KR20220089439A - 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 plated steel sheet for hot press forming with excellent surface quality and a method for manufacturing the same, and more particularly, for hot press forming excellent in surface quality because the plating layer does not peel off from a base material even after hot forming while having excellent productivity and corrosion resistance. It relates to a plated steel sheet and a method for manufacturing the same.

Description

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

The present invention relates to a plated steel sheet for hot press forming excellent in surface quality and a method for manufacturing the same.

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

A steel sheet for hot press forming is easy to form by press-working in a state of being heated to 800 to 950 ° C. 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, a zinc plating layer, or a mixture of both 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 lifespan of the sink roll used in the plating bath is only 3 to 4 days. Since the life 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 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 this problem, an attempt was made to increase productivity and improve corrosion resistance of the plating layer by increasing the cycle of use of the sink roll by lowering the temperature of the plating bath by using galvanium mixed with zinc and aluminum in the hot pressing process. According to the mixing of zinc, the temperature of the plating bath is lowered and the productivity of the plated steel sheet is increased, and the corrosion resistance of the plating layer is improved compared to aluminum even after hot press forming by zinc contained in the plating layer after plating.

However, since the zinc-added aluminum plating layer has a problem in that the plating layer is easily peeled off from the base material at the curvature portion of the mold after hot press forming, it has not been able to meet the demand for high-grade steel materials that require strict quality so far.

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

US Patent Publication No. 6,296,805

According to one aspect of the present invention, it is an object of the present invention to provide a plated steel sheet for hot press forming excellent in surface quality and a method for manufacturing the same.

Alternatively, according to another aspect of the present invention, it is an object to provide a plated steel sheet for hot press forming excellent in 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 method for manufacturing the same.

The subject of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the contents throughout the present specification.

One aspect of the present invention is

So Ji-cheol; and

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

including,

Provided is a plated steel sheet for hot press forming that satisfies the following Relations 1 and 2.

[Relational Expression 1]

0.09 ≤ [Zn]/[Si] ≤ 4.8

[Relational Expression 2]

3.6≤ [Al]/[Si] ≤8.5

(In Relations 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 in mine.)

In addition, another aspect of the present invention,

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

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

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

[Relational Expression 1]

0.09 ≤ [Zn]/[Si] ≤ 4.8

[Relational Expression 2]

3.6≤ [Al]/[Si] ≤8.5

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

In addition, another aspect of the present invention,

There is provided a hot press formed member obtained by hot press forming the plated steel sheet for hot press forming as described above.

In addition, another aspect of the present invention,

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

It provides a method of manufacturing a hot press-formed member, 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 plated steel sheet for hot press forming excellent in surface quality and a method for manufacturing the same.

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

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

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

The terminology used herein is for the purpose of describing specific embodiments, and is not intended to limit the present invention. Also, the singular forms used herein include the plural forms unless the relevant definition clearly indicates to the contrary.

As used herein, the meaning of "comprising" specifies an element and does not exclude the presence or addition of other elements.

Unless defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms defined in the dictionary are interpreted to have meanings consistent with the related technical literature and the presently disclosed content.

In order to improve the problem of insufficient corrosion resistance and poor productivity of an aluminum plated steel sheet mainly used as a conventional plated steel sheet for hot press forming, a Zn-Al-based plated steel sheet to which zinc and aluminum are added has been developed.

However, although 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, there was a problem in that the coating layer was easily peeled off from the base material at the curvature part of the mold during hot forming, resulting in poor adhesion.

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

Hereinafter, a plated 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: Soji-iron; and a plating layer provided on at least one surface of the base iron. At this time, as long as it can be used as a steel sheet for hot press forming with respect to the type of the base iron, all of it can be applied without limitation. Therefore, as the base iron, it is sufficient if the steel material can be a martensitic transformation by starting austenite transformation at a temperature of 800 ° C. or higher, and by quenching at the same time as forming. As a representative example, in wt%, 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 Substance iron having impurities can be used, and 22MnB5 and the like can be mentioned.

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. In this case, when the content of each element is expressed, it means weight % unless otherwise defined.

