Coated steel sheet for hot forming and method of manufacturing same and manufacturing method of hot press formed member

Abstract

One aspect of the present invention provides a coated steel sheet for hot forming, a method of manufacturing same, and a manufacturing method of a hot press formed member. One preferred aspect of the present invention provides a coated steel sheet for hot forming, a method of manufacturing same, and a manufacturing method of a hot press formed member, wherein productivity is excellent and it is possible to reduce manufacturing costs.

Description

Galvanized steel sheet for hot forming and its manufacturing method and method for manufacturing hot press-formed parts

[0001] The present invention relates to a plated steel sheet for hot forming and a method for manufacturing the same, and a method for manufacturing a hot press-formed member.

[0002] In line with efforts to improve fuel efficiency through lightweight automobiles and the trend toward strengthening crash regulations and passenger safety, the use of ultra-high-strength steel for hot forming is becoming more widespread.

[0003] When non-coated steels are used for hot press forming, scaling occurs and corrosion resistance is difficult to ensure, so there is a growing trend toward using coated steels instead. Furthermore, aluminum-coated steels are typically the most commonly used coated steels.

[0004] For example, to prevent the formation of surface oxide scale and decarburization that occurs during the process of heating steel plates to high temperatures exceeding 850°C, the use of an aluminum-silicon (Al-Si) coating was first proposed and commercially successful by ArcelorMittal.

[0005] However, the steel sheet having a plating layer mainly composed of Al as described above has a problem in that the Al plating layer liquefies and the Al is fused to the rolls in the furnace when heated above the melting point of the Al plating layer in the furnace during the heat treatment process for hot forming. In particular, this problem of plating layer melting becomes more serious when the heat treatment temperature increase rate is increased to increase productivity or when heating using an induced current that has excellent heating efficiency and enables rapid heating, so there is a limit to shortening the heat treatment time through rapid heating. In addition, since the steel sheet must be heated to 900℃ or higher to austenitize without melting the Al plating layer, the Al plating layer needs to be alloyed into an AlFe alloy during heating. As a result, 30-50% of the entire furnace must be used for AlFe alloying, which increases the furnace length and can cause problems such as high energy consumption and CO2 emissions during the manufacturing process.

[0006] To address these issues, prior technologies have proposed a method of performing a separate alloying annealing heat treatment prior to the heat treatment for hot forming. This process creates an aluminum-iron (Al-Fe) alloy phase through mutual diffusion between the Al plating layer and the steel sheet (Fe). This increases the melting point of the plating layer, preventing Al fusion and enabling rapid heating, thereby increasing productivity. However, this pre-alloying heat treatment leads to increased manufacturing costs, which in turn reduces market competitiveness. In particular, the pre-alloying heat treatment is also performed at a temperature below the melting point of the plating layer to prevent melting of the plating layer. This means that it takes several to several tens of hours for the plating layer to be completely alloyed. Furthermore, the continuous oxidation of the plating layer during the heat treatment process can deteriorate the surface quality and plating properties. Furthermore, if partial alloying is performed to reduce the alloying heat treatment time, the unalloyed plating layer can melt during rapid heating, potentially causing roll seizure.

[0007] Other prior art techniques have proposed a method of simultaneously coating an aluminum alloy and performing alloying annealing in a continuous hot-dip galvanizing line, thereby improving productivity during hot forming while reducing manufacturing costs. However, in order to increase the diffusion rate between the aluminum plating layer and the steel sheet, the content of silicon (Si), which inhibits the aluminum-iron diffusion reaction, is limited to 0.4-4%. This lowering of the Si content increases the melting point of the aluminum-silicon molten metal, which generates a large amount of fume during continuous hot-dip galvanizing, thereby reducing productivity and increasing line maintenance / repair costs. Furthermore, the reduced fluidity of the molten metal increases the problem of surface defects such as surface flow patterns, and the increase in brittle alloy layers after hot forming increases problems such as plating layer peeling and mold seizure during forming.

[0008] Meanwhile, the aluminum-plated steel material has a problem in that the heating time becomes long because the heating speed is rapidly slowed down in a specific temperature range as alloying occurs during the heating process, and a solution to this problem is also required.

[0009] One aspect of the present invention is to provide a hot-forming galvanized steel sheet and a method for manufacturing the same, and a method for manufacturing a hot press-formed member.

[0010] A desirable aspect of the present invention is to provide a hot-forming galvanized steel sheet and a manufacturing method thereof, and a manufacturing method of a hot press-formed member, which not only have excellent productivity but also enable reduction in manufacturing costs.

[0011] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall content of this specification.

[0012] A first aspect of the present invention provides a plated steel sheet for hot forming, comprising: a base steel sheet; and a plating layer formed on at least one surface of the base steel sheet; wherein the plating layer comprises: a first layer formed of an Fe2Al5 phase; and a second layer formed of an FeAl3 phase formed on the first layer; wherein an AlSiFe intermetallic compound (τ phase) is included in the first layer, which is present continuously or discontinuously.

[0013] The above plating layer may contain, in wt%, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities.

[0014] The above plating layer may have an average thickness of 7 to 30 μm.

[0015] The above plating layer may have a whiteness of 60 or less and a gloss of 6.0 or less.

[0016] A second aspect of the present invention provides a hot forming plated steel sheet, comprising: a base steel sheet; and a plating layer formed on at least one surface of the base steel sheet; wherein the plating layer comprises: a first layer comprising, in wt%, Al: 40 to 48%, Fe: 49 to 55%, and Si: 5% or less; a second layer formed on the first layer and comprising, in wt%, Al: 40 to 50%, Fe: 38 to 48%, and Si: 2 to 12%; a third layer formed on the second layer and comprising, in wt%, Al: 45 to 60%, Fe: 33 to 48%, and Si: 7% or less; and a fourth layer formed on the third layer and comprising, in wt%, Al: 50 to 65%, Fe: 25 to 35%, and Si: 15% or less.

[0017] The above plating layer may contain, in wt%, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities.

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

[0019] The second layer may be present in an area within 40% of the total average thickness (T) of the plating layer in the thickness direction from the bottom of the plating layer.

[0020] The ratio (T1 / T) of the total average thickness (T1) of the first layer, the second layer, and the third layer to the total average thickness (T) of the plating layer may be 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 plating layer may be 0.50 to 0.80.

[0022] The above plating layer may have an average thickness of 7 to 30 μm.

[0023] The above plating layer may have a whiteness of 60 or less and a gloss of 6.0 or less.

[0024] A third aspect of the present invention provides a method for manufacturing a plated steel sheet for hot forming, comprising the steps of: preparing a base steel sheet; immersing the base steel sheet in a plating bath to form a plating layer on at least one surface of the base steel sheet to obtain a plated steel sheet; performing a diffusion annealing heat treatment on the plated steel sheet; and cooling the plated steel sheet subjected to the diffusion annealing heat treatment, wherein the diffusion annealing heat treatment is performed at 650 to 850°C for 3.0 to 20.0 seconds, and during the cooling, the method comprises first cooling to a first cooling end temperature of 600°C and then second cooling to a second cooling end temperature less than 600°C, wherein the cooling rate during the first cooling is slower than the cooling rate during the second cooling.

[0025] The above plating bath may contain, in wt%, Si: 6 to 15%, Fe: 0.1 to 2.0%, the remainder Al, and other unavoidable impurities.

[0026] The above diffusion annealing heat treatment can be performed by induction heating.

[0027] The cooling rate during the first cooling may be 0.9 to 21°C / s, and the cooling rate during the second cooling may be 12 to 60°C / s.

[0028] The fourth aspect of the present invention provides a method for manufacturing a hot press-formed part, wherein, in the step of heating a plated steel sheet for hot forming, X expressed by the following [Formula 1] is performed at 1.50°C / s or less.

[0029] [Formula 1] X = A - B

[0030] (However, in the above [Formula 1], A is the heating rate at 600°C calculated from the straight line when the heating rate in the temperature range between 300°C and 550°C is fitted with a straight line, and B is the heating rate at 600°C calculated from the straight line when the heating rate in the temperature range between 600°C and 700°C is fitted with a straight line.)

