KR102501440B1 - Plated steel sheet for hot press forming having excellent surface property and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a coated steel sheet for hot press forming with excellent surface quality and a method for manufacturing the same, and more particularly, to a method for producing the same, and more particularly, for hot press forming with excellent productivity and corrosion resistance and excellent surface quality as the plating layer does not peel off from the base material even after hot forming. It relates to a coated steel sheet and a manufacturing method thereof.

Description

Plated steel sheet for hot press forming with excellent surface quality and method for manufacturing the same

The present invention relates to a coated steel sheet for hot press forming having excellent surface quality and a manufacturing method thereof.

In order to reduce the weight of automobiles, the use of high-strength steel is increasing. However, high-strength steel has a problem in that it is difficult to form a product having a complex and precise shape as breakage of the material occurs during processing or springback occurs after processing. Hot Press Forming (HPF) is a method for solving these problems, and its application is recently expanding.

The steel sheet for hot press forming is easy to form as it is press-worked in a state where it is usually heated to 800 to 950° C., and has the advantage of increasing the strength of molded products when rapidly cooled through a mold. However, when the steel material is heated to a high temperature, oxidation occurs on the surface of the steel material, and accordingly, there is a problem in that a process of removing oxides on the surface of the steel sheet must be added after press forming. In order to prevent this, as a material for hot press forming, a plated steel sheet having an aluminum plating layer or a zinc plating layer or a mixture of the two is used on the surface of the steel sheet.

However, the aluminum-coated steel sheet has a high melting point of the aluminum plating bath, and the lifetime of the sink roll used in the plating bath is only 3 to 4 days, so the productivity is very poor due to the need to periodically stop production and replace the sink roll. Since the lifetime of the sink roll is inversely proportional to the temperature of the plating bath, it is important to lower the temperature of the plating bath. In addition, there is also a problem that the aluminum plating layer is inferior in corrosion resistance because there is no sacrificial corrosion resistance of the plating layer after hot press forming.

In order to improve these problems, attempts have been made to increase productivity and improve corrosion resistance of the plating layer by increasing the use cycle of the sink roll by lowering the temperature of the plating bath by using galvalume mixed with zinc and aluminum in the hot press process. The mixing of zinc lowers the temperature of the plating bath, thereby increasing the productivity of the plated steel sheet, and the zinc included in the plated layer after plating improves the corrosion resistance of the plated layer compared to aluminum even after hot press forming.

However, the aluminum plating layer to which zinc is added has a problem in that the plating layer is easily peeled off from the base material at a curved portion of the mold after hot press forming, so that the demand for high-quality steel materials requiring strict quality has not been met.

Therefore, while securing corrosion resistance and productivity, there is a need to develop technology with excellent surface quality because the plating layer does not peel off from the base material during hot forming. The level of technology has not been developed.

US Patent Publication No. 6,296,805

According to one aspect of the present invention, it is intended to provide a coated steel sheet for hot press forming having excellent surface quality and a manufacturing method thereof.

Alternatively, according to another aspect of the present invention, it is intended to provide a coated steel sheet for hot press forming and a method for manufacturing the same, which is excellent in productivity and corrosion resistance and has excellent surface quality because the plating layer does not peel off from the base material even after hot forming.

The object of the present invention is not limited to the foregoing. Anyone with ordinary knowledge in the technical field to which the present invention belongs will have no difficulty in understanding the additional objects of the present invention from the contents throughout the present specification.

One aspect of the present invention,

So Ji-cheol; and

a plating layer provided on at least one surface of the base iron and containing Zn: 21 to 34%, Si: 6.3 to 15%, Fe: 3 to 13%, balance Al and other unavoidable impurities, by weight;

including,

A plated steel sheet for hot press forming that satisfies the following relational expressions 1 and 2 is provided.

[Relationship 1]

0.09 ≤ [Zn]/[Si] ≤ 4.8

[Relationship 2]

3.6 ≤ [Al]/[Si] ≤ 8.5

(In relational expressions 1 and 2, the [Zn] represents the average weight % content of Zn in the plating layer, the [Si] represents the average weight % content of Si in the plating layer, and the [Al] represents the average weight % content of Si in the plating layer It represents the average weight % content of Al within.)

In addition, another aspect of the present invention,

In weight percent, Zn: 21 to 34%, Si: 6.3 to 15%, Fe: 2% or less (including 0%), the balance including Al and other unavoidable impurities, and satisfying the following relations 1 and 2 A plating step of immersing the base iron to obtain a steel sheet having a hot-dip plating layer formed on its surface; and

Based on the surface temperature of the hot-dip layer of the steel sheet, the average cooling rate Vc1 in the first section of 630 to 530 ° C is 25 to 90 ° C / s, and the average cooling rate Vc2 in the second section of 529 to 350 ° C is 3 cooling to meet ~15°C/s;

It provides a method for manufacturing a plated steel sheet for hot press forming comprising a.

[Relationship 1]

0.09 ≤ [Zn]/[Si] ≤ 4.8

[Relationship 2]

3.6 ≤ [Al]/[Si] ≤ 8.5

(In the relational expressions 1 and 2, the [Zn] represents the average weight% content of Zn in the hot-dip layer, the [Si] represents the average weight% content of Si in the hot-dip layer, and the [Al] is It represents the average weight percent content of Al in the hot-dip layer.)

In addition, another aspect of the present invention,

A hot press formed part obtained by hot press forming the above-mentioned plated steel sheet for hot press forming is provided.

In addition, another aspect of the present invention,

heat-treating the coated steel sheet for hot press forming obtained by the method for manufacturing a coated steel sheet for hot press forming described above in the range of Ac3 to 950° C. for 1 to 1000 seconds; and

Provided is a method for manufacturing a hot press-formed part, in which the heat-treated plated steel sheet for hot press forming is hot press-formed using a mold.

According to one aspect of the present invention, it is possible to provide a coated steel sheet for hot press forming having excellent surface quality and a manufacturing method thereof.

