Hot-stamped plated steel sheet, method for manufacturing the same, and method for manufacturing hot-stamped formed part

Abstract

One aspect of the present invention aims to provide a hot-formed coated steel sheet, a method for manufacturing the same, and a method for manufacturing hot-stamped formed parts. A preferred aspect of the present invention aims to provide a hot-formed coated steel sheet, a method for manufacturing the same, and a method for manufacturing hot-stamped formed parts that not only offer excellent manufacturability but also reduce manufacturing costs.

Description

Technical Field

[0001] This invention relates to a galvanized steel sheet for hot forming, a method for manufacturing the same, and a method for manufacturing hot-stamped parts. Background Technology

[0002] With efforts to improve fuel efficiency through vehicle lightweighting and the trend of continuously strengthening collision regulations and upgrading passenger safety, the application of ultra-high strength steel for hot forming has become increasingly widespread.

[0003] In this type of hot-stamping steel, the use of uncoated materials can lead to the formation of iron oxide scale and compromised corrosion resistance. Therefore, there is a growing trend to replace uncoated steel with coated steel. Furthermore, the coated steel is typically made of aluminized steel.

[0004] For example, to prevent the formation of surface iron oxide scale and decarburization in steel plates during heating to temperatures above 850°C, ArcelorMittal first proposed and commercially succeeded in using an aluminum-silicon (Al-Si) coating.

[0005] However, as described above, steel sheets with an Al-based coating experience liquefaction during heat treatment for hot forming. When heated to temperatures above the melting point of the Al coating in the furnace, the Al coating tends to adhere to the rollers within the furnace. This melting problem is exacerbated when increasing the heat treatment heating rate to improve productivity or using efficient induction currents for rapid heating. Therefore, shortening heat treatment time through rapid heating has limitations. Furthermore, to austenitize the steel sheet without melting the Al coating, it needs to be heated to above 900°C. This requires alloying the Al coating into an AlFe alloy during heating. Consequently, 30-50% of the furnace area is dedicated to AlFe alloying, increasing furnace length and potentially leading to significant energy consumption and CO2 emissions during manufacturing.

[0006] To address these issues, existing technologies propose an additional alloying annealing heat treatment before heat treatment for hot forming. This involves the interdiffusion of the Al coating and the steel sheet (Fe) to generate an aluminum-iron (Al-Fe) alloy phase, raising the melting point of the coating and preventing Al fusion adhesion. Simultaneously, rapid heating improves productivity. However, this pre-alloying heat treatment increases manufacturing costs, thus reducing market competitiveness. In particular, the pre-alloying heat treatment also needs to be performed at temperatures below the coating's melting point to prevent melting. Therefore, complete alloying can take several hours to tens of hours, during which the coating continues to oxidize, leading to a decline in surface quality and coating properties. Furthermore, when alloying is performed only on a portion of the coating instead of completely alloying to shorten the alloying heat treatment time, the unalloyed coating may still melt during rapid heating, increasing the risk of roll sticking.

[0007] Another existing technology proposes a method to improve productivity and reduce manufacturing costs during hot forming by simultaneously applying alloying annealing to aluminum alloys in a continuous melt plating production line. However, to increase the diffusion rate between the aluminum coating and the steel sheet, this method limits the Si content, which hinders the aluminum-iron diffusion reaction, to 0.4-4%. This reduction in Si content raises the melting point of the aluminum-silicon melt, generates a large amount of fumes during continuous melt plating, leading to decreased productivity and increased production line maintenance costs. Furthermore, the decreased melt fluidity causes an increase in surface defects such as flow lines, and the increased proportion of brittle alloy layers after hot forming exacerbates problems such as coating peeling and mold adhesion during the forming process.

[0008] In addition, the aluminized steel will undergo alloying during the heating process, which will cause the heating rate to slow down sharply within a certain temperature range, thus leading to a longer heating time. Therefore, a solution to this problem is also needed. Summary of the Invention

[0009] (a) Technical problems to be solved One aspect of the present invention is to provide a galvanized steel sheet for hot forming, a method for manufacturing the same, and a method for manufacturing hot stamped parts.

[0010] A preferred aspect of the present invention aims to provide a hot-formed coated steel sheet that not only has excellent productivity but also reduces manufacturing costs, as well as a method for manufacturing the same and a method for manufacturing hot-stamped parts.

[0011] The technical problems addressed by this invention are not limited to those described above. Those skilled in the art will readily understand the additional technical problems addressed by this invention through the entirety of this specification.

[0012] (II) Technical Solution A first aspect of the present invention provides a clad steel sheet for hot forming, the clad steel sheet comprising: a base steel sheet; and a coating formed on at least one side of the base steel sheet, the coating comprising: a first layer composed of a Fe2Al5 phase; and a second layer formed on the first layer and composed of a FeAl3 phase, wherein the first layer contains an AlSiFe intermetallic compound (τ phase) present continuously or discontinuously.

[0013] The coating may contain, by weight percent: Si: 6-15%, Fe: 20-60%, balance Al and other unavoidable impurities.

[0014] The coating can have an average thickness of 7-30 μm.

[0015] The whiteness of the coating can be below 60, and the gloss can be below 6.0.

[0016] A second aspect of the present invention provides a galvanized steel sheet for hot forming, the galvanized steel sheet comprising: a base steel sheet; and a coating formed on at least one side of the base steel sheet; the coating comprising: a first layer, comprising, by weight%, Al: 40-48%, Fe: 49-55%, and Si: less than 5%; a second layer, formed on the first layer, comprising, by weight%, Al: 40-50%, Fe: 38-48%, and Si: 2-12%; a third layer, formed on the second layer, comprising, by weight%, Al: 45-60%, Fe: 33-48%, and Si: less than 7%; and a fourth layer, formed on the third layer, comprising, by weight%, Al: 50-65%, Fe: 25-35%, and Si: less than 15%.

[0017] The coating may contain, by weight percent: Si: 6-15%, Fe: 20-60%, balance Al and other unavoidable impurities.

[0018] The first layer may contain the Fe2Al5 phase, the second layer may contain the AlSiFe intermetallic compound (τ phase), the third layer may contain the Fe2Al5 phase, and the fourth layer may contain the FeAl3 phase.

[0019] The second layer may exist in the region within 40% of the total average thickness (T) of the coating from the bottom of the coating along the thickness direction.

[0020] The ratio (T1 / T) of the total average thickness (T1) of the first, second and third layers to the total average thickness (T) of the coating can be from 0.20 to 0.50.

[0021] The ratio (T2 / T) of the average thickness (T2) of the fourth layer to the total average thickness (T) of the coating can be from 0.50 to 0.80.

[0022] The coating can have an average thickness of 7-30 μm.

[0023] The whiteness of the coating can be below 60, and the gloss can be below 6.0.

