Steel sheet for hot forming, hot formed member, hot (PRESS) formed part, and manufacturing methods therefor, and forming device, forming method, bracket for battery case, and battery pack module

Abstract

An aspect of the present invention is to provide a steel sheet for hot forming. The steel sheet for hot forming may comprise, in weight%, carbon (C): 0.1-0.45%, silicon (Si): 0.8-3.0%, manganese (Mn): 0.5-4.0%, phosphorus (P): 0.001-0.05%, sulfur (S): 0.0001-0.02%, aluminum (Al): 0.001-0.1%, chromium (Cr): 1.0-5.0%, nitrogen (N): 0.001-0.02%, and the balance being Fe and inevitable impurities, and may have an R value of 2.8 or more.

Description

Steel sheets for hot forming, hot forming members, hot (press) forming parts and their manufacturing methods, forming devices, forming methods, brackets for battery cases and battery pack modules

[0001] The present invention relates to a steel sheet for hot forming, a hot forming member, a hot (press) forming part, and a method for manufacturing the same.

[0002] In addition, another aspect of the present invention relates to a forming device for forming high-strength steel, a forming method, and a bracket for a battery case formed thereby, and further, a battery pack module.

[0003] Recently, the application of high-strength steel has been actively pursued in response to the demand for vehicle weight reduction and improved safety.

[0004] In particular, efforts are ongoing to improve fuel efficiency by reducing the weight of automobiles and to enhance crashworthiness by increasing their strength. Accordingly, many structural components manufactured using hot forming methods are being used in automobiles, and research into this area is ongoing.

[0005] Hot press forming is a processing method that manufactures high-strength molded parts by heating a blank, forming the heated blank into a desired shape through press forming, and cooling the blank while keeping the mold closed.

[0006] This hot press forming method, when using a hot press forming method that forms at high temperatures, has an excellent characteristic of having an elongation of 50% or more at the high temperature where forming is performed, so that even parts with a small curvature radius can be formed without cracks, and thus the shape of the formed part that cannot be secured with cold ultra-high strength can be secured.

[0007] In addition, hot press-formed parts manufactured by hot-working (hot stamping) blanks can be manufactured into parts of various shapes compared to conventional giga-grade cold-formed materials because they are formed at high temperatures with excellent general formability.

[0008] As an example of a technology related to hot forming, Patent Document 1 discloses a technology of heating an Al-Si plated steel sheet to 850°C or higher, performing hot forming through a press, and forming the structure of the member into martensite through rapid cooling, thereby securing ultra-high strength.

[0009] In addition, Patent Document 1 discloses a technology for securing corrosion resistance and spot weldability without treatment such as shot blast by diffusing Fe from a base material to a plating layer through a heat treatment process to form an alloy layer and a diffusion layer.

[0010] However, Patent Document 1 requires the formation of an Al-Si plating layer, which requires a separate plating process, and this has the disadvantage of being disadvantageous in terms of economy and productivity.

[0011] Meanwhile, for unplated materials, the improvement in surface quality is limited by the iron-based oxide layer formed during hot forming. Furthermore, for unplated materials, a shot blasting process is essential to remove the iron-based oxide layer, which increases the manufacturing cost of the parts.

[0012] Accordingly, there is a growing demand for technologies that can improve surface quality without the need for processes such as plating or shot blasting. Furthermore, there is a growing need for technologies that can improve not only the strength of hot-formed parts but also their surface quality.

[0013] Meanwhile, the shape, material, and thickness of automotive structural components are determined by their intended purpose, such as functional structural components, high-rigidity structural components, compression-mode impact components, and bending-mode impact components. In particular, components that support directional impact loads and are connected to multiple surrounding components must have an ultra-high tensile strength of at least 1300 MPa to achieve thinness and weight reduction. Furthermore, the wrinkle height of the flange portion where it connects to surrounding components must be below the allowable standards for the product assembly line. For example, a center pillar component must withstand the impact load to prevent the deformed structure from injuring passengers in the event of a side impact. Spot welding must also be sufficiently applied to ensure a robust connection with surrounding components such as internal reinforcements, roof side rails, side outers, and side sills. Furthermore, if the component has a complex shape or requires a deep forming depth, the material inflow into the flange portion must be appropriately controlled to prevent cracking during forming. The center filler part, as an example above, also has a deep molding depth of approximately 100 to 150 mm, so the pressure pad and gap holder are arranged in a complex manner in the mold to control the material flow during molding.

[0014] Figure 3 is an example showing the changes in temperature, microstructure, and tensile strength during conventional hot press forming. As illustrated in Figure 3, the conventional method of imparting an ultra-high strength of 1300 MPa or more to a hot press formed part is to austenitize the material, form it at a high temperature, and then cool the material faster than the critical cooling rate through heat transfer with the mold surface maintained at a low temperature, thereby forming a hard martensite structure. In the case of conventional 22MnB5 material for hot press, it is common to produce it by utilizing a temperature cycle of austenizing at 850 to 1000°C, hot forming at 650 to 850°C, and then cooling at a critical cooling rate of 28°C / s or more to the martensite transformation end temperature of 150°C or less. The above material must be an austenite single phase without a soft phase occurring inside the material at the end of hot forming, and the alloy composition is designed so that the entire austenite transforms into martensite during rapid cooling thereafter.

[0015] Figure 4 is an example showing the temperature, microstructure, and ductility of each region of a part manufactured according to a conventional hot press forming process. As illustrated in Figure 4, when a blank holding force is used during high-temperature draw forming of a conventional 22MnB5 material, some regions of the material come into contact with the mold surface, which is maintained at a low temperature of 150℃ or lower, causing local rapid cooling. As a result, the region hardens before the surrounding region, reducing ductility, and there is a risk of tearing at the boundary with the surrounding region during forming. Therefore, in the hot draw forming process, it is difficult to use a blank holding force or draw bead that is effective in controlling material inflow and suppressing wrinkles, which complicates the mold structure and limits the maximum forming depth. For this reason, taking automotive body parts as an example, the hot press forming process is mainly applied to members and channels with shallow depths or relatively simple shapes, and is selectively applied to parts where flange wrinkles are a concern, parts with deep depths, large-area parts requiring control of material inflow, and parts with complex shapes.

[0016] To apply the hot press forming process to difficult-to-form parts, as described above, many manufacturers are utilizing complex mold structures utilizing multiple pads and shaped beads. Alternatively, they are utilizing a split process where the product shape is controlled through cold press forming before post-heat treatment to secure the material. Alternatively, they are developing and utilizing unconventional forming processes that utilize materials insensitive to temperature cycles. However, forming processes utilizing blank holding force, widely used in cold press forming, are not yet actively utilized due to the difficulty in managing conditions to prevent cracking.

[0017] Figure 5 is an example showing the changes in temperature, microstructure, and tensile strength during hot press forming for 22MnB5 material and low-temperature heating type material. Recently, steel manufacturers have been developing materials that can be hot stamped with different temperature cycles than before to overcome the limitations of conventional 22MnB5 material. For example, there are 'low-temperature heating' hot forming materials that relax the heat resistance standards of the material shown in Figure 3, and 'hardenability-improved' hot forming materials that relax the critical cooling rate standards for securing the material.

[0018] Fig. 6 is an example showing the changes in temperature, microstructure, and tensile strength during hot press forming for 22MnB5 material and hardenability-improved material. The 'hardenability-improved' material, as illustrated in Fig. 6, has the characteristic of being able to obtain the target ultra-high strength even under relatively slow cooling rate conditions compared to 22MnB5 by suppressing the occurrence of a soft phase in the continuous cooling phase transformation diagram by changing the metallurgical composition system. However, when a sheet material is generally formed into a part, a thickness reduction occurs in the shoulder R portion that has a curvature due to the wall surface or mold of the part. If the degree of reduction is large, a gap is generated between the mold surface and the material, which makes cooling relatively slow and may make it difficult to secure the target material.

[0019] Accordingly, even if a hot press forming method is used, there is a need for a method that can manufacture automotive filler parts with relatively deep forming depths or large panel parts requiring material inflow control, while ensuring ultra-high strength and resolving forming defects such as cracks, and sufficiently suppressing wrinkles formed in the flange area for assembly with surrounding parts.

[0020] Meanwhile, battery systems used in electric vehicles use battery cases to protect the battery cells inside, and these battery cases must ensure crash safety performance and watertightness.

[0021] Battery cases for electric vehicles are typically made from expensive aluminum extrusions or sheets, but efforts are being made to use steel, which offers competitive pricing and is easily recyclable. However, high-strength steel suffers from poor formability, making it difficult to form into the desired shape.

[0022] High-strength steel is often used only in simple, straight-section parts formed through roll forming. When manufacturing battery cases using these straight parts, alignment issues arise due to thin steel sheets and springback within the parts, making butt welding difficult at the corners where two straight parts meet.

[0023] To overcome this issue, a separate component (a bracket) can be used to support the two straight parts by wrapping around the corresponding corner area. This component has a longitudinal bend that reflects the required corner angle and also has a flange shape that connects to the upper and lower plates of the battery pack, making it difficult to mold.

[0024] Fig. 10 is a schematic diagram of a side frame of a battery case, and Fig. 11 is a cross-sectional view taken along line AA' of Fig. 10. As shown in Figs. 10 and 11, the side frame (B-1) of the battery case must have flanges formed at the top and bottom of the frame (B-10) in order to be connected to the upper and lower plates, and the same applies to the bracket (B-20). That is, the bracket (B-20) also includes a main body (B-23) and flanges (B-21, B-22) formed by bending from the main body (B-23). ​​The bracket (B-20) is difficult to form because it includes a curved portion having a radius of curvature (R1) and also has to have a curved surface having a radius of curvature (R2) formed at the top and bottom to form flanges.

[0025] When a part includes a curvature or dimensional transition, a stretch flange or a shrink flange is formed on the flange (B-21, B-22) depending on the geometric characteristics of the area. Furthermore, both types of flanges may form simultaneously, with one type dominating. In the stretch flange area, where thickness reduction is involved, necking, a localized reduction in cross-sectional area due to plastic instability, and subsequent fracture may occur if the molding amount is excessive or the material formability is insufficient compared to the required shape of the part. Meanwhile, in the shrink flange area, increased in-plane compressive load can lead to increased thickness, buckling, and wrinkles. These wrinkles or folds can create uneven gaps in the battery pack, potentially compromising its watertightness.

[0026] Meanwhile, in the case of the bracket (B-20), in order to ensure uniformity of strength and collision safety performance, it must be formed of the same material as the frame (B-10), which is a straight part. However, when forming with high-strength steel, there is a problem that it is extremely difficult to form a flange with a curved part.

[0027] In the case of conventional cold forming steel plates, when the tensile strength becomes ultra-high strength steel of 980 MPa or more, the risk of wrinkles and fractures increases due to the high strength and insufficient ductility of the material, making it difficult to process curved flanges. Therefore, the use of press-hardened steel as a substitute is increasing.

[0028] Press-hardened steel is formed at high temperatures, resulting in low strength and high ductility at the time of forming, enabling it to be formed into complex shapes. However, because it must be formed in one pass while maintaining high temperatures, it is difficult to apply blank holders or drawbeads, which are used to control fracture and wrinkles, limiting the freedom of forming methods.

[0029] In addition, high-strength steel generally has a problem in that it cannot form parts with a small curvature radius due to its poor bendability, but recently, due to the issues of vehicle weight reduction and safety improvement in electric vehicles, etc., there is a need for molded parts that have high strength but a small curvature radius compared to existing ones, making it easier to utilize space.

[0030] Meanwhile, the hot press forming method has the problem that it cannot be formed sequentially through multiple mold stages as is done in cold forming because the high-temperature material heated in the furnace must be formed before it cools in the air. This drawback also makes it difficult to form parts with multiple hat shapes using the hot press forming method. While it is possible to form each part separately and then attach them together during the assembly stage, this method has the problem of increasing performance and process costs due to the separate forming of each part.

[0031] (Patent Document 1) U.S. Patent Publication No. 6296805

[0032] (Patent Document 2) U.S. Patent Publication No. 2014-0056754

[0033] (Patent Document 3) Korean Patent Publication No. 10-2020-006635

[0034] (Patent Document 4) Korean Patent Publication No. 10-2023-0092430

[0035] The first aspect of the present invention is to provide a hot-forming steel sheet having high strength and excellent surface quality, a hot-forming member having high strength manufactured by hot-forming the steel sheet, and a method for manufacturing the same.

[0036] A second aspect of the present invention is to provide a hot press-formed part and a manufacturing method thereof capable of reducing wrinkles in a flange portion while ensuring ultra-high strength.

[0037] A third aspect of the present invention is to provide a molding device capable of molding a bracket in which a flange and a curved portion are formed, a molding method, and a bracket manufactured thereby.

[0038] The fourth aspect of the present invention is to provide a molded part having a high tensile strength and a small radius of curvature compared to existing parts, or a battery pack module including the same.

[0039] A fifth aspect of the present invention is to provide a press-formed part and a method for manufacturing the same, in which damage such as cracks is minimized during hot press forming to manufacture a part having a plurality of hat shapes.

[0040] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0041] Hereinafter, the technical solutions of the present invention will be described. Although not essential, it should be noted that the technical solutions according to each aspect of the present invention can be usefully applied to the technical solutions of other aspects. For example, the steel sheet for hot forming and / or the manufacturing method thereof according to the first aspect of the present invention can be usefully applied to the hot forming members according to the second to fifth aspects, various parts obtained therefrom, and the manufacturing methods thereof. In addition, the manufacturing method of the parts according to the second aspect of the present invention, or the heat treatment conditions used therefor, can be usefully applied in the process of manufacturing the brackets or molded parts for battery cases according to the third to fifth aspects. Furthermore, the compositions and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to obtain advantageous effects.

[0042] A first aspect of the present invention provides a steel sheet for hot forming. The steel sheet for hot forming contains, in wt%, carbon (C): 0.1 to 0.45%, silicon (Si): 0.8 to 3.0%, manganese (Mn): 0.5 to 4.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.02%, aluminum (Al): 0.001 to 0.1%, chromium (Cr): 1.0 to 5.0%, nitrogen (N): 0.001 to 0.02%, the remainder being iron and unavoidable impurities, and the value of R defined by the following [Relational Expression 1] may be 2.8 or more.

[0043] [Relationship 1]

[0044] R = ([Cr] + [Mn] + [Si]) / ([Cr*] + [Mn*] + [Si*])

[0045] (In the above [Relationship 1], [Cr], [Mn], and [Si] represent the maximum content (weight %) of each element within a range of 0.01 to 1.0 ㎛ in the thickness direction from the surface of the steel plate in the GDS profile, and [Cr*], [Mn*], and [Si*] represent the average content (weight %) of each element in the steel plate.)

[0046] In addition, in the steel sheet for hot forming described above, one or more selected from the following a) to f) may be further included.

[0047] a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.001 to 0.4 wt%,

[0048] b) Boron (B): 0.01 wt% or less (excluding 0%)

[0049] c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.0 wt%,

[0050] d) At least one of copper (Cu) and nickel (Ni): 0.005 to 2.0 wt%,

[0051] e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.0 wt%

[0052] f) Rare earth elements (REM): 0.0001 to 0.02 wt%

[0053] Additionally, in one of the hot forming steel sheets described above, the microstructure of the hot forming steel sheet may include ferrite and cementite.

[0054] Additionally, in one of the hot forming steel sheets described above, the sum of the area fractions of the ferrite and the cementite may be 5% or more.

[0055] In addition, in one of the hot forming steel sheets mentioned above, the hot forming steel sheet may have a tensile strength of 500 MPa or more.

[0056] Another aspect of the present invention provides a method for manufacturing a steel sheet for hot forming. The method comprises the steps of: heating a slab containing, in wt%, carbon (C): 0.1 to 0.45%, silicon (Si): 0.8 to 3.0%, manganese (Mn): 0.5 to 4.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.02%, aluminum (Al): 0.001 to 0.1%, chromium (Cr): 1.0 to 5.0%, nitrogen (N): 0.001 to 0.02%, the remainder being iron (Fe) and unavoidable impurities; finishing hot-rolling the heated slab to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet; coiling the cooled hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; The step of annealing the cold-rolled steel sheet at a dew point temperature of -30 to 0°C at 700 to 1000°C may be included; and the step of pickling the annealed cold-rolled steel sheet for 1 to 10 seconds may be included.

[0057] In addition, in the above-described method, in the annealing step, the temperature may be increased at a first rate of 3.0 to 20.0°C / s in a first temperature range of room temperature to 700°C, and then increased at a second rate of 0.015 to 10.0°C / s in a second temperature range of 700 to 1000°C.

[0058] Additionally, in one of the above-described methods, the slab may further include one or more selected from the following a) to f).

[0059] a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.001 to 0.4 wt%,

[0060] b) Boron (B): 0.01 wt% or less (excluding 0%)

[0061] c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.0 wt%,

[0062] d) At least one of copper (Cu) and nickel (Ni): 0.005 to 2.0 wt%,

[0063] e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.0 wt%

[0064] f) Rare earth elements (REM): 0.0001 to 0.02 wt%

[0065] In addition, in one of the above-described methods, the heating step may be performed at 1000 to 1300°C, and the step of obtaining the hot-rolled steel sheet may be performed by final hot rolling of the slab at Ar3 to 1000°C.

[0066] Additionally, in one of the above-described methods, the cooling step may be performed at a cooling rate of 20 to 100°C / s, and the coiling step may be performed at a temperature exceeding Ms and not exceeding 750°C.

[0067] Another aspect of the present invention provides a hot-formed member. The hot-formed member comprises, in wt%, carbon (C): 0.1 to 0.45%, silicon (Si): 0.8 to 3.0%, manganese (Mn): 0.5 to 4.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.02%, aluminum (Al): 0.001 to 0.1%, chromium (Cr): 1.0 to 5.0%, nitrogen (N): 0.001 to 0.02%, the remainder being iron and unavoidable impurities, and can satisfy the following [Relationship 2].

[0068] [Relationship 2]

[0069] Fe 40~80% Slope ≥ 200 (wt% / ㎛)

[0070] (In the above [Relationship 2], Fe 40~80% The slope represents the slope of the straight line in the section where the Fe content (weight %) is 40 to 80% within a range of 5 ㎛ or less in the thickness direction from the surface of the member in the GDS profile. In addition, the slope of the straight line here represents the slope derived after performing a simple linear regression on the graph in the section where the Fe content (weight %) is 40 to 80% in the GDS profile.)

[0071] In addition, in the hot-formed member described above, the hot-formed member may further include one or more selected from the following a) to f).

[0072] a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.001 to 0.4 wt%,

[0073] b) Boron (B): 0.01 wt% or less (excluding 0%)

[0074] c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.0 wt%,

[0075] d) At least one of copper (Cu) and nickel (Ni): 0.005 to 2.0 wt%,

[0076] e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.0 wt%

[0077] f) Rare earth elements (REM): 0.0001 to 0.02 wt%

[0078] Additionally, in one of the hot-formed parts described above, the microstructure of the hot-formed part may include martensite or bainite as a main phase.

[0079] Additionally, in one of the hot-formed members described above, the area fraction of the main phase may be 10% or more.

[0080] Additionally, in one of the hot-formed members described above, the hot-formed member may have a tensile strength of 1000 MPa or more.

[0081] Another aspect of the present invention provides a method for manufacturing a hot-formed part. The method may include the steps of: preparing a blank using one of the aforementioned hot-forming steel sheets; heat-treating the blank at a heat treatment temperature of 700 to 1000°C for 180 to 600 seconds; and hot-forming the heat-treated blank and then cooling it.

[0082] In addition, in the method described above, in the heat treatment step, heating can be performed to the heat treatment temperature at a heating rate of 1 to 1000°C / s.

[0083] Additionally, in one of the above-described methods, in the cooling step, cooling can be performed to a cooling end temperature lower than Mf.

[0084] In addition, in one of the above-described methods, in the cooling step, cooling can be performed to the cooling end temperature at a cooling rate of 5 to 1000°C / s.

[0085] Although not essential, the steel sheet for hot forming or the hot forming member according to the first aspect of the invention can secure improved performance when combined with the advantageous features of the other aspects described below.

[0086] A second aspect of the present invention provides a hot-formed member. The hot-formed member comprises, at a point t / 4 in the thickness direction (wherein t represents the thickness (mm) of the member), cementite having a Cr content of 1.20% or more and a short diameter of 100 nm or more, in a number of 10.00 / ㎛. 2 Below is the text.

[0087] According to one embodiment of the present invention, a hot press-formed part having ultra-high strength and reduced wrinkles in a flange portion can be provided.

[0088] In one embodiment of the present invention, the hot press formed part can be obtained from the hot forming steel sheet described above.

