Hot stamping component and hot stamping component manufacturing method

Abstract

The present invention provides a hot stamping component comprising a base material and a plating layer, which is formed on at least one surface of the base material and includes at least one from among a gamma phase, a delta phase, a zeta phase and an eta phase, wherein the amount of diffusible hydrogen in the hot stamping component is 0.3 wppm or less.

Description

Hot stamping parts and a method for manufacturing hot stamping parts

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

[0002] As environmental and fuel efficiency regulations become stricter globally, the need for lighter vehicle materials is increasing. Consequently, research and development on ultra-high-strength steel and hot-stamping steel is actively underway.

[0003] The hot stamping process generally consists of heating, forming, cooling, and trimming, and can utilize phase transformations and microstructural changes of the material during the process. During the heating process, moisture decomposes to form hydrogen; if this formed hydrogen flows into the base material and accumulates in one place, hydrogen embrittlement occurs.

[0004] In the case of aluminum-silicon (Al-Si) plated steel sheets, they consist mostly of a solid Al (FCC) plating layer prior to hot stamping. When the aluminum-silicon (Al-Si) plated steel sheet is heated, some or the entire plating layer transforms into liquid Al. At this time, because aluminum (Al) has a relatively low melting point, a large amount of the material transforms into the liquid state.

[0005] In addition, since the hydrogen diffusion rate of solid aluminum (Al) and liquid aluminum (Al) is fast, when hot stamping aluminum-silicon (Al-Si) plated steel sheets, hydrogen can relatively easily enter the base material during the heating process, and subsequently, when cooled to room temperature, the plating layer transforms into an aluminum-iron (Al-Fe) intermetallic compound layer (solid phase), and the hydrogen that entered the base material becomes trapped, which can cause hydrogen embrittlement.

[0006] Related technologies include Korean Patent Publication No. 10-2021-0129902 (Title of Invention: Hot Stamping Part and Method for Manufacturing the Same), etc.

[0007] Embodiments of the present invention can improve the hydrogen embrittlement resistance of manufactured hot stamping parts by controlling the heating temperature and heating time in the heating step.

[0008] According to one aspect of the present invention, a hot stamping part is provided, comprising: a base material; and a plating layer formed on at least one surface of the base material and comprising at least one of a gamma phase, a delta phase, a zeta phase, and an eta phase, wherein the diffusible hydrogen content of the hot stamping part is 0.3 wppm or less.

[0009] In one embodiment, the thickness of the gamma phase may be 4㎛ or more and 15㎛ or less.

[0010] In one embodiment, the sum of the thicknesses of the delta phase and the zeta phase may be 14 μm or less.

[0011] In one embodiment, the thickness of the eta phase may be 16 μm or less.

[0012] In one embodiment, the base material may comprise, in wt%, carbon (C): 0.25 wt% to 0.50 wt%, silicon (Si): 0.1 wt% to 0.8 wt%, manganese (Mn): 0.3 wt% to 3.0 wt%, phosphorus (P): greater than 0 and less than or equal to 0.05 wt%, sulfur (S): greater than 0 and less than or equal to 0.01 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): 0.01 wt% to 1.00 wt%, boron (B): greater than 0 and less than or equal to 0.006 wt%, and the remainder being iron (Fe) and other unavoidable impurities.

[0013] In one embodiment, the hot stamping part may have a tensile strength (TS) of 1680 MPa to 2300 MPa.

[0014] According to one aspect of the present invention, a method for manufacturing a hot stamping part comprises: a step of heating a steel plate for hot stamping at a temperature range of 870°C to 950°C for a time of 180 s to 360 s; a step of transferring the heated steel plate for hot stamping to a mold; and a step of pressurizing the steel plate for hot stamping transferred to the mold to form a molded body and cooling the formed molded body; wherein the hot stamping part comprises a base material and a plating layer formed on at least one surface of the base material, and the thickness of the plating layer and the hydrogen diffusion distance in the plating layer are given by Equation 1 (L < T total A method for manufacturing a hot stamping part satisfying ) is provided.

[0015] In one embodiment, the plating layer may include at least one of a gamma phase, a delta phase, a zeta phase, and an eta phase.

[0016] In one embodiment, the hydrogen diffusion distance (L) is relational expression 2 ( It can be derived through ).

[0017] In one embodiment, the thickness of the gamma phase may be 4㎛ or more and 15㎛ or less.

[0018] In one embodiment, the sum of the thicknesses of the delta phase and the zeta phase may be 14 μm or less.

[0019] In one embodiment, the thickness of the eta phase may be 16 μm or less.