[0024] A third aspect of the present invention provides a method for manufacturing a coated steel sheet for hot forming, the method comprising the following steps: preparing a base steel sheet; immersing the base steel sheet in a plating bath to form a coating on at least one side of the base steel sheet to obtain a coated steel sheet; subjecting the coated steel sheet to diffusion annealing heat treatment; and cooling the coated steel sheet after diffusion annealing heat treatment; wherein the diffusion annealing heat treatment is performed at 650-850°C for 3.0-20.0 seconds; the cooling includes: performing a first cooling to a first cooling termination temperature of 600°C, and then performing a second cooling to a second cooling termination temperature below 600°C, wherein the cooling rate during the first cooling is lower than the cooling rate during the second cooling.

[0025] The plating bath may contain, by weight percent: Si: 6-15%, Fe: 0.1-2.0%, balance Al and other unavoidable impurities.

[0026] The diffusion annealing heat treatment can be achieved through induction heating.

[0027] The cooling rate during the first cooling process can be 0.9-21℃ / second, and the cooling rate during the second cooling process can be 12-60℃ / second.

[0028] A fourth aspect of the present invention provides a method for manufacturing a hot-stamped part, wherein in the step of heating a plated steel sheet for hot forming, the step is performed such that X, as expressed by the following [Formula 1], reaches 1.50°C / second or less.

[0029] [Equation 1] X=AB (Whereinafter, in [Equation 1], A is the heating rate at 600℃ calculated based on the linear fit of the heating rate in the temperature range between 300℃ and 550℃, and B is the heating rate at 600℃ calculated based on the linear fit of the heating rate in the temperature range between 600℃ and 700℃.) The galvanized steel sheet for hot forming can satisfy one or more of the following (a) to (c).

[0030] (a) The hot-formed coated steel sheet comprises a base steel sheet and a coating formed on at least one side of the base steel sheet, the coating comprising, by weight %: Si: 6-15%, Fe: 20-60%, balance Al and other unavoidable impurities. (b) The coating adhesion amount on one side is 30 g / m 2 the following, (c) The whiteness of the coating is below 60.

[0031] X can be above 0℃ / second and below 1.50℃ / second.

[0032] The heating step can be performed in such a way that the cumulative value (Y) of the coating alloying history index at 550°C, as expressed by the following [Equation 2], reaches 0.14 or less.

[0033] [Equation 2] (Whereinafter, in [Equation 2], k is a parameter related to the alloying rate, with a value of 130.31; Q is a parameter related to the temperature effect affecting the alloying behavior at a given temperature, with a value of 62190.89 J / mol; R is the gas constant in the ideal gas law, representing the ratio of gas volume to amount of substance at constant temperature and pressure, with a value of 8.314 J / (mol·K); n is a parameter related to the time effect affecting the alloying rate, with a value of 0.644; and j represents T.) i i is at 550℃, and Δt is... i The time interval for temperature measurement during heating is 1 second (sec), where i is the temperature measurement time during heating, taking a positive integer value (seconds), and T... i This represents the heating temperature (K) measured in the i-th second. The value of Y can be less than 0.12.

[0034] The value of Y can be less than 0.10.

[0035] The value of Y can be greater than or equal to 0.01.

[0036] Following the heating step, a further step may be included: hot stamping the galvanized steel sheet for hot forming.

[0037] (III) Beneficial Effects According to one aspect of the present invention, a method for manufacturing a hot-formed coated steel sheet and a method for manufacturing a hot-stamped part can be provided.

[0038] According to a preferred aspect of the present invention, a hot-formed coated steel sheet with excellent productivity and reduced manufacturing costs, a method for manufacturing the same, and a method for manufacturing hot-stamped formed parts can be provided. Attached Figure Description

[0039] Figure 1 This is a schematic diagram showing a galvanized steel sheet for thermoforming according to a first aspect of the present invention.

[0040] Figure 2 This is a photograph of the cross-section of Invention Example 9 observed using a scanning electron microscope.

[0041] Figure 3 These are photographs of the cross-section of Comparative Example 1 observed using a scanning electron microscope.

[0042] Figure 4 This is a schematic diagram showing a galvanized steel sheet for thermoforming according to a second aspect of the present invention.

[0043] Figure 5 The photograph is a cross-section of Invention Example 18 observed using a scanning electron microscope.

[0044] Figure 6 These are photographs of the cross-section of Comparative Example 7 observed using a scanning electron microscope.

[0045] Figure 7 These are graphs showing the relationship between heating rate and temperature in Examples 19 and 20 of the Invention.

[0046] Figure 8 The graphs show the relationship between heating rate and temperature for Comparative Examples 13 and 14.

[0047] Figure 9 These are graphs relating heating time and temperature for Invention Examples 19, 20 and Comparative Examples 13 and 14.

[0048] Figure 10 The graph shows the cumulative values ​​of the coating alloying history index as a function of temperature for Invention Examples 19, 20 and Comparative Examples 13 and 14.

[0049] Figure 11 The graphs show the relationship between heating rate and temperature for Comparative Example 15 and Invention Example 21. Best practice

[0050] In describing embodiments of the present invention, detailed descriptions of well-known technologies related to the present invention will be omitted if it is determined that such detailed descriptions may unnecessarily obscure the main points of the invention. Furthermore, the terms used below are defined in consideration of the functionality within the present invention, and their meanings may change depending on the intentions or practices of users, operators, etc. Therefore, their definitions should be based on the entire contents of this specification. The terms used in the detailed description are only for describing embodiments of the present invention and should not be limiting in any way. Unless there is an explicit contrary usage, the singular form includes the meaning of the plural form.

[0051] Unless otherwise defined, all terms used herein, including technical and scientific terms, shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms as defined in common dictionaries shall be interpreted as having meanings consistent with relevant technical literature and current disclosure, and shall not be interpreted as having idealized or overly formal meanings unless otherwise defined.

[0052] In this specification, expressions such as “comprising,” “including,” or “possessing” are used to refer to the presence of a specific feature, number, step, action, element, part or combination thereof, and should not be construed as excluding the presence or possibility of one or more other features, numbers, steps, actions, elements, part or combination thereof.

[0053] Unless otherwise specified, all percentages in this specification refer to weight.

[0054] The present invention will be described in detail below through various embodiments or examples. It should be noted that the embodiments or examples described in this specification are not limited to a single embodiment or example, but can be combined with other embodiments or examples. Therefore, the claims referenced in the claims are only examples of one embodiment, and the technical concept of the present invention should not be interpreted solely as a combination with the referenced claims; combinations with each claim are also included within the scope of the technical concept of the present invention.

[0055] Figure 1 This is a schematic diagram illustrating a coated steel sheet for thermoforming according to the first aspect of the present invention. Hereinafter, reference will be made to... Figure 1 A thermoformed steel sheet according to the first aspect of the present invention will be described.

[0056] like Figure 1 As shown, a hot-formed coated steel sheet according to one embodiment of the present invention comprises: a base steel sheet 100; and a coating 200 formed on at least one side of the base steel sheet 100. The type of base steel sheet is not particularly limited in the present invention, and all steel sheets commonly used in this art can be used.