[0089] Another aspect of the present invention provides a method for manufacturing a hot press-formed part using a mold including an upper die; a lower punch provided at a central lower portion of the upper die; and a lower holder provided at a lower portion of the upper die and a side portion of the lower punch, the method comprising the steps of: austenizing a steel material; providing the austenized steel material on a lower punch and a lower folder; supporting and heating the steel material between the lower holder and the upper die by applying a blank holding force (F) and heat through the lower holder; and forming and cooling the steel material by applying pressure through the lower punch; wherein the steel material has a martensite transformation critical cooling rate of 15 to 25°C / s in a temperature range of 800 to 400°C.

[0090] In one embodiment of the present invention, the blank holding force (F) satisfies the following [Relationship 3], and the following P may be 1 to 10 MPa.

[0091] [Relationship 3] F = P × A (wherein, in the above [Relationship 3], P represents the blank holding pressure, and A represents the contact area between the lower holder and the steel.)

[0092] In one embodiment of the present invention, the minimum temperature value (T) of the lower holder min ) can be 300℃.

[0093] In one embodiment of the present invention, the maximum temperature value (T) of the lower holder max ) can satisfy [Relationship 4] and [Relationship 5] below.

[0094] [Relationship 4] T max = T blank - [(CR min ·w·ρ·C) / h]

[0095] [Relationship 5] h = A·P N

[0096] (However, in the above [Relationship 4] and [Relationship 5], h is the heat transfer coefficient between the steel and the lower holder, T blank is the forming temperature of steel, T max is the maximum temperature value of the lower holder, w is the thickness of the steel, ρ is the density of the steel, C is the specific heat of the steel, CR min (where a is the critical cooling rate for martensite transformation, a is the heat transfer coefficient of the steel, P is the blank holding pressure, and b is the heat transfer index of the steel.)

[0097] In one embodiment of the present invention, the temperature of the remaining mold excluding the lower holder may be 300°C or lower.

[0098] Although not necessarily essential, the hot press-formed part according to the second aspect of the invention can secure improved performance when combined with the advantageous features of the other aspects described below.

[0099] A third aspect of the present invention provides a molding device. The molding device comprises: a pad; a die disposed outside the first direction of the pad; And a punch including a body having a punch forming portion formed therein, the punch forming portion including a first punch forming portion having a straight first punch forming portion, a second punch forming portion having a curved second punch forming portion having a forming portion curvature radius (Rp), and a third punch forming portion having a straight third punch forming portion, when viewed in the first direction, the first to third punch forming portions being continuously arranged along a second direction perpendicular to the first direction on a horizontal plane, the first to third punch forming portions including first to third extension surface forming portions having a plane perpendicular to the first direction on both sides of the first direction, first to third body forming portions having a plane facing the pad, and first to third curved surface forming portions having a bending curvature radius (r) between the extension surface forming portion and the body forming portion, the pad including a pressing surface corresponding to the first to third body forming portions, and the die including, when viewed in the first direction, It comprises a first die forming portion having a straight shape, a second die forming portion having a curved shape having a die curvature radius (Rd), and a third die forming portion having a straight shape, wherein the first to third die forming portions are arranged continuously along a second direction, and when the upper positions of the third body forming portion and the third die forming portion are positioned on the same plane, the first body forming portion is positioned above the upper position of the first die forming portion when viewed in the first direction.

[0100] According to one embodiment of the present invention, there is an effect that enables forming of steel with poor formability (e.g., high-strength steel).

[0101] In one embodiment of the present invention, the punch further includes fourth and fifth punch forming portions symmetrical to the second and first punch forming portions with the third punch forming portion as the center, the die further includes fourth and fifth die forming portions symmetrical to the second and first die forming portions with the third die forming portion as the center, and the pressing surface of the pad may also include a surface corresponding to the main body forming portion of the fourth and fifth punch forming portions.

[0102] In one embodiment of the present invention, the punch may include a support member that supports the pre-molding material outside the first and fifth molding members with the third punch molding member as the center.

[0103] In one embodiment of the present invention, the curvature radius (Rp) of the molded portion may be smaller than the curvature radius (Rd) of the die.

[0104] In one embodiment of the present invention, the second direction length (lp) of the third body forming portion may be longer than the second direction length (ld) of the third die forming portion.

[0105] In one embodiment of the present invention, when viewed in the first direction, the center of the die curvature radius (Rd) in the second direction coincides with the center of the third body forming portion, and when viewed in the first direction, the angle (θ) at which the extension lines of imaginary tangents at both ends of the second and fourth punch forming portions intersect may be between 95 and 135°.

[0106] In one embodiment of the present invention, the first to fifth die forming portions include first to fifth die curved forming portions formed into a curved surface at a position relatively close to the punch in the first direction, and first to fifth flat forming portions formed at a position relatively far from the punch, and the curved surfaces of the first to fifth die curved forming portions increase in height as they get closer to the punch, and when the upper positions of the third body forming portion and the third die forming portion are positioned on the same plane, at least a portion of the second body forming portion may be positioned lower than the upper position of the second die forming portion when viewed in the first direction.

[0107] In one embodiment of the present invention, at least one of the punch and the die may include a cooling channel therein, and the curvature radius (Rp) of the forming portion and the bending curvature radius (r) may satisfy 25 mm ≤ Rp (mm) + r (mm) ≤ 30 mm.

[0108] Another aspect of the present invention provides a material forming method, comprising: a material forming step of forming a material longer in a first direction than a punch having a forming portion on the punch; a first forming step of forming the material into the shape of a main body forming portion of the punch through a pad positioned on the upper portion of the punch; and a second forming step of forming the material along the shape of the forming portion of the punch through a die positioned outside the pad in the first direction; wherein in the second forming step, the die forms an edge portion, a center portion, and an intermediate portion between the center and the edge of the material in that order in a second direction perpendicular to the first direction and the up-down direction.

[0109] In one embodiment of the present invention, the molding part includes, when viewed in the first direction, a first molding part having a straight shape, a second molding part having a curved shape having a curvature radius (Rp) of the molding part, a third molding part having a straight shape, a fourth molding part having a curved shape having a curvature radius (Rp) of the molding part, and a fifth molding part having a straight shape, and the middle part of the die is formed as a curved surface having a die curvature radius (Rd), and the die curvature radius (Rd) may be larger than the curvature radius (Rp) of the molding part.

[0110] In one embodiment of the present invention, the material is a steel plate, and the material settling step is performed while the material is heated to a temperature higher than AC3 of the material, and a cooling step may further be included in which the material is cooled to have a martensite structure by at least one of the punch and the die.

[0111] In one embodiment of the present invention, the material may be the aforementioned hot-forming steel sheet or a hot-formed member obtained by hot-forming the hot-forming steel sheet. As an example, the hot-forming steel sheet and the hot-formed member may have the aforementioned alloy composition and may be manufactured using the aforementioned manufacturing method.

[0112] In one embodiment of the present invention, in the material settling step, the material may include a notch formed in the first direction toward the center of the material at a position corresponding to the middle portion of the die.

[0113] In one embodiment of the present invention, the first to fifth molding portions include first to fifth extension surface molding portions on a surface perpendicular to the first direction on both sides of the first direction, first to fifth body molding portions on a surface facing the pad, and first to fifth curved surface molding portions having a bending radius of curvature (r) between the extension surface molding portions and the body molding portions, and the bending radius of curvature (Rp) of the molding portion and the bending radius of curvature (r) can satisfy 25 mm ≤ Rp (mm) + r (mm) < 30 mm, 3 mm ≤ r (mm) ≤ 10 mm.

[0114] Another aspect of the present invention provides a bracket for a battery case including a molding surface having a molded portion having a curvature radius (Rp), wherein the molding surface includes: a main body; a flange extending and bending from an end of the main body; and a curved surface connected between the main body and the flange having a bending curvature radius (r); wherein the bracket is formed of a steel material including boron having a tensile strength of 1200 MPa or more and has a martensite structure, and the molded portion having a curvature radius (Rp) and the bending curvature radius (r) satisfy 25 mm ≤ Rp (mm) + r (mm) ≤ 30 mm, 3 mm ≤ r (mm) ≤ 10 mm.

[0115] In one embodiment of the present invention, the minimum thickness (tmin) and the initial thickness (tini) before forming in the flange can satisfy (tini - tmin) / tini ≤ 0.2, and the maximum thickness (tmax) and the initial thickness (tini) before forming in the curved surface can satisfy (tini - tmax) / tini ≥ -0.4.

[0116] In one embodiment of the present invention, the bracket may have an angle (θ) at which the extension lines of the virtual contacts at both ends of the molding surface intersect each other between 95 and 135°.

[0117] A bracket for a battery case according to one embodiment of the present invention can be manufactured using the molding method described above.

[0118] According to one embodiment of the present invention, the weight reduction, watertightness, and collision safety performance of the battery pack can be secured through the bracket manufactured using the molding method described above.

[0119] Although not necessarily essential, the article according to the third aspect of the invention may achieve further improved performance when combined with the advantageous features of the other aspects described below.

[0120] A fourth aspect of the present invention provides a high-strength steel hot press-formed part. The high-strength steel hot press-formed part includes a first wall extending in a first direction, a second wall extending in a direction intersecting the first wall, and a third wall extending in the first direction and extending in a direction intersecting the second wall, a curved portion is formed between the first wall and the second wall, and the first wall, the second wall, and the curved portion are formed integrally, and the following [Relational Expression 6] is satisfied.

[0121] [Relationship 6]

[0122] Wall height (hw) ≤ 13.4 * wall angle (θw) + 182.4

[0123] (Here, the wall height (hw) is the length from the first wall to the third wall in a direction perpendicular to the first direction, and the unit is mm; the wall angle (θw) is the angle between an arbitrary line perpendicular to the first direction and the second wall, and the unit is degree; the left and right sides are calculated by dimensionlessly converting them.)

[0124] In one embodiment of the present invention, the high-strength steel may be the hot-forming steel plate described above.

[0125] According to one embodiment of the present invention, a molded part having high tensile strength and small curvature can be provided.

[0126] In one embodiment of the present invention, the first wall, the second wall, and the curved portion may be integrally formed by processing a plate material. In addition, the first wall, the second wall, and the curved portion may be integrally formed by processing a plate material, and the radius of curvature (r) of the curved portion may be greater than or equal to 0.5 mm, and the ratio (r / t) of the thickness (t) of the plate material to the radius of curvature (r) may be less than 2.5.

[0127] In one embodiment of the present invention, the molded part may have a tensile strength of 1300 to 2100 MPa.

[0128] In one embodiment of the present invention, the third wall and the fourth wall are parallel to the first direction, and the fifth wall is connected to the third wall and the fourth wall, the third wall extends from the second wall, and the first to fifth walls can be formed integrally.

[0129] In one embodiment of the present invention, the angle between an arbitrary line perpendicular to the first direction and the second wall may be 5° or less.

[0130] In one embodiment of the present invention, the microstructure of the molded part may be composed of martensite with an area fraction of 98% or more.

[0131] Another aspect of the present invention provides a battery pack module including a battery case formed to surround a battery pack, a base positioned at a lower portion of the battery case and connected to the battery case to form a space in which the battery pack is positioned, and a hot press-molded part positioned across the interior of the battery case and connected to the battery case and the base.

[0132] According to one embodiment of the present invention, a component with high space utilization can be manufactured when manufacturing a module such as a battery pack.

[0133] In one embodiment of the present invention, a plurality of hot press-formed parts may be arranged at regular intervals within the battery case.

[0134] Although not necessarily essential, the components according to the fourth aspect of the invention can be combined with the advantageous features of the other aspects described below to achieve further improved performance.

[0135] A fifth aspect of the present invention provides a press-formed part. The press-formed part has a hat shape including a top plate portion, a vertical wall portion continuous with the top plate portion, and a boundary portion continuous with the vertical wall portion, the hat shape including a plurality of deformed portions having a preset height, and a flat plate portion arranged between the plurality of deformed portions, connecting two adjacent boundary portions, and having a preset length.

[0136] In one embodiment of the present invention, the relationship between the preset length and the preset height can be determined based on the number of the deformation portions.

[0137] In one embodiment of the present invention, the deformation portion is provided in three numbers, and the relationship between the preset length and the preset height is defined by the following [Relationship 7], and w≤114.85e 0.0455h ... [Relationship 7], in the above [Relationship 7], w may mean the preset length, and h may mean the preset height.

[0138] In one embodiment of the present invention, the deformation portion is provided in four or more, and the relationship between the preset length and the preset height is defined by the following [Relational Expression 8], and w≤46.309e 0.105h ... [Relationship 8], in the above [Relationship 8], w may mean the preset length, and h may mean the preset height.

[0139] In one embodiment of the present invention, the preset length (w) may be 20 mm or more.

[0140] In one embodiment of the present invention, the press-formed part can be manufactured using a hot press forming method.

[0141] In one embodiment of the present invention, the hot press forming method may be the method described above.

[0142] Another aspect of the present invention provides a method for manufacturing a press-formed part, comprising the steps of: supplying a metal plate to a processing position; performing a first press forming on the metal plate using a first mold having a plurality of first press portions and an intermediate press portion; and, after the first press forming on the metal plate, performing a second press forming on the metal plate using the first mold and the second mold having a plurality of second press portions.

[0143] In one embodiment of the present invention, the metal plate may be the hot forming steel plate described above.

[0144] In one embodiment of the present invention, the press forming step may use the above-described forming device.

[0145] In one embodiment of the present invention, in the step of supplying the metal plate to the processing position, the metal plate may be supplied between the first mold and the intermediate pressurizing portion, and the central region of the metal plate may be positioned facing the intermediate pressurizing portion.

[0146] In one embodiment of the present invention, in the step of supplying the metal plate to the processing position, the intermediate pressurizing portion may be located above the second mold at the processing position.

[0147] In one embodiment of the present invention, in the step of supplying the metal plate to the processing position, the intermediate pressurizing part may be positioned at a position where the height difference between the intermediate pressurizing part and the second mold is 65% or more of the height of the press-molded part.

[0148] In one embodiment of the present invention, in the step of first press forming the metal plate, the intermediate press portion is introduced into the first press portion arranged in the middle among the plurality of first press portions to press the central region of the metal plate.

[0149] In one embodiment of the present invention, in the step of performing secondary press forming on the metal plate, the second pressing portion may be introduced into a first pressing portion arranged on the outside among the plurality of first pressing portions, thereby pressing an outer region of the metal plate.

[0150] In one embodiment of the present invention, in the step of first press forming the metal plate and the step of second press forming the metal plate, the metal plate can be formed using a hot press forming method.

[0151] In one embodiment of the present invention, between the step of first press-forming the metal plate and the step of second press-forming the metal plate, the method may further include a step of press-forming the metal plate using an additional intermediate press portion arranged outside the intermediate press portion and the first mold.

[0152] According to one aspect of the present invention, a hot-formed steel sheet, a hot-formed component, and a method for manufacturing the same can be provided, which possess high strength and excellent surface quality after hot forming. Furthermore, the hot-formed component can be suitably applied to automotive structural components or reinforcing materials. Furthermore, hot-formed steel sheets and hot-formed components with excellent surface quality and strength can be manufactured without processes such as plating or shot blasting.

[0153] According to another aspect of the present invention, a hot press-formed part and a method for manufacturing the same can be provided that can reduce wrinkles in a flange portion while ensuring ultra-strength.

[0154] According to another aspect of the present invention, a molding device capable of molding a bracket in which a flange and a curved portion are formed, a molding method, and a bracket manufactured thereby can be provided, and through such a bracket, it is possible to secure weight reduction, watertightness, and collision safety performance of a battery pack.

[0155] According to another aspect of the present invention, it is possible to provide a molded part having high tensile strength, small curvature, and being easy to process because it can be manufactured at one time, and also to manufacture a part with high space utilization when manufacturing a module such as a battery pack.

[0156] According to another aspect of the present invention, a press forming process can be performed on a metal plate by placing an intermediate press portion having a height difference between upper and lower molds, and using three press portions. At this time, a stepwise forming process in which the central region of the metal plate is first pressed and then the outer regions are sequentially pressed can prevent deformation from being concentrated at a specific location of the metal plate during press forming. This prevents damage to the press-formed part during the manufacturing process and improves the quality of the part.

[0157] The various advantageous and beneficial advantages and 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.

[0158] In order to more fully understand the drawings cited in the detailed description of the present invention, a brief description of each drawing is provided.

[0159] FIG. 1 is a graph showing the GDS profiles of Cr, Mn, and Si of a steel sheet for hot forming according to one embodiment of the present invention.

[0160] Figure 2 is a graph showing the GDS profile of Fe after hot forming for Invention Example A-1 and Comparative Example A-11.

[0161] Figure 3 shows changes in temperature, microstructure, and tensile strength during conventional hot press forming.

[0162] Figure 4 is an example showing the temperature, microstructure, and ductility of each region of a part manufactured according to a conventional hot press forming process.

[0163] Figure 5 is an example showing changes in temperature, microstructure, and tensile strength during hot press forming for 22MnB5 material and low-temperature heating material.

[0164] Figure 6 is an example showing changes in temperature, microstructure, and tensile strength during hot press forming for 22MnB5 material and hardenability-improved material.

[0165] Figure 7 is an example showing changes in temperature, microstructure, and tensile strength during hot press forming of a material with improved hardenability.

[0166] Figure 8 is a schematic diagram showing a hot press forming equipment applicable to the present invention.

[0167] Figure 9 is a schematic diagram showing a hot press forming facility applied to conventional 22MnB5 material.

[0168] Figure 10 is a schematic diagram of the side frame of the battery case.

[0169] Fig. 11 is a cross-sectional view taken along line A-A' of Fig. 10.

[0170] Figure 12 is an exploded perspective view of a molding device according to one embodiment of the present invention.

[0171] Figure 13 is a side view of a molding device according to one embodiment of the present invention.

[0172] Figure 14 is a flowchart of a molding method according to one embodiment of the present invention.

[0173] Figure 15 is a plan view of a material fed into a molding device according to one embodiment of the present invention.

[0174] Fig. 16 is a schematic perspective view showing a first molding step in a molding method according to one embodiment of the present invention.

[0175] Fig. 17 is a schematic perspective view showing a second forming step in a forming method according to one embodiment of the present invention.

[0176] FIG. 18a is a perspective view showing the second forming step in a forming method according to one embodiment of the present invention from another direction, and FIG. 18b is a front view showing the second forming step in a forming method according to one embodiment of the present invention.

[0177] Fig. 19 is a front view showing a cooling step in a molding method according to one embodiment of the present invention.

[0178] Fig. 20 is a perspective view showing a trimming step in a molding method according to one embodiment of the present invention.

[0179] Fig. 21 is a perspective view of a bracket manufactured using a molding method according to one embodiment of the present invention.

[0180] Figure 22 is a thickness reduction rate distribution diagram of a molded product manufactured using a molding method according to one embodiment of the present invention.

[0181] Figure 23 is a thickness reduction rate distribution diagram of a molded product manufactured using the molding method of Comparative Example 1-1.

[0182] Fig. 24 is a graph of the thickness reduction rate distribution in the cross section of the molded products of Figs. 22 and 23.

[0183] Figure 25 is a side view showing the forming method of Comparative Example 1-2.

[0184] Figure 26 is a perspective view of a molded product manufactured using the molding method of Comparative Example 1-2.

[0185] Figure 27 is a perspective view of a molded part according to one embodiment of the present invention.

[0186] Fig. 28 is a front view of a molded part according to one embodiment of the present invention.

[0187] FIGS. 29a, 29b, 30a, and 30b are drawings showing an interpretation according to an embodiment and a comparative example of the present invention. Referring to Table 1, FIG. 29a shows invention example 2-1, FIG. 29b shows comparative example 2-1, FIG. 30a shows invention example 2-3, and FIG. 30b shows comparative example 2-3.

[0188] Figure 31 is a perspective view of a battery pack module according to one embodiment of the present invention.

[0189] FIG. 32 is a drawing showing a cross-section of a portion of a battery pack module according to one embodiment of the present invention to show space utilization when the battery pack is positioned, where (a) is a case where the radius of curvature is small and (b) is a case where the radius of curvature is larger than that of (a).

[0190] Fig. 33 is a perspective view illustrating a press-molded part according to one embodiment of the present invention.

[0191] Fig. 34 is a perspective view illustrating a press-formed part according to another embodiment of the present invention.

[0192] FIG. 35 (a) is a graph showing the relationship between a preset length and a preset height of a press-molded part according to one embodiment of the present invention, and FIG. 35 (b) is a graph showing the relationship between a preset length and a preset height of a press-molded part according to another embodiment of the present invention.

[0193] Fig. 36 (a) is an example showing a strain applied to a press-formed part manufactured using a conventional manufacturing method, and Fig. 36 (b) is an example showing a strain applied to a press-formed part manufactured using a manufacturing method according to embodiments of the present invention.

[0194] Figure 37 is a flowchart showing a method for manufacturing a press-molded part according to one embodiment of the present invention.