In addition, according to another aspect of the present invention, it is possible to provide a coated steel sheet for hot press forming with excellent productivity and corrosion resistance, and excellent surface quality because the plating layer does not peel off from the base material even after hot forming, and a manufacturing method thereof.

Various and beneficial advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the process of describing specific embodiments of the present invention.

1 shows a photograph of Inventive Example 11 observed with a scanning electron microscope (SEM).

Terms used herein are for describing specific embodiments and are not intended to limit the present invention. Also, the singular forms used herein include the plural forms unless the related definition clearly dictates the contrary.

The meaning of "comprising" as used in the specification specifies a component, and does not exclude the presence or addition of other components.

Unless otherwise defined, all terms including technical terms and scientific terms used in this specification have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. The terms defined in the dictionary are interpreted to have a meaning consistent with the related technical literature and the currently disclosed content.

Zn-Al-based coated steel sheets to which zinc and aluminum are added have been developed in order to improve the problem of insufficient corrosion resistance and low productivity of aluminum-coated steel sheets, which were mainly used as conventional coated steel sheets for hot press forming.

However, even if the Zn-Al-based plated steel sheet has excellent corrosion resistance and productivity, there is a problem in that a material destruction phenomenon called liquid metal embrittlement (LME) occurs due to the addition of Zn. In addition, during hot forming, the plating layer is easily peeled off from the base material at a curved portion of the mold, resulting in poor adhesion.

Therefore, as a result of intensive studies, the inventors of the present invention have found that in the manufacture of a plated steel sheet having a plated layer of a component system containing Zn, Al, and Si, peeling of the plated layer is mainly a problem when aluminum primary crystals occur, By precisely controlling the components of Zn, Al, and Si, it was discovered that corrosion resistance and productivity as well as LME prevention and plating layer adhesion improvement were effective, and the present invention was completed.

Hereinafter, a coated steel sheet for hot press forming according to an aspect of the present invention will be described in detail.

The plated steel sheet for hot press forming according to the present invention includes iron base; and a plating layer provided on at least one surface of the base iron. At this time, with respect to the type of the base iron, any one that can be used as a steel sheet for hot press forming is applicable without limitation. Therefore, as the base iron, it is sufficient to start austenite transformation at a temperature of 800° C. or higher and to undergo martensitic transformation by rapid cooling simultaneously with forming. As a representative example, in weight percent, C: 0.15 to 0.39%, Mn: 0.5 to 3%, B: 0.01% or less (including 0%), Ti: 0.1% or less (including 0%), the balance Fe and other inevitable Base iron having impurities can be used, and examples thereof include 22MnB5.

In the present invention, the plating layer has a composition including, by weight, Zn: 21 to 34%, Si: 6.3 to 15%, Fe: 3 to 13%, the balance Al and other unavoidable impurities. Hereinafter, the reason for adding each component and the reason for limiting the content will be described in detail. At this time, when indicating the content of each element, it means weight% unless otherwise specifically defined.

Zn: 21-34%

Zinc (Zn) is an element added to improve corrosion resistance. Normally, since an aluminum-plated steel sheet does not have sacrificial corrosion resistance, corrosion resistance can be ensured by adding zinc. In addition, zinc is added to improve productivity by lowering the melting point of the plating bath to improve sink roll life. Therefore, in the present invention, 21% or more of Zn is added in order to secure a desired level of corrosion resistance and productivity. However, since excessive addition of Zn may cause liquid metal embrittlement (LME) during hot press molding, the content of Zn is limited to 34% or less in the present invention. However, from the viewpoint of further improving the desired effect of the present invention, the lower limit of the Zn content may be preferably 23% and more preferably 25%, and the upper limit of the Zn content may be 31.2%.

Si: 6.3 to 15%

Silicon (Si) is an element added to control the alloying of Al included in the plating layer and Fe included in the base iron, and excessive alloying can be prevented by adding Si. In addition, Si is an element that lowers the temperature of the plating bath by lowering the melting point of Al, thereby effectively suppressing the generation of ash in the plating bath. Therefore, in the present invention, 6.3% or more of Si is added in order to appropriately control the degree of alloying between Al and Fe. However, if the Si content in the plating layer is less than 6.3%, the brittle Fe-Al-based interface alloy layer is formed too thick due to excessive alloying, which may cause the plating layer to fall off while developing brittleness during processing. However, if the Si content exceeds 15%, the melting point of the plating bath is excessively higher than the melting point of pure aluminum, and the plating equipment in the plating bath may deteriorate, resulting in poor workability. In the present invention, the Si content is 15% limited to below However, from the viewpoint of further improving the desired effect of the present invention, the lower limit of the Si content may be more preferably 7.0%, and the upper limit of the Si content may be 13.1%.

Fe: 3-13%

The (Fe) content in the plating layer is preferably 3 to 13%. If the Fe content in the plating layer is less than 3%, Al in the plating bath and Fe in the base iron form a non-uniform alloy phase on the surface of the base iron, but a non-uniform Al-Fe alloy phase, resulting in a rough plating surface and increased plating defects. Problems can arise. In addition, when the Fe content in the plating layer exceeds 13%, an excessive Fe-Al-based interface alloy layer is formed, and there is a risk that the plating layer may be dropped during processing due to the brittle Fe-Al-based interface alloy layer. In addition, the excessive Fe-Al-based interface alloy layer formed during plating serves to suppress the diffusion of Fe in the heat treatment process performed during hot press molding, so that the plating layer remains in a liquid state for a long time, thereby solving the problem of seizure in the heating furnace. can cause Therefore, in the present invention, the Fe content in the plating layer is controlled to 3 to 13%. However, in view of further improving the desired effect of the present invention, the lower limit of the Fe content may be more preferably 3.8%, and the upper limit of the Fe content may be 11.2%. On the other hand, the Fe may be contained in a small amount in the plating bath, which may be a small amount of Fe flowing into the plating bath from the base iron during the manufacturing process.

balance Al and other unavoidable impurities

In addition to the composition of the plating layer described above, the remainder may be Al and other unavoidable impurities. Any unavoidable impurities may be included as long as they can be unintentionally incorporated in the manufacturing process of a conventional aluminum-based or aluminum-based coated steel sheet. Since these impurities can be easily understood by those skilled in the art, the present invention is not particularly limited thereto.

