KR20230090132A - A hot press formed member having excellent resistance against hydrogen embrittlement and impact resistance, a plated steel sheet therefor and methods for producting the same - Google Patents

Abstract

The present invention relates to a hot press forming member with excellent hydrogen embrittlement resistance and collision resistance, a coated steel sheet for hot press forming, and methods for manufacturing the same. The coated steel sheet for hot press forming according to one aspect of the present invention includes a base steel sheet and a plating layer formed on the surface of the base steel sheet, wherein the plating layer is an Al-Fe alloy plating layer containing more than 80wt% of Al and Fe in total, and a fraction of a BCC structure phase in the plating layer may be 30% or more based on area.

Description

Hot press formed member with excellent hydrogen embrittlement and crash resistance, plated steel sheet for hot press forming, and method for manufacturing them PRODUCTING THE SAME}

The present invention relates to a hot press formed member having excellent hydrogen embrittlement resistance and collision resistance, a plated steel sheet for hot press forming, and a method for manufacturing the same.

Hot press-formed members have recently been widely applied to structural members of automobiles for the purpose of improving fuel efficiency through weight reduction of automobiles and protecting passengers. Although not necessarily limited thereto, fields in which hot press-formed members are frequently used include structural members such as bumpers, doors, or pillar reinforcing materials or collision members that require ultra-high strength or high energy absorption capacity.

As a prior art related to hot press forming, Patent Document 1 can be cited. In Patent Document 1, after heating an Al-Si coated steel sheet to an austenitizing temperature (Ac3) or higher, it is rapidly cooled simultaneously with hot forming in a press to form a final As a result, a technique for obtaining a member having a martensitic structure is proposed. When this method is used, complex shapes can be easily molded because it is molded at a high temperature, and since rapid cooling is also performed by the mold of the press during molding, the effect of increasing the strength and reducing the weight of the member can be expected through the increase in strength.

However, the martensitic structure has low resistance to hydrogen embrittlement, and delayed fracture due to hydrogen embrittlement is a concern, especially when a significant amount of residual stress exists, such as in a part obtained by hot forming. As a result, in the case of an ultra-high-strength member having a tensile strength of 1800 MPa or more, the sensitivity to delayed hydrogen fracture is greatly increased, and even if a small amount of hydrogen remains in the steel sheet, fracture may occur, and thus use may be limited. In particular, such a problem may occur remarkably in a plated steel sheet having a plated layer formed on the surface thereof, because the plated layer causes hydrogen to penetrate and makes it difficult to discharge hydrogen.

In addition, the martensitic structure, which is essential for high strength, has a disadvantage of poor impact resistance compared to high strength. Therefore, in the case of high-strength products with high strength, many improvements are required to improve crash resistance to the level required by automobile companies.

[Patent Document 1] US Patent Registration No. 6,296,805

According to one aspect of the present invention, a hot press-formed member having excellent resistance to hydrogen embrittlement and collision resistance, a plated steel sheet for hot press-forming suitable for manufacturing such a hot-press-formed member, and a manufacturing method capable of providing them are provided. .

The object of the present invention is not limited to the above. Anyone with ordinary knowledge in the technical field to which the present invention belongs will have no particular difficulty in understanding the additional tasks of the present invention from the overall details of the present invention specification.

A plated steel sheet for hot press forming according to one aspect of the present invention includes a base steel plate and a plating layer formed on a surface of the base steel plate, wherein the plated layer contains Al-Fe containing at least 80% by weight of the total content of Al and Fe. It is a base alloy plating layer, and the fraction of the BCC structure phase in the plating layer may be 30% or more based on area.

A method of manufacturing a coated steel sheet for hot press forming according to another aspect of the present invention includes preparing an aluminum-coated steel sheet; and subjecting the aluminum-plated steel sheet to an annealing heat treatment, and controlling the annealing time and temperature so that λ represented by the following relational expression 1 is 32 to 37.

[Relationship 1]

Here, i means the elapsed time (minutes) from the start of annealing, n means the elapsed time (minutes) until the end of cooling, and λ _i means a parameter represented by the relational expression 2 below.

[Relationship 2]

Here, Ti means the absolute temperature (K) of the central part in the width direction and the length direction of the steel sheet (coil) at the elapsed time.

A hot press-formed member according to another aspect of the present invention is a holding steel sheet and

It includes a plating layer formed on the surface of the base steel sheet, the plating layer is an Al-Fe-based alloy plating layer containing 85% or more by weight of the total content of Al and Fe, and the fraction of the phase of the BCC structure in the plating layer is 75% based on area % or more, and α expressed by the following relational expression 3 may be 0.2 or less.

α = (Ci - Co)/Co

Here, Ci means the average value of the carbon content from the boundary between the base steel sheet and the plating layer to the inner depth of 20 μm of the base steel plate when measured by glow discharge spectroscopy (GDS), and Co means the average carbon content of the base steel plate .

As described above, by controlling the structure of the plating layer existing on the surface of the plated steel sheet for hot press forming to a BCC or orthorhombic structure through which hydrogen easily passes, it is possible to minimize the remaining hydrogen in the member and also to distribute Si in the plating layer. By controlling the structure, it is possible to transform the coating layer structure into a BCC structure more during heating for hot press forming.

In addition, the hot press formed member not only can minimize the amount of residual hydrogen by including a large amount of BCC structure in the plating layer, but also secures improved crash resistance even if it has high strength by controlling the carbon concentration at a specific location. can

1 is a photograph of a plated steel sheet for hot press forming according to Inventive Example 2 cut in the thickness direction and observed;
Figure 2 is a graph of the glow discharge spectroscopic analysis of the coated steel sheet for hot press forming according to Inventive Example 2 from the surface to the depth direction, and
3 is a photograph observed by cutting a hot press-formed member according to Inventive Example 2 in the thickness direction.

The terminology used herein is intended only to refer to specific embodiments and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite.

