Hot stamping parts and method for manufacturing the same

Abstract

To provide a hot-stamping component, and a method for manufacturing the same.SOLUTION: A method for manufacturing a hot-stamping component includes: a step of charging a blank into a heating furnace having a plurality of sections having different temperature ranges; a multi-stage heating step of gradually heating the blank; a uniform heating step of heating the blank at temperatures of Ac3 to 1,000°C; and a transfer step of transferring the heated blank to a mold from the heating furnace.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to hot stamping parts and a method for manufacturing the same.

Background Art

[0002] Worldwide, environmental regulations and fuel efficiency regulations are being strengthened, and the need for lighter vehicle materials is increasing. As a result, research and development on ultra-high strength steel and hot stamping steel have become active. Here, the hot stamping process generally consists of heating / formning / cooling / trimming, and during the process, the phase transformation of the material and the change in the microstructure are utilized.

[0003] Recently, research has been actively conducted to improve the delayed fracture, corrosion resistance, and weldability of hot stamping parts produced by the hot stamping process. As a related technology, there is Korean Patent Publication No. 10-2018-0095757 (title of the invention: Method for manufacturing hot stamping parts).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The problem to be solved by the present invention is to prevent or minimize the occurrence of quality differences between blanks even when at least two blanks, tailor welded blanks, or tailor rolled blanks, at least one of the thickness and size of which is different from each other, are simultaneously heated in a heating furnace, and to provide a hot stamping part and a method for manufacturing the same.

Means for Solving the Problems

[0006] One embodiment of the present invention includes the steps of charging a blank into a heating furnace having a plurality of sections with different temperature ranges; a multi-stage heating step of heating the blank step by step; a soaking heating step of heating the blank at a temperature of Ac3 to 1,000 ° C; and a transfer step of transferring the heated blank from the heating furnace to a mold. In the multi-stage heating step, the temperature conditions in the heating furnace satisfy the following mathematical formula, and a method for manufacturing hot stamping parts is disclosed.

Equation

[0007] In this embodiment, at least two blanks having different thicknesses can be simultaneously transferred into the heating furnace.

[0008] In this embodiment, the blank includes a first portion having a first thickness and a second portion having a second thickness different from the first thickness.

[0009] In this embodiment, the temperatures of the plurality of sections can increase in the direction from the inlet of the heating furnace to the outlet of the heating furnace.

[0010] In this embodiment, among the sections for multi-stage heating of the blank, the temperature difference between two adjacent sections is greater than 0 ° C and not more than 100 ° C.

[0011] In this embodiment, among the plurality of sections, the temperature of the section for soaking heating of the blank is higher than the temperature of the section for multi-stage heating of the blank.

[0012] In this embodiment, the blank can stay in the heating furnace for 180 seconds to 360 seconds.

[0013] In this embodiment, the air cooling time of the blank in the transfer stage satisfies the following formula (2).

Number

[0014] In this embodiment, in the formula (2), λ t is also 5 s or more and 20 s or less.

[0015] Another embodiment of the present invention discloses a hot stamping part in which the diffusible hydrogen content is less than 0.45 ppm and the corrosion rate measured through a potentiodynamic polarization test is 3×10 -6 A or less.

[0016] In this embodiment, the hot stamping part has a tensile strength of 500 MPa or more and less than 800 MPa and may have a composite structure of ferrite and martensite.

[0017] In this embodiment, the hot stamping part may have a tensile strength of 800 MPa or more and less than 1,200 MPa, and may have a composite structure of bainite and martensite.

[0018] In this embodiment, the hot stamping part may have a tensile strength of 1,200 MPa or more and less than 2,000 MPa, and may have a fully martensite structure.

Advantages of the Invention

[0019] According to the embodiment of the present invention, by multi-stage heating the blank in a heating furnace having a plurality of sections with different temperature ranges, the soaking temperature reaching time of the blank can be more precisely controlled.

[0020] Moreover, by more precisely controlling the soaking temperature reaching time of blanks having different thicknesses, the hydrogen embrittlement, corrosion resistance, and weldability of the parts manufactured by the manufacturing method of the hot stamping parts can be improved.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Mode for Carrying Out the Invention

[0022] The present invention can be subjected to various transformations and can have various embodiments. Specific embodiments are illustrated in the drawings and will be described in detail by detailed description. The effects, features, and the methods for achieving them of the present invention will become clear by referring to the embodiments described in detail later together with the drawings. However, the present invention is not limited to the embodiments disclosed below and can also be embodied in various forms.

[0023] In the following embodiments, terms such as first and second are not used in a limiting sense and are used for the purpose of distinguishing one component from other components.

[0024] In the following embodiments, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0025] In the following examples, terms such as "including" or "having" mean that the features or components described in the specification exist, and do not preclude in advance the possibility of adding one or more other features or components.

[0026] In the following examples, when a part such as a film, region, or component is said to be "above" or "upper" of another part, it includes not only the case where it is directly above the other part, but also the case where other films, regions, components, etc. are interposed in between.

[0027] In the drawings, for the sake of convenience of explanation, the sizes of the components may be exaggerated or reduced. For example, the sizes and thicknesses of the respective configurations shown in the drawings are arbitrarily shown for the convenience of explanation, and thus the present invention is not necessarily limited to the illustrated locations.