[0057] The coating 200 may contain, by weight percent: Si: 6-15%, Fe: 20-60%, balance Al and other unavoidable impurities.

[0058] Si: 6-15% Si can lower the melting point of Al melt and form a relatively stable alloy phase in the AlSiFe series, thereby reducing Fe diffusion. When the Si content is less than 6%, the melting point of the melt will rise, and a large amount of fumes will be generated during continuous melt plating, leading to reduced productivity and increased production line maintenance costs. In addition, besides the problem of reduced melt fluidity causing an increase in surface defects such as flow lines, increased Fe diffusion during hot forming will lead to a higher proportion of brittle alloy layers in the coating, causing problems such as coating peeling and mold adhesion during forming. When the Si content exceeds 15%, the melting point of Al melt will rise again, causing productivity problems, and Si crystallization in the coating will cause the coating itself to become brittle. Therefore, the Si content is preferably in the range of 6-15%. The lower limit of the Si content is more advantageously 6.5%, further more advantageously 7%, and most advantageously 7.5%. The upper limit of the Si content is more advantageously 13%, further more advantageously 11%, and most advantageously 9%.

[0059] Fe: 20-60% The Fe dissolves from the base steel plate entering the plating bath and is included in the coating. Through diffusion annealing heat treatment, the Fe in the base steel plate diffuses and moves into the coating, forming an Al-Fe alloy, which increases the melting point of the coating. When the Fe content is less than 20%, the Al-Fe alloy formation in the coating is insufficient, leaving residual Al(Si) coating, which cannot achieve a sufficient increase in heating rate during hot forming heat treatment, nor can it prevent the coating from melting. When the Fe content exceeds 60%, an excessive diffusion layer will form during hot forming, resulting in a narrowing of the welding current range and reduced weldability. Therefore, the Fe content is preferably in the range of 20-60%. The lower limit of the Fe content is more advantageously 23%, further more advantageously 25%, and most advantageously 28%. The upper limit of the Fe content is more advantageously 53%, further more advantageously 48%, and most advantageously 45%.

[0060] The remaining components are Al. However, during conventional manufacturing processes, unintentionally added impurities inevitably mix in with the raw materials or the surrounding environment, making it impossible to eliminate the presence of such impurities. These impurities are known to those skilled in the art during conventional manufacturing processes, therefore, their details are not specifically described in this specification.

[0061] The coating preferably comprises: a first layer 10 composed of the Fe2Al5 phase; and a second layer 20 formed on the first layer 10, composed of the FeAl3 phase. Typically, after diffusion annealing heat treatment, Fe from the base steel plate diffuses into the Al coating, changing sequentially according to the Fe concentration as FeAl3, Fe2Al5, FeAl2, and FeAl. According to the present invention, after the coating is formed during continuous melt plating, a short diffusion annealing heat treatment of 3-20 seconds is sufficient to transform the entire coating into an Al-Fe alloy phase. Therefore, the coating is composed of the Fe2Al5 and FeAl3 phases with relatively low Fe content. A layer composed of the Fe2Al5 phase is formed near the base steel plate in the first layer 10, where the Fe concentration is relatively high; on the first layer, a second layer 20 composed of the FeAl3 phase with a relatively low Fe concentration is formed.

[0062] The first layer 10 preferably contains a continuous or discontinuous AlSiFe intermetallic compound (τ phase) 30. The phase structure of the AlSiFe intermetallic compound varies slightly depending on the concentrations of Al, Fe, and Si, and is currently difficult to define precisely, but is generally referred to as the τ phase in AlSiFe ternary alloy systems. The AlSiFe intermetallic compound (τ phase) can be banded, exhibiting relatively good ductility, which can reduce the generation and propagation of coating cracks during production, while preventing excessive Fe diffusion during hot forming. Furthermore, the AlSiFe intermetallic compound (τ phase) forms on the surface of the base steel plate the instant it enters the plating bath, and then gradually moves from the surface into the coating interior through diffusion annealing heat treatment. However, the diffusion rate of the AlSiFe intermetallic compound (τ phase) is much lower than the diffusion rate of Fe atoms. Therefore, through only the short-time diffusion annealing heat treatment described in this invention, the AlSiFe intermetallic compound (τ phase) cannot move to the coating surface, but remains within the first layer 10.

[0063] The coating can have an average thickness of 7-30 μm. When the average thickness of the coating is less than 7 μm, it may be difficult to ensure sufficient corrosion resistance after thermoforming. When the average thickness of the coating exceeds 30 μm, it is difficult to achieve complete alloying through diffusion annealing heat treatment, which may prevent the achievement of the invention's objectives of improving productivity during thermoforming and preventing coating melt-out. Therefore, the coating can have an average thickness of 7-30 μm. The lower limit of the average thickness of the coating is more advantageously 8 μm, further more advantageously 9 μm, and most advantageously 10 μm. The upper limit of the average thickness of the coating is more advantageously 27 μm, further more advantageously 24 μm, and most advantageously 21 μm.

[0064] The whiteness (L value) of the coated steel sheet provided by the present invention, as described above, can be 60 or less, and the gloss can be 6.0 or less. Therefore, the heating efficiency during the heat treatment process for hot forming can be improved, resulting in a significant increase in heating speed and a reduction in heating time. Furthermore, the gloss measurement angle can be 60°.

[0065] The coated steel sheet of this invention, because the coating is fully alloyed throughout, has a melting point higher than the heat treatment temperature for hot forming, thus preventing coating melting and Al fusion adhesion within the hot forming furnace. Furthermore, it exhibits high heating efficiency during the heat treatment process for hot forming, eliminating the need for the latent heat of fusion required for coating melting and the energy required for Al-Fe alloying. Therefore, the austenitizing efficiency of the steel sheet is improved, ensuring material quality even with shortened heating times. Moreover, it can be manufactured in a continuous melt-coating production line, offering advantages such as ease of manufacturing and high economic efficiency. In other words, it not only boasts excellent productivity but also reduces manufacturing costs.

[0066] Figure 4 This is a schematic diagram of a galvanized steel sheet for thermoforming according to the second aspect of the present invention. Hereinafter, reference will be made to... Figure 4 A galvanized steel sheet for thermoforming according to the second aspect of the present invention will be described.

[0067] like Figure 4 As shown, the hot-formed coated steel sheet according to the second aspect of the present invention comprises: a base steel sheet 100'; and a coating 200' formed on at least one side of the base steel sheet 100'. The type of base steel sheet is not particularly limited in the present invention, and all steel sheets commonly used in this art can be used.

[0068] The coating 200' may contain, by weight percent: Si: 6-15%, Fe: 20-60%, balance Al and other unavoidable impurities.