As used herein, the meaning of "comprising" specifies specific characteristics, regions, integers, steps, operations, elements, and/or components, and other specific characteristics, regions, integers, steps, operations, elements, elements, and/or groups. does not exclude the presence or addition of

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted as having meanings consistent with related technical literature and currently disclosed content, and are not interpreted in ideal or very formal meanings unless defined.

Further, in the present invention, the steel sheet refers to a coil or sheet state before being processed into a specific shape, and a member refers to a steel sheet processed into a non-plate shape by a molding process. In addition, the plating layer referred to in the present invention means a layer of metal, alloy or intermetallic compound formed in contact with the base steel sheet.

It should be noted that in the present invention, when the content of each element is expressed, it is based on weight unless otherwise specified. In addition, the ratio of crystals or structures is based on area unless otherwise specified, and the content of gas is based on volume unless otherwise specified.

Hereinafter, the present invention will be described in detail.

[Coated steel sheet for hot press forming]

Hydrogen may cause hydrogen embrittlement if a large amount of hydrogen remains in the steel sheet. In order to prevent the residual of hydrogen that causes hydrogen embrittlement, it is necessary to reduce the amount of hydrogen penetrating into the steel sheet or to allow hydrogen to be easily discharged to the outside of the steel sheet.

However, the phenomenon in which hydrogen penetrates into the steel sheet is that hydrogen contained in the atmosphere directly penetrates into the steel sheet, or water vapor reacts with the plating layer on the surface to decompose into hydrogen and oxygen (oxide), and the decomposed hydrogen penetrates into the steel sheet. It can be explained by the process of Therefore, in order to suppress the penetration of hydrogen into the steel sheet, it is necessary to reduce the amount of hydrogen in the atmosphere or reduce the water vapor to suppress the oxidation reaction between the water vapor and the coating layer on the surface of the steel sheet. For this purpose, a method of controlling the atmosphere when heating the plated steel sheet for hot press forming or the blank for hot press forming to a nitrogen atmosphere or a non-hydrogen-containing atmosphere or controlling the dew point of the atmosphere to a low level may be mentioned. It may not be effective because it causes an increase in cost.

Instead of this method, the present inventors found that a method of reducing the amount of hydrogen remaining in a hot-press molded member by allowing hydrogen penetrating into the steel sheet to be easily discharged is effective in improving resistance to hydrogen embrittlement. did

In addition, as a result of the inventors of the present invention analyzing the cause of adversely affecting the collision characteristics of the hot press-formed member, the thickness of the plating layer increases due to diffusion when hot press forming by heating the plated steel sheet. The carbon contained in the increased thickness due to low solid solubility is pushed to the side of the base steel sheet. As a result, carbon is concentrated in the surface layer portion of the base steel sheet close to the interface between the base steel plate and the plating layer, which lowers the bendability of the surface. A decrease in bendability leads to a decrease in crash resistance.

Therefore, the inventors of the present invention alloy the surface plating layer of the coated steel sheet in advance by heat treating the coated steel sheet in advance through the analysis of these causes, and at the same time, the carbon concentrated in the surface layer of the steel sheet in contact with the plating layer is sufficiently diffused into the inside of the steel sheet. In other words, not only does the carbon concentration phenomenon not occur in the steel sheet stage where the alloy layer is formed, but even when heating is performed for subsequent hot press forming, a sufficient amount of iron is already diffused into the plating layer and alloying is performed, so additional alloying and consequently the plating layer It was found that the increase in thickness can be minimized and the thickening of carbon can be suppressed. However, it should be noted that pre-heat treatment is not essential in the present invention, and the scope of the present invention can be extended without limitation even when the same plating layer is formed by other alloy forming methods.

After all, the core of the present invention is to appropriately control the properties of the coating layer included on the surface of the coated steel sheet for hot press forming so that hydrogen can be easily discharged during heating and storage for hot press forming, as well as by alloying By minimizing the increase in thickness, additional concentration of carbon can be suppressed, so that resistance to hydrogen embrittlement and crash resistance can be secured at the same time.

The plating layer included in the steel sheet for hot forming of the present invention has Al (aluminum) and Fe (iron) as main components. The total content of these elements in the plating layer may be 80% or more by weight, and may be 85% or more or 90% or more in some cases.

According to one embodiment of the present invention, the plating layer may have a thickness of 5 to 20 μm. In order to secure corrosion resistance, the thickness of the plating layer may be 5 μm or more. If the thickness of the plating layer is too thick, the diffusion of Fe into the plating layer is not sufficient and the ratio of the orthorhombic (Fe ₂ Al ₅ ) phase increases, resulting in hydrogen emission. Since it may not be easy, the upper limit of the thickness of the plating layer may be set at 20 μm.

And, in order to advantageously control the phase distribution in the subsequent plating layer, the Fe in the plating layer may be included in an amount of 40% or more, in some cases, 45% or more, or 50% or more by weight. If the content of Fe is not sufficient, the fraction of the BCC phase described later may not be sufficient, and consequently, it may not be effective in improving hydrogen embrittlement or collision resistance.

In addition, in one embodiment of the present invention, the Fe content at a depth of 1 μm from the surface of the plating layer may be 20% or more by weight. In another embodiment, since the proportion of the phase of the BCC structure may be sufficient when it is sufficiently alloyed to the surface, the Fe content near the outermost layer may be set to 25% or more by weight.

In addition, the plating layer according to one embodiment of the present invention may further include 1 to 7%, or 1.5 to 7% of Si based on weight. The Si can prevent spot weldability defects due to an increase in welding resistance by preventing alloying from freezing too rapidly, and serves to reduce the temperature of the plating bath, and may be included in an amount of 1.5% or more. However, if the Si content is excessive, Si oxide is excessively generated in the surface layer, and thus not only weldability but also corrosion resistance may be deteriorated accordingly, so in one embodiment of the present invention, the Si content is limited to 7% or less. can do. In another embodiment of the present invention, the Si content may be limited to 2 to 7%.

