KR101461744B1 - Manufacturing method for hot press formed products of coated steel and hot press formed products using the same - Google Patents

Abstract

The present invention relates to a method of manufacturing a hot press molded product for a plated hot press forming steel product, and a hot press molded product which can be suitably used for automobile parts and the like, and a hot press molded product using the same.
To this end, in the present invention, the above object can be achieved by forming the organic coating layer and the oxide layer on the plating layer of the plated steel.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a hot press molded article and a hot press molded article using the same,

The present invention relates to a method of manufacturing a hot press molded product for a plated hot press forming steel product, and a hot press molded product which can be suitably used for automobile parts and the like, and a hot press molded product using the same.

In recent years, the use of high-strength steels has been increasing to lighten automobiles, but such high-strength steels are easily worn or fractured at room temperature. In addition, there is a problem that it is difficult to precisely process the dimension because spring back phenomenon occurs during processing. Accordingly, Hot Press Forming (HPF) has been applied as a preferable method for machining high strength steel.

The hot press forming (HPF) is a method of processing a steel sheet into a complicated shape at a high temperature by using a property of softening and high ductility at a high temperature, more specifically, At the same time, quenching is performed to transform the structure of the steel sheet into martensite to form a product having high strength and precise shape.

However, since the above-mentioned hot press forming (HPF) requires a heating process at a high temperature, it is difficult to avoid the formation of surface oxides during heating. Surface oxides formed during heating adhere to the mold during subsequent processing, which decreases the productivity and deteriorates the surface shape of the product.

In order to prevent this, there is a method of keeping the atmosphere of the heating step in a reducing atmosphere or a vacuum state, but this causes a problem that the manufacturing cost is increased.

As another method, there has been proposed a technique for preventing surface oxidation by using a plated steel sheet having an aluminum plating layer formed on a base steel sheet. The aluminum-plated steel sheet has an advantage that a thick oxide film is not formed on the plating layer due to the high melting point of aluminum and the dense and thin oxide film formed on the plating layer. However, as the weight of the steel plate is increased and the corrosion resistance of the steel plate is important, a technique for manufacturing a hot press formed product using a galvanized steel sheet has been introduced as a substitute for an aluminum-plated steel plate. In the galvanized steel sheet, zinc has an advantage of protecting the corrosion of the steel sheet by scratching the cross section or the surface by the self-sacrificing method, and is superior in the corrosion resistance to the aluminum-coated steel sheet.

However, when the galvanized steel sheet is subjected to hot press forming, cracks are formed in the galvanized layer by surface stress at a region where the degree of processing by the metal mold is severe and surface friction with the metal sheet is severe, And a fine crack is generated to the surface of the substrate. Such fine cracks are called microcracks or microcracks. These microcracks may act as a starting point for propagating cracks in the steel sheet, cause fatigue cracks, and may hinder durability of parts . Therefore, it is necessary to completely eliminate or minimize the formation of micro cracks that occurs when hot pressing (HPF) is performed to process the galvanized steel sheet into parts.

An aspect of the present invention is to provide a method of manufacturing a hot press molded product capable of reducing the occurrence of microcracks in the production of a hot press molded product by using a plated steel material for hot press and a hot press molded product using the method I would like to.

According to an aspect of the present invention, A plating layer formed on the base steel sheet; An organic coating layer formed on the plating layer; And an oxide layer formed on the organic coating layer, wherein the oxide layer has an average thickness of 0.1 to 20 占 퐉.

According to another aspect of the present invention, there is provided a method of manufacturing a steel sheet, comprising the steps of: preparing a plated steel material having a plated layer formed on a base steel sheet; Forming an organic coating layer on the plating layer of the plated steel material; Heating the plated steel material in a heating furnace to form an oxide layer on the organic coating layer; And a step of hot-pressing and cooling the heated plated steel material.

According to the present invention, in the hot press forming of the plated steel material, an organic coating layer is formed on the plating layer and the heating condition in the heating step is controlled to form an oxide layer mainly composed of Mn oxide on the most surface layer portion of the plating layer, Since the oxide layer can reduce the surface friction with the mold during the hot press forming process, it is possible to reduce the occurrence of cracks (microcracks) generated at the deeply processed portion during hot press forming, I can stop it.