[0031] The above-mentioned hot-forming galvanized steel sheet can satisfy one or more of the following (a) to (c).

[0032] (a) The hot-forming galvanized steel sheet comprises a base steel sheet; and a plating layer formed on at least one surface of the base steel sheet; wherein the plating layer contains, in wt%, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities.

[0033] (b) The plating layer has a plating weight of 30 g / m on one side. 2 Below

[0034] (c) The plating layer has a whiteness of 60 or less.

[0035] The above X may be 0℃ / s or more and 1.50℃ / s or less.

[0036] The above heating step can be performed so that the accumulated value (Y) of the plating layer alloying history index at 550°C, expressed by the following [Formula 2], becomes 0.14 or less.

[0037] [Formula 2] Cumulative value of plating layer alloying history index (Y) =

[0038] (However, in the above [Formula 2], the k is a factor related to the alloying speed and has a value of 130.31, the Q is a factor related to the temperature effect affecting the alloying behavior according to temperature and has a value of 62190.89 J / mol, the R is a gas constant indicating the ratio of the volume and mole number of gas at a constant temperature and pressure in the ideal gas law and has a value of 8.314 J / (mol·K), the n is a factor related to the time effect affecting the alloying speed and has a value of 0.644, and the j is T i i means when the temperature is 550℃, and the above Δt i has a value of 1 sec as the temperature measurement time interval during heating, and the i has a positive integer (sec) value as the temperature measurement time during heating, and the T i refers to the heating temperature (K) measured at i seconds.)

[0039] The above Y may be less than or equal to 0.12.

[0040] The above Y may be less than or equal to 0.10.

[0041] The above Y may be greater than or equal to 0.01.

[0042] After the heating step, a step of hot press forming the hot-forming plated steel sheet may be additionally included.

[0043] According to one aspect of the present invention, a hot-forming galvanized steel sheet and a manufacturing method thereof and a hot press-forming member manufacturing method can be provided.

[0044] According to a preferred aspect of the present invention, it is possible to provide a hot-forming galvanized steel sheet and a manufacturing method thereof, and a manufacturing method of a hot press-formed member, which not only have excellent productivity but also enable reduction in manufacturing costs.

[0045] Figure 1 is a schematic diagram showing a hot-forming galvanized steel sheet according to the first aspect of the present invention.

[0046] Figure 2 is a photograph of a cross-section of invention example 9 observed using a scanning electron microscope.

[0047] Figure 3 is a photograph of a cross-section of Comparative Example 1 observed using a scanning electron microscope.

[0048] Figure 4 is a schematic diagram showing a hot-forming galvanized steel sheet according to the second aspect of the present invention.

[0049] Figure 5 is a photograph of a cross-section of Invention Example 18 observed using a scanning electron microscope.

[0050] Figure 6 is a photograph of the cross-section of Comparative Example 7 observed using a scanning electron microscope.

[0051] Figure 7 is a graph related to the heating rate according to temperature for invention examples 19 and 20.

[0052] Figure 8 is a graph related to the heating rate according to temperature for comparative examples 13 and 14.

[0053] Figure 9 is a graph related to heating time and temperature for Invention Examples 19 and 20 and Comparative Examples 13 and 14.

[0054] Figure 10 is a graph related to the cumulative value of the plating layer alloying history index according to temperature for Invention Examples 19 and 20 and Comparative Examples 13 and 14.

[0055] Figure 11 is a graph related to the heating rate according to temperature for Comparative Example 15 and Invention Example 21.

[0056] In describing the embodiments of the present invention, if a detailed description of a known technology related to the present invention is judged to unnecessarily obscure the gist of the present invention, such detailed description will be omitted. In addition, the terms described below are terms defined in consideration of their functions in the present invention, and these may vary depending on the intention or custom of the user or operator. Therefore, the definitions should be made based on the contents throughout this specification. The terminology used in the detailed description is only for the purpose of describing the embodiments of the present invention and should in no way be limiting. Unless clearly defined otherwise, expressions in the singular form include plural meanings.

[0057] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or overly formal unless otherwise defined.

[0058] In this description, expressions such as "including" or "having" are intended to indicate certain features, numbers, steps, operations, elements, portions or combinations thereof, and should not be construed to exclude the presence or possibility of one or more other features, numbers, steps, operations, elements, portions or combinations thereof other than those described.

[0059] Unless otherwise specified in the specification of the present invention, the % unit means weight %.

[0060] Below, the present invention will be described in detail through each embodiment or example of the present invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may be combined with other embodiments or examples. Therefore, the citation of a claim in the patent claims is only an example of an embodiment, and the technical concept of the present invention should not be interpreted solely as a combination with the cited claim, and combinations with various claims also fall within the scope of the technical concept of the present invention.

[0061] Fig. 1 is a schematic diagram showing a hot-forming galvanized steel sheet according to a first aspect of the present invention. Hereinafter, the hot-forming galvanized steel sheet according to the first aspect of the present invention will be described with reference to Fig. 1.

[0062] As illustrated in Fig. 1, a hot-forming galvanized steel sheet according to one embodiment of the present invention includes a base steel sheet (100); and a galvanized layer (200) formed on at least one surface of the base steel sheet (100). The present invention does not specifically limit the type of the base steel sheet, and any steel sheet commonly used in the relevant technical field may be used.

[0063] The above plating layer (200) may contain, in wt%, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities.

[0064] Si: 6~15%

[0065] Si lowers the melting point of the molten Al and forms a relatively stable alloy phase of the AlSiFe series, thereby reducing the diffusion of Fe. When the Si content is less than 6%, the melting point of the molten metal increases, which reduces productivity and increases line maintenance / repair costs due to the generation of a large amount of fume during continuous hot-dip galvanizing. In addition, in addition to the problem of an increase in surface defects such as surface flow patterns due to reduced molten metal fluidity, the proportion of a brittle alloy layer in the plating layer increases as Fe diffusion increases during hot-forming, which causes problems of plating layer peeling and mold seizure during forming. When the Si content exceeds 15%, the melting point of the molten Al increases again, which causes a problem in productivity, and there is also the disadvantage of the plating layer itself becoming brittle due to Si crystallization within the plating layer. Therefore, the Si content is preferably in the range of 6 to 15%. The lower limit of the Si content is more advantageously 6.5%, 7% is even more advantageously, and 7.5% is most advantageously. The upper limit of the above Si content is more advantageous when it is 13%, more advantageous when it is 11%, and most advantageous when it is 9%.

[0066] Fe: 20~60%

[0067] The above Fe is eluted from the base steel sheet introduced into the plating bath and included in the plating layer, and through diffusion annealing heat treatment, Fe diffuses from the base steel sheet into the plating layer, forming an Al-Fe alloy to increase the melting point of the plating layer. When the content of Fe is less than 20%, the formation of the Al-Fe alloy in the plating layer is insufficient, so that an Al(Si) plating layer remains, making it impossible to sufficiently increase the heating rate and prevent the plating layer from melting during the hot forming heat treatment process. When the content of Fe exceeds 60%, there is a disadvantage in that the welding current range is reduced due to the formation of an excessive diffusion layer during hot forming, resulting in poor weldability. Therefore, the content of Fe is preferably in the range of 20 to 60%. The lower limit of the Fe content is more advantageously 23%, more advantageously 25%, and most advantageously 28%. The upper limit of the above Fe content is more advantageous when it is 53%, more advantageous when it is 48%, and most advantageous when it is 45%.

[0068] The remaining component is aluminum. However, since unintended impurities from raw materials or the surrounding environment can inevitably be mixed in during the normal manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0069] The plating layer is preferably composed of a first layer (10) formed of an Fe2Al5 phase; and a second layer (20) formed of an FeAl3 phase formed on the first layer (10). Generally, when diffusion annealing heat treatment is performed, Fe of the base steel sheet diffuses into the Al plating layer, and changes into FeAl3, Fe2Al5, FeAl2, and FeAl depending on the concentration of Fe. According to the present invention, after forming the plating layer in a continuous hot dip galvanizing process, the entire plating layer becomes an Al-Fe alloy phase only through a short diffusion annealing heat treatment of 3 to 20 seconds, so that the plating layer becomes composed of Fe2Al5 phase and FeAl3 phase with a low Fe content. The first layer (10) with a relatively high Fe concentration close to the base steel sheet is formed as a layer formed of an Fe2Al5 phase, and the second layer (20) formed of an FeAl3 phase with a relatively low Fe concentration is formed on the first layer.

