Method for manufacturing plated steel sheet for hot forming with excellent resistance to hydrogen embrittlement, and hot-formed member

Abstract

The present invention relates to a plated steel sheet for hot press forming (HPF) used for automobile body parts and the like, a hot-formed member, and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a plated steel sheet for hot press forming (HPF) used for automobile body parts and the like, a hot-formed member, and a method for manufacturing the same. [Background technology]

[0002] Recently, hot-formed members have been widely used in structural members of automobiles and the like for the purposes of improving fuel efficiency by reducing the weight of automobiles and protecting passengers, and are particularly widely used in bumpers, doors, filler reinforcements, etc., which require ultra-high strength or high energy absorption capacity. In particular, when using hot-formed members, plated steel sheets are often used to ensure corrosion resistance.

[0003] A representative technique relating to such hot forming technology is disclosed in Patent Document 1. In Patent Document 1, an Al-Si plated steel sheet is heated to 850°C or higher, and then hot formed by pressing and rapidly cooled to form the structure of the part into martensite, thereby ensuring ultra-high strength. When such hot forming technology is used to ensure ultra-high strength, complex shapes can be easily formed because the forming is performed at high temperatures. Furthermore, the increase in strength due to rapid cooling in a mold can be expected to result in both high strength and weight reduction.

[0004] However, the martensite structure is known to have low resistance to hydrogen embrittlement. In particular, parts manufactured after hot forming have residual stress due to rapid cooling after heating, and if the amount of diffusible hydrogen in the steel increases, delayed fracture due to hydrogen embrittlement may occur, limiting the application of the parts. Therefore, a solution to this problem is currently being sought. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 6,296,805 Summary of the Invention [Problem to be solved by the invention]

[0006] One aspect of the present invention relates to a plated steel sheet for hot forming that can reduce the occurrence of delayed fracture due to hydrogen embrittlement in hot-formed components and has excellent hydrogen embrittlement resistance, a hot-formed component manufactured using the same, and a method for manufacturing the same.

[0007] The object of the present invention is not limited to the above-mentioned content, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the overall matters of the specification of the present invention. [Means for solving the problem]

[0008] One aspect of the present invention is a base steel sheet; and an Al-based plating layer formed on the surface of the base steel sheet, The average Ni content in the Al-based plating layer is 0.05 to 0.35% by weight, and the plated steel sheet for hot forming has excellent resistance to hydrogen embrittlement.

[0009] Another aspect of the present invention is a method for manufacturing a steel sheet, comprising the steps of: preparing a base steel sheet; The above prepared base steel sheet is coated with a coating weight of 150 to 2500 mg / m 2 Ni plating step; and continuous annealing the Ni-plated cold-rolled steel sheet and plating it with an Al-based coating; The present invention relates to a method for producing a plated steel sheet for hot forming that is excellent in hydrogen embrittlement resistance.

[0010] Another aspect of the present invention is a steel sheet comprising: An alloy plating layer formed on the base steel, The hot-formed member has excellent resistance to hydrogen embrittlement, and the Ni content at the point of highest Ni content (Nimax) from the surface of the alloy plating layer to a depth of 15 μm is 0.1 to 0.75 wt %.

[0011] Another aspect of the present invention is a steel sheet manufacturing method, comprising the steps of: providing a blank, the blank including a base steel sheet and an Al-based plating layer formed on a surface of the base steel sheet, wherein an average Ni content in the Al-based plating layer is 0.05 to 0.35 wt%; Heating the blank at a temperature in the range of Ac3 to 975°C; and transferring the heated blank to a press to form it, and cooling it at a cooling rate of 20°C / s or more; The present invention relates to a method for producing a hot-formed member having excellent hydrogen embrittlement resistance, comprising: [Effects of the Invention]

[0012] When the plated steel sheet for hot forming of the present invention is hot formed, the amount of diffusible hydrogen in the steel can be reduced, and a hot-formed part having excellent resistance to hydrogen embrittlement can be provided.

