidizing power, and if at least one of the two elements Be and Sr is included in the plating bath, an oxide film of Be and Sr is formed on the surface of the plating bath to reduce the amount of ash generated by the evaporation of the plating bath. Can be reduced. In addition, since these elements stabilize the surface by forming an oxide film of Be and Sr on the surface of the plating layer, a small amount can be added. However, if the content of Be and Sr is too high, oxides of these elements may be excessively generated in the plating bath, causing defects, and the cost of component input increases. Therefore, in one embodiment of the present invention, the sum of the contents of these elements is It can be limited to 0.5% or less. These elements may be present as impurities as long as the above-described composition range is satisfied.

Impurities included in the plating layer include Mn, P, S, Cu, Co, Ca, Na, V, Ga, Ge, As, Se, In, Ag, W, Pb, and Cd, etc. may be mentioned, and these do not depart from the spirit of the present invention even if they are contained up to less than 0.1% each. In addition, Fe may also be included in the plating bath from the plating bath or the holding steel sheet, and up to about 5% may be allowed.

In the present invention, the type of the holding steel sheet is not particularly limited. However, since a steel sheet having a tensile strength of 1.0 GPa or more may be more sensitive to welding liquefaction embrittlement, in one embodiment of the present invention, when a steel sheet having a tensile strength of 1.0 GPa or more is used as the holding steel plate, the present invention has high resistance to welding liquefaction embrittlement. The effect of the plating layer of the invention can be obtained more reliably.

In addition, according to one embodiment of the present invention, the plated steel sheet may have a tensile strength of 80% or more compared to the tensile strength of the holding steel sheet before plating. That is, the plated steel sheet of the present invention is a temperature capable of minimizing the occurrence of dross even at a temperature in which the composition of the plating layer is low, and plating is possible even at a low plating bath temperature of 700°C or less, and accordingly, the steel sheet is not heated to a high temperature. Therefore, it is possible to minimize the material degradation of the steel plate.

The manufacturing method of the plated steel sheet of the present invention is not particularly limited as long as the above-described conditions can be realized. However, one non-limiting example of manufacturing the plated steel sheet of the present invention is as follows.

The method of manufacturing a plated steel sheet of the present invention comprises the steps of preparing a holding steel sheet; Heating the holding steel sheet to 700 ~ 1050 °C; Immersing the steel sheet in an aluminum plating bath maintained at 530 to 750°C to obtain an aluminum plating steel sheet; Adjusting the plating amount of the obtained aluminum-based plated steel sheet; Cooling the steel sheet with a controlled plating amount to 350° C. at a cooling rate of 7 to 25° C./sec; And it may include the step of cooling from 350 ℃ to 80 ℃ at a cooling rate of 5 ~ 15 ℃ / sec. In this case, the temperature at which the steel sheet is introduced into the plating bath may be adjusted to a plating bath temperature of -20°C to a plating bath temperature of +50°C. Although not necessarily limited thereto, according to one embodiment of the present invention, the amount of plating during plating may be controlled to 20 to 90 g/m ² based on one side. Hereinafter, it will be described in detail for each condition.

Heating temperature of holding steel sheet: 700~1050℃

The base steel sheet is heated to the above-described range before plating the base steel sheet. That is, in the case of high-strength steel, it is necessary to secure strength through heat treatment at an ideal temperature and to secure surface cleanliness through reduction heat treatment. To this end, it is necessary to heat the steel sheet in the above-described range. A more preferable temperature range is 750 to 1000°C.

Plating bath temperature: 530~750℃

The plating bath temperature is limited to 530~750℃, which increases the viscosity of aluminum at less than 530℃, resulting in poor driveability of the roll in the plating bath. If it exceeds 750℃, the amount of ash generated by the evaporation of the plating bath and the plating bath The amount of dross generated by the bond between Fe-Al increases. In addition, in the case of high-strength steel accompanied by phase transformation, high-temperature deterioration of the material may occur at a high temperature, resulting in a change in mechanical properties. In one embodiment of the present invention, the temperature of the plating bath may be limited to 600° C. or less in order to prevent a decrease in strength of the high-strength steel.