Mg: 1.4% or less

Although not particularly limited, the plating layer may optionally further include 1.4% or less of Mg in order to further improve corrosion resistance. The Mg may further improve corrosion resistance by serving as a buffer to suppress decomposition of zinc oxide in a corrosive environment. Therefore, Mg can optionally be added up to 1.4%. However, since Mg is an optional element, the lower limit of the content may not be separately limited. As an example, the lower limit of the Mg content may be 0.05%.

On the other hand, even if the Zn-Al-based coated steel sheet has excellent corrosion resistance and productivity, there is a problem in that a material destruction phenomenon called liquid metal embrittlement (LME) occurs due to the addition of Zn. The liquid metal brittleness refers to a phenomenon in which brittle fracture occurs due to intergranular cracking as zinc in the plating layer diffuses into the base iron during the plating process or the hot forming process. However, when the plated steel sheet with cracks connected from the plating layer to the base iron is used for automobile members, there is a risk of weakening the rigidity of the car body in the event of fatigue failure or vehicle collision due to accumulated stress generated during vehicle operation. , the LME phenomenon in hot press formed parts must be suppressed.

In general, the LME phenomenon does not occur in the case of an aluminum plating layer, but the LME phenomenon easily occurs in the case of adding zinc. Accordingly, as a result of intensive examination, the inventors of the present invention confirmed that the LME generation tendency differs depending on the component combination between Al and Zn.

In addition, when Si is added to prevent alloying in addition to Al and Zn, it has a ternary composition, and in the process of forming a coating layer by solidification after continuous plating, the combination of components of Al, Zn, and Si changes, resulting in aluminum primary crystal. In the case where this occurs, it was additionally found that the adhesion of the plating layer is mainly a problem.

That is, in order to simultaneously secure the corrosion resistance and productivity aimed at in the present invention, the LME phenomenon prevention and the adhesion of the plating layer, the relationship between the three components including Al, Zn, and Si is precisely controlled as shown in the following relational expressions 1 and 2 It was found to be a very important factor.

Therefore, the coated steel sheet for hot press forming according to the present invention preferably satisfies the following relational expressions 1 and 2.

[Relationship 1]

[Zn]/[Si] ≤ 4.8

[Relationship 2]

3.6 ≤ [Al]/[Si] ≤ 8.5

(In relational expressions 1 and 2, the [Zn] represents the average weight % content of Zn in the plating layer, the [Si] represents the average weight % content of Si in the plating layer, and the [Al] represents the average weight % content of Si in the plating layer It represents the average weight % content of Al within.)

Specifically, when the value of [Zn]/[Si] defined in relational expression 1 is less than 0.09, it may be difficult to secure a desired level of corrosion resistance. In addition, when the value of [Zn]/[Si] exceeds 4.8, even if the corrosion resistance of the plating layer is secured, the adhesion of the plating layer may decrease or the LME phenomenon may occur.

In addition, if the value of [Al] / [Si] defined in the relational expression 2 is less than 3.6, the surface shape may become rough because smooth aluminum oxide is not sufficiently generated on the surface after heat treatment. In addition, if the value of [Al]/[Si] exceeds 8.5, after hot press forming, a Fe-Al alloy phase with high corrosion resistance is generated in the coating layer, so that the coated steel sheet is blanked to fit the parts before hot press forming, or the steel sheet is There is a risk of peeling of the plating due to the Fe-Al-based alloy phase due to impact during transportation.

Meanwhile, according to one aspect of the present invention, the plating layer may include an Al-Zn-based plating layer; and a Fe-Al-based interfacial alloy layer provided between the Al-Zn-based plating layer and the base iron.

The Fe-Al-based interface alloy layer is a layer provided in contact with the base iron, and is a layer containing an intermetallic compound of Fe and Al formed through an alloying reaction of Fe and Al, and the intermetallic compound of Fe and Al may include, for example, FeAl, FeAl ₃ , Fe ₂ Al ₅ and the like. At this time, the matters commonly known in the art may be equally applied to the Fe-Al-based interfacial alloy layer. Therefore, the Fe-Al-based interface alloy layer may include, by weight, Fe: 30 to 50%, Al: 35 to 55%, and in addition to Fe and Al, some of the components that may be included in the plating layer, such as Zn and Si, It may further contain (eg, Si 27% or less).

In other words, the Fe-Al-based interface alloy layer, by weight, Fe: 30 to 50%, Al: 35 to 55%, Si: 5 to 27%, Zn: 4% or less (including 0%) and the rest May contain impurities.

In addition, the Al-Zn-based plating layer may mean a layer other than the Fe-Al-based suppression layer among the above-described plating layers. That is, among the plating layers formed while the hot-dip plating layer to be described later solidifies, it may mean a layer provided on the Fe-Al-based suppression layer and distinguished from the Fe-Al-based suppression layer by content. Specifically, the Al-Zn-based plating layer may include Zn: 1531%, Si: 2-12%, Fe: 10% or less, the balance Al and other unavoidable impurities, by weight%.

For example, when analyzing the plating layer by GDS, the point where the Fe content is 50% is determined as the interface between the base iron and the plating layer, and the thickness of the Fe-Al system is less than 30% compared to the Al-Zn system. The GDS value measured at the midpoint of the depth where the Fe content is 50% is determined as the value of the composition of the Al-Zn system. The composition can be determined as the average value of the values of EDS analysis more than once.