[0013] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a graph showing the results of analyzing the Fe and Ni contents of the Al-based coating layer of the coated steel sheet for hot forming of Example 1 in the Examples by glow discharge spectroscopy (GDS). [Figure 2] 1 is a graph showing the results of GDS analysis of the Ni content in the alloy plating layer of the hot-formed member of Example 3 among the Examples, and is a diagram showing the Ni-enriched layer. [Figure 3] 1 is a graph showing the results of GDS analysis of the Ni content in the alloy plating layer of the hot-formed members of Examples 1 to 3 and Comparative Example 2 among the working examples. [Figure 4] 1 is a graph showing the results of measuring the Ni content at the Nimax point and the diffusible hydrogen content of each hot-formed member of Examples 1 to 8 and Comparative Examples 1 and 2 among the working examples. DETAILED DESCRIPTION OF THE INVENTION

[0015] The terms used in this specification are for the purpose of describing the present invention and are not intended to limit the present invention. Furthermore, as used in this specification, the singular form includes the plural form unless the related definition clearly indicates otherwise.

[0016] The term "comprises" as used in the specification embodies features and does not exclude the presence or addition of other features.

[0017] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0018] The inventors of the present invention conducted in-depth research into hydrogen embrittlement in hot-formed components and found that during the heating step of a blank (manufactured from a plated steel sheet for hot forming) having a plated layer such as aluminum during hot forming, water vapor present in the heat treatment furnace is adsorbed on the surface of the blank. The hydrogen generated as the water dissociates is then absorbed into the austenite steel, which has high hydrogen solubility at high temperatures.

[0019] However, when rapid cooling occurs during hot forming and the steel transforms into a martensite phase, the solubility of hydrogen drops sharply, and the alloy plating layer formed during alloying acts as an obstacle to hydrogen release. Therefore, a significant amount of diffusible hydrogen remains in the steel, increasing the possibility of crack formation due to hydrogen-delayed fracture. Therefore, it was found that it is important to reduce the amount of diffusible hydrogen absorbed during heat treatment.

[0020] The inventors conducted an analysis of the amount of diffusible hydrogen in steel, which affects hydrogen embrittlement in steel for hot forming, and discovered that Ni plating can reduce the amount of diffusible hydrogen, leading to the present invention.

[0021] The present invention will be described in detail below.

[0022] A plated steel sheet for hot forming, which is one embodiment of the present invention, includes a base steel sheet and an Al-based plating layer formed on the base steel sheet, and it is preferable that the Ni content in the Al-based plating layer is 0.05 wt % or more.

[0023] The Al-based coating layer, excluding Ni, is sufficient as long as it is an Al-based coating widely used in hot-forming coated steel sheets by ordinary engineers in the technical field of hot-forming coated steel sheets to which the present invention pertains, and the content and type of components are not particularly limited. For example, it may include not only pure Al coating but also coatings in which Si is partially contained in Al.

[0024] The Al-based coating layer may include an Al-rich layer and a concentration gradient layer. The concentration gradient layer is a section containing Fe formed by the interaction of Fe in the base steel sheet with components of the Al coating bath during the Al-based coating process. It is not easy to clearly distinguish the boundary between the Al-rich layer and the concentration gradient layer in the Al-based coating layer. However, the Fe content in the Al-rich layer is lower than the Fe content in the concentration gradient layer. For example, when the Fe content in the Al-based coating layer is about 2 wt % or more, it can be understood that the content of Fe diffused in the base steel sheet becomes higher than the content of the coating components, and this can be considered to be the boundary between the Al-rich layer and the concentration gradient layer. The boundary between the concentration gradient layer and the base steel sheet can be considered to be up to the point where the Fe content reaches about 80 wt %, but it is not necessarily so clearly distinguished.

[0025] For example, referring to FIG. 1, the analysis results show that glow discharge optical emission spectrometry (GDS) was performed on the plated steel sheet for hot forming of Example 1 in the following Examples from the surface in the depth direction. From this, it can be understood that the Al-based plating layer includes an Al-rich layer and a concentration gradient layer in which the Fe content is up to 80 wt %.