Plating bath inlet temperature of steel sheet: Plating bath temperature-20°C to plating bath temperature + 50°C

If the inlet temperature of the steel sheet is lower than the plating bath temperature -20°C, the wettability of molten aluminum decreases, and if the plating bath temperature exceeds +50°C, the plating bath temperature is increased locally, making it difficult to manage the plating bath temperature.

Adjustment of adhesion amount after plating:

As in the present invention, in the Al-based plating including Si, Zn, and Mg, the cooling rate of each solid solution element or alloy phase is different, so care must be taken in controlling the amount of plating deposited using an air knife. That is, in order to obtain an advantageous alloy layer configuration to be implemented in the present invention, it is advantageous to divide the amount of plating deposited by an air knife into two steps. That is, when the solidification rate is slow at the initial stage of solidification of the plating layer, various phases are formed in the plating layer, and thus, partial non-plating may occur. In one embodiment of the present invention, in order to avoid such a problem, when the temperature of the steel sheet is reduced to 530 to 500°C, an air knife for controlling the amount of plating adhered is placed close to the steel sheet and blown at a high speed to promote rapid solidification of the plating layer. . That is, in one embodiment of the present invention, the distance between the steel plate and the air knife is adjusted to 6 to 10 mm up to the temperature, and the linear velocity of the gas discharged from the air knife is 150 to 250 m/s, preferably 180 to 220 m/s. Adjust quickly with s. After that, in order to precisely control the amount of adhesion, the distance between the steel plate and the air knife is adjusted within 10mm to 14mm, and the amount of gas blown is also adjusted to 30~70m/s.

Cooling to 350℃ at a rate of 7~25℃/sec

In order to form an appropriate amount of Mg ₂ Si phase in the plating layer, it is necessary to form Mg ₂ Si before the Fe component of the steel sheet is diffused into the plating layer. To this end, it is necessary to cool the plated steel sheet at a cooling rate of 7°C/sec or more, and preferably, it can be cooled at a cooling rate of 10°C/sec or more. In one embodiment of the present invention, cooling for forming the Mg ₂ Si phase in the plating layer by suppressing the diffusion of Fe may be controlled up to 350°C. If the cooling rate is fast, a phase close to the amorphous phase may be formed rather than the Mg ₂ Si phase, so the cooling rate is 25° C./sec or less, preferably 20° C./sec or less, more preferably for smooth formation of the Mg ₂ Si phase. It can be limited to 15 ℃ / sec or less. If necessary, a process of adjusting the amount of plating deposited after the plating and before starting cooling may be performed. In this case, the cooling rate means a cooling rate from 350°C to 350°C after the amount of plating deposited is adjusted. If there is no control of the amount of plating attached, it may mean the cooling rate from the point of exit from the plating bath to 350°C. In one embodiment of the present invention, the amount of plating deposited may be adjusted using an air knife, and in this case, the cooling rate in this step may be from immediately after the air knife to 350°C.

Cooling from 350℃ to 80℃ at a cooling rate of 5~15℃/sec

In addition, in one embodiment of the present invention, the cooling rate from 350°C to 80°C can be controlled at 5 to 15°C/sec. In the above-described cooling rate range, the MgZn ₂ phase may be continuously formed directly on the plating layer. If the cooling rate in the above temperature range is too fast, the MgZn ₂ phase, which is a kind of non-equilibrium phase, is difficult to be continuously formed, and thus the cooling rate may be limited to 15° C./sec or less. If the cooling rate is too slow, a large amount of unnecessary secondary phases, such as an additional Mg ₂ Si phase, are generated in the plating layer to hinder the growth of the MgZn ₂ phase, and thus the cooling rate may be limited to 5° C./sec or more. It should be noted that the cooling rate in the present invention is the average cooling rate in the corresponding temperature section. Therefore, it is possible to cool at a high cooling rate even in some temperature sections below 350°C, and only if the average cooling rate falls within the above range, it is assumed that the above-described conditions are satisfied. In addition, since the difference in physical properties of the steel sheet according to the cooling rate is not large in the temperature range of less than 80° C., this is not particularly limited in one embodiment of the present invention.