Zn: 21~34%

Zinc (Zn) is an element added to improve corrosion resistance. Usually, since an aluminum-coated 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 and improving the sink roll life. Therefore, in the present invention, 21% or more of Zn is added in order to secure the desired level of corrosion resistance and productivity. However, since there is a risk of liquid metal embrittlement (LME) occurring during hot press forming when Zn is added excessively, 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%, more preferably 25%, and the upper limit of the Zn content may be 31.2%.

Si: 6.3~15%

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 causes a decrease in the melting point of Al to lower the temperature of the plating bath, thereby effectively suppressing the generation of ash in the plating bath. Therefore, in the present invention, in order to properly control the degree of alloying between Al and Fe, 6.3% or more of Si is added. However, if the Si content in the plating layer is less than 6.3%, the brittle Fe-Al-based interfacial alloy layer is formed too thickly due to excessive alloying. 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 there may be a problem that the plating equipment in the plating bath is deteriorated and operability is deteriorated. Therefore, in the present invention, the Si content is 15% limited to below. However, from the standpoint of further improving the desired effect of the present invention, more preferably, the lower limit of the Si content may be 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%, a non-uniform Al-Fe alloy phase is formed on the surface of the base iron in which Al in the plating bath and Fe of the base iron are not uniform. Problems can arise. In addition, when the Fe content in the plating layer exceeds 13%, an excessive Fe-Al-based interfacial alloy layer is formed, and there is a fear that the plating layer may fall off during processing by the brittle Fe-Al-based interfacial alloy layer. In addition, since the excessive Fe-Al-based interfacial alloy layer formed during plating serves to suppress the diffusion of Fe in the heat treatment process performed during hot press forming, the plating layer remains in the liquid state for a long time, thereby reducing the problem of burning 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, from the standpoint of further improving the desired effect of the present invention, more preferably, the lower limit of the Fe content may be 3.8%, and the upper limit of the Fe content may be 11.2%. On the other hand, the Fe may be included in a small amount in the plating bath, which may include a small amount of Fe introduced into the plating bath from the base iron during the manufacturing process.

remainder Al and other unavoidable impurities

In addition to the composition of the above-described plating layer, the remainder may be Al and other unavoidable impurities. Inevitable impurities may be included as long as they may be unintentionally mixed in the manufacturing process of a typical aluminum-based or aluminum alloy-based plated steel sheet. Since the meaning of 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 Mg in an amount of 1.4% or less in order to further improve corrosion resistance. The Mg may serve as a buffer to inhibit the decomposition of zinc oxide in a corrosive environment, thereby further improving corrosion resistance. Accordingly, Mg may be optionally added in an amount of 1.4% or less. However, since Mg is an optional element, the lower limit of the content may not be separately limited, and as an example, the lower limit of the Mg content may be 0.05%.

On the other hand, although 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. The liquid metal brittleness refers to a phenomenon in which zinc in the plating layer diffuses into the base iron during a plating process or a hot forming process and brittle fracture occurs due to grain boundary cracking. However, when a plated steel sheet with cracks that is connected from the plating layer to the base iron is used for an automobile member, there is a risk of fatigue failure or weakening of the rigidity of the vehicle body during a vehicle collision due to accumulated stress during vehicle operation. , the LME phenomenon in hot press-formed parts should be suppressed.

In general, the LME phenomenon does not occur in the case of the aluminum plating layer, but the LME phenomenon easily occurs when zinc is added. Accordingly, as a result of intensive examination, the present inventors confirmed that the LME generation tendency was different 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 solidifying after continuous plating to form a plating layer, as the composition of Al, Zn, and Si changes, the aluminum primary crystal In this case, it was additionally found that adhesion of the plating layer was mainly a problem.

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

Accordingly, the plated steel sheet for hot press forming according to the present invention preferably satisfies the following Relations 1 and 2.

[Relational Expression 1]

[Zn]/[Si] ≤ 4.8

[Relational Expression 2]

3.6≤ [Al]/[Si] ≤8.5

(In Relations 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 in mine.)

Specifically, if the value of [Zn]/[Si] defined in Relation 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, adhesion of the plating layer may decrease or LME phenomenon may occur.