[0028] When a certain example can be embodied differently, the specific process order may be carried out differently from the described order. For example, two processes described consecutively may be carried out substantially simultaneously, and may also be advanced in the order opposite to the described order.

[0029] Hereinafter, embodiments of the present invention will be described in detail based on the accompanying drawings. When explaining with reference to the drawings, the same or corresponding components are given the same drawing reference numerals.

[0030] FIG. 1 is a flowchart schematically showing a method for manufacturing a hot stamping part according to an embodiment of the present invention. Hereinafter, with reference to FIG. 1, a method for manufacturing a hot stamping part will be described.

[0031] A method for manufacturing a hot stamping part according to an embodiment of the present invention includes a blank loading stage (S110), a multi-stage heating stage (S120), and a soaking heating stage (S130), and may further include a transfer stage (S140), a forming stage (S150), and a cooling stage (S160) after the soaking heating stage (S130).

[0032] First, the blank loading stage (S110) is also a stage of loading a blank into a heating furnace having a plurality of sections with different temperature ranges from each other.

[0033] The blank loaded into the heating furnace is also formed by cutting a plate material for forming hot stamping parts. The plate material can be manufactured through a process of performing hot rolling or cold rolling on a steel slab and then annealing heat treatment. Further, after the annealing heat treatment, an Al-Si based plating layer or a Zn plating layer can be formed on at least one surface of the annealed plate material.

[0034] FIG. 2 is a plan view schematically showing a blank used in a method for manufacturing a hot stamping part according to an embodiment of the present invention.

[0035] Referring to FIG. 2, a blank 200 according to an embodiment includes at least one of a blank 210 having a single thickness, a tailor welded blank 220 (Tailor Welded Blank, TWB) formed by cutting and welding different types of plate materials with different thicknesses into a required shape, a tailor rolled blank 230 (Tailor Rolled Blank, TRB) formed by rolling a single thickness plate material to have different thicknesses from each other, and a patchwork 240 (Patchwork) manufactured by welding a small patch blank to a large blank.

[0036] The tailor-welded blank 220 can be manufactured by welding a first plate material 221 and a second plate material 223 having different thicknesses from each other. The B-pillar, which is an important part for a vehicle collision member, can be manufactured by welding two plate materials in a form where plate materials with different strengths are joined to an upper collision support part and a lower shock absorption part, and then performing forming. At this time, the tailor-welded blank method mainly used means a series of processes of cutting dissimilar plate materials with different thicknesses, strengths, and materials into a required shape, welding them, and then performing press forming to manufacture parts. By welding plate materials with different thicknesses to manufacture blanks with different thicknesses, different characteristics can be given to different parts of the blank. For example, a 120-200K class ultra-high strength plate material is used for the upper collision support part of the B-pillar, and a plate material with excellent shock absorption performance is connected to the lower end part of the B-pillar where stress is concentrated, so that the shock absorption ability can be improved during a vehicle collision.

[0037] The tailor-rolled blank 230 is manufactured by rolling a steel material in a cold-rolled state so as to have a specific thickness profile. When manufacturing hot stamping parts using the tailor-rolled blank 230, the weight reduction effect is excellent. As an example, the thickness profile can be implemented by an ordinary method. For example, when cold rolling the cold-rolled state steel material, by adjusting the reduction ratio, a tailor-rolled blank 230 including a first region 231 having a first thickness, a second region 232 having a second thickness, a third region 233 having a third thickness, and a fourth region 234 having a fourth thickness can be formed. At this time, each of the first thickness, the second thickness, the third thickness, and the fourth thickness is different from each other, and a transition section 235 may exist between the first region 231 and the second region 232, between the second region 232 and the third region 233, and between the third region 233 and the fourth region 234. However, in FIG. 2, although the tailor-rolled blank 230 is illustrated as including the first region 231 to the fourth region 234, the present invention is not limited thereto. The tailor-rolled blank 230 can be formed including the first region 231, the second region 232,..., the nth region.

[0038] Patchwork 240 is a method of partially reinforcing a base material using at least two or more plate materials, and the patch can be joined to the base material before the forming process so that the base material and the patch can be formed simultaneously. As an example, after a patch 243 having a second size smaller than the first size is welded to a base material 241 having the first size, they can be formed simultaneously.

[0039] FIG. 3 is a plan view schematically showing a blank put into a heating furnace in a method for manufacturing a hot stamping part according to an embodiment of the present invention.

[0040] In the blank loading stage (S100), at least two blanks 200, at least one of the thickness and the size of which is different from each other, can be simultaneously loaded into the heating furnace.

[0041] As an example, FIG. 3 shows two first blanks 250 and two second blanks 260 that are simultaneously loaded into the heating furnace. At this time, the first blank 250 and the second blank 260 can have different sizes and different thicknesses. For example, the first blank 250 can have a thickness of 1.2 mm, and the second blank 260 can have a thickness of 1.6 mm. However, the present invention is not limited thereto, and one first blank 250 and one second blank 260 can be simultaneously loaded into the heating furnace. Also, the first blank 250 and the second blank 260 can be formed to have the same size and different thicknesses, or the same thickness and different sizes, and various deformations are possible.