According to one embodiment of the present invention, the cooling rate in the step of cooling to 350°C after adjusting the amount of plating deposited may be faster than the cooling rate from 350°C to 80°C. This is to induce dispersion formation of the Mg ₂ Si phase by relatively fast cooling rate of the previous step, and to form a continuous MgZn ₂ phase by relatively slowing the cooling rate of the later step.

Plating bath ingredients

The plating bath component of the present invention is substantially the same as the plating layer. However, since Fe may increase by 1 to 2% in the plating layer during the alloying process with the holding steel sheet after plating, the Fe content in the plating bath may be about 1 to 2% lower than the Fe content in the plating layer. The Fe content in the plating bath used in one embodiment of the present invention may be 4% or less, and in the other embodiment, the Fe content may be 0.5-3%.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for exemplifying the present invention and not limiting the scope of the present invention. This is because the scope of the rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

(Example)

A holding steel sheet (strength 1.2 GPa) was prepared, the temperature was raised to 950°C using a continuous annealing furnace, and then aluminum-based plating was performed by immersing in the aluminum-based plating bath shown in Table 1 below (the remaining components not shown in the table. Is Al except for impurities contained up to less than 0.1% and 20ppm Be and 40ppm Sr). The plating bath temperature and the temperature of the steel sheet introduced into the plating bath were adjusted as shown in Table 1. After adjusting the adhesion amount to 60g/m ² using an air knife for all the steel sheets exiting from the plating bath, the average cooling rate from 350°C to 80°C after that is as shown in Table 1. Controlled to obtain a plated steel sheet. When adjusting the adhesion amount by using an air knife, the distance between the air knife and the steel plate was adjusted to 8mm until the temperature of the steel plate reached 540℃, and the linear velocity of the injected gas was 200m/s. The distance between the steel plates was adjusted to 12 mm and the linear velocity of the gas was adjusted to 50 m/s. The composition of the plating layer (content of Si, Zn, Mg, Be, Sr and impurities) of the obtained plated steel sheet was substantially the same as that of the plating bath except for Fe, and about 1% of Fe increased (picked up). (Al content is the balance excluding additional elements and impurities). In addition, the plating layer of the obtained plated steel sheet was laminated in the order of the Fe-Al alloy layer, the MgZn ₂ alloy layer, and the Al alloy layer from the interface with the holding steel sheet in both the invention examples and comparative examples, and the Mg ₂ Si phase was present in the form of particles. I could confirm that I was doing it.

Table 2 shows the average thickness of the MgZn ₂ phase formed in the plating layer and the occupancy rate on the direct surface of Fe-Al. Further, Table 2 shows the average size and on the percentage of Mg ₂ Si in the plating layer within the Mg ₂ Si. Since the Fe ₂ Al ₅ or FeAl ₃ phase occupied all of the plating layer and the base metal interface, the area ratio of the phases in contact with the base steel sheet in both the MgZn ₂ and Mg ₂ Si phases was 0%. The thickness and fraction of the MgZn ₂ phase and the fraction of the Mg ₂ Si phase were measured in the following manner.

MgZn ₂ thickness measurement: After plating, the cross section of the specimen is observed with SEM, and the phase is identified through EDS. Measured in a direction perpendicular to the interface between the MgZn ₂ layer and the alloy layer (Fe-Al layer), and measured 10 times at 3 μm intervals based on the interface length of the steel plate and the Fe-Al phase. It is determined by the thickness, and the final thickness is obtained by average of the thickness obtained by measuring a total of 5 times for each location.

Mg ₂ Si size measurement: After plating, the cross section of the specimen is observed with SEM, and the phase is classified through EDS. After that, 10 or more pictures are obtained for each specimen, and the size of the major axis of the image (particle) is measured and the average value is used.

MgZn ₂ fraction (the ratio of the MgZn ₂ phase to the Fe-Al alloy phase) measurement: When the Al alloy layer formed on the top of the plating layer is removed and observed, it is determined as the ratio of the area occupied by the MgZn ₂ phase to the total area. It was also confirmed through cross section analysis that there was no other phase between the observed MgZn ₂ phase and the Fe-Al alloy phase.

Measurement of the Mg ₂ Si fraction in the plating layer: After plating, the cross section of the specimen is observed with SEM, and the phase is identified through EDS. After that, 10 or more pictures are obtained for each specimen, and the fraction of the phase is obtained through image analysis software, and the average value is used.