[0195] Figure 38 schematically illustrates the first step of manufacturing the press-formed part of Figure 33.

[0196] Figure 39 schematically illustrates the second step of manufacturing the press-formed part of Figure 33.

[0197] Figure 40 schematically illustrates the third step of manufacturing the press-formed part of Figure 33.

[0198] Figure 41 schematically illustrates the fourth step of manufacturing the press-formed part of Figure 33.

[0199] Figure 42 schematically illustrates the first step of manufacturing the press-formed part of Figure 34.

[0200] Figure 43 schematically illustrates the second step of manufacturing the press-formed part of Figure 34.

[0201] Figure 44 schematically illustrates the third step of manufacturing the press-formed part of Figure 34.

[0202] Figure 45 schematically illustrates the fourth step of manufacturing the press-formed part of Figure 34.

[0203] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified in various other forms, and the terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0204] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0205] In addition, unless specifically specified otherwise in the specification of the present invention, the unit of % means weight%, and 1 ppm is 0.0001 weight%.

[0206] 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.

[0207] The inventors of the present invention recognized the problem that, in the case of non-plated steel sheets for hot forming, an iron oxide layer is formed during the heat treatment process performed during the manufacture of hot-formed parts, making it impossible to secure excellent surface quality of the parts, and that a shot blasting process is necessarily required to remove the iron oxide layer. Accordingly, the inventors of the present invention studied a steel sheet for hot forming that can secure excellent surface quality while solving this problem.

[0208] As a result, the inventors of the present invention confirmed that by precisely controlling the alloy composition and manufacturing conditions, particularly the contents of Cr, Si, and Mn, and the temperature and heating rate during the annealing process, Cr, Si, and Mn can be concentrated on the surface layer of the steel sheet to a target composition. In addition, the inventors of the present invention confirmed that by controlling the temperature and time conditions during the hot forming process, excellent surface quality of the part can be secured even without the shot blasting process, and thus completed the present invention.

[0209] First, the alloy composition of the steel sheet for hot forming according to the first aspect of the present invention will be described. The content of the alloy composition refers to weight percent unless otherwise specified.

[0210] Carbon (C): 0.1 to 0.45%

[0211] Carbon (C) can improve the strength of parts manufactured through hot forming. If the C content is less than 0.1%, it may be difficult to secure sufficient strength. If the C content exceeds 0.45%, the strength of the hot-rolled steel sheet may be excessively high when cold-rolling the hot-rolled steel sheet, which may compromise cold-rollability or deteriorate spot weldability. That is, the C content may be 0.1 to 0.45%, specifically 0.10 to 0.45%, more specifically 0.11 to 0.40%, and even more specifically 0.12 to 0.38%.

[0212] Silicon (Si): 0.8 to 3.0%

[0213] Silicon (Si) not only plays a role in forming a Si-based oxide layer by concentrating on the surface during the annealing process of cold-rolled steel sheets in a continuous annealing line, but also can play a role in securing the surface quality and / or spot weldability of the member by suppressing the formation of an Fe oxide layer during the hot forming process. If the Si content is less than 0.8%, it may be difficult to sufficiently secure the above-described effect. In addition, if the Si content exceeds 3.0%, there may be a problem in that an excessive Si-based amorphous oxide layer is formed on the surface of the steel sheet, thereby deteriorating the surface quality and / or spot weldability. That is, the Si content may be 0.8 to 3.0%, more specifically, 0.9 to 2.9%, and even more specifically, 1.0 to 2.8%.

[0214] Manganese (Mn): 0.5 to 4.0%

[0215] Manganese (Mn) can secure a solid solution strengthening effect and can play a role in lowering the critical cooling rate for securing martensite in parts manufactured through hot forming. If the Mn content is less than 0.5%, it may be difficult to sufficiently secure the above-mentioned effect. If the Mn content exceeds 4.0%, the strength of the steel sheet for hot forming increases excessively, making blanking difficult, and the manufacturing cost may increase or spot weldability may deteriorate due to the excessive addition of alloy iron. In addition, if the Mn content exceeds 4.0%, segregation zones may be formed on the outside and / or inside of the steel sheet, causing the occurrence and propagation of cracks, which may deteriorate the mechanical properties (especially, bendability, etc.). That is, the Mn content may be 0.5 to 4.0%, more specifically, 0.6 to 3.9%, and even more specifically, 0.8 to 3.8%.

[0216] Phosphorus (P): 0.001 to 0.05%

[0217] Phosphorus (P) is an element included as an impurity in steel. When the P content is less than 0.001%, the manufacturing cost may increase excessively due to P content control. In addition, when the P content exceeds 0.05%, the weldability of the hot-formed part may deteriorate due to P segregation. That is, the P content may be 0.001 to 0.05%, specifically 0.001 to 0.050%, more specifically 0.001 to 0.04%, and even more specifically 0.001 to 0.03%.

[0218] Sulfur (S): 0.0001 to 0.02%

[0219] Sulfur (S) is an element included as an impurity in steel. When the S content is less than 0.0001%, the manufacturing cost may increase excessively in order to control the S content. In addition, when the S content exceeds 0.02%, the ductility, impact properties, or weldability of the steel sheet and / or hot-formed member may deteriorate. That is, the S content may be 0.0001 to 0.02%, specifically 0.0001 to 0.0200%, more specifically 0.0001 to 0.015%, and even more specifically 0.0001 to 0.010%.

[0220] Aluminum (Al): 0.001 to 0.1%

[0221] Aluminum (Al) can improve the cleanliness of steel by performing a deoxidizing effect in steelmaking. However, if the Al content is less than 0.001%, it may be difficult to sufficiently secure the above-mentioned effect. In addition, if the Al content exceeds 0.1%, there may be a problem in that the Ac3 temperature rises excessively, requiring an increase in the heating temperature during annealing. That is, the Al content may be 0.001 to 0.1%, specifically 0.001 to 0.100%, more specifically 0.003 to 0.09%, and even more specifically 0.005 to 0.08%.

[0222] Chromium (Cr): 1.0 to 5.0%

[0223] Chromium (Cr) can improve the hardenability of steel sheets, and can play a role in stably helping the formation of a Si-based amorphous oxide layer on the surface of the steel sheet through an appropriate reaction with Si. If the Cr content is less than 1.0%, it may be difficult to sufficiently secure the above-mentioned effect. If the Cr content exceeds 5.0%, the effect is saturated, and as a Cr-based amorphous oxide layer is excessively formed, there may be a problem in that the surface quality and / or spot weldability are rather deteriorated. In addition, if the Cr content exceeds 5.0%, there may be a problem in that the manufacturing cost excessively increases. That is, the Cr content may be 1.0 to 5.0%, more specifically, 1.2 to 4.8%, and even more specifically, 1.5 to 4.5%.

[0224] Nitrogen (N): 0.001 to 0.02%

[0225] Nitrogen (N) is an element included as an impurity in steel. When the N content is less than 0.001%, the manufacturing cost may increase excessively due to the control of the N content. In addition, when the N content exceeds 0.02%, not only may the slab become sensitive to cracking during casting, but the impact properties may also deteriorate. That is, the N content may be 0.001 to 0.02%, specifically 0.001 to 0.020%, more specifically 0.001 to 0.015%, and even more specifically 0.001 to 0.010%.

[0226] A steel sheet for hot forming according to one embodiment of the present invention may include the above components, as well as iron (Fe) as a remaining component. Furthermore, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during a typical manufacturing process, this cannot be excluded. For example, the steel sheet may further include elements that can be included in steel in a total amount of up to 1.0%. Since these impurities are readily apparent to anyone skilled in the art, their full content is not specifically discussed herein.

[0227] For example, a steel sheet for hot forming according to one embodiment of the present invention may further include one or more selected from the following a) to f).

[0228] a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V): 0.001 to 0.4 wt%

[0229] Titanium (Ti), niobium (Nb), zirconium (Zr), and / or vanadium (V) can form fine precipitates to improve the strength of hot-formed parts, and can stabilize retained austenite and improve impact toughness through grain refinement. When the content of Ti, Nb, Zr, and / or V (meaning the sum of their contents when two or more types are added) is less than 0.001%, it may be difficult to sufficiently secure the above-described effects. In addition, when the content of Ti, Nb, Zr, and / or V exceeds 0.4%, the above-described effects may be saturated, or the manufacturing cost may increase due to the addition of excessive alloy iron. That is, the content of Ti, Nb, Zr and / or V may be 0.001 to 0.4%, specifically 0.001 to 0.400%, more specifically 0.001 to 0.3%, and even more specifically 0.0015 to 0.27%.

[0230] b) Boron (B): 0.01 wt% or less (excluding 0%)

[0231] Boron (B) can improve hardenability even with a small amount of addition, and can suppress embrittlement of hot-formed parts caused by grain boundary segregation of P and / or S by segregating at the old austenite grain boundaries. When the B content exceeds 0.01%, the above-described effect becomes saturated, or hot embrittlement may occur during the hot rolling process. That is, the B content may be 0.01% or less, more specifically 0.008% or less, and even more specifically 0.005% or less.

[0232] c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.0 wt%

[0233] Molybdenum (Mo) and / or tungsten (W) can improve strength through improved hardenability and precipitation strengthening, and can refine grains. If the content of Mo and / or W is less than 0.001%, it may be difficult to sufficiently secure the above-described effects. In addition, if the content of Mo and / or W exceeds 1.0%, the above-described effects may be saturated or the manufacturing cost may excessively increase. That is, the content of Mo and / or W may be 0.001 to 1.0%, specifically 0.001 to 1.000%, more specifically 0.0015 to 0.9%, and even more specifically 0.002 to 0.8%.

[0234] d) At least one of copper (Cu) and nickel (Ni): 0.005 to 2.0 wt%

[0235] Copper (Cu) can form fine precipitates to improve strength. Since Cu alone can cause hot embrittlement, nickel (Ni) can be additionally added as needed. If the content of Cu and / or Ni is less than 0.005%, it may be difficult to sufficiently secure the above-described effects. In addition, if the content of Cu and / or Ni exceeds 2.0%, the manufacturing cost may excessively increase. That is, the content of Cu and / or Ni may be 0.005 to 2.0%, specifically 0.005 to 2.000%, more specifically 0.01 to 1.8%, and even more specifically 0.02 to 1.7%.

[0236] e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.0 wt%

[0237] Antimony (Sb) and / or tin (Sn) can minimize dent defects caused by surface grain boundary detachment during the annealing process of cold-rolled steel sheets by suppressing the formation of oxides that may be generated at the surface grain boundaries of hot-rolled steel sheets containing Si. If the content of Sb and / or Sn is less than 0.001%, it may be difficult to sufficiently secure the above-described effect. In addition, if the content of Sb and / or Sn exceeds 1.0%, the manufacturing cost may excessively increase, or the Sb and / or Sn may be dissolved in the slab grain boundaries, causing edge cracks of the coil during the hot rolling process. That is, the content of Sb and / or Sn may be 0.001 to 1.0%, specifically 0.001 to 1.000%, more specifically 0.0015 to 0.9%, and even more specifically 0.002 to 0.8%.

[0238] f) Rare earth elements (REM): 0.0001 to 0.02 wt%

[0239] Rare earth elements (REM) can control the activity of Fe in steel, thereby controlling the thickness of the Fe scale formed on the surface during the hot forming process. If the content of REM is less than 0.0001%, it may be difficult to sufficiently secure the above-mentioned effect. In addition, if the content of REM exceeds 0.02%, the ability to control the activity of Fe is lost, so the surface quality may deteriorate. The content of REM may be 0.0001 to 0.02%, specifically 0.0001 to 0.0200%, more specifically 0.0001 to 0.015%, and even more specifically 0.0001 to 0.01%.

[0240] A steel sheet for hot forming according to one embodiment of the present invention may have a value of R defined by the following [Relational Expression 1] of 2.8 or more, and more specifically, may have a value of 2.80 or more.

[0241] [Relationship 1]

[0242] R = ([Cr] + [Mn] + [Si]) / ([Cr*] + [Mn*] + [Si*])

[0243] In the above [Relationship 1], [Cr], [Mn], and [Si] represent the maximum content (weight %) of each element within a range of 0.01 to 1.0 ㎛ in the thickness direction from the surface of the steel plate in the GDS profile, and [Cr*], [Mn*], and [Si*] represent the average content (weight %) of each element in the steel plate.

[0244] The above [Relationship Equation 1] is a quantitative expression of how the contents of Cr, Si, and Mn can contribute to the surface quality of hot-formed parts. Specifically, by precisely controlling the contents of Cr, Si, and Mn, thereby concentrating Cr, Si, and Mn on the surface layer of the steel plate, the surface quality of the hot-formed part can be secured.

[0245] Fig. 1 is a graph showing the GDS profile of Cr, Mn, and Si of a steel sheet for hot forming according to one embodiment of the present invention. Specifically, Fig. 1 (a) is a graph showing the GDS profile of Cr, Mn, and Si within a region from the surface of the steel sheet to 1.2 μm in the thickness direction. In addition, Fig. 1 (b) is a graph showing an enlarged region in which the maximum contents of Cr, Mn, and Si appear in the GDS profile of Fig. 1 (a) in order to calculate the value of R defined by [Relational Expression 1].

[0246] Referring to Fig. 1, glow discharge emission spectrometry (GDS) analysis can be performed from the surface of the steel plate in the depth direction to measure the value of R defined by [Relationship 1]. For example, based on the GDS profile, the maximum contents (in weight%) of Cr, Mn, and Si within a range of 0.01 to 1.0 ㎛ in the thickness direction of the steel plate can be expressed as [Cr], [Mn], and [Si] in [Relationship 1]. In addition, for example, the contents (in weight%) of Cr, Mn, and Si contained in the steel slab during the manufacture of each steel plate can be expressed as [Cr*], [Mn*], and [Si*].

[0247] When the value of R defined by the above [Relationship 1] is less than 2.8, a sufficient protective film is not formed on the surface of the steel plate, and thus Fe oxide is excessively formed on the surface of the member during the hot forming process, which may deteriorate the surface quality of the member. That is, the value of R may be 2.8 or more, and more specifically, may be 2.80 or more.

[0248] Hereinafter, the microstructure of a steel sheet for hot forming according to one embodiment of the present invention will be described. The microstructure fraction above refers to area % unless otherwise specified. In addition, the microstructure fraction can be measured by analyzing the matrix structure at a point 1 / 4 the thickness of the steel sheet, and specifically, the microstructure fraction can be measured using FE-SEM and optical microscopy.

[0249] For example, the microstructure of a steel sheet for hot forming may include ferrite and cementite. Ferrite is a soft structure and can reduce the load of the blanking process of the steel sheet when producing a blank for obtaining a hot forming part.

[0250] The sum of the area fractions of ferrite and cementite may be 5% or more. If the sum of the area fractions of ferrite and cementite is less than 5%, the strength of the steel sheet may increase excessively, which may cause a problem of mold wear during blank production. That is, the sum of the area fractions of ferrite and cementite may be 5% or more, more specifically 10% or more, and even more specifically 20% or more.

[0251] A steel sheet for hot forming according to one embodiment of the present invention may include the above-described microstructure and pearlite as the remaining microstructure. However, this is not a limitation, and the steel sheet for hot forming may include bainite, martensite, etc. In this case, the fraction of each phase is not particularly limited.

[0252] According to one embodiment of the present invention, a steel sheet for hot forming can secure strength of the steel sheet by controlling the value of R defined by [Relational Formula 1] to an appropriate value by adjusting the content of the alloying component. For example, the steel sheet for hot forming can have a tensile strength of 500 MPa or more.

[0253] Hereinafter, a method for manufacturing a steel sheet for hot forming according to one embodiment of the present invention will be described in detail. However, the manufacturing method described below is merely one embodiment among all possible embodiments, and does not necessarily imply that the steel sheet for hot forming must be manufactured using the manufacturing method described below.

[0254] [Slab heating stage]

[0255] A slab having the aforementioned composition can be heated. Heating the slab facilitates the subsequent rolling process and ensures sufficient physical properties of the steel plate. At this time, the composition of the slab is identical to that of the aforementioned steel plate, and the explanations for the aforementioned steel plate can be applied equally to the reasons for adding each component and limiting its content in the slab.

[0256] For example, the heating step may be performed at 1000 to 1300°C. If the heating temperature is lower than 1000°C, it may be difficult to homogenize the slab's structure. Furthermore, if the heating temperature exceeds 1300°C, excessive oxide formation may occur or manufacturing costs may increase. That is, the heating temperature may be 1000 to 1300°C, more specifically 1050 to 1280°C, and even more specifically 1080 to 1270°C.

[0257] [Hot-rolled steel sheet production stage]

[0258] In the above-described step, the heated slab can be hot-rolled at a normal hot-rolling temperature to obtain a hot-rolled steel sheet.

[0259] For example, in the step of obtaining a hot-rolled steel sheet, the finishing hot rolling of the slab may be performed at Ar3 to 1000°C. If the finishing rolling temperature is lower than Ar3, two-phase rolling is likely to occur, so a mixed grain structure may occur on the surface, and shape control of the hot-rolled steel sheet may be difficult. In addition, if the finishing rolling temperature exceeds 1000°C, the grains may become coarser. That is, the finishing rolling temperature may be Ar3 to 1000°C, more specifically, 850 to 980°C, and even more specifically, 870 to 960°C.

[0260] [Cooling stage]

[0261] The hot-rolled steel sheet obtained through the aforementioned process can be cooled. The cooling step can appropriately secure the microstructure of the steel sheet. For example, the cooling step can be performed at a cooling rate of 20 to 100°C / s. If the cooling rate is less than 20°C / s, an excessive amount of Fe scale is formed on the surface of the steel sheet, and the scale is not sufficiently removed during cold rolling, which may deteriorate the surface quality of the cold-rolled steel sheet. In addition, if the cooling rate exceeds 100°C / s, the strength of the steel sheet excessively increases, which significantly increases the cold-rolling load, which may deteriorate the cold-rollability.

[0262] For example, in the cooling step, the hot-rolled steel sheet can be cooled to the temperature at which the subsequent coiling process is performed. Additionally / alternatively, the cooling method is not specifically limited, but various methods known in the art, such as air cooling or water cooling, can be applied.

[0263] [Winding stage]

[0264] The cooled hot-rolled steel sheet can be coiled in the aforementioned step. For example, the coiling temperature may be higher than Ms (the initiation temperature of martensitic transformation) and lower than 750°C. If the coiling temperature is lower than Ms, the strength of the hot-rolled steel sheet may increase excessively, which may reduce cold-rollability. If the coiling temperature exceeds 750°C, the oxide layer thickness may increase or grain boundary oxidation may occur in the surface layer, resulting in poor pickling properties and problems such as grain boundary detachment in the surface layer during the continuous annealing process.

[0265] [Cold-rolled steel sheet production steps]

[0266] The hot-rolled steel sheet coiled in the above-described step can be cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio during cold rolling is not specifically limited, but, for example, the step of obtaining a cold-rolled steel sheet can be performed at a reduction ratio of 30 to 80%. If the reduction ratio is less than 30%, it may be difficult to secure the target thickness. In addition, if the reduction ratio exceeds 80%, the possibility of cracks occurring at the edge of the steel sheet increases, or there may be a problem accompanying the rolling load during the cold rolling process.

[0267] For example, prior to cold rolling, a pickling process may further include a pickling process for the coiled hot-rolled steel sheet. Pickling can remove the oxide layer formed on the surface of the steel sheet. The pickling process is not particularly limited, and any method commonly used in the art (e.g., immersing the steel sheet in a hydrochloric acid bath) can be applied.

[0268] [Annealing stage]

[0269] The cold-rolled steel sheet obtained in the above-described step can be annealed. During the annealing step, water vapor is supplied to the steel sheet, so that the alloy components of the steel sheet and the water vapor can react under a certain dew point and steel sheet temperature. Specifically, the water vapor in the furnace and the Cr, Mn, and Si of the base material can form oxides and concentrate on the surface layer of the steel sheet. For example, by controlling the temperature, dew point temperature, and / or heating rate during the annealing step, the concentration of Cr, Mn, and Si can be precisely controlled, thereby improving the surface quality of the component manufactured by hot forming.

[0270] The annealing step may be performed at a temperature of 700 to 1000°C. If the annealing temperature is lower than 700°C, the rolled structure produced during the cold rolling process may be difficult to recover and recrystallize. Furthermore, if the annealing temperature exceeds 1000°C, the manufacturing cost may increase due to deterioration of the annealing equipment, etc. In other words, the annealing temperature may be 700 to 1000°C, more specifically 710 to 980°C, and even more specifically 720 to 970°C.

