STEEL MATERIALS USED FOR HOT FORMING, HOT-FORMED PARTS (COMPONENTS), MANUFACTURING METHODS, FORMING EQUIPMENT, FORMING METHODS, SUPPORTS FOR BATTERY HOUSINGS, AND BATTERY MODULES

Abstract

The invention relates to a hot-forming steel material used as a material for motor vehicles and similar applications, a hot-formed part, a hot-formed (pressed) part, and a method of manufacture. In addition, another aspect of the invention relates to a forming apparatus and a forming method for forming high-strength steel, a support for a battery housing formed from it, and a battery module.

Description

Steel for hot forming, hot (press) forming member (part), and manufacturing method thereof, forming device, forming method, bracket for battery case, and battery pack module

[0001] One aspect of the present invention relates to hot-forming steel used as a material for automobiles, hot-forming members, hot-forming (press) forming parts, 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] High-strength steel is being actively applied to meet the demand for vehicle weight reduction and improved safety.

[0004] In particular, in order to improve fuel efficiency through lightweight automobiles and to enhance crashworthiness through ultra-high strength, many parts manufactured through hot forming methods are being used as structural members (structural parts) of automobiles, and much research is being conducted on this.

[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 the excellent characteristic of having an elongation of over 50% at the high temperature where forming is performed, so even parts with a small curvature radius can be formed without cracks, and it is possible to secure the shape of the formed part that cannot be secured with cold ultra-high strength. In addition, hot press formed parts manufactured by hot working (hot stamping) a blank can be formed at high temperatures with excellent general formability, so parts with various shapes can be manufactured compared to typical giga-grade cold formed materials.

[0007] As an example of a technology related to hot forming, Patent Document 1 discloses a technology for forming a martensite structure of a member by heating an Al-Si plated steel sheet to 850°C or higher, hot forming by pressing, and rapid cooling, thereby securing an ultra-high strength of 1600 MPa or higher in tensile strength. In addition, it discloses that corrosion resistance and spot weldability can be secured without a treatment such as shot blast due to an alloy layer and a diffusion layer formed by Fe diffusion from the base metal to the plated layer during heat treatment.

[0008] However, Patent Document 1 requires the formation of an Al-Si plating layer, which requires a separate plating process, which is disadvantageous in terms of economy and productivity.

[0009] Meanwhile, in the case of non-plated materials that have not been plated, the surface quality cannot be improved due to the Fe-based oxide layer that is created during hot forming, and a shot blasting process is absolutely required to remove it, which increases the manufacturing cost of the parts.

[0010] Accordingly, there is a need for the development of a technology that can improve surface quality without performing processes such as the aforementioned plating process and shot blasting.

[0011] In addition, hot-formed components used for purposes such as passenger protection must have excellent crashworthiness characteristics. Reducing the anisotropy of the material (component) after hot forming is considered crucial as a means of improving these crashworthiness characteristics. For example, in the case of an automobile B-pillar, if the anisotropy of the material causes different crashworthiness in different directions during a vehicle collision, passenger protection may become difficult. Therefore, to ensure consistent crashworthiness in all collisions, it is necessary to reduce the anisotropy of the material.

[0012] Furthermore, ensuring excellent bendability after hot forming is considered crucial for improving the crashworthiness of hot-formed components. For example, in the case of automotive B-pillars, poor bendability in the event of a vehicle collision reduces crash energy absorption, posing challenges in ensuring crashworthiness. Therefore, there is a need for technology that can secure bendability while improving the surface quality and superior strength of hot-formed components obtained through hot forming.

[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 ultra-high strength of 1300 MPa or more to hot press formed parts 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 upper and lower portions of the frame (B-10) in order to be connected to the upper and lower portions, 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 a curved surface having a radius of curvature (R2) in order to form flanges at the upper and lower portions.

[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 is 980 MPa or higher, 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 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 material capable of producing a hot-formed part having high strength, excellent surface quality, and significantly reduced anisotropy of the material, a hot-formed part manufactured using the same, and a method for manufacturing the same.

[0036] A second aspect of the present invention is to provide a hot-formed member having high strength, excellent surface quality and improved bendability, and a method for manufacturing the same.

[0037] A third 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.

[0038] A fourth 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.

[0039] The fifth aspect of the present invention is to provide a molded part having a high tensile strength and a small radius of curvature compared to conventional parts, or a battery pack module including the same.

[0040] The sixth aspect of the present invention is to provide a press-formed part and a manufacturing method thereof in which damage such as cracks is minimized during hot press forming to manufacture a part having a plurality of hat shapes.

[0041] 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.

[0042] 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 hot-forming steel and / or manufacturing method according to the first aspect of the present invention can be usefully applied to the hot-formed members according to the second to sixth aspects, various parts obtained therefrom, and manufacturing methods thereof. In addition, the manufacturing methods of the parts according to the second and third aspects of the present invention, or the heat treatment conditions used therein, can be usefully applied in the process of manufacturing brackets or molded parts for battery cases according to the fourth to sixth 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.

[0043] The first aspect of the present invention is that the intensity of the Rotated-Cube aggregate structure on the ODF (Orientation Distribution Function) map is 2.50 or less, and in the thickness direction. <111> A hot forming steel is provided in which a cluster of faces perpendicular to the direction exceeds 6.00% of the total aggregate fabric area.

[0044] According to one embodiment of the present invention, in the direction of strength and thickness of the aggregate structure <111> By controlling the cluster area of ​​faces perpendicular to the direction, not only can anisotropy be reduced, but also excellent surface quality can be secured.

[0045] In one embodiment of the present invention, the steel for hot forming may include, in wt%, carbon (C): 0.040 to 0.450%, silicon (Si): 0.80 to 3.00%, manganese (Mn): 4.00% or less (excluding 0%), phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 1.000 to 5.000%, nitrogen (N): 0.020% or less (excluding 0%), the remainder being iron and unavoidable impurities.

[0046] According to another embodiment of the present invention, the steel for hot forming may further include one or more elements selected from the following a) to f).

[0047] a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.005 to 0.400 wt%,

[0048] b) Boron (B): 0.0001 to 0.0100 wt%,

[0049] c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.000 wt%,

[0050] d) At least one of copper (Cu) and nickel (Ni): 0.003 to 2.000 wt%,

[0051] e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.000 wt%

[0052] f) Rare earth elements (REM): 0.0001 to 0.0200 wt%

[0053] According to another embodiment of the present invention, the microstructure of the steel for hot forming may include ferrite or a composite structure of ferrite and cementite as a main phase, and in addition, at least one of pearlite, bainite, and martensite, and the main phase may be included at an area fraction of 30% or more.

[0054] Another aspect of the present invention provides a method for manufacturing a steel for hot forming, comprising the steps of: preparing a steel slab; heating the steel slab in a temperature range of 1000 to 1300°C; finishing hot rolling the heated steel slab in a temperature range of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling the steel slab at a cooling rate of 20 to 100°C / s after the finishing hot rolling; coiling the steel slab after the cooling in a temperature range of more than Ms and less than or equal to 750°C; cold rolling the steel slab after the coiling to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet in a temperature range of 700 to 900°C.

[0055] In one embodiment of the present invention, the cold rolling may be performed at a reduction ratio of 5% or more and less than 35% in the first cold rolling mill, and at a reduction ratio of 0.1% or more and less than 10.0% in the last cold rolling mill.

[0056] According to one embodiment of the present invention, by controlling the cold rolling process to specific conditions, the effect of improving material anisotropy can be obtained.

[0057] Another aspect of the present invention provides a hot-formed member that satisfies the following [Formula 1] of 15 or less.

[0058] [Formula 1]

[0059] {|Bending angle in the rolling direction - Bending angle in the rolling direction| / Bending angle in the rolling direction}×100

[0060] In one embodiment of the present invention, the hot-formed member may have a microstructure of a single phase of martensite or a composite structure of bainite with an area fraction of 40% or less and the remainder martensite.

[0061] Although not essential, the steel plate or hot-formed 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.

[0062] The second aspect of the present invention is that, at a point t / 4 in the thickness direction (wherein t means the thickness (mm) of the member), the number of cementite having a Cr content of 1.20% or more and a short diameter of 100 nm or more is 10.00 / ㎛. 2 The following hot-formed member is provided.

[0063] According to one embodiment of the present invention, the bendability of a hot-formed member can be improved by controlling the number of cementite having a Cr content of a certain amount or more and a short diameter of a certain amount or more.

[0064] In one embodiment of the present invention, the hot-formed member can be obtained from the hot-formed steel material described above.

[0065] In one embodiment of the present invention, the hot-formed member may include, in wt%, carbon (C): 0.040 to 0.450%, silicon (Si): 0.80 to 3.00%, manganese (Mn): 4.00% or less (excluding 0%), phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 1.00 to 5.00%, nitrogen (N): 0.020% or less (excluding 0%) or less, the remainder being iron and unavoidable impurities.

[0066] In one embodiment of the present invention, the hot-formed member may further include one or more elements selected from the following a) to f).

[0067] a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.005 to 0.400 wt%,

[0068] b) Boron (B): 0.0001 to 0.0100 wt%,

[0069] c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.000 wt%,

[0070] d) At least one of copper (Cu) and nickel (Ni): 0.003 to 2.000 wt%,

[0071] e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.000 wt%

[0072] f) Rare earth elements (REM): 0.0001 to 0.0200 wt%

[0073] In one embodiment of the present invention, a hot-formed member having a microstructure of a single phase of martensite or a composite structure of bainite with an area fraction of 50% or less and the remainder of martensite can be provided.

[0074] In one embodiment of the present invention, a hot-formed member having a maximum bending angle of 40° or more can be provided.

[0075] In one embodiment of the present invention, a hot-formed member having a standard gloss of 25.0 GU or more can be provided.

[0076] Another aspect of the present invention provides a method for manufacturing a hot-formed part, comprising the steps of: preparing a blank using steel; charging the blank into a furnace having a dew point temperature of -15 to 20°C and heating and maintaining the blank at a temperature of Ae3 or higher; and hot-forming the heated and maintained blank and then cooling it at a cooling rate of 10 to 1000°C / s.

[0077] In one embodiment of the present invention, the steel for obtaining the hot-formed member may be the hot-formed steel described above.

[0078] According to one embodiment of the present invention, by controlling the conditions inside the furnace during heating and maintaining of the blank, the effect of improving surface quality and bendability can be obtained.

[0079] In one embodiment of the present invention, the heating and maintaining steps can be performed under conditions satisfying the following relationships 1 and 2.