[0070] It is preferable that the first layer (10) contains an AlSiFe intermetallic compound (τ phase) (30) that exists continuously or discontinuously. The AlSiFe intermetallic compound is difficult to clearly define as the phase slightly changes depending on the concentration of Al, Fe, and Si, but is generally referred to as the τ phase for the AlSiFe ternary alloy phase. The AlSiFe intermetallic compound (τ phase) may have a band shape and is relatively ductile, which reduces the occurrence and propagation of cracks in the plating layer during the production process and prevents excessive diffusion of Fe during hot forming. Meanwhile, the AlSiFe intermetallic compound (τ phase) is formed on the surface of the base steel sheet the moment the base steel sheet is introduced into the plating bath, and thereafter, slowly moves from the surface into the plating layer through diffusion annealing heat treatment. However, the AlSiFe intermetallic compound (τ phase) has a very low diffusion rate compared to the diffusion rate of Fe atoms. Therefore, with only a short-time diffusion annealing heat treatment like the present invention, the AlSiFe intermetallic compound (τ phase) does not move to the surface of the plating layer, but exists inside the first layer (10).

[0071] The plating layer may have an average thickness of 7 to 30 μm. If the average thickness of the plating layer is less than 7 μm, it may be difficult to secure sufficient corrosion resistance after hot forming. If the average thickness of the plating layer exceeds 30 μm, complete alloying through diffusion annealing heat treatment may be difficult, making it difficult to improve productivity and prevent melting of the plating layer during hot forming, which are the purposes of the present invention. Therefore, the average thickness of the plating layer may have an average thickness of 7 to 30 μm. The lower limit of the average thickness of the plating layer is more advantageously 8 μm, more advantageously 9 μm, and most advantageously 10 μm. The upper limit of the average thickness of the plating layer is more advantageously 27 μm, more advantageously 24 μm, and most advantageously 21 μm.

[0072] As described above, the galvanized steel sheet of the present invention may have a whiteness (L value) of 60 or less and a glossiness of 6.0 or less. This allows for increased heating efficiency during the heat treatment process for hot forming, thereby achieving a sufficient increase in heating speed and shortening of heating time. Meanwhile, the glossiness may be measured at a 60° angle.

[0073] The plated steel sheet of the present invention has an advantage in that, since the entire plated layer is completely alloyed, its melting point is higher than the heat treatment temperature for hot forming, thereby preventing melting of the plated layer and Al deposition in a heating furnace for hot forming. In addition, since the heat treatment process for hot forming has high heating efficiency, the latent heat of fusion required for the melting process of the plated layer and the energy consumption for AlFe alloying are not required, thereby increasing the austenitizing efficiency of the steel sheet, thereby enabling securing material even with a shortened heating time. In addition, since it can be implemented in a continuous hot-dip galvanizing production line, it has the advantage of being easy to manufacture and highly economical. In other words, it has the advantage of not only excellent productivity but also reducing manufacturing costs.

[0074] Fig. 4 is a schematic diagram showing a hot-forming galvanized steel sheet according to a second aspect of the present invention. Hereinafter, the hot-forming galvanized steel sheet according to the second aspect of the present invention will be described with reference to Fig. 4.

[0075] As illustrated in Fig. 4, a hot-forming galvanized steel sheet according to a second aspect of the present invention comprises a base steel sheet (100'); and a galvanized layer (200') formed on at least one surface of the base steel sheet (100'). The present invention does not specifically limit the type of the base steel sheet, and any steel sheet commonly used in the relevant technical field can be used.

[0076] The above plating layer (200') may contain, in wt%, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities.

[0077] Si: 6~15%

[0078] Si lowers the melting point of the molten Al and forms a relatively stable alloy phase of the AlSiFe series, thereby reducing the diffusion of Fe. When the Si content is less than 6%, the melting point of the molten metal increases, which reduces productivity and increases line maintenance / repair costs due to the generation of a large amount of fume during continuous hot-dip galvanizing. In addition, in addition to the problem of an increase in surface defects such as surface flow patterns due to reduced molten metal fluidity, the proportion of a brittle alloy layer in the plating layer increases as Fe diffusion increases during hot-forming, which causes problems of plating layer peeling and mold seizure during forming. When the Si content exceeds 15%, the melting point of the molten Al increases again, which causes a problem in productivity, and there is also the disadvantage of the plating layer itself becoming brittle due to Si crystallization within the plating layer. Therefore, the Si content is preferably in the range of 6 to 15%. The lower limit of the Si content is more advantageously 6.5%, 7% is even more advantageously, and 7.5% is most advantageously. The upper limit of the above Si content is more advantageous when it is 13%, more advantageous when it is 11%, and most advantageous when it is 9%.

[0079] Fe: 20~60%

[0080] The above Fe is eluted from the base steel sheet introduced into the plating bath and included in the plating layer, and through diffusion annealing heat treatment, Fe diffuses from the base steel sheet into the plating layer, forming an Al-Fe alloy to increase the melting point of the plating layer. When the content of Fe is less than 20%, the formation of the Al-Fe alloy in the plating layer is insufficient, so that an Al(Si) plating layer remains, making it impossible to sufficiently increase the heating rate and prevent the plating layer from melting during the hot forming heat treatment process. When the content of Fe exceeds 60%, there is a disadvantage in that the welding current range is reduced due to the formation of an excessive diffusion layer during hot forming, resulting in poor weldability. Therefore, the content of Fe is preferably in the range of 20 to 60%. The lower limit of the Fe content is more advantageously 23%, more advantageously 25%, and most advantageously 28%. The upper limit of the above Fe content is more advantageous when it is 53%, more advantageous when it is 48%, and most advantageous when it is 45%.

[0081] The remaining component is aluminum. However, since unintended impurities from raw materials or the surrounding environment can inevitably be mixed in during the normal manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0082] The plating layer may include a first layer (10') comprising, in wt%, Al: 40 to 48%, Fe: 49 to 55%, and Si: 5% or less; a second layer (20') formed on the first layer (10') and comprising, in wt%, Al: 40 to 50%, Fe: 38 to 48%, and Si: 2 to 12%; a third layer (30') formed on the second layer (20') and comprising, in wt%, Al: 45 to 60%, Fe: 33 to 48%, and Si: 7% or less; and a fourth layer (40') formed on the third layer (30') and comprising, in wt%, Al: 50 to 65%, Fe: 25 to 35%, and Si: 15% or less.

[0083] In general, when diffusion annealing heat treatment is performed, Fe of the base steel sheet diffuses into the Al plating layer, and changes into FeAl3, Fe2Al5, FeAl2, and FeAl depending on the concentration of Fe. According to the present invention, after forming the plating layer in the continuous hot dip galvanizing process, the entire plating layer becomes an Al-Fe alloy phase only through diffusion annealing heat treatment for a short time of 3 to 20 seconds, so that the plating layer is composed of Fe2Al5 phase and FeAl3 phase having a low Fe content. That is, the first layer (10) having a relatively high Fe concentration because it is close to the base steel sheet may include the Fe2Al5 phase, the second layer may include the AlSiFe intermetallic compound (τ phase), the third layer may include the Fe2Al5 phase, and the fourth layer (40) having a relatively low Fe concentration may include the FeAl3 phase. At this time, the AlSiFe intermetallic compound (τ phase) may exist continuously or discontinuously.