[0271] The annealing step can be performed at a dew point temperature of -30 to 0°C at 700 to 1000°C. By controlling the dew point temperature within the above range, the degree of enrichment of Cr, Mn, and Si in the surface layer can be more precisely controlled. When the dew point temperature is less than -30°C, the enrichment of Cr, Mn, and Si is not sufficient, and it may be difficult to secure the surface quality of the part manufactured by the hot forming process. In addition, when the dew point temperature exceeds 0°C, Fe oxide is formed on the surface of the cold-rolled steel sheet, which deteriorates the surface quality of the steel sheet, and thus it may also be difficult to secure the surface quality of the hot-formed part. That is, the dew point temperature at 700 to 1000°C may be -30 to 0°C, more specifically -27 to -5°C, and even more specifically -25 to -7°C.

[0272] For example, in the annealing step, the temperature may be increased at a first rate of 3.0 to 20.0°C / s in a first temperature range of room temperature to 700°C, and then increased at a second rate of 0.015 to 10.0°C / s in a second temperature range of 700 to 1000°C. Here, room temperature may mean a temperature range of 15 to 25°C. By controlling the heating rates in the first temperature range and the second temperature range differently, the degree of enrichment of Cr, Mn, and Si in the surface layer can be controlled more precisely.

[0273] If the first speed in the first temperature section is less than 3.0°C / s, the time for the steam inside the furnace to come into contact with the parent material increases, which may make it difficult to secure sufficient surface enrichment due to excessive internal oxide formation. In addition, if the first speed exceeds 20.0°C / s, a problem may arise in that sufficient Cr, Mn, and / or Si enrichment in the surface layer is not secured. That is, the first speed may be 3.0 to 20.0°C / s.

[0274] If the second speed in the second temperature range is less than 0.015°C / s, excessive decarburization may occur due to internal oxidation of the surface layer, thereby failing to satisfy the mechanical properties and possibly resulting in poor fatigue characteristics. In addition, if the second speed exceeds 10.0°C / s, a problem may arise in that sufficient concentrations of Cr, Mn, and / or Si may not be secured in the surface layer. That is, the second speed may be 0.015 to 10.0°C / s.

[0275] For example, the annealing step can be performed for 1 to 1,000 seconds. If the annealing time is less than 1 second, it may be difficult to fully achieve the desired effects of the annealing process. Furthermore, if the annealing time exceeds 1,000 seconds, productivity may decline.

[0276] [Sanse stage]

[0277] The annealed cold-rolled steel sheet can be pickled in the aforementioned steps. There are no specific limitations on the pickling method, and any method commonly used in the relevant technical field (e.g., immersing the steel sheet in a hydrochloric acid bath) can be applied.

[0278] The pickling step can be performed for 1 to 10 seconds. If the pickling treatment time exceeds 10 seconds, the Cr, Mn and / or Si concentrated layer densely concentrated on the surface of the steel plate after annealing may be excessively pickled, which may result in a problem of not being able to secure good surface quality during hot forming.

[0279] Hereinafter, a hot-formed component according to one embodiment of the present invention will be described in detail. The hot-formed component is manufactured by hot-pressing one of the aforementioned hot-formed steel sheets, and the alloy composition of the hot-formed component may be identical to the alloy composition of the aforementioned hot-formed steel sheet. Accordingly, the explanation for the aforementioned steel sheet can be applied equally to the reasons for adding each component of the hot-formed component and the reasons for limiting the content.

[0280] A hot-formed member according to one embodiment of the present invention can satisfy the following [Relationship Expression 2].

[0281] [Relationship 2]

[0282] Fe 40~80% Slope ≥ 200 (wt% / ㎛)

[0283] In the above [Relationship 2], Fe 40~80% The slope represents the slope of the straight line in the section where the Fe content (weight %) is 40 to 80% within a range of 5 ㎛ or less in the thickness direction from the surface of the member in the GDS profile. In addition, the slope of the straight line here represents the slope derived after performing simple linear regression on the graph in the section where the Fe content (weight %) is 40 to 80% in the GDS profile.

[0284] [Relationship Equation 2] quantitatively expresses the correlation between the content of Fe formed on the surface of a member and its surface quality. Specifically, the more Fe oxide is formed on the surface of a member, the more the surface quality of the member may deteriorate.

[0285] Fe 40~80% When the slope is less than 200 wt% / ㎛, Fe oxide is excessively formed on the surface, which may deteriorate the surface quality of the member. That is, Fe 40~80% The slope may be 200 wt% / ㎛ or more, more specifically 220 wt% / ㎛ or more, and even more specifically 240 wt% / ㎛ or more. Fe 40~80% There is no need to separately limit the upper bound of the slope, but as a non-limiting example, Fe 40~80% The upper limit of the slope may be 1000 wt% / ㎛, and more specifically 950 wt% / ㎛.

[0286] Hereinafter, the microstructure of a hot-formed component according to one embodiment of the present invention will be described. The microstructure fraction refers to area % unless otherwise specified. In addition, the microstructure fraction can be measured by analyzing the matrix structure at a point 1 / 4 the thickness of the steel plate, and specifically, the microstructure fraction can be measured using FE-SEM and optical microscopy.

[0287] For example, the microstructure of a hot-formed component may include martensite or bainite as the main phase. Here, the main phase may refer to the phase with the largest area fraction among the various phases forming the microstructure. Martensite or bainite can enhance the strength of the component.

[0288] The area fraction of the columnar phase may be 10% or more. If the area fraction of the columnar phase is less than 10%, it may be difficult to secure the strength of the member at an appropriate level. That is, the area fraction of the columnar phase may be 10% or more, more specifically 30% or more, and even more specifically 40% or more. There is no need to separately limit the upper limit of the area fraction of the columnar phase, but as a non-limiting example, the upper limit of the area fraction of the columnar phase may be 95%, and more specifically 90%.

[0289] A hot-formed member according to one embodiment of the present invention may include the above-described structure and, as the remaining microstructure, ferrite or the like. In this case, the fraction of each phase is not particularly limited.

[0290] A hot-formed component according to one embodiment of the present invention can secure its strength by controlling the alloy composition and microstructure fraction. For example, the hot-formed component can have a tensile strength of 1000 MPa or more. In this way, the hot-formed component can secure excellent strength and crash resistance.

[0291] Hereinafter, a method for manufacturing a hot-formed part according to one embodiment of the present invention will be described in detail. However, the manufacturing method described below is merely one embodiment among all possible embodiments, and does not necessarily imply that the hot-formed part must be manufactured using the manufacturing method described below.

[0292] [Blank Preparation Stage]

[0293] A blank can be prepared using a steel plate for hot forming. Here, the steel plate for hot forming may be a steel plate for hot forming according to an embodiment of the present invention, but is not limited thereto.

[0294] For example, there is no particular limitation on the blank manufacturing process, and a method commonly applied in the relevant technical field, such as manufacturing a blank according to the shape of the final intended member, can be used.

[0295] [Heat treatment stage]

[0296] The blank prepared in the aforementioned steps can be heat-treated. This heat treatment step can secure the strength of the hot-formed part. Furthermore, by utilizing a hot-formed steel sheet enriched with Cr, Si, and Mn on the surface, the formation of Fe oxides on the blank's surface during the heat treatment process can be minimized, thereby ensuring the surface quality of the part.

[0297] By controlling the heat treatment conditions such as temperature and duration during the heat treatment step, excellent surface quality of the member can be secured even without the shot blasting process. For example, the blank can be heat treated at 700 to 1000°C. If the heat treatment temperature is lower than 700°C, it may be difficult to secure strength due to the presence of untransformed ferrite. In addition, if the heat treatment temperature exceeds 1000°C, even though Cr, Si, and Mn are concentrated in the surface layer, Fe oxide may be excessively formed on the surface of the member, which may deteriorate the surface quality. That is, the heat treatment temperature may be 700 to 1000°C, more specifically 800 to 990°C, and even more specifically 810 to 980°C.

[0298] In the heat treatment step, the blank may be heat treated for a time of 180 to 600 seconds. If the heat treatment time is less than 180 seconds, the blank may not be sufficiently heat treated, making it difficult to secure strength, which may result in deterioration of physical properties. In addition, if the heat treatment time exceeds 600 seconds, it may be difficult to suppress the formation of Fe oxide on the surface of the member even though Cr, Si, and Mn are concentrated in the surface layer, which may result in deterioration of surface quality after hot forming. That is, the heat treatment time may be 180 to 600 seconds, more specifically, 200 to 580 seconds, and even more specifically, 240 to 540 seconds.

[0299] For example, in the heat treatment step, heating may be performed at a heating rate of 1 to 1000°C / s to the heat treatment temperature. Here, the heating rate may refer to the heating rate of the atmosphere of the furnace. If the heating rate is less than 1°C / s, it may be difficult to secure sufficient productivity. In addition, if the heating rate exceeds 1000°C / s, there may be a problem of increased manufacturing costs because excessively expensive equipment is required. That is, the heating rate may be 1 to 1000°C / s, more specifically 3 to 500°C / s, and even more specifically 5 to 300°C / s.

[0300] [Hot forming and cooling stages]

[0301] The blank heat-treated in the aforementioned step can be cooled after hot forming. For example, the hot forming process is not particularly limited, and any method commonly used in the relevant technical field, such as forming into a desired shape using a mold, can be utilized.

[0302] For example, in the cooling step, cooling can be performed to a cooling end temperature below Mf (the end temperature of martensite transformation). If the cooling end temperature exceeds Mf, it may be difficult to secure the strength of the hot-formed part because cooling must be performed again to room temperature after the end of cooling.

[0303] For example, in the cooling step, cooling may be performed at a cooling rate of 5 to 1000°C / s to the cooling end temperature. If the cooling rate is less than 5°C / s, undesired ferrite and / or pearlite may be formed, making it difficult to secure the tensile strength at the desired level. In addition, if the cooling rate exceeds 1000°C / s, expensive cooling equipment is required to control the rate, which may lower productivity. That is, the cooling rate may be 5 to 1000°C / s, more specifically 7 to 950°C / s, and even more specifically 10 to 900°C / s.

[0304] For example, in the cooling step, first cooling may be performed to a temperature between Ms (the transformation start temperature of martensite) and Mf, then reheating to a temperature higher than the first cooling temperature or Mf, heat treatment may be performed for 1 to 1000 seconds, and final cooling may be performed to a temperature lower than Mf.

[0305] Hereinafter, a hot press-formed part according to the second aspect of the present invention will be described.

[0306] Fig. 7 is an example showing changes in temperature, microstructure, and tensile strength during hot press forming for a material with improved hardenability. A hot press formed part according to an embodiment of the present invention is characterized by using a material with improved hardenability that has changes in temperature, microstructure, and tensile strength during hot press forming as illustrated in Fig. 7. The material with improved hardenability has the characteristic of being able to obtain an ultra-high strength of 1300 MPa or more, and more advantageously, 1500 MPa or more, even under a relatively slow cooling rate condition compared to the 22MnB5 that was mainly used in the past. In addition, when forming a sheet material into a part, there is an advantage in that the target material can be secured even in a thickness reduction area in the shoulder R portion that has a curvature due to the product wall or mold that generally occurs.

[0307] Meanwhile, the above hardenability-improved material has a critical cooling rate (CR) for martensite transformation in the temperature range of 800 to 400°C. min ) may be 15 to 25°C / s. The temperature range of 800 to 400°C is the temperature at which martensite transformation occurs. The critical cooling rate for martensite transformation refers to a cooling rate at which a martensite single-phase structure can be obtained, and more specifically, refers to a cooling rate at which no structure other than martensite is generated within the CCT curve. When the critical cooling rate for martensite transformation is at a low level as described above in the temperature range of 800 to 400°C, it is easy to secure a martensite single-phase structure even at a low cooling rate. When the critical cooling rate for martensite transformation is less than 15°C / s, there may be a disadvantage in that early transformation due to air cooling may occur during the material transfer stage, making forming difficult. When the critical cooling rate for martensite transformation exceeds 25°C / s, it is difficult to utilize the blank holding force, which may cause wrinkles in the flange portion to increase.

[0308] In addition, the hot press-formed component according to one embodiment of the present invention may have a wrinkle height of 1 mm or less in the flange portion. In this way, by controlling the wrinkle height of the flange portion to 1 mm or less, assembly with other components can be facilitated. Meanwhile, the flange portion refers to the area secured by the upper die and the lower holder, and spot welding can be applied to the flange portion to ensure a strong connection with other components. In addition, the wrinkle height refers to the difference between the lowest and highest points of the wrinkle.

[0309] As described above, the hot press-formed component according to one embodiment of the present invention can be applied to components requiring a deep forming depth or controlled material flow. Furthermore, while ensuring ultra-high strength, the component avoids forming defects such as cracks, and the wrinkle depth formed in the flange is also low, thereby improving weldability with other components.

[0310] Fig. 8 is a schematic diagram illustrating a hot press forming equipment applicable to the present invention. Hereinafter, with reference to Fig. 8, a method for manufacturing a hot press forming part according to one aspect of the present invention will be described.

[0311] As illustrated in Fig. 8, a hot press forming equipment applicable to the present invention, i.e., a mold (A-100), may include an upper die (A-10); a lower punch (A-20) provided at a central lower portion of the upper die; and a lower holder (A-30) provided at a lower portion of the upper die (A-10) and a side portion of the lower punch (A-20). Meanwhile, a cooling channel (A-12) for cooling steel may be provided within the upper die (A-10). A cooling channel (A-22) for cooling steel may also be provided within the lower die (A-20). In addition, a heating heater (A-32) for heating a flange portion of the steel may be provided within the lower holder (A-30).

[0312] First, the steel is austenitized. The purpose of this austenitizing process is to transform the microstructure of the steel into a martensite single-phase structure through a post-process, hot press forming. In one embodiment of the present invention, the austenizing process is not particularly limited, and any conventional method used in the art can be utilized. However, as an example, the austenizing process may include heating to 850 to 950°C and then maintaining the temperature for 30 to 180 seconds.

[0313] In one embodiment of the present invention, the steel may be the hot-forming steel sheet described above and may have the alloy composition described above. In addition, the steel may be obtained by the manufacturing method described above, for example, the cold-rolled steel sheet manufacturing method described above.

[0314] Thereafter, the austenitized steel (A-200) is placed on the lower punch (A-20) and the lower folder (A-30). The present invention does not specifically limit the method for placing the steel, and any conventional method used in the relevant technical field can be used. However, the steel is a material with improved hardenability, and the critical cooling rate for martensite transformation may be 15 to 25°C / s in the temperature range of 500 to 200°C.

[0315] Thereafter, the steel (A-100) is supported and heated between the lower holder (A-30) and the upper die (A-10) by applying blank holding force (F) and heat through the lower holder (A-30). The blank holding force (F) and heat control the inflow of material to prevent cracks from occurring while forming the steel into a part, and serve to lower the wrinkle depth of the flange portion.

[0316] The above blank holding force (F) satisfies the following [Relationship 3], and the blank holding pressure (P) may be 1 to 10 MPa. If the blank holding pressure is less than 1 MPa, the wrinkle suppression effect cannot be sufficiently obtained, and if the blank holding pressure exceeds 10 MPa, supercooling may occur by the upper die, causing cracks to occur in the steel.

[0317] [Relationship 3] F = P × A

[0318] (However, in the above [Relationship 3], P represents the blank holding pressure, and A represents the contact area between the lower holder and the steel.)

[0319] The above heat can be added by a heating heater (A-32) provided in the lower holder (A-30). For this purpose, the minimum temperature value (T) of the lower holder (A-30) min ) can be 300℃. Through this, cracks that may occur due to the temperature difference between the area in contact with the lower holder and the area not in contact can be suppressed. If the temperature of the lower holder (A-30) is less than 300℃, cracks in the steel may be caused. In addition, the maximum temperature value (T) of the lower holder max ) can satisfy the following [Relationship 4] and [Relationship 5]. The temperature of the lower holder is the maximum temperature value (T max ) exceeds the critical cooling rate of the material, the material may be cooled slower than the critical cooling rate, and the target material may not be obtained.

[0320] [Relationship 4] T max = T blank - [(CR min ·w·ρ·C) / h]

[0321] [Relationship 5] h = a·P b

[0322] (However, in the above [Relationship 4] and [Relationship 5], h is the heat transfer coefficient between the steel and the lower holder, T blank is the forming temperature of steel, T maxis the maximum temperature value of the lower holder, w is the thickness of the steel, ρ is the density of the steel, C is the specific heat of the steel, CR min (where a is the critical cooling rate for martensite transformation, a is the heat transfer coefficient of the steel, P is the blank holding pressure, and b is the heat transfer index of the steel.)

[0323] At this time, the above T blank can have a range of 650 to 850℃, the above w can have a range of 0.9 to 1.8 mm, and the above ρ is 7.5×10 -9 ~8.1×10 -9 ton / mm 3 , and C is the specific heat of the material, which is 6.3×10 8 ~8.3×10 8 It can have a range of mJ / ton·K, a can have a range of 2.0 to 2.5, and n can have a range of 0.16 to 0.28.

[0324] Thereafter, the steel material is formed by applying pressure through the lower punch, and then cooled. The present invention does not specifically limit the method of applying pressure through the lower punch or the cooling method, and any conventional method used in the relevant technical field can be used.

[0325] Meanwhile, the temperature of the remaining molds excluding the lower holder may be 300°C or lower. Through this, the area of ​​the steel material that does not come into contact with the lower holder can be cooled faster than the critical internal velocity after forming is completed, thereby ensuring an ultra-high-strength material. Meanwhile, the present invention does not specifically limit the temperature control method of the remaining molds excluding the lower holder, and all conventional methods used in the relevant technical field can be used. However, for example, a fluid coolant may be continuously supplied to a cooling channel (A-12) provided in the upper die (A-10) and a cooling channel (A-22) provided in the lower punch (A-20), or the fluid coolant may be continuously brought into direct contact with the steel material, or a mold with excellent heat transfer performance may be used so that a cooling rate within a target range can be secured without a separate cooling device.

[0326] Hereinafter, a molding device according to a third aspect of the present invention and a molding method using the same will be described.

[0327] According to one embodiment of the present invention, there is provided a device and method for forming a bracket having a curved portion and a flange at the same time using a high-strength steel material, and a bracket for a battery case formed thereby, thereby forming a bracket for a battery case that could not be formed using high-strength steel in the past.

[0328] In one embodiment of the present invention, the high-strength steel material may be the aforementioned hot-forming steel plate and may have the aforementioned alloy composition. In addition, the material may be manufactured using the aforementioned manufacturing methods.

[0329] Figures 12 and 13 illustrate a molding apparatus according to one embodiment of the present invention. Specifically, Figure 12 is an exploded perspective view of a molding apparatus according to one embodiment of the present invention, and Figure 13 is a side view of a molding apparatus according to one embodiment of the present invention. In this embodiment, the molding apparatus is illustrated with a focus on the exterior of the mold.

[0330] As shown in FIG. 12, a molding device according to one embodiment of the present invention includes a pad (B-200); a die (B-300) disposed outside the first direction (2) of the pad (B-200); and a punch (B-100) disposed below the pad (B-200) and including a body in which first to fifth punch forming portions (B-120, B-130, B-140, B-150, B-160) are formed. Although not shown in FIGS. 12 and 13, the molding device according to one embodiment of the present invention is a hot press forming device, and cooling channels (B-170, B-370; see FIG. 10) through which cooling water flows are disposed on the inside of the punch (B-100) and the die (B-300), so that the material can be hardened through heat treatment during or after molding.

[0331] The pad (B-200) and the die (B-300) are connected to the upper part of the press, and the punch (B-100) is connected to the lower part of the press, so that the material is formed while the pad (B-200) and the die (B-300) move in the third direction (4) that is the up-and-down direction toward the punch (B-100). Although not shown, the pad (B-200) and the die (B-300) are configured to be able to move independently or to move relative to each other in the up-and-down direction. For example, an elastic member may be attached to the upper part of the pad (B-200), so that the pad (B-200) and the die (B-300) move together, and after the forming of the pad (B-200) is completed, a structure may be applied in which only the die (B-300) moves downward with respect to the pad (B-200) by the elastic member.

[0332] The punch (B-100) is formed with a structure that forms two brackets simultaneously. Accordingly, it includes two forming sections corresponding to the curved portions of the brackets. Specifically, the punch (B-100) includes, when viewed from the first direction (2), a first punch forming portion (B-110) that is linear, a second punch forming portion (B-120) that is curved and has a forming portion curvature radius (Rp) along a second direction (3) that is perpendicular to the first direction (2) on a horizontal plane, a third punch forming portion (B-130) that is linear, a fourth forming portion (B-140) that is curved and has the forming portion curvature radius (Rp), and a fifth punch forming portion (B-150) that is linear, and a support portion (B-160) that supports the material before forming is arranged on the outside of the first forming portion and the fifth forming portion (B-150).

[0333] The first and fifth punch forming parts (B-110, B-150) have a structure that is inclined toward the third punch forming part (B-130) as a whole, and the third punch forming part (B-130) has a structure that is parallel to a horizontal plane at the center, and the second and fourth punch forming parts (B-120, B-140) have a structure that connects the first and fifth punch forming parts (B-110, B-150) and the third punch forming part (B-130) by bending.