[0069] Si: 6-15% Si can lower the melting point of Al melt and form a relatively stable alloy phase in the AlSiFe series, thereby reducing Fe diffusion. When the Si content is less than 6%, the melting point of the melt will rise, and a large amount of fumes will be generated during continuous melt plating, leading to reduced productivity and increased production line maintenance costs. In addition, besides the problem of reduced melt fluidity causing an increase in surface defects such as flow lines, increased Fe diffusion during hot forming will lead to a higher proportion of brittle alloy layers in the coating, causing problems such as coating peeling and mold adhesion during forming. When the Si content exceeds 15%, the melting point of Al melt will rise again, causing productivity problems, and Si crystallization in the coating will cause the coating itself to become brittle. Therefore, the Si content is preferably in the range of 6-15%. The lower limit of the Si content is more advantageously 6.5%, further more advantageously 7%, and most advantageously 7.5%. The upper limit of the Si content is more advantageously 13%, further more advantageously 11%, and most advantageously 9%.

[0070] Fe: 20-60% The Fe dissolves from the base steel plate entering the plating bath and is included in the coating. Through diffusion annealing heat treatment, the Fe in the base steel plate diffuses and moves into the coating, forming an Al-Fe alloy, which increases the melting point of the coating. When the Fe content is less than 20%, the Al-Fe alloy formation in the coating is insufficient, leaving residual Al(Si) coating, which cannot achieve a sufficient increase in heating rate during hot forming heat treatment, nor can it prevent the coating from melting. When the Fe content exceeds 60%, an excessive diffusion layer will form during hot forming, resulting in a narrowing of the welding current range and reduced weldability. Therefore, the Fe content is preferably in the range of 20-60%. The lower limit of the Fe content is more advantageously 23%, further more advantageously 25%, and most advantageously 28%. The upper limit of the Fe content is more advantageously 53%, further more advantageously 48%, and most advantageously 45%.

[0071] The remaining components are Al. However, during conventional manufacturing processes, unintentionally added impurities inevitably mix in with the raw materials or the surrounding environment, making it impossible to eliminate the presence of such impurities. These impurities are known to those skilled in the art during conventional manufacturing processes, therefore, their details are not specifically described in this specification.

[0072] The coating may include: a first layer 10', which, by weight percent, comprises: Al: 40-48%, Fe: 49-55%, and Si: less than 5%; a second layer 20', formed on top of the first layer 10', which, by weight percent, comprises: Al: 40-50%, Fe: 38-48%, and Si: 2-12%; a third layer 30', formed on top of the second layer 20', which, by weight percent, comprises: Al: 45-60%, Fe: 33-48%, and Si: less than 7%; and a fourth layer 40', formed on top of the third layer 30', which, by weight percent, comprises: Al: 50-65%, Fe: 25-35%, and Si: less than 15%.

[0073] Typically, after diffusion annealing heat treatment, Fe from the base steel plate diffuses into the Al coating, changing sequentially according to Fe concentration as FeAl3, Fe2Al5, FeAl2, and FeAl. According to the present invention, after the coating is formed during continuous melt plating, a short diffusion annealing heat treatment of 3-20 seconds is sufficient to transform the entire coating into an Al-Fe alloy phase. Therefore, the coating consists of Fe2Al5 and FeAl3 phases with relatively low Fe content. That is, the first layer 10', which is closer to the base steel plate and has a relatively high Fe concentration, may contain the Fe2Al5 phase; the second layer may contain the AlSiFe intermetallic compound (τ phase); the third layer may contain the Fe2Al5 phase; and the fourth layer 40', with a relatively low Fe concentration, may contain the FeAl3 phase. In this case, the AlSiFe intermetallic compound (τ phase) may exist continuously or discontinuously.

[0074] The second layer can exist in a region within 40% of the total average thickness (T) of the coating from the bottom along the thickness direction. If the second layer exists in a region exceeding 40% of the total average thickness (T) of the coating from the bottom along the thickness direction, the diffusion of Fe from the base steel plate within the coating cannot be suppressed during hot forming, resulting in over-alloying and poor spot weldability. More advantageously, the second layer can exist in a region within 20% of the total average thickness (T) of the coating from the bottom along the thickness direction. The lower limit of the location of the second layer is not particularly limited in this invention; the second layer can exist in a region at intervals of more than 1% of the total average thickness (T) of the coating from the bottom along the thickness direction.

[0075] The ratio (T1 / T) of the total average thickness (T1) of the first, second, and third layers to the total average thickness (T) of the coating can be from 0.20 to 0.50. When T1 / T is less than 0.20, the adhesion between the base steel plate and the coating deteriorates. When T1 / T exceeds 0.50, the diffusion of Fe from the base steel plate within the coating cannot be suppressed during hot forming, resulting in over-alloying and poor spot weldability. The lower limit of T1 / T is more advantageously 0.23, further more advantageously 0.25, and most advantageously 0.27. The upper limit of T1 / T is more advantageously 0.48, further more advantageously 0.46, and most advantageously 0.44.

[0076] The ratio (T2 / T) of the average thickness (T2) of the fourth layer to the total average thickness (T) of the coating can be between 0.50 and 0.80. When T2 / T is less than 0.50, excessive Fe diffusion occurs within the coating, resulting in poor spot weldability after hot forming. When T2 / T exceeds 0.80, the adhesion between the base steel plate and the coating deteriorates. The lower limit of T2 / T is more advantageously 0.52, further more advantageously 0.54, and most advantageously 0.56. The upper limit of T2 / T is more advantageously 0.77, further more advantageously 0.76, and most advantageously 0.73.

[0077] The coating can have an average thickness of 7-30 μm. When the average thickness of the coating is less than 7 μm, it may be difficult to ensure sufficient corrosion resistance after thermoforming. When the average thickness of the coating exceeds 30 μm, it is difficult to achieve complete alloying through diffusion annealing heat treatment, which may prevent the achievement of the objectives of this invention, namely, improving productivity during thermoforming and preventing coating melt. Therefore, the coating can have an average thickness of 7-30 μm. The lower limit of the average thickness of the coating is more advantageously 8 μm, further more advantageously 9 μm, and most advantageously 10 μm. The upper limit of the average thickness of the coating is more advantageously 27 μm, further more advantageously 24 μm, and most advantageously 21 μm.

[0078] The whiteness (L value) of the coated steel sheet provided by the present invention, as described above, can be 60 or less, and the gloss can be 6.0 or less. Therefore, the heating efficiency during the heat treatment process for hot forming can be improved, resulting in a significant increase in heating speed and a reduction in heating time. Furthermore, the gloss measurement angle can be 60°.

[0079] The coated steel sheet of this invention, because the coating is fully alloyed throughout, has a melting point higher than the heat treatment temperature for hot forming, thus preventing coating melting and Al adhesion within the hot forming furnace. Furthermore, it exhibits high heating efficiency during the heat treatment process for hot forming, eliminating the need for the latent heat of fusion required for coating melting and the energy required for Al-Fe alloying. Therefore, the austenitizing efficiency of the steel sheet is improved, ensuring material quality even with shortened heating times. Moreover, it can be manufactured in a continuous melt-coating production line, offering advantages of simple manufacturing and high economy. In other words, it not only boasts excellent productivity but also reduces manufacturing costs.