[0084] The second layer may be present in a region within 40% in the thickness direction from the bottom of the plating layer among the total average thickness (T) of the plating layer. If the second layer is present in a region exceeding 40% in the thickness direction from the bottom of the plating layer among the total average thickness (T) of the plating layer, overalloying may occur due to failure to suppress Fe diffusion of the base steel sheet in the plating layer during hot forming, thereby deteriorating spot weldability. More advantageously, the second layer may be present in a region within 20% in the thickness direction from the bottom of the plating layer among the total average thickness (T) of the plating layer. In the present invention, there is no particular limitation on the lower limit of the position of the second layer, but the second layer may be present in a region that is 1% or more in the thickness direction from the bottom of the plating layer among the total average thickness (T) of the plating layer.

[0085] The ratio (T1 / T) of the total average thickness (T) of the first layer, the second layer, and the third layer to the total average thickness (T) of the plating layer may be 0.20 to 0.50. When the T1 / T is less than 0.20, the adhesion between the base steel sheet and the plating layer may be poor. When the T1 / T exceeds 0.50, the diffusion of Fe of the base steel sheet into the plating layer cannot be suppressed during hot forming, resulting in overalloying and poor spot weldability. The lower limit of the T1 / T is more advantageously 0.23, more advantageously 0.25, and most advantageously 0.27. The upper limit of the T1 / T is more advantageously 0.48, more advantageously 0.46, and most advantageously 0.44.

[0086] The ratio (T2 / T) of the average thickness (T2) of the fourth layer to the total average thickness (T) of the plating layer may be 0.50 to 0.80. When the T2 / T is less than 0.50, Fe diffusion within the plating layer may be excessive, resulting in poor spot weldability after hot forming. When the T2 / T exceeds 0.80, the adhesion between the base steel sheet and the plating layer may be poor. The lower limit of the T2 / T is more advantageously 0.52, more advantageously 0.54, and most advantageously 0.56. The upper limit of the T2 / T is more advantageously 0.77, more advantageously 0.76, and most advantageously 0.73.

[0087] The plating layer may have an average thickness of 7 to 30 μm. If the average thickness of the plating layer is less than 7 μm, it may be difficult to secure sufficient corrosion resistance after hot forming. If the average thickness of the plating layer exceeds 30 μm, complete alloying through diffusion annealing heat treatment may be difficult, making it difficult to improve productivity and prevent melting of the plating layer during hot forming, which are the purposes of the present invention. Therefore, the average thickness of the plating layer may have an average thickness of 7 to 30 μm. The lower limit of the average thickness of the plating layer is more advantageously 8 μm, more advantageously 9 μm, and most advantageously 10 μm. The upper limit of the average thickness of the plating layer is more advantageously 27 μm, more advantageously 24 μm, and most advantageously 21 μm.

[0088] As described above, the galvanized steel sheet of the present invention may have a whiteness (L value) of 60 or less and a glossiness of 6.0 or less. This allows for increased heating efficiency during the heat treatment process for hot forming, thereby achieving a sufficient increase in heating speed and shortening of heating time. Meanwhile, the glossiness may be measured at a 60° angle.

[0089] The plated steel sheet of the present invention has an advantage in that, since the entire plated layer is completely alloyed, its melting point is higher than the heat treatment temperature for hot forming, thereby preventing melting of the plated layer and Al deposition in a heating furnace for hot forming. In addition, since the heat treatment process for hot forming has high heating efficiency, the latent heat of fusion required for the melting process of the plated layer and the energy consumption for AlFe alloying are not required, and thus the austenitizing efficiency of the steel sheet increases, thereby enabling the securing of material even with a shortened heating time. In addition, since it can be implemented in a continuous hot-dip galvanizing production line, it has the advantage of being easy to manufacture and highly economical. In other words, it has the advantage of not only excellent productivity but also reducing manufacturing costs.

[0090] Hereinafter, a method for manufacturing a hot-forming plated steel sheet according to one embodiment of the present invention will be described.

[0091] First, prepare a steel plate. As previously mentioned, the present invention does not specifically limit the type of steel plate, and any steel plate commonly used in the relevant technical field can be used.

[0092] For the above steel plate, heat treatment can be performed in an annealing furnace to secure the desired material.

[0093] Thereafter, the base steel sheet is immersed in a plating bath to form a plating layer on at least one surface of the base steel sheet, thereby obtaining a plated steel sheet. The present invention does not specifically limit the method for forming the plating layer, and any hot dip plating method commonly used in the relevant technical field can be used. Meanwhile, the composition of the plating bath for forming the plating layer is not specifically limited, but, for example, the composition of the plating bath may include, in wt%, Si: 6 to 15%, Fe: 0.1 to 2.0%, and the remainder Al, and the temperature of the plating bath may be 600 to 680°C.

[0094] Thereafter, the plated steel sheet is subjected to a diffusion annealing heat treatment. The diffusion annealing heat treatment is intended to cause mutual diffusion between the base steel sheet and the plating layer. The diffusion annealing heat treatment is preferably performed at 650 to 850°C for 3.0 to 20.0 seconds. If the diffusion annealing heat treatment temperature is lower than 650°C, an unalloyed plating layer remains, which has the disadvantage of causing roll sticking due to heating caused by a sufficient increase in the heating rate and melting of the residual plating layer during hot forming. If the diffusion annealing heat treatment temperature exceeds 850°C, the plating layer becomes overalloyed, which may result in a decrease in weldability due to an increase in the diffusion layer during hot forming. Therefore, the diffusion annealing heat treatment temperature is preferably in the range of 650 to 850°C. The lower limit of the diffusion annealing heat treatment temperature is more advantageously 670°C, more advantageously 680°C, and most advantageously 690°C. The upper limit of the above diffusion annealing heat treatment temperature is more advantageously 830°C, more advantageously 820°C, and most advantageously 810°C. If the diffusion annealing heat treatment time is less than 3.0 seconds, complete alloying may not be possible when the plating layer is thick, and problems such as increased equipment costs and decreased operational safety may occur due to the need for excessive heating power. If the diffusion annealing heat treatment time exceeds 20.0 seconds, problems such as excessively low productivity, difficulty in implementing in a continuous hot-dip plating line, and increased production costs may occur. Therefore, the diffusion annealing heat treatment time is preferably in the range of 3.0 to 20.0 seconds. The lower limit of the above diffusion annealing heat treatment time is more advantageously 4 seconds, more advantageously 4.8 seconds, and most advantageously 5.3 seconds. The upper limit of the above diffusion annealing heat treatment time is more advantageously 12 seconds, more advantageously 10 seconds, and most advantageously 8 seconds.Meanwhile, the present invention does not specifically limit the heating method of the steel sheet during the diffusion annealing heat treatment. However, preferably, a heating method using induction heating can be applied to secure sufficient heating capacity in a limited heating section of a continuous hot-dip galvanizing production line. More specifically, if heating above the Curie temperature of the steel sheet is required, a longitudinal or transverse flux type induction heating device can be used alone or in combination in some or all of the heating sections.

[0095] Thereafter, the plated steel sheet subjected to the diffusion annealing heat treatment is cooled. The cooling includes first cooling to a first cooling end temperature of 600°C, and then second cooling to a second cooling end temperature of less than 600°C, and it is preferable that the cooling rate during the first cooling is slower than the cooling rate during the second cooling. After the diffusion annealing heat treatment, the diffusion reaction occurs up to 600°C. That is, by controlling the cooling rate during the first cooling to be slower than the cooling rate during the second cooling as described above, the heating time required for complete alloying of the plated layer can be reduced, thereby further improving productivity. If the cooling rate (C1) during the first cooling end temperature of 600°C is higher than the cooling rate (C2) during the second cooling end temperature of less than 600°C, it may be difficult to sufficiently obtain the above-described effect. In the present invention, the cooling speed during the cooling is not specifically limited, but for example, the first cooling speed (C1) may be 0.9 to 21°C / s, and the second cooling speed (C2) may be 12 to 60°C / s.

[0096] Hereinafter, a method for manufacturing a hot press-formed member according to one embodiment of the present invention will be described.

[0097] The inventors of the present invention have completed the present invention based on the insight that it is important to suppress the phenomenon of rapid decrease in heating speed that occurs in the temperature range between 550°C and 600°C in order to shorten the heating time when heating a plated steel sheet for hot forming.