[0080] [Relationship 1]

[0081] 0.722260 + (0.773168×Si) + (2.073977×Mn) + (1.667101×Cr) - (0.196843×O2) - (0.031066×DP) - (0.002384×T furnace ) - (0.001400×t total ) ≥ 0.8

[0082] (In equation 1, Si, Mn and Cr represent the content (weight%) of the alloy composition of the absence, O2 represents the oxygen concentration (%) in the furnace, DP represents the dew point temperature (℃) in the furnace, T furnace is the set temperature (℃) within the furnace, t total means the total heating time (sec) within the furnace.)

[0083] [Relationship 2]

[0084]

[0085] (In relation 2, t total Total heating time (sec) within the furnace, A e3 is the equilibrium temperature (℃) at which transformation to austenite is completed, T furnace refers to the set temperature (℃) within the furnace.)

[0086] In one embodiment of the present invention, when the blank is loaded into a furnace and heated, the residence time in the furnace may be 180 seconds or more.

[0087] In one embodiment of the present invention, the cooling can be performed to a temperature range below Mf.

[0088] In one embodiment of the present invention, the steel may be a cold-rolled steel sheet. As an example, the cold-rolled steel sheet may be the hot-forming steel described above, and the steel may be manufactured using the manufacturing method described above.

[0089] In one embodiment of the present invention, the cold rolled steel sheet can be manufactured through the steps of: preparing a steel slab; heating the steel slab in a temperature range of 1000 to 1300°C; finishing hot rolling the heated steel slab in a temperature range of Ar3 to 1000°C to obtain a hot rolled steel sheet; cooling at a cooling rate of 20 to 100°C / s after the finishing hot rolling; coiling at a temperature range of more than Ms and less than or equal to 750°C after the cooling; cold rolling at a cumulative reduction ratio of 30 to 80% after the coiling to obtain a cold rolled steel sheet; and annealing the cold rolled steel sheet in a temperature range of 700 to 900°C.

[0090] Although not necessarily essential, the steel plate or hot-formed member according to the second aspect of the invention can secure improved performance when combined with the advantageous features of the other aspects described below.

[0091] A third aspect of the present invention provides a hot press-formed part having a martensite transformation critical cooling rate of 15 to 25°C / s in a temperature range of 800 to 400°C and a wrinkle wave height of a flange portion of 1 mm or less.

[0092] 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.

[0093] In one embodiment of the present invention, the hot press-formed part can be obtained from the hot forming steel material described above.

[0094] 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.

[0095] In one embodiment of the present invention, the blank holding force (F) satisfies the following equation 2, and the following P may be 1 to 10 MPa.

[0096] [Formula 2] F = P × A (wherein, in the above formula 2, P represents the blank holding pressure, and A represents the contact area between the lower holder and the steel.)

[0097] In one embodiment of the present invention, the minimum temperature value (T) of the lower holder min ) can be 300℃.

[0098] In one embodiment of the present invention, the maximum temperature value (T) of the lower holder max ) can satisfy [Equation 3] and [Equation 4] below.

[0099] [Formula 3] T max = T blank - [(CR min ·w·ρ·C) / h]

[0100] [Equation 4] h = A·P N

[0101] (However, in the above equations 3 and 4, h is the heat transfer coefficient between the steel and the lower holder, Tblank is the forming temperature of the 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.)

[0102] In one embodiment of the present invention, the temperature of the remaining mold excluding the lower holder may be 300°C or lower.

[0103] Although not necessarily essential, the hot press-formed part according to the third aspect of the invention can secure improved performance when combined with the advantageous features of the other aspects described below.

[0104] A fourth aspect of the present invention 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, A forming device comprising 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 third body forming portion and the upper position of 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.

[0105] 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).

[0106] 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.

[0107] In one embodiment of the present invention, the punch may include a support portion that supports the pre-molding material outside the first and fifth molding portions with the third punch molding portion as the center.

[0108] 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.

[0109] 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.

[0110] 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°.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] In one embodiment of the present invention, the material may be the aforementioned hot-forming steel or a hot-formed member obtained by hot-forming the hot-forming material. As an example, the hot-forming steel and the hot-formed member may have the aforementioned alloy composition and may be manufactured using the aforementioned manufacturing method.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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°.

[0122] A bracket for a battery case according to one embodiment of the present invention can be manufactured using the molding method described above.

[0123] 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.

[0124] Although not necessarily essential, the article according to the fourth aspect of the invention may achieve further improved performance when combined with the advantageous features of the other aspects described below.

[0125] A fifth aspect of the present invention provides a high-strength steel hot press-formed part comprising 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, wherein 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 integrally formed, and the following relational expression 3 is satisfied.

[0126] [Relationship 3]

[0127] Wall height (hw) ≤ 13.4 * wall angle (θw) + 182.4

[0128] (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.)

[0129] In one embodiment of the present invention, the high-strength steel may be the hot-forming steel described above.

[0130] According to one embodiment of the present invention, a molded part having high tensile strength and small curvature can be provided.

[0131] 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.

[0132] In one embodiment of the present invention, the molded part may have a tensile strength of 1300 to 2100 MPa.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] In one embodiment of the present invention, a plurality of hot press-formed parts may be arranged at regular intervals within the battery case.

[0139] Although not necessarily essential, the components according to the fifth aspect of the invention can be combined with the advantageous features of the other aspects described below to achieve further improved performance.

[0140] A sixth aspect of the present invention provides a press-molded part having 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 press-molded part including a plurality of deformable portions having a preset height, and a flat plate portion arranged between the plurality of deformable portions, connecting two adjacent boundary portions, the flat plate portion having a preset length.

[0141] 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.

[0142] 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 equation (1), where w≤114.85e 0.0455h ... Formula (1), in the above formula (1), w may mean the preset length, and h may mean the preset height.

[0143] 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 equation (2), where w≤46.309e 0.105h ... Formula (2), in the above formula (2), w may mean the preset length, and h may mean the preset height.

[0144] In one embodiment of the present invention, the preset length (w) may be 20 mm or more.

[0145] In one embodiment of the present invention, the press-formed part can be manufactured using a hot press forming method.

[0146] In one embodiment of the present invention, the hot press forming method may be the method described above.

[0147] 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.

[0148] In one embodiment of the present invention, the metal plate may be the hot forming steel material described above.

[0149] In one embodiment of the present invention, the press forming step may use the above-described forming device.

[0150] 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.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] According to one aspect of the present invention, a hot-formed member can be provided that has high strength after hot forming, while also possessing excellent surface quality and excellent anisotropy. In addition, a hot-formed steel material for obtaining the hot-formed member, and a method for manufacturing the steel material and the member can be provided.

[0158] According to another aspect of the present invention, a hot-formed member can be provided that has both high strength and excellent bendability after hot forming. Furthermore, the hot-formed member according to the present invention can have excellent surface quality.

[0159] According to another aspect of the present invention, a hot press-formed part and a manufacturing method thereof can be provided that can reduce wrinkles in a flange portion while ensuring ultra-high strength.

[0160] 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 lightweight, watertight, and collision safety performance of a battery pack.

[0161] According to another aspect of the present invention, not only can a molded part be provided that has high tensile strength, small curvature, and can be manufactured at one time, making it easy to process, but also a part that has high space utilization when manufacturing modules such as battery packs can be manufactured.

[0162] According to another aspect of the present invention, a middle press section having a height difference is arranged between upper and lower molds, and press forming can be performed on a metal plate using three press sections. 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.

[0163] 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.

[0164] FIG. 1 is a drawing showing the ODF measurement results of hot-forming steel of Invention Example 1 and Comparative Example 1 according to one embodiment of the present invention.

[0165] Figure 2 shows a surface observation photograph of a hot-formed member according to one embodiment of the present invention.

[0166] Figure 3 shows changes in temperature, microstructure, and tensile strength during conventional hot press forming.

[0167] 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.

[0168] 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.

[0169] Figure 6 is an example showing changes in temperature, microstructure, and tensile strength during hot press forming for 22MnB5 material and hardenability-improved material.

[0170] Figure 7 is an example showing changes in temperature, microstructure, and tensile strength during hot press forming of a material with improved hardenability.

[0171] Figure 8 is a schematic diagram showing a hot press forming equipment applicable to the present invention.

[0172] Figure 9 is a schematic diagram showing a hot press forming facility applied to conventional 22MnB5 material.

[0173] Figure 10 is a schematic diagram of the side frame of the battery case.

[0174] Fig. 11 is a cross-sectional view taken along line A-A' of Fig. 10.

[0175] Fig. 12 is an exploded perspective view of a molding device according to one embodiment of the present invention.

[0176] Figure 13 is a side view of a molding device according to one embodiment of the present invention.

[0177] Figure 14 is a flowchart of a molding method according to one embodiment of the present invention.

[0178] Figure 15 is a plan view of a material fed into a molding device according to one embodiment of the present invention.

[0179] FIG. 16 is a schematic perspective view showing a first molding step in a molding method according to one embodiment of the present invention.

[0180] Figure 17 is a schematic perspective view showing a second molding step in a molding method according to one embodiment of the present invention.

[0181] FIG. 18a is a perspective view showing the second molding step in a molding method according to an embodiment of the present invention from a different direction, and FIG. 18b is a front view showing the second molding step in a molding method according to an embodiment of the present invention.

[0182] Fig. 19 is a front view showing a cooling step in a molding method according to one embodiment of the present invention.

[0183] FIG. 20 is a perspective view showing a trimming step in a molding method according to one embodiment of the present invention.

[0184] Fig. 21 is a perspective view of a bracket manufactured using a molding method according to one embodiment of the present invention.

[0185] Figure 22 is a thickness reduction rate distribution diagram of a molded product manufactured using a molding method of one embodiment of the present invention.

[0186] Figure 23 is a thickness reduction rate distribution diagram of a molded product manufactured using the molding method of Comparative Example 1.

[0187] Fig. 24 is a graph of the thickness reduction rate distribution in the cross section of the molded products of Figs. 22 and 23.

[0188] Figure 25 is a side view showing the molding method of Comparative Example 2.

[0189] Figure 26 is a perspective view of a molded product manufactured using the molding method of Comparative Example 2.

[0190] Fig. 27 is a perspective view of a molded part according to one embodiment of the present invention.

[0191] Fig. 28 is a front view of a molded part according to one embodiment of the present invention.

[0192] 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.

[0193] Fig. 31 is a perspective view of a battery pack module according to one embodiment of the present invention.

[0194] FIG. 32 is a drawing showing a cross-section of a portion of a battery pack module according to an 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).