[0080] The following describes a method for manufacturing a thermoformed coated steel sheet according to one embodiment of the present invention.

[0081] First, prepare the base steel plate. As mentioned above, the type of base steel plate is not particularly limited in this invention, and any steel plate commonly used in this technical field can be used.

[0082] To obtain the desired material, the base steel plate can be heat-treated in an annealing furnace.

[0083] Subsequently, the base steel plate is immersed in a plating bath to form a coating on at least one side of the base steel plate to obtain a coated steel plate. The method for forming the coating is not particularly limited in this invention; any melt plating method commonly used in this art can be used. Furthermore, the composition of the plating bath used to form the coating is not particularly limited. For example, the composition of the plating bath, by weight percent, may include: Si: 6-15%, Fe: 0.1-2.0%, and the balance Al, and the temperature of the plating bath may be 600-680°C.

[0084] Subsequently, the coated steel sheet undergoes diffusion annealing heat treatment. This diffusion annealing heat treatment aims to induce interdiffusion between the base steel sheet and the coating. The diffusion annealing heat treatment is preferably performed at 650-850°C for 3.0-20.0 seconds. When the diffusion annealing heat treatment temperature is below 650°C, unalloyed coating will remain, hindering the achievement of a sufficient heating rate during hot forming and potentially causing the furnace rolls to stick due to melting of the residual coating. When the diffusion annealing heat treatment temperature exceeds 850°C, the coating will become overalloyed, resulting in a thicker diffusion layer during hot forming, which may reduce weldability. Therefore, the diffusion annealing heat treatment temperature is preferably in the range of 650-850°C. The lower limit of the diffusion annealing heat treatment temperature is more advantageously 670°C, further more advantageously 680°C, and most advantageously 690°C. The upper limit of the diffusion annealing heat treatment temperature is more advantageously 830°C, further more advantageously 820°C, and most advantageously 810°C. When the diffusion annealing heat treatment time is less than 3.0 seconds, complete alloying cannot be achieved if the coating thickness is thick, and excessively high heating capacity is required, which may lead to increased equipment costs and reduced operational safety. When the diffusion annealing heat treatment time exceeds 20.0 seconds, productivity will decrease significantly, making it difficult to implement in a continuous melt coating production line, and may also lead to increased production costs. Therefore, the diffusion annealing heat treatment time is preferably in the range of 3.0-20.0 seconds. The lower limit of the diffusion annealing heat treatment time is more advantageously 4 seconds, further more advantageously 4.8 seconds, and most advantageously 5.3 seconds. The upper limit of the diffusion annealing heat treatment time is more advantageously 12 seconds, further more advantageously 10 seconds, and most advantageously 8 seconds. In addition, the heating method of the steel plate during the diffusion annealing heat treatment is not particularly limited in this invention. However, preferably, induction heating can be used to ensure sufficient heating capacity within the limited heating range of the continuous melt coating production line. More specifically, since it is necessary to heat the steel plate to above the Curie temperature, induction heating devices using longitudinal or transverse flux methods can be used alone or in combination in part or all of the heating range.

[0085] The coated steel sheet after the diffusion annealing heat treatment is then cooled. The cooling process includes: a first cooling to a first cooling termination temperature of 600°C, followed by a second cooling to a second cooling termination temperature below 600°C. Preferably, the cooling rate during the first cooling is lower than the cooling rate during the second cooling. After the diffusion annealing heat treatment, the diffusion reaction continues until the temperature drops to 600°C. That is, by controlling the cooling rate during the first cooling to be lower than the cooling rate during the second cooling, as described above, the heating time required for complete alloying of the coating can be shortened, further improving productivity. If the cooling rate (C1) to the first cooling termination temperature of 600°C is higher than the cooling rate (C2) to the second cooling termination temperature below 600°C, the above-mentioned effects may not be fully achieved. The present invention does not specifically limit the cooling rate during the cooling process; for example, the first cooling rate (C1) can be 0.9-21°C / second, and the second cooling rate (C2) can be 12-60°C / second.

[0086] The following describes a method for manufacturing a hot-stamped part according to an embodiment of the present invention.

[0087] The inventors completed this invention based on the following understanding: In order to shorten the heating time when heating coated steel sheets for hot forming, it is crucial to suppress the phenomenon of a sharp decrease in heating rate that occurs in the temperature range of 550°C to 600°C.

[0088] First, a galvanized steel sheet for hot forming is prepared. The galvanized steel sheet for hot forming includes a base steel sheet and a coating formed on at least one side of the base steel sheet.

[0089] The galvanized steel sheet for hot forming can satisfy one or more of the following (a) to (c).

[0090] (a) The hot-formed coated steel sheet comprises a base steel sheet and a coating formed on at least one side of the base steel sheet, the coating comprising, by weight %: Si: 6-15%, Fe: 20-60%, balance Al and other unavoidable impurities. (b) The coating adhesion amount on one side is 30 g / m 2 the following, (c) The whiteness of the coating is below 60.

[0091] (a) The hot-formed coated steel sheet comprises a base steel sheet and a coating formed on at least one side of the base steel sheet, the coating comprising, by weight %: Si: 6-15%, Fe: 20-60%, balance Al and other unavoidable impurities. Si: 6-15% The Si content within the coating serves to homogenize the alloying with Fe. When the Si content is less than 6%, the aforementioned effect may not be fully achieved. However, Si also inhibits Fe diffusion; therefore, when the Si content exceeds 15%, excessive suppression of Fe diffusion may occur. Therefore, the Si content is preferably in the range of 6-15%. The lower limit of the Si content is more advantageously 8%. The upper limit of the Si content is more advantageously 12%, and even more advantageously 10%.

[0092] Fe: 20-60% The Fe mentioned refers to the element that diffuses from the base material into the coating. When the Fe content is less than 20%, the effect of suppressing the decrease in heating rate in the temperature range of 550°C to 600°C when heating the aluminum coating may be insufficient. When the Fe content exceeds 60%, disadvantages such as reduced corrosion resistance and weldability may occur. Therefore, the Fe content is preferably in the range of 20-60%. The lower limit of the Fe content is more advantageously 25%, and even more advantageously 30%. The upper limit of the Fe content is more advantageously 55%, and even more advantageously 50%.

[0093] (b) The coating adhesion of the aluminum plating layer on one side is 30 g / m². 2 the following The coating adhesion of the aluminum plating layer on one side can be 30 g / m². 2 The following refers to the coating adhesion amount, which is a factor that may affect the alloying rate of the aluminum coating. The coating adhesion amount exceeds 30 g / m². 2 When heating the aluminum coating, the effect of suppressing the decrease in heating rate within the temperature range of 550°C to 600°C may be insufficient. Therefore, the coating adhesion amount can be 30 g / m². 2 The following is a preferred coating adhesion amount: 25 g / m². 2 The following is even more favorable at 20g / m 2 The following is a separate point. Furthermore, the present invention does not specifically limit the lower limit of the coating adhesion amount; the lower limit of the coating adhesion amount can be 10 g / m². 2 .