The plating layer of the present invention has orthorhombic and body centered cubic (BCC) phases as main phases. The orthorhombic system refers to a phase containing 25 to 60% of Fe and 10 to 55% of Al by weight. Representatively, Fe ₂ Al ₅ may be present, but industrially, it must be stoichiometrically 2: It is not required to have a ratio of 5. In addition, the phase of the body-centered cubic structure means FeAl, Fe ₃ Al, or a phase in which Al is dissolved in Fe (α-Fe), and based on weight, Fe is 50 to 99% and Al is 1 to 30%. It means the prize that is %. In some cases, 3 to 7% of Si may be included.

The orthorhombic and body-centered cubic structures need not consist of only Fe and Al, and may contain up to 15% or less of elements such as Mg, Zn, Mn, Sb, Sn, etc. that may be included in the plating layer. In addition, although the compositions of the orthorhombic and body-centered cubic structures may partially overlap, there is no particular problem in distinguishing the two structures because their crystal structures are fundamentally different.

In one embodiment of the present invention, the total occupancy of the orthorhombic and body-centered cubic structures may be 90% or more, or 95% or more, based on area. The upper limit of the ratio of these phases is not limited and may include up to 100%. When the ratio of these alloy phases is low, alloying is not sufficient, and iron may additionally diffuse from the base steel sheet during the heating process for hot press forming. As a result, the thickness of the alloyed plating layer increases and the boundary between the base steel sheet and the plating layer becomes By moving to the base steel sheet side, carbon can be concentrated toward the base steel plate side. In the embodiment of the present invention, the ratio of each phase can be obtained from the area occupied by each phase in the cross section observed by cutting the coated steel sheet in the thickness direction.

In addition, in order to ensure that hydrogen is sufficiently discharged from the steel sheet during the heat treatment process for hot press forming or the storage process, it is advantageous to have a high BCC structure phase ratio. In addition, the phase of the BCC structure is a phase with a high Fe content in the Al-Fe alloy system, and can suppress additional diffusion of Fe during heating for hot press forming, thereby maximally preventing the phenomenon of carbon concentration. To this end, it is necessary to set the ratio of the BCC phase in the plating layer to 25% or more relative to the total area of the plating layer, and in another embodiment, it may be 30% or more.

Therefore, a steel sheet according to one aspect of the present invention includes a base steel sheet and a plating layer formed on a surface of the base steel sheet, and the plating layer includes an Al-Fe-based alloy plating layer containing 80% or more by weight of the total content of Al and Fe. And, the fraction of the phase of the BCC structure in the plating layer may be 25% or more based on area.

In addition, according to one embodiment of the present invention, the Al content of the plating layer can be controlled to 70% or less at all depths at which GDS analysis is performed in the depth direction.

If the Al content is excessive, sufficient alloying may not be achieved, and as a result, an unalloyed layer remains in the plating layer. , the Al content may be 70% or less at all locations. The Al content can be obtained by measuring at a depth interval of 0.005 μm from the surface by a glow discharge spectroscopy (GDS) method.

Further, according to another aspect of the present invention, in the plated steel sheet for hot press forming of the present invention, the content of Si that may be included in the orthorhombic system in the plating layer may be 3% or less. This is in consideration of the alloying process that can occur during hot press forming. In the case of the BCC structure, since sufficient alloying has already occurred, alloying in the heating process for hot press forming takes place in the orthorhombic system, which is the ratio of the BCC structure with smooth hydrogen emission. Elevation can be beneficial. However, when a large amount of Si is included in the orthorhombic system, alloying is delayed during heating, so that a sufficient amount of BCC may not be formed. Accordingly, the amount of Si that may be included in the orthorhombic system may be less than 3%, and in one embodiment may be less than 2.5%. In addition, the Si content in the phase of the BCC structure may exceed 3%, and in one embodiment may be greater than 5%, in order to achieve a low Si distribution in the orthorhombic system.

Further, in another embodiment of the present invention, 50 or more voids having an equivalent circle diameter of 5 μm or less may exist in a region within a depth of 5 μm from the surface of the plating layer per 200 μm ² of the cross-sectional area of the plating layer. When the voids exist, heat transfer proceeds smoothly through the voids, so that the alloying rate can be increased. The void in the present invention means a closed empty space surrounded by a plating layer, not open, when the steel sheet is cut in the thickness direction and observed. In one embodiment of the present invention, the density of the pores may be 70 or more per 200 μm ² . The more voids, the more advantageous in terms of alloying effect, but if too many voids exist in the plating layer, the bonding strength between the surface layer and the inside of the plating layer decreases, which may cause a part of the alloyed surface layer to peel off. Therefore, the upper limit of the void density Silver can be set to 200 per 200 μm ² .

The type of the base steel sheet of the coated steel sheet targeted in the present invention is not particularly limited as long as it can be used for hot press forming. However, the holding steel sheet that can be used as one embodiment of the present invention contains C: 0.14 ~ 0.5%, Si: 0.05 ~ 1%, Mn: 0.5 ~ 4%, P: 0.001 ~ 0.03%, S: 0.0001 ~ 0.02%, Al: 0.01 ~ 0.1%, Cr: 0.01 ~ 1%, and N: may have a composition containing 0.001 ~ 0.02%.

Carbon (C): 0.14 to 0.5%

The C is an element that increases the strength of the heat treated member and improves the hardenability, and must be appropriately added as an essential element for strength control. If the C content is less than 0.14%, the hardenability is low, so when the cooling rate is reduced, sufficient martensite cannot be secured and the collision resistance is inferior due to the formation of ferrite, so 0.14% or more must be added. The upper limit may be 0.5% or less because it may cause brittleness and poor weldability.