Therefore, the hot press member manufactured according to the present invention can secure excellent durability.

Fig. 1 shows an example of a shape used in hot press forming (HPF).
Fig. 2 shows the results of observation of the cross sections of the steel grades 1 and 4 of the Examples by hot press molding (HPF) with an optical microscope.
3 shows the results of measurement of the content of Mn existing in the depth of 5 mu from the oxide layer in the outermost layer portion of the plating layer of the steel sheets of Examples 1 to 4 in the direction of the base steel sheet with GDS.
4 is a graph showing the relationship between the maximum Mn content in the oxide layer of the outermost layer of the plating layer and the average depth of generated micro cracks after hot pressing (HPF) the steel pieces 1 to 8 of the embodiment.

The inventors of the present invention have found that when performing hot press forming (HPF) for the production of hot press forming products by using a plated steel material, the surface stress at the deep processing region where the degree of processing by the mold is severe and surface friction with the mold is severe, And suggests a method for minimizing the occurrence of microcracks formed on the surface of the steel sheet due to the formed cracks.

The microcracks formed on the surface of the steel sheet by the cracks formed on the plating layer are formed in a size of several micrometers to several tens of micrometers. Such micro cracks mainly occur in the plating layer, but when the surface stress is large, And connected to the surface of the connected ground steel sheet. This is because when the surface of the metal mold is in contact with the surface of the coated steel sheet during the hot press forming and the surface of the coated layer is strongly scratched off, cracks are formed in the coated layer. When the surface stress reaches the base steel sheet after cracking developed in the plating layer If the stress is not dispersed at the interface of the plating layer or is not absorbed at the surface of the base steel sheet, it is propagated to the base steel sheet.

The inventors of the present invention have found that it is possible to minimize the friction with the metal mold during processing and effectively prevent cracking and propagation if a stable oxide layer is formed at a high temperature, .

Hereinafter, the present invention will be described in detail.

[Method of Manufacturing Hot Press Molded Parts]

First, a method of manufacturing a hot press molded article which is one aspect of the present invention will be described in detail.

A method of manufacturing a hot press formed article according to the present invention comprises the steps of preparing a plated steel material having a plated layer formed on a base steel sheet; Forming an organic coating layer on the plating layer of the plated steel material; Heating the plated steel material in a heating furnace to form an oxide layer on the organic coating layer; And hot-pressing and cooling the heated plated steel material.

The base steel sheet to be applied in the method of the present invention is not particularly limited as long as it can be hot-dip galvanized. However, in order to have high strength even after processing, which is a steel characteristic of hot press forming, To a martensite transformation is preferable. Typically, 22MnB 5 steel species are representative. As one example, the content of carbon (C) is 0.1-0.3 wt%, the content of manganese (Mn) is 1.0-1.5 wt%, the content of silicon (Si) is 0.02-0.30 wt% , The content of boron (B) is 5 to 45 ppm, and it may be a steel containing some other components.

Preferably, the base steel sheet is plated with a plated steel material prior to the hot press forming (HPF) of the base steel sheet, and the plated steel material is a zinc-based plated steel material, and the plating method is a hot-dip galvanizing method, Plating, or the like, and is not particularly limited. The plated layer formed after plating is a zinc-based plated layer and may be a hot-dip galvanized layer or a hot-dip galvanized layer.

It is preferable that the zinc-based plated layer is secured at 30 g / m ² or more so as to secure corrosion resistance even after hot press forming (HPF), but if it is more than 150 g / m ² , the heating time for alloying becomes long, It is preferable to perform at least 150 g / m ² only in the case where extreme corrosion resistance is required, since there may be a Lipid Metal Embrittlement (LME) due to alloying.

The composition of the zinc-based plated layer is not particularly limited, but may include one or more alloying elements such as Al, Mg, Si, Sn, and Pb. However, for the purpose of improving the adhesion with the base steel sheet, the zinc-based plated layer may further contain aluminum (Al) in an amount of 0.05 to 1.0% by weight.