[0042] Alternatively, in the blank loading stage (S100), at least two blanks 200 having a single thickness can be simultaneously loaded into the heating furnace. For example, at least two or more blanks 250 having a thickness of 1.2 mm can be simultaneously loaded, and at least two or more blanks 260 having a thickness of 1.6 mm can be simultaneously loaded. Also, in the blank loading stage (S100), the aforementioned tailor welded blank 220 (see FIG. 2) or tailor rolled blank 230 (see FIG. 2) can be loaded into the heating furnace.

[0043] The blank introduced into the heating furnace can be transferred along the transfer direction after being mounted on the roller.

[0044] After the blank input stage (S110), a multi-stage heating stage (S120) and a soaking heating stage (S130) can be performed. The multi-stage heating stage (S120) and the soaking heating stage (S130) are also the stages in which the blank is heated while passing through a plurality of sections provided in the heating furnace.

[0045] Specifically, in the multi-stage heating stage (S120), the blank can be gradually heated up while passing through a plurality of sections provided in the heating furnace. Among the plurality of sections provided in the heating furnace, there are a plurality of sections where the multi-stage heating stage (S120) is performed, and the temperature is set for each section so as to increase from the inlet of the heating furnace where the blank is introduced to the outlet of the heating furnace where the blank is taken out, and the blank can be gradually heated up.

[0046] After the multi-stage heating stage (S120), a soaking heating stage (S130) can be performed. In the soaking heating stage (S130), the blank that has been multi-stage heated can be heat-treated while passing through a section of the heating furnace set at a temperature of Ac3 to 1,000 °C. Desirably, in the soaking heating stage (S130), the blank that has been multi-stage heated can be soaked and heated at a temperature of 930 °C to 1,000 °C. More desirably, in the soaking heating stage (S130), the blank that has been multi-stage heated can be soaked and heated at a temperature of 950 °C to 1,000 °C. Also, among the plurality of sections provided in the heating furnace, the section where the soaking heating stage (S130) is performed is at least one or more.

[0047] FIG. 4 is a graph showing the temperature change of the blank when the blank is singly heated by the conventional method. Specifically, FIG. 4 shows the temperature change of the blanks over time when the temperature of the heating furnace is set so that the internal temperature of the heating furnace is kept the same as the target temperature T t of the blank, and then a 1.2-mm-thick blank and a 1.6-mm-thick blank are simultaneously singly heated (320).

[0048] At this time, the target temperature T of the blank t is also above Ac3. Desirably, the target temperature T of the blank t is also 930°C. More desirably, the target temperature T of the blank t is also 950°C. However, the present invention is not limited thereto. Also, single heating does not mean that blanks having a thickness of 1.2 mm and blanks having a thickness of 1.6 mm are respectively charged into the heating furnace and heated. After setting the temperature of the heating furnace to a single temperature, it means the case where blanks having a thickness of 1.2 mm and blanks having a thickness of 1.6 mm are simultaneously charged into the heating furnace and heated.

[0049] Referring to FIG. 4, after setting the temperature inside the heating furnace to the same temperature as the target temperature T of the blank t when simultaneously single-heating a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm, it can be seen that the blank having a thickness of 1.2 mm reaches the target temperature T t earlier than the blank having a thickness of 1.6 mm.

[0050] That is, the blank having a thickness of 1.2 mm reaches the target temperature T t earlier, the blank having a thickness of 1.2 mm is soaked and heated during the first hour S1, and the blank having a thickness of 1.6 mm can be soaked and heated during a second hour S2 shorter than the first hour S1. Since the soaking time is adjusted based on the blank that reaches the target temperature late, the blank having a thickness of 1.2 mm that reaches the target temperature earlier may be overheated, increasing the delayed fracture of the blank having a thickness of 1.2 mm and reducing the weldability.

[0051] FIG. 5 is a graph showing temperature changes when a blank is multi-stage heated and soaking heated in a method for manufacturing a hot stamping part according to an embodiment of the present invention. FIG. 5 is a graph showing the temperature changes over time when a blank having a thickness of 1.2 mm is multi-stage heated at 330 and a blank having a thickness of 1.6 mm is multi-stage heated (340) in an embodiment of the present invention.

[0052] Referring to FIG. 5, a heating furnace according to an embodiment can include a plurality of sections having different temperature ranges. More specifically, the heating furnace can include a first section P1 having a first temperature range T1, a second section P2 having a second temperature range T2, a third section P3 having a third temperature range T3, a fourth section P4 having a fourth temperature range T4, a fifth section P5 having a fifth temperature range T5, a sixth section P6 having a sixth temperature range T6, and a seventh section P7 having a seventh temperature range T7.

[0053] The first section P1 to the seventh section P7 can be arranged in the heating furnace in order. The first section P1 having the first temperature range T1 can be adjacent to the inlet of the heating furnace where the blank is inserted, and the seventh section P7 having the seventh temperature range T7 can be adjacent to the outlet of the heating furnace where the blank is discharged. Therefore, the first section P1 having the first temperature range T1 is also the first section of the heating furnace, and the seventh section P7 having the seventh temperature range T7 is also the last section of the heating furnace. As will be described later, in a plurality of sections of the heating furnace, the fifth section P5, the sixth section P6, and the seventh section P7 are also sections where soaking heating is performed, which is not a section where multi-stage heating is performed.