Area ratio of Mg ₂ Si phase in contact with Fe-Al: When the Al alloy layer formed on the top of the plating layer is removed and observed, it is determined as the ratio of the area occupied by the MgZn ₂ phase to the total area.

With respect to the plated steel sheet obtained by this process, whether or not welding liquid brittleness (LME) has occurred, plating adhesion, change in tensile strength after plating, corrosion resistance, etc. were evaluated based on the criteria described below, and the results are shown in FIG.

LME evaluation: The spot point was performed according to the ISO 18278-2 standard, and the welding current was set as low as 0.5kA in the expulsion current. After welding, the cross section of the steel plate was observed with OM or SEM, and the occurrence of LME cracks in the specimen was determined based on whether the heat affected zone was cracked.

Plating adhesion evaluation: After applying the automotive structural adhesive on the surface of the steel sheet, it was carried out by drying it to complete solidification and then bending it at 90 degrees to separate the adhesive from the plated steel sheet, and checking whether the plated layer is peeled off and smeared on the adhesive to determine the plating adhesion Evaluate. A good case in which the plating layer did not peel off and the adhesive was not adhered was indicated as non-peeling, and a poor case in which the plated layer was peeled and the adhesive adhered was indicated as peeling.

Mechanical property evaluation: Measure the change in tensile strength by performing a tensile test on an aluminum plated specimen at a strain rate of 10 ^-2 /s. Cold-rolled steel sheet (CR) specimens without plating were also tested in the same way and the results were compared.

Good (Tensile strength of plating material / Tensile strength of CR material = 0.80 or more)

Inferiority (Tensile strength of plating material / Tensile strength of CR = less than 0.80)

Corrosion resistance evaluation: The salt spray test (SST) was performed in a 3.5% NaCl solution, and it was judged as the time taken until the occurrence of red rust.

Excellent: more than 2000h

Normal: Between 1000h and 2000h

Inferior: less than 1000h

division Plating bath composition
(weight%) Plating bath temperature
(℃) Steel plate inlet temperature
(℃) Cooling rate up to ~350℃ (℃/s) Cooling rate from 350℃ to 80℃ (℃/s) Si Zn Mg Comparative Example 1 One 20 3 663 660 15 10 Comparative Example 2 One 20 5 651 650 15 10 Comparative Example 3 One 40 5 620 620 15 10 Invention Example 1 5 20 3 585 580 15 10 Comparative Example 4-1 5 20 3 585 580 15 20 Comparative Example 4-2 5 20 3 585 580 30 10 Inventive Example 2 5 20 5 567 560 15 1O Invention Example 3 7 20 3 561 560 15 10 Comparative Example 5-1 7 20 3 561 560 15 20 Comparative Example 5-2 7 20 3 561 560 15 3 Comparative Example 5-3 7 20 3 561 560 30 10 Comparative Example 5-4 7 20 3 561 560 5 10 Comparative Example 5-5 7 20 3 561 560 30 20 Invention Example 4 7 20 5 553 550 15 10 Comparative Example 6 7 20 10 544 540 15 10 Invention Example 5 7 30 3 534 535 15 10 Invention Example 6 7 30 5 542 540 15 10 Comparative Example 7 7 30 10 532 530 15 10 Comparative Example 8 7 40 3 528 530 15 10 Invention Example 7 9 20 3 545 540 15 10 Invention Example 8 9 20 5 546 540 15 10 Comparative Example 9 11 30 10 548 550 15 10 Comparative Example 10 15 25 3 641 640 15 10 Comparative Example 11 15 25 5 620 620 15 10 Comparative Example 12 15 25 10 599 600 15 10

