KR101528107B1 - High strength galvanealed steel sheet with good coatability and coating adhesion - Google Patents

Abstract

The present invention relates to a high-strength hot-dip galvanized steel sheet used for automobile materials, building materials, household appliances, and the like.
More specifically, it is possible to improve the wettability of the steel sheet by suppressing the formation of oxides by diffusing the elements of the noble metals contained in the steel during annealing to the surface of the steel sheet to thereby improve the plating ability and the plating adhesion in the plating step The present invention relates to a hot-dip galvanized steel sheet having excellent properties.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-dip galvanized steel sheet excellent in plating ability and plating adhesion,

The present invention relates to a high-strength hot-dip galvanized steel sheet used for automobile materials, building materials, home appliances, and the like, and more particularly to a high-strength hot-dip galvanized steel sheet excellent in plating ability and plating adhesion.

In accordance with the recent environmental problems, regulations on automobile fuel economy are strictly regulated. As a countermeasure to this, there have been increasing cases of applying high-strength steel sheets to materials such as automobiles, and the demand for high-strength steel sheets is increasing rapidly.

Typical high-strength steel sheets that are currently being commercialized include low-temperature steels such as martensite, bainite and retained austenite such as dual phase steel (DP steel) and transformed induced plasticity steel (TRIP steel) There is a metamorphic fortified steel using tissue. In order to increase the strength of such transformation-strengthened steels, it is common to add elements such as Si, Mn, Cr and the like. When a steel sheet containing these elements is subjected to a hot dip galvanizing or galvannealing hot dip galvanizing process, There is a problem in that the oxide is formed on the surface of the steel sheet and the wettability with molten zinc is lowered during plating.

Generally, during the hot dip galvanizing process, the steel sheet is subjected to heat treatment up to 900 ° C. in a reducing atmosphere containing hydrogen (H ₂ ). At this time, elements such as Si and Mn contained in the steel have a very high affinity with oxygen Even in a reducing atmosphere containing at least 5% of hydrogen (H ₂ ), and is concentrated on the surface of the steel sheet. Oxides formed on the surface of the steel sheet lower the wettability of the molten zinc and the steel sheet during galvanizing, thereby causing unplated or plating peeling. In addition, when Si or Mn oxide is present at the interface between the steel sheet and the plated layer after the hot dip galvanizing, the reaction of Fe and Zn is inhibited during the heat treatment to delay the alloying of the plated layer. As a result, the productivity of the galvannealed galvanized steel sheet is reduced, and plating defects such as powder ring are generated.

As a method for preventing the deterioration of the plating ability by the Si or Mn oxide described above, there has been proposed a method of plating a metal such as Fe and Ni by electroplating before the annealing of the steel sheet proceeds. However, in this case, additional facilities are required and the cost is raised, which is inefficient in terms of economy.

In addition, a method of increasing the formation of the Fe-Al alloy layer at the interface between the steel sheet and the plating layer by increasing the amount of Al added to the hot dip galvanizing bath was proposed. This is because if the content of Al increases on the surface of the steel sheet as the content of Al in the plating bath is increased, the easily oxidizable Al reduces the oxide layer formed on the surface of the steel sheet, thereby improving the hot dip galvanizing. However, if the content of Al in the plating bath is increased, there arises a problem of deteriorating the weldability in steel sheet processing, and it is difficult to actually apply this method.

As another method, there has been proposed a method of improving the plating ability by reducing the oxide in an annealing furnace after forming a Fe oxide on the surface by heating the steel plate in an oxidizing atmosphere in advance. As an example, Patent Document 1 discloses that, in the case of Patent Document 1, an iron oxide is produced by increasing the air / fuel ratio from the normal 0.9 to 1.05 in the pre-annealing furnace (DFF) It is possible to secure a stable plating ability since it is formed on the surface of the steel sheet. However, in the case of a high-strength steel having a high content of Si and Mn, when the above-described technique is applied, the Si or Mn oxide layer is concentrated at the interface, and plating peeling may occur.

Japanese Patent Application Laid-Open No. 2001-226742

One aspect of the present invention relates to a method for improving the wettability of a steel sheet by suppressing the formation of oxides by diffusing the elements of stagnant elements contained in the steel into the surface of the steel sheet during the annealing of the hot-dip galvanized steel sheet, And to provide a technique capable of ensuring excellent adhesion.