[0026] In the plated steel sheet for hot forming, the Al-based plating layer preferably contains an average Ni content of 0.05 wt. % or more. As shown in FIG. 1, the Ni contained in the Al-based plating layer is preferably distributed in the depth direction. However, referring to FIG. 1, the Ni content is not constant from the surface of the plated steel sheet to the depth direction of the plating layer. The average Ni content refers to the average of the results of an analysis from the surface to the depth direction of the Al-based plating layer. As a preferred example of measuring this, it can be calculated as the average value of a graph using a GDS analysis Ni profile as shown in FIG. 1. As described below, the Ni contained in the Al-based plating layer forms a Ni-enriched layer in the alloy plating layer formed during hot forming. This Ni-enriched layer prevents external hydrogen, which may be absorbed during heat treatment for hot forming, from penetrating into the base steel sheet. This improves the hydrogen embrittlement resistance of the hot-formed component.

[0027] If the Ni content is less than 0.05 wt%, a sufficient Ni-enriched layer is not formed during hot forming, making it difficult to effectively prevent external hydrogen from being absorbed during the hot forming heat treatment. On the other hand, the higher the Ni content, the greater the effect of preventing external hydrogen. However, if the Ni content exceeds 0.35 wt%, the Ni coating weight increases, resulting in increased costs and increased non-uniformity in the width direction, making it difficult to produce uniform steel sheets. Therefore, it is preferable that the Ni content does not exceed 0.35 wt%. A more preferred range is 0.055 to 0.335 wt%.

[0028] In the present invention, the type of base steel sheet and the alloy composition are not particularly limited, as long as they are usable as hot-formed members. As a preferred example, the base steel sheet in the present invention contains, by weight, 0.07-0.5% carbon (C), 0.05-1% silicon (Si), 0.5-4% manganese (Mn), 0.001-0.015% phosphorus (P), 0.0001-0.02% sulfur (S), 0.01-0.1% aluminum (Al), 0.01-1% chromium (Cr), 0.001-0.02% nitrogen (N), with the remainder being iron (Fe) and unavoidable impurities. The steel sheet may further contain up to 0.1% (inclusive) titanium (Ti) and up to 0.01% (inclusive) boron (B). Each alloy component will be briefly described below.

[0029] Carbon (C): 0.07~0.5% by weight (hereinafter referred to as %) C is an element that improves the strength and hardenability of hot-formed members and must be appropriately included as an essential element for strength adjustment. If the C content is less than 0.07%, the hardenability is low and sufficient martensite cannot be obtained when the cooling rate is reduced, making it difficult to obtain the desired strength through ferrite formation. Therefore, it is preferable to include 0.07% or more of C. If the C content exceeds 0.5%, the strength increases excessively, causing brittleness and deteriorating weldability, so it is preferable not to exceed 0.5%.

[0030] Silicon (Si): 0.05 to 1% The above-mentioned Si is not only necessary as a deoxidizer in steelmaking, but is also a solid-solution strengthening element that suppresses carbide formation and is effective in homogenizing the internal structure. It also contributes to increasing the strength of hot-formed parts and is an effective element in homogenizing material properties. However, if the Si content is less than 0.05%, the above effects cannot be expected, and if it exceeds 1%, excessive Si oxides formed on the surface of the steel sheet during annealing significantly reduce galvanic properties. Therefore, a Si content of 1% or less is preferred.

[0031] Manganese (Mn): 0.5-4% Mn is an essential element that not only ensures the desired strength through its solid solution strengthening effect, but also suppresses the formation of ferrite during hot forming by improving hardenability. If the Mn content is less than 0.5%, it is difficult to obtain sufficient hardenability, and the insufficient hardenability requires excessive use of other expensive alloy elements, resulting in a significant increase in manufacturing costs. On the other hand, if the Mn content exceeds 4%, the band-like structure aligned in the rolling direction in the microstructure deepens, causing non-uniformity in the internal structure and resulting in reduced impact resistance. Therefore, a Mn content of 4% or less is preferred.