In addition, if the value of [Al]/[Si] defined in Equation 2 is less than 3.6, the formation of smooth aluminum oxide on the surface after heat treatment may not be sufficient, and the shape of the surface may be rough. In addition, when the value of [Al]/[Si] exceeds 8.5, a Fe-Al alloy phase with high brittle resistance to the plating layer is generated after hot press forming, so that the coated steel sheet is blank processed to fit the parts before hot press forming of the coated steel sheet or the steel sheet There is a risk of plating peeling due to Fe-Al alloy phase due to impact during transportation.

On the other hand, according to an aspect of the present invention, the plating layer, Al-Zn-based plating layer; and an Fe-Al-based interfacial alloy layer provided between the Al-Zn-based plating layer and the base iron.

The Fe-Al-based interfacial 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. is, for example, FeAl, FeAl ₃ , Fe ₂ Al ₅ and the like. At this time, with respect to the Fe-Al-based interfacial alloy layer, matters commonly known in the art may be equally applied. Accordingly, the Fe-Al-based interfacial alloy layer may include, by weight, Fe: 30-50%, Al: 35-55%, and in addition to Fe and Al, some of the components that may be included in the plating layer, such as Zn, Si, etc. It may further include (eg, Si 27% or less).

In other words, the Fe-Al-based interfacial alloy layer is, by weight, Fe: 30-50%, Al: 35-55%, Si: 5-27%, Zn: 4% or less (including 0%), and the balance other It may contain impurities.

In addition, the Al-Zn-based plating layer may refer to 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 plated layer to be described later is solidified, it may refer to a layer provided on the Fe-Al-based suppression layer and separated by content from the Fe-Al-based suppression layer. Specifically, the Al-Zn-based plating layer may include, by weight%, Zn: 1531%, Si: 2 to 12%, Fe: 10% or less, and the remainder Al and other unavoidable impurities.

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 since the thickness of the Fe-Al type is less than 30% compared to the thickness of the Al-Zn type, the surface of the plating layer and the The GDS value measured at the midpoint of the depth where the Fe content is 50% is determined as an Al-Zn-based compositional component value, and the Fe-Al-based composition is determined using a scanning electron microscope at a magnification of 1500 times or more. The composition can be determined by the average value of the EDS-analyzed values more than once.

According to an 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, an excessive Si phase is formed on the Al-Zn 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 with an excessive Si content in the Fe-Al-based alloy layer to inhibit the Fe-Al-based diffusion coefficient of Fe-Al-based and hot press During molding, the alloying of the initial plating layer may be inhibited, which may cause problems such as burning, and thus there is a risk of impairing the adhesion of the plating layer.

That is, the plated steel sheet for hot press forming is usually used in automobiles and is 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. These 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 above-described uniform alloying can be achieved.

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

In addition, according to one aspect of the present invention, based on the cross-section in the thickness direction of the Al-Zn-based plating layer, the Si phase may include 3 to 30% by area fraction. This ‹š, the Si phase means a phase containing 40 to 99.9% Si by weight%.

In the present invention, by controlling the Si phase to contain about 3 to 30%, the drop-off phenomenon derived from the brittle Fe-Al-based interfacial 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, it is possible to simultaneously prevent peeling of the plating layer, which easily occurs during hot forming. Accordingly, even when a plated steel sheet having a component-based plating layer containing Zn and Al is used for hot press forming, corrosion resistance, productivity, and LME prevention as well as adhesion of the plating layer can be further improved.

That is, hot press-formed parts are mainly applied to parts with a complex shape and high strength compared to cold-formed products. For this reason, deep machining is required to produce complex shapes and due to the nature of hot press forming, which uses a lot of molds, the brittle Fe-Al interface alloy layer is excessively formed due to friction and tolerance between the mold and the plating layer. Apart from the peel-off phenomenon caused by , there is a problem that the plating layer tends to peel off after hot press forming is applied.

So far, no attention has been paid to the problem that the plating layer is easily peeled off after such hot press forming, but the present inventors have conducted intensive studies. By precisely controlling conditions such as the area fraction of Si phase, not only the drop-off phenomenon derived from the base iron and the Fe-Al-based interface alloy layer, but also the Al-Zn-based plating layer and the Fe-Al-based interfacial alloy layer mainly generate parts forming. It was confirmed that even the later defect problem could be solved.