Al in the plated layer is thickened at the interface between the plated layer and the base steel sheet to form an inhibiting layer (Al-enriched layer) to improve the adhesion between the plated layer and the base steel sheet and also to control diffusion of Fe diffused from the base steel sheet to the plated layer Also. That is, in the process of heating the plated steel for the hot press forming (HPF), alloying is performed by Fe diffused from the base steel sheet to the plating layer, which has an effect of preventing LME embrittlement phenomenon (LME). In order to obtain such an effect, it is necessary to control the concentration of Fe to be alloyed, preferably to 10 to 90%, and more preferably to 50% or less to maintain corrosion resistance. Thus, the preferable concentration of Fe is obtained as an inhibiting layer formed of Al contained in an appropriate amount.

In addition, in heating the plated steel, in order to suppress the evaporation of Zn during heating, a ZnO layer must be formed on the surface of the plating layer or the Fe alloying by the diffusion of Fe from the base steel sheet during the heating process must be uniform. Particularly, in order to uniformize the Fe alloying, it is necessary to uniformly form the inhibiting layer (Al-enriched layer) described above.

The inhibiting layer has an average thickness of 0.01 to 0.10 mu m while mainly containing Al and Fe at the interface between the base steel sheet and the plating layer. Preferably, the inhibiting layer contains 95% or more of crystal grains having a diameter of 500 nm or less, The fine particles are generally formed in a continuous manner as compared with the zinc-based plated layer. If the ratio of the crystal grains having a diameter exceeding 500 nm is more than 5%, Fe is diffused unequally to the zinc based plating layer during the heating process in the heating furnace for the subsequent hot press forming (HPF) There is a problem that a liquid phase containing 50% of Zn is formed. When a liquid phase is formed in the plating layer, a liquid-phase embrittlement phenomenon (LME) may occur during the subsequent molding process, thereby causing macroscopic defects.

In order to control the grain size of the crystal grains of the inhibiting layer, the temperature at which the steel sheet is heated before plating the base steel sheet is controlled to 600 ° C or less, or the specific metal such as Ni is diffused on the surface of the steel sheet before plating, And a method of growing crystal grains small and fine can be applied.

When the plated steel material having the zinc-based plated layer as described above is prepared, an organic coating layer can be formed on the plated layer of the plated steel material.

The constitution of forming the organic coating layer on the plating layer in the present invention is a constitution which constitutes the core of the present invention. When the plated steel material having the organic coating layer is subjected to the heating process in the heating furnace for the subsequent hot press forming (HPF) An oxide layer containing an oxide layer, particularly a Mn-based oxide layer, is formed in the surface layer portion of the plating layer while suppressing oxygen penetration into the atmosphere by the organic coating layer. The oxide layer containing the Mn-based oxide at this time is stable at a high temperature and may be improved in lubrication characteristics with the mold during hot press forming (HPF), as will be described in detail below.

In order to form the organic coating layer on the plating layer, the organic coating composition may be first coated on the plating layer. At this time, the organic film composition may include various metal ion compounds. For example, the organic film composition may be selected from among PTFE (Polytetrafluoroethylene) and PVA (Polyvinyl alcoho) as a metal ion compound such as Mn, Ni, It is preferable to apply the organic film composition containing one substance on the plating layer. At this time, the coating amount of the organic coating composition is preferably applied in an application amount of 500 to 5500 mg / m < ² > so as to form an organic coating layer having a thickness enough to suppress oxygen penetration into the atmosphere. The application method is not particularly limited, but a method such as a bar coater, a roll coater or a curtain coater can be used, and preferably a roll coater method can be used.

Thereafter, in order to perform the hot press forming (HPF) of the plated steel material having the organic coating layer on the plating layer as described above, it is possible to first carry out the step of heating in the heating furnace.

In this case, it is preferable that the step of heating the plated steel material controls the total holding time in the heating furnace and the holding time when the surface temperature of the plated steel reaches the target temperature. That is, the heating time at the time of heating is a time during which the material is held from being charged into the heating furnace until the material is introduced, and the time during which the temperature of the steel material charged into the heating furnace reaches a target temperature (for example, 880 to 950 ° C) .

In the present invention, the total holding time in the heating furnace is preferably set to 4 to 15 minutes, and the holding time after reaching the target temperature is set to 1 to 10 minutes.

The present invention can form an oxide layer containing a Mn-based oxide in the surface layer portion of the plating layer of the plated steel by performing heating in the heating furnace under the above-described conditions. Therefore, when the target temperature and time do not satisfy the above-mentioned range when heating in the heating furnace, formation of the desired oxide layer becomes difficult.