[0094] (c) The whiteness of the aluminum plating layer is below 60. The whiteness of the aluminum plating layer can be 60 or less. Whiteness is a factor that may affect the absorption and reflection of radiant energy by the aluminum plating layer. When the whiteness exceeds 60, the effect of suppressing the decrease in heating rate within the temperature range of 550°C to 600°C when heating the aluminum plating layer may be insufficient. Therefore, the whiteness can be 60 or less. More preferably, the whiteness is 55 or less, and even more preferably 50 or less. Furthermore, the lower limit of the whiteness is not particularly limited in this invention; the lower limit of the whiteness can be 0. The whiteness refers to the L value measured by a colorimeter commonly used in this technical field.

[0095] Next, the prepared hot-formed coated steel sheet is heated. This heating step can be performed such that X, as expressed in [Formula 1] below, reaches 1.50°C / second or less. When X exceeds 1.50°C / second, the effect of suppressing the decrease in heating rate within the temperature range of 550°C to 600°C is insufficient, resulting in reduced productivity. Therefore, X can be 1.50°C / second or less. Alternatively, X can be 1.0°C / second or less. Alternatively, X can be 0.5°C / second or less. Furthermore, there is no particular limitation on the lower limit of X; for example, the lower limit of X can be 0°C / second.

[0096] [Equation 1] X=AB In [Equation 1], A is the heating rate at 600℃ calculated based on the linear trend line of the heating rate in the temperature range between 300℃ and 550℃ (i.e., the heating rate at 600℃ after extending the linear trend line), when the heating rate in the temperature range between 300℃ and 550℃ is linearly fitted (i.e., the linear trend line of the heating rate between 300℃ and 550℃ is a linear trend line). B is the heating rate at 600℃ calculated based on the linear trend line of the heating rate in the temperature range between 600℃ and 700℃ (i.e., the heating rate at 600℃ on the linear trend line).

[0097] The heating step can be performed such that the cumulative value (Y) of the coating alloying history index at 550°C reaches 0.14 or less, as expressed by [Equation 2] below. [Equation 2] below is a regression equation related to the decrease in heating rate when heating the aluminum coating. For example, the cumulative value (Y) of the coating alloying history index at 550°C can be 0.14 or less. When the cumulative value (Y) of the coating alloying history index at 550°C exceeds 0.14, the effect of suppressing the decrease in heating rate in the temperature range of 550°C to 600°C when heating the aluminum coating may be insufficient. Therefore, the cumulative value (Y) of the coating alloying history index at 550°C can be 0.14 or less. More advantageously, the cumulative value (Y) of the coating alloying history index at 550°C is 0.12 or less, and even more advantageously, 0.10 or less. Furthermore, there is no particular limitation on the lower limit of the cumulative value (Y) of the coating alloying history index; for example, the lower limit of the cumulative value (Y) of the coating alloying history index can be 0.01. The cumulative value of the coating alloying history index refers to the sum of the coating alloying history indices measured at fixed temperature measurement time intervals up to a specific measurement time. For example, when the temperature measurement time interval is 1 second, the cumulative value of the coating alloying history index at 10 seconds refers to the sum of the coating alloying history indices measured every second within the 0-10 second interval. Furthermore, if the temperature measurement time for a measured temperature of 550℃ is not a positive integer, i.e., the temperature measurement time for a measured temperature of 550℃ falls between a specific temperature measurement time j seconds and j+1 seconds, it is assumed that the cumulative value of the coating alloying history index between j seconds and j+1 seconds exhibits a linear change, and its value is calculated proportionally.

[0098] [Equation 2] In [Equation 2], k is a parameter related to the alloying rate, with a value of 130.31; Q is a parameter related to the temperature effect affecting the alloying behavior at different temperatures, with a value of 62190.89 J / mol; R is the gas constant in the ideal gas law, representing the ratio of gas volume to amount of substance at constant temperature and pressure, with a value of 8.314 J / (mol·K); n is a parameter related to the time effect affecting the alloying rate, with a value of 0.644; and j represents T. i i is at 550℃, and Δt is... i The time interval for temperature measurement during heating is 1 second, where i is the temperature measurement time during heating, and is a positive integer (second). i This represents the heating temperature (K) measured in the i-th second. K refers to absolute temperature.

[0099] Following the heating step, a further step of hot stamping the coated steel sheet for hot forming may be included. The hot stamping method is not particularly limited in this invention; all methods commonly used in this technical field may be used.

[0100] According to the method for manufacturing a hot-stamped part according to one embodiment of the present invention provided above, when heating the coated steel sheet for hot forming, the phenomenon of a sharp decrease in heating rate within the temperature range of 550°C to 600°C can be suppressed. Thus, by increasing the heating rate, productivity can be improved. Detailed Implementation

[0101] The present invention will now be specifically described through embodiments. However, it should be noted that the embodiments described below are merely examples and illustrations of the invention and are not intended to limit the scope of the invention. The scope of the invention is determined by the matters set forth in the claims and those reasonably deduced therefrom.

[0102] (Example 1) A base steel sheet is immersed in a 640°C plating bath containing, by weight, 9.5% Si, 1.5% Fe, with the balance being Al and other unavoidable impurities. After a coating is formed on the base steel sheet, the coating thickness is adjusted to the desired thickness using an air knife to produce a coated steel sheet. The coated steel sheet is then subjected to diffusion annealing heat treatment and cooling according to the conditions in Table 1 below. The cooling is divided into a first cooling to 600°C and a second cooling to 300°C. The alloy composition of the coating, whether the coating is fully alloyed, the average thickness of the coating, whiteness, and emissivity are measured for the coated steel sheet thus manufactured, and the results are recorded in Table 2 below. At this point, the fully alloyed coating includes: a first layer composed of the Fe2Al5 phase; and a second layer formed on the first layer and composed of the FeAl3 phase, wherein the first layer contains AlSiFe intermetallic compounds (τ phase) that exist continuously or discontinuously, and the incompletely alloyed coating has an unalloyed Si-Fe-Al coating remaining on the second layer.

[0103] The alloy composition of the coating was measured using a wet analytical method based on inductively coupled plasma (ICP).

[0104] Whether the coating is fully alloyed is confirmed by polishing the cross-section of the coating and then observing it with an optical microscope and a scanning electron microscope (SEM).

[0105] The average thickness of the coating was obtained by etching the cross-section of the coating with a nitric acid-alcohol solution and then measuring the average value at any six points using an optical microscope.

[0106] Whiteness was determined using a colorimeter by measuring six points along the width of the coating surface and calculating the average value.

[0107] The gloss level was measured using a gloss meter at any six points along the width of the coating surface, and the average value was calculated. The measurement angle was 60°.