division MgZn ₂ average
Thickness(㎛) The proportion of MgZn ₂ phase in the direct phase of the Fe-Al alloy phase (%) Mg ₂ Si average
Size (㎛) Mg ₂ Si in the plating layer
Fraction (%) Fe-Al alloy layer share of Mg ₂ Si (%) Thickness of Fe-Al phase (㎛) Comparative Example 1 3.8 99 6.4 2.1 0.7 11.8 Comparative Example 2 4.1 99 6.3 2.6 0.8 10.3 Comparative Example 3 5.7 99 6.1 2.6 0.8 10.0 Invention Example 1 2.7 99 5.3 5.1 0.9 7.3 Comparative Example 4-1 1.8 83 5.3 4.8 0.9 7.9 Comparative Example 4-2 2.8 99 2.7 2.0 0.6 7.3 Inventive Example 2 3.1 98 5.1 5.3 1.1 7.1 Invention Example 3 1.4 98 5.1 5.7 1.1 6.9 Comparative Example 5-1 1.0 81 5.0 5.6 1.2 6.9 Comparative Example 5-2 3.9 98 5.1 5.8 1.0 6.9 Comparative Example 5-3 1.3 98 2.7 2.1 0.7 6.9 Comparative Example 5-4 1.4 97 7.1 9.8 2.5 6.9 Comparative Example 5-5 1.0 87 6.8 2.3 0.6 6.9 Invention Example 4 1.8 98 5.0 6.0 1.3 6.7 Comparative Example 6 3.7 96 6.1 10.1 2.8 6.7 Invention Example 5 2.2 98 4.8 5.7 1.0 6.5 Invention Example 6 2.5 98 5.0 5.9 1.2 6.4 Comparative Example 7 5.3 96 6.2 10.3 3.0 6.4 Comparative Example 8 4.3 87 2.8 4.4 0.9 6.4 Invention Example 7 2.3 97 5.1 6.0 1.1 6.5 Invention Example 8 1.9 97 5.1 6.2 1.3 6.4 Comparative Example 9 5.5 88 5.2 10.1 2.5 6.6 Comparative Example 10 3.8 89 6.3 10.0 1.3 10.1 Comparative Example 11 3.9 88 6.2 9.5 1.4 9.8 Comparative Example 12 5.4 86 5.8 12.3 2.1 9.8

division LME Occurrence Plating adhesion Corrosion resistance evaluation Mechanical properties Comparative Example 1 Not occurring Unremoved Inferiority Inferiority Comparative Example 2 Not occurring Unremoved Inferiority Inferiority Comparative Example 3 Not occurring Unremoved Inferiority Inferiority Invention Example 1 Not occurring Unremoved usually Good Comparative Example 4-1 Occur Peeling usually Good Comparative Example 4-2 Not occurring Unremoved Inferiority Good Inventive Example 2 Not occurring Unremoved usually Good Invention Example 3 Not occurring Unremoved Great Good Comparative Example 5-1 Occur Peeling Great Good Comparative Example 5-2 Not occurring Unremoved Great Good Comparative Example 5-3 Not occurring Unremoved Inferiority Good Comparative Example 5-4 Occur Unremoved Great Good Comparative Example 5-5 Occur Peeling Great Good Invention Example 4 Not occurring Unremoved Great Good Comparative Example 6 Not occurring Peeling Great Good Invention Example 5 Not occurring Unremoved Great Good Invention Example 6 Not occurring Unremoved Great Good Comparative Example 7 Not occurring Peeling Great Good Comparative Example 8 Occur Peeling usually Good Invention Example 7 Not occurring Unremoved Great Good Invention Example 8 Not occurring Unremoved Great Good Comparative Example 9 Occur Peeling usually Good Comparative Example 10 Occur Peeling usually Inferiority Comparative Example 11 Occur Peeling usually Inferiority Comparative Example 12 Occur Peeling usually Inferiority

In the case of the inventive examples satisfying the conditions of the present invention, LME did not occur, and the plating adhesion was also excellent. In addition, the corrosion resistance evaluation results were above average, and the mechanical properties also showed good results. 1 is a SEM photograph showing a cross section of an aluminum-based hot-dip plated steel sheet manufactured according to Inventive Example 3. As can be seen from the figure, it is confirmed that the Fe-Al phase is continuously formed at the interface between the holding steel sheet (substrate iron) and the plating layer, and the MgZn ₂ phase is distributed directly above the phase, and the Mg ₂ Si phase is distributed in the upper layer of the plating layer. I can. Fig. 2 shows a cross-sectional photograph of a steel sheet manufactured according to Inventive Example 1 as an exemplary photograph of an inventive example according to the present invention.