Silicon (Si): 0.05 to 1%

The Si not only needs to be added as a deoxidizer in steelmaking, but also is effective in uniformizing the internal structure as a solid-solution strengthening element and a carbide formation inhibiting element, contributes to increasing the strength of a hot-formed member, and is added as an effective element in material uniformity. However, if the content is less than 0.05%, the above effect cannot be expected, and if the Si content exceeds 1%, plating properties are greatly reduced due to excessive Si oxide generated on the surface of the steel sheet during annealing, so 1% or less is added.

Manganese (Mn): 0.5 to 4%

The Mn needs to be added not only to secure desired strength due to the solid solution strengthening effect but also to suppress ferrite formation during hot forming through hardenability improvement. If the Mn content is less than 0.5%, it is difficult to obtain a sufficient hardenability effect, and an excessive amount of other expensive alloy elements are required for insufficient hardenability, resulting in a large increase in manufacturing cost. The band structure arranged in the direction deepens and causes non-uniformity of the internal structure, which can lead to inferior crash resistance, so 4% or less is added.

Phosphorus (P): 0.001 to 0.03% by weight

The P exists as an impurity in steel, and it is undesirable because a lot of manufacturing cost is required for the minimum content of less than 0.001%, and if the maximum content exceeds 0.03%, the weldability of the hot-formed member and material properties due to high-temperature grain boundary segregation Since this deteriorates, the upper limit is made into 0.03%.

Sulfur (S): 0.0001 to 0.02% by weight

The S is an impurity, and the maximum content is limited to 0.02% because it is an element that inhibits the ductility, impact characteristics and weldability of the member. In addition, if the minimum content is less than 0.0001%, it is not preferable because the manufacturing cost is greatly increased.

Aluminum (Al): 0.01 to 0.1%

The Al is an element that, together with Si, acts as a deoxidizer in steelmaking to increase the cleanliness of the steel. If the Al content is less than 0.01%, it is difficult to obtain the above effect, and if the Al content exceeds 0.1%, high-temperature ductility due to excessive AlN precipitates formed during the casting process is reduced, and slab cracks may occur, which may cause manufacturing problems. is set to 0.1%.

Chromium (Cr): 0.01~1%

Cr, like Mn, is added as an element for securing hardenability of steel and suppressing ferrite generation after hot forming. If the Cr content is less than 0.01%, it is difficult to secure the above effect, and if the Cr content exceeds 1%, the effect of improving the hardenability compared to the amount added is insignificant, and excessive formation of coarse iron carbide may cause cracks when stress is applied. The upper limit is set to 1%.

Nitrogen (N): 0.001 to 0.02%

The N is included as an impurity in steel. If the N content is less than 0.001%, excessive manufacturing costs are involved, and if the content exceeds 0.02%, slab cracks due to AlN formation are likely to occur, such as added Al, so the upper limit is set to 0.02%.

The remaining components other than the above elements may be Fe and impurities unavoidably included in the steel manufacturing process.

In the present invention, in addition to the above-mentioned steel composition, the sum of one or more selected from the group consisting of Cr, Mo, and W: 0.01 to 4.0%, the sum of one or more selected from the group consisting of Ti, Nb, Zr, and V on a weight basis: 0.001 to One or more of 0.4%, Cu + Ni: 0.005 to 2.0%, Sb + Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01% may be further added.

The sum of one or more selected from the group consisting of Cr, Mo, and W: 0.01 to 4.0% Since Cr, Mo, and W can improve hardenability and secure strength and crystal grain refinement through precipitation strengthening effects, one or more of these may be added in an amount of 0.01% or more based on the total content. In addition, the content may be limited to 4.0% or less in order to secure the weldability of the member. In addition, if the content of these elements exceeds 4.0%, further increase in effect is insignificant, so if the content is limited to 4.0% or less, cost increase due to the addition of additional elements can be prevented. According to one embodiment of the present invention, since Cr is included in 0.01 to 1%, the content of the remaining components may be determined so that the sum of the component contents is 0.01 to 4.0% while including Cr in this range.

Sum of at least one selected from the group consisting of Ti, Nb, Zr, and V: 0.001 to 0.4%

The Ti, Nb, and V are effective in improving the steel sheet of the heat treated member by forming fine precipitates and stabilizing retained austenite and improving impact toughness by grain refinement, so one or more of them may be added in an amount of 0.001% or more in total. there is. However, if the added amount exceeds 0.4%, the effect is saturated and the cost may increase due to the excessive addition of ferroalloy.

Cu + Ni: 0.005~2.0%

The Cu and Ni are elements that improve strength by forming fine precipitates. In order to obtain the above-mentioned effect, the sum of one or more of these components may be 0.005% or more. However, if the value exceeds 2.0%, excessive cost increases, so the upper limit is set at 2.0%.

Sb+Sn: 0.001~1.0%

During the annealing heat treatment for Al-Si plating, the Sb and Sn are concentrated on the surface to suppress the formation of Si or Mn oxide on the surface, thereby improving plating properties. In order to obtain such an effect, 0.001% or more may be added. However, if the addition amount exceeds 1.0%, the upper limit is set to 1.0% because not only excessive ferroalloy cost but also solid solution at the grain boundary of the slab can cause coil edge cracks during hot rolling.

B: 0.0001 to 0.01%

The B is an element that can improve hardenability even with a small amount of addition, and can suppress brittleness of hot-press-formed parts due to grain boundary segregation of P or S by being segregated at old austenite grain boundaries. Accordingly, B may be added in an amount of 0.0001% or more. However, if it exceeds 0.01%, the effect is not only saturated, but also causes brittleness in hot rolling, so the upper limit may be 0.01%, and in one embodiment, the B content may be 0.005% or less.

The remainder other than the above components may be composed of iron and unavoidable impurities, and further addition is not particularly limited as long as it is a component that can be added to the coated steel sheet for hot forming.

[Method of manufacturing coated steel sheet for hot press forming]

The manufacturing method of the coated steel sheet for hot press forming of the present invention is not particularly limited as long as the above conditions can be satisfied. However, if a method for manufacturing a coated steel sheet for hot press forming according to one aspect of the present invention is proposed, it is as follows.