[0195] Fig. 33 is a perspective view illustrating a press-molded part according to one embodiment of the present invention.

[0196] Fig. 34 is a perspective view illustrating a press-formed part according to another embodiment of the present invention.

[0197] 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.

[0198] 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.

[0199] Figure 37 is a flowchart showing a method for manufacturing a press-molded part according to one embodiment of the present invention.

[0200] Figure 38 schematically illustrates the first step of manufacturing the press-formed part of Figure 33.

[0201] Figure 39 schematically illustrates the second step of manufacturing the press-formed part of Figure 33.

[0202] Figure 40 schematically illustrates the third step of manufacturing the press-formed part of Figure 33.

[0203] Figure 41 schematically illustrates the fourth step of manufacturing the press-formed part of Figure 33.

[0204] Figure 42 schematically illustrates the first step of manufacturing the press-formed part of Figure 34.

[0205] Figure 43 schematically illustrates the second step of manufacturing the press-formed part of Figure 34.

[0206] Figure 44 schematically illustrates the third step of manufacturing the press-formed part of Figure 34.

[0207] Figure 45 schematically illustrates the fourth step of manufacturing the press-formed part of Figure 34.

[0208] As described above, hot-formed parts used as materials for automobiles, etc., need to have improved crash resistance, and the inventors of the present invention recognized that securing anisotropy of the material or securing bendability of the material is an important factor in improving the crash resistance of the hot-formed parts.

[0209] Accordingly, the inventors of the present invention have conducted in-depth research and have confirmed that a hot-formed member obtained by hot forming not only has high strength, but also has low anisotropy and thus has favorable crash resistance, and further has improved surface quality, and that a hot-formed member obtained using the same can be provided, thereby completing the present invention.

[0210] In addition, the inventors of the present invention have confirmed that a hot-formed member having excellent bendability can be provided, and have thus provided the present invention.

[0211] Hereinafter, the present invention will be described in detail.

[0212] The steel for hot forming according to the first aspect of the present invention may contain, in wt%, carbon (C): 0.040 to 0.450%, silicon (Si): 0.80 to 3.00%, manganese (Mn): 4.00% or less (excluding 0%), phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 1.00 to 5.00%, and nitrogen (N): 0.020% or less (excluding 0%) or less.

[0213] Below, the reasons for limiting the alloy composition of steel and components according to one embodiment of the present invention are described in detail. Unless otherwise specified, the content of each element is based on weight, and the ratio of the structure is based on area.

[0214] Carbon (C): 0.040~0.450%

[0215] Carbon (C) is an essential element added to improve the strength of hot-formed parts obtained through hot forming. If the C content is less than 0.040%, it is difficult to secure sufficient strength, resulting in a reduction in impact energy absorption capacity. On the other hand, if the C content exceeds 0.450%, while it is advantageous for securing the strength of the hot-formed part, its bendability deteriorates, resulting in a lower impact energy absorption capacity.

[0216] Therefore, according to one embodiment of the present invention, the C may be included in an amount of 0.040 to 0.450%, according to another embodiment, it may be included in an amount of 0.050% or more, and according to another embodiment, it may be included in an amount of 0.400% or less.

[0217] Silicon (Si): 0.80~3.00%

[0218] Silicon (Si) plays an important role in forming a Si-based amorphous oxide layer by concentrating on the surface of the obtained cold-rolled steel sheet during annealing treatment in a continuous annealing line, and also plays a role in ensuring spot weldability of hot-formed parts by suppressing the formation of (Fe, Mn, Cr)-based oxide layers during the hot-forming process.

[0219] If the content of Si is less than 0.80%, the above-described effect cannot be sufficiently obtained, and on the other hand, if the content exceeds 3.00%, there is a problem in that the Si-based amorphous oxide layer is formed too thickly, which actually reduces the spot weldability.

[0220] Accordingly, according to one embodiment of the present invention, the Si may be included in an amount of 0.80 to 3.00%, and according to another embodiment, the Si may be included in an amount of 0.85% or more and 2.80% or less. According to yet another embodiment, the Si may be included in an amount of 2.50% or less.

[0221] Manganese (Mn): 4.00% or less (excluding 0%)

[0222] Manganese (Mn) not only enhances the solid-solution strengthening effect of steel, but also effectively suppresses the formation of ferrite during hot forming by improving hardenability. If the Mn content exceeds 4.0%, the steel's strength may increase, reducing its cold-rollability. Furthermore, the microstructure may become more band-like, with bands arranged in the rolling direction, potentially reducing impact energy absorption.

[0223] According to one embodiment of the present invention, there is no particular limitation on the lower limit content of Mn, and if it is added in excess of 0%, the present invention can be achieved.

[0224] According to another embodiment of the present invention, the Mn may be included in an amount of 0.10% or more, and according to another embodiment, in an amount of 0.30% or more.

[0225] Phosphorus (P): 0.05% or less

[0226] Phosphorus (P) is an impurity that is inevitably added during the steel manufacturing process, and if its content exceeds 0.05%, it can significantly impair the weldability of hot-formed parts. Since P is an unavoidable impurity, there is no particular limitation regarding its lower limit content. However, since controlling the P content to less than 0.001% may require significant manufacturing costs, taking this into consideration, the content may be 0.001% or more.

[0227] Sulfur (S): 0.0200% or less

[0228] Sulfur (S) is also an impurity that is inevitably added during the steel manufacturing process, and is an element that impairs the ductility, impact properties, and weldability of hot-formed parts. Therefore, it is advantageous to limit its content to a maximum of 0.0200%. Since the above S is an unavoidable impurity, there is no specific limitation on its lower content. However, since controlling the S content to less than 0.0001% may require significant manufacturing costs, taking this into consideration, the content may be 0.0001% or more.

[0229] Aluminum (Al): 0.010~0.100%

[0230] Aluminum (Al) is an element that acts as a deoxidizer during the steelmaking process, enhancing the purity of steel. To fully achieve the aforementioned benefits, it is advantageous to include at least 0.010% Al. However, if the content exceeds 0.100%, excessive AlN formation during the casting process can lead to reduced high-temperature ductility, which can lead to slab cracking and other problems.

[0231] According to one embodiment of the present invention, the Al may be included in an amount of 0.010 to 0.100%.

[0232] Chromium (Cr): 1.000~5.000%

[0233] Chromium (Cr), like the aforementioned Mn, is advantageous in securing the hardenability of steel, and is added in the present invention to secure a beautiful surface during the hot forming process. If the Cr content is less than 1.000%, it is difficult to secure a beautiful surface after hot forming, and on the other hand, if the Cr content exceeds 5.000%, the effect of improving the hardenability relative to the amount added is insignificant, and there is a concern that the formation of coarse Cr-based carbides may be encouraged, thereby lowering the impact energy absorption capacity of the member.

[0234] According to one embodiment of the present invention, Cr may be included in an amount of 1.000 to 5.000%. According to another embodiment, Cr may be included in an amount of 1.100% or more, and according to another embodiment, Cr may be included in an amount of 4.800% or less.

[0235] Nitrogen (N): 0.020% or less (excluding 0%)

[0236] Nitrogen (N) is an element that is inevitably added to steel. If the content of N exceeds 0.020%, it combines with Al in the steel to form AlN, which can easily cause slab cracks. Since N is an unavoidable impurity, there is no particular limitation on its lower content. However, since controlling the content of N to less than 0.001% may incur significant manufacturing costs, taking this into consideration, the content may be 0.001% or more.

[0237] In one embodiment of the present invention, the remaining components, other than those mentioned above, are iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during the typical manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the art, their full details are not specifically discussed in this specification.

[0238] The steel and member for hot forming according to one embodiment of the present invention may further include one or more of the following elements, i.e., one or more selected from the following a) to f). By optionally adding these elements, the surface quality of the steel and other properties such as hot formability can be further improved.

[0239] a) At least one of titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V): 0.005 to 0.400 wt%

[0240] Titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V) improve the strength of hot-formed parts by forming fine precipitates within the steel, and are effective in stabilizing retained austenite and improving impact toughness by refining crystal grains.

[0241] When containing one or more of these elements, if the content is less than 0.005%, the above-described effect cannot be sufficiently obtained, and on the other hand, if the sum of the contents exceeds 0.400%, not only will the intended effect be saturated, but there is also a concern that the addition of excessive alloying elements may lead to an increase in cost.

[0242] In one embodiment of the present invention, when one or more of the above elements are contained, the content may be 0.005 to 0.400%. In another embodiment of the present invention, the lower limit content of the elements may be 0.008% or more, and the upper limit content may be 0.380% or less, or 0.350% or less.

[0243] In another embodiment of the present invention, when only one of the above elements is contained, it may contain Ti: 0.005 to 0.100%, Nb: 0.005 to 0.100%, Zr: 0.005 to 0.100%, and V: 0.005 to 0.100%, respectively.

[0244] b) Boron (B): 0.0001~0.0100%

[0245] Boron (B) can not only improve the hardenability of steel even with a small amount of addition, but is also useful in suppressing embrittlement of hot-formed parts caused by grain boundary segregation of P and / or S by segregating at the grain boundaries of old austenite. When adding B, if the content is less than 0.0001%, the above-described effect cannot be sufficiently obtained, and on the other hand, if the content exceeds 0.0100%, not only is the above-described effect saturated, but hot embrittlement may also occur during hot rolling.

[0246] In one embodiment of the present invention, the addition of B may include 0.0001 to 0.0100%. In another embodiment of the present invention, the B may be 0.00012% or more, 0.00015% or more in terms of the lower limit content, and may be 0.0050% or less in terms of the upper limit content.

[0247] c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.000 wt%

[0248] Molybdenum (Mo) and tungsten (W) can be added to improve the hardenability of steel, improve strength through precipitation strengthening, and refine grain size. When containing at least one of the above Mo and W, if the content is less than 0.001%, the above-described effects cannot be sufficiently obtained, whereas if the content exceeds 1.000%, the above-described effects may be saturated and costs may increase significantly.

[0249] In one embodiment of the present invention, when one or more of the above elements are contained, it may be contained at 0.001 to 1.000%. In another embodiment of the present invention, when one or more of the above elements are contained, the lower limit content may be 0.0015% or more, 0.0020% or more, and the upper limit content may be 0.950% or less, 0.900% or less.

[0250] According to another embodiment of the present invention, when only one of the above elements is added, each may be included in an amount of 0.001 to 0.500%.