However, in Comparative Examples 1 to 3, when the Si content of the plating bath and the plating layer was low, Mg ₂ Si was not well formed. As a result, it was difficult to secure corrosion resistance due to a low Mg2Si fraction. In addition, in the comparative examples, the Fe-Al phase was formed with an excessive thickness due to the low Si content that suppresses the alloying reaction, resulting in poor adhesion. In addition, due to insufficient Si content, the plating bath temperature was set at 620 to 660°C during plating, and as a result, the tensile strength of the plated material was relatively lower than that of the CR material, resulting in a side problem.

In Comparative Examples 4-1 and 4-2, the composition was the same as Inventive Example 1, but the cooling rate of each section was different. Comparative Example 4-1 is a result of rapidly increasing the cooling rate to 350 ℃ ~ 80 ℃. MgZn not be formed on the _second time is not enough low percentage of MgZn ₂ phase Fe-Al alloy phase immediately above. As a result, LME occurred, and the plating adhesion was inferior. Comparative Example 4-2 is a result of rapidly increasing the cooling rate from plating temperature to 350°C. As a result, the average size of the Mg ₂ Si phase was somewhat small, and the fraction was also low due to insufficient formation time. This resulted in poor corrosion resistance.

In Comparative Examples 5-1 to 5-5, the composition was the same as Inventive Example 3, but the cooling conditions were different. Among them, Comparative Example 1 was a case in which the cooling rate from 350°C to 80°C was faster than the range specified in one embodiment of the present invention, and the ratio of the MgZn ₂ phase in the direct phase of the Fe-Al alloy phase was low, resulting in LME. , The plating adhesion showed poor results. Comparative Example 5-2 is a case in which the cooling rate from 350° C. to 80° C. was made slower than the range specified in one embodiment of the present invention, and as a result, the MgZn ₂ layer was formed relatively thick. In this case, the various physical properties shown in Table 3 were not significantly deteriorated, but the result of deteriorating the workability of the plating layer was confirmed. Comparative Example 5-3 is a case where the cooling rate from the plating temperature to 350°C is faster than the range specified in the present invention, and as a result, the average size of the Mg ₂ Si phase was somewhat smaller, and the formation time was insufficient, so the fraction was also low. This resulted in poor corrosion resistance. Comparative Example 5-4 is a case in which the cooling rate from the plating temperature to 350°C was made slower than the range specified in the present invention. As a result, not only the Mg ₂ Si phase was excessive, but also the Mg ₂ Si phase in contact with the Fe-Al phase. The area ratio was also formed in excess of the preferred range defined in the present invention. For this reason, LME occurred during welding in the steel sheet manufactured according to Comparative Example 5-4. Comparative Example 5-5 not only made the cooling rate from plating temperature to 350°C faster than the range specified in the present invention, but also made the cooling rate from 350°C to 80°C slower than the range specified in one embodiment of the present invention. In one case, the proportion of the MgZn ₂ phase in the direct phase of the Fe-Al alloy phase was not sufficient, and the size of the Mg ₂ Si phase was coarse beyond the range specified in the present invention. As a result, it was not possible to prevent the LME phenomenon from occurring during welding. 3 and 4 show photographs of observing cross-sections of steel sheets manufactured by Comparative Examples 4-3 and 5-4, respectively.

In Comparative Examples 6 and 7, the Mg content was very high. In this case, the MgZn ₂ layer was formed to have a thick thickness, and the Mg ₂ Si phase was coarse and formed to have a high fraction. In addition, the area ratio of the Mg ₂ Si phase in contact with the Fe-Al phase was also high. As a result, it was found that the plating adhesion was poor.