[0108] [Table 1] [Table 2] As can be confirmed from Tables 1 and 2 above, in Invention Examples 1 to 9, which meet the conditions of this invention, the coatings are all fully alloyed, and the whiteness and gloss are also relatively low. These values ​​are extremely low compared to ordinary Al-Si coatings.

[0109] On the other hand, in Comparative Examples 1 to 6, which do not meet the conditions of the present invention, not only is the coating not fully alloyed, but the whiteness and gloss are also high values.

[0110] Figure 2 This is a photograph of the cross-section of Invention Example 9 observed using a scanning electron microscope. (Through...) Figure 2 It can be confirmed that the coating of Example 9 of the invention achieves complete alloying. The coating includes a first layer composed of Fe2Al5 phase and a second layer formed on the first layer composed of FeAl3 phase. At the same time, the first layer contains continuously existing AlSiFe intermetallic compound (τ phase).

[0111] Figure 3 These are photographs of the cross-section of Comparative Example 1 observed using a scanning electron microscope. Figure 3 It can be confirmed that although the coating of Comparative Example 1 includes a first layer composed of Fe2Al5 phase and a second layer composed of FeAl3 phase formed on the first layer, there is a discontinuous AlSiFe intermetallic compound (τ phase) formed on the upper part of the first layer, and due to the incomplete alloying of the coating, there is an unalloyed Si-Fe-Al coating remaining on the second layer.

[0112] (Example 2) A base steel plate is immersed in a 680°C plating bath containing, by weight, 9.5% Si, 1.5% Fe, with the balance being Al and other unavoidable impurities. After a coating is formed on the base steel plate, the coating thickness is adjusted to the desired thickness using an air knife to produce a coated steel plate. The coated steel plate is then subjected to diffusion annealing heat treatment and cooling according to the conditions in Table 3 below. The cooling is divided into a first cooling to 600°C and a second cooling to 300°C. For the coated steel plates thus manufactured, the alloy composition of the coating, whether the coating is fully alloyed, the average thickness of the coating, whiteness, and emissivity are measured, and the results are recorded in Tables 4 to 6 below. At this point, the fully alloyed coating comprises: a first layer containing the Fe2Al5 phase; a second layer formed on the first layer and containing the AlSiFe intermetallic compound (τ phase); a third layer formed on the second layer and containing the Fe2Al5 phase; and a fourth layer formed on the third layer and containing the FeAl3 phase, wherein the incompletely alloyed coating has an unalloyed Al-Si system coating remaining on the fourth layer.

[0113] The alloy composition of the coating was measured using a wet analytical method based on inductively coupled plasma-atomic emission spectrometry (ICP-AES).

[0114] Whether the coating is fully alloyed is confirmed by polishing the cross-section of the coating and then observing it with an optical microscope and a scanning electron microscope.

[0115] The average thickness of the coating was obtained by etching the cross-section of the coating with a nitric acid alcohol solution and then measuring the average value at any six points using an optical microscope.

[0116] Whiteness was determined using a colorimeter by measuring six points along the width of the coating surface and calculating the average value.

[0117] The gloss level was measured using a gloss meter at any six points along the width of the coating surface, and the average value was calculated. The measurement angle was 60°.

[0118] [Table 3] [Table 4] [Table 5] [Table 6] As can be confirmed from Tables 3 to 6 above, in Invention Examples 10 to 18, which meet the manufacturing conditions of the present invention, the coatings are all fully alloyed, and a first, second, third, and fourth layer with suitable Al, Fe, and Si contents are formed. Therefore, the whiteness and gloss are both low. These values ​​are extremely superior compared to ordinary Al-Si coatings. On the other hand, in Comparative Examples 7 to 12, which do not meet the manufacturing conditions of the present invention, not only are the coatings not fully alloyed, but a first, second, third, and fourth layer with suitable Al, Fe, and Si contents are also not formed. Therefore, the whiteness and gloss are both high.

[0119] Figure 5 This is a photograph of the cross-section of Invention Example 18 observed using a scanning electron microscope. (Through...) Figure 5 It can be confirmed that the coating of Example 18 achieves complete alloying. The coating consists of a first layer containing the Fe2Al5 phase, a second layer formed on the first layer and containing a continuously existing AlSiFe intermetallic compound (τ phase), a third layer formed on the second layer and containing the Fe2Al5 phase, and a fourth layer formed on the third layer and containing the FeAl3 phase.

[0120] Figure 6 These are photographs of the cross-section of Comparative Example 7, observed using a scanning electron microscope. Figure 6 It can be confirmed that although the coating of Comparative Example 7 includes a first layer containing the Fe2Al5 phase, a second layer formed on the first layer and containing discontinuous AlSiFe intermetallic compounds (τ phase), and a fourth layer formed on the third layer and containing the FeAl3 phase, the fourth layer has unalloyed Al-Si coating remaining due to incomplete alloying.

[0121] (Example 3) First, a hot-formed coated steel sheet is prepared, wherein both sides of the steel sheet are formed with an aluminum plating layer having the alloy composition, whiteness, and coating adhesion amount based on one side as described in Table 7 below, and the steel sheet thickness is as described in Table 7 below. Then, the hot-formed coated steel sheet is heated from room temperature to 935°C. For the hot-formed coated steel sheet thus heated, the values ​​of [Equation 1] and [Equation 2] at 550°C are measured, and the results are shown in Table 7 below.

[0122] The value of [Equation 1] is obtained by obtaining the curve of the heating rate of each hot-formed coated steel sheet as a function of temperature, and then performing linear fitting on the heating rate in the temperature range between 300℃ and 550℃ and the heating rate in the temperature range between 600℃ and 700℃. The fitting is performed using the linear fitting method of Excel.

[0123] In addition, for each hot-formed coated steel sheet, the temperature measurement time interval is set to 1 second to measure the temperature, and then the cumulative value of [Equation 2] at 550°C is calculated.

[0124] [Table 7] Figure 7 These are graphs showing the relationship between heating rate and temperature in Examples 19 and 20 of the Invention. Figure 8 The graphs show the relationship between heating rate and temperature for Comparative Examples 13 and 14. Figure 9 These are graphs relating heating time and temperature for Invention Examples 19, 20 and Comparative Examples 13 and 14.

[0125] Through the above Table 7 and Figures 7 to 9 It can be confirmed that the heating rate of Comparative Examples 13 and 14 decreased sharply in the temperature range of 550°C to 600°C, while the heating rate of Invention Examples 19 and 20 did not decrease sharply in the same temperature range. Therefore, Invention Examples 19 and 20 can heat to 935°C at a significantly faster rate than Comparative Examples 13 and 14. Furthermore, Figure 7 In Invention Example 19, A is designated as A1 and B as B1; in Invention Example 20, A is designated as A2 and B as B2. Figure 8 In Comparative Example 13, A is labeled A3 and B is labeled B3; in Comparative Example 14, A is labeled A4 and B is labeled B4.