[0251] d) At least one of copper (Cu) and nickel (Ni): 0.003 to 2.000 wt%

[0252] Copper (Cu) forms fine precipitates in steel to improve strength, and nickel (Ni) can be added as needed to prevent hot embrittlement when Cu is added alone. When containing at least one of Cu and Ni, if the total content is less than 0.003%, the above-described effect cannot be sufficiently obtained, and on the other hand, if the content exceeds 2.000%, there is a concern that it will cause excessive cost increase.

[0253] According to one embodiment of the present invention, when one or more of the above elements are contained, it may be contained at 0.003 to 2.000%. According to another embodiment of the present invention, when one or more of the above elements are contained, the lower limit content may be 0.005% or more, 0.006% or more, or 0.007% or more, and the upper limit content may be 1.950% or less, or 1.900% or less.

[0254] According to another embodiment of the present invention, when only one of the above elements is added, each can be included in an amount of 0.003 to 1.500%.

[0255] e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.000 wt%

[0256] Antimony (Sb) and tin (Sn) have the effect of suppressing the formation of oxides at the grain boundaries of the surface layer of hot-rolled steel to which silicon (Si) is added, and can also suppress dent defects caused by the detachment of the surface grain boundaries during annealing of cold-rolled steel. In order to obtain the above-described effect, at least one of the Sb and Sn may be included in an amount of 0.001% or more. However, if the content exceeds 1.000%, not only will the cost increase significantly, but the elements may be dissolved in the slab grain boundaries, increasing the possibility of causing edge cracks in the coil during hot rolling.

[0257] According to one embodiment of the present invention, in further including the above elements, the total content may be 0.001 to 1.000%. According to another embodiment of the present invention, the lower limit content may be 0.002% or more, 0.030% or more, and the upper limit content may be 0.950% or less, 0.900% or less.

[0258] According to another embodiment of the present invention, when only one of the above elements is added, each may be included in an amount of 0.001 to 0.500%.

[0259] f) Rare earth elements (REM): 0.0001 to 0.0200 wt%

[0260] Rare earth elements (REMs) can control the activity of iron (Fe) in steel, thereby controlling the thickness of the Fe scale formed on the surface during hot forming. To achieve this effect, REMs can be included at levels greater than 0.0001%. However, if the content exceeds 0.0200%, the ability to control Fe activity is lost, potentially degrading the surface quality of hot-formed parts.

[0261] According to one embodiment of the present invention, in further including the REM, it may be included at 0.0001 to 0.0200%. According to another embodiment of the present invention, in terms of the lower limit content, it may be 0.00015% or more, 0.0002% or more, and in terms of the upper limit content, it may be 0.0100% or less.

[0262] In one embodiment of the present invention, the steel for hot forming has a strength of Rotated-Cube aggregate structure of 2.50 or less on the ODF (Orientation Distribution Function) map, and in the thickness direction <111> The cluster of planes perpendicular to the direction may exceed 6.00% of the total aggregate tissue area. Here, ODF means the ODF result of the ferrite phase.

[0263] The strength of the Rotated-Cube aggregate structure on the above ODF map and the thickness direction of the steel <111> The cluster area of ​​faces perpendicular to the direction is used as an indicator of anisotropy.

[0264] Specifically, the intensity of the Rotated-Cube aggregate structure on the ODF map exceeds 2.50, and in the thickness direction <111> If the cluster of planes perpendicular to the direction is less than 6.00% of the total texture area, the bending anisotropy of the steel after hot forming increases, and the bending angle deviation in the rolling direction and the direction perpendicular to the rolling direction exceeds 15.0, resulting in poor anisotropy. In other words, the bending anisotropy increases in each direction, making it impossible to secure crashworthiness properties in various impact modes when a hot-formed member collides.

[0265] In one embodiment of the present invention, the bending angle deviation can be measured through a three-point bending test according to the VDA standard (VDA238-100), and the bending angle deviation result can be calculated by the following [Formula 1]. When the bending line (plane) in the three-point bending test is parallel to the rolling direction, it is expressed as the bending angle in the direction perpendicular to the rolling direction (C direction), and when the bending line (plane) is perpendicular to the rolling direction, it is defined as the bending angle in the rolling direction (L direction).

[0266] [Formula 1]

[0267] {|Bending angle in the rolling direction - Bending angle in the rolling direction| / Bending angle in the rolling direction}×100

[0268] In one embodiment of the present invention, the microstructure composition of the steel for hot forming is not particularly limited, and it is to be noted that the appropriate phases and the fraction of each phase will be determined by the alloy composition described above and the manufacturing process described below.

[0269] To give an example, the microstructure of the above hot forming steel may include ferrite or a composite structure of ferrite and cementite as the main phase. In addition, it may further include one or more of pearlite, bainite, and martensite.

[0270] In one embodiment of the present invention, when the carbon (C) content is relatively low within the range of the carbon (C) content contained in the steel for hot forming, the main phase may be composed of ferrite and cementite, and the main phase may be included at an area fraction of 30% or more. Here, in the case of a composite structure of ferrite and cementite as the main phase, it refers to the combined fraction of the two phases.

[0271] In one embodiment of the present invention, the steel for hot forming may include one or more of pearlite, bainite, and martensite as a structure other than the main phase, and the fraction of each phase is not particularly limited. However, although not necessarily limited thereto, the pearlite may be included in an area fraction of 30% or less, bainite in an area fraction of 20% or less, and martensite in an area fraction of 20% or less.

[0272] In one embodiment of the present invention, the steel for hot forming has high strength, and specifically, can have a tensile strength of 550 MPa or more.

[0273] Hereinafter, a method for manufacturing a steel material for hot forming according to another aspect of the present invention will be described in detail.

[0274] According to one embodiment of the present invention, the steel can be obtained through a process such as [steel slab heating - hot rolling - cooling - coiling - cold rolling - annealing], and the steel can be a cold-rolled steel sheet obtained through a series of processes. Each process step is described in detail below, and it is to be noted that the following manufacturing process corresponds to one example for manufacturing the steel of the present invention.

[0275] [Heating of steel slabs]

[0276] First, a steel slab having the above-described alloy composition is prepared and then heated. This heating process of the steel slab is intended to facilitate the subsequent hot rolling process. Here, the steel slab alloy composition may refer to the alloy composition of the hot-forming steel material mentioned above, and the description of each element is replaced with the aforementioned information.

[0277] In one embodiment of the present invention, the heating process may be performed at a temperature range of 1000 to 1300°C. If the heating temperature of the steel slab is below 1000°C, it is difficult to homogenize the slab structure. On the other hand, if it exceeds 1300°C, excessive oxide formation is likely to cause surface defects after hot rolling. In addition, there is the problem of increased manufacturing costs.

[0278] [Hot rolling]

[0279] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0280] In one embodiment of the present invention, the finishing hot rolling can be performed in a temperature range of Ar3 to 1000°C. If the temperature of the finishing hot rolling is lower than Ar3, there is a risk that mixed grain structure may be generated on the surface of the hot-rolled steel sheet due to two-phase rolling, and there is also difficulty in controlling the shape of the hot-rolled steel sheet. On the other hand, if the temperature during the finishing hot rolling exceeds 1000°C, the grains of the hot-rolled steel sheet become coarser, making it difficult to obtain the intended physical properties.

[0281] [cooling]

[0282] The above hot-rolled steel sheet can be cooled.

[0283] In one embodiment of the present invention, the cooling may be performed at a rate of 20 to 100°C / s. If the cooling rate is less than 20°C / s during cooling after the hot rolling, the formation of pearlite and cementite on the surface of the steel sheet is insufficient, making it difficult to secure sufficient strength after subsequent hot forming. On the other hand, if the cooling rate exceeds 100°C / s, pearlite and cementite are formed at an excessive area ratio on the surface during the subsequent annealing process, resulting in a large hardness difference between the surface martensite formed by the hot forming, which may result in poor bendability.

[0284] In one embodiment of the present invention, cooling may be performed to a temperature of a subsequent coiling process.

[0285] [Winding]

[0286] The above cooled hot-rolled steel sheet can be coiled.

[0287] In one embodiment of the present invention, the coiling can be performed at a temperature range of Ms to 750°C. If the coiling temperature is lower than Ms during the coiling, the strength of the hot-rolled steel sheet becomes too high, which can cause a problem of reduced cold-rollability. On the other hand, if the coiling temperature exceeds 750°C, a thick oxide layer is formed, which causes grain boundary oxidation in the surface layer, resulting in poor pickling properties. In addition, there is a risk of defects such as surface grain boundaries being removed during annealing in a continuous annealing furnace. Here, Ms refers to the martensite transformation initiation temperature.

[0288] [Cold rolling]

[0289] The inventors of the present invention recognized that when a cold-rolled steel sheet is manufactured under conventional cold rolling conditions and then a hot forming process is performed, there is a problem in that the anisotropy of the hot-formed member increases, resulting in a large deviation in bendability in each direction.

[0290] From this, the inventors of the present invention attempted to secure bending anisotropy of a hot-formed member obtained through a subsequent hot-forming process by optimizing the conditions of the cold rolling process.

[0291] According to one embodiment of the present invention, in performing cold rolling while uncoiling a coiled hot-rolled steel sheet, there is a technical significance in controlling the cold rolling in the first cold rolling mill to a reduction ratio of 5% or more and less than 35%, and controlling the cold rolling in the last cold rolling mill to a reduction ratio of 0.1% or more and less than 10.0%.

[0292] In one embodiment of the present invention, the number of rolling mills (i.e., the number of cold rolling rolls) during the cold rolling is not limited, but cold rolling can be performed in at least four cold rolling mills, and the reduction ratio of the initial cold rolling and the reduction ratio of the final cold rolling can be controlled as described above.

[0293] If the reduction ratios of the first and final cold rolling do not meet the set lower limits, sufficient recrystallization driving force will not be secured, making it difficult to secure material anisotropy in hot-formed parts. On the other hand, if each reduction ratio exceeds the set upper limits, sufficient recrystallization driving force can be secured in the subsequent annealing process, which is advantageous for improving material anisotropy. However, there is a concern that excessive rolling load will be generated, which will lower productivity.

[0294] That is, by controlling the reduction ratio of a specific cold rolling mill during the cold rolling process according to one embodiment of the present invention, an appropriate crystal orientation can be secured after subsequent annealing, thereby reducing the deviation of the maximum bending angle in each direction of the hot-formed member obtained after hot forming, in other words, there is an effect of excellently securing anisotropy.

[0295] In one embodiment of the present invention, when cold rolling is performed under the conditions described above, the total reduction ratio of the cold rolling is not particularly limited. As a non-limiting example, cold rolling can be performed at a total reduction ratio of 30 to 80%, thereby securing a predetermined target thickness.

