Hot press formed part and fabrication method therefor

Abstract

The present disclosure relates to a hot press formed part having improved hydrogen embrittlement resistance. Specifically, the hot press formed part comprises: a base steel sheet; and a plating layer on at least one surface of the base steel sheet, the hot press formed part including a nanovoid region in which a nano Kirkendall void having an equivalent circular diameter of 5 nm to 100 nm with respect to a thickness-direction cross-section is positioned.

Description

Hot press-formed member and manufacturing method thereof

[0001] The present disclosure relates to a hot press-formed member, and more particularly, to a hot press-formed member having improved hydrogen embrittlement resistance and a method for manufacturing the same.

[0002] Hot press-formed components are primarily used in automotive structural components, aiming to improve passenger protection and fuel efficiency by reducing vehicle weight. They can be used in components requiring high toughness, such as bumpers, doors, and pillar reinforcements. A representative example of hot press-formed technology and hydrogen embrittlement prevention in hot press-formed components is Patent Document 1 (U.S. Publication No. 6,296,805).

[0003] According to the above patent document 1, ultra-high strength can be secured by heating an aluminum-plated steel sheet to 850°C or higher, hot forming it using a press, and then rapidly cooling it to form a martensite structure into the member. When this hot-formed ultra-high strength steel is applied, complex shapes can be easily formed because it is formed at high temperatures, springback can be minimized, and the weight reduction effect due to high strength can be expected.

[0004] However, martensite structure is known to be vulnerable to hydrogen embrittlement, and parts manufactured after hot forming have residual stress from rapid cooling, so there are concerns about delayed fracture due to hydrogen embrittlement as the amount of diffusible hydrogen in the steel increases.

[0005] In addition, since changes in process parameters can cause changes in mechanical properties within the sheet during coil manufacturing, a stress that is less sensitive to changes in specific manufacturing parameters for manufacturing sheets with good mechanical properties and homogeneity and resistance to delayed fracture due to hydrogen embrittlement is required. In particular, since diffusible hydrogen in dislocations or grain boundaries promotes crack formation at stress concentration areas when stress occurs, there is a need for technology development that can reduce the amount of diffusible hydrogen introduced into the steel after hot press forming and increase the fraction of harmless, non-diffusible hydrogen.

[0006] One aspect of the present disclosure is to provide a hot press-formed member having improved hydrogen embrittlement resistance and a method for manufacturing the same.

[0007] The objectives of the present invention are not limited to the above-described content. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall description of the present invention.

[0008] A hot press forming according to the invention comprises a base steel sheet and a plating layer on at least one surface of the base steel sheet, and a nano void region in which nano Kerkendahl voids having a diameter of 5 nm to 100 nm in the thickness direction are located.

[0009] In one specific example, the nano void region may be a region within 20 μm from the interface between the base steel plate and the plating layer toward the plating layer.

[0010] In one specific example, the maximum diameter (D) of the nano Kerkendal void m ) is the minimum diameter (D) in the direction perpendicular to the maximum diameter n ) divided by the diameter ratio (D m / D n) may be between 1.1 and 3.0. Here, the maximum diameter may refer to the major axis length, and the minimum diameter may refer to the minor axis length.

[0011] In one specific example, the number of voids per unit area of ​​the nano Kerkendal void located in the nano void region is 300 to 1000 / 100 ㎛. 2 It could be.

[0012] In one specific example, the unit area of ​​the nano void region is 100 ㎛ based on the cross-section in the thickness direction of the hot press-formed member. 2 The sum of the areas occupied by the above nano Kerkendal voids is 8 ㎛. 2 It could be as follows:

[0013] In one specific example, the hot press-formed member can satisfy the following relationship 1.

[0014] (Relationship 1)

[0015] 0.45 ≤ C nH / C H ≤ 0.80

[0016] In equation 1, C H is the total amount of diffusible and non-diffusible hydrogen (mass ppm) contained in the hot press-formed part, and C nH is the amount of non-diffusible hydrogen (mass ppm) absorbed in the hot press-formed part.

[0017] In one specific example, the C nH / C H can be between 0.55 and 0.65.

[0018] In one specific example, the hot press-formed member can satisfy the following relationship 2.

[0019] (Relationship 2)

[0020] 0.3 ≤ L 2 / 2[1 / (6t l ) +1 / (15.3t b )]×10 11 ≤ 1.0

[0021] In equation 2, L is the thickness (m) of the hot press-formed part, and t b is the break-through time (sec) in the electrochemical hydrogen permeation test, and t l In the same electrochemical hydrogen permeation test, the permeation current density (I) / saturated permeation current density (I) ss ) means the time (sec) at which the value becomes 0.61 to 0.64.

[0022] In one specific example, the steel sheet contains, in wt%, C: 0.03 to 0.5%, Sb: 0.01 to 0.1%, Si: 0.01 to 2%, Al: 0.001 to 1%, Mn: 0.2 to 4%, P: 0.05% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), Ti: 0.15% or less (including 0%), B: 0.0001 to 0.01%, Mo: 1.00% or less (including 0%), V: 1.00% or less (including 0%), Ca: 0.01% or less (including 0%), Nb: 0.1% or less (including 0%), W: 1% or less (including 0%), REM: 0.3% or less (including 0%), and the remainder Fe. and may contain other unavoidable impurities.

[0023] In one specific example, the plating layer may be an aluminum-based plating layer.

[0024] A method for manufacturing a hot press-formed member according to the invention comprises S1) in wt%, C: 0.03 to 0.5%, Sb: 0.01 to 0.1%, Si: 0.01 to 2%, Al: 0.001 to 1%, Mn: 0.2 to 4%, P: 0.05% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), Ti: 0.15% or less (including 0%), B: 0.0001 to 0.01%, Mo: 1.00% or less (including 0%), V: 1.00% or less (including 0%), Ca: 0.01% or less (including 0%), Nb: 0.1% or less (including 0%), W: 1% or less (including 0%), REM: 0.3% or less (including 0%) Step S1) Reheating a steel slab containing iron (Fe), residual iron (Fe), and other unavoidable impurities at 1100 to 1350°C; Step S2) Finish-rolling the reheated steel slab at 800 to 950°C to obtain a hot-rolled steel sheet; Step S3) Cooling the hot-rolled steel sheet to a coiling temperature, wherein the step is a multi-stage cooling including a rapid cooling section having a relatively fast cooling rate and a slow cooling section having a relatively slow cooling rate, to a coiling temperature of less than 500°C; Step S4) Coiling the cooled hot-rolled steel sheet at the coiling temperature; Step S5) Pickling the coiled hot-rolled steel sheet so that the product of the acid concentration and the pickling time becomes 1,000 to 10,000 g / L·s; Step S6) Continuously annealing the pickled steel sheet in two or more temperature zones; Step S7) Plating by immersing the continuously annealed steel sheet in an aluminum-based plating bath; And S8) a step of heat-treating the plated steel plate at 800°C to 950°C for 1 to 600 seconds and then hot press forming.

[0025] In a manufacturing method according to one specific example, step S3) may include the step S3-1) of first cooling the hot-rolled steel sheet to a temperature T1 below Bs at an average cooling rate of 50°C / s or more; S3-2) of second cooling the first-cooled hot-rolled steel sheet to a temperature T2 below Bs and above Ms at an average cooling rate of 20°C / s or more and 30°C / s or less; and S3-3) of third cooling the second-cooled hot-rolled steel sheet to a coiling temperature below 500°C at an average cooling rate of 30°C / s or more.

[0026] In a manufacturing method according to one specific example, during the continuous annealing in step S6), the average heating rate for each temperature zone can be controlled and performed so as to satisfy the following equations 1 and 2.

[0027] [Formula 1]

[0028] 1.0 ℃ / s ≤ Average heating rate of temperature zone 1 ≤ 10.0 ℃ / s

[0029] [Formula 2]

[0030] 0.1 ℃ / s ≤ Average heating rate of temperature zone 2 ≤ 1.0 ℃ / s

[0031] In Equation 1, temperature zone 1 means the section until the steel plate surface temperature reaches 600℃, and in Equation 2, temperature zone 2 means the section until the steel plate surface temperature reaches the maximum temperature from 600℃.

[0032] A hot press-formed member according to the invention can have improved hydrogen embrittlement resistance due to the nano-sized Kerkendall voids (nano Kerkendall voids) acting as irreversible hydrogen trap sites.

[0033] A hot press formed member according to one embodiment may have a reduced diffusible hydrogen ratio and, further, a reduced hydrogen diffusion rate due to nano Kerkendal voids.

[0034] A hot press-formed member according to one specific example can have improved hydrogen embrittlement resistance while suppressing deterioration of mechanical properties of the hot press-formed member, as nano Kerkendal voids act as irreversible hydrogen trap sites.

[0035] A hot press-formed member according to one specific example can have significantly improved hydrogen embrittlement resistance even with a minute amount of voids, as the nano-sized anisotropic Kirkendall voids irreversibly trap hydrogen.