As described above, the present invention not only facilitates the release of hydrogen during heating for subsequent hot press forming and storage of members by forming a large amount of phases of the BCC structure on the plating layer present on the surface of the steel sheet, but also alloy alloys. It is an object of the present invention to provide a coated steel sheet for hot press forming capable of preventing carbon from concentrating on the surface of a base steel sheet by suppressing additional layer growth. For this purpose, the present invention allows the plating layer to be alloyed by performing heat treatment under appropriate conditions on the coated steel sheet on which the aluminum-based plating layer is formed. It is explained in detail below.

The manufacturing method of the present invention starts from preparing an aluminum-coated steel sheet. The aluminum-coated steel sheet is not particularly limited as long as it is an aluminum-coated steel sheet used during hot press forming. For example, an aluminum-coated steel sheet containing 5 to 20% Si based on weight may be used. In addition to Si, it goes without saying that alloy elements or other impurity elements that may be included in the aluminum plating layer may be added without limitation. However, in one embodiment of the present invention, the total content of elements other than Al and Si in the plating layer may not exceed 15% by weight.

In one embodiment of the present invention, the coating amount of the aluminum-coated steel sheet may be 5 to 40 g/m ² based on one side of the steel sheet. In order for the plating layer to have a uniform thickness and exhibit sufficient corrosion resistance, the plating amount may be 5 g/m ² or more. However, when the coating amount is too large, it is difficult to obtain a phase with a sufficient BCC structure, and since an excessive amount of time is required for alloying, the upper limit of the coating amount may be set at 40 g/m ² . More than the range of the coating amount is 10 ~ 35g / m ² It may be.

In one embodiment of the present invention, the plated steel sheet may be prepared by a hot-dip aluminum plating method in which the base steel sheet is immersed in an aluminum plating bath. Cooling conditions after plating are not particularly limited, and any of air cooling or other cooling methods can be applied without particular restrictions. If necessary, temper rolling (skin pass rolling) may be performed on the plated steel sheet, and the reduction ratio at this time may be 3% or less.

A batch annealing heat treatment is performed on the aluminum-plated steel sheet. Phase annealing refers to an annealing treatment in which a coil of coated steel sheet is charged into a box-type annealing furnace and then maintained at a high temperature for a long time. In the present invention, the maximum temperature during the annealing may be 600 to 800 ° C, and the holding time at 600 to 800 ° C may be 1 to 100 hours. When the heating temperature is low, sufficient alloying is not achieved, and non-alloyed Al, etc. may remain, and thus the plating layer may be peeled off during roll leveling, leaving an uneven plating layer. Conversely, when the heating temperature is too high, oxide is excessively formed on the surface layer, and spot weldability deteriorates, which is not preferable. In order to achieve a sufficient alloying effect and prevent productivity from deteriorating, the holding time at 600 to 800 ° C. may be 1 to 100 hours.

According to one embodiment of the present invention, the average temperature increase rate from room temperature (25° C. as a non-limiting example) to the highest normal annealing temperature may be 1 to 75° C./h. If the heating rate is too slow, productivity may decrease, and conversely, if the heating rate is too fast, there may be a problem in that the plating layer is not uniformly alloyed.

In another embodiment of the present invention, the following parameter λ obtained by the relationship between the temperature and time from the start of heating for phase annealing to the end of cooling, represented by the following relational expression 1, is controlled to have a value between 32 and 37, so that the inside of the plating layer The distribution of phases can be appropriately controlled.

[Relationship 1]

Here, i means the elapsed time (minutes) from the start of annealing, n means the elapsed time (minutes) until the end of cooling, and λ _i means a parameter represented by the relational expression 2 below.

[Relationship 2]

Here, Ti means the absolute temperature (K) of the central part in the width direction and the length direction of the steel sheet (coil) at the elapsed time.

By controlling the parameter λ derived from the above formula to be 32 or more, sufficient alloying of the plating layer can be expected, and thus a uniformly alloyed plating layer can be obtained. On the other hand, when the parameter λ has an excessively high value, excessive oxides are formed on the surface and the contact resistance value increases during welding, resulting in deterioration in weldability. A preferable contact resistance value of the hot press-formed part may be 0.25 Ωm or less. In addition, Si distribution in the orthorhombic system and the BCC structure can be appropriately controlled by controlling the parameter λ within an appropriate range. Therefore, in one embodiment of the present invention, the parameter λ can be limited to 32 to 37, and in some cases, to 32.5 to 36.5.

After the top annealing, it is advantageous to cool the temperature range from 600 ° C to 400 ° C as slowly as possible. When the cooling in the temperature range from 600 ° C to 400 ° C is performed slowly, sticking defects in which steel sheets adhere to each other can be minimized, and fine pores can be sufficiently formed in the plating layer. Accordingly, the average cooling rate in the temperature range may be set to 50° C./h or less. There is no need to specifically set the lower limit of the average cooling rate, but it may be set to 1 ° C./h or more in consideration of productivity.

[Hot press forming member]

Next, a hot press-formed member according to one embodiment of the present invention will be described. The hot press formed member of the present invention is a member obtained by hot press forming, and is composed of a base steel sheet (same as the base steel sheet composition of the steel sheet for hot press forming) and an Al-Fe alloy plating layer formed on the surface of the base steel sheet.

The plating layer of the hot press-formed member of the present invention has an alloy of Al (aluminum) and Fe (iron) as a main component. The total content of these elements in the plating layer may be 85% or more by weight, and may be 90% or more or 95% or more in some cases. Since the occupancy rate of the phase in the BCC structure increases when the proportion of Fe in the plating layer is high, the content of Fe in the plating layer may be 50% or more by weight, and in some cases may be 55% or more.

In one embodiment of the present invention, the total occupancy of the orthorhombic and body-centered cubic structures may be 95% or more, or 97% or more, based on area. The upper limit of the ratio of these phases is not limited and may include up to 100%. In the present invention, the ratio of each phase can be obtained from the area occupied by each phase in the cross section observed by cutting the coated steel sheet in the thickness direction.