More specifically, if the heating target temperature is lower than 880 DEG C, alloying of the plating layer may not be sufficiently performed. If the total holding time is less than 4 minutes, diffusion of Mn in the steel sheet is insufficient, And there is a fear that the heat treatment is not uniform and the material becomes uneven. On the other hand, if the heating target temperature exceeds 950 DEG C or the total holding time exceeds 15 minutes, the sufficient oxide layer is maintained, but the plating layer may be partially deteriorated, which is undesirable. Furthermore, a minimum holding time is required for the ferrite, which is the initial steel structure after reaching the target temperature, to be transformed into austenite, and it is preferably maintained for 1 to 10 minutes. At this time, if the holding time exceeds 10 minutes, there is a problem that the plating layer is thermally damaged or the productivity is lowered.

The formation of the oxide layer on the surface layer portion of the plating layer by heating can be explained by the following principle.

That is, when the plated steel material having the organic coating layer on the plating layer is subjected to a heating process in a heating furnace for subsequent hot press forming (HPF), the metal ion materials contained in the organic coating layer are heated in a heating process for subsequent hot press forming A thin oxide film is formed on the surface layer of the plating layer, and this oxide film prevents oxygen from diffusing into the plating layer, and the oxide film mainly consists of Zn oxide. In addition, during the heating process, diffusion of Mn into the plating layer occurs from the base steel sheet simultaneously with formation of the oxide film. In the case where there is no such oxide film, the diffused Mn bonds with oxygen in the plating layer to form Mn oxide, In the present invention, since the oxygen partial pressure inside the plating layer is lowered due to the inhibition of oxygen penetration by the oxide film, Mn diffused into the plating layer diffuses to the polar surface layer of the plating layer to balance Gibbs energy, Respectively. Therefore, the oxide layer formed on the surface layer of the plating layer mainly contains Zn oxide, but the oxide layer contains Mn oxide.

At this time, the formed Mn oxide is MnO, MnO _2, Mn ₂ SiO ₄ and MnSiO may be made in one or two or more of the ₃ Mn-based oxide, and the order to obtain the effect of the Mn-based oxide the total inside of the Mn-based oxide Mn It is preferable that the content is from 4.5 to 9.5% by weight. This can be confirmed by the following examples.

That is, the oxide layer formed in the surface layer portion of the plating layer according to the present invention contains Mn in an amount of 4.5 to 9.5%, and further contains about 90% of Zn and the remaining Al.

As described above, the Mn-based oxide in the oxide layer formed densely and uniformly in the surface layer of the plated layer has a strong property at high temperature and a high hardness. Such a Mn-based oxide can impart high temperature lubrication properties to the zinc- .

On the other hand, if a plated steel material having no organic coating layer on the upper surface of the plating layer is heated in a heating furnace, Mn in the base steel sheet naturally diffuses through the inhibiting layer to the plating layer and diffuses into the surface layer portion of the plating layer. That is, Mn has a lower amount of Gibbs free energy change than Fe or Zn and moves to the surface layer of the plating layer to react with oxygen in the atmosphere to form Mn oxide. However, since oxygen diffuses into the surface of the plating layer while Mn diffuses to the surface layer of the plating layer, the point where the concentration of Mn is highest is inside the plating layer, not in the surface layer portion of the plating layer, and Mn oxide is mainly present in the plating layer do.

On the other hand, it can be suggested that a method of forming a Mn oxide by applying metal ions containing Mn directly to the upper part of the plating layer without forming an organic coating layer may be suggested. In this case, Mn applied to the surface layer of the plating layer rapidly reacts with oxygen It is difficult to expect lubrication characteristics due to Mn oxide because ununiform and inefficient Mn oxide is formed into a group.

Further, although the coated metal ion particles may be thickly coated so as to have a lubricating property by itself, in this case, coating coating may be limited and the cost is increased. In addition, since the thick coating layer adheres to the mold at the time of the subsequent hot press forming (HPF), the work efficiency may be hindered, which is not preferable.