[0036] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0037] Figure 1 is a transmission electron microscope photograph observing a nano-void region of a hot press-formed member manufactured according to an embodiment.

[0038] Figure 2 is a drawing showing a graph of the hydrogen desorption rate of a hot press-formed part manufactured according to an implementation example.

[0039] Figure 3 is a drawing showing the results of an electrochemical hydrogen permeation test of a hot press-formed part manufactured according to an implementation example.

[0040] Hereinafter, preferred embodiments of the present invention will be described with reference to the attached drawings. However, the embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0041] In addition, the embodiments of the present invention are provided to more completely explain the present invention to a person having average knowledge in the relevant technical field.

[0042] The shape and size of elements in the drawing may be exaggerated for clearer explanation.

[0043] 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.

[0044] 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.

[0045] Unless otherwise specified in the specification of the present invention, the % unit means weight %.

[0046] In this specification, terms such as 'top', 'upper part', 'top surface', 'bottom', 'lower part', 'bottom', 'side', etc. are based on the drawings, and in reality, they may vary depending on the direction in which the elements or components are arranged.

[0047] Additionally, throughout the specification, when we say that a part is 'connected' to another part, this includes not only cases where it is 'directly connected', but also cases where it is 'indirectly connected' with other elements in between.

[0048] 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.

[0049] The terms used herein are for the purpose of describing the invention and are not intended to limit the invention. Furthermore, the singular forms used herein also include the plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0050] The meaning of 'comprising' as used in the specification is to specify a configuration and not to exclude the presence or addition of other configurations.

[0051] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0052] The present inventors have observed that the amount of diffusible hydrogen absorbed during the heat treatment of a hot press-formed component directly affects hydrogen retardation. Specifically, during the heating step of a blank having a plating layer made of aluminum or an aluminum alloy during hot press forming, water vapor present within the heat treatment furnace is adsorbed onto the surface of the blank. At this time, if the temperature within the heat treatment furnace is sufficiently high to melt the plating layer, a portion of the surface of the aluminum plating layer remains molten, resulting in explosive generation of hydrogen and absorption into the steel through the following reaction formula.

[0053] Reaction formula

[0054] 2Al(l) + 3H20 (g) → Al2O3(s) + 3H2(g)

[0055] Once an Al2O3 layer begins to form on the surface, the reaction between molten aluminum and water vapor decreases, suppressing hydrogen generation. However, the Al2O3 oxide layer formed at this time acts as a barrier, preventing hydrogen from being released back out of the steel. Consequently, a significant amount of hydrogen remains in the steel, increasing the risk of hydrogen embrittlement and delayed fracture. Therefore, inhibiting hydrogen diffusion into the steel during heat treatment and utilizing irreversible trapping sites are crucial factors for improving hydrogen embrittlement resistance.

[0056] The inventors of the present invention have confirmed that when a certain level of Kerkendall voids are formed within a plating layer to act as irreversible hydrogen trap sites, hydrogen penetrating into steel can be effectively reduced.

[0057] However, when a large amount of Kerkendal voids are formed to obtain the effect of reducing hydrogen penetration into the steel, there is a risk that the plating layer may peel off or cracks may form during hot press forming, resulting in deterioration of the corrosion resistance of the paint. Therefore, it was confirmed that there is a limit to the amount of Kerkendal voids formed, and accordingly, it was confirmed that there is a limit to the degree of reducing hydrogen penetration into the steel using Kerkendal voids.

[0058] As a result of conducting in-depth research to solve this problem, the inventors of the present invention discovered that when nano-sized ultra-fine Kerkendall voids are formed instead of conventional macro-sized Kerkendall voids and used as hydrogen trap sites, the mechanical properties deterioration caused by Kerkendall voids can be prevented and excellent hydrogen penetration reduction effects can be achieved simultaneously, and the inventors established a technology for forming nano-sized ultra-fine Kerkendall voids (nano Kerkendall voids) and completed the present invention.

[0059] Based on the above-described findings, a hot press formed member according to the invention is described.

[0060] A hot press-formed member according to the invention is a hot press-formed member including a base steel sheet and a plating layer located on at least one surface of the base steel sheet, and includes a nano void region in which nano Kerkendahl voids having a circular diameter of 5 nm to 100 nm are located based on a cross-section in the thickness direction of the hot press-formed member.

[0061] Nano-Kerkendal voids, with their ultra-fine size, exhibit a greater surface area per volume than macro-Kerkendal voids, allowing them to exhibit a more powerful reduction in hydrogen penetration into steel and a slower diffusion rate of hydrogen within the steel. Furthermore, they are free from the risk of void-induced deterioration of the mechanical properties of the plating layer, allowing for simultaneous enhancement of hydrogen embrittlement resistance and prevention of deterioration of the mechanical properties of the plating layer.

[0062] First, the alloy composition of the exemplary base steel plate of the present invention will be described below, but the alloy composition of the base steel plate is not particularly limited. In the present invention, when each element is expressed in content, % means weight % unless specifically limited.

[0063] Carbon (C): 0.03~0.50 wt%

[0064] Carbon (C) is an element that increases the strength of a member and improves hardenability, and is an essential element for strength control, so it must be added appropriately. If the C content is less than 0.03%, the hardenability is low, making it difficult to secure sufficient strength, making it difficult to apply it to hot-pressed members. Therefore, it must be added in an amount of 0.03% or more. In addition, if the content exceeds 0.50%, the strength may increase excessively, causing brittleness and deteriorating weldability. Therefore, the upper limit is set to 0.50% or less. More advantageously, the lower limit of the C content may be 0.07%, or the upper limit of the C content may be 0.40%.

[0065] Silicon (Si): 0.01 to 2.00 wt%

[0066] Silicon (Si) must be added as a deoxidizer in steelmaking, and as a solid solution strengthening element and an element that suppresses carbide formation, it is effective in uniformizing internal structures, and it contributes to increasing the strength of hot press-formed parts and is added as an effective element for material uniformity. However, if the content is less than 0.01%, not only cannot the above effect be expected, but the process cost for controlling the Si content is excessively incurred, and if the Si content exceeds 2%, the plating and weldability are significantly reduced due to excessive Si oxides formed on the surface of the steel sheet during annealing. Therefore, the addition is made in an amount of 2% or less. More advantageously, it can be 0.1% or more, or the upper limit of the Si content can be 1.5%.

[0067] Manganese (Mn): 0.20~4.00 wt%

[0068] Manganese (Mn) is necessary to secure the desired strength due to the solid solution strengthening effect, and to suppress ferrite formation during hot press forming by improving hardenability. If the Mn content is less than 0.2%, it is difficult to obtain sufficient hardenability, and other expensive alloying elements are excessively required to compensate for the insufficient hardenability, which causes a significant increase in manufacturing cost. In addition, if the Mn content exceeds 4%, the band-like structure arranged in the rolling direction in the microstructure is aggravated, causing non-uniformity of the internal structure and thereby deteriorating the crash resistance. Therefore, the Mn content is added in an amount of 4% or less. More advantageously, the lower limit of the Mn content can be 0.5%, or the upper limit of the Mn content can be 2.5%.

[0069] Phosphorus (P): 0.05 wt% or less (excluding 0%)

[0070] Phosphorus (P) is an unavoidable impurity found in steel and is a major factor in reducing steel workability through segregation. Therefore, it is desirable to keep its content as low as possible.

[0071] In theory, it is advantageous to limit the content of P to 0%, but since controlling the content of P to less than 0.0001% requires excessive manufacturing costs, the lower limit may be set to 0.0001%. However, since there is a concern that processability may deteriorate if the content exceeds 0.05%, the upper limit of P may be limited to 0.05%. However, more advantageously, the lower limit of the P content may be 0.0005%, or the upper limit of the P content may be 0.013%.

[0072] Sulfur (S): 0.02 wt% or less (excluding 0%)

[0073] Sulfur (S) is an unavoidable impurity in steel. It combines with manganese (Mn) and other elements to form non-metallic inclusions, which reduces the steel's workability. Therefore, it is desirable to keep its content as low as possible.

[0074] In theory, it is advantageous to limit the S content to 0%, but controlling the S content to less than 0.0001% requires excessive manufacturing costs, so the lower limit may be set to 0.0001%. However, there is a concern that processability may deteriorate if the content exceeds 0.02%, so the upper limit of S may be limited to 0.02%. However, more advantageously, the lower limit of the S content may be 0.0003%, or the upper limit of the S content may be 0.0012%.

[0075] Aluminum (Al): 0.001 to 1 wt%

[0076] Aluminum (Al) is an element added to molten steel for deoxidation, and some of it remains in the steel after deoxidation. If the Al content exceeds 1%, it causes an increase in oxide and nitride inclusions in the steel, which deteriorates the formability of the steel sheet. On the other hand, if the Al content is less than 0.001%, it is economically disadvantageous because it causes unnecessary increases in refining costs due to excessive reduction in Al. Advantageously, Al can be included at 0.001 to 0.1%, and more advantageously, the lower limit of the Al content can be 0.005%, or the upper limit of the Al content can be 0.05%.