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

The plated 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 remainder Al and other unavoidable impurities, and a plating step of immersing the base iron in a plating bath satisfying the following Relations 1 and 2 to obtain a steel sheet having a hot-dip plated layer formed on its surface. At this time, for the reason for adding the plating bath component and the reason for limiting the content, the description of the plating layer described above is equally applicable. That is, since the composition of the plating layer includes components derived from base iron, the plating bath and its components may vary slightly, but the reason for adding each component in the plating bath and numerical limitation other than those derived from base iron is the lower limit. and the reason for limiting the upper limit is the same.

[Relational Expression 1]

0.09 ≤ [Zn]/[Si] ≤ 4.8

[Relational Expression 2]

3.6≤ [Al]/[Si] ≤8.5

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

Then, although not particularly limited, according to one aspect of the present invention, after the plating step, the average cooling rate Vc1 is 25 to 90 in the first section of 630 to 530° C. based on the surface temperature of the hot-dip plated layer for the steel sheet. ℃/s, and in the second section of 529 ~ 350 ℃, cooling may be performed so that the average cooling rate Vc2 meets 3 ~ 15 ℃ / s. In this case, the cooling may be controlled to satisfy the following relational expression (3).

[Relational Expression 3]

Vc2 ≤ Vc1/7

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

Specifically, by controlling the cooling rate to be 25 ~ 90 ℃ / s in the first section largely, the size of the sequins unique to aluminum at the time of solidification of the plating layer of the first section becomes small and fine, so that the surface roughness becomes uniform and the surface quality is improved can have the effect of being In addition, in the second section, by controlling the average cooling rate as low as 3 to 15° C./s, the amount of air blown by the cooling tower is reduced, thereby reducing the mixing of foreign substances into the plating layer.

In addition, according to one aspect of the present invention, before the plating step, the temperature is 650 ~ 850 ℃, in an annealing furnace in which the ratio of hydrogen is 0.1 ~ 30%, the base iron may be heat treated. The heat treatment is performed at a temperature above recrystallization to prevent work hardening of the material in the continuous plating process, and maintain the base steel sheet at a higher temperature than the plating bath to improve plating properties. If the heat treatment temperature is less than 650 ℃, there is a disadvantage 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 steel sheet may be concentrated on the surface of the substrate to form an oxide, thereby making the properties inferior. Therefore, the temperature of the heat treatment before plating is preferably controlled to be 650 ~ 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 may be difficult to maintain a reducing atmosphere in the heat treatment furnace, so cleaning the surface of the steel sheet may be difficult. At this time, if more than 0.1% of hydrogen gas is added, it is possible to lower the dew point in the heat treatment furnace and maintain a reducing atmosphere to prevent further oxidation of the base iron and clean the surface of the base material. However, when the fraction of the hydrogen gas exceeds 30%, there is a risk of explosion due to contact with oxygen when leaking into the atmosphere. Accordingly, in the present invention, the ratio of hydrogen during heat treatment may be controlled to 0.1 to 30%, and 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 the annealing is less than -30 ℃, a sufficient reducing atmosphere may not be created, and there may be a problem in reducing the Fe oxide generated on the surface. If it exceeds -60 ℃, the use of high-purity nitrogen, etc. Costly problems can arise.

According to an 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 plating bath melting point (Tb). If the temperature of the plating bath is less than Tb+20° C., the plating bath may harden at a location where heat can escape to the outside, such as a sink roll in the plating pot. 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 equipment, etc., and operation may be difficult due to high temperature. 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.

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

In addition, another aspect of the present invention, heat-treating the plated steel sheet for hot press forming obtained by the above-described method for manufacturing the plated steel sheet for hot press forming in the range of Ac3 ~ 950 ℃ 1 ~ 1000 seconds; and

It provides a method of manufacturing a hot press-formed member, in which the heat-treated plated steel sheet for hot press forming is hot press-formed using a mold.

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

(Example)

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

(Experimental Example 1)

By weight%, C: 0.23%, Mn: 1.3%, B: 0.002%, Ti: 0.03%, Si: 0.25%, Cr: 0.15%, balance Fe and other impurities containing iron were prepared. After heat treatment of 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, immersed in a plating bath under the conditions shown in Table 1 below to melt 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 treating with an air knife to control the plating adhesion amount per side and then cooling.