According to one aspect of the present invention, the weight ratio (Sb/Sa) of the average Si content (Sb) in the Fe-Al-based alloy layer and the average Si content (Sa) in the Al-Zn-based plating layer may be 1 to 3.1. . If the value of Sb/Sa is less than 1, excessive Si phase is formed on the Al—Zn phase during solidification of the plating layer, and there is a risk of surface defects due to the hard Si phase on the surface of the plating layer. On the other hand, when the value of Sb/Sa is 3.1 or more, Si penetrates into the Fe-Al-based crystal structure in a state where the Si content in the Fe-Al-based alloy layer is excessive, thereby inhibiting the Fe diffusion coefficient of the Fe-Al-based Fe-Al system and hot pressing. During molding, alloying of the initial plating layer may be inhibited to cause problems such as burning, and accordingly, there is a concern that adhesion of the plating layer may be impaired.

That is, usually, the plated steel sheet for hot press forming is used for automobiles and then used after electrodeposition coating. However, if there is a severe color difference on the surface or non-uniform oxide is generated during the heat treatment process, it becomes difficult to use it as a part. Such surface defects occur when the components present in the plating layer are not uniformly alloyed during the heat treatment process and different components are locally gathered. It was confirmed that the aforementioned uniform alloying could be achieved.

In addition, according to one aspect of the present invention, based on the cross section of the Al-Zn-based plating layer in the thickness direction, the average Si content of the Al-Zn-based plating layer per unit area of 100 μm ² of the plating layer compared to the Si content It may include areas enriched by 40% or more. As such, by including the Si-enriched region, the Si phase concentrated at the interface at the beginning of heat treatment and the concentrated Si phase of the plating layer are simultaneously dissolved in molten aluminum while maintaining a uniform Si content throughout the plating layer, thereby improving plating adhesion after hot press molding. can be improved

Further, according to one aspect of the present invention, based on the cross section of the Al-Zn-based plating layer in the thickness direction, the Si phase may be included in an area fraction of 3 to 30%. In this case, the Si phase means a phase containing 40 to 99.9% of Si in weight percent.

In the present invention, by controlling the Si phase to include about 3 to 30%, the drop-off phenomenon derived from the highly brittle Fe-Al-based interface alloy layer at the interface between the base iron and the plating layer before hot forming is prevented, and at the same time, zinc is included. By doing so, peeling of the plating layer, which easily occurs during hot forming, can be prevented at the same time. Therefore, even if a plated steel sheet having a plated layer of a component system containing Zn and Al is used for hot press forming, corrosion resistance, productivity, LME prevention, and adhesion of the plated layer can be further improved.

That is, hot-press-formed parts are mainly applied to parts with more complex shapes and higher strength than cold-formed products. For these reasons, due to the nature of hot press molding in which deep processing is required to manufacture complex shapes and molds are frequently used, a highly brittle Fe-Al-based interface alloy layer is excessively formed due to friction and tolerance between the mold and the plating layer. Apart from the peeling-off phenomenon caused by this, there is a problem that the plating layer tends to peel off after applying the hot press forming.

Until now, attention has not been paid to the problem that the plating layer is easily peeled off after such hot press forming, but as a result of intensive examination, the present inventors have found that the above-mentioned value of Sb / Sa, presence or absence of a region in which the Si content is concentrated (C), and By precisely controlling the conditions such as the area fraction of the Si phase, part molding mainly occurs in the Al-Zn-based plating layer and the Fe-Al-based interface alloy layer as well as the drop-off phenomenon derived from the base iron and the Fe-Al-based interface alloy layer. It was confirmed that even the later defect problem could be solved.

Next, a method for manufacturing a coated steel sheet for hot press forming, which is another aspect of the present invention, will be described in detail. However, this does not mean that the coated steel sheet for hot press forming of the present invention must be manufactured by the following manufacturing method.

The coated steel sheet for hot press forming according to the present invention contains, by weight, Zn: 21 to 34%, Si: 6.3 to 15%, Fe: 2% or less (including 0%), the balance Al and other unavoidable impurities, A plating step of immersing the base iron in a plating bath that satisfies the following relational expressions 1 and 2 to obtain a steel sheet having a hot-dip plating layer formed on a surface thereof. At this time, the description of the plating layer described above can be equally applied to the reason for adding the plating bath component and the reason for limiting the content. That is, since the composition of the plating layer includes components derived from the base iron, the plating bath and its components may be slightly different. and the reason for limiting the upper limit is the same.

[Relationship 1]

0.09 ≤ [Zn]/[Si] ≤ 4.8

[Relationship 2]

3.6 ≤ [Al]/[Si] ≤ 8.5

(In the relational expressions 1 and 2, the [Zn] represents the average weight% content of Zn in the hot-dip layer, the [Si] represents the average weight% content of Si in the hot-dip layer, and the [Al] is It represents the average weight percent content of Al in the hot-dip layer.)

Subsequently, although not particularly limited, according to one aspect of the present invention, after the plating step, the average cooling rate Vc1 in the first section of 630 to 530 ° C. is 25 to 90 °C/s, and cooling may be performed such that the average cooling rate Vc2 satisfies 3 to 15 °C/s in the second section of 529 to 350 °C. At this time, the cooling may be controlled to satisfy the following relational expression 3.

[Relationship 3]

Vc2 ≤ Vc1/7

As described above, by precisely controlling the cooling rate of the hot-dip plating layer after the plating step, an effect of improving the surface quality of the plating layer can be exhibited.

Specifically, by greatly controlling the cooling rate to 25 to 90° C./s in the first section, the size of spangles inherent in aluminum becomes smaller and finer at the time of solidification of the plating layer in the first section, resulting in uniform surface roughness and improved surface quality. effect can be exerted. In addition, by controlling the average cooling rate to a low level of about 3 to 15° C./s in the second section, it is possible to reduce the amount of air blown from the cooling tower and to reduce contamination of the plating layer.