[0020] In one embodiment, the base material may comprise, in wt%, carbon (C): 0.25 wt% to 0.50 wt%, silicon (Si): 0.1 wt% to 0.8 wt%, manganese (Mn): 0.3 wt% to 3.0 wt%, phosphorus (P): greater than 0 and less than or equal to 0.05 wt%, sulfur (S): greater than 0 and less than or equal to 0.01 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): 0.01 wt% to 1.00 wt%, boron (B): greater than 0 and less than or equal to 0.006 wt%, and the remainder being iron (Fe) and other unavoidable impurities.

[0021] In one embodiment, the hot stamping part may have a tensile strength (TS) of 1680 MPa to 2300 MPa.

[0022] Other aspects, features, and advantages other than those described above will become clear from the specific details, claims, and drawings for implementing the invention below.

[0023] According to one embodiment of the present invention as described above, a hot stamping part having excellent hydrogen embrittlement resistance and a method for manufacturing it can be provided. Of course, the scope of the present invention is not limited by these effects.

[0024] FIG. 1 is a cross-sectional view schematically illustrating a hot stamping part according to one embodiment.

[0025] FIG. 2 is a flowchart schematically illustrating a method for manufacturing a hot stamping part according to one embodiment.

[0026] FIG. 3 is a flowchart schematically illustrating the steps for manufacturing a steel plate for hot stamping according to one embodiment.

[0027] Figure 4 is a schematic cross-sectional view of an aluminum-silicon (Al-Si) plated steel sheet.

[0028] Figure 5 is a diagram showing the phase diagram of aluminum-iron (Al-Fe).

[0029] Figure 6 is a diagram showing the phase diagram of zinc-iron (Zn-Fe).

[0030] The present invention will be described in detail below. However, in describing the present invention, if it is determined that a detailed description of related known technologies or configurations may unnecessarily obscure the essence of the present invention, such detailed description will be omitted.

[0031] In the following embodiments, terms such as first, second, etc. are used not in a limiting sense, but for the purpose of distinguishing one component from another component.

[0032] In the following examples, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0033] In the following embodiments, when various components such as layers, films, regions, and plates are described as being "on" another component, this includes not only cases where they are "directly on" another component, but also cases where another component is interposed between them.

[0034] In the drawings, the size of components may be exaggerated or reduced for convenience of explanation. For example, the size and thickness of each component shown in the drawings are depicted arbitrarily for convenience of explanation, and therefore the present invention is not necessarily limited to what is illustrated.

[0035] In the following embodiments, terms such as "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0036] In this specification, "A and / or B" indicates the case where it is A, B, or both A and B. Additionally, in this specification, "at least one of A and B" indicates the case where it is A, B, or both A and B.

[0037] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with relevant technical literature and the present disclosure, and are not interpreted in an ideal or highly formal sense unless otherwise defined.

[0038] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0039] FIG. 1 is a cross-sectional view schematically illustrating a hot stamping part according to one embodiment.

[0040] Referring to FIG. 1, a hot stamping part (10) according to one embodiment may include a base material (100) and a plating layer (200). The plating layer (200) may be formed on at least one surface of the base material (100).

[0041] In one embodiment, the base material (100) may comprise, in wt%, carbon (C): 0.25 wt% to 0.50 wt%, silicon (Si): 0.1 wt% to 0.8 wt%, manganese (Mn): 0.3 wt% to 3.0 wt%, phosphorus (P): greater than 0 and less than or equal to 0.05 wt%, sulfur (S): greater than 0 and less than or equal to 0.01 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): 0.01 wt% to 1.00 wt%, boron (B): greater than 0 and less than or equal to 0.006 wt%, and the remainder being iron (Fe) and other unavoidable impurities.

[0042] In one embodiment, the base material (100) may further include one or more of titanium (Ti), molybdenum (Mo), and niobium (Nb), and the sum of the contents of one or more of titanium (Ti), molybdenum (Mo), and niobium (Nb) may be 0.01 wt% to 0.10 wt%.

[0043] Carbon (C) can be an element that increases the strength of steel and stabilizes austenite. Carbon (C) may be included in an amount of 0.25 wt% to 0.50 wt%. If the carbon (C) content is less than 0.25 wt%, it may be difficult to achieve the target mechanical strength. On the other hand, if the carbon (C) content exceeds 0.50 wt%, it may cause problems with controlling the toughness or brittleness of the steel and may reduce weldability.

[0044] Silicon (Si) may be added to prevent material degradation by inhibiting the formation of carbides (Fe3C). Silicon (Si) may be included in an amount of 0.1 wt% to 0.8 wt%. If the silicon (Si) content is less than 0.1 wt%, it may be difficult to achieve the target mechanical strength. On the other hand, if the silicon (Si) content exceeds 0.8 wt%, the plating quality may deteriorate.