[0098] First, a galvanized steel sheet for hot forming is prepared. The galvanized steel sheet for hot forming includes a base steel sheet; and a galvanized layer formed on at least one surface of the base steel sheet.

[0099] The above-mentioned hot-forming galvanized steel sheet can satisfy one or more of the following (a) to (c).

[0100] (a) The hot-forming galvanized steel sheet comprises a base steel sheet; and a plating layer formed on at least one surface of the base steel sheet; wherein the plating layer contains, in wt%, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities.

[0101] (b) The plating layer has a plating weight of 30 g / m on one side. 2 Below

[0102] (c) The plating layer has a whiteness of 60 or less.

[0103] (a) The hot-forming galvanized steel sheet comprises a base steel sheet; and a plating layer formed on at least one surface of the base steel sheet; wherein the plating layer contains, in wt%, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities.

[0104] Si: 6~15%

[0105] The above Si plays a role in uniformly alloying with Fe within the plating layer. If the content of the Si is less than 6%, it may be difficult to sufficiently obtain the above-described effect. However, since the Si also plays a role in inhibiting the diffusion of Fe, if the content of the Si exceeds 15%, there may be a problem in that the diffusion of Fe is excessively inhibited. Therefore, the content of the Si is advantageously in the range of 6 to 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%.

[0106] Fe: 20~60%

[0107] The above Fe is an element that is included in the plating layer by diffusion of Fe of the base material. When the content of the Fe is less than 20%, the effect of suppressing the phenomenon of a decrease in the heating rate in the temperature range between 550℃ and 600℃ when heating the aluminum plating layer may not be sufficient. When the content of the Fe exceeds 60%, there may be disadvantages such as a decrease in corrosion resistance and weldability. When the content of the Fe exceeds 60%, problems such as a decrease in corrosion resistance and weldability may occur. Therefore, the content of the Fe is advantageously in the range of 20 to 60%. The lower limit of the Fe content is more advantageously 25%, and the upper limit is more advantageously 30%. The upper limit of the Fe content is more advantageously 55%, and the upper limit is more advantageously 50%.

[0108] (b) The aluminum plating layer has a plating amount of 30 g / m on one side. 2 Below

[0109] The above aluminum plating layer has a plating amount of 30 g / m on one side. 2 It may be less than or equal to the above plating amount. The above plating amount is a factor that can affect the alloying speed of the aluminum plating layer. The above plating amount is 30 g / m 2If it exceeds , the effect of suppressing the decrease in heating rate in the temperature range between 550℃ and 600℃ when heating the aluminum plating layer may not be sufficient. Therefore, the plating adhesion amount is 30g / m 2 It may be less than 25g / m. 2 Less than 20g / m is more advantageous. 2 The following is more advantageous. Meanwhile, in the present invention, there is no particular limitation on the lower limit of the plating amount, but the lower limit of the plating amount is 10 g / m. 2 It could be.

[0110] (c) The aluminum plating layer has a whiteness of 60 or less.

[0111] The above aluminum plating layer may have a whiteness of 60 or less. The whiteness is a factor that can affect the absorption and reflection of radiant energy of the aluminum plating layer. If the whiteness exceeds 60, the effect of suppressing the phenomenon of a decrease in the heating rate in the temperature range between 550°C and 600°C when heating the aluminum plating layer may not be sufficient. Therefore, the whiteness may be 60 or less. It is more advantageous for the whiteness to be 55 or less, and even more advantageous for it to be 50 or less. Meanwhile, the present invention does not specifically limit the lower limit of the whiteness, but the lower limit of the whiteness may be 0. The whiteness refers to the L value measured through a colorimeter commonly used in the relevant technical field.

[0112] Thereafter, the prepared hot forming galvanized steel sheet is heated. The heating step may be performed so that X expressed by the following [Formula 1] becomes 1.50°C / s or less. If the following X exceeds 1.50°C / s, the effect of suppressing the decrease in heating speed in the temperature range between 550°C and 600°C when heating the aluminum plating layer is not sufficient, which has the disadvantage of lowering productivity. Therefore, the following X may be 1.50°C / s or less. Alternatively, the below X may be 1.0°C / s or less. Alternatively, the below X may be 0.5°C / s or less. Meanwhile, the below X is not particularly limited with respect to its lower limit, but as an example, the lower limit of the above X may be 0°C / s.

[0113] [Formula 1] X = A - B

[0114] At this time, in the above [Formula 1], A is the heating rate at 600°C calculated from the straight line when the heating rate in the temperature section between 300°C and 550°C is fitted with a straight line (i.e., the straight line means the linear trend line of the heating rate between 300°C and 550°C) (i.e., when the linear trend line is extended, it means the heating rate of 600°C), and B is the heating rate at 600°C calculated from the straight line when the heating rate in the temperature section between 600°C and 700°C is fitted with a straight line (i.e., the straight line means the linear trend line of the heating rate between 600°C and 700°C) (i.e., the heating rate of 600°C on the linear trend line).

[0115] The heating step may be performed so that the cumulative value (Y) of the plating layer alloying history index at 550°C, expressed by the following [Formula 2], becomes 0.14 or less. The following [Formula 2] is a regression equation related to the decrease in heating rate when heating the aluminum plating layer. For example, the cumulative value (Y) of the plating layer alloying history index at 550°C may be 0.14 or less. If the cumulative value (Y) of the plating layer alloying history index at 550°C exceeds 0.14, the effect of suppressing the phenomenon of decreasing the heating rate in the temperature range between 550°C and 600°C when heating the aluminum plating layer may not be sufficient. Therefore, the cumulative value (Y) of the plating layer alloying history index at 550°C may be 0.14 or less. It is more advantageous for the cumulative value (Y) of the plating layer alloying history index at 550°C to be 0.12 or less, and even more advantageously, it is 0.10 or less. Meanwhile, there is no particular limitation on the lower limit of the accumulated value (Y) of the plating layer alloying history index, but, for example, the lower limit of the accumulated value (Y) of the plating layer alloying history index may be 0.01. The accumulated value of the plating layer alloying history index refers to the sum of the plating layer alloying history indices measured at regular temperature measurement time intervals until a specific measurement time. For example, when the temperature measurement time interval is 1 second, the accumulated value of the plating layer alloying history index at 10 seconds refers to the sum of the plating layer alloying history indices measured at every second in the range of 0 to 10 seconds. Meanwhile, when the temperature measurement time when the measured temperature is 550°C is not a positive integer, that is, when the temperature measurement time when the measured temperature is 550°C falls between j seconds and j+1 seconds, which are specific temperature measurement times, the accumulated value of the plating layer alloying history index between j seconds and j+1 seconds can be assumed to change linearly and then its value can be obtained proportionally.

[0116] [Formula 2] Cumulative value of plating layer alloying history index (Y) =

[0117] At this time, in the above [Formula 2], the k is a factor related to the alloying speed and has a value of 130.31, the Q is a factor related to the temperature effect affecting the alloying behavior according to the temperature and has a value of 62190.89 J / mol, the R is a gas constant indicating the ratio of the volume and mole number of gas at a constant temperature and pressure in the ideal gas law and has a value of 8.314 J / (mol·K), the n is a factor related to the time effect affecting the alloying speed and has a value of 0.644, and the j is T i i means when the temperature is 550℃, and the above Δt i has a value of 1 sec as the temperature measurement time interval during heating, and the i has a positive integer (sec) value as the temperature measurement time during heating, and the T i refers to the heating temperature (K) measured at i seconds. The above K refers to the absolute temperature.

[0118] After the heating step, a step of hot press forming the hot-forming plated steel sheet may be additionally included. The present invention does not specifically limit the hot press forming method, and any method commonly used in the relevant technical field may be used.

[0119] According to the method for manufacturing a hot press-formed part according to one embodiment of the present invention as described above, when heating a plated steel sheet for hot forming, the phenomenon of a rapid decrease in heating speed in the temperature range between 550°C and 600°C can be suppressed. In this way, an improvement in productivity can be expected through an increase in heating speed.