[0054] The temperatures of the plurality of sections provided in the heating furnace, for example, the temperatures of the first section P1 to the seventh section P7, can increase in the direction from the inlet of the heating furnace where the blank is inserted to the outlet of the heating furnace where the blank is taken out. However, the temperatures of the fifth section P5, the sixth section P6, and the seventh section P7 are also the same. Also, among the plurality of sections provided in the heating furnace, the temperature difference between two adjacent sections is greater than 0°C and less than or equal to 100°C. For example, the temperature difference between the first section P1 and the second section P2 is greater than 0°C and less than or equal to 100°C.

[0055] In one embodiment, the first temperature range T1 of the first section P1 is also 820°C to 860°C and also 835°C to 865°C. The second temperature range T2 of the second section P2 is also 850°C to 890°C and also 865°C to 895°C. The third temperature range T3 of the third section P3 is also 880°C to 920°C and also 895°C to 925°C. The fourth temperature range T4 of the fourth section P4 is also 910°C to 940°C and also 915°C to 945°C. The fifth temperature range T5 of the fifth section P5 is also Ac3 to 1,000°C. Desirably, the fifth temperature range T5 of the fifth section P5 is also 930°C or higher and 1,000°C or lower. More desirably, the fifth temperature range T5 of the fifth section P5 is also 950°C or higher and 1,000°C or lower. The sixth temperature range T6 of the sixth section P6 and the seventh temperature range T7 of the seventh section P7 are the same as the fifth temperature range T5 of the fifth section P5. However, the present invention is not limited thereto.

[0056] In FIG. 5, a heating furnace according to an embodiment of the present invention is illustrated as having seven sections with different temperature ranges, but the present invention is not limited thereto. The heating furnace may be provided with five, six, eight, or the like sections having different temperature ranges.

[0057] A blank according to an embodiment can be heated step by step by passing through a plurality of sections defined in the heating furnace. In one embodiment, in a multi-stage heating step in which the blank is heated step by step by passing through a plurality of sections in the heating furnace, the temperature conditions in the heating furnace can satisfy the following mathematical formula 1.

Equation

[0058] When the value of the formula (1) exceeds 0.025 °C / mm, the initial temperature of the heating furnace becomes low, the temperature rising rate of the blank decreases, and it is impossible to secure a sufficient soaking heating time. In order to secure the soaking heating time, when the driving speed of the roller is decreased for operation, the productivity may decrease. Further, when the value of the formula (1) is 0 °C / mm, it corresponds to single heating, and as described above, there may be a case where the thin and thick blanks reach the target temperature T t first, and overheating may occur in the thin and thick blanks.

[0059] Referring to FIGS. 4 and 5, the blank passes through a plurality of sections (for example, the first section P1 to the fourth section P4) defined in the heating furnace and is gradually heated in multiple stages. When the temperature conditions of the multiple-stage heating satisfy the above formula, compared with the case where the blank is heated by single heating, the temperature change graphs of blanks having different thicknesses may show similar behaviors. For example, when the same time has elapsed after the blank is charged into the heating furnace, the temperature difference between the blanks when the blank having a thickness of 1.2 mm is heated by single heating (310) and the blank having a thickness of 1.6 mm is heated by single heating (320) is smaller than the temperature difference between the blanks when the blank having a thickness of 1.2 mm is heated in multiple stages (330) and the blank having a thickness of 1.6 mm is heated in multiple stages (340). Therefore, when the blank is heated in multiple stages, by controlling the temperature rising rates of blanks having different thicknesses in the same manner, the time difference for each blank to reach the target temperature can be reduced, and overheating of the thin and thick blanks can be prevented.

[0060] After the multi-stage heating step (S120), a soaking heating step (S130) may be performed. The soaking heating step (S130) is also a step of soaking and heating the blank at a temperature of 950 °C to 1,000 °C in the last part of the plurality of sections provided in the heating furnace.

[0061] The soaking heating stage (S130) can be carried out in the last part among the plurality of sections of the heating furnace. As an example, the soaking heating stage (S130) can be carried out in the fifth section P5, the sixth section P6, and the seventh section P7 of the heating furnace. When a plurality of sections are provided in the heating furnace, if the length of one section is long, there may be problems such as temperature changes occurring within the section. Therefore, the section where the soaking heating stage (S130) is carried out is divided into the fifth section P5, the sixth section P6, and the seventh section P7, but the fifth section P5, the sixth section P6, and the seventh section P7 may have the same temperature range in the heating furnace.

[0062] In the soaking heating stage (S130), the multi-stage heated blank can be soaked and heated at a temperature of Ac3 to 1,000 °C. Desirably, in the soaking heating stage (S130), the multi-stage heated blank can be soaked and heated at a temperature of 930 °C to 1,000 °C. More desirably, in the soaking heating stage (S130), the multi-stage heated blank can be soaked and heated at a temperature of 950 °C to 1,000 °C.