In addition, according to one aspect of the present invention, before the plating step, the base iron may be heat treated in an annealing furnace having a temperature of 650 to 850 ° C and a hydrogen ratio of 0.1 to 30%. The heat treatment is performed at a temperature above recrystallization to prevent work hardening of the material in a continuous plating process, and to improve plating properties by maintaining the base steel sheet at a temperature higher than that of the plating bath. If the heat treatment temperature is less than 650 ° C., there is a disadvantage in that there may be deformation and meandering of the material due to work hardening when passing through a roll such as a continuous process. In addition, when the heat treatment temperature exceeds 850° C., Mn and Si present in the base steel sheet may be concentrated on the base surface to form oxides, thereby deteriorating properties. Therefore, the temperature of the heat treatment before plating is preferably controlled to 650 to 850 ° C, more preferably, the lower limit of the heat treatment temperature may be 700 ° C, and the upper limit of the heat treatment temperature may be 810 ° C.

In addition, the heat treatment may be controlled in a reducing atmosphere in which the ratio of hydrogen is 0.1 to 30% and the balance is composed of nitrogen gas. If high-purity nitrogen gas is not used, it is difficult to maintain a reducing atmosphere in the heat treatment furnace, and it may be difficult to clean the surface of the steel sheet. At this time, when hydrogen gas is added in an amount of 0.1% or more, the dew point in the heat treatment furnace can be lowered, and a reducing atmosphere can be maintained to prevent further oxidation of the base iron and to clean the base surface. However, if the fraction of the hydrogen gas exceeds 30%, there is a risk of explosion due to heat treatment due to contact with oxygen when it leaks into the atmosphere. Therefore, in the present invention, the hydrogen ratio during heat treatment may be controlled to 0.1 to 30%, more preferably, the lower limit of the hydrogen ratio may be 5%, and the upper limit of the hydrogen ratio may be 25%.

At this time, although not particularly limited, the annealing may be performed in an atmosphere having a dew point temperature of -30 to -60 °C. If the dew point temperature during annealing is less than -30 ° C, a sufficient reducing atmosphere may not be created, and there may be a problem with the reducibility of Fe oxide generated on the surface. This can lead to costly problems.

According to one aspect of the present invention, the temperature of the plating bath may be controlled to be Tb+20°C to Tb+80°C based on the melting point (Tb) of the plating bath. If the temperature of the plating bath is less than Tb + 20 ° C, the plating bath hardens at a location where heat can escape to the outside, such as a sink roll in the plating pot, and a problem of low fluidity may occur at a low temperature as a whole. In addition, when the temperature of the plating bath exceeds Tb + 80 ° C., the plating bath temperature is high, and the roll replacement cycle is shortened due to deterioration of facilities and the like, and high temperature operation may be difficult. Therefore, in the present invention, it is preferable to control the temperature of the plating bath in the range of Tb+20°C to Tb+80°C.

Further, another aspect of the present invention provides a hot press formed member obtained by hot press forming the above-described plated steel sheet for hot press forming.

In addition, another aspect of the present invention includes the steps of heat-treating the coated steel sheet for hot press forming obtained by the method for manufacturing a coated steel sheet for hot press forming described above in the range of Ac3 to 950° C. for 1 to 1000 seconds; and

Provided is a method for manufacturing a hot press-formed part, in which the heat-treated plated steel sheet for hot press forming is hot press-formed using a mold.

However, for the hot press-formed member and its manufacturing method, matters commonly known in the art can be applied without limitation. Therefore, it is not specifically limited in this specification.

(Example)

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

(Experimental Example 1)

In weight percent, a base iron containing C: 0.23%, Mn: 1.3%, B: 0.002%, Ti: 0.03%, Si: 0.25%, Cr: 0.15%, the balance Fe and other impurities was prepared. After heat-treating the base iron at 750 ° C in an annealing furnace with a dew point temperature of -50 ° C and a mixed atmosphere of 5% hydrogen and 95% nitrogen, it was immersed in a plating bath under the conditions shown in Table 1 below, and melted on the surface of the base iron. A plating layer was formed. Thereafter, under the conditions shown in Table 1 below, the coated steel sheet for hot press forming was manufactured by processing with an air knife to control the coating amount per side and then cooling.

No. Plating bath composition [wt%]
(residual impurity) Plating bath melting point (Tb) [℃] plating bath temperature
[℃] Plating per side
Coating weight [g/m ² ] Al Zn Si Fe Comparative Example 1 77 16.5 5.7 0.7 587 620 94 Comparative Example 2 73 22.1 4.1 0.6 585 620 123 Comparative Example 3 67 27.5 4.7 0.7 567 610 93 Comparative Example 4 72 25.4 2.0 0.8 594 620 102 Comparative Example 5 73 25.9 1.1 0.2 600 630 106 Comparative Example 6 65 29.2 5.6 0.6 555 590 43 Comparative Example 7 65 29.5 5.0 0.5 559 590 61 Comparative Example 8 59 38.0 2.0 0.8 565 600 81 Comparative Example 9 70 20.8 9.0 0.2 561 600 62 Comparative Example 10 79 20.2 0 0.8 621 650 63 Invention Example 1 70 22.6 6.7 0.5 563 620 37 Invention example 2 69 21.5 9.3 0.5 568 630 39 Inventive example 3 64 27.0 8.1 0.8 558 600 58 Inventive example 4 64 27.0 8.1 0.8 558 600 98 Inventive Example 5 61 26.1 12.5 0.6 639 670 60 Inventive example 6 61 32.0 6.8 0.7 539 590 93

After preparing specimens for each plated steel sheet for hot press forming obtained from the method of Table 1, the alloy composition of the plating layer was analyzed. Specifically, the alloy composition of the plating layer was first dissolved for 10 minutes with NaOH solution to remove the aluminum oxide film present on the surface, then dissolved for 30 minutes using HCl solution and inhibitor, and then combined with the two solutions dissolved in ICP -OES (Inductively Coupled Plasma Optical Emission Spectroscopy) was measured and shown in Table 2 below.