[0077] Nitrogen (N): 0.02 wt% or less (excluding 0%)

[0078] Nitrogen (N) is an unavoidable impurity in steel. It combines with aluminum and other elements to form nitrides, which can impair the steel's workability. Therefore, it is desirable to keep its content as low as possible. In practice, N can range from 0.0001% to 0.02%.

[0079] Titanium (Ti): 0.15 wt% or less (including 0%)

[0080] Titanium (Ti) combines with nitrogen remaining as an impurity in steel to form TiN, thereby protecting B from forming compounds that are necessary for securing hardenability. Therefore, it can be selectively added to secure the aforementioned effects, and the formation of TiC precipitates due to the addition of Ti can be expected to have precipitation strengthening and grain refinement effects. However, if the content exceeds 0.15%, a large amount of coarse TiN is formed, which degrades the material quality of the steel, so the upper limit is set at 0.1%.

[0081] Boron (B): 0.0001 to 0.01 wt%

[0082] Boron (B) is an element that can effectively improve hardenability, and is an element that can suppress embrittlement of hot-formed parts due to grain boundary segregation of P or / and S, which are impurities, by segregating at the grain boundaries of old austenite. In order to sufficiently obtain the desired effect, it is preferable that the concentration of B be contained at 0.0001% or more. In addition, if the content exceeds 0.01%, embrittlement may be caused in hot rolling due to the formation of Fe23CB6 complex compounds, so the upper limit is set to 0.01%. Advantageously, B may be contained in an amount of 0.0005 to 0.003%, and more advantageously, the lower limit of the B content may be 0.001%, or the upper limit of the B content may be 0.003%.

[0083] Molybdenum (Mo): 1.00 wt% or less (including 0%)

[0084] Molybdenum (Mo) is an element that can effectively improve hardenability and can be optionally included. To prevent excessive manufacturing costs, the upper limit of the amount added may be 1.00%. Meanwhile, the present invention includes a case where the Mo content is 0%, as the intended physical properties can be secured without significant problems even if Mo is not included. However, it should be noted that when adding Mo, it is effective to add it at a minimum of 0.01%.

[0085] Vanadium (V): 1.00 wt% or less (including 0%)

[0086] Vanadium (V) is an element that can improve hydrogen embrittlement resistance by forming carbides and reducing grain boundaries, and can be optionally included. In order to suppress the problem of excessive manufacturing costs, the upper limit of the amount added can be 1.00%.

[0087] Calcium (Ca): 0.01 wt% or less (including 0%)

[0088] Calcium (Ca) is an element added to improve steel cleanliness by removing a certain level of P and S during steelmaking, and can be optionally included. In order to suppress the problem of excessive manufacturing costs, the upper limit of the amount added can be 0.01%.

[0089] Neobium (Nb): 0.1 wt% or less (including 0%)

[0090] Neobium (Nb) is an element that can improve hydrogen embrittlement resistance by forming carbides and reducing grain boundaries, and can be optionally included. In order to suppress the problem of excessive manufacturing costs, the upper limit of the amount added can be 0.1%.

[0091] Tungsten (W): 1 wt% or less (including 0%)

[0092] Tungsten (W) is an element that can improve hydrogen embrittlement resistance by forming carbides and making grain boundaries fine, and can be optionally included. In order to suppress the problem of excessive manufacturing costs, the upper limit of the amount added can be 1%.

[0093] Antimony (Sb): 0.01~0.1 wt%

[0094] When antimony (Sb) is added, in the plating layer of a hot press-formed part, a region where the Sb concentration is concentrated may exist in the surface region from the interface between the base steel sheet and the plating layer toward the plating layer. As this concentrated region is formed, the diffusion speed of aluminum (Al) and iron (Fe), which diffuse together during high-temperature heat treatment, becomes non-uniform, which promotes the formation of Kerkendahl voids in the plating layer after hot press forming. On the other hand, if the Sb content is less than 0.01%, the concentrated layer is not sufficiently formed, and thus the promotion of the formation of the Kerkendahl voids described above may not sufficiently occur. On the other hand, if the Sb content exceeds 0.1%, not only does the cost of the alloy steel increase, but the formation of voids in the plating layer may become excessive.

[0095] Rare earth elements (REM): 0.3 wt% or less (including 0%)

[0096] Rare earth metals (REM) are elements that can be included in steelmaking, and may be included optionally. However, if the content of REM exceeds 0.3%, REM may segregate excessively at grain boundaries, degrading hydrogen embrittlement resistance. Therefore, the content of REM may be limited to 0.3% or less.

[0097] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0098] The type of plating layer is not particularly limited, and any plating layer that is applied to a conventional hot-forming plating steel sheet can be applied to the present invention without limitation.

[0099] However, as an example, the plating layer may be an aluminum-based plating layer, and the aluminum-based plating layer may be aluminum or an aluminum alloy. As an example, the plating layer may include, by weight %, Si: 6 to 12%, Fe: 4% or less (including 0%), the remainder Al, and other unavoidable impurities. Or, as another example, the plating layer may be an aluminum-iron alloy plating layer, and as an example, the plating layer may include, by weight %, Fe: 8 to 62%, the remainder Al, and other unavoidable impurities. As another example, in the case of a hot-dip plating layer, a hot-dip aluminum plating layer, a hot-dip Al-Si plating layer, a hot-dip Al-Si-Mg plating layer, a hot-dip Al-Zn plating layer, a hot-dip Al-Mg plating layer, etc. may be applied. As an alloyed hot-dip plating layer, an alloyed hot-dip aluminum plating layer, an alloyed hot-dip Al-Si plating layer, an alloyed hot-dip Al-Si-Mg plating layer, an alloyed hot-dip Al-Zn plating layer, an alloyed hot-dip Al-Mg plating layer, etc. are exemplified. The plating layer may include at least one of Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, Sr, Mg, Be, Li, and Na. The thickness of the plating layer may be, for example, 5 to 50 ㎛, but is not necessarily limited thereto, and any thickness commonly applied in the art may be sufficient.

[0100] A hot press-formed member may include a nano void region. The nano Kerkendal voids located within the nano void region may have a circular diameter of 5 nm to 100 nm based on a cross-section in the thickness direction of the hot press-formed member (hereinafter collectively referred to as the member cross-section).

[0101] When the nano Kerkenthal void diameter is excessively small, less than 5 nm, it cannot effectively perform the role of a hydrogen trap, so the improvement effect of hydrogen embrittlement resistance may be minimal. On the other hand, when the nano Kerkenthal void diameter exceeds 100 nm, it increases the possibility that the trapped atomic-state diffusible hydrogen will react and generate molecular-state hydrogen gas. Therefore, due to the volume of the generated hydrogen molecules, not only does the Kerkenthal void not effectively perform hydrogen trapping, but it also causes plating peeling due to hydrogen blistering in a hydrogen permeation environment, and there is a risk that partial plating peeling or cracking may occur during hot press forming, and the corrosion resistance of the paint may deteriorate.

[0102] For effective hydrogen trapping action, suppression of hydrogen molecular gas generation, and prevention of plating peeling or crack generation, the diameter (circular diameter) of the nano Kerkendal voids based on the cross-section of the member may be 10 to 80 nm, advantageously 10 to 60 nm, and more advantageously 15 to 50 nm.

[0103] The nano-void region may be a region within 20 ㎛ from the interface between the base steel sheet and the plating layer toward the plating layer. The region within 20 ㎛ toward the plating layer is an interface-adjacent region. If Kerkendall voids having a size of several hundred nanometers to several micrometers are located in this region, it is difficult to suppress peeling or crack formation in the plating layer during hot press forming, and in particular, as the forming curvature becomes severe, peeling or crack formation in the plating layer inevitably becomes severe. However, if nano-Kerkendall voids having an ultra-fine size of 5 nm to 100 nm, advantageously 10 to 80 nm, more advantageously 10 to 60 nm, and even more advantageously 15 to 50 nm in diameter are located in the region within 20 ㎛ toward the plating layer, peeling or crack formation in the plating layer can be effectively prevented even when the forming curvature becomes severe.

[0104] The number of voids per unit area of ​​nano-Kerkendal voids located in the nano-void region (hereinafter, nano-Kerkendal void density) is 300 to 1000 / 100㎛. 2 It can be 350 to 900 / 100㎛, advantageously 2 , more advantageously 350 to 800 / 100㎛ 2 This nano-Kerkendal void density is a density that can completely prevent the infiltration of diffusible hydrogen into the steel by trapping diffusible hydrogen that penetrates into the steel, and at the same time, it is a density that can prevent the voids from being interconnected and increasing in size due to the formation of an excessive number of nano-Kerkendal voids.

[0105] The minimum diameter of the Kerkendal voids used to prevent hydrogen penetration into the steel is approximately 500 nm, and the density of Kerkendal voids (the number of voids per unit area) required to prevent hydrogen penetration is at least 300 / 100 μm. 2 It is a level. Considering this, 100㎛ 2 The total void area occupied by the Kirkendall void in the area (cross-sectional area) is approximately 58 ㎛. 2 At the level of 100 ㎛ 2 More than half of the area is occupied by voids.