[0063] FIG. 6 is a graph showing the high-temperature tensile properties according to the forming start temperature of the heated blank. FIG. 6 is a high-temperature tensile test graph for the blank 410 that was soaked and heated at a temperature of 950 °C, taken out, and then air-cooled and exposed for 10 seconds, and the blank 420 that was soaked and heated at a temperature of 900 °C, taken out, and then air-cooled and exposed for 10 seconds. At this time, the forming start temperature of the blank 410 that was soaked and heated at a temperature of 950 °C, taken out, and then air-cooled and exposed for 10 seconds is 650 °C to 750 °C, and the forming start temperature of the blank 420 that was soaked and heated at a temperature of 900 °C, taken out, and then air-cooled and exposed for 10 seconds is also 550 °C to 650 °C.

[0064] Referring to FIG. 6, it was confirmed that the blank 410, which was soaked and heated at a temperature of 950° C., taken out, and then air-cooled and exposed for 10 seconds, had a lower true stress than the blank 420, which was soaked and heated at a temperature of 900° C., taken out, and then air-cooled and exposed for 10 seconds. Therefore, when the soaking temperature in the heating furnace is less than 950° C., after the heated blank is taken out of the heating furnace, the press-forming start temperature becomes excessively low depending on the air-cooling exposure time, the elongation of the heated blank decreases, and during forming, thickness reduction or breakage occurs. During the air-cooling exposure time, as the heated blank is cooled, the strength of the blank increases, a large force is required to simultaneously form a plurality of blanks, and an overload is applied to the press equipment. Further, when the soaking temperature exceeds 1,000° C., carbide-forming elements and nitride-forming elements such as Ti, V, Nb, and Mo in the blank are dissolved into the base material, making it difficult to suppress grain coarsening.

[0065] In one embodiment, among a plurality of sections in the heating furnace, the temperature of the section for soaking and heating the blank is higher than or the same as the temperature of the section for multi-stage heating the blank.

[0066] In one embodiment, the blank stays in the heating furnace for 180 seconds to 360 seconds. More specifically, the time for the blank to be multi-stage heated and soaked in the heating furnace is also 180 seconds to 360 seconds. When the residence time of the blank in the heating furnace is less than 180 seconds, it is difficult to be sufficiently soaked at the desired soaking temperature. Further, when the residence time of the blank in the heating furnace exceeds 360 seconds, the amount of hydrogen penetrating into the blank increases, the risk of delayed fracture becomes high, and the corrosion resistance after hot stamping may decrease.

[0067] FIG. 7 is a graph showing the temperature change when the blank is multi-stage heated and soaked in the manufacturing method of a hot stamping part according to an embodiment of the present invention. The graph of FIG. 7 is different from the graph of FIG. 5 and shows the temperature of the blank depending on the distance.

[0068] Referring to FIG. 7, in one embodiment, the heating furnace may have a length of 20 m to 40 m along the transfer path of the blank. The heating furnace includes a plurality of sections having different temperature ranges. Among the plurality of sections, the ratio of the length D1 of the section for multi-stage heating of the blank to the length D2 of the section for soaking heating of the blank satisfies 1:1 to 4:1. For example, among the plurality of sections, the section for soaking heating of the blank is also the last part of the heating furnace (for example, the fifth section P5 to the seventh section P7). When the length of the section for soaking heating of the blank increases and the ratio of the length D1 of the section for multi-stage heating of the blank to the length D2 of the section for soaking heating of the blank exceeds 1:1, an austenite (FCC) structure is generated in the soaking heating section, the amount of hydrogen penetration into the blank increases, and delayed fracture may increase. Also, when the length of the section for soaking heating of the blank decreases and the ratio of the length D1 of the section for multi-stage heating of the blank to the length D2 of the section for soaking heating of the blank is less than 4:1, the soaking heating section (time) is not sufficiently ensured, and the strength of the parts manufactured by the manufacturing process of the hot stamping parts becomes non-uniform.

[0069] In one embodiment, among the plurality of sections provided in the heating furnace, the length of the uniform heating section may have a length of 20% to 50% of the total length of the heating furnace.

[0070] After the soaking heating stage (S130), a transfer stage (S140), a forming stage (S150), and a cooling stage (S160) may be further performed.

[0071] In one embodiment, the transfer stage (S140) is also the stage of transferring the heated blank from the heating furnace to the mold. At this time, in the transfer stage (S140), while the heated blank is being transferred to the mold, it can be cooled at the atmospheric temperature (or, normal temperature). The heated blank can be air-cooled during transfer. If the heated blank is not air-cooled, the mold entry temperature (for example, the forming start temperature) will be high, and wrinkles (or, buckling) may occur on the surface of the manufactured hot stamping parts. Also, when using a refrigerant, it may affect the subsequent process (hot stamping), so it is desirable that the heated blank be air-cooled during transfer.

[0072] The forming stage (S150) is also a stage of hot stamping the transferred blank to form a molded body. The cooling stage (S160) is also a stage of cooling the formed molded body.