[0334] The first and second punch forming parts (B-110, B-120) and the fourth and fifth punch forming parts (B-140, B-150) have a symmetrical structure centered on the third punch forming part (B-130), and thus, one bracket is formed centered on the second punch forming part (B-120), and the other bracket is formed centered on the fourth punch forming part (B-140).

[0335] The first punch forming portion (B-110) includes a first extension surface forming portion (B-111) extending along the third direction (4) to form a flange (B-21, B-23; see FIG. 11) of a bracket (B-20; see FIG. 10) on both sides of the first direction (2), a first body forming portion (B-113) formed to form a body (B-23; see FIG. 11) of the bracket (B-20), and a first curved surface forming portion (B-112) having a bending radius of curvature (r) between the first extension surface forming portion (B-111) and the first body forming portion (B-113).

[0336] The second and third punch forming portions (B-120) and the third punch forming portions (B-130) also form the flanges (B-21, B-22) of the bracket (B-20) on both sides of the first direction (2), the second and third extension surface forming portions (B-121, B-131) extending along the third direction (4) to form the body (B-23) of the bracket (B-20), and the second and third body forming portions (B-123, B-133) arranged below the pad (B-200) and the second and third curved surfaces having the bending radius of curvature (r) between the second extension surface forming portion (B-121) and the second body forming portion (B-113) and between the third extension surface forming portion (B-131) and the third body forming portion (B-133) It includes a molding part (B-122, B-132). At this time, the third main body molding part (B-133) and the support part (B-160) can be parallel to the horizontal plane.

[0337] Since the fourth and fifth punch forming parts (B-140, B-150) have a structure symmetrical to the first and second punch forming parts (B-110, B-120), the fourth and fifth punch forming parts (B-140, B-150) also include the fourth and fifth extension surface forming parts (B-141, B-151), the fourth and fifth curved surface forming parts (B-142, B-152), and the fourth and fifth body forming parts (B-143).

[0338] Meanwhile, the pad (B-200) includes first to fifth pressing surfaces (B-210, B-220, B-230, B-240, B-250) having shapes corresponding to the first to fifth body forming portions (B-113, B-123, B-133, B-143) of the punch (B-100). The pad (B-200) presses the material toward the top of the punch (B-100) to primarily form the material to have an overall curved portion. The pad (B-200) may have a length from a length corresponding to the third body forming portion (B-133) in the first direction (2) to the third extension surface forming portion (B-131).

[0339] The die (B-300) is arranged on both sides of the first direction of the pad (B-200) and has a structure that forms flanges (B-21, B-22). In this embodiment, the die (B-300) is arranged on both sides to form the flanges (B-21, B-22) formed on both sides of the bracket (B-20), but is not limited thereto and may be arranged on only one side. Since the shapes of the two sides of the die (B-300) are symmetrical with respect to the pad (B-200), the description will be centered on one side of the die (B-300).

[0340] When viewed from the first direction (2), the die (B-300) includes a first die forming portion (B-310) having a straight shape, a second die forming portion (B-320) having a curved shape having a die curvature radius (Rd), a third die forming portion (B-330) having a straight shape, a fourth die forming portion (B-340) having a curved shape having the die curvature radius (Rd), and a fifth die forming portion (B-310) having a straight shape, corresponding to first to fifth forming portions (B-110, B-120, B-130, B-140, B-150) of the punch (B-100). The first to fifth die forming portions (B-310, B-320, B-330, B-340, B-350) are connected via a vertical plane (B-360).

[0341] Similar to the above punch (B-100), the first and second die forming sections (B-310, B-320) and the fourth and fifth die forming sections (B-340, B-350) in the die (B-300) have a structure that is symmetrical with respect to the third die forming section (B-330).

[0342] The first die forming portion (B-310) is adjacent to the punch (B-100) and includes a first die curved forming portion (B-311) formed into a curved surface at a position relatively close to the punch (B-100) in the first direction and a first flat forming portion (B-312) at a position relatively far from the punch (B-100).

[0343] Similarly, the second to fifth die forming sections (B-320, B-330, B-340, B-350) include second to fifth die curved forming sections (B-321, B-331, B-341, B-351) and second to fifth flat forming sections (B-322, B-332, B-342, B-352). The curved surfaces of the first to fifth die curved forming sections (B-311, B-321, B-331, B-341, B-351) increase in height as they get closer to the punch (B-100).

[0344] In this embodiment, the radius of curvature (Rd) in the second and fourth die forming portions (B-320, B-340) refers to the radius of curvature at the upper portion (B-321a, B-341a) closest to the punch (B-100) in the second and fourth curved forming portions (B-321, B-341) when viewed in the first direction (2).

[0345] As shown in Fig. 13, in this embodiment, the die (B-300) is configured so that when forming a flange, the first and fifth die forming sections (B-310, B-350) come into contact with the material first, the third die forming section (B-330) comes into contact with the material next, and the second and fourth die forming sections (B-320, B-340) come into contact with the material last, and by forming sequentially in this manner, the material between the second and fourth die forming sections (B-320, B-340) and the second and fourth punch forming sections (B-120, B-140) is secured so that stretching / contraction occurs, thereby preventing wrinkles or tears from occurring during the forming process.

[0346] Specifically, when the third body forming part (B-133) and the upper part (B-331a) of the third die forming part (B-330) are positioned (a) on the same plane, when viewed in the first direction (2), the first body forming part (B-113) is positioned (c) above the upper part (B-311a) of the first die forming part (B-310), and the second body forming part (B-123) is positioned (b) below the upper part (B-321a) of the second die forming part (B-320). The relationship between the symmetrical fourth and fifth die forming parts (B-340, B-350) and the fourth and fifth body forming parts (B-143, B-153) is also the same.

[0347] That is, when the third die forming section (B-330) starts flange forming the material, the first die forming section (B-310) is already performing flange forming, and the second die forming section (B-320), which is a curved section, is not yet performing forming.

[0348] In this embodiment, the length (lp) of the third body forming part (B-133) is longer than the length (ld) of the third die forming part (B-330), and the forming part curvature radius (Rp) of the second body forming part (B-123) is configured to be smaller than the die curvature radius (Rd) of the second die forming part (B-320), so that the above-described relationship can be achieved.

[0349] Since the die curvature radius (Rd) is formed to be larger than the die curvature radius (Rp) of the above-described forming part, the center of the die curvature radius (Rd) can be located at the center of the second direction (3) of the third die forming part (B-330). Accordingly, the centers of the die curvature radii (Rd) of the second die forming part (B-320) and the fourth die forming part (B-340) can be located at the same position, but are not limited thereto.

[0350] At this time, when viewed from the first direction, the angle (θ) at which the extension lines of the imaginary tangents of both ends intersect in the fourth punch forming section (B-143) may be 90° or more, and specifically, may be between 95 and 135°, and of course, the same applies to the second punch forming section (B-123).

[0351] Meanwhile, a forming device according to one embodiment of the present invention achieves a structure that is difficult to achieve through press forming in a conventional high-strength steel of 1200 MPa or more through hot press forming and the above-described structure, and the forming portion curvature radius (Rp) and the bending curvature radius (r) can satisfy 25 mm ≤ Rp (mm) + r (mm) ≤ 30 mm.

[0352] The bracket produced by the forming device can be used for a battery case as mentioned above. However, if the radius of curvature in the battery case is large, dead space will be generated, so the bending radius of curvature (r) that is practically permissible for forming a flange in the battery case is 10 mm or less. On the other hand, if the radius of curvature in the battery case is less than 3 mm, forming is difficult even with a hot press forming steel of 1200 MPa or more. Even if the radius of curvature in the battery case is 3 mm, whether forming is possible or not is determined depending on the radius of curvature (Rp) of the forming portion. This will be explained again when explaining the bracket for the battery case.

[0353] FIGS. 14 to 20 illustrate a molding method according to an embodiment of the present invention, wherein FIG. 14 is a flowchart of a molding method according to an embodiment of the present invention, FIG. 15 is a plan view of a material input into a molding device according to an embodiment of the present invention, FIG. 16 is a schematic perspective view showing a first molding step in a molding method according to an embodiment of the present invention, and FIGS. 17, 18a, and 18b are a schematic perspective view, another perspective view, and a front view showing a second molding step in a molding method according to an embodiment of the present invention, and FIG. 19 is a front view showing a cooling step in a molding method according to an embodiment of the present invention, and FIG. 20 is a perspective view showing a trimming step in a molding method according to an embodiment of the present invention.

[0354] A molding method according to another aspect of the present invention can be performed by the molding device described above, and will therefore be described with reference to FIG. 12.

[0355] A forming method according to one embodiment of the present invention comprises: a material placing step (S110) of placing a heated material (B-400) on a punch (B-100) having a forming portion and having a length longer in a first direction (2) than the punch (B-100); a first forming step (S120) of forming the material (B-400) into the shape of a main body forming portion (B-113, B-123, B-133, B-143) of the punch (B-100) through a pad (B-200) positioned above the punch (B-100); a second forming step (S130) of forming the material (B-400) to follow the shape of the forming portion of the punch (B-100) through a die (B-300) positioned outside the pad (B-200) in the first direction (2); The above material includes a cooling step (S140) in which the material is cooled to have a martensite structure by at least one of the punch and the die, and a trimming step (S150) in which the cooled molded product is cut into a bracket shape.

[0356] The material settling step (S110) is a step of feeding the material into a forming device and settling it on a punch (B-100). At this time, the material is a steel plate, and the material is settling on the punch (B-100) in an austenitic state, i.e., in a state heated to a temperature higher than AC3 of the material, so that a structural change can occur during or after forming. The material may be hot press forming steel, for example, 22MnB5.

[0357] In one embodiment of the present invention, the material may be the hot forming steel sheet described above and may have the alloy composition described above.

[0358] In addition, in order to form two brackets at a time in the present invention, the material (B-400) includes notches (B-410) formed on both sides toward the center of the material in the second direction (3). The notches (B-410) cause deformation to be concentrated during forming, thereby preventing cracks from occurring in areas other than the notches (B-410).

[0359] As shown in Fig. 16, the first forming step (S120) is a step of forming the material (B-400) into a curve through the pad (B-200) after the material (B-400) is settled, and the material (B-400) is formed by pressing the material (B-400) with the pad (B-200) from above the punch (B-100).

[0360] As shown in FIGS. 17 to 18a and 18b, the secondary forming step (S130) is a step of forming a flange on a curved material through a die (B-300), which is a step of deforming the material by applying pressure to the die (B-300) on a portion of the material (B-400) that is not in contact with the pad (B-200) that is formed primarily by the pad (B-200). At this time, as described in FIG. 13, in the secondary forming step (S130), the die (B-300) forms the edge portion, the center portion, and the middle portion between the center and the edge of the material in that order in the second direction (3), so that the curved portion of the primarily formed material (B-400), i.e., the second and fourth punch forming portions (B-120, B-140), can be formed last, thereby forming a flange on the material (B-400). At this time, the shape of the forming part of the punch (B-100) and die (B-300) has been described through Figs. 12 and 13.

[0361] The cooling step (S140) is a step of cooling the material (B-400) during or after forming, and cools the material (B-400) having an austenite structure by circulating a cooling fluid through a cooling channel (B-170, B-370) formed in at least one of the punch and the die, thereby transforming the austenite structure into a martensite structure, thereby making the material (B-400) a high-strength steel having a tensile strength of 1200 MPa or more. In this embodiment, the cooling channels (B-170, B-370) are formed in both the punch (B-100) and the die (B-300). If the austenite structure can be transformed into a martensite structure, the cooling channels (B-170, B-370) may be provided in only one of them, and if necessary, the cooling channels (B-170, B-370) may also be provided in the pad (B-200).

[0362] When the cooling step (S140) is completed, a trimming step (S150) is performed to cut the formed material (B-400) along a cutting line (B-420) so that only the portions corresponding to the two brackets remain. The two brackets are cut around the second and fourth punch forming portions (B-120, B-140) of the punch (B-100).

[0363] A forming method according to one embodiment of the present invention enables forming a bracket having a curved portion and a flange from high-strength steel. Furthermore, by applying a hot press forming method, springback occurring during forming in high-strength steel can be prevented, and forming errors, such as wrinkles or thinning, caused by forming a flange on the curved portion can be prevented.

[0364] FIG. 21 shows a perspective view of a bracket manufactured using a molding method according to one embodiment of the present invention.

[0365] The bracket (B-20) of Fig. 21 has a main body (B-23) and flanges (B-21, B-22), and is curved overall with a curvature radius (Rp) of the molded portion. At this time, the flanges (B-21, B-22) are connected to the main body (B-23) by a curved surface (B-24) that is curved with a curvature radius (r). The surface of the main body (B-23), the flanges (B-21, B-22) of the bracket (B-20), and the curved surface (B-24) therebetween that is formed with the curvature radius (Rp) of the molded portion can be called a molded surface.

[0366] FIG. 22 shows a thickness reduction rate distribution diagram of a molded product manufactured by a molding method of one embodiment of the present invention (embodiment 1-1), FIG. 23 shows a thickness reduction rate distribution diagram of comparative example 1-1 manufactured by another molding method, and FIG. 24 shows a thickness reduction rate distribution graph in a cross section of the molded products of FIGS. 22 and 23.

[0367] The thickness reduction rate distribution diagram is a diagram showing how the thickness of the material (B-400, see Figure 15) changes after molding. The thickness reduction rate is obtained by subtracting the thickness of the bracket (B-20) at the corresponding location from the initial thickness (tini) of the material (B-400) and dividing the result by the initial thickness (tini). A positive number means that the thickness has decreased, and a negative number means that the thickness has increased.

[0368] The embodiment of Fig. 22 was manufactured using the molding device of Fig. 12, and a material heated to have a martensite structure with 22MnB5 steel was supplied and the cooling rate was adjusted, and the tensile strength of the bracket was 1500 MPa. At this time, the thickness of the material was 1.2 t, the bending radius of curvature (r) was 5 mm, and the forming portion's curvature radius of curvature (Rp) was 22 mm.

[0369] As shown in Figs. 22 and 24, since the thickness of the main body (B-23) in the bracket (B-20) is not different from the initial thickness (tini), the average thickness (tav) of the main body (B-23) of the bracket (B-20) can be regarded as the initial thickness (tini) of the material (B-400), and the value obtained by subtracting the thickness of each position from the average thickness (tav) of the main body (B-23) and dividing it by the average thickness (tav) can be regarded as the thickness reduction rate.

[0370] Comparative Example 1-1 is a product formed by supplying 1500 MPa mart steel as a material through the same forming device.

[0371] As shown in Fig. 24, when manufacturing a bracket using the molding device and molding method according to one embodiment of the present invention (embodiment 1-1), the maximum thickness reduction rate, that is, the thickness reduction rate of the portion having the minimum thickness (tmin), does not exceed 0.2. However, in the case of Comparative Example 1-1, it can be confirmed that the thickness reduction rate of the portion with the thinnest thickness exceeds 0.2. When the thickness reduction rate exceeds 0.2, a height difference may occur at the flanges (B-21, B-22), which may deteriorate the sealing performance when manufacturing the battery case.

[0372] In addition, as shown in Fig. 24, even if the bracket is curved and elongation is dominant, shrinkage occurs in certain areas, and if buckling occurs due to this shrinkage, material damage may occur. In the case of the bracket according to the forming method according to one embodiment of the present invention, a thickness reduction rate of -0.3 or less did not occur even at the maximum thickness (tmax) position on the curved surface, and thus, it was confirmed that stable forming was achieved even in the curved portion.

[0373] However, in the case of Comparative Example 1-1, a thickness reduction rate of -0.8 occurs at the maximum thickness (tmax) position, indicating buckling in that area. Such buckling not only leads to cracks that compromise the structural performance of the battery case, but also potentially allows water to enter that area, making brackets like Comparative Example 1-1 inapplicable to battery cases. To maintain sealing properties, it is advantageous to maintain a thickness reduction rate of -0.4 or higher even in the curved area.

[0374] Meanwhile, the sum of the curvature radius (Rp) of the molded part and the bending curvature radius (r) had to be 25 mm or more to enable molding. However, if the curvature radius (r) of the molded part is less than 3 mm, it is difficult to mold a bracket applicable to a battery case even with the molding device and method according to an embodiment of the present invention. Even if flange molding is possible because the curvature radius (r) of the molded part is 3 mm, molding was not properly performed at the flange of the curved part when the curvature radius (Rp) of the molded part was less than 22 mm. Meanwhile, if the curvature radius (r) of the molded part and the curvature radius (Rp) of the molded part increased, there was no difficulty in molding itself, but there was a problem that the curvature radius of the battery case increased, creating a dead space, which made it impractical. Therefore, it is preferable that the sum of the curvature radius (Rp) of the molded part and the curvature radius (r) of the bending part be 30 mm or less for a bracket applicable to a battery case.

[0375] In addition, in the bracket (B-20), the angle (θ) at which the extension lines of the virtual contacts intersect at both ends of the molding surface may be between 95 and 135°. If the angle (θ) is less than 95°, the molding is not properly performed even if the conditions of the molding portion curvature radius (Rp) and the bending curvature radius (r) are satisfied. If the angle (θ) exceeds 135°, the battery case must be configured with an angle exceeding an octagon, resulting in a dead space where the battery cells, which are roughly rectangular in shape, are not filled, making it impractical.

[0376] Meanwhile, Fig. 25 shows a side view showing the molding method of Comparative Example 1-2, and Fig. 26 shows a perspective view of a molded product manufactured using the molding method of Comparative Example 1-2.

[0377] The molding method of Fig. 25 is formed in the same order as the molding method of the present invention, but in the second molding step (S130), the flange molding is not performed in the order of edge-center-curved part in the second direction (3), but in the order of center-curved part-edge. In Comparative Example 1-2, the die (B-300) is formed so that the center has the lowest upper position (B-301a), and the molding is performed sequentially as it goes outward.

[0378] Even if the hot press forming process is applied in the same way in this case, as shown in Fig. 26, a folding area (B) is created, and a bracket that cannot be used as a battery case is produced.

[0379] Although the present invention has been described above with reference to embodiments thereof, it is to be understood that the present invention is not limited thereto and may be implemented in various modified forms.

[0380] Hereinafter, a hot press-formed part according to the fourth aspect of the present invention will be described.

[0381] FIG. 27 illustrates a perspective view of a molded part according to one embodiment of the present invention, and FIG. 28 illustrates a front view of a molded part according to one embodiment of the present invention.

[0382] A hot press-formed part according to one embodiment of the present invention includes a first wall (C-110) extending in a first direction (Y direction) and a second wall (C-120) extending in a direction intersecting the first wall (C-110), a curvature (C-160) is formed between the first wall (C-110) and the second wall (C-120), an angle between an arbitrary line perpendicular to the first direction (a line parallel to the Z direction) and the second wall (C-120) is 0° or more and 5° or less, and the first wall (C-110), the second wall (C-120) and the curvature (C-160) are formed integrally.

[0383] A hot press-formed part according to one embodiment of the present invention includes a first wall (C-110), a second wall (C-120), and a curved portion (C-160) positioned between the first wall (C-110) and the second wall (C-120). The first wall (C-110), the curved portion (C-160), and the second wall (C-120) are integrally formed in that order.

[0384] A hot press-formed part has the characteristic of being formed as a single body, including a first wall (C-110) formed parallel to a first direction, a curved portion (C-160) curved from the first wall (C-110), and a second wall (C-120) connected to one end of the curved portion (C-160).

[0385] Assuming an arbitrary line perpendicular to the first direction, the second wall (C-120) may have an angle of 0° or more and 5° or less with the arbitrary line.

[0386] In response to the demand for vehicle weight reduction and improved safety, many parts using high-strength steel are being produced. However, due to the poor bendability of high-strength steel, it is difficult to produce parts with a small curvature radius.

[0387] Hot press forming is a processing method that heats a blank, press-forms the heated blank into the desired shape, and then cools the blank with the mold closed to produce a high-strength molded part. This hot press forming method, which uses a hot press forming method that forms at high temperatures, has excellent characteristics such as an elongation of over 50% at the high temperature during forming, allowing for the formation of parts with a small curvature radius without cracks, making it possible to secure the shapes of molded parts that cannot be achieved with cold ultra-high strength.

[0388] One embodiment of the present invention relates to a molded part (C-10) formed as an integral part including a first wall (C-110), a curved portion (C-160), and a second wall (C-120) through a hot press forming method, wherein the second wall (C-120) has an angle of 0° to 5° with respect to the arbitrary line, which is smaller than a conventional angle. Accordingly, the first wall (C-110) and the second wall (C-120) have a shape close to a right angle, so that the space surrounded by the first wall (C-110) and the second wall (C-120) can be efficiently used.