[0296] In one embodiment of the present invention, prior to cold rolling the coiled hot-rolled steel sheet, a pickling process may be further performed, thereby further improving surface quality. The pickling process may be performed under normal conditions, and thus, there are no particular limitations on the conditions.

[0297] [Sodun]

[0298] The cold rolled steel sheet obtained by the above cold rolling can be annealed.

[0299] In one embodiment of the present invention, the annealing treatment may be performed at a temperature range of 700 to 900°C. If the temperature during the annealing treatment is lower than 700°C, it is difficult for the rolled structure created in the cold rolling process to recover and recrystallize. On the other hand, if the temperature exceeds 900°C, the annealing equipment may deteriorate, which may lead to increased manufacturing costs due to frequent equipment replacement.

[0300] In one embodiment of the present invention, the annealing treatment may be performed for 1 to 1000 seconds. If the annealing treatment time is less than 1 second, the annealing treatment effect cannot be obtained, whereas if it exceeds 1000 seconds, there is a risk of reduced productivity.

[0301] Hereinafter, a hot-formed member according to another aspect of the present invention will be described in detail.

[0302] According to one embodiment of the present invention, a hot-formed member can be obtained by hot press forming a steel material for hot forming.

[0303] In one embodiment of the present invention, the alloy composition of the hot-formed member may be the same as that of the hot-formed steel material, and as an example, the hot-formed steel material may have the aforementioned alloy composition, and the description of each element is replaced with the aforementioned matter.

[0304] In one embodiment of the present invention, the hot-formed member can satisfy the following [Formula 1] of 15.0 or less.

[0305] [Formula 1]

[0306] {|Bending angle in the rolling direction - Bending angle in the rolling direction| / Bending angle in the rolling direction}×100

[0307] In one embodiment of the present invention, [Formula 1] is a formula defining a bending angle deviation of a hot-formed member, and a value of [Formula 1] exceeding 15.0 means that anisotropy is inferior, and ultimately, bendability or collision characteristics may be inferior.

[0308] As a non-limiting example, the above bending angle deviation can be confirmed through a three-point bending test according to VDA standard (VDA238-100).

[0309] In one embodiment of the present invention, the hot-formed member may have a microstructure of a single phase of martensite or a composite structure of bainite with an area fraction of 40% or less and the remainder martensite.

[0310] The above martensite is an effective structure for securing the target strength. On the other hand, the above bainite is a structure with somewhat lower strength than martensite, and when formed above a certain fraction, it not only reduces the bendability of the material but also can have a negative effect on anisotropy. Therefore, the hot-formed member according to one embodiment of the present invention may include a bainite phase as a microstructure, but in this case, the fraction may be 40% or less. If the fraction of the bainite exceeds 40%, there is a concern that the bendability and anisotropy may be reduced.

[0311] According to one embodiment of the present invention, a hot-formed member can have a tensile strength of 1000 MPa or more.

[0312] Hereinafter, a method for manufacturing a hot-formed part according to another aspect of the present invention will be described. The manufacturing method described below is merely one embodiment among all possible embodiments, and does not necessarily imply that the hot-formed part of the present invention must be manufactured exclusively by the manufacturing method described below.

[0313] In one embodiment of the present invention, a steel material for hot forming is prepared and a blank can be manufactured using the steel material. Here, the steel material for hot forming may be a steel material according to one embodiment of the present invention.

[0314] In accordance with one embodiment of the present invention, the blank may be heated to a temperature of 700°C or higher, and at a heating rate of 1 to 1000°C / s. If the heating temperature of the blank is lower than 700°C, recrystallization of ferrite is insufficient, which causes a problem of increased bending anisotropy after hot forming. On the other hand, if the heating rate is lower than 1°C / s when heating to the above temperature, there is a problem of reduced productivity, while if it exceeds 1000°C / s, there is a problem of requiring expensive equipment.

[0315] In one embodiment of the present invention, a heated blank may be hot-formed into an intended shape using a mold or the like, and then the hot-formed blank may be cooled. As an example, the cooling of the hot-formed blank may be performed at a cooling rate of 5 to 1000°C / s. If the cooling rate is less than 5°C / s, unintended ferrite and pearlite structures may be formed, making it difficult to secure strength. On the other hand, if the cooling rate exceeds 1000°C / s, there is a problem in that expensive special cooling equipment is required. At this time, the cooling end temperature may be lower than Mf, but if cooling is completed at a temperature exceeding Mf, it must be cooled back to room temperature, and in this case, it is difficult to secure the shape freezing property of the hot-formed part. Here, Mf refers to the martensite transformation end temperature.

[0316] The above cooling process may be, for example, a mold cooling method.

[0317] Meanwhile, according to the second aspect of the present invention, at a point t / 4 in the thickness direction (wherein t means the thickness of the member), the number of cementite having a Cr content of 1.20% or more and a short diameter of 100 nm or more is 10.00 / ㎛. 2 The following hot-formed member can be provided.

[0318] If Cr present in steel becomes concentrated within cementite, remelting may be slowed, potentially degrading the material's hardenability. Therefore, controlling the number of cementite particles enriched with Cr above a certain level holds technological significance in securing bendability.

[0319] In one embodiment of the present invention, the number of cementite having a Cr content of 1.20% or more and a short diameter of 100 nm or more at a point t / 4 in the thickness direction of the hot-formed member is 10.00 / ㎛. 2 If it exceeds, the hardening ability of the member is deteriorated, and there is a risk that ferrite will be formed during the cooling process after hot forming, and the stress concentration phenomenon in cementite during bending may become severe, resulting in poor bendability.

[0320] According to one embodiment of the present invention, a hot-formed member may include a single-phase martensite or a composite structure of martensite and bainite as a microstructure.

[0321] In one embodiment of the present invention, when the microstructure of the hot-formed member is a composite structure, the martensite phase may be included at an area fraction of 50% or more. The martensite is an effective structure for securing strength. On the other hand, the bainite structure has a somewhat lower strength than martensite, and therefore, when formed at a fraction exceeding a certain level, there is a concern that the material's bendability may be inferior.

[0322] Accordingly, a hot-formed member according to one embodiment of the present invention may include a bainite phase as a microstructure, but the fraction thereof may be 50% or less. If the fraction of bainite exceeds 50%, there is a risk that bendability may be poor.

[0323] According to one embodiment of the present invention, a hot-formed member may have a tensile strength of 1000 MPa or more and may have excellent bendability and surface quality.

[0324] In one embodiment of the present invention, the hot-formed member may have a maximum bending angle of 40° or more and a surface gloss of 25.0 GU or more. If the surface gloss of the hot-formed member is less than 25.0 GU, the surface quality is not secured, and the iron oxide on the surface must be removed, and for this purpose, a shot blasting process is essentially required.

[0325] In one embodiment of the present invention, the alloy composition of the hot-formed member may have the alloy composition described above, and the description of each element is replaced with the aforementioned matters.

[0326] Hereinafter, a method for manufacturing a hot-formed part according to another aspect of the present invention, specifically a method for manufacturing a hot-formed part with excellent bendability, will be described.

[0327] In one embodiment of the present invention, after preparing a steel material, a blank can be manufactured using the steel material. Here, the steel material may be a steel material (steel material for hot forming) according to one embodiment of the present invention, and as a non-limiting example, the steel material may have the aforementioned alloy composition, and the description of each element is replaced with the aforementioned details.

[0328] In one embodiment of the present invention, a step of loading the blank into a furnace having a dew point temperature of -15 to 20°C and heating it to a temperature of Ae3 or higher and then maintaining it may be performed.

[0329] According to one embodiment of the present invention, the heating and maintenance of the blank can be performed under conditions that satisfy both the following relations 1 and 2. That is, the blank can be kept in a furnace that satisfies the heat treatment conditions of the following relation 1 for a time defined by the following relation 2.

[0330] [Relationship 1]

[0331] 0.722260 + (0.773168×Si) + (2.073977×Mn) + (1.667101×Cr) - (0.196843×O2) - (0.031066×DP) - (0.002384×T furnace ) - (0.001400×t total ) ≥ 0.8

[0332] (In equation 1, Si, Mn and Cr represent the content (weight%) of the alloy composition of the absence, O2 represents the oxygen concentration (%) in the furnace, DP represents the dew point temperature (℃) in the furnace, T furnace is the set temperature (℃) within the furnace, t total means the total heating time (sec) within the furnace.)

[0333] [Relationship 2]

[0334]

[0335] (In relation 2, t total Total heating time (sec) within the furnace, A e3 is the equilibrium temperature (℃) at which transformation to austenite is completed, T furnace refers to the set temperature (℃) within the furnace.)

[0336] In one embodiment of the present invention, in the case of a heating facility consisting of a furnace connected to a roller hearth type furnace as a facility for heating a blank, T of relational expression 2 furnace Indicates the temperature of the furnace with the highest setting temperature among the connected furnaces.

[0337] In one embodiment of the present invention, if the heating is performed for a time shorter than the total heating time obtained from relational expression 2, i.e., if the residence time in the furnace is less than the value of relational expression 2, sufficient reverse transformation austenite cannot be secured, and there is a concern that the intended properties of the hot-formed part may not be secured after the subsequent cooling process is completed.

[0338] In one embodiment of the present invention, when heating and maintaining the blank under the conditions satisfying both the above equations 1 and 2, if the total residence time in the furnace is less than 180 seconds, the reverse transformation into austenite may not be sufficiently achieved, and there is a concern that ferrite may be included in the microstructure after the subsequent cooling process. In addition, since remelting of cementite is not actively achieved, the number of cementite containing a certain amount or more of Cr and having a minor diameter of 100 nm or more may be excessive, and as a result, the hardenability may be deteriorated, and thus, the bendability may be deteriorated due to the presence of ferrite after cooling. Therefore, in one embodiment of the present invention, the residence time in the furnace when heating the blank may be 180 seconds or more, and in another embodiment, it may be set to 200 seconds or more, and in yet another embodiment, it may be set to 240 seconds or more.

[0339] In one embodiment of the present invention, if the dew point temperature within the furnace is below -15°C, excessive dew point-controlled air must be injected, which may result in increased process costs. On the other hand, if the temperature exceeds 20°C, it becomes difficult to ensure good surface quality, and the amount of diffusible hydrogen introduced during hot forming increases, which may result in poor bendability.

[0340] In one embodiment of the present invention, the oxygen concentration (O2) within the furnace may be 15% or less. As another example, the oxygen concentration may be 10% or less.