No. Plating layer composition [wt%] Al Zn Si Fe [Zn]/[Si] [Al]/[Si] Comparative Example 1 77.6 14.9 4.2 3.3 3.5 18.5 Comparative Example 2 69.4 24.0 3.8 2.8 6.3 18.3 Comparative Example 3 67.4 26.3 3.6 2.7 7.3 18.7 Comparative Example 4 72.5 17.3 2.5 7.7 6.9 29.0 Comparative Example 5 73.9 17.0 1.5 6.6 11.3 49.9 Comparative Example 6 58.1 32.3 5.9 3.7 5.5 9.8 Comparative Example 7 63.3 26.5 5.6 4.7 4.7 11.3 Comparative Example 8 54.1 35.0 2.6 8.3 13.5 20.8 Comparative Example 9 66.5 19.8 10.1 3.6 2.0 6.6 Comparative Example 10 64.1 20.1 0.1 14.6 201.0 641.0 Invention Example 1 62.8 21.9 7.4 7.9 3.0 8.5 Invention example 2 62.7 21.1 10.8 5.4 2.0 5.8 Inventive example 3 60.5 22.5 10.3 6.7 2.2 5.9 Inventive example 4 60.4 25.9 9.9 3.8 2.6 6.1 Inventive Example 5 58.3 24.1 13.1 4.6 1.8 4.4 Inventive example 6 55.3 31.2 7.0 6.5 4.5 7.9

After taking specimens of the coated steel sheet manufactured by the above-described method, they were cut into blanks, and after the blank specimens reached 900 °C in a furnace at 900 °C, they were maintained for 2 minutes. Thereafter, the heated blank specimen was transferred to the mold within 20 seconds, hot-formed using the mold, and then cooled at a cooling rate of 20° C./s to obtain a hot-formed member.

Next, whether or not LME occurred, corrosion resistance after hot press forming, whether detachment occurred at the interface between the base iron and the plating layer before hot forming, and plating adhesion after hot press forming were evaluated according to the following criteria, and are shown in Table 3 below.

[Whether or not LME has occurred]

The cross section in the thickness direction (meaning the direction perpendicular to the rolling direction) of each specimen subjected to hot press molding was observed with an optical microscope to measure the crack depth of the base material. Occurrence' specimen was marked.

[Corrosion resistance after hot press forming]

After the specimen was heat-treated to reach 900 ° C., maintained for 2 minutes and rapidly cooled, the specimen was evaluated using SST (salt spray test), one of the accelerated corrosion evaluations. That is, after removing the corrosion products of the specimens subjected to salt spray for 30 days in SST by shot blasting, the maximum depth of cracks generated by corrosion in the base steel was measured.

○: When the crack depth is 30% or less of the base metal thickness

Х: When the crack depth exceeds 30% of the base metal thickness

[whether drop-off occurs at the interface between the base iron and the plating layer]

In order to evaluate the occurrence of drop-off at the interface between the base iron and the plating layer, a 90-degree bending process was performed on the specimen for the plated steel sheet before hot press forming, and then a Fe-Al-based interface alloy layer with strong brittleness at the interface between the base iron and the plating layer. It was evaluated whether dropout by . The case where dropout occurred was marked as 'dropout'.

[Plating adhesion after hot press molding]

After the specimen was subjected to heat treatment to reach 900 ° C., the specimen heated for 2 minutes was press-worked to measure the peeling phenomenon of the plating layer occurring on the surface. Among the specimens, the length of the major axis of the part peeled off from the part was measured and evaluated according to the following criteria.

◎: Less than 1 mm of the long axis of the peeled area

○: The length of the major axis of the peeled area is 1 mm or more and less than 3 mm

Х: The length of the major axis of the peeled area is 3 mm or more

No. LME occurrence
Whether hot press
After molding
corrosion resistance Base iron and plating layer
drop-off at the interface
Occurrence hot press
After molding
Plating Adhesion Comparative Example 1 - Х - Х Comparative Example 2 - ○ - Х Comparative Example 3 - ○ - Х Comparative Example 4 - Х leaving out Х Comparative Example 5 - Х leaving out Х Comparative Example 6 - ○ - Х Comparative Example 7 - ○ - Х Comparative Example 8 generation ○ leaving out Х Comparative Example 9 - Х leaving out Х Comparative Example 10 - ○ leaving out Х Invention Example 1 - ○ - ○ Invention example 2 - ○ - ○ Inventive example 3 - ○ - ○ Inventive example 4 - ○ - ○ Inventive Example 5 - ○ - ○ Inventive example 6 - ○ - ○

As can be seen from Table 3, the inventive examples satisfying all of the composition of the plating layer, relational expression 1, relational expression 2, and manufacturing conditions specified in the present invention do not cause drop-off at the interface between the base iron and the plating layer, while suppressing the occurrence of LME, and It was confirmed that both corrosion resistance after press molding and plating adhesion after hot press molding were excellent.

On the other hand, in the case of Comparative Examples 1 to 9 that do not satisfy at least one of the plating layer composition, relational expression 1, relational expression 2, and manufacturing conditions specified in the present invention, LME characteristics, whether or not detachment occurs at the interface between the base iron and the plating layer, after hot press forming It was confirmed that at least one of corrosion resistance and coating adhesion after hot press molding was inferior.

(Experimental Example 2)

Except for heat-treating the base iron at 750 ° C. in an annealing furnace with a dew point temperature of -50 ° C and a mixed atmosphere of 5% hydrogen and 95% nitrogen, and cooling the hot-dipped plating layer under the conditions shown in Table 4 below after plating, the above-mentioned A coated steel sheet for hot press forming was manufactured in the same manner as in Experimental Example 1.