No. Plating bath composition [wt%]
(residual impurities) Plating bath melting point (Tb) [℃] plating bath temperature
[℃] Plating per side
Amount [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 Invention example 3 64 27.0 8.1 0.8 558 600 58 Invention Example 4 64 27.0 8.1 0.8 558 600 98 Invention Example 5 61 26.1 12.5 0.6 639 670 60 Invention example 6 61 32.0 6.8 0.7 539 590 93

After preparing a specimen for each plated steel sheet for hot press forming obtained by the method of Table 1, the alloy composition of the plating layer was analyzed. Specifically, the alloy composition of the plating layer is first dissolved with NaOH solution for 10 minutes to remove the aluminum oxide film present on the surface, then the plating layer is dissolved using HCl solution and inhibitor for 30 minutes. -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 Invention example 3 60.5 22.5 10.3 6.7 2.2 5.9 Invention Example 4 60.4 25.9 9.9 3.8 2.6 6.1 Invention Example 5 58.3 24.1 13.1 4.6 1.8 4.4 Invention example 6 55.3 31.2 7.0 6.5 4.5 7.9

After taking a specimen of the plated steel sheet prepared by the above method, it was cut into a blank, and the blank specimen reached 900°C in a furnace at 900°C and then maintained for 2 minutes. Thereafter, the heated blank specimen was transferred to the mold within 20 seconds, and then hot-formed using the mold, followed by cooling at a cooling rate of 20° C./s to obtain a hot-formed member.

Then, whether LME occurs, corrosion resistance after hot press forming, whether drop-off occurs at the interface between the base iron and the plating layer before hot forming, and plating adhesion after hot press forming were evaluated based on the following criteria, respectively, and are shown in Table 3 below.

[Whether LME occurs]

The cross section in the thickness direction (meaning the direction perpendicular to the rolling direction) of each specimen subjected to hot press forming was observed with an optical microscope to measure the crack depth of the base material, and the case where the crack depth of the base material was 30 μm or more 'occurrence' was indicated as a specimen.

[Corrosion resistance after hot press forming]

The specimen was heat-treated to reach 900° C., maintained for 2 minutes, and then rapidly cooled. The specimen was evaluated using SST (salt spray test), which is 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 iron was measured.

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

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

[Whether or not there is a drop-off 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 the Fe-Al interface alloy layer with strong brittleness at the interface between the base iron and the plating layer. Whether or not dropout occurred was evaluated. The case where dropout occurred was marked as 'dropout'.

[Plating adhesion after hot press forming]

After heat-treating the specimen and reaching 900° C., the specimen heated for 2 minutes was press-processed to measure the delamination of the plating layer occurring on the surface. Among the specimens, the long axis length of the part peeled 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 long axis length of the peeled part is 1 mm or more and less than 3 mm

Х: The long axis length of the exfoliated area is 3 mm or more

No. LME occurrence
Whether hot press
after molding
corrosion resistance Substance iron and plating layer
dropout at the interface
Whether it occurs 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 Occur ○ leaving out Х Comparative Example 9 - Х leaving out Х Comparative Example 10 - ○ leaving out Х Invention Example 1 - ○ - ○ Invention Example 2 - ○ - ○ Invention example 3 - ○ - ○ Invention Example 4 - ○ - ○ Invention Example 5 - ○ - ○ Invention example 6 - ○ - ○

As can be seen in Table 3, the invention examples that satisfy all of the composition of the plating layer, Relational Expression 1, Relational Expression 2, and manufacturing conditions specified in the present invention suppress LME generation, and do not cause drop-off at the interface between the base iron and the plating layer, 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, which do not satisfy one or more of the plating layer composition, Relational Expression 1, Relational Expression 2, and manufacturing conditions specified in the present invention, LME characteristics, whether or not falling off occurs at the interface between the base iron and the plating layer, and after hot press forming It was confirmed that one or more properties of corrosion resistance and plating adhesion after hot press forming were inferior.

(Experimental Example 2)

The dew point temperature is -50 ° C., heat treatment of the base iron at 750 ° C. in an annealing furnace that is a mixed atmosphere of 5% hydrogen and 95% nitrogen, and cooling the hot-dip plated layer under the conditions described in Table 4 below after plating. A plated steel sheet for hot press forming was manufactured in the same manner as in Experimental Example 1.