Comparative Example 8 was a case where the Zn content was very high In this case, LME was generated by unalloyed Zn. In Comparative Example 9, the content of Si and Zn was excessive, and the Fe-Al thickness was insufficient, resulting in LME, as well as poor plating adhesion due to insufficient formation of an Fe-Al alloy phase.Comparative Example 10 And 11 are cases in which the Si content was somewhat high, and the plating bath temperature was set to 620 to 645°C due to dross generation, and as a result, the tensile strength of the plated steel sheet was decreased. In addition, in these comparative examples, the size of Mg ₂ Si was somewhat coarse and the fraction was excessively formed, and as a result, the corrosion resistance was averaged. This is an interfacial alloy phase (Fe-Al alloy phase) due to the high content of Si in the plating bath. The formation was suppressed, and accordingly, the MgZn ₂ layer was very non-uniformly formed directly on the alloy, and the surface occupancy of the base iron was low, so it was judged that the corrosion resistance decreased and the LME generation was affected. In Comparative Example 12, the Si content was excessive, and Mg was also added in a somewhat excessive amount. As a result, Mg ₂ Si was formed excessively, and the thickness of the MgZn ₂ layer was somewhat excessive and the occupancy rate was also low. In this comparative example, LME occurred, the surface share of MgZn ₂ was low, so that the plating layer was peeled off, and the corrosion resistance was also at a normal level. In addition, since the plating bath temperature was high, the tensile strength was greatly reduced after plating.

5 is a photograph showing the results of a plating adhesion test on the steel sheets manufactured according to the inventive examples and comparative examples of the present invention. As can be seen from the drawings, peeling of the plating layer was not observed at all in each of the inventive examples satisfying the conditions of the present invention, whereas peeling was observed in Comparative Examples 3, 5, 6, 7, 8, which did not satisfy the conditions of the present invention. You can see that it is. The reason for this peeling is that the formation of the Fe-Al phase is suppressed and thus the surface occupancy of the MgZn ₂ phase is low or Mg ₂ Si is formed excessively.

Therefore, it was possible to confirm the advantageous effects of the present invention.

Claims (11)

A high corrosion-resistant plated steel sheet consisting of a plated layer including an Fe-Al alloy layer and a MgZn ₂ layer sequentially from an interface between the holding steel plate and the holding steel plate.
The high corrosion-resistant plated steel sheet according to claim 1, wherein the Fe-Al alloy layer has a thickness of 1 to 8 μm.
The high corrosion-resistant plated steel sheet according to claim 1, wherein the MgZn ₂ layer has a thickness of 0.5 μm or more.
The high corrosion-resistant plated steel sheet according to claim 1, wherein the plating layer further comprises an Al alloy layer on top of the MgZn ₂ layer.
The highly corrosion-resistant plated steel sheet according to claim 4, wherein the MgZn ₂ phase occupies 90% or more of the Fe-Al alloy phase in the plating layer.
(However, the ratio is determined as the ratio of the area occupied by the MgZn ₂ phase relative to the total area when the layer formed above MgZn ₂ is removed and observed)
The high corrosion-resistant plated steel sheet according to any one of claims 1 to 5, wherein the plated layer comprises an Mg ₂ Si phase in an area of 10% or less.
(However, the proportion of the Mg ₂ Si is determined by the ratio of the area for cutting by the cutting surface when observed, compared to the area of the entire plating layer Mg ₂ Si phase occupies the plated steel sheet)
The high corrosion-resistant plated steel sheet according to claim 6, wherein the area ratio of the Mg ₂ Si phase in contact with the holding steel sheet is 2% or less.
(However, the proportion of the Mg ₂ Si is determined by the ratio of the area of removing the layer formed on more Mg ₂ Si and time, compared to the total area occupied by Mg ₂ Si phase was observed)
The highly corrosion-resistant plated steel sheet according to any one of claims 1 to 5, wherein the plating layer has a composition comprising Si: 5 to 10%, Zn: 5 to 30%, and Mg: 1 to 7% by weight.
The highly corrosion-resistant plated steel sheet according to claim 8, wherein the composition of the plating layer further comprises 0.5% or less in total of Be and Sr based on weight.
Preparing a holding steel plate;
Heating the holding steel sheet to 700 ~ 1050 °C;
Obtaining an aluminum plated steel plate by immersing the steel plate in an aluminum-based plating bath maintained at 530 to 750°C;
Adjusting the plating amount of the obtained aluminum-based plated steel sheet;
Cooling to 350° C. at a cooling rate of 7 to 25° C./sec; And
Cooling from 350℃ to 80℃ at a cooling rate of 5~15℃/sec
Method of manufacturing a high corrosion-resistant plated steel sheet comprising a.
The method of claim 10, wherein the temperature at which the steel sheet is introduced into the plating bath is a plating bath temperature of -20°C to a plating bath temperature of +50°C.

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