No. Plating bath composition [wt%]
(residual impurity) Plating bath melting point (Tb)
[℃] plating bath temperature
[℃] per side
Plated
Coating weight [g/m ² ] Vc1*
[℃/s] Vc2*
[℃/s] Al Zn Si Fe Comparative Example 11 77 16.5 5.7 0.7 587 620 94 60 5 Comparative Example 12 73 22.1 4.1 0.6 585 620 123 60 5 Comparative Example 13 67 27.5 4.7 0.7 567 610 93 50 10 Comparative Example 14 72 25.4 2.0 0.8 594 620 102 60 5 Comparative Example 15 73 25.9 1.1 0.2 600 630 106 60 10 Comparative Example 16 65 29.2 5.6 0.6 555 590 43 30 5 Comparative Example 17 65 29.5 5.0 0.7 559 590 61 30 5 Comparative Example 18 59 38.0 2.0 0.8 565 600 81 40 5 Inventive Example 7 70 22.6 6.7 0.5 563 620 37 60 5 Inventive Example 8 69 21.5 9.3 0.2 568 630 37 70 10 Inventive Example 9 64 27.0 8.1 0.8 558 600 58 40 5 Inventive Example 10 64 27.0 8.1 0.8 558 600 98 80 5 Inventive Example 11 61 26.1 12.5 0.6 639 670 60 80 10 Inventive Example 12 61 32.0 6.8 0.7 539 590 93 305 5

Vc1*: average cooling rate in the range of 630 to 530 ° C based on the surface temperature of the hot-dip layer

Vc2*: average cooling rate in the range of 530 to 350 ° C based on the surface temperature of the hot-dip layer

In the same manner as in Experimental Example 1, the composition of the plating layer of the plated steel sheet for hot press forming obtained from each invention example and example was analyzed and shown in Table 5 below.

No. Plating layer composition [wt%] Al Zn Si Fe [Zn]/[Si] [Al]/[Si] Comparative Example 11 77.6 14.9 4.2 3.3 3.5 18.5 Comparative Example 12 69.4 24.0 3.8 2.8 6.3 18.3 Comparative Example 13 67.4 26.3 3.6 2.7 7.3 18.7 Comparative Example 14 72.5 17.3 2.5 7.7 6.9 29.0 Comparative Example 15 73.9 17.0 1.5 6.6 11.3 49.9 Comparative Example 16 58.1 32.3 5.9 3.7 5.5 9.8 Comparative Example 17 63.3 26.5 5.6 4.7 4.7 11.3 Comparative Example 18 54.1 35.0 2.6 8.3 13.5 20.8 Inventive Example 7 59.5 21.9 7.4 11.2 3.0 8.0 Inventive Example 8 59.1 21.1 10.8 9.0 2.0 5.5 Inventive Example 9 60.5 22.5 10.3 6.7 2.2 5.9 Inventive Example 10 60.4 25.9 9.9 3.8 2.6 6.1 Inventive Example 11 58.3 24.1 13.1 4.6 1.8 4.4 Inventive example 12 55.3 31.2 7.0 6.5 4.5 7.9

Subsequently, cross-sectional specimens cut in the thickness direction (meaning a direction perpendicular to the rolling direction of the steel sheet) for each of the above-described plated steel sheets were made, and then observed with a scanning electron microscope (SEM). Through this, a plating layer is formed on the base iron, a Fe-Al-based interface alloy layer in contact with the base iron is formed as the plating layer, and an Al-Zn plating layer is formed on the Fe-Al-based interface alloy layer. Confirmed.

In addition, with respect to the plating layer of each plated steel sheet, the average value of Si was measured at points of 1/2t based on the total thickness t of the plating layer using GDS, and expressed as Sa below. In addition, for the cross-sectional specimens in the thickness direction of each plated steel sheet, EDS point analysis was performed three or more times on the Fe-Al-based interface alloy layer formed adjacent to the base iron by observing with an electron microscope, and the average of the measured values was Sb indicated by

In addition, with respect to the cross-sectional specimen, through SEM and EDS point analysis in the same manner as described above, the region in which the Si content is concentrated by 40% or more compared to the average Si content of the Al-Zn-based plating layer per unit area of 100 μm ² of the plating layer was present (C). In addition, similarly, using SEM and EDS, the Si phase containing 40 to 99.9% of Si was detected, and the area fraction (D) of the Si phase was measured based on the cross-section of the plating layer. At this time, the value of D was measured in a region secured to a total length of 75 μm or more in the rolling direction. These measured values are shown in Table 6 below.

No. Sa* Sb* Sb/Sa C* D* Comparative Example 11 3.4 13.0 3.8 Х - Comparative Example 12 3.2 10.4 3.2 Х - Comparative Example 13 2.8 16.4 5.8 Х - Comparative Example 14 2.2 12.7 5.9 Х - Comparative Example 15 1.0 10.3 10.0 Х - Comparative Example 16 5.1 18.0 3.5 Х - Comparative Example 17 4.9 15.8 3.2 Х - Comparative Example 18 2.2 12.4 5.6 Х - Inventive Example 7 5.9 17.6 3.0 ○ 4 Inventive Example 8 10.3 12.0 1.2 ○ 12 Inventive Example 9 9.6 18.1 1.9 ○ 10 Inventive Example 10 9.5 13.0 1.4 ○ 7 Inventive Example 11 8.6 16.7 1.9 ○ 19 Inventive example 12 6.3 15.4 2.4 ○ 5

Sa*: Average Si content in Al-Zn-based plating layer [wt%]

Sb*: Average Si content in Fe-Al-based alloy layer [wt%]

C*: Based on the section in the thickness direction (direction perpendicular to the rolling direction) of the Al-Zn-based plating layer, there is a region where the Si content is concentrated by 40% or more compared to the average Si content of the plating layer per unit area of 100 μm ² of the plating layer Whether or not [○/Х]

D*: Area fraction of Si phase based on the cross section in the thickness direction of the Al-Zn-based plating layer [%]

For each plated steel sheet thus obtained, the characteristics shown in Table 7 were evaluated in the same manner as in Experimental Example 1. In addition, surface quality characteristics were additionally evaluated according to the following criteria and are shown in Table 7.

[Surface quality]

With respect to the specimen, it was visually observed to evaluate whether there was a surface color difference and whether an overall staining phenomenon was found due to excessively generated zinc oxide on the surface, and evaluated according to the following criteria.

Good: No surface color differences and surface stains observed

Surface Color Difference: The presence of an area of 50 mm ² or more on the surface that differs in color from the surrounding area.

Surface Staining: Staining is found on the surface.