[0106] On the other hand, the hot press formed member according to one specific example has a maximum void size and maximum void density of 100 ㎛, even assuming the maximum void size and maximum void density of the nano Kerkendahl void. 2 The total void area occupied by nano voids in the area (cross-sectional area) is 8 ㎛. 2 It is only 0.005 ㎛ or less, practically 2 8 ㎛ 2 It could be.

[0107] By significantly reducing the total nano-Kerkendal void area compared to the conventional one, the problem of deterioration of corrosion resistance of the component due to plating peeling or micro-cracks occurring due to stress applied during hot press forming can be fundamentally prevented.

[0108] In an advantageous example, a nano-Kerkendal void may be anisotropic. That is, among the diameters across the center of a nano-Kerkendal void, at least one diameter is larger than the other diameters, or at least one diameter is smaller than the other diameters. In a specific example, based on the cross-section of the member, the nano-Kerkendal void may be rod-shaped.

[0109] In detail, based on the absence cross-section, the maximum diameter of the nano-Kerkendal void (D m ) is the minimum diameter (D n ) divided by the diameter ratio (D m / D n ) may have an average value of 1.1 to 3.0, specifically 1.1 to 2.5, more specifically 1.2 to 2.0, and even more specifically 1.2 to 1.8.

[0110] The anisotropy of nano-Kerkendal voids increases the surface area of ​​the nano-voids per nano-void volume along with the miniaturization (nanoization) of the voids, allowing the nano-Kerkendal voids to trap diffusing hydrogen more effectively.

[0111] Here, the absence cross-section may mean a cross-section of a member in the thickness direction of the base steel plate of a hot press-formed member including a base steel plate and a plating layer provided on at least one surface of the base steel plate.

[0112] Here, the diameter of the nano Kerkendal void may be the diameter of a circle (circle conversion diameter) when converted to a circle having the same area as the area of ​​the nano Kerkendal void in the absence cross-section.

[0113] Here, the anisotropy of the nano-Kirkendal void (D m / D nThe average value of) is the maximum diameter (D) among the diameters passing through the center of a nano Kerkendal void for a single nano Kerkendal void. m ) and find the diameter in the direction perpendicular to the maximum diameter, which is the minimum diameter (D n ) is obtained by D m D n It can be calculated by dividing by the diameter ratio and calculating the average of the obtained diameter ratios.

[0114] Experimentally, the circular diameter of the nano-Kerkendal void, void density, anisotropy of the nano-Kerkendal void, etc. can be calculated through electron microscope images of the nano-void region observed using a transmission electron microscope, a scanning electron microscope, etc.

[0115] In detail, a cross-section of a tissue including a nano-void region within 20 ㎛ in the direction of the plating layer at the interface between the base steel sheet and the plating layer (a specimen is collected using a focused ion beam in a direction perpendicular to the rolling direction) is observed with a transmission electron microscope, and a transmission electron microscope photograph is obtained, and a part with black contrast existing in the observed image can be specified as a Kirkendall void. Kirkendall voids and fine precipitates can be distinguished by the contrast of the image, but they can also be distinguished by component analysis using an elemental analysis device attached to an electron microscope (e.g., TEM-EDS). That is, if the content of Ti, Nb, V, Mo, Cr, etc., which are components of fine precipitates other than carbon or iron, is less than 1 wt% in component analysis using point analysis, etc., it can be distinguished as a Kirkendall void.

[0116] D m / D nThe 'average' of the average value, etc. can be calculated by observing 30 or more, practically 50 or more, more practically 100 or more, and practically 200 or less nano Kerkendahl voids in transmission electron microscope observation photographs of the nano void region, and averaging the measured values ​​for each nano Kerkendahl void.

[0117] The void density was measured using a transmission electron microscope with a total density of 100 ㎛. 2 1000 ㎛ 2 Area, specifically a total of 200 ㎛ 2 1000 ㎛ 2 After observing the nano void area reaching an area of ​​100㎛, the number of nano Kerkendal voids that enter the observation area completely was measured. 2 It can be calculated by taking the value per area.

[0118] At this time, the interface between the base steel sheet and the plating layer can be distinguished by the difference in contrast through observation with a scanning electron microscope or transmission electron microscope when collecting a sample using a focused ion beam, but it can also be distinguished by component analysis using SEM-EDS (Scanning Electron Microscope - Energy Dispersive Spectrometer), TEM-EDS (Transmission Electron Microscope - Energy Dispersive Spectrometer), EPMA (Electron Probe Micro Analyzer), GDS (Glow Discharge Spectrometer), etc. That is, when the components are measured across the base steel sheet - plating layer using a line profile, etc., the interface between the base steel sheet and the plating layer can be determined as the point where the Al content is 2 wt%, and if it exceeds 2%, it can be distinguished as a plating layer, and if it is less than 2%, it can be distinguished as base iron. As a practical example, the interface between the base steel and the plating layer may be at a point where Al is 2 wt% in the Al line profile from the plating layer to the base steel based on Glow Discharge Spectrometer (GDS) analysis.

[0119] In one specific example, the hot press formed member can satisfy the following relationship 1.

[0120] 0.45 ≤ C nH / C H ≤ 0.80

[0121] In equation 1, C H is the total amount of diffusible and non-diffusible hydrogen (mass ppm) contained in the hot press-formed part, and C nH is the amount of non-diffusible hydrogen (mass ppm) absorbed in the hot press-formed part.

[0122] Experimentally, C defined from relation 1 H and C nHIt can be a value calculated from a hydrogen desorption rate graph obtained when heating to 600℃ at a heating rate of 100 to 300℃ / hour using a quadrupole mass spectrometer. The hydrogen desorption rate graph is a graph with the y-axis of the hydrogen desorption rate (mass ppm / sec) and the x-axis of the elapsed time during heating (sec). At this time, it goes without saying that a hydrogen desorption rate graph according to the heating temperature can be obtained by converting the axes by multiplying the elapsed time during heating (sec) and the heating rate (℃ / s).

[0123] In detail, the total amount of hydrogen (C) is the sum of the diffusible and non-diffusible hydrogen amounts. H , mass ppm) can be defined as the area (area below) of the graph of hydrogen desorption rate versus elapsed time during heating when heated to 600°C.

[0124] Non-diffusible hydrogen content (C nH) can be defined as the area in the range of boundary temperature (℃) to 600℃ among the graph of hydrogen desorption rate according to the elapsed time during heating obtained through a quadrupole mass spectrometer, and the amount of diffusible hydrogen (CdH) can be defined as the area in the range of room temperature (℃) to boundary temperature (℃). At this time, the boundary temperature dividing diffusible hydrogen and non-diffusible hydrogen is the point dividing the peak in the low-temperature region (T < 300℃) and the peak in the high-temperature region (T > 300℃) that appear in the hydrogen desorption rate graph, and can be defined as the point where the slope of the hydrogen desorption rate graph within the range of 220℃ to 330℃, which is the region between the low-temperature peak and the high-temperature peak, is substantially 0, that is, the lowest point between the adjacent low-temperature region peak and the high-temperature region peak. At this time, for the sake of clarity of understanding, the boundary dividing diffusible hydrogen and non-diffusible hydrogen was described based on temperature, but the boundary temperature (℃) was converted to boundary time (sec) by the formula of temperature (℃) = elapsed time during heating (sec) X heating rate (℃ / s), so that the area of ​​the hydrogen desorption rate graph from 0 sec to the boundary time (sec) was the amount of diffusible hydrogen (CdH), and the area of ​​the hydrogen desorption rate graph from the boundary time (sec) to the end of analysis time (sec) reaching 600℃ was the amount of non-diffusible hydrogen (C nH ) is of course produced.

[0125] Relationship 1 represents the ratio of non-diffusible hydrogen to the total amount of hydrogen. C nH / C H A value of less than 0.45 means that the Kerkendal void within the absence has not been formed to a certain level. nH / C H When C is less than 0.45, it is difficult to effectively trap hydrogen that penetrates during hot press forming due to undeveloped Kerkendal voids, which may cause a problem of high diffusible hydrogen content in the steel and deterioration of hydrogen embrittlement resistance of the material. On the other hand, C defined by relational expression 1 nH / C HIf the value exceeds 0.80, it means that Kerkendal voids are excessively formed, and there is a risk that Kerkendal voids will be over-generated and over-grow, resulting in coarsening of the Kerkendal voids. In this respect, C nH / C H may be 0.45 to 0.80, advantageously 0.50 to 0.75, more advantageously 0.55 to 0.70, and even more advantageously 0.55 to 0.65.

[0126] In one specific example, the hot press formed member can satisfy the following relationship 2.