The oxide layer containing the Mn-based oxide formed on the surface layer of the plating layer serves to show the lubrication characteristics with the mold during the subsequent hot press forming (HPF). The Mn-based oxide in the oxide layer is formed uniformly and densely because the previously formed organic coating layer effectively prevents the penetration of oxygen and Mn in the coated steel sheet is continuously and smoothly diffused to the surface of the plating layer. Therefore, the oxide layer including the Mn-based oxide thus formed uniformly and densely can minimize the surface friction with the mold during the subsequent hot press forming (HPF), thereby causing a problem in the conventional hot press forming (HPF) The generation of cracks in the plating layer can be suppressed.

In order to obtain the above effect, it is necessary to form the oxide layer with a thickness of 0.1 탆 or more, more preferably 0.5 탆 or more, and preferably up to 20 탆.

Thereafter, the plated steel material having the oxide layer formed on the plating layer by the above heating can be hot-pressed (HPF) and cooled. At this time, the above-described molding and cooling is sufficient by a conventional hot press forming (HPF) method, and thus the present invention does not limit it.

[Hot press molded article]

A hot press molded product which is another aspect of the present invention will be described in detail.

The hot press formed article according to the present invention comprises a base steel sheet; A plating layer formed on the base steel sheet; An organic coating layer formed on the plating layer; And an oxide layer formed on the organic coating layer.

According to the present invention, the oxide layer formed on the outermost layer of the plating layer minimizes the surface friction between the plated steel and the mold during the hot press forming (HPF), and can prevent the occurrence of cracks in the plating layer after final molding. That is, when the conventional plated steel material is subjected to hot press forming (HPF), cracks are generated in the plated layer due to surface friction between the plated layer and the mold and stress applied by the mold, and such cracks have a problem of propagating to the plated steel sheet , In the present invention, by using a plated steel material having an oxide layer containing an Mn-based oxide in the outermost part of the plated layer, it is possible not only to suppress the occurrence of cracks in the plated layer but also to prevent such cracks from being propagated to the steel sheet .

For this purpose, the oxide layer in the hot press-molded article preferably has a thickness of 0.1 탆 or more, more preferably 0.1 to 20 탆.

Further, the oxide layer within the Mn-based oxide is preferably from MnO, MnO _2, Mn ₂ SiO ₄ and MnSiO comprises _three or more of one or two of the Mn oxide in the Mn content is 4.5 ~ 9.5 wt% .

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 )

22MnB5 steel which is galvanized with a plating amount of 50 g / m < ² > on one side was used as the coated steel sheet for the practice of the present invention. After treating the plated steel material with the organic film composition according to the conditions shown in Table 1 below, the plated steel materials were heated in a heating furnace at 900 ° C or 930 ° C for 6 minutes according to the conditions shown in Table 1 below, 1 was used for hot press forming.

Then, the thicknesses of the formed oxide layers and the average depth and maximum depth of the cracks (microcracks) were measured and are shown in Table 1 below.

division Organic coating weight
(mg / m ² ) Heating furnace heating condition
(° C, min) Micro crack
Average depth (μm) Micro crack
Maximum depth (μm) Mn content
(weight%) Grade 1 - 900 ° C, 6 15.5 32.4 3.53 Gangjong 2 200 900 ° C, 6 12.4 29.4 9.09 Gangjong 3 350 900 ° C, 6 8.9 21.0 7.66 Grade 4 500 900 ° C, 6 7.4 13.8 6.44 Grade 5 - 930 ° C, 6 10.5 16.2 3.46 Grade 6 200 930 ° C, 6 7.2 11.5 10.55 Grade 7 350 930 ° C, 6 8.5 12.6 9.41 Grade 8 500 930 ° C, 6 10.8 21.0 8.38

As shown in Table 1, in the case of the steel sheet 1 not treated with the organic coating composition, the average depth of micro cracks formed after molding was 15.5 占 퐉, which indicates that cracks were generated severely due to no organic coating layer formed . On the other hand, in the steel types 2 to 4, the formation of the organic coating layer resulted in the formation of the oxide layer in the surface layer portion of the plating layer during the heating process, and it was confirmed that the micro crack formation was reduced as compared with the steel type 1. Particularly, when the organic coating layer is formed with an adhesion amount of 500 mg / m < ² > as in the case of the steel type 4, sufficient diffusion of Mn in the base steel sheet is ensured while effectively preventing oxygen penetration from the atmosphere, It can be seen that micro cracks are very effectively prevented from occurring.