No. LME crack After hot press forming
corrosion resistance Occurrence of drop-off at the interface between the base steel and the plating layer After hot press forming
Plating Adhesion surface quality Comparative Example 11 - Х - Х Good Comparative Example 12 - ○ - Х Good Comparative Example 13 - ○ - Х Good Comparative Example 14 - Х leaving out Х surface color difference Comparative Example 15 - Х leaving out Х surface color difference Comparative Example 16 - ○ - Х Good Comparative Example 17 - ○ - Х Good Comparative Example 18 generation ○ leaving out Х surface stains Inventive Example 7 - ○ leaving out ◎ Good Inventive Example 8 - ○ leaving out ◎ Good Inventive Example 9 - ○ - ◎ Good Inventive Example 10 - ○ - ◎ Good Inventive Example 11 - ○ - ◎ Good Inventive example 12 - ○ - ◎ Good

As can be seen in Table 7, the plating layer composition, relational expression 1, and relational expression 2 specified in the present invention are satisfied, and the value of Sb/Sa specified in Table 6: 1 to 3.1, C exists, and D is 3 In the case of inventive examples satisfying at least one of ~30%, it was confirmed that not only LME, corrosion resistance, plating adhesion, and surface quality, but also surface properties were very excellent.

In particular, a photograph taken by observing a cross-sectional specimen in the thickness direction of the plated steel sheet obtained from Inventive Example 11 with a scanning electron microscope (SEM) is shown in FIG. 1 . Through this, it was confirmed that there was a region in which the Si content was concentrated by 40% or more compared to the average Si content of the plating layer in the plating layer, and the Si phase with the Si content of 40% or more contained 3 to 30% in area fraction.

On the other hand, the plating layer composition specified in the present invention, relational expressions 1 and 2, and the value of Sb / Sa specified in Table 6: 1 to 3.1, C exists, and D does not satisfy at least one of 3 to 30%. In the case of the examples, one or more of the aforementioned properties were found to be inferior.

Claims (14)

So Ji-cheol; and
A plating layer provided on at least one surface of the base iron and containing, by weight, Zn: 21 to 34%, Si: 6.3 to 15%, Fe: 3 to 13%, the balance Al and other unavoidable impurities,
Satisfy the following relations 1 and 2,
The plating layer is
Al-Zn-based plating layer; and
Including; Fe-Al-based interface alloy layer provided between the Al-Zn-based plating layer and the base iron,
The weight ratio (Sb / Sa) of the average Si content (Sb) in the Fe-Al-based interfacial alloy layer and the average Si content (Sa) in the Al-Zn-based plating layer is 1 to 3.1, plated steel sheet for hot press forming.
[Relationship 1]
0.09 ≤ [Zn]/[Si] ≤ 4.8
[Relationship 2]
3.6 ≤ [Al]/[Si] ≤ 8.5
(In relational expressions 1 and 2, the [Zn] represents the average weight % content of Zn in the plating layer, the [Si] represents the average weight % content of Si in the plating layer, and the [Al] represents the average weight % content of Si in the plating layer It represents the average weight % content of Al within.)
The method of claim 1,
The plated layer is a plated steel sheet for hot press forming containing 25 to 34% of Zn in weight%.
delete delete The method of claim 1,
Based on the cross section of the Al-Zn-based plating layer in the thickness direction, the Si content is concentrated by 40% or more relative to the average Si content of the Al-Zn-based plating layer per unit area of 100 μm ² of the plating layer. Plated steel sheet for hot press forming.
The method of claim 1,
A plated steel sheet for hot press forming containing, in area fraction, 3 to 30% of a Si phase based on a cross section in the thickness direction of the Al-Zn-based plating layer.
The method of claim 6,
The Si phase is a plated steel sheet for hot press forming containing 40 to 99.9% of Si in weight%.
In weight percent, Zn: 21 to 34%, Si: 6.5 to 15%, Fe: 2% or less (including 0%), the balance including Al and other unavoidable impurities, and satisfying the following relations 1 and 2 A plating step of dipping the base iron to obtain a steel sheet having a hot-dip plating layer formed on its surface;
After the plating step, controlling the amount of plating per side by performing an air knife treatment; and
Based on the surface temperature of the hot-dipped plating layer for the steel sheet whose coating amount is controlled, the average cooling rate Vc1 is 25 to 90 °C/s in the first section of 630 to 530 °C and the average in the second section of 529 to 350 °C Cooling so that the cooling rate Vc2 meets 3 to 15 ° C / s;
The method of manufacturing a plated steel sheet for hot press forming, wherein the temperature of the plating bath is controlled to be Tb + 20 ° C to Tb + 80 ° C based on the melting point (Tb) of the plating bath.
[Relationship 1]
0.09 ≤ [Zn]/[Si] ≤ 4.8
[Relationship 2]
3.6 ≤ [Al]/[Si] ≤ 8.5
(In the relational expressions 1 and 2, the [Zn] represents the average weight% content of Zn in the hot-dip layer, the [Si] represents the average weight% content of Si in the hot-dip layer, and the [Al] is It represents the average weight percent content of Al in the hot-dip layer.)
delete The method of claim 8,
A method for manufacturing a coated steel sheet for hot press forming that satisfies the following relational expression 3.
[Relationship 3]
Vc2 ≤ Vc1/7
In claim 8,
Before the plating step, the hot press further comprising heat treating the base iron in an annealing furnace having a temperature of 650 to 850 ° C, a dew point temperature of -30 to -60 ° C, and a hydrogen ratio of 0.1 to 30%. Method for manufacturing a coated steel sheet for forming.
delete A hot press-formed part obtained by hot press-forming the coated steel sheet for hot press forming according to any one of claims 1, 2, and 5 to 7.
Heat-treating the plated steel sheet for hot press forming obtained by the method for manufacturing a coated steel sheet for hot press forming according to any one of claims 8, 10, and 11 in the range of Ac3 to 950° C. for 1 to 1000 seconds; and
A method of manufacturing a hot press-formed member, wherein the heat-treated plated steel sheet for hot press forming is hot press-formed using a mold.

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