[0127] (Relationship 2)

[0128] 0.3 ≤ L 2 / 2[1 / (6t l ) +1 / (15.3t b )]×10 11 ≤ 1.0 [unit is m 2 / s]

[0129] In equation 2, L is the thickness (m) of the hot press-formed part, and t b is the break-through time (sec) in the electrochemical hydrogen permeation test, and t l In the same electrochemical hydrogen permeation test, the permeation current density (I) / saturated permeation current density (I) ss ) means the time (in seconds) at which the value becomes 0.6100 to 0.6400, practically 0.6200 to 0.6350, and more practically 0.6299.

[0130] t defined from relation 2 l and t bis the time obtained through calculations of the time-lag method and breakthrough method in a hydrogen permeation test using the Devanathan-Stachurski double cell (also called the Devanathan-Stachurski battery), which is an electrochemical hydrogen permeation test method, and is expressed as I / I in a hydrogen permeation curve (y-axis permeation current density - x-axis time). ss The breakout time is the x-axis intercept of the tangent at the point where the slope is 0.610 to 0.640, practically 0.625 to 0.635, and more practically 0.63.

[0131] Experimentally, a 1 M NaOH solution was added to the hydrogen measurement cell among the two types of cells to create an anodic polarization state. Ideally, 0 coverage should be achieved, but since complete 0 coverage is difficult in reality, in this case I = 0.3 μA / cm 2 The time when it becomes can be defined as the anode polarization state. After this, 30g / L NaCl + 3g / L NH4SCN aqueous solution is added to the charging cell and 1.5mA / cm 2 By maintaining a constant coverage, the cathodic polarization state is created. At this time, the point at which constant coverage is achieved is set as the start time of the hydrogen permeation test. After a certain period of time has elapsed, when the current density value measured in the hydrogen measurement cell becomes saturated, this is called the saturated permeation current density, I SS is defined as . However, as the penetration test time elapses, hydrogen molecules are generated within the measurement cell, so I SS The initial I may vary as the value may vary. SS Value I SS is defined as I / I in the measured hydrogen permeation curve. SSt at the point where it is between 0.610 and 0.640, practically between 0.625 and 0.635, more practically 0.63 l can be obtained by t b is I SS From the point of applying constant coverage to the charging cell, I can measure the permeation rate even in an unstable hydrogen permeation curve. SS Within the range of elapsed time to reach t, ​​the point where the slope of the hydrogen permeation curve is maximum can be found, and tb can be found by calculating the x-axis (elapsed time) intercept of the tangent line at this time. Two hours t l , t b are values ​​corresponding to the hydrogen diffusion rate, respectively. The smaller the value, the faster the hydrogen diffusion rate, and the larger the value, the slower the hydrogen diffusion rate.

[0132] L defined from relation 2 2 / 2(1 / 6t l + 1 / 15.3t b )×10 11 (m 2 / s) is 0.3 m 2 If it is less than / s, Kerkendal voids are excessively formed, and there is a risk that Kerkendal voids will be over-generated and over-grow, resulting in coarsening. Such coarsening of Kerkendal voids may cause plating peeling or cracking due to stress applied during hot press hardening, thereby deteriorating the corrosion resistance of the component. On the other hand, L defined by Equation 2 2 / 2(1 / 6t l + 1 / 15.3t b )×10 11 (m 2 / s) has a value of 1.0 m 2If / s is exceeded, it is difficult for the Kirkendall void to effectively trap hydrogen that penetrates into the steel during hot press forming, which may increase the amount of diffusible hydrogen in the steel and cause a problem of deterioration in the hydrogen embrittlement resistance of the material. In this respect, L defined by Equation 2 2 / 2(1 / 6t l + 1 / 15.3t b )×10 11 (m 2 The value of / s) is 0.3 to 1.0 m 2 / s, preferably 0.4 to 1.0 m 2 / s, more advantageously 0.5 to 0.9 m 2 / s, more advantageously more advantageously 0.6 to 0.9 m 2 It could be / s.

[0133] Although not particularly limited, in a hot press-formed member according to one specific example, the microstructure of the base steel plate may include at least one selected from the group consisting of martensite and bainite, or may be formed of at least one selected from the group consisting of martensite and bainite.

[0134] The present invention includes a method for manufacturing the hot press-formed member described above.

[0135] A method for manufacturing a hot press-formed member according to the present invention comprises the steps of: S1) reheating a steel slab at 1100 to 1350°C; S2) finish-rolling the reheated steel slab at 800 to 950°C to obtain a hot-rolled steel sheet; S3) cooling the hot-rolled steel sheet to a coiling temperature, wherein the cooling is performed through multi-stage cooling including a rapid cooling section having a relatively fast cooling rate and a slow cooling section having a relatively slow cooling rate, to a coiling temperature of less than 500°C; S4) coiling the cooled hot-rolled steel sheet at the coiling temperature; S5) pickling the coiled hot-rolled steel sheet so that the product of the acid concentration and the pickling time becomes 1,000 to 10,000 g / L·s; S6) continuously annealing the pickled steel sheet in two or more temperature zones; S7) plating the continuously annealed steel sheet by immersing it in an aluminum-based plating bath; And S8) a step of heat-treating the plated steel plate at 800°C to 950°C for 1 to 600 seconds and then hot press forming.

[0136] Below, each step is explained in more detail.

[0137] S1) Reheating of steel slabs

[0138] First, the steel slab can be reheated at 1100 to 1350°C. If the reheating temperature is below 1100°C, the slab structure will not be sufficiently homogenized, making it difficult to remelt the steel using precipitated elements. Conversely, if the reheating temperature exceeds 1350°C, an excessive oxide layer will form, increasing manufacturing costs for its removal and increasing the likelihood of surface defects after finish rolling.

[0139] Steel slabs shall contain, in wt%, C: 0.03 to 0.5%, Sb: 0.01 to 0.1%, Si: 0.01 to 2%, Al: 0.001 to 1%, Mn: 0.2 to 4%, P: 0.05% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), Ti: 0.15% or less (including 0%), B: 0.0001 to 0.01%, Mo: 1.00% or less (including 0%), V: 1.00% or less (including 0%), Ca: 0.01% or less (including 0%), Nb: 0.1% or less (including 0%), W: 1% or less (including 0%), REM: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities. It may, but is not necessarily limited to,

[0140] S2) Finish rolling

[0141] Reheated steel slabs can be finished rolled at 800 to 950°C to obtain hot-rolled steel sheets. If the finishing rolling temperature is below 800°C, abnormal rolling occurs, introducing ferrite into the surface layer of the steel sheet and making it difficult to control the sheet shape. Conversely, if the finishing rolling temperature exceeds 950°C, hot-rolled grain coarsening can occur.

[0142] S3) and S4) Cooling and coiling of hot rolled steel sheets

[0143] After manufacturing a hot-rolled steel sheet by final rolling, the hot-rolled steel sheet can be cooled to a coiling temperature of less than 500°C. Cooling of the hot-rolled steel sheet can be performed through multi-stage cooling, including a rapid cooling section with a relatively fast cooling rate and a slow cooling section with a relatively slow cooling rate.

[0144] In detail, the cooling of the hot-rolled steel sheet may be performed including the steps of S3-1) first cooling the hot-rolled steel sheet to a temperature T1 lower than or equal to Bs at an average cooling rate of 50°C / s or more; S3-2) second cooling the first-cooled hot-rolled steel sheet to a temperature T2 lower than or equal to Bs and higher than Ms at an average cooling rate of 20°C / s or more and 30°C / s or less; and S3-3) third cooling the second-cooled hot-rolled steel sheet to a coiling temperature lower than 500°C at an average cooling rate of 30°C / s or more.

[0145] Through primary cooling, the manufactured hot-rolled steel sheet can be quickly cooled below the temperature (Bs) at which bainite begins to form, thereby suppressing the formation of ferrite (granular ferrite).

[0146] When performing primary cooling at a temperature below Bs after completing hot rolling, if the cooling rate is less than 50 ℃ / s, there is a problem of excessive formation of ferrite phase during cooling. In this case, the upper limit of the primary cooling rate is not specifically limited, but since there is a concern that the shape of the steel plate may be distorted if it is cooled too excessively, it can be limited to 200 ℃ / s or less.

[0147] There is no particular limitation on the lower limit of the cooling end temperature (T1) during the first cooling, but if it is excessively low, there is a concern that the cooling time during the subsequent second cooling may not be sufficient, so it may be limited to Bs-100℃.

[0148] When the temperature of the hot-rolled steel sheet becomes Bs or lower through the above primary cooling, the steel cooling is terminated, and secondary cooling can be performed at a cooling rate of 25 ℃ / s or lower to a temperature (T2) higher than Ms and lower than Bs.

[0149] During the secondary cooling according to the conditions described above, the temperature of the steel sheet may rise due to transformation heat generated by the bainite phase transformation. At this time, since the dislocation density may decrease excessively due to excessive heat generation, the cooling rate during the secondary cooling may be controlled to 25 ℃ / s or less in order to minimize the temperature rise of the steel sheet due to transformation heat generation. If the cooling rate during the secondary cooling exceeds 25 ℃ / s, there is a risk of distortion of the plate shape. The secondary cooling may include air cooling. The lower limit of the secondary cooling rate is not particularly limited, but may be limited to 0.5 ℃ / s or more in order to prevent excessive time from being consumed during the cooling process.