[0120] Hereinafter, the present invention will be described in detail through examples. However, it should be noted that the examples described below are intended only to illustrate and concretize the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0121] (Example 1)

[0122] A base steel sheet was immersed in a plating bath (640°C) containing, by weight%, 9.5% Si, 1.5% Fe, the remainder Al, and other unavoidable impurities to form a plating layer on the base steel sheet, and then adjusted to a desired plating thickness through an air knife to manufacture a plated steel sheet. Thereafter, the plated steel sheet was subjected to diffusion annealing heat treatment and cooling under the conditions shown in Table 1 below. At this time, the cooling was performed by dividing into a first cooling and a second cooling, and the first cooling was performed up to 600°C, and the second cooling was performed up to 300°C. For the plated steel sheet manufactured in this way, the alloy composition of the plating layer, whether the plating layer was completely alloyed, the average thickness of the plating layer, whiteness, and emissivity were measured, and the results are shown in Table 2 below. At this time, the plating layer that was completely alloyed included a first layer composed of Fe2Al5 phase; And a second layer formed of FeAl3 phase on the first layer; and the first layer contained an AlSiFe intermetallic compound (τ phase) that existed continuously or discontinuously, and a plating layer that was not completely alloyed remained as an unalloyed Si-Fe-Al plating layer on the second layer.

[0123] The alloy composition of the plating layer was measured using a wet analysis method using ICP (Inductively coupled plasma).

[0124] Complete alloying of the plating layer was measured by observing the cross-section of the plating layer using an optical microscope and SEM after polishing.

[0125] The average thickness of the plating layer was calculated by etching the cross-section of the plating layer with a natal solution and then using an optical microscope to calculate the average value for six arbitrary points.

[0126] Whiteness was calculated as the average value for six arbitrary points on the surface of the plating layer along the width direction using a colorimeter.

[0127] The gloss was calculated by using a gloss meter and taking the average value for six random points on the surface of the plating layer along the width direction. At this time, the measurement angle was 60°.

[0128] Diffusion annealing heat treatment Cooling temperature (℃) Time (sec) First cooling rate (℃ / s) Second cooling rate (℃ / s) Relationship 1 Comparative example 1--4.0 12.0 Satisfactory invention example 16503.24.3 12.0 Satisfactory invention example 26754.05.7 16.0 Satisfactory invention example 37004.87.120.0 Satisfactory invention example 47255.38.5 24.0 Satisfactory invention example 57506.0 10.28.0 Satisfactory invention example 67756.9 11.432.0 Satisfactory invention example 78009.6 12.836.0 Satisfactory invention example 882512.014.340.0 Satisfactory invention example 985016.017.048.0 Satisfactory comparison example 282512.040.014.3 Unsatisfactory comparison example 37756.920.07.1 Unsatisfactory comparison example 47504.028.010.0 Unsatisfactory comparison example 57254.08.52.4 Unsatisfactory comparison example 67004.820.07.1 Unsatisfactory [Relationship 1] Cooling rate during first cooling < Cooling rate during second cooling

[0129] Alloy composition of plating layer (weight%)Complete alloying of plating layerAverage thickness of plating layer (㎛)Whiteness (L value)GlossSiFeAlComparative example18.919Remainder×5.170.89.2Invention example1623Remainder○7.250.52.5Invention example26.326Remainder○8.552.52.5Invention example36.629Remainder○10.156.75.5Invention example4731Remainder○11.858.35.4Invention example5733Remainder○13.958.75.7Invention example67.639Remainder○16.259.05.6Invention example77.8 41 Balance ○ 19.45 8.65.8 Invention Example 87.543 Balance ○ 24.26 0.05.8 Invention Example 97.622 Balance ○ 30.06 0.06.0 Comparative Example 29.115 Balance × 21.08 3.05 2.9 Comparative Example 37.819 Balance × 15.66 1.56 3.9 Comparative Example 48.617 Balance × 12.27 6.36 9.3 Comparative Example 57.918 Balance × 10.67 0.87 0.8 Comparative Example 67.719 Balance × 9.76 2.39.2

[0130] As can be seen from Tables 1 and 2 above, in the case of Invention Examples 1 to 9 that satisfy the conditions of the present invention, the plating layer is completely alloyed, and the whiteness and gloss also have low values. This is a very low level compared to a typical Al-Si plating layer.

[0131] On the other hand, in the case of Comparative Examples 1 to 6 that do not satisfy the conditions of the present invention, it can be seen that not only was the plating layer not completely alloyed, but also the whiteness and gloss also had high values.

[0132] Fig. 2 is a photograph of a cross-section of Invention Example 9 observed using a scanning electron microscope. As can be seen from Fig. 2, in the case of Invention Example 9, the plating layer is completely alloyed, and it can be confirmed that the plating layer includes a first layer formed of an Fe2Al5 phase and a second layer formed of an FeAl3 phase formed on the first layer, and that an AlSiFe intermetallic compound (τ phase) is continuously present within the first layer.

[0133] Fig. 3 is a photograph of a cross-section of Comparative Example 1 observed using a scanning electron microscope. As can be seen from Fig. 3, in the case of Comparative Example 1, the plating layer includes a first layer formed of an Fe2Al5 phase and a second layer formed of an FeAl3 phase formed on the first layer, but an AlSiFe intermetallic compound (τ phase) that exists discontinuously on the upper portion of the first layer is formed, and it can be confirmed that an Si-Fe-Al plating layer that is not alloyed remains on the second layer due to incomplete alloying of the plating layer.

[0134] (Example 2)

[0135] A base steel sheet was immersed in a plating bath (680°C) containing, by weight%, 9.5% Si, 1.5% Fe, the remainder Al, and other unavoidable impurities to form a plating layer on the base steel sheet, and then adjusted to a desired plating thickness through an air knife to manufacture a plated steel sheet. Thereafter, the plated steel sheet was subjected to diffusion annealing heat treatment and cooling under the conditions shown in Table 3 below. At this time, the cooling was performed by dividing into a first cooling and a second cooling, and the first cooling was performed up to 600°C, and the second cooling was performed up to 300°C. For the plated steel sheet manufactured in this way, the alloy composition of the plating layer, whether the plating layer was completely alloyed, the average thickness of the plating layer, whiteness, and emissivity were measured, and the results are shown in Tables 4 to 6 below. At this time, the plating layer in which complete alloying was achieved includes a first layer including an Fe2Al5 phase; a second layer including an AlSiFe intermetallic compound (τ phase) on the first layer; The second layer comprises a third layer including an Fe2Al5 phase; the third layer comprises a fourth layer including an FeAl3 phase; and the plating layer that is not completely alloyed remains as an unalloyed Al-Si plating layer on the fourth layer.

[0136] The alloy composition of the plating layer was measured using a wet analysis method using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy).

[0137] Complete alloying of the plating layer was measured by observing the cross-section of the plating layer using an optical microscope and SEM after polishing.

[0138] The average thickness of the plating layer was calculated by etching the cross-section of the plating layer with a natal solution and then using an optical microscope to calculate the average value for six arbitrary points.

[0139] Whiteness was calculated as the average value for six arbitrary points on the surface of the plating layer along the width direction using a colorimeter.

[0140] The gloss was calculated by using a gloss meter and taking the average value for six random points on the surface of the plating layer along the width direction. At this time, the measurement angle was 60°.