[0126] Figure 10 This is a graph showing the cumulative value of the coating alloying history index as a function of temperature for Invention Examples 19, 20, 13, and 14. Figure 10 It can be confirmed that the cumulative value of the coating alloying history index of Comparative Examples 13 and 14 at 550°C exceeds 0.30, while the cumulative value of the coating alloying history index of Invention Examples 19 and 20 at 550°C is less than 0.30.

[0127] Figure 11 This is a graph showing the relationship between heating rate and temperature for Comparative Example 15 and Invention Example 21. (By...) Figure 11 It can be confirmed that Comparative Example 15 exhibited a sharp decrease in heating rate within the temperature range of 550°C to 600°C, while Invention Example 21 did not show a sharp decrease in heating rate within the same temperature range. Furthermore, Figure 11 In Invention Example 21, A is labeled A5 and B is labeled B5, while in Comparative Example 15, A is labeled A6 and B is labeled B6.

[0128] [Explanation of reference numerals in the attached figures] 10: First floor 20: Second layer 30: AlSiFe intermetallic compound (τ phase) 100: Foundation steel plate 200: Coating 10': First layer 20': Second layer 30': Third layer 40': Fourth layer 100': Foundation steel plate 200': Coating

Claims

1. A galvanized steel sheet for hot forming, comprising: Foundation steel plate; as well as A coating formed on at least one side of the base steel plate; The coating includes: The first layer, by weight percent, comprises: Al: 40-48%, Fe: 49-55%, Si: less than 5%; The second layer, formed on the first layer, comprises, by weight %: Al: 40-50%, Fe: 38-48%, Si: 2-12%; A third layer, formed on the second layer, comprises, by weight percent: Al: 45-60%, Fe: 33-48%, Si: less than 7%; and The fourth layer, formed on the third layer, comprises, by weight percent: Al: 50-65%, Fe: 25-35%, and Si: less than 15%.

2. The galvanized steel sheet for hot forming according to claim 1, wherein, The coating, by weight percent, comprises: Si: 6-15%, Fe: 20-60%, with the balance being Al and other unavoidable impurities.

3. The galvanized steel sheet for hot forming according to claim 1, wherein, The first layer contains the Fe2Al5 phase, the second layer contains the AlSiFe intermetallic compound, i.e., the τ phase, the third layer contains the Fe2Al5 phase, and the fourth layer contains the FeAl3 phase.

4. The galvanized steel sheet for hot forming according to claim 1, wherein, The second layer exists in the region within 40% of the total average thickness T of the coating from the bottom of the coating along the thickness direction.

5. The galvanized steel sheet for hot forming according to claim 1, wherein, The ratio of the total average thickness T1 of the first, second, and third layers to the total average thickness T of the coating, T1 / T, is between 0.20 and 0.

50.

6. The galvanized steel sheet for hot forming according to claim 1, wherein, The ratio of the average thickness T2 of the fourth layer to the total average thickness T of the coating, T2 / T, is between 0.50 and 0.

80.

7. The galvanized steel sheet for hot forming according to claim 1, wherein, The coating has an average thickness of 7-30 μm.

8. The galvanized steel sheet for hot forming according to claim 1, wherein, The whiteness of the coating is below 60, and the gloss is below 6.

0.

9. A method for manufacturing a galvanized steel sheet for hot forming, comprising the following steps: Prepare the foundation steel plate; The base steel plate is immersed in a plating bath to form a coating on at least one side of the base steel plate to obtain a coated steel plate. The coated steel sheet is subjected to diffusion annealing heat treatment; as well as The coated steel sheet that has undergone the diffusion annealing heat treatment is then cooled. The diffusion annealing heat treatment is performed at 650-850℃ for 3.0-20.0 seconds. The cooling process includes: performing a first cooling to a first cooling termination temperature of 600°C, and then performing a second cooling to a second cooling termination temperature below 600°C. The cooling rate during the first cooling process is lower than the cooling rate during the second cooling process.

10. The method for manufacturing galvanized steel sheet according to claim 9, wherein, The plating bath, by weight percent, contains: Si: 6-15%, Fe: 0.1-2.0%, balance Al and other unavoidable impurities.

11. The method for manufacturing galvanized steel sheet according to claim 9, wherein, The diffusion annealing heat treatment is achieved through induction heating.

12. The method for manufacturing galvanized steel sheet according to claim 9, wherein, The cooling rate during the first cooling process is 0.9-21℃ / second, and the cooling rate during the second cooling process is 12-60℃ / second.

13. A method for manufacturing a hot-stamped part, wherein in the step of heating a galvanized steel sheet for hot forming, the method is carried out such that X, as expressed by the following [Formula 1], reaches 1.50°C / second or less. [Equation 1] X=AB in, In the above [Equation 1], A is the heating rate at 600℃ calculated based on the straight line when the heating rate in the temperature range between 300℃ and 550℃ is linearly fitted, and B is the heating rate at 600℃ calculated based on the straight line when the heating rate in the temperature range between 600℃ and 700℃ is linearly fitted.

14. The method for manufacturing a hot-stamped part according to claim 13, wherein, The hot-formed coated steel sheet satisfies one or more of the following (a) to (c): (a) The hot-formed coated steel sheet comprises a base steel sheet and a coating formed on at least one side of the base steel sheet, the coating comprising, by weight %: Si: 6-15%, Fe: 20-60%, balance Al and other unavoidable impurities. (b) The coating adhesion amount on one side is 30 g / m 2 the following, (c) The whiteness of the coating is below 60.

15. The method for manufacturing a hot-stamped part according to claim 13, wherein, X is above 0°C / second and below 1.50°C / second.

16. The method for manufacturing a hot-stamped part according to any one of claims 13 to 15, wherein, The heating step is performed such that the cumulative value (Y) of the coating alloying history index at 550°C, as expressed by the following [Equation 2], reaches 0.14 or less. [Equation 2] , In [Equation 2], k is a parameter related to the alloying rate, with a value of 130.31; Q is a parameter related to the temperature effect affecting the alloying behavior at different temperatures, with a value of 62190.89 J / mol; R is the gas constant in the ideal gas law, representing the ratio of gas volume to amount of substance at constant temperature and pressure, with a value of 8.314 J / (mol·K); n is a parameter related to the time effect affecting the alloying rate, with a value of 0.644; and j represents T. i i is at 550℃, and Δt is... i The time interval for temperature measurement during heating is 1 second, where i is the temperature measurement time during heating, and is a positive integer value (seconds). i This indicates the heating temperature (K) measured in the i-th second.

17. The method for manufacturing a hot-stamped part according to claim 13, wherein, The value of Y is below 0.

12.

18. The method for manufacturing a hot-stamped part according to claim 13, wherein, The value of Y is below 0.

10.

19. The method for manufacturing a hot-stamped part according to claim 13, wherein, The value of Y is greater than or equal to 0.

01.

20. The method for manufacturing a hot-stamped part according to claim 13, wherein, Following the heating step, the process further includes a hot stamping step of hot-forming the galvanized steel sheet for hot forming.

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