[0150] After completing the secondary cooling, the hot-rolled steel sheet is cooled for the third time at a cooling rate of 30 ℃ / s or more from T2 to the coiling temperature, which is less than 500 ℃, and then coiling can be performed at the coiling temperature, which is the temperature at which the third cooling is completed. The upper limit of the third cooling rate is not particularly limited, but it can be limited to 200 ℃ / s or less in order to prevent an increase in process costs due to excessively rapid cooling. During the third cooling, transformation of low-temperature bainite progresses, and some of the untransformed austenite may transform into martensite even after coiling.

[0151] The cooling end temperature during the third cooling, i.e. the coiling temperature, may be below 500°C. If the coiling temperature exceeds 500°C, the dislocation density within the low-temperature bainite and martensite may drop excessively, and carbides may coarsen, potentially resulting in lower yield and tensile strengths. Conversely, if the coiling temperature is below Ms, martensitic transformation may occur immediately before coiling, potentially leading to shape defects and temperature variations, potentially increasing material deviations within the steel sheet.

[0152] As noted, Bs is the onset temperature of bainite transformation (℃), and Ms is the onset temperature of martensite transformation (℃), which are temperatures determined by the composition of the steel. As noted, Bs and Ms can be derived by the equations below, and each element represents a weight content.

[0153] Bs(℃)=830 - 320×[C] - 90×[Mn] - 35×[Si] - 70×[Cr] - 120×[Mo]

[0154] Ms(℃)= 550 - 330×[C] - 41×[Mn] - 20×[Si] - 20×[Cr] - 10×[Mo] + 30×[Al]

[0155] S5) Acid treatment

[0156] After coiling, the coiled hot-rolled steel sheet can be pickled so that the product of the acid concentration and the pickling time becomes 1,000 to 10,000 g / L·s. As a steel sheet that has gone through the aforementioned reheating, finish rolling, and coiling steps, a concentrated region of Sb is formed within the plating layer. When the product of the acid concentration and the pickling time in the pickling treatment process is applied within the range of 1,000 to 10,000 g / L·s, the formation of the concentrated region described above is effectively promoted, thereby exhibiting the effect of reducing the amount of diffusible hydrogen in the steel.

[0157] Specifically, if the product of the acid concentration and the pickling time is less than 1,000 g / L·s, the scale generated during the finishing rolling may not be sufficiently removed, which may cause product quality problems. On the other hand, if the product of the acid concentration and the pickling time exceeds 10,000 g / L·s, all or part of the concentrated area may be lost during pickling, making it impossible to achieve the expected effect. In addition, it may cause an increase in manufacturing costs, so the upper limit is set to 10,000 g / L·s. However, if there is more than one pickling tank and the acid concentration and pickling time are different accordingly, the above value can be expressed by adding the products of the acid concentration and the pickling time for each tank.

[0158] Acids commonly used in the art can be used for pickling. Representative acids include hydrochloric acid (HCl) and sulfuric acid (H2SO4). In particular, using hydrochloric acid (HCl) offers superior pickling performance, economical process costs, and a reduced risk of surface foreign matter after pickling, making it easier to secure surface quality.

[0159] Meanwhile, although not particularly limited, the acid concentration during pickling treatment may range from 40 to 500 g / L. If the acid concentration is less than 40 g / L, the surface scale generated during hot rolling may not be sufficiently removed during the limited pickling time, which may cause defects on the surface of the steel sheet. On the other hand, if the acid concentration exceeds 500 g / L, it may be difficult to achieve the intended effect of the invention in the final hot press-formed part due to loss of the concentrated region, and surface defects due to overpickling may be induced.

[0160] Additionally, although not specifically limited, the pickling time during pickling treatment may be 5 to 60 seconds (s). If the pickling time is less than 5 seconds, the surface scale of the steel plate may not be sufficiently removed, which may cause defects on the surface. If the pickling time exceeds 60 seconds, the loss of the concentrated area may reduce productivity, thereby increasing the process cost.

[0161] Additionally, although not specifically limited, the pickling temperature can range from 40 to 120°C, and in practice, it can range from 50 to 100°C. If the pickling temperature is below 40°C, the pickling power may be insufficient, which may adversely affect product quality. On the other hand, if the pickling temperature exceeds 120°C, not only will the fixed costs increase due to maintaining the high temperature, but the high temperature may also increase the vaporization of the pickling solution, which may lead to increased costs for replenishing the lost pickling solution.

[0162] S6) Continuous annealing

[0163] Continuous annealing can be performed on a steel plate that has been pickled by controlling the average heating rate for each temperature zone so as to satisfy Equations 1 and 2 below.

[0164] During continuous annealing, continuous annealing can be performed by controlling the average heating rate for each temperature zone so as to satisfy Equations 1 and 2 below.

[0165] [Formula 1]

[0166] 1.0 ℃ / s ≤ Average heating rate of temperature zone 1 ≤ 10.0 ℃ / s

[0167] [Formula 2]

[0168] 0.1 ℃ / s ≤ Average heating rate of temperature zone 2 ≤ 1.0 ℃ / s

[0169] In Equation 1, temperature zone 1 means the section until the steel plate surface temperature reaches 600℃, and temperature zone 2 means the section until the steel plate surface temperature reaches the maximum temperature from 600℃.

[0170] In the continuous annealing described above, each temperature zone can be divided based on the surface temperature of the steel sheet. The temperature range from the starting point (room temperature) to the surface temperature of the steel sheet reaching 600°C can be defined as temperature zone 1 (Zone 1). Next, the temperature range from the surface temperature of the steel sheet reaching 600°C to the maximum target annealing temperature of the steel sheet can be defined as temperature zone 2 (Zone 2). That is, since the conditions from the starting point to the annealing temperature reaching the maximum target annealing temperature satisfy Equations 1 and 2, Sb, which is an additive element for enrichment, is sufficiently enriched in the surface layer, thereby ensuring excellent hydrogen embrittlement resistance after heat treatment for hot forming. Specifically, since the average heating rate in temperature zone 1 increases rapidly, the time maintained at high temperature can be secured more, and since the diffusion rate of the enriching elements into the plating layer region in contact with the base steel sheet at high temperatures is fast, the surface enriching elements can be sufficiently formed in the hot-pressed member after annealing. At this time, the above-mentioned temperature zones 1 and 2 can be sequentially provided in the transport direction of the steel sheet.

[0171] Although not specifically limited, the average heating rate in each temperature zone can be calculated by measuring the surface temperature of the steel plate through the non-contact heat treatment temperature at the middle or end of each temperature zone. Knowing the steel plate surface temperature at each measurement location and the sheet-passing time based on line speed, the average heating rate in each temperature zone can be calculated inversely.

[0172] Meanwhile, during continuous annealing, the heat treatment area (surface thickening heat treatment area) over time at 700°C or higher can be controlled to satisfy 4,000°C·s to 25,000°C·s. Specifically, the surface thickening heat treatment area corresponds to the area of ​​the area where the area above the reference line of y=700°C and the area below the heat treatment temperature profile overlap in the heat treatment temperature profile where the y-axis represents the heat treatment temperature (°C) and the x-axis represents the heat treatment time (sec).

[0173] The pickled steel sheet should be heat treated so that the maximum temperature of the steel sheet exceeds 700℃ during the continuous annealing step, and it is preferable that the surface thickening heat treatment area in the section above 700℃ s be in the range of 4,000℃·s or more and 25,000℃·s or less. This allows the added Sb to be effectively diffused into the plating layer area in contact with the base steel sheet, thereby effectively forming and promoting the Sb thickening area.

[0174] If the heat treatment area over time (i.e., the surface concentration heat treatment area) is less than 4,000°C·s, there is a risk that the Sb concentration layer will not be sufficiently concentrated on the surface, and if it exceeds 25,000°C·s, not only will the process cost and time for continuous annealing increase, but the plating property may deteriorate, resulting in uneven plating or no plating.

[0175] S7) Plating

[0176] The continuously annealed steel sheet can be plated by immersing it in an aluminum-based plating bath, and for example, the plating can be performed by immersing it in an aluminum or aluminum alloy-based plating bath. The plating bath can be, for example, an Al-Si plating bath. Specifically, a plated steel sheet can be manufactured by passing the hot-rolled steel sheet through a plating bath made of aluminum or an aluminum alloy after annealing. At this time, the plating conditions can be applied to the present invention without limitation as long as they are plating conditions typically applied to steel sheets for hot press forming. However, as an example, the composition of the plating bath can include, based on weight %, Si: 6 to 12%, Fe: 4% or less (including 0%), the remainder Al, and other unavoidable impurities.