[0141] Diffusion annealing heat treatmentCooling temperature (℃)Time (sec)First cooling rate (℃ / s)Second cooling rate (℃ / s)Relationship 1Comparative example 7--4.012.0Satisfactory invention example 106503.24.312.0Satisfactory invention example 116754.05.716.0Satisfactory invention example 127004.87.120.0Satisfactory invention example 137255.38.524.0Satisfactory invention example 147506.010.28.0Satisfactory invention example 157756.911.432.0Satisfactory invention example 168009.612.836. 0 Satisfactory invention example 1782512.014.340.0 Satisfactory invention example 1885016.017.048.0 Satisfactory comparison example 882512.040.014.3 Unsatisfactory comparison example 97756.920.07.1 Unsatisfactory comparison example 107504.028.010.0 Unsatisfactory comparison example 117254.08.52.4 Unsatisfactory comparison example 127004.820.07.1 Unsatisfactory [Relationship 1] Cooling rate during first cooling < Cooling rate during second cooling

[0142] Alloy composition of plating layer (weight %)Whether the plating layer is completely alloyed or not1st layer alloy composition (weight %)SiFeAlSiFeAlComparative example78.919Remainder×11.534.853.7Invention example10623Remainder○4.549.346.2Invention example116.326Remainder○4.349.646.1Invention example126.629Remainder○3.850.246.0Invention example13731Remainder○3.551.145.4Invention example14733Remainder○3.252.144.7Invention example157.639Remainder○2.953.243. 9 Invention Example 167.841 Remainder ○ 2.55 4.143.4 Invention Example 177.543 Remainder ○ 2.25 542.8 Invention Example 187.622 Remainder ○ 1.95 543.1 Comparative Example 89.115 Remainder × 11.23 2.55 6.3 Comparative Example 97.819 Remainder × 2.64 453.4 Comparative Example 108.617 Remainder × 2.54 6.45 1.1 Comparative Example 117.918 Remainder × 247.45 0.6 Comparative Example 127.719 Remainder × 3.44 551.6

[0143] Classification 2nd layer 3rd layer 4th layer SiFeAlSiFeAlSiFeAlComparative example 77.744.747.611.534.853.701.998.1Invention example 102.24849.86.533.759.814.225.560.3Invention example 112.847.949.34.235.860.05.134.260.7Invention example 12 6.544.648.92.940.556.64.330.765Invention Example 136.245.8483.142.354.64.533.162.4Invention Example 148.342.449.33.644.252.27.829.362.9Invention Example 158.642.249.22.84849.26.230.563.3Invention Example 1 610.443.6464.846.348.910.831.557.7Invention Example 1711.841.346.95.747.247.113.13452.9Invention Example 181247.640.46.44845.614.534.351.2Comparative Example 810.944.744.411.555.133.43.2096.8B Comparative Example 910.842.147.112.656.131.30.2297.8Comparative Example 109.543.746.812.23255.83.33.693.1Comparative Example 114.644.151.34.439.356.310.328.860.9Comparative Example 121044.145.93.44551.69.423.866.8

[0144] Total average thickness of plating layer (T) (㎛) Ratio of the total average thickness of the plating layer (T) to the average thickness of the first, second and third layers (T1) (T1 / T) Ratio of the total average thickness of the plating layer (T) to the average thickness of the fourth layer (T2) T2 / T Whiteness (L value) Gloss Comparison Example 75.10.2 90.7 170.89.2 Invention Example 107.20.5 0.5 5 0.5 2.5 Invention Example 118.5 0.4 4 0.5 6 52.5 2.5 Invention Example 12 10.10.4 3 0.5 7 5 6.7 5.5 Invention Example 13 11.80.4 10.5 9 5 8.3 5.4 Invention Example 14 13.90.3 9 0.6 1 5 8.7 5.7 Invention Example 15 16.20.3 5 0.6 5 9.0 5.6 Invention Example 16 19.40.320.6858.65.8Invention Example 1724.20.30.760.05.8Invention Example 1830.00.270.7360.06.0Comparative Example 821.00.420.5883.052.9Comparative Example 915.60.550.4561.563.9Comparative Example 1012.20.570.4376.369.3Comparative Example 1110.60.220.7870.870.8Comparative Example 129.70.200.862.39.2

[0145] As can be seen from Tables 3 to 6 above, in the case of Inventive Examples 10 to 18 that satisfy the manufacturing conditions of the present invention, the plating layer was completely alloyed, and the first, second, third, and fourth layers having appropriate levels of Al, Fe, and Si contents were formed, so it can be seen that the whiteness and gloss also had low values. This is a very excellent level compared to a general Al-Si-based plating layer. On the other hand, in the case of Comparative Examples 7 to 12 that do not satisfy the manufacturing conditions of the present invention, the plating layer was not completely alloyed, and the first, second, third, and fourth layers having appropriate levels of Al, Fe, and Si contents were not formed, so it can be seen that the whiteness and gloss had high values.

[0146] Fig. 5 is a photograph of a cross-section of Invention Example 18 observed using a scanning electron microscope. As can be seen from Fig. 5, in the case of Invention Example 18, the plating layer is completely alloyed, and it can be confirmed that the plating layer is composed of a first layer including an Fe2Al5 phase, a second layer including an AlSiFe intermetallic compound (τ phase) continuously present on the first layer, a third layer including an Fe2Al5 phase on the second layer, and a fourth layer including an FeAl3 phase on the third layer.

[0147] Fig. 6 is a photograph of a cross-section of Comparative Example 7 observed using a scanning electron microscope. As can be seen from Fig. 6, in the case of Comparative Example 7, the plating layer is composed of a first layer including an Fe2Al5 phase, a second layer including an AlSiFe intermetallic compound (τ phase) that exists discontinuously on the first layer, and a fourth layer including an FeAl3 phase on the third layer. It can be confirmed that an Al-Si-based plating layer that is not alloyed remains on the fourth layer due to incomplete alloying of the plating layer.

[0148] (Example 3)

[0149] First, an aluminum-based plating layer having the alloy composition, whiteness, and single-sided plating amount described in Table 7 below was formed on both sides of a base steel sheet to prepare a hot-forming plated steel sheet having the steel sheet thickness described in Table 7 below. Thereafter, the hot-forming plated steel sheet was heated from room temperature to 935°C. The values ​​of [Formula 1] and [Formula 2] at 550°C were measured for the hot-forming plated steel sheet thus heated, and the results are shown in Table 7 below.

[0150] The value of [Formula 1] was obtained by obtaining a graph related to the heating rate according to temperature for each hot-forming galvanized steel sheet, and then fitting the heating rate in the temperature range between 300°C and 550°C with a straight line and fitting the heating rate in the temperature range between 600°C and 700°C with a straight line. The fitting was performed using the linear fitting method of Excel.

[0151] In addition, for each hot-forming galvanized steel sheet, the temperature was measured at a temperature measurement time interval of 1 second, and then the accumulated value of [Equation 2] at 550℃ was calculated.

[0152] Alloy composition (weight %)Whiteness (L)Single-sided plating amount (g / m) 2 ) Steel plate thickness (mm) [Formula 1] [Formula 2] SiFeAl Comparative example 13938884751.41.980.16 Comparative example 14938877751.41.820.17 Invention example 199405150751.40.380.12 Invention example 209405145751.40.140.11 Comparative example 15938880401.21.750.14 Invention example 21938886301.20.480.18 [Formula 1] X = A - B (However, in the above [Formula 1], A is the heating rate at 600°C calculated from the straight line when the heating rate in the temperature section between 300°C and 550°C is fitted with a straight line, and B is the heating rate at 600°C and 700°C When the heating rate of the temperature interval between is fitted with a straight line, it is the heating rate at 600℃ calculated from the straight line. [Formula 2] Cumulative value of the plating layer alloying history index (Y) = (However, in the above [Formula 2], the k is a factor related to the alloying speed and has a value of 130.31, the Q is a factor related to the temperature effect affecting the alloying behavior according to temperature and has a value of 62190.89 J / mol, the R is a gas constant indicating the ratio of the volume and mole number of gas at a constant temperature and pressure in the ideal gas law and has a value of 8.314 J / (mol·K), the n is a factor related to the time effect affecting the alloying speed and has a value of 0.644, and the Δt i has a value of 1 sec as the temperature measurement time interval during heating, and the i has a positive integer (sec) value as the temperature measurement time during heating, and the T i refers to the heating temperature (K) measured at i seconds.)

[0153] Fig. 7 is a graph related to the heating rate according to temperature for Inventive Examples 19 and 20. Fig. 8 is a graph related to the heating rate according to temperature for Comparative Examples 13 and 14. Fig. 9 is a graph related to the heating time and temperature for Inventive Examples 19 and 20 and Comparative Examples 13 and 14.