According to an aspect of the present invention, Comprising a Fe layer and a zinc plating layer reduced in the possession of the steel sheet surface, within the said carrying plate direction 1μm from the interface of the reduced Fe layer and the carrying plate SiO _2, MnO, MnO _2, Mn ₂ SiO _4, and MnSiO _{And the} total amount of at least one of them is 1 g / m < ² > or less. BRIEF DESCRIPTION OF THE DRAWINGS Fig.

According to the present invention, when a steel sheet containing an alloy element that forms an oxide on the surface of a steel sheet at a high temperature is used as a base steel sheet to produce a coated steel sheet, the conditions of heating in a direct heating furnace and the conditions of the annealing heat treatment step are controlled, A hot-dip galvanized steel sheet excellent in characteristics can be obtained.

Fig. 1 is a cross-sectional photograph in the thickness direction of a hot-dip galvanized steel sheet produced by a conventional annealing treatment in a reducing atmosphere and a schematic diagram thereof.
2 is a schematic view of the hot dip galvanized steel sheet produced in accordance with the conventional method and the method proposed in the present invention in the thickness direction.
3 is a cross-sectional photograph of the hot-dip galvanized steel sheet produced in accordance with the present invention in the thickness direction.
Fig. 4 is a graph showing the distribution of alloy elements in the thickness direction of an unplated annealed steel sheet produced by annealing in a non-reducing atmosphere according to the present invention.

The inventors of the present invention have conducted intensive studies to solve the problem of deterioration of the plating characteristics of the steel sheet due to the formation of oxides on the surface of the steel sheet when elements such as Si or Mn included for the production of high- Is controlled in the direct heating furnace before annealing and the conditions of the annealing heat treatment step to be carried out are controlled to prevent the diffusion of the elements of stagnant elements contained in the steel to the surface of the steel sheet to thereby control the formation of oxides on the surface of the steel sheet And reached the present invention.

Hereinafter, a method for manufacturing the hot-dip galvanized steel sheet of the present invention will be described in detail. However, it should be noted that the present invention is not limited to such a manufacturing method.

In producing a hot-dip galvanized steel sheet according to the present invention, a steel sheet is first prepared.

More specifically, a steel sheet containing at least one of Si, Mn and Al is preferably used as the base steel sheet. As an example, the base steel sheet may be DP steel, TRIP steel, CP steel, MART steel or TWIP steel.

The base steel sheet preferably contains at least one member selected from the group consisting of Si: 0.1 to 3.0%, Mn: 0.1 to 6.0% and Al: 1% or less (excluding 0) in weight percent, Is to obtain a steel sheet excellent in plating characteristics by limiting the plating process and the plating peeling phenomenon as main problems.

The base steel sheet may be a cold rolled steel sheet or a cold rolled steel sheet subjected to a degreasing process by a conventional degreasing method.

Thereafter, the prepared base steel sheet can be directly heated in the heating furnace.

Conventionally, a steel sheet containing an alloy element such as Si or Mn, which easily forms an oxide on the surface of a steel sheet at high temperature, is directly heated in a heating furnace to form a Fe oxide layer on the surface of the steel sheet, The formed Fe oxide layer was reduced to a reduced Fe layer. The reduced Fe layer may be formed by a single oxide such as Si or Mn oxide layer formed during annealing, for example, SiO ₂ , MnO or the like, or an oxide layer formed by mixing two or more complex oxides such as Mn ₂ SiO ₄ , MnSiO ₃ , The Si or Mn oxide layer and the Fe layer were inferior in adhesion so that peeling easily occurred at the interface between the Si or Mn oxide layer and the reduced Fe layer (Fig. 1 And Fig. 2 (a)).

In order to solve this problem, in the present invention, when the base steel sheet is directly heated in the heating furnace, the thickness of the Fe oxide layer is controlled so that the oxidation of the noble metal by the annealing heat treatment is generated in the Fe oxide layer, Thereby preventing diffusion of the Venustic oxide into the surface of the steel sheet. In addition, a part of the Fe oxide layer remains on the surface of the steel sheet, thereby exhibiting improved plating performance in the subsequent plating step (see FIG. 2 (b)).