No. Plating bath composition [wt%]
(residual impurities) Plating bath melting point (Tb)
[℃] plating bath temperature
[℃] per side
Plated
Amount [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 Invention Example 7 70 22.6 6.7 0.5 563 620 37 60 5 Invention Example 8 69 21.5 9.3 0.2 568 630 37 70 10 Invention Example 9 64 27.0 8.1 0.8 558 600 58 40 5 Invention Example 10 64 27.0 8.1 0.8 558 600 98 80 5 Invention Example 11 61 26.1 12.5 0.6 639 670 60 80 10 Invention 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 ℃ based on the surface temperature of the hot-dip plated layer

Vc2*: Average cooling rate in the range of 530~350℃ based on the surface temperature of the hot-dip plated layer

In the same manner as in Experimental Example 1, the composition of the coating 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 Invention Example 7 59.5 21.9 7.4 11.2 3.0 8.0 Invention Example 8 59.1 21.1 10.8 9.0 2.0 5.5 Invention Example 9 60.5 22.5 10.3 6.7 2.2 5.9 Invention Example 10 60.4 25.9 9.9 3.8 2.6 6.1 Invention Example 11 58.3 24.1 13.1 4.6 1.8 4.4 Invention Example 12 55.3 31.2 7.0 6.5 4.5 7.9

Then, a cross-sectional specimen 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 was prepared, and then observed with a scanning electron microscope (SEM). Through this, on the base iron, a plating layer is formed, as the plating layer, a Fe-Al-based interfacial alloy layer in contact with the base iron is formed, and an Al-Zn plating layer is formed on the Fe-Al-based interfacial alloy layer. Confirmed.

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

In addition, with respect to the cross-sectional specimen, through SEM and EDS point analysis in the same manner as described above, per 100 μm ² of the unit area of the plating layer, the Si content is concentrated by 40% or more compared to the average Si content of the Al-Zn-based plating layer. The presence (C) was measured. In addition, using SEM and EDS, a 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 with 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 - Invention Example 7 5.9 17.6 3.0 ○ 4 Invention Example 8 10.3 12.0 1.2 ○ 12 Invention Example 9 9.6 18.1 1.9 ○ 10 Invention example 10 9.5 13.0 1.4 ○ 7 Invention Example 11 8.6 16.7 1.9 ○ 19 Invention 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 cross section in the thickness direction (direction perpendicular to the rolling direction) of the Al-Zn-based plating layer, there is a region in which the Si content is concentrated by 40% or more compared to the average Si content of the plating layer per 100 μm ² of the unit area of the plating layer Whether to [○/Х]

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

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

[Surface Quality]

With respect to the specimen, it was evaluated whether there was a color difference on the surface and whether a staining phenomenon was found as a whole due to the excessively generated zinc oxide on the surface by visual observation, and the evaluation was performed according to the following criteria.

Good: No difference in surface color and no surface staining

Surface color difference: the presence of an area of 50 mm ² or more on the surface that is different in color from the surrounding area

Surface Staining: Smudges found on the surface

No. LME crack After hot press forming
corrosion resistance Whether or not there is a drop-off at the interface between the base iron 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 Occur ○ leaving out Х surface stain Invention Example 7 - ○ leaving out ◎ Good Invention Example 8 - ○ leaving out ◎ Good Invention Example 9 - ○ - ◎ Good Invention example 10 - ○ - ◎ Good Invention Example 11 - ○ - ◎ Good Invention example 12 - ○ - ◎ Good

As can be seen in Table 7, the plating layer composition, Relation 1 and Relation 2 specified in the present invention are satisfied, and the values of Sb/Sa defined in Table 6: 1 to 3.1, C exist, and D is 3 In the case of the invention example satisfying at least one of ~30%, it was confirmed that LME, corrosion resistance, plating adhesion, surface quality as well as surface properties were very excellent.

In particular, a photograph taken by observing the cross-sectional specimen in the thickness direction of the plated steel sheet obtained in 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 the plating layer in which the Si content was concentrated by 40% or more relative to the average Si content of the plating layer, and the Si phase having a Si content of 40% or more was included in an area fraction of 3 to 30%.