[0389] According to one embodiment of the present invention, the first wall (C-110), the second wall (C-120) and the curved portion (C-160) can be formed integrally by processing a plate material.

[0390] The first wall (C-110), the curved portion (C-160), and the second wall (C-120) can be formed as a single body using a blank formed by joining a single body or multiple bodies in the thickness direction using a hot press forming method. Since the blank is formed as a single body using a hot press, processing is convenient, and processing problems or breakage due to stress concentration at joints such as welds can be prevented, while a molded part (C-10) having a relatively high tensile strength can be manufactured.

[0391] In one embodiment of the present invention, the plate may be the hot forming steel plate described above and may have the alloy composition described above.

[0392] According to one embodiment of the present invention, the radius of curvature (r) of the curved portion (C-160) may be greater than or equal to 0.5 mm, and the limit bending (r / t), which is the ratio of the thickness (t) of the plate material and the radius of curvature (r), may be formed to be less than 2.5.

[0393] The ratio (r / t) of the thickness (t) of the sheet metal before processing to the radius of curvature (r) is generally determined by the limit bending value of the raw material. The limit bending can be viewed as a numerical value expressing the maximum radius of curvature without lag in relation to the thickness of the material. Since the thickness (t) of the sheet metal is usually set before processing, if the radius of curvature is reduced to a value lower than the ratio (r / t) that can be formed in the sheet metal, cracks will occur on the molded part (C-10).

[0394] According to one embodiment of the present invention, the ratio (r / t) of the thickness (t) of the plate and the radius of curvature (r) may be less than 2.5. In comparison, the limit bending of existing ultra-high strength materials with a tensile strength of 1500 MPa is known to be 2.5 or more. For example, this means that the minimum radius of curvature of a part that can be formed with an ultra-high strength of 1500 MPa from a material having a thickness of 1.0 mm is 2.5 mm or more. However, according to the present invention, even when forming with ultra-high strength, the ratio (r / t) may have a value less than 2.5, so that a product with a ratio (r / t) of less than 2.5 can be formed even from an ultra-high strength material with a tensile strength of 1500 MPa. Therefore, a molded part according to one embodiment of the present invention may be a part with excellent formability and high space utilization. According to one embodiment of the present invention, the third wall (C-130) and the fourth wall (C-140) are parallel to the first direction, and the fifth wall (C-150) is connected to the third wall (C-130) and the fourth wall (C-140), and the third wall (C-130) extends from the second wall (C-120), and the first wall (C-110) to the fifth wall (C-150) can be formed integrally.

[0395] According to the hot press forming process, a hat-shaped shape with multiple walls can be formed at once, resulting in excellent formability.

[0396] For example, a third wall (C-130) and a fourth wall (C-140) may be formed parallel to the first direction, and the first wall (C-110) and the fourth wall (C-140) may have the same height in a direction perpendicular to the first direction, but this is not limited thereto. In addition, the third wall (C-130) may have a different height. The fifth wall may be positioned between the third wall (C-130) and the fourth wall (C-140), the second wall (C-120) and the third wall (C-130) may be connected by a first connecting portion (C-170), the third wall (C-130) and the fifth wall (C-150) may be connected by a second connecting portion (C-180), and the fifth wall (C-150) and the fourth wall (C-140) may be connected by a third connecting portion (C-190). Accordingly, the first to fifth walls (C-150), the curved portion (C-160), and the first connecting portion (C-170) to the third connecting portion (C-190) may be integrally formed by processing a plate material.

[0397] Here, the first connecting portion (C-170) to the third connecting portion (C-190) can be formed into a shape having a curvature radius like the curved portion (C-160), and at this time, the curvature radius may have all the characteristics of the curved portion (C-160) of the first wall (C-110) and the second wall (C-120). However, it is not limited to this shape.

[0398] According to one embodiment of the present invention, the tensile strength of the molded part may be 1300 to 2100 MPa.

[0399] The tensile strength referred to as ultra-high strength is generally around 1500 MPa. According to the present invention, a molded part having a tensile strength of around 1300 to 2100 MPa, which can be considered high strength or ultra-high strength, can be manufactured. This is because the formability is improved by hot press forming, and thus the molded part according to one embodiment of the present invention can be manufactured as an integral part and have an ultra-high tensile strength while forming a part with a relatively small radius of curvature.

[0400] In addition, by utilizing the excellent formability at high temperatures for hot press forming at the strengths of 1800 MPa and 2000 MPa, parts with a small radius of curvature as described above can be formed. Therefore, according to the present invention, molded parts with a small radius of curvature having a tensile strength of 1700 MPa to 1900 MPa, or even 1900 MPa to 2100 MPa, can also be produced.

[0401] In addition, the microstructure forming the molded part (C-10) may be composed of martensite with an area fraction of 98% or more. The molded part (C-10) according to the present invention may form a tensile strength of the ultra-high strength level as described above by having martensite, a hard phase with high strength, occupying most of the area fraction.

[0402] For example, more than 98% of the area fraction may be formed as martensite, and other phases such as trace amounts of ferrite or bainite may be included.

[0403] According to one embodiment of the present invention, when a third wall (C-130) is further included that is parallel to the first direction and connected to the second wall (C-120), and a length in a direction perpendicular to the first direction from the first wall (C-110) to the third wall (C-130) is defined as a wall height (hw), and an angle between an arbitrary line perpendicular to the first direction and the second wall (C-120) is defined as a wall angle (θw), the following [Relational Expression 6] can be satisfied.

[0404] [Relationship 6]

[0405] hw ≤ 13.4 * θw + 182.4

[0406] Here, the unit of hw is mm, the unit of θw is degree, and the left and right sides are nondimensionalized and calculated.

[0407] A molded part processed by hot press forming that satisfies the above [Relationship 6] can be manufactured with excellent quality without defects such as cracks.

[0408] Referring to Table 1 below, it can be seen that formability is excellent when the above formula is satisfied.

[0409] Division θw (°) hw (mm) Calculated value of the right side [Relationship 6] Satisfaction Formability Invention Example 2-10 180 182.4 Satisfactory Good Invention Example 2-21 190 195.8 Satisfactory Good Invention Example 2-33 220 222.6 Satisfactory Good Invention Example 2-45 240 249.4 Satisfactory Good Comparison Example 2-10 190 182.4 Unsatisfactory Poor Comparison Example 2-21 200 195.8 Unsatisfactory Poor Comparison Example 2-33 230 222.6 Unsatisfactory Poor Comparison Example 2-45 260 249.4 Unsatisfactory Poor

[0410] Comparative Examples 2-1 to 2-4 were formed identically to Invention Examples 2-1 to 2-4, except for the wall heights indicated above. However, the comparative examples had problems in that the forming results revealed defects in formability, such as cracks.

[0411] Fig. 29a is a drawing interpreted in the same manner as the conditions of Invention Example 2-1, and Fig. 29b is a drawing interpreted in the same manner as the conditions of Comparative Example 2-1. Comparing Figs. 29a and 29b, it can be seen that in Comparative Example 2-1, a crack occurs in the upper part of the second wall (C-120) or the fifth wall (C-150), indicated in red, and some parts are interpreted as parts at risk of crack occurrence, indicated in yellow.

[0412] Fig. 30a is a drawing interpreted in the same manner as the conditions of Invention Example 2-3, and Fig. 30b is a drawing interpreted in the conditions of Comparative Example 2-3. Comparing Figs. 30a and 30b, it can be seen that in Comparative Example 2-3, a crack, indicated in red, occurs in the lower part of the second wall (C-120) or the fifth wall (C-150), and some parts are interpreted as parts at risk of crack occurrence in yellow. In comparison, in Invention Example 2-3, there are some parts at risk of crack occurrence in yellow, but no cracks occur.

[0413] Hereinafter, another aspect of the present invention, a battery pack module, will be described.

[0414] The battery pack module below includes the molded part described above, and the description of the molded part is cited above.

[0415] FIG. 31 is a drawing illustrating a battery pack module according to one embodiment of the present invention, and FIG. 32 is a drawing expressing a cross-section of a portion of the battery pack module to show space utilization when the battery pack is positioned, where (a) illustrates a case where the radius of curvature of a hot press-formed part is small, and (b) illustrates a case where the radius of curvature is larger than that of (a).

[0416] A battery pack module (C-1) according to one embodiment of the present invention includes a battery case (C-20) formed to surround a battery pack, a base (C-30) positioned at a lower portion of the battery case (C-20) and connected to the battery case (C-20) to form a space in which the battery pack is positioned, and a hot press-molded part (C-10) positioned across the interior of the battery case (C-20) and connected to the battery case (C-20) and the base (C-30).

[0417] The battery case (C-20) is formed to surround the side of the battery pack, and the base (C-30) is formed of a general plate material or has a curve or groove as needed, and is configured to place the battery pack.

[0418] For example, a reinforcing member (C-40) may be further included to be bonded to the outer surface of the battery case (C-20) to prevent collision.

[0419] For example, a hot press-formed part (C-10) can serve as a cross member to reinforce rigidity against impact caused by collision of the battery frame and can be positioned inside the battery case (C-20).

[0420] For example, the hot press-formed parts (C-10) may be arranged in multiple numbers at regular intervals within the battery case (C-20).

[0421] The battery pack is positioned in the space formed between the base (C-30), the battery case (C-20), and the cross member, and the battery pack module (C-1) is intended to stably position the battery in the vehicle body, including the battery pack.

[0422] The radius of curvature within the battery pack is formed smaller than before, and high tensile strength molded parts are positioned, thereby increasing the rigidity of the battery pack module (C-1) itself, thereby preventing fires caused by damage to the battery due to impact.

[0423] In addition, since a plurality of cross members are generally provided within the battery case (C-20), the size of the battery pack can be increased according to the benefit of increased space utilization, and therefore, if the battery pack module (C-1) has the same volume, it provides the effect of increasing the battery capacity.

[0424] As an example of a case where it is used as a cross member, when the radius of curvature r2 of the lower end of the hot press-formed part (C-10b) of embodiment 2-2 of FIG. 32 (b) is formed to be larger than the radius of curvature r1 of the lower end of the hot press-formed part (C-10a) of embodiment 2-1 of FIG. 32 (a), the gap, which is the remaining space after the battery pack (b) is positioned, can be compared. It can be seen that the gap g2 of FIG. 32 (b) is formed to be larger than the gap g1 of FIG. 32 (a), and as the radius of curvature (r) increases, the space that cannot be used for mounting the battery pack (b) becomes larger.

[0425] And as the radius of curvature (r) decreases, the space utilization increases, which can be seen to have greater utility when multiple cross members are mounted within the same battery pack module (C-1).

[0426] Hereinafter, a hot press-formed part according to the fifth aspect of the present invention will be described.

[0427] Fig. 33 is a perspective view illustrating a press-molded part according to one embodiment of the present invention. Fig. 34 is a perspective view illustrating a press-molded part according to another embodiment of the present invention. Fig. 35 (a) is a graph showing the relationship between a preset length and a preset height of a press-molded part according to one embodiment of the present invention, and Fig. 35 (b) is a graph showing the relationship between a preset length and a preset height of a press-molded part according to another embodiment of the present invention. In addition, Fig. 36 (a) is an example showing a strain applied to a press-molded part manufactured using a conventional manufacturing method, and Fig. 36 (b) is an example showing a strain applied to a press-molded part manufactured using a manufacturing method according to embodiments of the present invention.

[0428] Referring to FIGS. 33 and 34, press-formed parts (D-10, D-10') according to embodiments of the present invention can be manufactured by performing press-forming processing on a metal plate. At this time, a hot press forming method can be used to manufacture the press-formed parts (D-10, D-10').

[0429] The press-formed part (D-10, D-10') may include a flat portion (D-110) that is not press-formed and thus has a flat shape, and a deformed portion (D-120) that is a portion that is convex in one direction due to press-forming. At this time, a plurality of deformed portions (D-120) may be formed. In this case, the flat portions (D-110) may be arranged between a plurality of deformed portions (D-120).

[0430] The deformation portion (D-120) may have a convex shape protruding upward (+Z) based on the drawing. At this time, the deformation portion (D-120) may include a top plate portion (D-121), a longitudinal wall portion (D-122), and a boundary portion (D-123).

[0431] The top plate (D-121) may be the upper end of a deformation portion (D-120) extending in a first direction. Here, the first direction may be, for example, a direction parallel to the longitudinal direction (Y) of the press-formed part (D-10, D-10').

[0432] The longitudinal wall portion (D-122) may be a side wall portion of a deformation portion (D-120) extending along a second direction. Here, the second direction may be a different direction from the first direction described above. For example, the second direction may be a direction inclined at a predetermined angle with respect to the first direction. As another example, the second direction may be a direction perpendicular to the first direction and parallel to the vertical direction (Z) in the drawing. Meanwhile, the vertical direction (Z) described above may be parallel to the height direction of the deformation portion (D-120).

[0433] The longitudinal wall portion (D-122) may be continuous with the top plate portion (D-121). More specifically, the longitudinal wall portion (D-122) may be arranged so that its upper end is continuous with one side end of the top plate portion (D-121). A pair of such longitudinal wall portions (D-122) may be provided. A pair of longitudinal wall portions (D-122) may be arranged so as to be continuous with each of the two side ends of the top plate portion (D-121), thereby forming two side wall portions of the deformation portion (D-120).

[0434] The portion (hereinafter, shoulder portion) (D-121a) where the side end of the top plate portion (D-121) and the upper end of the longitudinal wall portion (D-122) are connected may have a round shape having a first radius of curvature. In this case, the shoulder portion (D-121a) may be convexly curved toward the outside of the deformation portion (D-120).

[0435] The boundary portion (D-123) may be a boundary between a deformation portion (D-120) and a flat portion (D-110) that are continuous along the longitudinal direction (Y) of the press-molded part (D-10, D-10'). One end of the boundary portion (D-123) may be continuous with the lower end of the longitudinal wall portion (D-122), and the other end of the boundary portion (D-123) may be continuous with one end of the flat portion (D-110). At this time, the other end of the flat portion (D-110) may be continuous with a boundary portion (D-123) provided in another deformation portion (D-120). A pair of such boundary portions (D-123) may be provided, and arranged to be continuous with each of the lower ends of the pair of longitudinal wall portions (D-122).

[0436] The boundary portion (D-123) may be rounded with a second radius of curvature. The boundary portion (D-123) may be concavely curved inwardly of the deformation portion (D-120). In this case, the second radius of curvature may be the same as or similar to the first radius of curvature.

[0437] As described above, the deformation portion (D-120) may have a shape in which a boundary portion (D-123), a longitudinal wall portion (D-122), a top plate portion (D-121), a longitudinal wall portion (D-122), and a boundary portion (D-123) are sequentially connected along the longitudinal direction (Y). Accordingly, the deformation portion (D-120) may have a shape like a hat or a similar shape.

[0438] As described above, a plurality of deformation sections (D-120) may be provided. More specifically, at least three deformation sections (D-120) may be provided. In this case, a flat section (D-110) may be arranged between two adjacent deformation sections (D-120) among the plurality of deformation sections (D-120). In this case, the two adjacent deformation sections (D-120) may be connected by the flat section (D-110) located at the center thereof.

[0439] Accordingly, the press-formed part (D-10, D-10') may have a form in which three or more deformed parts (D-120) and a plurality of flat parts (D-110) are arranged alternately in a row along the longitudinal direction (Y). At this time, the flat parts (D-110) may be arranged on both outer sides of the press-formed part (D-10, D-10') based on the longitudinal direction (Y), but the present invention is not limited thereto.

[0440] The deformation portion (D-120) can be formed with a preset height (h). At this time, the preset height (h) is the height of the deformation portion (D-120), and may mean the length from the lower surface of the flat portion (D-110) to the upper surface of the top plate portion (D-121).

[0441] Two adjacent deformation sections (D-120) may be spaced apart by a preset length (w) along the longitudinal direction (Y). Here, the preset length (w) may refer to the length of the flat section (D-110) measured with respect to the longitudinal direction (Y). At this time, the preset length (w) may be measured with respect to the flat section (D-110) positioned between the two adjacent deformation sections (D-120).

[0442] As illustrated in FIG. 33, a press-formed part (D-10) according to one embodiment of the present invention (hereinafter, Embodiment 3-1) may include three deformed portions (D-120). Accordingly, the press-formed part (D-10) of Embodiment 3-1 may include at least two flat portions (D-110) so as to be arranged between the three deformed portions (D-120).

[0443] As illustrated in FIG. 34, a press-formed part (D-10') according to another embodiment of the present invention (hereinafter, Embodiment 3-2) may include four or more deformed portions (D-120). Accordingly, the press-formed part (D-10) of Embodiment 1 may include at least three flat portions (D-110) so as to be arranged between the four or more deformed portions (D-120).

[0444] Meanwhile, although the drawing only illustrates a case where the press-formed part (D-10') has four deformation sections (D-120), the present invention is not limited thereto. Although not illustrated in the drawing, as another example, the press-formed part (D-10') may have five or more deformation sections (D-120), and in this case, the upper limit of the number of deformation sections (D-120) is not limited.

[0445] Referring to FIGS. 35 and 36, in the case of the conventional hot press forming method, the deformation portion (D-120) is first formed, and then a separate flat metal plate is bonded to the lower end of the formed deformation portion (D-120). In this case, as the number of deformation portions (D-120) to be formed increases, as exemplarily illustrated in (a) of FIG. 36, the force (deformation force) (stress) is concentrated on the vertical wall portion (D-122) to which the flat metal plates are connected and its surrounding area [A1 and B1 of FIG. 36 (a)] during the manufacturing process. As a result, there is a disadvantage in that cracks, etc. occur in the deformation portion (D-120) of the press-formed part, causing damage. To prevent this, the distance between the connected deformation parts (D-120) must be increased, but since the size of the press-formed part (D-10, D-10') to be manufactured is inevitably limited, there is a problem that the number of deformation parts (D-120) that can be included in it is also limited.

[0446] Accordingly, in one embodiment of the present invention, a press-formed part (D-10, D-10') capable of preventing damage to a deformation part (D-120) due to hot press forming is provided by utilizing the relationship between the height (preset height) (h) of a deformation part (D-120) and the distance (preset length) (w) between two adjacent deformation parts (D-120). At this time, the "relationship between the preset height (h) and the preset length (w)" can be determined based on the number of deformation parts (D-120) formed in the press-formed part (D-10, D-10').

[0447] In the case of the press-formed part (D-10) according to embodiment 3-1 of the present invention, since three deformation parts (D-120) are formed, the relationship between the “preset height (h)” of the deformation parts (D-120) and the “preset length (w)” of the distance between two adjacent deformation parts (D-120) can be defined by the following [Relational Expression 7].

[0448] w min≤w≤114.85e 0.0455h ... [Relationship 7]

[0449] Here, w may mean the preset length of embodiment 3-1, and h may mean the preset height of embodiment 3-1. And, w min may mean the minimum value (i.e., minimum interval) of the preset length (w) of embodiment 3-1.

[0450] As shown in the graph of Fig. 35 (a) and Fig. 36 (b), when three deformation sections (D-120) included in a press-formed part (D-10) are designed so that the distance (predetermined length) (w) apart from each other satisfies [Relationship 7], the occurrence of damage such as cracks in the deformation sections (D-120) and their surroundings [A2 and B2 in Fig. 36 (b)] during the hot press forming process can be minimized. At this time, the manufacturing method of the press-formed part (D-10) according to embodiment 3-1 will be described in detail below.

[0451] In addition, in the case of the press-formed part (D-10) according to embodiment 3-2 of the present invention, since four or more deformation parts (D-120) are formed, the relationship between the “preset height (h)” of the deformation parts (D-120) and the “preset length (w)” of the distance between two adjacent deformation parts (D-120) can be defined by the following [Relational Expression 8].

[0452] w min ≤w≤46.309e 0.105h ... [Relationship 8]

[0453] Here, w may mean the preset length of embodiment 3-2, and h may mean the preset height of embodiment 3-2. And, w min may mean the minimum value of the preset length of embodiment 3-2.

[0454] As shown in the graph of Fig. 35 (b) and Fig. 36 (b), when three deformation sections (D-120) included in a press-formed part (D-10) are designed so that the distance (predetermined length) (w) apart from each other satisfies [Relationship 8], the occurrence of damage such as cracks in the deformation sections (D-120) and their surroundings [A2 and B2 in Fig. 36 (b)] can be minimized during the hot press forming process. At this time, the manufacturing method of the press-formed part (D-10) according to embodiment 3-1 will be described in detail below.

[0455] Meanwhile, in the above-described embodiments 3-1 and 3-2, the preset length (w) may be 20 mm or more. That is, the minimum distance (w) between two adjacent deformation parts (D-120) min ) may be 20 mm. This may be the minimum gap required to secure space for arranging cooling holes (not shown) between the first pressurizing parts (D-210) provided in the molds (D-200, D-400) and / or between the second pressurizing parts (D-410) and the intermediate pressurizing parts (D-300) in the manufacturing device (D-20, D-20') of the press-molded parts (D-10, D-10') to be described later.