[0073] After being formed into the final part shape in the press mold, the molded body can be cooled to form the final product. The press mold may be provided with cooling channels through which a refrigerant circulates inside. By circulating the refrigerant supplied through the cooling channels provided in the press mold, the heated blank can be rapidly cooled. At this time, in order to prevent the spring back phenomenon of the plate material and maintain the desired shape, rapid cooling can be carried out while pressurizing the press mold in the closed state. When performing the forming and cooling operations on the heated blank, it can be cooled with an average cooling rate of at least 10 °C / s or more until the martensite finish temperature. The blank can be held in the press mold for 3 to 20 seconds. If the holding time in the press mold is less than 3 seconds, sufficient cooling of the material is not achieved, and thermal deformation may occur due to the residual heat of the product and the temperature deviation by part, resulting in a decrease in dimensional quality. Also, if the holding time in the press mold exceeds 20 seconds, the holding time in the press mold becomes long and productivity decreases.

[0074] FIG. 8 is a drawing showing the air cooling time according to the material thickness and the air cooling time according to the heating temperature. Specifically, FIG. 8 is a graph shown to explain the maximum allowable air cooling time according to the material thickness and the maximum allowable air cooling time according to the heating temperature. In FIG. 8, it can be understood that a high heating temperature means a high heating furnace take-out temperature.

[0075] Referring to FIGS. 1 and 8, it can be confirmed that the maximum allowable air cooling time increases as the heating temperature decreases with the same material thickness. Also, it can be confirmed that the maximum allowable air cooling time increases as the material thickness increases at the same heating temperature.

[0076] When a heated blank is excessively exposed to room temperature, not only does productivity decrease, but also a phase transformation occurs in the blank during air cooling, resulting in a decrease in formability and difficulty in ensuring the desired material quality. On the other hand, when the room temperature exposure time of the heated blank is short, wrinkles (or buckling) may occur in the hot stamping parts manufactured by starting forming at an excessively high temperature. Also, the plating layer of the blank may adhere to the mold. Therefore, it is necessary to adjust the air cooling time in the transfer stage (S140). However, in order to adjust the air cooling time in the transfer stage (S140), not only the heating temperature and the thickness of the blank (e.g., the thickness of the material), but also various variables such as the components of the blank, the thickness of the blank, the plating amount and the thermal conductivity due to the surface emissivity, the thermal conductivity and the heat transfer amount, and the heating furnace extraction temperature and the atmospheric temperature of the blank must be considered.

[0077] Therefore, the present inventor derived Equation 2 that can easily control the air cooling time through excessive repeated experiments. In one embodiment, the air cooling time of the blank in the transfer stage (S140) can satisfy the following Equation 2.

Equation

[0078] a t is a correction coefficient considering the heating furnace extraction temperature and the atmospheric temperature of the heated blank, and can have a value of about 0.0160 or more and about 0.0165 or less. At this time, a t can have a unit of s / (°C x mm).

[0079] b ttakes into account the case where the components of each material are different from each other, and b t is a correction coefficient considering the material components. At this time, b t can have a value of about -10.0 or more and about 0.5 or less. At this time, b t can have the unit of s / mm.

[0080] Also, even if the amount of heat transfer transmitted from inside the material varies depending on the thickness of the material. c t can have a value of about 0.7 or more and about 0.9 or less as a correction coefficient considering the difference in the amount of heat transfer due to the thickness of the material at high temperature. At this time, high temperature means 600 °C or more. However, high temperature means 500 °C or more or 700 °C or more.

[0081] Heating temperature T t means the soaking heating temperature in the soaking heating stage (S130), and the heating temperature T t can have a value of about Ac3 or more and about 1000 °C or less. At this time, the heating temperature T t can mean the heating furnace extraction temperature. Also, the material thickness (t) can have a value of about 1 mm or more and about 2.6 mm or less.

[0082] In one embodiment, the air cooling time λ according to Equation 2 t is also about 5 s or more and about 20 s or less. The air cooling time λ t If it is less than 5 s, the forming start temperature at which the forming of the blank starts is excessively high, the forming of the blank proceeds at a high temperature, wrinkles (or buckling) occur in the manufactured hot stamping part, and it is difficult to implement an air cooling time λ of less than 5 s in terms of equipment. On the other hand, when the air cooling time λ t is more than 20 s, not only does the productivity decrease, but during the process of transporting the blank, a phase transformation occurs in the blank, the formability of the blank decreases, and the manufactured hot stamping part does not have the desired material. Therefore, when the air cooling time λ t satisfies the range of about 5 s or more and about 20 s or less, it is possible to improve the formability of the blank and the productivity of the process, and make the manufactured hot stamping part have the desired material. t

[0083] In one embodiment, a hot stamping part manufactured by the method for manufacturing a hot stamping part described above may have a tensile strength of 500 MPa or more and less than 800 MPa, and may have a composite structure of ferrite and martensite. A hot stamping part manufactured by the method for manufacturing a hot stamping part may have a tensile strength of 800 MPa or more and less than 1,200 MPa, and may have a composite structure of bainite and martensite. A hot stamping part manufactured by the method for manufacturing a hot stamping part may have a tensile strength of 1,200 MPa or more and less than 2,000 MPa, and may have a structure of full martensite.