[0341] According to one embodiment of the present invention, a heated and maintained blank can be hot-formed into a desired shape using a mold or the like. Thereafter, the hot-formed blank can be cooled, and the cooling can be performed at a cooling rate of 10 to 1000°C / s.

[0342] In one embodiment of the present invention, if the cooling rate is less than 10°C / s during the cooling process, unintended ferrite and pearlite structures are formed, making it difficult to secure strength. On the other hand, if the cooling rate exceeds 1000°C / s, there is a problem in that expensive special cooling equipment is required.

[0343] In one embodiment of the present invention, the cooling end temperature during the cooling may be lower than Mf. However, if cooling is terminated at a temperature exceeding Mf, it must be cooled back to room temperature, and in this case, it is difficult to secure shape freezing properties of the hot-formed part. Here, Mf refers to the martensite transformation end temperature.

[0344] In another embodiment of the present invention, in order to secure better elongation and impact resistance in a hot-formed member, martensite may be tempered and retained austenite stabilized through a process of stopping cooling in a temperature range of Mf to Ms and then maintaining the temperature therein, or a process of stopping cooling in a temperature range of Mf to Ms and then reheating (reheating) to a temperature lower than Ac1. Here, Ms refers to the martensite transformation initiation temperature, and Ac1 refers to the temperature at which transformation from ferrite + cementite to austenite begins during heating.

[0345] According to one embodiment of the present invention, the steel used to obtain a blank in the manufacture of a hot-formed member may be a cold-rolled steel sheet. In this case, the cold-rolled steel sheet may be manufactured using the method presented for manufacturing the aforementioned steel, i.e., steel for hot forming, but is not limited thereto.

[0346] As an example, the cold-rolled steel sheet can be manufactured through the steps of: preparing a steel slab; heating the steel slab in a temperature range of 1000 to 1300°C; finishing hot-rolling the heated steel slab in a temperature range of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling at a cooling rate of 20 to 100°C / s after the finishing hot-rolling; coiling at a temperature range of more than Ms and less than or equal to 750°C after the cooling; cold-rolling at a cumulative reduction ratio of 30 to 80% after the coiling to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet in a temperature range of 700 to 900°C. At this time, the cold rolling after the coiling can be performed at a cumulative reduction ratio of 30 to 80% in order to secure a predetermined target thickness.

[0347] Hereinafter, a hot press-formed part according to the third aspect of the present invention will be described.

[0348] 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.

[0349] 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.

[0350] 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.

[0351] 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.

[0352] 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.

[0353] 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).

[0354] 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.

[0355] In one embodiment of the present invention, the steel may be the aforementioned hot-forming steel and may have the aforementioned alloy composition. In addition, the steel may be obtained by the aforementioned manufacturing method, for example, the aforementioned cold-rolled steel sheet manufacturing method.

[0356] 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.

[0357] 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.

[0358] The above blank holding force (F) satisfies the following [Formula 2], 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.

[0359] [Formula 2] F = P × A

[0360] (However, in the above formula 2, P represents the blank holding pressure, and A represents the contact area between the lower holder and the steel.)

[0361] 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 [Equation 3] and [Equation 4]. 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.

[0362] [Formula 3] T max = T blank - [(CR min ·w·ρ·C) / h]

[0363] [Equation 4] h = a·P b

[0364] (However, in the above equations 3 and 4, h is the heat transfer coefficient between the steel and the lower holder, T blank is the forming temperature of the 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.)

[0365] 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.

[0366] 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.

[0367] 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 the end of forming, 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 of the target range can be secured without a separate cooling device.

[0368] Hereinafter, a molding device according to the fourth aspect of the present invention and a molding method using the same will be described.

[0369] 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.

[0370] In one embodiment of the present invention, the high-strength steel material may be the aforementioned hot-forming steel material and may have the aforementioned alloy composition. In addition, the material may be manufactured by the aforementioned manufacturing methods.

[0371] Figures 12 and 13 illustrate a molding apparatus according to one embodiment of the present invention. Specifically, Figure 12 illustrates an exploded perspective view of the molding apparatus according to one embodiment of the present invention, and Figure 13 illustrates a side view of the 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.

[0372] 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.

[0373] 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.

[0374] 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).

[0375] 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.

[0376] 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).

[0377] 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).

[0378] 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.

[0379] 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).

[0380] 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).

[0381] 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).

[0382] 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).

[0383] 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).

[0384] 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).

[0385] 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).

[0386] 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).

[0387] 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.

[0388] 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.

[0389] 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.

[0390] 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.

[0391] 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.

[0392] 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).

[0393] 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.

[0394] 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 it is 3 mm, whether forming is possible or not is determined depending on the radius of curvature (Rp) of the forming part. This will be explained again when explaining the bracket for the battery case.

[0395] FIGS. 14 to 20 are views showing a molding method according to an embodiment of the present invention. FIG. 14 is a flowchart showing a molding method according to an embodiment of the present invention. FIG. 15 is a plan view showing a material fed 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. 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. FIG. 19 is a front view showing a cooling step in a molding method according to an embodiment of the present invention. FIG. 20 is a perspective view showing a trimming step in a molding method according to an embodiment of the present invention.

[0396] A molding method according to another aspect of the present invention can be performed by the molding device described above, and will be described with reference to FIG. 12.

[0397] 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.

[0398] 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.

[0399] In one embodiment of the present invention, the material may be the hot forming steel described above and may have the alloy composition described above.

[0400] 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).

[0401] 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).

[0402] 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) is described through Figs. 12 and 13.

[0403] 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).

[0404] After the cooling step (S140) is completed, a trimming step (S150) is performed to cut the formed material (B-400) along the 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).

[0405] 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.

[0406] FIG. 21 shows a perspective view of a bracket manufactured using a molding method according to one embodiment of the present invention.

[0407] 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) with 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.

[0408] 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 the cross section of the molded products of FIGS. 22 and 23.

[0409] 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.

[0410] 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 curvature radius of curvature (Rp) was 22 mm.

[0411] As shown in Figures 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 location 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.

[0412] Comparative Example 1-1 is a product formed by supplying 1500 MPa mart steel as a material through the same forming device.

[0413] As shown in Fig. 24, when manufacturing a bracket using the molding device and molding method according to one embodiment (embodiment 1-1) of the present invention, the maximum thickness reduction rate, that is, the thickness reduction rate of the part 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 part 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.

[0414] In addition, as shown in Fig. 24, even if the bracket is curved and elongation is dominant, shrinkage occurs in certain parts, and if buckling occurs due to this shrinkage, damage to the material 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.

[0415] However, in the case of Comparative Example 1-1, a thickness reduction rate of -0.8 occurs at the maximum thickness (tmax) position, and buckling occurs in that area. Such buckling leads to cracks, which not only harm the structural performance of the battery case, but also may allow water to flow into that area, so a bracket like Comparative Example 1-1 cannot be applied to a battery case. In order to maintain sealing properties, it is advantageous to maintain a thickness reduction rate of -0.4 or higher even in the curved area.

[0416] 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, when the curvature radius (r) of the molded part was less than 3 mm, it was 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. In addition, even if flange molding was possible when the curvature radius (r) of the molded part was 3 mm and the bending curvature radius (r) was less than 22 mm, molding was not properly performed at the flange of the curved part. Meanwhile, when the curvature radius (r) of the molded part and the curvature radius (Rp) of the molded part increased, the molding itself was not difficult, but there was a problem that the curvature radius increased in the battery case, resulting in a dead space, which was impractical. Therefore, it is preferable that the sum of the curvature radius (Rp) of the molded part and the bending curvature radius (r) be 30 mm or less for a bracket applicable to a battery case.

[0417] 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.

[0418] 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.

[0419] 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.

[0420] 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.

[0421] 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.

[0422] Hereinafter, a hot press-formed part according to the fifth aspect of the present invention will be described.

[0423] 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.

[0424] 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.

[0425] 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.

[0426] 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).

[0427] 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.

[0428] 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.

[0429] 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.

[0430] 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.

[0431] 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.

[0432] 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.

[0433] In one embodiment of the present invention, the plate may be the hot forming steel described above and may have the alloy composition described above.

[0434] 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.

[0435] 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).

[0436] 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), 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.

[0437] According to the hot press forming process, a hat-shaped shape with multiple walls can be formed at once, resulting in excellent formability.

[0438] 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.

[0439] 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.

[0440] According to one embodiment of the present invention, the tensile strength of the molded part may be 1300 to 2100 MPa.

[0441] 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.

[0442] 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.

[0443] 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.

[0444] 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.

[0445] 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 3 can be satisfied.

[0446] [Relationship 3]

[0447] hw ≤ 13.4 * θw + 182.4

[0448] Here, the unit of hw is mm, the unit of θw is degree, and the left and right sides are nondimensionalized and calculated.

[0449] A molded part processed by hot press forming that satisfies the above relational expression 3 can be manufactured with excellent quality without defects such as cracks.

[0450] Referring to Table 1 below, it can be seen that formability is excellent when the above formula is satisfied.

[0451] θw (°)hw(mm)Calculated value of the right sideRelationship 3 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

[0452] 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.

[0453] Fig. 29a is a drawing interpreted in the same manner as the conditions of Invention Example 1, and Fig. 29b is a drawing interpreted in 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.

[0454] 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.

[0455] Hereinafter, another aspect of the present invention, a battery pack module, will be described.

[0456] The battery pack module below includes the molded part described above, and the description of the molded part is cited above.

[0457] FIG. 31 is a drawing illustrating a battery pack module according to an 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-molded part is small, and (b) illustrates a case where the radius of curvature is larger than that of (a).

[0458] 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).

[0459] 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 or has a curve or groove as needed, and is configured to place the battery pack.

[0460] 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.

[0461] 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).

[0462] For example, the hot press-formed parts (C-10) may be arranged in multiple numbers at regular intervals within the battery case (C-20).

[0463] 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.

[0464] 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.

[0465] 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.

[0466] 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.

[0467] 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).

[0468] Hereinafter, a hot press-formed part according to the sixth aspect of the present invention will be described.

[0469] 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.

[0470] 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').

[0471] 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).

[0472] 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).

[0473] 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').

[0474] 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).

[0475] 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).

[0476] 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).

[0477] 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).

[0478] 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.

[0479] 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.

[0480] 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.

[0481] 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.

[0482] 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).

[0483] 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).

[0484] 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).

[0485] 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).

[0486] 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.

[0487] 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.

[0488] 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').

[0489] 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 equation (1).