[0456] Fig. 37 is a flowchart illustrating a method for manufacturing a press-formed part according to one embodiment of the present invention. Fig. 38 schematically illustrates a first step for manufacturing the press-formed part of Fig. 33. Fig. 39 schematically illustrates a second step for manufacturing the press-formed part of Fig. 33. Fig. 40 schematically illustrates a third step for manufacturing the press-formed part of Fig. 33. And, Fig. 41 schematically illustrates a fourth step for manufacturing the press-formed part of Fig. 33.

[0457] Referring to FIGS. 37 to 41, a manufacturing device (D-20) for manufacturing a press-molded part (D-10) according to embodiment 3-1 may include a first mold (D-200), an intermediate pressurizing part (D-300), and a second mold (D-400). In this case, one intermediate pressurizing part (D-300) may be provided. At this time, based on the vertical direction (Z), the first mold (D-200) may be an upper mold, and the second mold (D-400) may be a lower mold.

[0458] The first mold (D-200) and the second mold (D-400) may be arranged to face each other along the pressing direction (-Z). The first mold (D-200) may be arranged above the second mold (D-400). At this time, the first mold (D-200) may be installed on an upper support (not shown) that can be elevated. Accordingly, the first mold (D-200) may be lowered toward the second mold (D-400) or elevated away from the second mold (D-400). In addition, the second mold (D-400) may be arranged on a lower support (not shown) that is fixedly installed on the floor surface of a building where the manufacturing device (D-20) is installed, for example.

[0459] The first mold (D-200) may be provided with a first pressurizing portion (D-210). The first pressurizing portion (D-210) may pressurize the upper surface of a metal plate during press forming. Here, the metal plate may be a material used to manufacture a press-formed part (D-10). Before press forming, the metal plate may have, for example, a flat upper surface and a flat lower surface. At this time, the first pressurizing portion (D-210) may be configured to have a shape and size corresponding to the "upper surface of the deformed portion (D-120) of the press-formed part (D-10)" described above.

[0460] The manufacturing device (D-20) is for manufacturing the press-molded part (D-10) of embodiment 3-1, and may be provided with three first pressurizing parts (D-210). At this time, the three first pressurizing parts (D-210) may be spaced apart from each other by a first distance. The first distance may be the same distance as the preset length (w) described above. In addition, the first mold (D-200) may be provided with a first flat part (D-220). The first flat part (D-220) may be arranged between two adjacent first pressurizing parts (D-210). The first flat part (D-220) may extend parallel to the longitudinal direction (Y) between the two first pressurizing parts (D-210), thereby connecting the first pressurizing parts (D-210). As described above, since three first pressurized portions (D-210) are provided, at least two first flat portions (D-220) may be provided.

[0461] Accordingly, the first mold (D-200) may be formed such that three first pressurizing portions (D-210) are spaced apart from each other by a first distance (i.e., a preset length) (W) along the longitudinal direction (Y), and a first flat portion (D-220) is arranged between them to connect the first pressurizing portions (D-210).

[0462] The second mold (D-400) may be provided with a second pressurizing portion (D-410) and a second flat portion (D-420a). At this time, the second pressurizing portion (D-410) may be formed in a shape and size corresponding to the lower surface of the press-molded part (D-10).

[0463] Unlike the first mold (D-200), the second mold (D-400) may be provided with only two second pressurizing portions (D-410). In this case, the two second pressurizing portions (D-410) may be arranged symmetrically with the two first pressurizing portions (D-210) located on both sides among the three first pressurizing portions (D-210) along the longitudinal direction (Y). Accordingly, in the second mold (D-400), the second pressurizing portion (D-410) may not exist in the area (hereinafter, the center area) facing the first pressurizing portion (D-210) located in the middle among the three first pressurizing portions (D-210).

[0464] Accordingly, the second mold (D-400) may be a single structure having a form connected by two second pressurizing portions (D-410) and a second flat portion (D-420a) disposed therebetween. At this time, the flat portion (D-420a) may be provided with a penetrating portion (D-420aa). When press forming the outer region of the metal plate (D-10A), an intermediate pressurizing portion (D-300) may be inserted into the penetrating portion (D-420aa). The penetrating portion (D-420aa) may be formed by penetrating a portion of the flat portion (D-420a) along the vertical direction (Z). This penetrating portion (D-420aa) may have a cross-sectional shape and an area corresponding to the shape and size of the intermediate pressurizing portion (D-300). Additionally, a second-second flat portion (D-420b) may be further provided on each side of the first pressurized portions (D-210).

[0465] The intermediate pressurizing portion (D-300) may be placed between the first mold (D-200) and the second mold (D-400). As illustrated in Fig. 38, the metal plate (D-10A), which is a press-molding target, may be placed on the lower side of the first mold (D-200). In this case, the intermediate pressurizing portion (D-300) is positioned between the metal plate (D-10A) and the second mold (D-400), and may pressurize a portion of the lower surface of the metal plate (D-10A) when the first mold (D-200) is lowered. The intermediate pressurizing portion (D-300) may be, for example, a pad for pressing and bending the lower surface of the metal plate (D-10A).

[0466] The intermediate pressurizing portion (D-300) may be positioned directly above the above-described central region of the second mold (D-400). At this time, the intermediate pressurizing portion (D-300) may have the same shape as the second pressurizing portion (D-410). That is, the intermediate pressurizing portion (D-300) may have a shape and size corresponding to the lower surface of the press-molded part (D-10). During press molding, the intermediate pressurizing portion (D-300), together with the middle first pressurizing portion (D-210) among the three first pressurizing portions (D-210), may pressurize the metal plate (D-10A) upward and downward to form the deformation portion (D-120) located in the center.

[0467] The intermediate pressurizing section (D-300) may be configured to be elevable. Accordingly, the intermediate pressurizing section (D-300) can descend along the pressing direction (-Z) during press forming. In addition, the intermediate pressurizing section (D-300) can rise in the opposite direction (Z) to the pressing direction after press forming is completed. This elevation of the intermediate pressurizing section (D-300) can be realized by an elastic member (not shown) installed in the intermediate pressurizing section (D-300).

[0468] The “manufacturing method of a press-molded part (D-10) according to embodiment 3-1” using the above manufacturing device (D-20) may be as follows.

[0469] First, a metal plate (D-10A) can be supplied to a manufacturing device (D-20) (S100). Before press forming begins, as illustrated in Fig. 38, the metal plate (D-10A) can be placed at a position (hereinafter referred to as a processing position) between the first mold (D-200) and the intermediate pressurizing portion (D-300). Meanwhile, the metal plate (D-10A) can be supplied to the manufacturing device (D-20) while being heated by a separate heating device (not illustrated).

[0470] For example, when the metal plate (D-10A) is the processing position, the upper surface of the metal plate (D-10A) may be positioned so that it contacts the lower surface of the first mold (D-200) or there is only a slight gap therebetween. At this time, the second mold (D-400) may be positioned symmetrically with the first mold (D-200), with the metal plate (D-10A) and the intermediate pressurizing portion (D-300) interposed therebetween.

[0471] In addition, when the metal plate (D-10A) is placed at the processing position (i.e., before press forming begins), the intermediate pressurizing portion (D-300) may be placed on the upper side of the second mold (D-400). Accordingly, a height difference (hi) may exist between the intermediate pressurizing portion (D-300) and the second pressurizing portion (D-410). Here, the height difference (hi) may mean the distance from the upper surface of the second pressurizing portion (D-410) to the upper surface of the intermediate pressurizing portion (D-300). In this case, the height difference (hi) between the intermediate pressurizing portion (D-300) and the second pressurizing portion (D-410) may be smaller than the “preset height (h)”, which is the height of the press-formed part (D-10). At this time, the height difference (hi) may be 65% or more of the preset height (h). When the manufacturing device (D-20) is configured to have such a height difference (hi), damage such as cracks in the deformed portion (D-120) formed through press forming can be minimized.

[0472] Next, the metal plate (D-10A) can be first press-formed by the first mold (D-200) and the intermediate pressurizing portion (D-300) (S200). As illustrated in Fig. 39, the first mold (D-200) can be lowered along the pressing direction (-Z) to pressurize the upper surface of the metal plate (D-10A). In this case, the first mold (D-200) can be lowered to a position where the lower surface of the metal plate (D-10A) does not come into contact with the second mold (D-400). At this time, since the intermediate pressurizing portion (D-300) is positioned higher than the second pressurizing portion (D-410) by the height difference (hi), only the intermediate pressurizing portion (D-300) is inserted into the first pressurizing portion (D-210) and can pressurize the center region of the metal plate (D-10A). Accordingly, only the central region of the metal plate (D-10A) can be pressed in the vertical direction by the central first pressing portion (D-210) among the three first pressing portions (D-210) and the intermediate pressing portion (D-300). Accordingly, only the central region of the metal plate (D-10A) can be bent, thereby forming one deformed portion (D-120).

[0473] Next, the metal plate (D-10A) can be secondarily press-formed by the first mold (D-200) and the second mold (D-400) (S300). After the first press-formed is performed in step S200, the first mold (D-200) can be further lowered along the pressing direction (-Z), as illustrated in FIG. 40. In this process, the intermediate pressing portion (D-300) can be lowered together with the first mold (D-200) while being inserted into the central first pressing portion (D-210) together with the central region of the metal plate (D-10A). Accordingly, the outer region of the metal plate (D-10A) can come into contact with the upper surfaces of the two second pressing portions (D-410).

[0474] Thereafter, as the first mold (D-200) continues to descend, the second pressurizing portion (D-410) is introduced into the first pressurizing portion (D-210) and can pressurize both outer regions of the metal plate (D-10A). In this process, the intermediate pressurizing portion (D-300) can be accommodated into the through portion (D-420aa) of the second mold (D-400). Both outer regions of the metal plate (D-10A) can be simultaneously pressed in the vertical direction by the two outer first pressurizing portions (D-210) and the two outer second pressurizing portions (D-410). As a result, both outer regions of the metal plate (D-10A) are bent, so that two additional deformed regions (D-120) can be formed.

[0475] Next, the first mold (D-200) and the intermediate pressurizing portion (D-300) can be returned to their original positions (S400). After the "stepwise press forming" in steps S200 and S300 as described above, as illustrated in FIG. 9, the first mold (D-200) can be raised in the opposite direction (+Z) to the pressing direction (-Z). In this process, the deformation portions (D-120) can be separated from the first pressure portions (D-210). In addition, the intermediate pressurizing portion (D-300) can be separated from the central deformation portion (D-120) and returned to its original position. As a result, a press-formed part (D-10) having three deformation portions (D-120) having a preset height (h) and spaced apart from each other by a preset length (w) can be manufactured.

[0476] Then, the press-formed part (D-10) that has been manufactured can be discharged (S500). After being discharged from the manufacturing device (D-20), the press-formed part (D-10) can be transported for the next processing or manufacturing process. Thereafter, the next metal plate (D-10A) is supplied, so that the aforementioned manufacturing method can be repeatedly performed.

[0477] Fig. 42 schematically illustrates the first step of manufacturing the press-formed part of Fig. 34. Fig. 43 schematically illustrates the second step of manufacturing the press-formed part of Fig. 34. Fig. 44 schematically illustrates the third step of manufacturing the press-formed part of Fig. 34. And Fig. 45 schematically illustrates the fourth step of manufacturing the press-formed part of Fig. 34.

[0478] Referring to FIG. 37 and FIG. 42 to FIG. 45, a press-molded part (D-10) according to embodiment 3-2 can be manufactured through the following manufacturing method. A manufacturing device (D-20') for a press-molded part can include a first mold (D-200), an intermediate pressurizing part (D-300), and a second mold (D-400). At this time, at least two intermediate pressurizing parts (D-300) can be provided. In this case, most of the features of the first mold (D-200), the intermediate pressurizing part (D-300), and the second mold (D-400) are the same or similar to those of the aforementioned embodiment 3-1, and therefore, a duplicate description thereof will be omitted.

[0479] Since the press-molded part (D-10') according to embodiment 3-2 is provided with four or more deformation sections (D-120), the first mold (D-200) may be provided with four or more first pressurizing sections (D-210). For convenience of explanation, the following description will focus on a case where four first pressurizing sections (D-210) are provided and are spaced apart from each other by a preset length (w). In this case, two first pressurizing sections (D-210) may be arranged in the center area of ​​the first mold (D-200), and one first pressurizing section (D-210) may be arranged on each side of the center.

[0480] The second mold (D-400) may be provided with two second pressurizing parts (D-410). At this time, the two second pressurizing parts (D-410) may be arranged to face the two first pressurizing parts (D-210) arranged on each side of the first mold (D-200).

[0481] When four first pressurizing parts (D-210) are provided, two intermediate pressurizing parts (D-300) may be provided. In this case, the two intermediate pressurizing parts (D-300) may be arranged to face the two first pressurizing parts (D-210) arranged in the center area of ​​the first mold (D-200). As in the case described above, the intermediate pressurizing parts (D-300) are arranged between the first mold (D-200) and the second mold (D-400).

[0482] In addition, in the case of embodiment 3-2, the second flat portion (D-420a) of the second mold (D-400) may be provided with two through-hole portions (D-420aa). Accordingly, when the outer region of the metal plate (D-10A) is press-formed by the first mold (D-200) and the second mold (D-400), the two intermediate pressurizing portions (D-300) can be accommodated in the through-hole portions (D-420aa), respectively.

[0483] In the case of the "method for manufacturing a press-molded part (D-10') according to Embodiment 3-2" using the above-described manufacturing device (D-20'), steps S100, S200, S300, and S400 can be sequentially performed, similarly to the method for manufacturing the press-molded part (D-10) of Embodiment 3-1. At this time, since the specific manufacturing method performed at each step is the same or similar to that of Embodiment 3-1, the description will focus on the differences.

[0484] First, in step S100, as illustrated in FIG. 42, the metal plate (D-10') may be supplied to the aforementioned processing position. In this case, the metal plate (D-10A) may be supplied to the manufacturing device (D-20') in a heated state by a separate heating device (not illustrated). When supplied to the processing position, two intermediate pressurizing portions (D-300) may be positioned below the center region of the metal plate (D-10'). At this time, the two intermediate pressurizing portions (D-300) may be arranged at the same height. In this case, the height difference (hi) between the intermediate pressurizing portions (D-300) and the second pressurizing portions (D-410) is the same as in embodiment 3-1.

[0485] Next, in step S200, as illustrated in FIG. 43, as the first mold (D-200) is lowered, the intermediate pressurizing portions (D-300) are each introduced into the two first pressurizing portions (D-210) arranged in the central region of the first mold (D-200), and the central region of the metal plate (D-10') can be pressed. By this first press forming step, two deformation portions (D-120) can be formed in the central region of the metal plate (D-10A).

[0486] Next, in step S300, as illustrated in FIG. 44, the first mold (D-200) may be lowered further, and the second pressurizing portions (D-410) may be introduced into the two first pressurizing portions (D-210) arranged in the outer region of the first mold (D-200), respectively. Accordingly, the outer region of the metal plate (D-10') may be pressed in the vertical direction by the first pressurizing portions (D-210) and the second pressurizing portions (D-410). By this second press forming step, two deformation portions (D-120) may be additionally formed in the outer region of the metal plate (D-10A).

[0487] Next, in step S400, as illustrated in FIG. 45, the first mold (D-200) can be raised in the opposite direction (+Z) to the pressing direction (-Z). Accordingly, the press-formed part (D-10') in a press-formed state can be separated from the first mold (D-200), the intermediate pressurizing part (D-300), and the second mold (D-400). In this process, the two intermediate pressurizing parts (D-300) can be raised and returned to their original positions at the processing positions described above.

[0488] Then, after the press-formed part (D-10') that has been manufactured is discharged from the manufacturing device (D-20'), the next press-formed process can begin as a new metal plate (D-10') is supplied.

[0489] Meanwhile, when the number of deformed parts (D-120) formed in the press-molded part (D-10') increases, the number of intermediate pressurized parts (D-300) may also increase accordingly.

[0490] For example, although not shown in the drawing, if five deformation portions (D-120) are formed in a press-molded part (D-10'), the first mold (D-200) must be provided with five first pressurizing portions (D-210). Accordingly, three intermediate pressurizing portions (D-300) may be provided, and may be arranged to face the three first pressurizing portions (D-210) arranged in the center area of ​​the first mold (D-200). In this case, the three intermediate pressurizing portions (D-300) may be arranged between the first mold (D-200) and the second mold (D-400).

[0491] Among the three intermediate pressurizing parts (D-300), one intermediate pressurizing part (hereinafter, the first intermediate pressurizing part) (D-300) arranged in the middle may be arranged at the highest position. The remaining two intermediate pressurizing parts (hereinafter, the second intermediate pressurizing parts) (D-300) may be arranged between the first intermediate pressurizing part (D-300) and the second mold (D-400). At this time, the two second intermediate pressurizing parts (D-300) may be arranged at the same height. The height difference between the first intermediate pressurizing part (D-300) and the second intermediate pressurizing part (D-300) may be the same as or similar to the height difference (hi) between the aforementioned intermediate pressurizing part (D-300) and the second pressurizing part (D-410).

[0492] In this arranged state, first press forming can be performed by the first mold (D-200) and the first intermediate press portion (D-300). When the first press forming is completed or immediately before completion, the first mold (D-200) is lowered further together with the first intermediate press portion (D-300), so that second press forming can be performed by the first mold (D-200) and the two second intermediate press portions (D-300). By these two press formings, forming of the center region of the metal plate (D-10A) can be performed first. Thereafter, after the second press forming is completed or immediately before completion, the first mold (D-200) is lowered further, so that third press forming of the outer region of the metal plate (D-10A) can be performed by the first mold (D-200) and the second mold (D-400).

[0493] As another example, although not shown in the drawing, if six deformation portions (D-120) are formed in the press-molded part (D-10'), the first mold (D-200) must be provided with six first pressurizing portions (D-210). Accordingly, four intermediate pressurizing portions (D-300) may be provided, and may be arranged to face the four first pressurizing portions (D-210) arranged in the center area of ​​the first mold (D-200). In this case, the four intermediate pressurizing portions (D-300) may be arranged between the first mold (D-200) and the second mold (D-400).

[0494] Among the four intermediate pressurizing parts (D-300), the two intermediate pressurizing parts (hereinafter, first intermediate pressurizing parts) (D-300) arranged in the middle may be arranged at the highest position. The remaining two intermediate pressurizing parts (hereinafter, second intermediate pressurizing parts) (D-300) may be arranged between the first intermediate pressurizing part (D-300) and the second mold (D-400). At this time, the two first intermediate pressurizing parts (D-300) may be arranged at the same height. And, the two second intermediate pressurizing parts (D-300) may also be arranged at the same height. The height difference between the first intermediate pressurizing part (D-300) and the second intermediate pressurizing part (D-300) may be the same as or similar to the height difference (hi) between the aforementioned intermediate pressurizing part (D-300) and the second pressurizing part (D-410).

[0495] In this arrangement, first press forming can be performed by the first mold (D-200) and two first intermediate press portions (D-300). When the first press forming is completed, or immediately before completion, the first mold (D-200) is lowered further together with the first intermediate press portions (D-300), so that second press forming can be performed by the first mold (D-200) and two second intermediate press portions (D-300). By these two press formings, forming of the central region of the metal plate (D-10A) can be performed first. Thereafter, after the second press forming is completed or just before completion, the first mold (D-200) is lowered further, whereby the third press forming can be performed on the outer area of ​​the metal plate (D-10A) by the first mold (D-200) and the second mold (D-400).

[0496] In addition, when the number of deformation sections (D-120) formed in the press-molded part (D-10') increases to 7 or more, the number of first press sections (D-210) and intermediate press sections (D-300) must be further increased similarly to the above. Accordingly, the number of penetration sections (D-420aa) in the second mold (D-400) can also be increased. With the manufacturing device configured in this manner, by adding a press-molding step, press-molding can be performed first on the center region of the metal plate (D-10A), and then press-molding can be performed on the outer region of the metal plate (D-10A).

[0497] The press-formed parts (D-10, D-10') and the manufacturing method (S10) thereof according to the embodiments of the present invention as described above can perform press forming on a metal plate (D-10, D-10') by arranging an intermediate press portion (D-300) having a height difference (hi) between upper / lower molds (D-200, D-400) and using three press portions (D-210, D-300, D-410). At this time, through a step-by-step forming process of first pressing the center region of the metal plate (D-10, D-10') and then sequentially pressing the outer region, it is possible to prevent deformation force from being concentrated at a specific location of the metal plate (D-10, D-10') during press working. As a result, the press-formed parts (D-10, D-10') can be prevented from being damaged during the manufacturing process, and the quality of the parts can be improved.