On the other hand, the plating layer composition defined in the present invention, the relational expressions 1 and 2, and the values of Sb/Sa defined in Table 6: 1 to 3.1, C exist, and D does not satisfy one or more of 3 to 30% In the case of the examples, it was confirmed that one or more of the above-described characteristics were inferior.

Claims (14)

So Ji-cheol; and
a plating layer provided on at least one surface of the base iron, by weight%, Zn: 21 to 34%, Si: 6.3 to 15%, Fe: 3 to 13%, the remainder Al and other unavoidable impurities;
including,
A plated steel sheet for hot press forming that satisfies the following Relations 1 and 2.
[Relational Expression 1]
0.09 ≤ [Zn]/[Si] ≤ 4.8
[Relational Expression 2]
3.6≤ [Al]/[Si] ≤8.5
(In Relations 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 in mine.)
The method according to claim 1,
The plating layer is a plated steel sheet for hot press forming, including 25 to 34% of Zn by weight%.
The method according to claim 1,
The plating layer is
Al-Zn-based plating layer; and
A plated steel sheet for hot press forming, including; a Fe-Al-based interfacial alloy layer provided between the Al-Zn-based plating layer and the base iron.
4. The method according to claim 3,
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 plated layer is 1 to 3.1, a plated steel sheet for hot press forming.
4. The method according to claim 3,
Based on the cross-section in the thickness direction of the Al-Zn-based plating layer, per 100 μm ² of the unit area of the plating layer, including a 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, Plated steel sheet for hot press forming.
4. The method according to claim 3,
Based on the cross-section in the thickness direction of the Al-Zn-based plating layer, the Si phase as an area fraction, including 3 to 30%, plated steel sheet for hot press forming.
7. The method of claim 6,
The Si phase is a plated steel sheet for hot press forming, comprising 40 to 99.9% of Si by weight%.
In weight %, Zn: 21~34%, Si: 6.5~15%, Fe: 2% or less (including 0%), the balance contains Al and other unavoidable impurities, and in a plating bath satisfying the following Relations 1 and 2 A plating step of immersing the base iron to obtain a steel sheet having a hot-dip plated layer formed on its surface; and
A method of manufacturing a plated steel sheet for hot press forming, comprising a.
[Relational Expression 1]
0.09 ≤ [Zn]/[Si] ≤ 4.8
[Relational Expression 2]
3.6≤ [Al]/[Si] ≤8.5
(In Relations 1 and 2, the [Zn] represents the average wt% content of Zn in the hot-dip plated layer, the [Si] represents the average wt% content of Si in the hot-dip plated layer, and the [Al] is It represents the average weight % content of Al in the hot-dip plated layer.)
9. The method of claim 8,
After the plating step, based on the surface temperature of the hot-dip plated layer for 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 in the second section is 529 to 350 ° C. The method of manufacturing a plated steel sheet for hot press forming, further comprising the step of cooling so that the rate Vc2 satisfies 3 to 15°C/s.
9. The method of claim 8,
A method of manufacturing a plated steel sheet for hot press forming, which satisfies the following relational expression (3).
[Relational Expression 3]
Vc2 ≤ Vc1/7
In claim 8,
Before the plating step, the temperature is 650 to 850 ℃, the dew point temperature is -30 to -60 ℃, in an annealing furnace in which the ratio of hydrogen is 0.1 to 30%, further comprising the step of heat-treating the base iron, hot A method for manufacturing a plated steel sheet for press forming.
9. The method of claim 8,
The temperature of the plating bath is controlled to be Tb+20°C to Tb+80°C based on the plating bath melting point (Tb), a method of manufacturing a plated steel sheet for hot press forming.
A hot press formed member obtained by hot press forming the plated steel sheet for hot press forming according to any one of claims 1 to 7.
13. A method of manufacturing a plated steel sheet for hot press forming according to any one of claims 8 to 12, heat-treating the plated steel sheet for hot press forming in the range of Ac3 to 950° C. for 1 to 1000 seconds; and
A method for manufacturing a hot press-formed member, wherein the heat-treated plated steel sheet for hot press forming is subjected to hot press forming using a die.

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