[0032] Phosphorus (P): 0.001 to 0.015% P exists as an impurity in steel, and a minimum content of less than 0.001% is undesirable because it requires a large manufacturing cost, while a maximum content exceeding 0.015% reduces the weldability of hot-formed parts and material properties due to high-temperature grain boundary segregation, so it is preferable that the content does not exceed 0.015%.

[0033] Sulfur (S): 0.0001 to 0.02% Since S acts as an impurity and impairs the ductility, impact properties, and weldability of the member, the maximum content is preferably set to 0.02%. Furthermore, setting the minimum content to less than 0.0001% is not preferable because it would significantly increase manufacturing costs.

[0034] Aluminum (Al): 0.01 to 0.1% Al, together with Si, acts as a deoxidizer during steelmaking to enhance the cleanliness of steel. If the Al content is less than 0.01%, it is difficult to obtain this effect. If the Al content exceeds 0.1%, excessive AlN precipitates formed during the continuous casting process may reduce high-temperature ductility, causing cracks in the slab and resulting in manufacturing problems. Therefore, the upper limit of Al content is preferably 0.1%.

[0035] Chromium (Cr): 0.01 to 1% Cr, like Mn, is added as an element to ensure the hardenability of steel and suppress the formation of ferrite after hot forming. If the Cr content is less than 0.01%, it is difficult to ensure the above effects. If the Cr content exceeds 1%, not only is the hardenability improvement effect relative to the amount added small, but also coarse iron carbides are excessively formed, which may cause cracks when stress is applied, thereby degrading the material. Therefore, it is preferable that the Cr content does not exceed 1%.

[0036] Nitrogen (N): 0.001-0.02% N is contained in steel as an impurity. Setting the N content to less than 0.001% would result in excessive manufacturing costs, and if the N content exceeds 0.02%, like added Al, cracks in the slab due to the formation of AlN are likely to occur. Therefore, it is preferable that the N content does not exceed 0.02%.

[0037] Titanium (Ti): 0.1% or less (including 0) The Ti combines with nitrogen remaining as an impurity in the steel to form TiN, thereby preventing the B, which ensures hardenability, from forming compounds. For this reason, Ti may be further included. Furthermore, the formation of TiC precipitates can be expected to have the effects of precipitation strengthening and grain refinement. However, if the Ti content exceeds 0.1%, a large amount of coarse TiN is formed, degrading the quality of the steel. Therefore, it is preferable that the upper limit of the Ti content does not exceed 0.1%.

[0038] Boron (B): 0.01% or less (including 0) B is an element that can effectively improve hardenability and segregates at prior austenite grain boundaries to suppress brittleness of hot-formed parts caused by grain boundary segregation of impurities P and / or S, so it can be further included. However, if B exceeds 0.01%, Fe 23 The upper limit is preferably 0.01% since the formation of CB6 complex compounds may cause embrittlement during hot rolling.

[0039] The remainder includes iron (Fe) and inevitable impurities. Unavoidable impurities are sometimes unintentionally mixed in during the normal steel manufacturing process, and cannot be completely eliminated. This meaning is easily understood by engineers in the field of normal steel manufacturing. Note that the present invention does not completely exclude the addition of other components than those described above.

[0040] Next, a method for producing a plated steel sheet for hot forming, which is another embodiment of the present invention, will be described in detail. The method for producing the plated steel sheet for hot forming described above is a preferred example, but it is not necessarily required to produce the plated steel sheet in this manner.

[0041] The manufacturing method includes the steps of preparing a base steel sheet; Ni-plating the surface of the prepared base steel sheet; and continuously annealing the Ni-plated base steel sheet and applying an Al-based plating.

[0042] The base steel sheet is as described above, and the process for preparing the base steel sheet involves heating a steel slab having the above-described steel composition, followed by hot rolling, coiling, and cold rolling.