In addition, it is advantageous to have a high BCC phase ratio in order to ensure that hydrogen can be sufficiently discharged from the base steel sheet during the storage process of hot-press-formed parts, etc. It is necessary, and in some cases, the lower limit of the area ratio of the BCC phase may be set to 75%. Since resistance to hydrogen embrittlement can be secured as the area ratio of the BCC phase increases, there is no need to specifically set an upper limit for the area ratio of the BCC phase. However, if the BCC area ratio is excessive, weldability may deteriorate, so the upper limit of the ratio may be set at 90%.

Therefore, a hot press-formed member according to one aspect of the present invention includes a base steel sheet and a plating layer formed on a surface of the base steel sheet, wherein the plating layer includes Al-Fe containing at least 85% by weight of the total content of Al and Fe. It is a based alloy plating layer, and in one embodiment, the fraction of the BCC structure phase in the plating layer may be 70% or more based on area.

According to one embodiment of the present invention, α expressed by the following relational expression 3 of the base plate may be 0.2 or less.

[Relationship 3]

α = (Ci - Co)/Co

Here, Ci means the average value of the carbon content from the boundary between the base steel sheet and the plating layer to the inner depth of 20 μm of the base steel plate when measured by glow discharge spectroscopy (GDS), and Co means the average carbon content of the base steel plate .

When α exceeds 0.2, carbon is excessively concentrated in the surface layer of the base steel sheet, increasing the hardness of the surface layer. Bendability, which is an index of toughness, deteriorates. It is not necessary to set a lower limit for α because the lower the carbon content in the surface layer portion of the base steel sheet, the more advantageous it is. However, if the value of α is too low, the hardness of the surface layer portion may be excessively reduced and the fatigue properties of the material may deteriorate. Therefore, in one embodiment of the present invention, the lower limit of α may be set to -0.1.

[Method of manufacturing hot press-formed parts]

As long as the steel sheet for hot press forming of the present invention is used, the hot press forming method is not particularly limited. As a commonly known method, a steel sheet for hot press forming is heated for 5 to 15 minutes in a furnace maintained at a temperature between Ac3 and about 1000 ° C, transferred to a press, and formed while forming at a temperature of 27 ° C / s or more to a temperature of Ms or less. A method of cooling at a rate may be mentioned.

(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 exemplifying and specifying the present invention, 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.

A cold-rolled steel sheet for hot press forming having the composition (unit: weight%) shown in Table 1 below was prepared, and the surface of the steel sheet was plated with a plating bath having a composition of Al-9%Si-2%Fe on the surface of the steel sheet. During plating, the coating amount was adjusted to 20 g/m ² , and after plating, the average cooling rate was set to 10° C./s up to 250° C., and the coil was wound after temper rolling at a reduction rate of 1.0%.

element C Si Mn Al P S N Ti B Cr content 0.22 0.2 1.2 0.03 0.01 0.002 0.004 0.03 0.003 0.15

The plated steel sheet was heated to a maximum temperature of 750 ° C. as a target temperature in a normal annealing furnace, and the average heating rate of the steel plate from room temperature (20 ° C.) to the target temperature was 15 to 30 ° C. for each example, as shown in Table 2 I was able to confirm that it was in the /h range. During heating, the coil was maintained for 20 hours in a temperature range of 600° C. or higher. Thereafter, the coil was cooled, and the cooling rate from 600° C. to 400° C. during cooling was set to an average of 30° C./h.

By controlling the temperature raising, maintenance, and cooling processes in the entire heat treatment for each example, the value of the parameter λ calculated by the relational expression 1 could be adjusted to be within or outside the range specified in the present invention, and their specific values are shown in Table 2. shown in

The maximum Al content in the plating layer of the iron-aluminum alloy plated steel sheet obtained after the phase annealing was performed according to the above conditions was evaluated through GDS (glow discharge spectroscopy), and the location was determined through scanning electron microscope observation and component analysis through EDS. The phase that exists as a star was specified, and the area ratio of the phase was derived through image analysis. In image analysis, since all regions within the boundary divided by light and shade have the same type of image, the image occupancy rate can be obtained by performing image analysis on some of them and calculating the area of the same region. In addition, the Si content in the orthorhombic system and BCC could be determined through EDS. Specifically, at the position where the thickness in the depth direction is the thickest on each phase to be analyzed, a line segment is drawn connecting points 0.5 μm apart from the upper boundary and the lower boundary of the phase to the inside in the depth direction, respectively, and then the line segment is divided into 4 parts. The Si content of each phase can be obtained by measuring the Si component at the number of points by EDS and averaging the contents. When several phases enter the field of view, the average value of the same phase can be obtained and the Si content of each phase can be obtained. In addition, a roll leveling operation for shape correction was performed on the obtained plated steel sheet, and if there was an area where plating peeling of 3 mm or more occurred during visual observation, it was indicated as O, and if it was not, it was indicated in Table 2 as X. Al and Fe contents in the plating layer were not separately shown in Table 2 because they exceeded 90% in all examples.

division Average heating rate (℃/h) λ Si content in orthorhombic system (%) Si content in BCC (%) Maximum amount of Al in plating layer (%) BCC area ratio in plating layer (%) The number of pores having a size of 5 μm or less in diameter equivalent to a circle per 200 μm ² of cross-sectional area of the coating layer plating peeling Invention example 1 24.2 32.2 2.8 5.7 69.5 27.3 62 X Invention example 2 23.2 35 2.3 6.3 58.6 34.6 115 X Inventive example 3 20.8 36.7 1.9 6.6 58.9 40.7 153 X Comparative Example 1 25.1 31.8 3.2 4.9 71.9 24.7 48 O Comparative Example 2 19.9 37.2 1.6 7.1 54.3 45.9 203 O