More specifically, in order to finally improve the plating characteristics of the steel sheet, it is necessary to control the thickness of the Fe oxide layer formed on the surface of the steel sheet, and it is preferable to form the Fe oxide layer with a thickness of preferably 50 to 500 nm.

At this time, if the thickness of the Fe oxide layer is less than 50 nm, the surface diffusion of Si or Mn can not be effectively suppressed, and Si or Mn oxide is present on the surface of the steel sheet during plating. On the other hand, if the thickness of the Fe oxide layer is too thick to be more than 500 nm, it is difficult to reduce the Fe oxide layer in the atmosphere, so that the Fe oxide form is present as it is and the plating is difficult during plating, There is a problem that Fe oxide is present to cause plating peeling.

Therefore, it is preferable to control the thickness of the Fe oxide layer. At this time, a method for controlling the thickness of the Fe oxide layer is not particularly limited, but it is preferable to control the air / fuel ratio (air-fuel ratio) and the steel sheet temperature of the direct heating furnace as a preferable example. Since the air / fuel ratio (air-fuel ratio) and the steel sheet temperature directly affect the formation of the Fe oxide layer in the direct heating furnace, the air / fuel ratio is limited to 1.01 to 1.40, It is preferable to control the temperature to 750 deg. On the other hand, the control of the thickness of the Fe oxide layer can also be performed by directly injecting O ₂ or air or water vapor in the annealing furnace. This is a method of oxidizing the steel sheet surface by injecting an oxidizable gas at the front end of the annealing furnace, and it is possible to control the thickness of the Fe oxide layer according to the amount of oxygen. Normally, the oxygen atmosphere is controlled to be less than 1% in order to suppress the abrupt reaction of the oxidizing gas injected in the annealing furnace.

If the air-fuel ratio of the direct heating furnace is less than 1.01, oxidation of Fe does not occur sufficiently and it is difficult to obtain a sufficient Fe oxide layer. On the other hand, if the air-fuel ratio exceeds 1.40, back- , And the air-fuel ratio is limited to 1.01 to 1.40. When the temperature of the steel sheet on the outlet side of the direct heating furnace is less than 550 캜, the Fe is not sufficiently oxidized in the steel sheet. Therefore, it is difficult to suppress the diffusion of Si or Mn oxide onto the surface during annealing, , The diffusion rate of the Fe oxide to the surface is excessively increased to excessively increase the amount of the agglomerate while reducing the production speed because the line speed must be decreased to reach the target temperature. Therefore, it is preferable to control the steel sheet temperature at the exit side of the direct heating furnace to 550 to 750 占 폚.

The direct heating furnace used for the heating of the steel sheet may be a DFF (Direct Fired Furnace) or a DFI (Direct Flame Impingement). The DFF or DFI heating equipment is mainly located in the first half of the heating zone, It is independent of any position in securing the target heat treatment temperature and forming the Fe oxide layer.

As described above, it is possible to carry out the heat treatment for annealing the base steel sheet heated in the direct heating furnace. At this time, it is preferable to perform in a nitrogen atmosphere.

Usually, in the annealing heat treatment, heat treatment is performed in a reducing atmosphere, that is, an atmosphere containing 5 to 15% H _{2. In} this atmosphere, the Fe oxide layer formed on the surface of the steel sheet is easily reduced. More specifically, since the partial pressure of oxygen in the reducing atmosphere is as low as about 10 ^-23 atm, the Fe oxide layer on the surface of the steel sheet is reduced and oxygen in the atmosphere gas permeates into the reduced Fe layer, Mn oxide layer is formed (Fig. 2 (a)).

On the other hand, in the present invention, it is preferable to apply the atmosphere in the annealing heat treatment in a nitrogen atmosphere. In the nitrogen atmosphere, since the oxygen partial pressure is in the range of 10 ^-8 atm to 10 ^-11 atm, the Fe oxide layer on the surface of the steel sheet is not reduced by the atmospheric gas, and the Fe oxide layer Since Si or Mn has good affinity with oxygen, Si or Mn oxide is formed near the interface between the base steel sheet and the reduced Fe layer and is distributed discontinuously. In this case, in the heating step, the inside of the Fe oxide layer formed to a thickness of 50 to 500 nm is reduced by annealing heat treatment to form a reduced Fe (Reduced Fe) layer, and the uppermost layer of the steel sheet is a non- (Fig. 2 (b)). The Fe oxide layer remaining on the uppermost layer of the steel sheet suppresses the surface diffusion of the Si or Mn oxide, and the Fe oxide layer is reduced by Al in the plating bath in the subsequent plating process. At this time, the plating ability is not affected. Al in the plating bath is a metal having excellent oxidizing power, and after the Fe oxide is reduced, it is diffused in the plating bath in the form of Al ₂ O ₃ .