[0498] Although the embodiments of the present invention have been described with reference to the attached drawings, those skilled in the art will appreciate that the present invention can be implemented in other specific forms without altering the technical spirit or essential characteristics thereof. Therefore, the embodiments described above should be understood to be illustrative in all respects and not restrictive.

[0499] [Explanation of symbols]

[0500] A-10: Upper die

[0501] A-12: Cooling Channel

[0502] A-20: Lower punch

[0503] A-22: Cooling channel

[0504] A-30: Lower holder

[0505] A-32: Heating heater

[0506] A-34: Cooling channel

[0507] A-100: Mold

[0508] A-200: Steel

[0509] A-300: 22MnB5 material

[0510] B-1: Side Frame B-10: Frame

[0511] B-20: Bracket B-21, B-22: Flange

[0512] B-23: Body B-24: Curved surface

[0513] B-100: Punch

[0514] B-110, B-120, B-130, B-140, B-150: 1st to 5th punch forming sections

[0515] B-111, B-121, B-131, B-141, B-151: First to fifth extension surface forming parts

[0516] B-112, B-122, B-132, B-142, B-152: First to fifth curved forming sections

[0517] B-113, B-123, B-133, B-143: First to fourth main body molding parts

[0518] B-200: Pad

[0519] B-300: Die

[0520] B-310, B-320, B-330, B-340, B-350: 1st to 5th die forming sections

[0521] B-311, B-321, B-331, B-341, B-351: 1st to 5th die curved forming sections

[0522] B-312, B-322, B-332, B-342, B-352: 1st to 5th die flat forming sections

[0523] B-360: Vertical plane

[0524] B-400: Material B-410: Notch

[0525] C-1: Battery pack module C-10: Hot press molded part

[0526] C-20: Battery Case C-30: Base

[0527] C-40: Reinforcing member C-110: First wall

[0528] C-120: Second wall C-130: Third wall

[0529] C-140: 4th wall C-150: 5th wall

[0530] C-160: Curved section C-170: First connecting section

[0531] C-180: Second connection C-190: Third connection

[0532] b: battery pack r: radius of curvature

[0533] t: plate thickness θw: wall angle

[0534] hw: wall height

[0535] D-10, D-10': Press-formed parts

[0536] D-10A: Metal plate

[0537] D-110: Reputation Department

[0538] D-120: Transformation section

[0539] D-20, D-20': Press forming device

[0540] D-200: Mold 1

[0541] D-210: First projection

[0542] D-220: First plane section

[0543] D-300: Intermediate pressurization section

[0544] D-400: Second mold

[0545] D-410: Second protrusion

[0546] D-420: Second plane

[0547] 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 further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0548] (Example 1)

[0549] Manufacturing of steel sheets for hot forming

[0550] A steel slab (thickness: 100 mm) having the composition (remaining components of the alloy composition: Fe and unavoidable impurities) disclosed in Table 2 below was manufactured through vacuum melting. The steel slab was heated to 1250°C, finish-hot rolled at 900°C, cooled at a cooling rate of 30°C / s, and coiled at 640°C to obtain a hot-rolled steel sheet (final thickness: 2.5 mm). Thereafter, the hot-rolled steel sheet was pickled and cold-rolled at a reduction ratio of 45% to obtain a cold-rolled steel sheet. Thereafter, the cold-rolled steel sheet was annealed at 780°C under a 5% hydrogen-95% nitrogen atmosphere under the conditions of the dew point temperature described in Table 3 below, pickled under the conditions described in Table 3 below, and cooled to room temperature to manufacture a steel sheet for hot forming.

[0551] Manufacturing of hot-formed parts

[0552] After preparing a blank using a steel plate for hot forming, the blank was transferred into a heating furnace and heat-treated under the conditions described in Table 3 below. Thereafter, the heat-treated blank was hot-formed using a hot-forming mold and then cooled to manufacture a hot-formed part.

[0553] Steel alloy composition (weight %) CSiMnPSAlCrTiBNA0.2551.550.830.010.0020.03971.530.03070.00280.0035B0.31.530.670.0070.00080.0351.00.0140.00250.004C0.251.480.7930.01010.00250.0274.50.0280.00280.0056D0.120.961.60.0080.0030.0511.93 0.0150.0020.001E0.31.530.390.0070.00080.0351.00.0140.00250.004F0.291.490.730.00920.00070.0520.90.0150.0 0260.0037G0.2910.51.210.0080.0020.0532.00.0160.00180.001H0.122.893.840.0080.0030.00342.30.0150.0020.001

[0554] ClassificationSteel gradeManufacturing conditions of steel sheets for hot formingManufacturing conditions of hot forming membersDew point temperature (℃)Pickling time (sec)Heat treatment temperature (℃)Heat treatment time (sec)Invention example A-1A-105900300Comparative example A-1A-1014900300Comparative example A-2A55900300Invention example A-2B-257900300Comparative example A-3B-105900820Comparative example A-4B-2861120192Invention example A-3C-207930270Comparative example A-5C-405930270Comparative example A-6C-2018930270Invention example A-4D-258930360Comparative example A-7D-258930660Comparative example A-8E-205900420Comparative Example A-9F-205900420Comparative Example A-10G-205900420Invention Example A-5H-106900600Comparative Example A-11H-1061050650

[0555] For each hot forming steel sheet, the value of R defined by the following [Relational Expression 1] was calculated to evaluate the degree of formation of a concentrated layer on the surface, and the results are shown in Table 4.

[0556] [Relationship 1]

[0557] R = ([Cr] + [Mn] + [Si]) / ([Cr*] + [Mn*] + [Si*])

[0558] For each steel plate, glow discharge emission spectrometry (GDS) analysis was performed from the surface to the depth direction. Based on the GDS profile, the maximum contents (in wt%) of Cr, Mn, and Si within the range of 0.01 to 1.0 ㎛ in the thickness direction of the steel plate were expressed as [Cr], [Mn], and [Si] in [Relationship 1], and the contents (in wt%) of Cr, Mn, and Si contained in the steel slab during the manufacture of each steel plate were expressed as [Cr*], [Mn*], and [Si*].

[0559] In addition, for each hot-formed member, the physical properties (yield strength (YS), tensile strength (TS), yield ratio (YS) and elongation (EL)) and surface quality (Fe 40~80% The results are shown in Table 4, and the results are evaluated by visual inspection and standard gloss.

[0560] Specifically, to evaluate the physical properties, a tensile test was conducted at room temperature using JIS-5 specimens according to the ISO6892 standard.

[0561] Fe 40~80% In the case of slope, glow discharge emission spectrometry (GDS) analysis was performed in the depth direction from the surface of the member. Based on the GDS profile, the slope of the straight line in the section where the Fe content (weight %) is 40 to 80% within the range of 5.0 ㎛ or less in the thickness direction of the member was measured to obtain Fe 40~80% It is expressed as a slope. More specifically, the slope of the straight line is expressed as the slope derived after simple linear regression of the graph in the section where the Fe content (weight %) is 40 to 80% in the GDS profile.

[0562] For visual inspection, the surface quality was evaluated as good when light was reflected by shining light on the surface of each sample, and the surface quality was evaluated as poor when no light was reflected.

[0563] For standard gloss, the standard gloss (angle between the incident angle and the normal vector to the surface of the material: 60°) of each material surface was measured using the ISO2813 standard. The standard gloss standard is 25 GU. If the standard gloss is 25 GU or higher, good surface quality can be secured, and good surface quality can be secured even without the essential shot blasting process.

[0564] Hot forming steel plate for hot forming [Relationship 1] Surface quality YS (MPa) TS (MPa) YRE l (%) Fe 40~80% Tilt Visual inspection Gloss (GU) Invention example A-13.2 1198 169 50.7 16.1 230 9.0 Good 59.2 Comparative example A-12.1 1150 166 00.696 84 95.3 Poor 24.7 Comparative example A-21.8 1054 166 00.63 6.64 23.9 Poor 11.9 Invention example A-26.5 129 119 050.686 128 7.8 Good 66.0 Comparative example A-35.2 130 8 189 60.696 26 1.4 Poor 19.9 Comparative example A-44.7 135 5 19 03 0.716 4 32.9 Poor 14.7 Invention example A-33.6116818990.625.9216.6Good62.1Comparative Example A-51.6126519000.676.239.0Poor19.5Comparative Example A-62.4112518010.626.769.3Poor24.5Invention Example A-43.285312670.679.8375.4Good84.3Comparative Example A-73.689412530.719.2768.2Poor10.6Comparative Example A-82.7122319150.645.9384.0Poor6.5Comparative Example A-92.2121518970.646.0139.0Poor5.2Comparative Example A-102.6121019090.635.956.3 Defective 4.7 Invention example A-54.895013870.688.1617.6 Good 109.3 Comparative example A-114.786213590.638.313.5 Good 3.8

[0565] As shown in Table 4, in the case of invention examples A-1 to A-5 satisfying the alloy composition and manufacturing conditions of the present invention, Cr, Si, and Mn were appropriately concentrated on the surface layer of the steel sheet for hot forming, and thus, excellent surface quality of the hot-formed part could be secured even without a shot blasting process. In addition, it was confirmed that the physical properties of the hot-formed part were also secured at the target level.

[0566] Meanwhile, the hot-formed steel sheet of Comparative Example A-1 had a low R value because the pickling time exceeded the range of the example, confirming that Cr, Si, and Mn were not properly concentrated on the surface. This was found to be because the long pickling time caused erosion by the pickling solution, removing the concentrated layer. Accordingly, the hot-formed member of Comparative Example A-1 had poor surface quality due to the formation of excessive Fe scale on the surface.

[0567] Since the hot-formed steel sheet of Comparative Example A-2 had a dew point temperature exceeding the range of the example, the value of R defined by [Relationship 1] was measured to be low, and it was confirmed that Cr, Si, and Mn were not properly concentrated on the surface, and Fe oxides were formed on the surface. Accordingly, the hot-formed member of Comparative Example A-2 had excessive Fe scales formed on the surface, failed to secure a surface gloss of 25 GU or higher, and had poor surface quality.

[0568] Although the hot-forming steel sheets of Comparative Examples A-3 and A-7 had R values ​​within the example range, the hot-forming members of Comparative Examples A-3 and A-7 had excessive Fe oxides formed on the surface because the heat treatment time exceeded the example range, and thus the surface quality deteriorated.

[0569] In the hot-formed member of Comparative Example A-4, since the heat treatment temperature exceeded the example range, Fe oxide was excessively formed on the surface, and thus the surface quality was deteriorated.

[0570] Since the hot-formed steel sheet of Comparative Example A-5 had a dew point temperature below the range of the example, the value of R defined by [Relationship 1] was measured to be low, confirming that the concentration of Cr, Si, and Mn was not sufficient. Accordingly, the hot-formed member of Comparative Example A-5 had excessive Fe scale formed on the surface, failed to secure a surface gloss of 25 GU or higher, and had poor surface quality.

[0571] It was confirmed that the hot-formed steel sheet of Comparative Example A-6 had uneven surface shape, color, etc. because the pickling time exceeded the range of the example. Accordingly, the hot-formed member of Comparative Example A-6 had poor surface quality due to the formation of excessive Fe scale on the surface.

[0572] Since the contents of Cr, Si, and Mn in Comparative Examples A-8 to A-10 were below the range of the example, the value of R defined by [Relationship 1] was measured to be low, confirming that the concentration of Cr, Si, and Mn was not sufficient. Accordingly, the hot-formed parts of Comparative Examples A-8 to A-10 had excessive Fe oxide formed on the surface, failed to secure a surface gloss of 25 GU or higher, and had poor surface quality.

[0573] Since the hot-formed member of Comparative Example A-11 had a heat treatment temperature and time exceeding the range of the example, excessive Fe oxide was formed on the surface, a surface gloss of 25 GU or higher was not secured, and the surface quality was deteriorated.

[0574] Meanwhile, Fig. 2 is a graph showing the GDS profile of Fe after hot forming for Invention Example A-1 and Comparative Example A-11. Specifically, Fig. 2 (a) is a graph showing the GDS profile of the hot-formed part of Invention Example A-1, and Fig. 2 (b) is a graph showing the GDS profile of the hot-formed part of Comparative Example A-11.

[0575] Referring to (a) of Fig. 2, in the case of invention example A-1, Cr, Si and Mn were sufficiently concentrated on the surface of the steel plate, thereby minimizing the formation of Fe oxide on the surface of the member during the hot forming process, and therefore Fe 40~80% It was found that the slope was 200 wt% / ㎛ or more.

[0576] Referring to (b) of Fig. 2, in the case of Comparative Example A-11, excessive Fe oxide was formed on the surface during the hot forming process, and therefore Fe 40~80% It was found that the slope was measured to be less than 200 wt% / ㎛.

[0577] (Example 2)

[0578] Manufacturing of Invention Example B

[0579] A steel material having a martensitic transformation critical cooling rate of 18°C / s in the temperature range of 800 to 400°C was heated to 930°C and maintained for 60 seconds, and then air-cooled to 730°C at a cooling rate of 15°C / s. Thereafter, the steel material was placed in a hot press forming facility as shown in Fig. 8, and the steel material was supported between the lower holder and the upper die while being heated by applying a blank holding force (F) and heat through the lower holder. At this time, the blank holding pressure (P) was 5 MPa, and the contact area (A) between the lower holder and the steel material was 30,000 mm. 2 That is, the blank holding force (F) was 150 kN. The temperature of the lower holder was 600°C. The forming temperature of the steel (T blank ) is 730℃, the thickness of the steel (w) is 1.5 mm, and the density of the steel (ρ) is 7.7×10 -9 ton / mm 3 , the specific heat (C) of steel is 6.7×10 8mJ / ton K. The heat transfer coefficient (a) of the steel was 2.3, and the heat transfer index (b) of the steel was 0.18. Afterwards, the steel was formed and cooled at a cooling rate of 20°C / s. Meanwhile, the temperature of all molds except the lower holder was set to 80°C. The flange wrinkle height of the hot press-formed part manufactured through this was measured and found to be 0.5 mm. At this time, the wrinkle height was calculated as the difference between the maximum and minimum heights of the flange wrinkle.

[0580] Manufacturing of Comparative Example B

[0581] Fig. 9 is a schematic diagram showing a hot press forming equipment applied to 22MnB5 material. 22MnB5 material (A-300) was heated to 930°C and maintained for 60 seconds, and then air-cooled to 730°C at a cooling rate of 15°C / s. Thereafter, the 22MnB5 material (A-300) was placed in a hot press forming equipment as illustrated in Fig. 7, and the steel material was formed and cooled at a cooling rate of 50°C / s. At this time, the gap between the upper die and the lower holder was set to 3 mm, and the temperatures of all molds were set to 80°C. The flange wrinkle height of the hot press formed part manufactured through this was measured and found to be 3.0 mm. At this time, the wrinkle height was calculated as the difference between the maximum and minimum heights of the flange wrinkle.

[0582] 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. Contains, by weight%, carbon (C): 0.1 to 0.45%, silicon (Si): 0.8 to 3.0%, manganese (Mn): 0.5 to 4.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.02%, aluminum (Al): 0.001 to 0.1%, chromium (Cr): 1.0 to 5.0%, nitrogen (N): 0.001 to 0.02%, the remainder being iron and unavoidable impurities. A steel sheet for hot forming, having a value of R defined by the following [Relational Formula 1] of 2.8 or more. [Relationship 1] R = ([Cr] + [Mn] + [Si]) / ([Cr*] + [Mn*] + [Si*]) (In the above [Relational Expression 1], [Cr], [Mn], and [Si] represent the maximum content (weight %) of each element within a range of 0.01 to 1.0 ㎛ in the thickness direction from the surface of the steel plate in the GDS profile, and [Cr*], [Mn*], and [Si*] represent the average content (weight %) of each element in the steel plate.) 2. In paragraph 1, The hot forming steel sheet further comprises at least one selected from the following a) to f). a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.001 to 0.4 wt%, b) Boron (B): 0.01 wt% or less (excluding 0%) c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.0 wt%, d) At least one of copper (Cu) and nickel (Ni): 0.005 to 2.0 wt%, e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.0 wt% f) Rare earth elements (REM): 0.0001~0.02 wt% 3. In paragraph 1, The microstructure of the hot forming steel sheet comprises ferrite and cementite.

4. In paragraph 3, A steel sheet for hot forming, wherein the sum of the area fractions of the above ferrite and the above cementite is 5% or more.

5. In paragraph 1, The above hot forming steel sheet is a hot forming steel sheet having a tensile strength of 500 MPa or more.

6. A step of heating a slab containing, by weight%, carbon (C): 0.1 to 0.45%, silicon (Si): 0.8 to 3.0%, manganese (Mn): 0.5 to 4.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.02%, aluminum (Al): 0.001 to 0.1%, chromium (Cr): 1.0 to 5.0%, nitrogen (N): 0.001 to 0.02%, the remainder iron and unavoidable impurities; A step of final hot rolling the above heated slab to obtain a hot rolled steel sheet; A step of cooling the above hot-rolled steel plate; A step of coiling the cooled hot-rolled steel plate; A step of cold rolling the above-mentioned coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; A step of annealing the above cold rolled steel sheet at a dew point temperature of -30 to 0°C at 700 to 1000°C; and A method for manufacturing a steel sheet for hot forming, comprising a step of pickling the annealed cold rolled steel sheet for 1 to 10 seconds.

7. In paragraph 6, A method for manufacturing a steel sheet for hot forming, wherein in the annealing step, the temperature is increased at a first rate of 3.0 to 20.0°C / s in a first temperature range of room temperature to 700°C, and then the temperature is increased at a second rate of 0.015 to 10.0°C / s in a second temperature range of 700 to 1000°C.

8. In paragraph 6, A method for manufacturing a steel plate for hot forming, wherein the above slab further includes at least one selected from the following a) to f). a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.001 to 0.4 wt%, b) Boron (B): 0.01 wt% or less (excluding 0%) c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.0 wt%, d) At least one of copper (Cu) and nickel (Ni): 0.005 to 2.0 wt%, e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.0 wt% f) Rare earth elements (REM): 0.0001~0.02 wt% 9. In paragraph 6, A method for manufacturing a steel sheet for hot forming, wherein the heating step is performed at 1000 to 1300°C, and the step of obtaining the hot-rolled steel sheet is performed by final hot rolling of the slab at Ar3 to 1000°C.

10. In paragraph 6, A method for manufacturing a steel sheet for hot forming, wherein the cooling step is performed at a cooling rate of 20 to 100°C / s, and the coiling step is performed at a temperature exceeding Ms and below 750°C.

11. Contains, by weight%, carbon (C): 0.1 to 0.45%, silicon (Si): 0.8 to 3.0%, manganese (Mn): 0.5 to 4.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.02%, aluminum (Al): 0.001 to 0.1%, chromium (Cr): 1.0 to 5.0%, nitrogen (N): 0.001 to 0.02%, the remainder being iron and unavoidable impurities. A hot-formed member satisfying [Relationship 2] below. [Relationship 2] Fe 40~80% Slope ≥ 200 (wt% / ㎛) (In the above [Relationship 2], Fe 40~80% The slope represents the slope of the straight line in the section where the Fe content (weight %) is 40 to 80% within a range of 5 ㎛ or less in the thickness direction from the surface of the absence in the GDS profile. In addition, the slope of the straight line here represents the slope derived after performing a simple linear regression on the graph of the section where the Fe content (weight %) is 40 to 80% in the GDS profile.) 12. In paragraph 11, A hot-formed member further comprising at least one selected from the following a) to f). a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.001 to 0.4 wt%, b) Boron (B): 0.01 wt% or less (excluding 0%) c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.0 wt%, d) At least one of copper (Cu) and nickel (Ni): 0.005 to 2.0 wt%, e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.0 wt% f) Rare earth elements (REM): 0.0001~0.02 wt% 13. In paragraph 11, A hot-formed member having a microstructure that mainly includes martensite or bainite.

14. In paragraph 13, A hot-formed member having an area fraction of the above main component of 10% or more.

15. In paragraph 11, The above hot-formed member is a hot-formed member having a tensile strength of 1000 MPa or more.

16. A step of preparing a blank using a hot forming steel plate according to any one of clauses 1 to 5; A step of heat treating the above blank at a heat treatment temperature of 700 to 1000°C for 180 to 600 seconds; and A method for manufacturing a hot-formed part, comprising the step of hot-forming the heat-treated blank and then cooling it.

17. In paragraph 16, A method for manufacturing a hot-formed part, wherein in the heat treatment step, heating is performed at a heating rate of 1 to 1000°C / s to the heat treatment temperature.

18. In paragraph 16, A method for manufacturing a hot-formed part, wherein in the cooling step, cooling is performed to a cooling end temperature below Mf.

19. In paragraph 18, A method for manufacturing a hot-formed part, wherein in the cooling step, cooling is performed at a cooling rate of 5 to 1000°C / s to the cooling end temperature.

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