As can be seen from the results of Table 2, when the value of the parameter (λ) decreases due to the decrease in heat treatment temperature and time, as in Comparative Example 1, diffusion of Fe in the base iron during heat treatment does not sufficiently proceed, resulting in Al content in the plating layer. increases and the area ratio on the BCC structure tends to decrease. In particular, when the λ value is less than 32, it is difficult to secure an area ratio of 30% or more of the phase of the BCC structure, and there is a point where the Al content exceeds 70% inside the plating layer, which occurs during shape correction by the roll leveler. The plating layer was peeled off in some areas due to the physical stress caused by the physical stress, resulting in non-uniformity in the plating layer of the coil. In the case of Comparative Example 2, the Al content in the plating layer, the BCC fraction, and the Si content in each phase met the ranges specified in the present invention, but the plating layer had a problem of peeling due to the excessive number of pores per unit area. 1 shows a photograph of a cross section obtained by cutting the steel sheet for hot press forming obtained in Inventive Example 2 in the thickness direction. In the figure, a portion marked with ① is a region in which a phase of the BCC structure exists, and a portion marked with ② represents a region in which an orthorhombic phase exists. As can be seen in the figure, the plating layer was substantially composed of only orthorhombic and BCC structure phases, and it can be seen that the BCC structure phase occupied 34.6% of the total plating layer cross-sectional area. 2 shows the results of analyzing Al and Fe components by glow discharge spectroscopy (GDS) in the depth direction from the surface of the plating layer of Example 2. As can be seen from the figure, the maximum Al content was 58.6% by weight, and sufficient alloying occurred, and as a result, the ratio of the BCC phase was secured at a ratio greater than or equal to the ratio specified in the present invention.

A blank was taken from the iron-aluminum alloy plated steel sheet obtained in each of the above examples and heated in a furnace maintained at a temperature of 930° C. for 6 minutes, then taken out of the furnace and subjected to press forming. At the time of heating, the atmosphere was an atmospheric atmosphere. In addition, the elapsed time from taking out the furnace to the start of press molding was 10 seconds, and the cooling rate to the Ms point during molding was set to 30 ° C. / s or more.

The occupancy rate of each phase was analyzed in the same way as in the steel plate, and the amount of diffusible hydrogen in the member after 10 days was evaluated using TDA gas chromatography equipment.

division Al content in plating layer (%) Fe content in plating layer (%) BCC fraction in member (%) Diffused hydrogen amount after 10 days (ppm) Invention example 1 22.91 66.50 72.1 0.13 Invention example 2 22.45 68.56 80.2 0.10 Inventive example 3 22.26 69.34 86.9 0.08 Comparative Example 1 23.27 65.11 69.8 0.17 Comparative Example 2 21.85 70.50 90.7 0.06

As in Comparative Example 1, when the area ratio of the phase having a BCC structure in the plated steel sheet for hot press forming is low, the BCC area ratio of the member after heat treatment is less than 70%, and hydrogen release is not relatively easy after hot press forming, so diffusion The amount of hydrogen produced was higher than 0.15 ppm. However, when the BCC occupancy rate of the plated steel sheet for hot press forming was 25% or more, the BCC area ratio could be secured at 70% or more even in the member, and as a result, the amount of diffusible hydrogen could be stably maintained at 0.15 ppm or less. Therefore, it was confirmed that it is important to set the BCC occupancy rate of the plated steel sheet for hot press forming to 25% or more, and to set the BCC occupancy rate to 70% or more in the hot press formed part. FIG. 3 shows a photograph of a cross-section of a hot press-formed member manufactured by Inventive Example 2 cut in the thickness direction. In the figure, a portion marked with ① is a region in which a phase of the BCC structure exists, and a portion marked with ② represents a region in which an orthorhombic phase exists. As can be seen from the figure, the plating layer was substantially composed of only orthorhombic and BCC structure phases, and it can be seen that the BCC structure phase occupies 80.2% of the total plating layer cross-sectional area.

In addition, by measuring the carbon of the base steel sheet through GDS in the members obtained from each example, the carbon concentration expressed by α in the relational expression 3 was calculated, and the values are shown in Table 4. In order to examine the bendability (crash resistance) according to the carbon concentration, the three-point bendability (bending angle) according to the VDA238-100 standard was evaluated, and the value of tensile strength (TS) × bending angle of the material was indicated. The unit of tensile strength in the table is MPa. In addition, the contact resistance value was measured to confirm the weldability of the member. Contact resistance means the resistance generated during welding. When the contact resistance increases at the same welding voltage, the current (=V/R) decreases, and as a result, the calorific value (=VI) decreases, resulting in poor weldability. fall out Since the method of measuring the contact resistance value is well shown in the ISO 18594 standard, it is not described in detail in the present invention.

division carbon enrichment Tensile strength (TS)ס¿bending angle (°¡Æ) Contact resistance value (Ωm) Invention example 1 0.198 60,642 0.195 Invention example 2 0.175 61,021 0.221 Inventive example 3 0.161 61,588 0.240 Comparative Example 1 0.207 59,051 0.178 Comparative Example 2 0.155 62,050 0.262

If the BCC ratio of the steel sheet for hot press forming is not sufficient, the thickness of the alloy layer becomes thick during heating for hot press forming, and as a result, carbon is pushed to the side of the steel sheet, resulting in a high carbon concentration (see Comparative Example 1). ). As a result, the tensile strength (TS) × bending angle according to the inferior bending angle is less than 60,000. Conversely, when the value of parameter (λ) exceeds 37 as in Comparative Example 2, oxide is slightly generated in the surface layer due to excessive oxidation during annealing, which is not appropriate because the contact resistance value increases. The relatively bright area corresponds to the base steel plate (base iron). From the drawing, it can be seen that the concentration of carbon is very low on the surface of the base steel sheet, and as a result, excellent bendability can be secured.