[0043] A steel slab satisfying the above-mentioned composition is heated to 1050 to 1300°C and then hot-rolled. If the heating temperature is less than 1050°C, the steel slab structure is not sufficiently homogenized, making it difficult to redissolve precipitated elements when they are used. If the heating temperature exceeds 1300°C, an excessive oxide layer is formed, which increases the manufacturing cost for removing the oxide layer and increases the possibility of surface defects occurring after hot-rolling.

[0044] The finish rolling temperature of the hot rolling is preferably in the range of 800 to 950°C. If the finish rolling temperature is less than 800°C, rolling in the two-phase region progresses, ferrite is introduced into the surface layer of the steel sheet, and it is difficult to control the sheet shape. On the other hand, if the temperature exceeds 950°C, coarsening of the crystal grains in the hot rolling may occur.

[0045] After hot rolling, coiling is performed at a temperature in the range of 500 to 700°C. If the coiling temperature is less than 500°C, martensite is formed in the entire or partial steel sheet, making it difficult to control the coil shape, and the subsequent cold rolling properties are reduced due to an excessive increase in strength of the hot-rolled steel sheet. On the other hand, if the coiling temperature exceeds 700°C, excessive coarse carbides are formed, which promotes cracking when stress is generated in the hot-formed part, and this may reduce crash resistance.

[0046] This is followed by a cold rolling process. However, before cold rolling, a pickling process can be further carried out to remove surface oxides. If necessary, the steel sheet can be used as a base steel sheet for hot forming by plating without cold rolling.

[0047] The cold rolling reduction rate during the cold rolling is set to obtain a predetermined thickness, and is preferably set to a reduction rate of 30 to 80%.

[0048] The surface of the base steel sheet prepared as described above is subjected to Ni plating. One example of the Ni plating is Ni flash plating. Ni flash plating is a type of electroplating method performed before annealing, and is performed by using a Ni electrolyte and adjusting the amount of current during electroplating. The Ni plating is performed by plating the surface of the base steel sheet with 150 to 2500 mg / m 2 It is preferable to apply the coating amount of 100%.

[0049] The Ni plating coating weight in the above Ni plating is 150 to 2500 mg / m 2 It is preferable that the Ni plating coverage is 150 mg / m 2 If the Ni content is less than 2500 mg / m, Ni cannot be concentrated in the plating layer by the heat treatment for hot forming, and the effect of the present invention cannot be fully exhibited. 2 If it exceeds 2500mg / m, excessive current concentration during electroplating will cause localized unplated areas inside the steel sheet, and the manufacturing cost will increase excessively due to excessive use of electroplating solution for Ni coating. 2On the other hand, a more preferable Ni plating coverage is 200 to 2000 mg / m 2 is preferred.

[0050] The Ni-plated base steel sheet is then continuously annealed and subjected to Al-based plating. The annealing temperature is preferably 740 to 860°C. If the annealing temperature is below 740°C, the recrystallization of the cold-rolled structure is not fully completed, resulting in poor sheet shape or excessively high post-plating strength, which may cause die wear during the blanking process. On the other hand, if the annealing temperature exceeds 860°C, the formation of surface oxides during the annealing process is promoted, which may cause defects on the plated surface during Al-based plating. Furthermore, the annealing heat treatment is preferably performed in a non-oxidizing atmosphere, such as a hydrogen-nitrogen mixed gas. In this case, the dew point of the atmospheric gas is preferably −75 to −20°C. A dew point below −75°C requires additional equipment for dew point temperature control, which increases manufacturing costs. If the dew point exceeds −20°C, annealing oxides may form on the surface of the steel sheet during annealing, resulting in poor surface quality, such as undercoating.

[0051] After the annealing, an Al-based plating is performed. The Al-based plating means an aluminum or aluminum alloy plating, and any plating layer that can be applied to a plated steel sheet for hot forming can be applied to the present invention without any restrictions.

[0052] The Al-based plating is preferably carried out by immersion in an Al-based plating bath, and as a preferred example, the plating bath composition preferably contains 6 to 12 wt % silicon (Si), 1 to 4 wt % iron (Fe), and the remainder aluminum (Al) and unavoidable impurities.