[0490] w min ≤w≤114.85e 0.0455h ... Formula (1)

[0491] 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.

[0492] 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 (pre-set length) (w) apart from each other satisfies the formula (1), the occurrence of damage such as cracks in the deformation section (D-120) and its surrounding area [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.

[0493] 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 equation (2).

[0494] w min ≤w≤46.309e 0.105h ... Formula (2)

[0495] 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.

[0496] 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 the formula (1), the occurrence of damage such as cracks in the deformation section (D-120) and its surrounding area [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.

[0497] Meanwhile, in the above-described embodiment 3-1 and embodiment 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.

[0498] 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.

[0499] 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.

[0500] 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.

[0501] 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.

[0502] 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.

[0503] 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).

[0504] 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).

[0505] 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).

[0506] 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).

[0507] 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).

[0508] 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.

[0509] 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).

[0510] 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.

[0511] 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).

[0512] 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.

[0513] 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.

[0514] 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).

[0515] 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).

[0516] 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.

[0517] 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.

[0518] 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.

[0519] 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.

[0520] 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.

[0521] 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.

[0522] 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).

[0523] 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).

[0524] 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.

[0525] 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.

[0526] 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.

[0527] 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).

[0528] 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).

[0529] 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.

[0530] Then, after the manufactured press-formed part (D-10') is discharged from the manufacturing device (D-20'), the next press-formed process can begin as a new metal plate (D-10') is supplied.

[0531] 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.

[0532] 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).

[0533] 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).

[0534] 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).

[0535] 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).

[0536] 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).

[0537] 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).

[0538] 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).

[0539] 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.

[0540] 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.

[0541] [Explanation of symbols]

[0542] A-10: Upper die

[0543] A-12: Cooling Channel

[0544] A-20: Lower punch

[0545] A-22: Cooling channel

[0546] A-30: Lower holder

[0547] A-32: Heating heater

[0548] A-34: Cooling channel

[0549] A-100: Mold

[0550] A-200: Steel

[0551] A-300: 22MnB5 material

[0552] B-1: Side Frame B-10: Frame

[0553] B-20: Bracket B-21, 22: Flange

[0554] B-23: Body B-24: Curved surface

[0555] B-100: Punch

[0556] B-110, B-120, B-130, B-140, B-150: 1st to 5th punch forming sections

[0557] B-111, B-121, B-131, B-141, B-151: First to fifth extension surface forming parts

[0558] B-112, B-122, B-132, B-142, B-152: First to fifth curved forming sections

[0559] B-113, B-123, B-133, B-143: First to fourth main body molding parts

[0560] B-200: Pad

[0561] B-300: Die

[0562] B-310, B-320, B-330, B-340, B-350: 1st to 5th die forming sections

[0563] B-311, B-321, B-331, B-341, B-351: 1st to 5th die curved forming sections

[0564] B-312, B-322, B-332, B-342, B-352: 1st to 5th die flat forming sections

[0565] B-360: Vertical plane

[0566] B-400: Material B-410: Notch

[0567] C-1: Battery pack module C-10: Hot press molded part

[0568] C-20: Battery Case C-30: Base

[0569] C-40: Reinforcing member C-110: First wall

[0570] C-120: Second wall C-130: Third wall

[0571] C-140: 4th wall C-150: 5th wall

[0572] C-160: Curved section C-170: First connecting section

[0573] C-180: Second connection C-190: Third connection

[0574] b: battery pack r: radius of curvature

[0575] t: plate thickness θw: wall angle

[0576] hw: wall height

[0577] D-10, D-10': Press-formed parts

[0578] D-10A: Metal plate

[0579] D-110: Reputation Department

[0580] D-120: Transformation section

[0581] D-20, D-20': Press forming device

[0582] D-200: Mold 1

[0583] D-210: First projection

[0584] D-220: First plane section

[0585] D-300: Intermediate pressurization section

[0586] D-400: Second mold

[0587] D-410: Second protrusion

[0588] D-420: Second plane

[0589] Hereinafter, the present invention will be described in detail through examples. However, it should be noted that the following examples are intended only to illustrate the present invention through examples 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.

[0590] (Example 1)

[0591] Steel slabs having the alloy compositions shown in Table 2 and a thickness of 100 mm were manufactured through vacuum melting. Each steel slab was heated to 1250°C, then final 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 with a final thickness of 2.5 mm. Thereafter, each of the hot-rolled steel sheets was pickled and then cold-rolled under the conditions shown in Table 2 to manufacture a cold-rolled steel sheet. At this time, the total reduction ratio during cold rolling was set to 45%. Thereafter, each of the cold-rolled steel sheets was annealed at an annealing temperature of 780°C in a 5% hydrogen-95% nitrogen atmosphere, and then cooled to manufacture annealed cold-rolled steel sheets.

[0592] Each annealed cold-rolled steel sheet manufactured as described above was made into a blank, and then hot-formed and cooled using a hot-forming mold to obtain a hot-formed part. At this time, the heating temperature of the blank was 900°C, the holding time at that temperature was set to 5 minutes, and the cooling rate was 40°C / s.

[0593] In this way, when performing hot forming, the transfer time from the heating furnace to hot forming using the mold was applied equally at 10 seconds.

[0594] The physical properties of the annealed cold-rolled steel sheet (steel for hot forming) and hot-formed member according to the above were measured.

[0595] Specifically, the strength and thickness direction of the Rotated-Cube texture on the ODF (Orientation Distribution Function) map were determined by applying the EBSD (Electron Backcattered Diffraction) technique to the annealed cold-rolled steel sheet. <111> The cluster area fraction of faces perpendicular to the direction was measured. For data analysis, EDAX OIM Analysis was used. TM An ODF map on ferrite was constructed using the v7.3 program, and the derived values ​​are shown in Table 3 below. At this time, the angular deviation of the direction for measuring the cluster area fraction was set to ±5°, and then the area fraction was calculated.

[0596] In addition, in order to evaluate the bending anisotropy of the hot-formed member, a bending evaluation was conducted according to the VDA238-100 standard. Specifically, if the bending angle deviation derived from [Equation 1] exceeded 15.0 after a three-point bending test, the anisotropy was evaluated as poor.

[0597] [Formula 1]

[0598] {|Bending angle in the rolling direction - Bending angle in the rolling direction| / Bending angle in the rolling direction}×100

[0599] In addition, mechanical properties (tensile strength (TS), yield strength (YS) and elongation (El)) were measured using a universal tensile tester according to the ISO6892 standard using JIS-5 specimens.

[0600] Steel alloy composition (weight %) CSiMnPSAlCrNA0.4351.791.540.0070.0040.062.1200.004 B0.3111.201.870.0030.0030.063.9900.008 C0.2071.051.480.0090.0100.081.1570.003 D0.0841.091.000.0050.0070.013.6100.008 E0.3752.711.250.0080.0050.044.2910.003

[0601] Steel gradeFirst cold rolling mill reduction ratio (%)Last cold rolling mill reduction ratio (%)ClassificationA19.398.61Invention example A-1A2.770.07Comparative example A-1A0.393.28Comparative example A-2A8.100.03Comparative example A-3B14.515.88Invention example A-2B4.886.29Comparative example A-4B24.460.07Comparative example A-5C30.703.50Invention example A-3C4.710.06Comparative example A-6C2.761.20Comparative example A-7C32.570.02Comparative example A-8D25.808.81Invention example A-4D1.430.09Comparative example A-9D3.163.84Comparative example A-10D30.410.08 Comparative Example A-11E15.983.70 Invention Example A-5E1.390.01 Comparative Example A-12E2.805.25 Comparative Example A-13E22.920.09 Comparative Example A-14

[0602] Steel for hot forming (annealed cold rolled steel sheet)Hot forming memberRotated-cube strength <111> Vertical plane cluster area (%)YS (MPa)TS (MPa)El (%)Rolling perpendicular direction bending angle (°)Rolling direction bending angle (°)Bending angle deviation (%)Invention example A-12.1117.20134119705.943.043.92.1Comparative example A-13.605.90101318184.026.241.236.4Comparative example A-22.962.69129918586.235.942.615.7Comparative example A-31.734.74134919966.332.940.619.0Invention example A-21.859.56130518606.444.048.59.3Comparative example A-49.882.96108117426.731.037.717.8Comparative example A-53.759.27111017866.731.337.917.4Inventive example A-32.147.98105314897.356.456.90.9Comparative example A-61.125.93107415257.242.858.026.2Comparative example A-78.143.29106115166.946.656.717.8Comparative example A-83.317.89105715077.646.855.816.1Inventive example A-41.529.2485511006.9100.1100.80.7Comparative example A-94.702.4984810917.084.7102.517.4Comparative example A-101.054.0888611135.976.7104.026.3Comparative example A-114.306.3381310497.186.9105.017.2Invention example A-52.348.05134119737.157.957.60.5Comparative example A-128.034.17139020377.348.157.516.3Comparative example A-133.116.12139620797.043.952.115.7Comparative example A-142.334.49138920307.445.553.815.4

[0603] As shown in Tables 2 to 4 above, invention examples A-1 to A-5 that satisfy the alloy composition and manufacturing conditions (particularly, cold rolling conditions) of the present invention are steels for hot forming (annealed cold rolled steel sheets) in which the strength of the Rotated-Cube on the ODF map of ferrite is 2.50 or less in the thickness direction. <111> The cluster fraction of faces perpendicular to the direction was obtained to exceed 6.00%. As a result, excellent bending anisotropy of a hot-formed member obtained by hot-forming the above-mentioned hot-forming steel can be secured.

[0604] On the other hand, although the alloy compositions of Comparative Examples A-1 to A-14 all satisfy the present invention, the manufacturing conditions (cold rolling conditions) deviate from the present invention, and thus the characteristics of the steel for hot forming could not be secured. As a result, it can be confirmed that the bending anisotropy is inferior in the hot forming member, with the bending angle deviation exceeding 15.0.

[0605] Figure 1 shows the ODF map results of Invention Example A-1 and Comparative Example A-1. As the color on the drawing changes from blue to red, the cluster area increases, and the values ​​are as described.

[0606] As shown in Fig. 1, in invention example A-1 compared to comparative example A-1, <111> It can be confirmed that the cluster area fraction of faces perpendicular to the direction is higher.

[0607] (Example 2)

[0608] Steel slabs having the alloy compositions shown in Table 5 below and a thickness of 100 mm were manufactured through vacuum melting. Each steel slab was heated to 1250°C, final 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 with a final thickness of 2.5 mm. Thereafter, each of the hot-rolled steel sheets was pickled and then cold-rolled at a cold reduction ratio of 45% to manufacture a cold-rolled steel sheet. Thereafter, each of the cold-rolled steel sheets was annealed at an annealing temperature of 780°C in a 5% hydrogen-95% nitrogen atmosphere, and then cooled to manufacture annealed cold-rolled steel sheets.