As described above, according to the present invention, hot-formed products having excellent collision resistance as well as resistance to delayed destruction by hydrogen due to a reduction in the amount of diffusible hydrogen can be manufactured, and these parts are structural materials or reinforcement As a part, it can be applied and utilized in various fields including the automobile manufacturing field.

Claims (17)

base steel plate and
Including a plating layer formed on the surface of the base steel sheet,
The plating layer is an Al-Fe-based alloy plating layer containing 80% or more by weight of the total content of Al and Fe,
The fraction of the phase of the BCC structure in the plating layer is 30% or more based on area
Coated steel sheet for hot press forming.
The coated steel sheet for hot press forming according to claim 1, wherein the fraction of the phase of the BCC structure is 35% or more on an area basis.
The plated steel sheet for hot press forming according to claim 1, wherein the plating layer contains 90% or more of the orthorhombic and BCC structure phases based on area.
The plated steel sheet for hot press forming according to claim 3, wherein the orthorhombic system contains less than 3% by weight of Si, and the phase of the BCC structure contains more than 3% by weight of Si.
The coated steel sheet for hot press forming according to claim 1, wherein 50 or more voids having an equivalent circular diameter of 5 μm or less exist in a region within 5 μm in depth from the surface of the plating layer per 200 μm ² of cross-sectional area of the coating layer.
The coated steel sheet for hot press forming according to claim 1, wherein the Al content measured by GDS at all depths in the plating layer is 70% or less by weight.
The method according to any one of claims 1 to 6, wherein the base steel sheet contains, by weight, carbon (C): 0.14-0.5%, silicon (Si): 0.05-1%, manganese (Mn): 0.5-4% , phosphorus (P): 0.001 to 0.03%, sulfur (S): 0.0001 to 0.02%, aluminum (Al): 0.01 to 0.1%, nitrogen (N): 0.001 to 0.02%, the balance Fe and unavoidable impurities Steel sheet for hot press forming.
The steel sheet for hot press forming according to claim 7, wherein the base steel sheet has a composition further comprising one or two or more of the following.
(1) The sum of one or more selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%
(2) The sum of at least one selected from the group consisting of Ti, Nb, Zr, and V: 0.001 to 0.4%
(3) Cu + Ni: 0.005~2.0%
(4) Sb + Sn: 0.001~1.0%
(5) B: 0.0001~0.01%
Preparing an aluminum-coated steel sheet; and
Phase annealing heat treatment of the aluminum-plated steel sheet
including,
Method for producing a steel sheet for hot press forming to control the phase annealing time and temperature so that λ represented by the following relational expression 1 is 32 to 37.
[Relationship 1]

Here, i means the elapsed time (minutes) from the start of annealing, n means the elapsed time (minutes) until the end of cooling, and λ _i means a parameter represented by the relational expression 2 below.
[Relationship 2]

Here, Ti means the absolute temperature (K) of the central part in the width direction and the length direction of the steel sheet (coil) at the elapsed time.
10. The method of claim 9, wherein the maximum temperature during the annealing is 600 to 800 ° C based on the temperature of the center of the steel sheet (coil) in the width direction and longitudinal direction, and the time maintained in the temperature range of 600 to 800 ° C is 1 to 100 hours A method for producing a steel sheet for phosphorus hot press forming.
[Claim 10] The method of manufacturing a steel sheet for hot press forming according to claim 9, wherein an average temperature increase rate from room temperature to the maximum temperature during normal annealing is 1 to 75 °C/h.
The method for manufacturing a steel sheet for hot press forming according to claim 9, wherein the average cooling rate from 600°C to 400°C is 50°C/h or less.
The method according to any one of claims 9 to 12, wherein the aluminum-plated steel sheet contains, by weight, carbon (C): 0.14-0.5%, silicon (Si): 0.05-1%, manganese (Mn): 0.5~4%, Phosphorus (P): 0.001~0.03%, Sulfur (S): 0.0001~0.02%, Aluminum (Al): 0.01~0.1%, Nitrogen (N): 0.001~0.02%, balance Fe and unavoidable impurities Method for producing a steel sheet for hot press forming having a composition comprising a.
The method of claim 13, wherein the base steel sheet has a composition further comprising one or two or more of the following.
(1) The sum of one or more selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%
(2) The sum of at least one selected from the group consisting of Ti, Nb, Zr, and V: 0.001 to 0.4%
(3) Cu + Ni: 0.005~2.0%
(4) Sb + Sn: 0.001~1.0%
(5) B: 0.0001~0.01%
base steel plate and
Including a plating layer formed on the surface of the base steel sheet,
The plating layer is an Al-Fe-based alloy plating layer containing 85% or more by weight of the total content of Al and Fe,
The fraction of the phase of the BCC structure in the plating layer is 75% or more based on area,
α expressed by the following relational expression 3 is 0.2 or less
Hot press forming part.
α = (Ci - Co)/Co
Here, Ci means the average value of the carbon content from the boundary between the base steel sheet and the plating layer to the inner depth of 20 μm of the base steel plate when measured by glow discharge spectroscopy (GDS), and Co means the average carbon content of the base steel plate .
16. The method of claim 15, wherein the base steel sheet contains, by weight, carbon (C): 0.14-0.5%, silicon (Si): 0.05-1%, manganese (Mn): 0.5-4%, phosphorus (P): 0.001-0.001 0.03%, sulfur (S): 0.0001 to 0.02%, aluminum (Al): 0.01 to 0.1%, nitrogen (N): 0.001 to 0.02%, the balance Fe and unavoidable impurities.
The hot press-formed part according to claim 16, wherein the base steel sheet has a composition further comprising one or two or more of the following.
(1) The sum of one or more selected from the group consisting of Cr, Mo and W: 0.01 to 4.0%
(2) The sum of at least one selected from the group consisting of Ti, Nb, Zr, and V: 0.001 to 0.4%
(3) Cu + Ni: 0.005~2.0%
(4) Sb + Sn: 0.001~1.0%
(5) B: 0.0001~0.01%

Images