[0053] The coating weight for the above Al-based plating is 20 to 140 g / m per side 2 It is preferable that the plating coverage per side is 20 g / m 2 If the thickness is less than 140 g / m, it is difficult to ensure the desired corrosion resistance of the hot-formed part. 2If the thickness exceeds 1000 nm, not only will the manufacturing cost increase due to the excessive plating amount, but it will also be difficult to uniformly plate the plating layer over the entire width and length of the coil.

[0054] Next, a hot-formed member, which is another embodiment of the present invention, will be described in detail. As a preferred example, the hot-formed member can be obtained by hot forming the above-mentioned plated steel sheet for hot forming.

[0055] The hot-formed member includes a steel substrate and an alloy plating layer formed on the steel substrate. The Ni content of the alloy plating layer is measured from the surface to a depth of 15 μm, and the Ni content at the point with the highest Ni content (Nimax) is preferably 0.1 wt % or more. One method for determining Nimax is to analyze a graph obtained from the results of glow discharge spectroscopy (GDS). If the Ni content in Nimax is less than 0.1 wt %, a Ni-enriched layer is not sufficiently formed, and external hydrogen absorption during hot forming cannot be effectively prevented, making it difficult to ensure hydrogen embrittlement resistance. The higher the Ni content in Nimax, the greater the effect of inhibiting hydrogen absorption. However, an excessive Ni content increases the cost due to increased Ni deposition and may cause environmental problems during welding. Therefore, it is preferable that the Ni content not exceed 0.75 wt %.

[0056] The alloy plating layer is a layer obtained by alloying the Al-based plating layer through mutual reaction of the plating layer components, the base iron, etc. during the heating process due to hot forming, and the present invention does not particularly limit the type of alloy plating layer.

[0057] On the other hand, it is preferable that a Ni-enriched layer is included in the alloy plating layer. There are various methods for identifying the Ni-enriched layer. As a preferred example, a graph of a concentration profile derived by glow discharge optical emission spectrometry (GDS) on the surface of the alloy plating layer in the thickness direction of the component can be used. In the concentration profile graph, the Ni-enriched layer can be defined as the width between the point (Nimax) with the highest Ni content, the first point (Ni) where the slope of the graph becomes 0 in the direction toward the surface of the component, and the point (Nio') where the Ni content is the same as Ni in the depth direction of the component from Nimax.

[0058] Figure 2 is a concentration profile graph obtained by GDS analysis of the Ni content in the hot-formed part of Example 3 in the following Examples. From Figure 2, the Nimax, Ni, and Ni' can be derived, and the Ni-enriched layer can be identified.

[0059] The thickness of the Ni-enriched layer is not particularly limited and can vary depending on the heat treatment conditions during plating and hot forming. As a specific example, the thickness of the Ni-enriched layer is preferably 1 to 15 μm. If the thickness of the Ni-enriched layer is less than 1 μm, it is difficult to ensure sufficient hydrogen embrittlement resistance, and if it exceeds 15 μm, excessive Ni flash plating is required, which can increase costs and cause environmental problems during welding.

[0060] The diffusible hydrogen content of the hot-formed part is preferably 0.3 wt ppm or less. The diffusible hydrogen content of the hot-formed part can be measured using a thermal desortion analysis (TDA) device, specifically by measuring the diffusible hydrogen curve by heating the part to about 400°C at a rate of 20°C / s and maintaining the time until a sufficient diffusible hydrogen peak appears, and integrating the curve.

[0061] Although the difference in alloy composition between the base steel sheet of a plated steel sheet for hot forming and the base iron of a hot-formed member is not large, there may be differences in the microstructure, so it is preferable to distinguish between them. Therefore, in the present invention, the base steel sheet is used in the plated steel sheet for hot forming, and the base iron is used in the hot-formed member. On the other hand, the base steel sheet of a plated steel sheet for hot forming has a ferrite and pearlite structure, but the base iron of a hot-formed member formed through hot forming has martensite as the main phase, and bainite can be formed in part.