[0084] By multi-stage heating blanks having different thicknesses simultaneously in a heating furnace, the time to reach the target temperature (for example, soaking temperature) of the blanks can be controlled more precisely. By more precisely controlling the time to reach the target temperature (for example, soaking temperature) of blanks having different thicknesses, the hydrogen embrittlement, corrosion resistance, and weldability of parts manufactured by the method for manufacturing hot stamping parts can be improved. More specifically, when a thin material and a thick material are simultaneously heated singly in a heating furnace, there may be a case where the thin material reaches the target temperature earlier than the thick material and overheating occurs in the thin material. According to an embodiment of the present invention, even when a thin material and a thick material are simultaneously heated in a heating furnace, by multi-stage heating the thin material and the thick material, the time to reach the target temperature (for example, soaking temperature) of the thin material and the thick material can be controlled in the same way. Therefore, by controlling the time to reach the target temperature (for example, soaking temperature) of the thin material and the thick material in the same way, the hydrogen embrittlement, corrosion resistance, and weldability of parts manufactured by the hot stamping manufacturing method can be improved.

[0085] Experimental Example After preparing blanks having the alloy compositions shown in Table 1, in a heating furnace set according to the standards shown in Table 2, after setting the temperature settings for each section shown in Table 3, hot stamping parts were manufactured according to the conditions of Comparative Examples 1 and 2 and the Example. On the other hand, the total length of the heating furnace is 22,000 mm.

Table 1

Table 2

Table 3

[0086] Referring to Table 3, hot stamping parts (Examples) were manufactured using the manufacturing method of hot stamping parts according to an embodiment, and Comparative Example 1 and Comparative Example 2 manufactured hot stamping parts by single heating at temperatures of 950 °C and 930 °C, respectively.

[0087] Hydrogen embrittlement evaluation, corrosion resistance evaluation, and welding evaluation were performed on the parts manufactured under the conditions of the Example, Comparative Example 1, and Comparative Example 2.

[0088] 1. Hydrogen embrittlement evaluation Hydrogen embrittlement of the parts manufactured under the conditions of the Example, Comparative Example 1, and Comparative Example 2 was evaluated using TDS (Thermal Desorption Spectroscopy) equipment in accordance with the provisions of ISO16573-2015. That is, under a vacuum atmosphere, the parts manufactured under the conditions of the Example, Comparative Example 1, and Comparative Example 2 were heated respectively, and the amount of diffusible hydrogen released from the parts was measured at 300 °C or lower.

[0089] FIG. 9 is a graph showing the hydrogen release rate released from the parts manufactured under the conditions of the Example, Comparative Example 1, and Comparative Example 2, and Table 4 is a table showing the results of calculating the amount of diffusible hydrogen at 300 °C or lower and the results of the delayed fracture experiment based on the hydrogen release rate results of the Example, Comparative Example 1, and Comparative Example 2 in FIG. 9.

Table 4

[0090] 2. Corrosion Resistance Evaluation For the hot stamping parts manufactured under the conditions of the example, Comparative Example 1, and Comparative Example 2, a corrosion resistance evaluation experiment was carried out according to the ASTM G59-97(2014) standard. More specifically, for the corrosion resistance evaluation experiment, a three-electrode electrochemical cell was constructed using the test piece as the working electrode, a high-purity carbon rod as the counter electrode, and a saturated calomel electrode as the reference electrode, and a potentiodynamic polarization test was carried out. The potentiodynamic polarization test was carried out after measuring the open-circuit potential (OCP) in a 3.5% NaCl solution for 10 hours to confirm electrochemical stabilization, and then applying a potential at a scanning rate of 0.166 mV / s from -250 mV to 0 mV SCE based on the corrosion potential (Ecorr) to proceed with the corrosion resistance evaluation experiment.

[0091] FIG. 10 is a graph showing the corrosion resistance evaluation results for the parts manufactured under the conditions of the example, Comparative Example 1, and Comparative Example 2, and Table 5 is a table calculating the corrosion rates of the parts manufactured under the conditions of the example, Comparative Example 1, and Comparative Example 2 based on the polarization curves in FIG. 10. At this time, the corrosion rate in Table 5 is a numerical value corresponding to the current density at the time when a branch of the potential stably held occurs from the polarization curves of the example, Comparative Example 1, and Comparative Example 2.

Table 5

[0092] Referring to FIG. 10 and Table 5, in the case of Comparative Example 1 and Comparative Example 2, the lower the single heating temperature, the lower the corrosion rate and the better the corrosion resistance. However, when multi-stage heating is used as in the examples, it can be seen that it is possible to ensure even better corrosion resistance compared to using single heating.

[0093] 3. Weldability evaluation Weldability evaluation was carried out on the parts manufactured under the conditions of the examples, Comparative Example 1, and Comparative Example 2. In the weldability evaluation, a pair of parts manufactured under the conditions of the examples, Comparative Example 1, and Comparative Example 2 were prepared respectively, and spot welding was carried out for 300 ms while applying a pressure of 350 kgf and a current of 5.5 kA with an electrode rod made of a chromium-copper alloy having a diameter of 6 mm. The resistance was measured while performing spot welding.