As a result, in the conventional reducing atmosphere, Si or Mn in the steel migrates to the surface of the steel sheet to form oxides when the annealing heat treatment is performed. In the present invention, annealing is performed in a nitrogen atmosphere to diffuse to the surface of a steel sheet of Si or Mn oxide It is possible to prevent the deterioration of the plating ability in the subsequent plating step.

In order to maintain the nitrogen atmosphere in the annealing heat treatment, it is preferable to control the dew point temperature to -60 to 0 占 폚, and the annealing heat treatment is preferably performed at the annealing temperature of 750 to 850 占 폚.

When the dew point temperature is less than -60 DEG C, the Fe oxide on the surface of the steel sheet is easily reduced by the atmosphere, and the Fe layer reduced to the uppermost layer of the steel sheet is formed, and it is difficult to suppress diffusion of Si and Mn into the surface. On the other hand, when the dew point temperature exceeds 0 ° C, it is difficult to reduce the Fe oxide upon annealing due to partial pressure of oxygen and water vapor.

Further, in order for the Fe oxide on the surface of the steel sheet to be reduced by Si and Mn in the base iron, the annealing temperature is preferably set to 750 to 850 캜. If the annealing temperature is less than 750 ° C, the Fe oxide on the surface of the steel sheet reacts with a small amount of Si or Mn and is not sufficiently reduced. On the other hand, when the annealing temperature exceeds 850 ° C, Si or Mn is rapidly oxidized under the Fe oxide, and is formed in a film shape, thereby deteriorating the plating adhesion.

The equipment of the annealing heat treatment step is not particularly limited and can be carried out in a normal continuous annealing line. That is, the type of equipment and the operating conditions are irrelevant if there is no problem in ensuring the mechanical properties of the steel sheet by continuously performing the heat treatment and forming the nitrogen atmosphere. For example, a typical annealing heat treatment facility may include a preheating section, a heating section, a soaking section, a fast cooling section, a slow cooling section, an Overage section, and a reheating section, and the arrangement and quantity of the heat treatment sections can be changed as needed.

Thereafter, the base steel sheet after the annealing heat treatment step is immersed in a plating bath to form a plating layer. The method of forming the plating layer is not particularly limited.

For example, when the plating process is a hot dip galvanizing process, hot dip galvanizing can be performed by immersing in a hot dip galvanizing bath containing 0.12 to 0.25 wt% of Al and the remainder of Zn at 460 to 480 캜 for 3 to 5 seconds have.

In the present invention, a hot-dip galvanized steel sheet is described as an example of the hot-dip galvanized steel sheet. However, the hot-dip galvanized steel sheet may be a hot-dip galvanized steel sheet, a hot-dip galvanized steel sheet, -Mg, Zn-Mg, or the like. For example, in the case of a hot-dip galvanized steel sheet, the hot-dip galvanized steel sheet contains 50 to 60% by weight of Al-40 to 50% by weight of a steel sheet containing 5 to 12% by weight of Si and the remainder of Al with a plating bath at 660 to 680 캜 The plating process can be performed by changing the operating conditions according to the melting point of the alloy element and the facility capability in the plating bath containing 5% Zn and 590 ~ 610 ° C.

The alloyed heat treatment may be further performed on the hot-dip galvanized steel sheet produced by forming the zinc-plated layer in the hot-dip galvanizing bath.

At this time, it is possible to secure a sufficient Fe content in the zinc plating layer by controlling the alloying heat treatment temperature to 480 DEG C or more, and by controlling the upper limit of the heat treatment temperature to 560 DEG C or less, the Fe content in the zinc plating layer is excessively excessive, It is possible to prevent the powdering phenomenon from falling out.