[0062] Hereinafter, a method for producing a hot-formed member according to another embodiment of the present invention will be described. To this end, a blank is provided, which includes a base steel sheet and an Al-based plating layer formed on the surface of the base steel sheet, the Al-based plating layer having an average Ni content of 0.05 to 0.35 wt%. As an example of providing the blank, the blank can be produced using the above-mentioned plated steel sheet for hot forming.

[0063] The blank is preferably heated to a temperature above the austenite single-phase region, more specifically, in the Ac3 to 975°C temperature range. If the heating temperature is below Ac3, the presence of untransformed ferrite in the two-phase region makes it difficult to ensure strength and crash resistance. On the other hand, if the heating temperature exceeds 975°C, excessive oxides are formed on the surface of the part, making it difficult to ensure spot weldability and increasing manufacturing costs due to maintaining a high temperature. The blank thus heated is preferably maintained within this temperature range for 1 to 1,000 seconds. If the maintenance time is less than 1 second, uniform temperature distribution across the entire blank is difficult, resulting in material variations at different locations. If the maintenance time exceeds 1,000 seconds, excessive oxides are formed on the surface, making it difficult to ensure spot weldability and increasing manufacturing costs. Ar3 can be calculated using the following formula: Ar3=902-255C+19Si-11Mn-5Cr[℃] (The above C, Si, Mn, and Cr refer to the content of each component.)

[0064] The heated blank is transferred to a press where it is formed and cooled. The cooling rate is preferably 20°C / s or more. If the cooling rate is less than 20°C / s, ferrite phases may be introduced during cooling and formed at grain boundaries, resulting in reduced physical properties such as strength and impact resistance. The blank transfer, forming, and cooling processes are not particularly limited, and methods commonly used in hot forming processes may be applied. [Example]

[0065] The present invention will be described in detail below with reference to examples. The following examples are provided for the purpose of understanding the present invention, but are not intended to interpret the scope of the present invention.

[0066] (Example) First, a steel slab having the steel composition (unit: wt%) shown in Table 1 below was vacuum melted to a thickness of 60 mm. The remainder of the steel composition not listed in Table 1 was Fe and unavoidable impurities. The steel slab thus prepared was heated to 1200°C and maintained for 1 hour, and then hot-rolled. The hot-rolling finish temperature was 900°C, and the slab was coiled at 600°C. Then, cold-rolled steel sheets having a thickness of 1.5 mm were produced.

[0067] The cold-rolled steel sheet was subjected to Ni plating by Ni flash treatment. The Ni plating weights are shown in Table 1. The Ni flash treatment was performed by electroplating using a Ni electrolytic solution, and the amount of current during electroplating was adjusted to control the Ni plating weight.

[0068] The Ni-flash treated cold-rolled steel sheet was annealed and coated at a coating weight of 75 g / m per side at a coating temperature of approximately 660°C using an Al-based coating bath containing 9 wt% Si, 2 wt% Fe, and the remainder being Al and unavoidable impurities. 2 The continuous annealing was carried out at a temperature of 800°C.

[0069] The average Ni content distributed in the Al-based coating layer of the coated steel sheets manufactured as described above was measured and is shown in Table 2. As described above, the Al-based coating layer was measured, including the coating layer and the concentration gradient layer (up to the point where the Fe content in the Al-based coating layer becomes 80 wt% or less), and analyzed using glow discharge optical emission spectrometry (GDS) with GDS850A (model name, manufactured by LECO) DC and RF equipment.

[0070] In addition, to examine the plating adhesion after Ni electroplating, the produced plated steel sheets were visually observed and the major axis length of the area peeled from the steel sheet was measured. The results were evaluated according to the following criteria and shown in Table 2. ○: The major axis length of the detached area is 3 mm or less ×: The longitudinal length of the detached area exceeds 3 mm

[0071] A blank was produced using the above-mentioned plated steel sheet, heated to 900°C and maintained for 6 minutes, and then transferred to a mold within 10 seconds for forming and cooling. Cooling was carried out at a rate of 20-40°C / s. The Ni content at the point with the highest Ni content (Nimax) from the surface of the alloy plating layer to a depth of 15 μm in the hot-formed part produced in this manner was analyzed by GDS as described above. The results are shown in Table 2. The amount of diffusible hydrogen within the part was also measured and is also shown in Table 2.