[0154] As can be seen from Table 7 and FIGS. 7 to 9, in the case of Comparative Examples 13 and 14, the heating rate decreased rapidly in the temperature range between 550°C and 600°C, whereas in the case of Inventive Examples 19 and 20, the phenomenon of a rapid decrease in the heating rate did not occur in the temperature range between 550°C and 600°C, and thus, it can be seen that Inventive Examples 19 and 20 were heated to 935°C much faster than Comparative Examples 13 and 14. Meanwhile, in FIG. 7, A of Inventive Example 19 is depicted as A1, B as B1, A of Inventive Example 20 is depicted as A2, B as B2, and in FIG. 8, A of Comparative Example 13 is depicted as A3, B as B3, and A of Comparative Example 14 is depicted as A4, B as B4.

[0155] Fig. 10 is a graph related to the cumulative value of the plating layer alloying history index according to temperature for Inventive Examples 19 and 20 and Comparative Examples 13 and 14. As can be seen from Fig. 10, Comparative Examples 13 and 14 have cumulative values ​​of the plating layer alloying history index exceeding 0.30 at 550°C, whereas Inventive Examples 19 and 20 have cumulative values ​​of the plating layer alloying history index of 0.30 or less at 550°C.

[0156] Fig. 11 is a graph related to the heating rate according to temperature for Comparative Example 15 and Inventive Example 21. As can be seen from Fig. 11, in the case of Comparative Example 15, the heating rate decreased rapidly in the temperature range between 550°C and 600°C, whereas in the case of Inventive Example 21, the phenomenon of a rapid decrease in the heating rate did not occur in the temperature range between 550°C and 600°C. Meanwhile, in Fig. 11, A of Inventive Example 21 was depicted as A5, B as B5, and A of Comparative Example 15 was depicted as A6, B as B6.

[0157] [Explanation of symbols]

[0158] 10: 1st floor

[0159] 20: 2nd floor

[0160] 30: AlSiFe intermetallic compound (τ phase)

[0161] 100: Steel plate

[0162] 200: Plating layer

[0163] 10': 1st floor

[0164] 20': 2nd floor

[0165] 30': 3rd floor

[0166] 40': 4th floor

[0167] 100': Steel plate

[0168] 200': Plating layer

Claims

1. Steel plate; and A plating layer formed on at least one surface of the above steel plate; The above plating layer is, A first layer containing, in weight %, Al: 40 to 48%, Fe: 49 to 55%, and Si: 5% or less; A second layer formed on the first layer and containing, in wt%, Al: 40 to 50%, Fe: 38 to 48%, and Si: 2 to 12%; A third layer formed on the second layer and containing, in wt%, Al: 45 to 60%, Fe: 33 to 48%, and Si: 7% or less; and A hot-forming galvanized steel sheet comprising a fourth layer formed on the third layer and containing, in weight %, Al: 50 to 65%, Fe: 25 to 35%, and Si: 15% or less.

2. In paragraph 1, A hot-forming galvanized steel sheet, wherein the above-mentioned galvanized layer contains, in weight %, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities.

3. In paragraph 1, The above first layer comprises a Fe2Al5 phase, The second layer above includes an AlSiFe intermetallic compound (τ phase), The above third layer contains Fe2Al5 phase, The fourth layer is a hot-forming galvanized steel sheet containing an FeAl3 phase.

4. In paragraph 1, The second layer is a hot-forming galvanized steel sheet, wherein the second layer exists in an area within 40% in the thickness direction from the bottom of the galvanized layer among the total average thickness (T) of the galvanized layer.

5. In paragraph 1, A hot-forming galvanized steel sheet, wherein the ratio (T1 / T) of the total average thickness (T) of the first layer, the second layer, and the third layer to the total average thickness (T) of the galvanized layer is 0.20 to 0.

50.

6. In paragraph 1, A hot-forming galvanized steel sheet, wherein the ratio (T2 / T) of the average thickness (T2) of the fourth layer to the total average thickness (T) of the galvanized layer is 0.50 to 0.

80.

7. In paragraph 1, The above plating layer is a hot-forming plating steel sheet having an average thickness of 7 to 30 ㎛.

8. In paragraph 1, The above plating layer is a hot-forming galvanized steel sheet having a whiteness of 60 or less and a glossiness of 6.0 or less.

9. Step for preparing the steel plate; A step of immersing the steel sheet in a plating bath to form a plating layer on at least one surface of the steel sheet to obtain a plated steel sheet; A step of performing diffusion annealing heat treatment on the above-mentioned galvanized steel sheet; and A step of cooling the above diffusion annealing heat-treated plated steel sheet; The above diffusion annealing heat treatment is performed at 650 to 850°C for 3.0 to 20.0 seconds. In the above cooling, the first cooling is performed to a first cooling end temperature of 600°C, and then the second cooling is performed to a second cooling end temperature of less than 600°C. A method for manufacturing a hot-forming galvanized steel sheet, wherein the cooling speed during the first cooling is slower than the cooling speed during the second cooling.

10. In paragraph 9, A method for manufacturing a plated steel sheet, wherein the above plating bath contains, in wt%, Si: 6 to 15%, Fe: 0.1 to 2.0%, the remainder Al, and other unavoidable impurities.

11. In paragraph 9, A method for manufacturing a plated steel sheet, wherein the above diffusion annealing heat treatment is performed by induction heating.

12. In paragraph 9, A method for manufacturing a plated steel sheet, wherein the cooling rate during the first cooling is 0.9 to 21°C / s, and the cooling rate during the second cooling is 12 to 60°C / s.

13. In the step of heating the galvanized steel sheet for hot forming, A method for manufacturing a hot press-formed member, wherein X expressed by the following [Formula 1] is performed at 1.50°C / s or less. [Formula 1] X = A - B (However, in the above [Formula 1], A is the heating rate at 600°C calculated from the straight line when the heating rate in the temperature range between 300°C and 550°C is fitted to a straight line, and B is the heating rate at 600°C calculated from the straight line when the heating rate in the temperature range between 600°C and 700°C is fitted to a straight line.) 14. In paragraph 13, The above hot-forming galvanized steel sheet is a method for manufacturing a hot press-formed member that satisfies at least one of the following (a) to (c). (a) The hot-forming galvanized steel sheet comprises a base steel sheet; and a plating layer formed on at least one surface of the base steel sheet; wherein the plating layer contains, in wt%, Si: 6 to 15%, Fe: 20 to 60%, the remainder Al, and other unavoidable impurities. (b) The plating layer has a plating weight of 30 g / m on one side. 2 Below is (c) The plating layer has a whiteness of 60 or less.

15. In paragraph 13, The above X is a method for manufacturing a hot press-formed part having a temperature of 0℃ / s or more and 1.50℃ / s or less.

16. In any one of paragraphs 13 to 15, A method for manufacturing a hot press-formed part, wherein the heating step is performed so that the accumulated value (Y) of the plating layer alloying history index at 550°C, expressed by the following [Formula 2], becomes 0.14 or less. [Formula 2] Cumulative value of plating layer alloying history index (Y) = (However, in the above [Formula 2], the k is a factor related to the alloying speed and has a value of 130.31, the Q is a factor related to the temperature effect affecting the alloying behavior according to temperature and has a value of 62190.89 J / mol, the R is a gas constant indicating the ratio of the volume and mole number of gas at a constant temperature and pressure in the ideal gas law and has a value of 8.314 J / (mol K), the n is a factor related to the time effect affecting the alloying speed and has a value of 0.644, and the j is T i i means when the temperature is 550℃, and the above Δt i is a temperature measurement time interval during heating and has a value of 1 sec, and the i is a temperature measurement time during heating and has a positive integer (sec) value, and the T i refers to the heating temperature (K) measured at i seconds.) 17. In paragraph 13, A method for manufacturing a hot press-formed member wherein the above Y is 0.12 or less.

18. In paragraph 13, A method for manufacturing a hot press-formed member wherein the above Y is 0.10 or less.

19. In paragraph 13, A method for manufacturing a hot press-formed member wherein the above Y is 0.01 or greater.

20. In paragraph 13, A method for manufacturing a hot press-formed part, further comprising, after the heating step, a step of hot press-forming the hot-forming plated steel sheet.

Images