[0094] Normally, the change in resistance value up to the initial 30 ms affects the generation of spatter and the characteristics of weldability. The lower the resistance, the better the weldability.

[0095] FIG. 11 is a graph showing the resistance values of the parts manufactured under the conditions of the examples, Comparative Example 1, and Comparative Example 2. Referring to FIG. 11, it can be confirmed that the hot stamping parts (examples) manufactured through multi-stage heating have lower resistance compared to the hot stamping parts (Comparative Example 1) manufactured by single heating at a temperature of 950°C and the hot stamping parts (Comparative Example 2) manufactured by single heating at a temperature of 930°C. Therefore, it can be confirmed that the weldability of the hot stamping parts (examples) manufactured through multi-stage heating is relatively excellent compared to the weldability of the hot stamping parts (Comparative Example 1) manufactured by single heating at a temperature of 950°C and the hot stamping parts (Comparative Example 2) manufactured by single heating at a temperature of 930°C.

[0096] As described above, the present invention has been described based on one embodiment illustrated in the drawings, but this is merely exemplary, and those having ordinary knowledge in the art will understand that various modifications and variations of the embodiments are possible from these. Therefore, the true technical protection scope of the present invention must be determined by the technical idea of the claims.

Explanation of Reference Numerals

[0097] 200 Blank 210 Blank having a single thickness 220 Tailor-welded blank 221 First plate material 223 Second plate material 230 Tailor-rolled blank 231 First region 232 Second region 233 Third region 234 Fourth region 240 Patchwork 241 Base material 243 Patch

Claims

1. Charging at least two blanks into a heating furnace having a plurality of sections with different temperature ranges; A multi-stage heating step of stepwise heating the blank by passing it through the plurality of sections in the heating furnace; A soaking heating step of heating the blank at a temperature of Ac3 to 1,000 °C; A transfer step of transferring the heated blank from the heating furnace to a mold; including; In the transfer step, the heated blank is air-cooled; In the multi-stage heating step, the temperature conditions in the heating furnace satisfy the following Equation 1, a method for manufacturing hot stamping parts; 【Number 1】 (In Equation 1, Tg is the soaking heating temperature (°C), Ti is the initial temperature of the heating furnace, which is the temperature of the heating path (°C), and Lt is the length (mm) of the section where the multi-stage heating step is performed).

2. The method for manufacturing hot stamping parts according to claim 1, wherein in the plurality of sections, the ratio of the length of the section for multi-stage heating the blank to the length of the section for soaking heating the blank satisfies 1:1 to 4:

1.

3. The method for manufacturing hot stamping parts according to claim 1, wherein at least two blanks having different thicknesses are simultaneously transferred into the heating furnace.

4. The method for manufacturing hot stamping parts according to claim 1, wherein the blank includes a first part having a first thickness and a second part having a second thickness different from the first thickness.

5. The method for manufacturing hot stamping parts according to claim 2, wherein the temperatures of the plurality of sections increase in the direction from the inlet of the heating furnace to the outlet of the heating furnace.

6. The method for manufacturing hot stamping parts according to claim 5, wherein the temperature difference between two adjacent sections among the sections for multi-stage heating the blank is greater than 0 °C and 100 °C or less.

7. The method for manufacturing hot stamping parts according to claim 2, wherein the temperature of the section for soaking heating the blank among the plurality of sections is higher than the temperature of the section for multi-stage heating the blank.

8. The method for manufacturing hot stamping parts according to claim 1, wherein the blank stays in the heating furnace for 180 seconds to 360 seconds.

9. The method for manufacturing hot stamping parts according to claim 1, wherein the air-cooling time of the blank in the transfer step satisfies the following Equation 2; 【Number 2】 (At this time, λ ｔ is the air cooling time (s), a ｔ is the correction coefficient considering the heating furnace outlet temperature and the atmospheric temperature, T ｔ is the heating temperature (°C), b ｔ is the correction coefficient considering the material components, t is the material thickness (mm), c ｔ is the correction coefficient considering the high-temperature material thickness sensitivity).

10. In Equation 2, The aforesaid a ｔ is 0.0160 or more and 0.0165 or less, T ｔ is Ac3 or more and 1000 °C or less, b ｔ is -10 or more and 0.5 or less, t is 1 mm or more and 2.6 mm or less, c ｔ is 0.7 or more and 0.9 or less, The method for manufacturing a hot stamping part according to claim 9.

11. In the formula (2), λ ｔ is from 5 s to 20 s, and the method for manufacturing a hot stamping part according to claim 9.

12. A hot stamping part manufactured according to any one of claims 1 to 11, The diffusible hydrogen content is less than 0.45 ppm, and the corrosion rate measured through the potentiodynamic polarization test is 3 x 10 －６ A or less, hot stamping parts.

13. The hot stamping part according to claim 12, having a tensile strength of 500 MPa or more and less than 800 MPa and having a composite structure of ferrite and martensite.

14. The hot stamping part according to claim 12, having a tensile strength of 800 MPa or more and less than 1,200 MPa and having a composite structure of bainite and martensite.

15. The hot stamping part according to claim 12, having a tensile strength of 1,200 MPa or more and less than 2,000 MPa and having a fully martensite structure.

Images