[0072] [Table 1]

[0073] [Table 2]

[0074] Meanwhile, Fig. 1 is a graph showing the results of GDS analysis of the Fe and Ni contents in the Al-based plating layer of the plated steel sheet for hot forming of the above-mentioned Invention Example 1. Also, Fig. 2 is a graph showing the results of GDS analysis of the Ni contents in the alloy plating layers of the hot-formed members of Invention Examples 1 to 3 and Comparative Example 2. Fig. 3 is a graph showing the maximum Ni contents and diffusible hydrogen contents in the alloy plating layers of the hot-formed members of the above-mentioned Invention Examples 1 to 8 and Comparative Examples 1 and 2.

[0075] As can be seen from the results of Table 2 and Figures 1 to 4 obtained from the above examples, the invention examples within the scope of the present invention have excellent coating adhesion in plated steel sheets, and when these are hot formed, they have excellent hydrogen embrittlement resistance.

[0076] However, in Comparative Examples 1 and 2, which were outside the scope of the present invention and in which no Ni flash plating was applied or only a small amount was applied, it was found that sufficient Ni was not secured even in the alloy plating layer after hot forming, making it difficult to secure the hydrogen embrittlement resistance required in the present invention.

[0077] On the other hand, in Comparative Examples 2 and 3, in which the amount of Ni flash plating was excessive, hydrogen embrittlement resistance was ensured, but it was confirmed that unplated areas occurred due to the burning phenomenon within the steel sheet during plating, resulting in poor plating adhesion.

Claims

1. preparing a base steel sheet; The prepared base steel sheet is coated with a coating weight of 150 to 2500 mg / m ２ Ni plating step; and a step of continuously annealing the Ni-plated cold-rolled steel sheet and plating it with an Al-based coating.

2. The step of preparing the base steel sheet includes: preparing a steel slab containing, in weight percent, C: 0.07 to 0.5%, Si: 0.05 to 1%, Mn: 0.5 to 4%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, Cr: 0.01 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, and the remainder being Fe and unavoidable impurities; heating the steel slab to 1050-1300°C; Finish hot rolling the heated steel slab at a temperature in the range of 800 to 950°C to produce a hot-rolled steel sheet; Coiling the hot-rolled steel sheet at a temperature in the range of 500 to 700°C; and 2. The method for producing a plated steel sheet for hot forming having excellent hydrogen embrittlement resistance according to claim 1, further comprising the steps of pickling the steel sheet after coiling and cold rolling it at a reduction of 30 to 80%.

3. 2. The method for producing a plated steel sheet for hot forming having excellent hydrogen embrittlement resistance according to claim 1, wherein the continuous annealing is carried out in a temperature range of 740 to 860°C and a dew point temperature of -75 to -25°C.

4. bare steel; and An alloy plating layer formed on the base steel, the point (Nimax) where the Ni content is highest in the alloy plating layer is observed from the surface of the alloy plating layer to 15 μm in the depth direction, and the Ni content at the point (Nimax) where the Ni content is highest is 0.1 to 0.75 wt %, The alloy plating layer includes a Ni-enriched layer, and the Ni-enriched layer has a thickness of 1 to 15 μm, The Ni-enriched layer is a width in a concentration profile graph obtained by deriving the Ni content of the alloy plating layer using glow discharge spectroscopy (GDS), between a point (Nio) where the Ni content is highest (Nimax) in the surface direction of the component and a point (Nio') where the Ni content is the same as Ni in the depth direction of the component from Nimax.

5. 5. The hot-formed part having excellent resistance to hydrogen embrittlement according to claim 4, wherein the amount of diffusible hydrogen in the hot-formed part is 0.3 ppm by weight or less.

6. 5. The hot-formed member having excellent hydrogen embrittlement resistance according to claim 4, wherein the main phase of the base steel is martensite.

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