[0177] Alternatively, as an example, plating can be performed by immersing in a plating bath made of an aluminum alloy. At this time, the plating bath can be an aluminum-based plating bath, for example, an Al-Fe plating bath. Specifically, a plated steel sheet can be manufactured by passing a hot-rolled steel sheet through a plating bath made of an aluminum alloy after annealing. At this time, the plating conditions can be applied to the present invention without limitation as long as they are plating conditions typically applied to steel sheets for hot press forming. However, as an example, the composition of the plating bath can include Fe: 8 to 62%, the remainder Al, and other unavoidable impurities. As another example, the composition of the plating bath can include Fe: 8 to 62%, Si: 10% or less (including 0%), the remainder Al, and other unavoidable impurities.

[0178] At this time, although not particularly limited, the plating amount in the plating step is usually 20 to 140 g / m on one side. 2 It can be made with 20 g / m on one side. 2 Below 140 g / m, it may be difficult to secure the desired corrosion resistance of hot-formed parts. 2In excess, not only does the manufacturing cost increase due to excessive plating attachment, but it may also be difficult to uniformly plate the plating amount across the entire width and length of the coil.

[0179] cold rolling

[0180] In one specific example, the method for manufacturing a hot press-formed member may further include a step of manufacturing a cold-rolled steel sheet by cold-rolling a hot-rolled steel sheet after the aforementioned pickling treatment.

[0181] S8) Hot press forming

[0182] A final member (hot press-formed member) with excellent hydrogen embrittlement resistance can be manufactured by hot forming and die quenching using a plated steel sheet manufactured by the above-described method.

[0183] A blank for hot forming can be manufactured using the plated steel sheet manufactured according to the aforementioned steel composition and manufacturing method. The blank can be heated at a temperature above the austenite single-phase temperature, more specifically, within a temperature range of 800°C to 950°C. If the heating temperature is below 800°C, diffusion between the base metal and the plated layer may not be sufficient, making it difficult to form Kerkendall voids. On the other hand, if the heating temperature exceeds 950°C, excessive oxides are generated on the surface of the member, making it difficult to secure spot weldability and increasing manufacturing costs due to maintaining a high temperature.

[0184] It is preferable to maintain the heated blank within the above temperature range for 1 to 600 seconds. If the holding time is less than 1 second, it is difficult to achieve a uniform temperature distribution across the blank temperature, which may cause material deviations at different locations. On the other hand, if the holding time exceeds 600 seconds, not only is it difficult to secure spot weldability due to excessive oxide formation on the surface of the component, as when the heating temperature is exceeded, but it may also lead to an increase in the manufacturing cost of the component.

[0185] Meanwhile, although not particularly limited, according to one aspect of the present invention, the heated blank described above may be transferred to a press and hot press forming and die quenching may be performed at a cooling rate of 20°C / s or more, specifically, a cooling rate of 20°C / s to 200°C / s. At this time, at a cooling rate of less than 20°C / s, a ferrite phase may be introduced during cooling and formed at grain boundaries, thereby deteriorating strength and crash resistance. There are no particular limitations on the transfer, hot press forming, and cooling steps of the described blank, and a commonly utilized hot press forming method may be applied as is.

[0186] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples 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 claims and matters reasonably inferred therefrom.

[0187] (Example)

[0188] A steel sheet containing 0.32% C - 0.54% Si - 0.84% ​​Mn - 0.02% Al - 0.01% P - 0.002% S - 0.03% Ti - 0.0025% B - 0.004% N - Sb according to Table 1 - the balance Fe (and other unavoidable impurities) was used. Slabs having each composition were manufactured through vacuum melting with a thickness of 40 mm. These slabs were maintained at 1200°C for 1 hour and then hot-rolled to a hot-rolling end temperature of 900°C. After hot rolling, the steel sheet was first cooled from 900°C to 550(T1)°C at a cooling rate of 70°C / sec, second cooled from 550(T1)°C to 510(T2)°C at a cooling rate of 20°C / sec, and third cooled from 510(T2)°C to the coiling temperature of 475°C at a cooling rate of 35°C / sec, and then coiled at a temperature of 475°C. Subsequently, pickling treatment, continuous annealing, and plating were sequentially applied to the coiled cold-rolled steel sheet obtained by cold rolling. At this time, the pickling conditions within the scope of the present invention and the continuous annealing conditions for each zone were applied, and more specifically, the HCl concentration and pickling time were 4,000 g / L·s (however, the process was performed with an HCl concentration of 200 g / L and a pickling time of 20 seconds, and the pickling temperature was applied uniformly at 80°C), and the annealing conditions for each temperature zone for continuous annealing were uniformly applied as follows: temperature zone 1: 3.0°C / s, temperature zone 2: 0.5°C / s, and a surface thickening heat treatment area of ​​13687°C·s.

[0189] In addition, during plating, plating was performed by immersing in a plating bath (Al-Si in Table 1) consisting of 9% Si-2% Fe - the remainder Al and trace impurities.

[0190] Alternatively, during plating, plating was performed by immersing in a plating bath (Al-Fe in Table 1) consisting of 48% Fe-1% Si - the remainder Al and trace impurities.

[0191] Afterwards, hot press forming was performed by heat treating at the heating temperature in Table 1 and then press forming, and at this time, a heating time of 6 minutes and a transfer time to the mold of 10 seconds were applied, and then die quenching was applied.

[0192] Example Sb composition (weight%) Plating bath Heating temperature during hot forming (℃) 10.022Al-Si88020.021Al-Si90030.034Al-Si93040.038Al-Fe90050.001Al-Si88060.006Al-Si93070.020Al-Si75080.002Al-Fe90090.118Al-Si880

[0193] In the manufactured hot press-formed parts, the nano void region (region within 20 μm from the interface between the base steel sheet and the plating layer toward the plating layer) was observed using a transmission electron microscope, and the number of voids per unit area of ​​nano Kerkendahl voids (nano Kerkendahl void density) was measured and summarized in Table 2.

[0194] Number of Jena Nano Kerkendal voids per unit area (units / 100μm) 2 )1572262936434667

[0195] Figure 1 shows a cross-sectional specimen obtained from Example 1, which is an implementation example, within a 20 ㎛ region from the interface between the base steel and the plating layer in the direction perpendicular to the rolling direction of the hot press-formed member, collected through a focused ion beam (FIB), and then examined with a transmission electron microscope (TEM) to obtain a 2 ㎛ 2 The photograph shows the area. As shown in Fig. 1, in Examples 1 to 4, the nano Kerkendal voids with a circle diameter of 5 nm to 100 nm are 572 to 667 / μm. 2 It was confirmed that it was formed with a density of . In addition, as can be seen in Fig. 1, the nano Kerkendal voids of Examples 1 to 4 had anisotropy in the range of 1.3 to 1.5.

[0196] In order to evaluate the amount of diffusible hydrogen in the manufactured hot press-formed parts, TDS (Thermal Desorption Spectroscopy) equipment (Quadrupole mass spectroscopy) was used to measure the amount of hydrogen. The amount of hydrogen was measured 3 days after the hot forming heat treatment. The conditions for analyzing the amount of hydrogen were as follows: the temperature was increased to 600°C at 200°C / hour, and the hydrogen desorption curve was measured while maintaining the temperature for a sufficient time for the hydrogen peak to appear. The total amount of hydrogen and the amount of non-diffusible hydrogen in the steel were obtained by integrating the hydrogen desorption curve based on the above-mentioned contents.

[0197] In addition, in order to evaluate the hydrogen embrittlement resistance, a bending stress was applied to a test piece with a width of 30 mm and a length of 180 mm, and after applying a strain of 1.0 YS at once, the fracture time was measured after immersion in 0.1 N hydrochloric acid. If no fracture occurred for more than 120 hours, it was considered good, and if fracture occurred in less than 120 hours, it was considered bad.

[0198] In order to confirm whether the concentrated layer was sufficiently formed on the surface of the substrate in the specimen manufactured as above, the content of Sb element was analyzed by glow discharge optical emission spectrometry (GDS) using GDS850A (model name, manufactured by LECO), DC and RF equipment.

[0199] In addition, in order to evaluate the adhesion of the electroplated coating, the ISO2409 standard was applied, a 10x10 (1 mm wide) grid was applied to the painted surface with a blade, and an adhesion test tape was attached by placing it parallel to the grid pattern, and then removed. The area of ​​the peeled coating layer was observed using a magnifying glass. At this time, if it was separated by less than 5% of the grid area, it was evaluated as good, and if it was separated by more than 5%, it was evaluated as bad, and this was indicated in Table 3 below. Table 3 shows the Sb composition of Table 1 and the heating temperature (T) during hot forming. H) are shown together. Relationship 1 in Table 3 is the above-mentioned C nH / C H is the value of , and the relational expression 2 in Table 3 is the L described above. 2 / 2[1 / (6t l ) +1 / (15.3t b )]×10 11 The value (unit m) 2 / s) is.