Fig. 2 shows the results of observing the cross sections of the steel types 1 and 4 by an optical microscope. In the case of steel type 1, the cracks propagated to the steel sheet along the cracks formed in the steel plate after molding, In case of steel type 4, most of the cracks formed on the plating layer were found to be stopped on the surface of the base steel sheet. Even if it propagated to the base steel sheet, the depth was found to be about 7.4 μm on average.

As a result of measuring the content of Mn existing in a depth of 5 mu from the oxide layer formed on the outermost surface layer portion of the steel plates 1 to 4 in the direction of the base steel sheet by GDS, as shown in Fig. 3, It can be confirmed that the Mn content in the oxide layer is high in the steel types 2 to 4.

According to the above results, even in the case of the steel types 5 to 8, the depth of micro cracks will be reduced in the steel type in which the organic film layer is formed as compared with the steel type in which the organic film layer is not formed. In the case of forming the organic film layer, The depth of micro cracks will be reduced.

However, if the results of Table 1 are actually taken into consideration, it can be confirmed that the steel species 5 to 8 do not satisfy the above result inference. That is, even if the organic coating layer is formed at an adhesion amount of 500 mg / m ² (steel type 8), it can be confirmed that the depth of micro cracks formed after molding is similar to the case where the organic coating layer is not formed (steel type 5). These results indicate that when the Mn content of the oxide layer formed in the surface layer of the plating layer is controlled in accordance with the heating conditions and the amount of the organic coating composition applied for forming the organic coating layer in the heating process, It can be expected that it is excellent.

As a result of observing the relationship between the maximum Mn content in the oxide layer formed in the surface layer portion of the plating layer and the average depth of generated microcracks after forming each steel sheet, In the case of 6 minutes, microcracking was reduced as the amount of the organic coating composition was increased. However, when the organic coating composition was heated at 930 ° C for 6 minutes, the effect of reducing the microcracking was excellent even with a relatively small amount of the organic coating composition . According to these results, it is required that the optimum Mn content of the oxide layer necessary for reducing microcracks is required, rather than simply maximizing the Mn content in the surface layer portion of the plating layer by forming the organic coating layer on the surface.

That is, as shown in FIG. 4, when the Mn content in the oxide layer is 4.5 to 9.5 wt%, it can be confirmed that the effect of microcrack reduction is remarkable.

Claims (7)

Base steel sheet;
A plating layer formed on the base steel sheet;
An organic coating layer formed on the plating layer; And
And an oxide layer formed on the organic coating layer,
Wherein the oxide layer is 0.1 ~ 20μm average thickness of the oxide layer is MnO, MnO _2, Mn ₂ SiO ₄ and MnSiO includes a Mn-based oxide composed of one or two or more of ₃ and, Mn in the Mn-based oxide And the content is 4.5 to 9.5% by weight.
delete The method according to claim 1,
Wherein the plating layer is a hot-dip molded product of a hot-dip galvanized layer, a hot-dip galvanized layer, a hot-dip aluminum layer or a hot-rolled aluminum alloy layer.
The method according to claim 1,
Wherein the plating layer is formed with a plating amount of 30 g / m ² to 150 g / m ² .
The method according to claim 1,
Wherein the organic coating layer comprises an organic coating composition comprising one or more metal ions selected from the group consisting of Mn, Ni, B and Mo, and at least one binder selected from the group consisting of PTFE (Polytetrafluoroethylene) and PVA (Polyvinyl alcoho) A hot press molded product formed by coating at a coverage of 5500 mg / m ² .
Preparing a plated steel material having a plated layer formed on a base steel sheet;
Forming an organic coating layer on the plating layer of the plated steel material;
Heating the plated steel material in a heating furnace to form an oxide layer on the organic coating layer; And
And hot-pressing and cooling the heated plated steel material,
The oxide layer is a hot press molded product prepared MnO, MnO _2, Mn ₂ SiO ₄ and MnSiO includes a Mn-based oxide composed of one or two or more of _3, and the Mn content in the Mn-based oxide 4.5 ~ 9.5 wt% Way.
The method according to claim 6,
Wherein the heating step includes a total holding time in the heating furnace and a holding time when the surface temperature of the plated steel reaches 880 to 950 占 폚 is 4 to 15 minutes and 10 minutes or less, respectively.

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