[0609] Each annealed cold-rolled steel sheet manufactured as described above was made into a blank, and then the blank was transferred into a heating furnace, and then hot-formed and cooled using a hot-forming mold according to the conditions shown in Table 6 below, thereby obtaining a hot-formed member.

[0610] The microstructure, surface quality, and physical properties of each hot-formed member were evaluated and are shown in Table 7 below.

[0611] Specifically, in order to measure the microstructure of the hot-formed member, the cross-section of the specimen at the t / 4 point in the thickness direction was polished and etched, and then observed using a scanning electron microscope (SEM). At this time, the major and minor axes were measured after magnifying 10,000 times to analyze the size of cementite. In addition, the composition of the cementite was measured using energy-dispersive X-ray spectroscopy (EDS) at a magnification of 10,000 times.

[0612] In addition, in order to inspect the surface quality of hot-formed parts, the standard gloss (angle of 60° between the incident angle and the normal vector to the surface of the part) of each part surface was measured in accordance with the ISO2813 standard, and in order to evaluate the tensile properties, a room temperature tensile test was performed in accordance with the ISO6892 standard using a JIS-5 specimen.

[0613] In addition, to evaluate the bendability, a bendability evaluation was conducted according to the VDA238-100 standard to obtain the maximum bending angle. At this time, the angle at which the maximum bending strength specified in the standard is secured was measured, then converted into a bending angle to derive the maximum bending angle, and the value was displayed.

[0614] Steel alloy composition (weight %) CSiMnPSAlCrTiBNF0.3450.201.250.0100.00100.0350.400.0300.00250.003G0.2910.000.010.0100.00200.0532.000.0160.00180.001H0.2900.500.010.0100.00300.0501.900.0150.00200.002I0.2931.490.590.0100.00080.0460.690.0150 .00290.004J0.0700.960.020.0100.00300.0511.930.0150.00200.001K0.2501.530.390.0100.00080.0351.000.0140.0025 0.004L0.3141.500.080.0100.00060.0571.500.0150.00260.003M0.2541.490.790.0100.00200.04054.320.0320.00280.005

[0615] Kang Jong A e3 (℃) Heating condition relation equation 1 Relation equation 2 Oxygen concentration (%) Dew point temperature (℃) T furnace (℃)t total(sec)F780559005000.15142.61G810559004000.23163.62H821559004000.45173.33I851559004000.4 0210.84J918559503000.90248.55J918205950400-2.19248.56J9185109504000.60248.57K8801590040 01.32300.48K880159001501.60300.49K8801590010000.48300.410K880559004000.54300.411L854159004001.49216.112L854559004000.70216.113M827559004006.87179.314M827559501007.17140.815- A in Table 6 e3 The temperature is calculated using the Thermo_calc software, and the database uses TCFE6. - The value in equation 1 is rounded to the third decimal place, and the value in equation 2 is rounded to the second decimal place.

[0616] Tensile properties Gloss (GU) Maximum bending angle (°) Classification Ferrite fraction (area %) Number of cementite (units / ㎛) 2)YS(MPa)TS(MPa)YREl(%)10.08.47132419830.676.13.845Invention example B-120.03.45115216620.697.65.656Invention example B-230.04.06113417130.667.811.353Invention example B-340.03.59127219020.677.110.148Invention example B-450.04.7879111650.687.727.983Invention example B-560.03.5778711410.697.0-54.982Invention example B-670.04.5775911430.667.115.184 Invention example B-780.03.89118217540.677.233.153 Invention example B-898.112.3085812520.6911.241.831 Comparative example B-1100.00.87114616900.687.812.155 Invention example B-9110.04.54118617560.686.313.452 Invention example B-10120.03.98129118990.687.437.347 Invention example B-11130.03.25130719050.697.917.646Invention Example B-12140.03.16123217970.697.9171.852Invention Example B-131511.218.3076511210.6811.2179.334Comparative Example B-2- Except for the ferrite fraction among the structural characteristics, the remaining structures are martensite phases in all examples. That is, if the ferrite fraction is 0%, it means that the microstructure is a martensite single-phase structure. - The number of cementite represents the number of cementite having a Cr content of 1.20% or more and a minor diameter of 100 nm or more at the t / 4 point in the thickness direction.

[0617] As shown in Tables 5 to 7, according to one embodiment of the present invention, the number of specific cementites is 10.00 μm. 2 The following invention examples B-1 to B-13 have bending characteristics of 40° or more, confirming excellent bendability.

[0618] Among these, examples that deviate from some of the alloy composition according to one embodiment of the present invention and / or manufacturing conditions according to one embodiment of the present invention showed a gloss of less than 25.0 GU.

[0619] In contrast, examples satisfying both the alloy composition and manufacturing conditions according to one embodiment of the present invention (Invention Examples B-5, B-8, B-11 and B-13) showed excellent results in both bendability and surface quality.

[0620] Meanwhile, according to one embodiment of the present invention, the number of specific cementite is 10.00㎛ 2 Comparative examples B-1 and B-2, which exceeded the value, showed inferior bendability results. Comparative examples B-1 and B-2 were not only excessively formed of specific cementite due to being in the furnace for a time shorter than the value derived by relational expression 2, i.e., the heat treatment time was insufficient, but also ferrite was transformed in the microstructure, resulting in inferior bendability.

[0621] Fig. 2 shows surface observation photographs of Invention Example B-5 and Invention Example B-1. As shown in Fig. 2, it can be confirmed with the naked eye that the surface quality of Invention Example B-5 is superior to that of Invention Example B-1.

[0622] (Example 3)

[0623] Invention Example C

[0624] 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. 2That 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 8 mJ / 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.

[0625] Comparative Example C

[0626] 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.

[0627] Although the present invention has been described above with reference to examples, the present invention is not limited to the above-described examples, and it goes without saying that modifications can be made and implemented by those skilled in the art without changing the technical idea of ​​the present invention as claimed in the claims.

Claims

1. On the ODF (Orientation Distribution Function) map, the intensity of the Rotated-Cube aggregate structure is 2.50 or less, and in the thickness direction. <111> Hot-forming steel in which the cluster of planes perpendicular to the direction exceeds 6.00% of the total aggregate fabric area.

2. In paragraph 1, The above steel is a hot forming steel containing, in wt%, carbon (C): 0.040 to 0.450%, silicon (Si): 0.80 to 3.00%, manganese (Mn): 4.00% or less (excluding 0%), phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 1.000 to 5.000%, nitrogen (N): 0.020% or less (excluding 0%), the remainder being iron and unavoidable impurities.

3. In paragraph 1, The above steel is a hot forming steel further comprising at least one element selected from among the following a) to f). a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.005 to 0.400 wt%, b) Boron (B): 0.0001 to 0.0100 wt%, c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.000 wt%, d) At least one of copper (Cu) and nickel (Ni): 0.003 to 2.000 wt%, e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.000 wt% f) Rare earth elements (REM): 0.0001 to 0.0200 wt% 4. In paragraph 1, The microstructure of the above steel mainly includes ferrite or a composite structure of ferrite and cementite, and in addition, includes at least one of pearlite, bainite, and martensite. Hot forming steel containing the above main component in an area fraction of 30% or more.

5. In paragraph 1, The above steel is a hot forming steel with a tensile strength of 550 MPa or more.

6. Steps to prepare the steel slab; A step of heating the above steel slab in a temperature range of 1000 to 1300°C; A step of obtaining a hot-rolled steel sheet by finishing hot rolling the above-mentioned heated steel slab in a temperature range of Ar3 to 1000℃; A step of cooling at a cooling rate of 20 to 100°C / s after the above finishing hot rolling; A step of winding in a temperature range of from above Ms to below 750℃ after the above cooling; A step of obtaining a cold rolled steel sheet by cold rolling after the above coiling; and It includes a step of annealing the above cold rolled steel sheet at a temperature range of 700 to 900°C, A method for manufacturing a steel for hot forming, wherein the above cold rolling is performed at a reduction ratio of 5% or more and less than 35% in the first cold rolling mill and at a reduction ratio of 0.1% or more and less than 10.0% in the last cold rolling mill.

7. In paragraph 6, The above steel slab contains, in wt%, carbon (C): 0.040 to 0.450%, silicon (Si): 0.80 to 3.00%, manganese (Mn): 4.00% or less (excluding 0%), phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 1.000 to 5.000%, nitrogen (N): 0.020% or less (excluding 0%), the remainder being iron and unavoidable impurities, a method for manufacturing a steel for hot forming.

8. In paragraph 6, A method for manufacturing a steel for hot forming, wherein the above steel slab further includes at least one element selected from among the following a) to f). a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.005 to 0.400 wt%, b) Boron (B): 0.0001 to 0.0100 wt%, c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.000 wt%, d) At least one of copper (Cu) and nickel (Ni): 0.003 to 2.000 wt%, e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.000 wt% f) Rare earth elements (REM): 0.0001 to 0.0200 wt% 9. [Formula 1] A hot-formed member satisfying 15.0 or less. [Formula 1] {|Bending angle in the rolling direction - Bending angle in the rolling direction| / Bending angle in the rolling direction}×100 10. In paragraph 9, A hot-formed member comprising the above-mentioned member in weight %, carbon (C): 0.040 to 0.450%, silicon (Si): 0.80 to 3.00%, manganese (Mn): 4.00% or less (excluding 0%), phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 1.000 to 5.000%, nitrogen (N): 0.020% or less (excluding 0%)), the remainder being iron and unavoidable impurities.

11. In paragraph 9, The above-mentioned member is a hot-formed member whose microstructure is a single phase of martensite or a composite structure of bainite with an area fraction of 40% or less and the remainder of martensite.

12. In paragraph 9, A hot-formed member, wherein the above-mentioned member further comprises at least one element selected from among the following a) to f). a) At least one selected from titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V): 0.005 to 0.400 wt%, b) Boron (B): 0.0001 to 0.0100 wt%, c) At least one of molybdenum (Mo) and tungsten (W): 0.001 to 1.000 wt%, d) At least one of copper (Cu) and nickel (Ni): 0.003 to 2.000 wt%, e) At least one of antimony (Sb) and tin (Sn): 0.001 to 1.000 wt% f) Rare earth elements (REM): 0.0001 to 0.0200 wt%

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