Hereinafter, a hot-dip galvanized steel sheet as one aspect of the present invention will be described in detail. In the present invention, the hot-dip galvanized steel sheet may be an alloyed hot-dip galvanized steel sheet.

The hot-dip galvanized steel sheet according to the present invention may comprise a base steel sheet, wherein the base steel sheet contains 0.1 to 3.0% of Si, 0.1 to 6.0% of Mn, 1% or less of 1% Species or two or more species.

Wherein the hot dip galvanized steel sheet comprises a reduced Fe layer formed on the base steel sheet and a zinc plated layer formed on the reduced Fe layer, wherein Si, Mn Based oxide and MnSi-based oxide in a total amount of 1 g / m ² or less.

The Si-based, Mn-based, MnSi based oxide may be _{SiO 2, MnO, MnO 2,} Mn 2 SiO 4 and MnSiO _3.

The concentrated hot-dip galvanized steel sheet according to the present invention has a concentrated layer formed by agglomeration of the oxides having a maximum length of 1 μm or less in the longitudinal direction at a maximum depth of 1 μm in the direction of the base steel sheet from the interface of the reduced Fe layer, The oxides are distributed with an average spacing of less than 500 nm. The density of the oxide on the cross section is 10% or less in area ratio within 1 占 퐉 in the direction of the base steel sheet.

In order to form oxides at a depth exceeding 1 占 퐉 from the interface of the reduced Fe layer toward the base steel sheet, the oxidizing atmosphere must be high during the annealing process. In this case, since a large amount of Fe oxide is formed on the surface of the steel sheet, . Therefore, it is preferable that the oxide formed in the inner side of the steel sheet is formed to a depth of 1 mu m at the maximum.

If the maximum length of the Si or Mn oxide existing within 1 占 퐉 in the direction of the base steel sheet from the interface of the reduced Fe layer exceeds 1 占 퐉 in the longitudinal direction, the adhesion between the oxide and the plating layer is lowered, It is difficult to secure. Therefore, in order to secure plating adhesion, it is preferable to form such oxides to be dispersed.

On the other hand, the total amount of oxides formed in the base steel sheet may be less than 1 g / m ² , and the total amount of the oxides may vary depending on the heat treatment temperature, so that the maximum total amount is preferably controlled to 1 g / m ² or less.

As described above, the hot-dip galvanized steel sheet according to the present invention can provide a steel sheet having excellent surface quality after hot-dip coating by improving the plating property in hot-dip coating by forming the metal oxide on the surface of the steel sheet.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

In the case of using TRIP, DP, and CP steels, full hard steel sheets having a thickness of 1.0 mm were used as the base steel sheets, Were heated in a direct-heating furnace under the conditions shown in Table 1 below. After the heat treatment was performed under the annealing heat treatment conditions shown in Table 1 below, hot dip galvanizing was performed under normal operating conditions to produce a hot-dip galvanized steel sheet.

Then, the plating quality of the hot-dip galvanized steel sheet, that is, the plating property and the plating adhesion property were evaluated, and the results are shown in Table 1 below.

At this time, the method and criteria for evaluating the plating quality were set as follows.

Plating property: The appearance of the coated steel sheet was visually observed to the extent that the molten plated material was well coated on the base steel sheet. The evaluation criteria are as follows: No bare spots; Class 1 or 2, unalloyed micro-observation, automotive interior trim and other product levels; Grades 3 to 5, and uncoating is somewhat observed and can not be used as a product; Grade.

Plating Adhesiveness: A property of evaluating whether or not a plating layer adhered to a base steel sheet causes a peeling problem in processing and use, is a method of OT bending a plated steel sheet, and then attaching a tape to the bending portion, It was observed whether or not the plated layer was sticking out. The evaluation criteria are very good when there is no peeling phenomenon at all; ◎, peeling occurs only within 5mm of the edge (in this case, no problem if site trimming is done in production); ○, when peeling phenomenon as a whole occurs; X.