[0200] Example Sb composition (weight %)T H (℃)C dH (ppm)C nH (ppm) [Relationship 1] [Relationship 2] Hydrogen embrittlement resistance Electrodeposition coating adhesion 10.02 28 800.33 60.46 60.58 10.87 Good Good 20.02 19 000.32 30.42 50.56 80.74 Good Good 30.03 49 300.31 70.51 80.62 0.72 Good Good 40.03 89 000.29 30.44 10.60 10.63 Good Good 50.0018800.4310.2810.3953.22 Poor Good60.0069300.4790.2550.3473.04 Poor Good70.0207500.3940.2660.4031.97 Poor Good80.0029000.4150.2280.3552.36 Poor Good90.1188800.1630.6970.8100.17 Good Poor

[0201] As can be seen from the results in Tables 2 and 3, in the cases of Examples 1 to 4 where nano-Kerkendahl voids were formed and equations 1 and 2 were satisfied, not only was the amount of diffusible hydrogen reduced and the fraction of non-diffusible hydrogen increased, but also the adhesion of electrodeposition coating was excellent.

[0202] On the other hand, in the case of examples 5 to 9, it was confirmed that at least one of the characteristics of hydrogen embrittlement resistance and electrodeposition coating adhesion was inferior compared to examples 1 to 4.

[0203] In the case of Example 7, the Sb concentration present in the steel was sufficient, but the heating temperature during hot forming was low, so not only was the concentrated layer not sufficiently formed on the steel surface, but diffusion between the plating layer and the base steel sheet did not sufficiently occur, and thus the hydrogen embrittlement resistance was confirmed to be inferior.

[0204] In the case of Example 9, the hydrogen embrittlement resistance was evaluated as good due to the formation of a large amount of Kerkendal voids, but it was confirmed that the adhesion of the electrodeposition coating was poor due to the excessively formed Kerkendal voids.

[0205] Fig. 2 shows thermal desorption graphs (TDS curves) obtained by electrochemically charging specimens obtained from Example 1, which is an implementation example, and Example 5, which is a comparative example, and analyzing them using a quadrupole mass spectrometer up to 600°C. In Fig. 2, the amount of diffusible hydrogen and the amount of non-diffusible hydrogen were calculated as the area under the hydrogen desorption rate graph based on the boundary temperature dividing diffusible hydrogen and non-diffusible hydrogen (Example 1: 249.1°C, Example 5: 312.3°C). At this time, since the heating rate during the TDS experiment of Fig. 2 is 200°C / s, the x-axis can be converted to elapsed time when calculating the area, and the amounts of diffusible and non-diffusible hydrogen can be obtained. As shown in Table 3, Example 1 satisfied Equation 1. On the other hand, Example 5 did not satisfy Equation 1 because the high-temperature peak was low and the fraction of non-diffusible hydrogen was low.

[0206] Figure 3 shows the I (current density) vs. t (time) graph (i.e., transmission curve) of the specimens obtained from Example 1, which is an implementation example, and Example 5, which is a comparative example. In Figure 3, I of Example 1 ss is A, I of example 5 ss is indicated by B. As shown in Table 3, Example 1 satisfies Relationship 2, while Example 5 has a penetration curve that is faster than Example 1. ss to reach the hydrogen diffusion rate parameter (in this case t l , t b) is low, it can be confirmed that it exceeds the upper limit of relational expression 2.

[0207] The above embodiments are merely examples and the present invention is not limited thereto. Anything that has substantially the same configuration and achieves the same operational effects as the technical concepts described in the claims of the present invention is included within the technical scope of the present invention.

Claims

1. A steel plate comprising a base plate and a plating layer on at least one surface of the base plate, A hot press-formed member comprising a nano-void region in which nano-Kernandall voids having a circular conversion diameter of 5 nm to 100 nm in a thickness direction cross-section are located.

2. In paragraph 1, A hot press-formed member, wherein the above nano void region is a region within 20 ㎛ from the interface between the base steel plate and the plating layer toward the plating layer.

3. In paragraph 1, The maximum diameter (D) of the above nano-Kernanda void m ) is the minimum diameter (D) in the direction perpendicular to the maximum diameter. n ) divided by the diameter ratio (D) m / D n ) is a hot press formed member having an average value of 1.1 to 3.

0.

4. In paragraph 2, The number of voids per unit area of ​​the nano Kerkendal void located in the above nano void region is 300 to 1000 / 100㎛. 2 People, hot press formed parts.

5. In paragraph 2, Based on the cross-section in the thickness direction of the hot press-formed member, the unit area of ​​the nano void region is 100 ㎛ 2 The sum of the area occupied by the above nano Kerkendal voids is 8 ㎛. 2 Below, hot press formed member.

6. In paragraph 1, The above hot press formed member is a hot press formed member that satisfies the following relational expression 1. (Relationship 1) 0.45 ≤ C nH / C H ≤ 0.80 (In relation 1, C H is the total amount of diffusible and non-diffusible hydrogen (in mass ppm) absorbed in the hot press-formed part, and C nH is the amount of non-diffusible hydrogen (in mass ppm) absorbed in the hot press-formed part.

7. In paragraph 6, C above nH / C H A hot press formed member having a thickness of 0.55 to 0.

65.

8. In paragraph 1, The above hot press formed member is a hot press formed member that satisfies the following relationship 2. (Relationship 2) 0.3 ≤ L 2 / 2[1 / (6t l ) + 1 / (15.3t b )]×10 11 ≤ 1.0 (In relational expression 2, L is the thickness (m) of the hot press-formed member, and t b is the break-through time (sec) in the electrochemical hydrogen permeation test, and t l Silver is the permeation current density (I) / saturated permeation current density (I) in the same electrochemical hydrogen permeation test. ss ) means the time (sec) at which the point becomes 0.61 to 0.

64.

9. In any one of paragraphs 1 to 8, The above steel sheet contains, in wt%, C: 0.03 to 0.5%, Sb: 0.01 to 0.1%, Si: 0.01 to 2%, Al: 0.001 to 1%, Mn: 0.2 to 4%, P: 0.05% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), Ti: 0.15% or less (including 0%), B: 0.0001 to 0.01%, Mo: 1.00% or less (including 0%), V: 1.00% or less (including 0%), Ca: 0.01% or less (including 0%), Nb: 0.1% or less (including 0%), W: 1% or less (including 0%), REM: 0.3% or less (including 0%), the remainder being Fe and other unavoidable impurities. A hot press formed member, comprising:

10. In any one of paragraphs 1 to 8, A hot press-formed member, wherein the above plating layer is an aluminum-based plating layer. 11.S1) In weight%, C: 0.03~0.5%, Sb: 0.01~0.1%, Si: 0.01~2%, Al: 0.001~1%, Mn: 0.2~4%, P: 0.05% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), Ti: 0.15% or less (including 0%), B: 0.0001~0.01%, Mo: 1.00% or less (including 0%), V: 1.00% or less (including 0%), Ca: 0.01% or less (including 0%), Nb: 0.1% or less (including 0%), W: 1% or less (including 0%), REM: 0.3% or less (including 0%), the remainder being Fe and other unavoidable A step of reheating a steel slab containing impurities at 1100 to 1350°C; S2) A step of finishing rolling the reheated steel slab at 800 to 950°C to obtain a hot-rolled steel sheet; S3) A step of cooling the hot-rolled steel sheet to the coiling temperature, which is less than 500℃, through multi-stage cooling including a rapid cooling section having a relatively fast cooling rate and a slow cooling section having a relatively slow cooling rate; S4) Step of coiling the cooled hot-rolled steel sheet at the coiling temperature; S5) A step of pickling the coiled hot-rolled steel sheet so that the product of acid concentration and pickling time becomes 1,000 to 10,000 g / L·s; S6) A step of continuously annealing the pickled steel sheet in two or more temperature zones; S7) A step of plating by immersing a continuously annealed steel plate in an aluminum-based plating bath; and S8) A step of heat-treating the plated steel plate at 800°C to 950°C for 1 to 600 seconds and then hot press forming. A method for manufacturing a hot press-formed member including:

12. In paragraph 11, S3) Step is, S3-1) A step of first cooling the hot-rolled steel sheet to a temperature T1 below Bs at an average cooling rate of 50 ℃ / s or more; S3-2) A step of secondary cooling the first-cooled hot-rolled steel sheet to a temperature T2 of Bs or lower and Ms or higher at an average cooling rate of 20 ℃ / s or higher and 30 ℃ / s or lower; and S3-3) A step of cooling the secondarily cooled hot-rolled steel sheet for the third time at an average cooling rate of 30°C / s or more to a coiling temperature of less than 500°C; A method for manufacturing a hot press-formed member including:

13. In paragraph 11 or 12, S6) A method for manufacturing a hot press-formed part, wherein the continuous annealing in step S6 is performed by controlling the average heating rate for each temperature zone so as to satisfy the following equations 1 and 2. [Formula 1] 1.0 ℃ / s ≤ Average heating rate of temperature zone 1 ≤ 10.0 ℃ / s [Formula 2] 0.1 ℃ / s ≤ Average heating rate of temperature zone 2 ≤ 1.0 ℃ / s (In Equation 1, temperature zone 1 means the section until the steel plate surface temperature reaches 600℃, and in Equation 2, temperature zone 2 means the section until the steel plate surface temperature reaches the maximum temperature from 600℃.)

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