Steel composition Steel grade Direct heating furnace Fe layer
thickness Annealing furnace atmosphere
Plating quality division Si
(wt%) Mn
(wt%) Air-fuel ratio Temperature
(° C) H ₂
(%) N ₂
(%) Dew point temperature Plating property Plated
Adhesiveness One 1.5 1.5 TRIP 0.95 630 - 5 95 -50 Outside grade X Comparative Example 2 1.5 1.5 TRIP 1.1 700 225 5 95 -50 1-3 X Comparative Example 3 1.5 1.5 TRIP 1.1 750 280 5 95 -50 1-3 X Comparative Example 4 1.5 1.5 TRIP 1.2 630 230 5 95 -30 1-3 X Comparative Example 5 1.5 1.5 TRIP 1.1 630 160 10 90 0 1-3 X Comparative Example 6 1.5 1.5 TRIP 1.1 630 160 0 100 -50 1-3 ◎ Honor 7 0.1 2.0 DP 1.1 630 420 0 100 -50 1-3 ◎ Honor 8 0.2 2.5 CP 1.1 630 310 0 100 -50 1-3 ◎ Honor 9 1.5 1.5 TRIP 1.1 580 60 0 100 -50 3 to 5 ○ Honor 10 1.5 1.5 TRIP 1.1 650 190 0 100 -50 1-3 ◎ Honor 11 1.5 1.5 TRIP 1.1 680 210 0 100 -50 1-3 ◎ Honor 12 1.5 1.5 TRIP 1.2 630 230 0 100 -50 1-3 ◎ Honor

As shown in Table 1, in the case where the atmosphere of the annealing furnace is formed into a reducing atmosphere without satisfying the air-fuel ratio of the direct heating furnace proposed in the present invention, the plating performance is out of the grade or even if the plating performance is excellent, And the plating adhesion is not excellent.

However, when the conditions proposed in the present invention are satisfied, it is possible to produce a hot-dip galvanized steel sheet having not only the appearance of the surface of the steel sheet, but also the coating adhesion property, so that the quality can be applied to products.

3, the cross-section of the hot-dip galvanized steel sheet produced in accordance with the present invention was observed. As a result, the Fe oxide layer was reduced by Si or Mn contained in the steel during the annealing heat treatment, And it is confirmed that the Si or Mn oxide layer is discontinuously formed in a net shape at the interface between the base steel sheet and the reduced Fe layer or in the vicinity of the reduced Fe layer. In other words, the diffusion of the Si or Mn oxide layer to the surface of the steel sheet is suppressed in this manner, whereby excellent plating characteristics can be secured.

4 is a result of analyzing a cross section of an unplated steel sheet annealed according to the present invention, and discontinuous Si and Mn oxides shown in FIG. 3 can be more clearly identified. That is, it can be seen that Si or Mn oxides are formed on and under the Fe layer formed to a thickness of about 200 nm, and these Si or Mn oxides are present as complex oxides at the same position rather than in a single state have.

As described above, according to the present invention, it is possible to effectively suppress the diffusion of the Si or Mn oxide layer, which lowers the surface quality of the steel sheet, to the surface of the steel sheet, thereby securing excellent plating characteristics.

Claims (4)

Base steel sheet; An Fe layer and a zinc plating layer reduced on the surface of said backing steel sheet,
And a concentrated layer containing 1 g / m ² or less of the total amount of one or more oxides of Si, Mn and MnSi within 1 μm from the interface between the base steel sheet and the reduced Fe layer toward the base steel sheet ,
Wherein the concentrated layer is formed by annealing annealing in a nitrogen atmosphere at a dew point temperature of -60 to 0 占 폚, and is excellent in plating ability and plating adhesion.
The method according to claim 1,
Wherein the maximum length of the oxide existing within 1 占 퐉 in the direction of the base steel sheet from the interface between the base steel sheet and the reduced Fe layer on the cross section of the hot-dip galvanized steel sheet is 1 占 퐉 or less in the longitudinal direction Excellent high strength hot dip galvanized steel sheet.
The method according to claim 1,
Wherein an average interval of the oxides existing within 1 占 퐉 in the direction of the base steel sheet from the interface between the base steel sheet and the reduced Fe layer on the cross section of the hot-dip galvanized steel sheet is 500 nm or less. galvanized steel.
The method according to claim 1,
Wherein the density of the oxide within 1 占 퐉 in the direction of the base steel sheet from the interface between the base steel sheet and the reduced Fe layer on the section of the hot-dip galvanized steel sheet is 10% or less in terms of area ratio. Hot - dip galvanized steel sheet.

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