[0045] Manganese (Mn) can be an element that increases the strength of steel and stabilizes austenite. Manganese (Mn) may be included in an amount of 0.3 wt% to 3.0 wt%. If the manganese (Mn) content is less than 0.3 wt%, it may be difficult to achieve the target mechanical strength. On the other hand, if the manganese (Mn) content exceeds 3.0 wt%, formability may be reduced.

[0046] Phosphorus (P) can reduce weldability and increase the brittleness of steel sheets. Phosphorus (P) may be present in an amount greater than 0 and less than or equal to 0.05 wt%. If the phosphorus (P) content exceeds 0.05 wt%, brittleness may increase and workability may be reduced.

[0047] Sulfur (S) can reduce weldability and increase the brittleness of steel sheets. Sulfur (S) may be included in an amount greater than 0 and less than or equal to 0.01 wt%. If the sulfur (S) content exceeds 0.01 wt%, brittleness may increase and workability may decrease.

[0048] Aluminum (Al) can suppress solid solution strengthening and carbide formation. Additionally, aluminum (Al) may be an element primarily used as a carbonate. Aluminum (Al) may be included in an amount of 0.01 wt% to 0.05 wt%. If the aluminum (Al) content is less than 0.01 wt%, the deoxidation effect may be negligible. On the other hand, if the aluminum (Al) content exceeds 0.05 wt%, excessive inclusions may be formed during steelmaking and continuous casting operations.

[0049] Chromium (Cr) may be added to improve the hardenability and strength of steel. Chromium (Cr) may be included in an amount of 0.01 wt% to 1.00 wt%. If the chromium (Cr) content is less than 0.01 wt%, the effect of improving hardenability and strength may be negligible. On the other hand, if the chromium (Cr) content exceeds 1.00 wt%, it may lead to a degradation of the steel sheet material and an increase in cost.

[0050] Boron (B) is added for the purpose of securing the hardenability and strength of the steel by securing a martensite structure, and can have a grain refinement effect by increasing the austenite grain growth temperature. Boron (B) may be included in an amount greater than 0 and less than or equal to 0.006 wt%. If the content of boron (B) exceeds 0.006 wt%, hard phase intergranular brittleness may occur.

[0051] Titanium (Ti), molybdenum (Mo), and niobium (Nb) may be added for precipitation strengthening and grain refinement. The sum of the contents of titanium (Ti), molybdenum (Mo), and niobium (Nb) may be 0.01 wt% to 0.10 wt% or less. If the sum of the contents of titanium (Ti), molybdenum (Mo), and niobium (Nb) is less than 0.01 wt%, the effects of precipitation strengthening and grain refinement may be negligible. On the other hand, if the sum of the contents of titanium (Ti), molybdenum (Mo), and niobium (Nb) exceeds 0.10 wt%, it may lead to a degradation of the steel sheet material quality and an increase in cost.

[0052] In one embodiment, a plating layer (200) may be formed on at least one surface of the base material (100). The plating layer (200) may be an alloyed plating layer in which zinc (Zn) and iron (Fe) are alloyed. The plating layer (200) may comprise, in wt%, iron (Fe): 10 wt% or more and 70 wt% or less, aluminum (Al): greater than 0 and 5 wt% or less, manganese (Mn): greater than 0 and 1 wt% or less, silicon (Si): greater than 0 and 1 wt% or less, and the remainder being zinc (Zn) and other unavoidable impurities.

[0053] The average thickness of the plating layer (200) may be 7㎛ or more and 20㎛ or less. If the thickness of the plating layer (200) is less than 7㎛, the sacrificial protection capability may be insufficient. Additionally, if the thickness of the plating layer (200) is less than 7㎛, the resistance to hydrogen embrittlement may be inferior. If the thickness of the plating layer (200) exceeds 20㎛, the cost of forming the plating layer (200) increases, which may reduce economic efficiency. At this time, the average thickness of the plating layer (200) can be measured through Scanning Electron Microscope (SEM) analysis.

[0054] In one embodiment, the plating layer (200) may include a gamma phase, a delta phase, a zeta phase, and an eta phase. However, the present invention is not limited thereto. Some of the gamma phase, delta phase, zeta phase, and eta phase may be omitted. For example, the plating layer (200) may include only the gamma phase and the eta phase.

[0055] The gamma phase is a zinc (Zn)-iron (Fe) alloy phase and may be a phase formed closest to the base material (100) in the plating layer (200). The gamma phase may be formed on the surface of the base material (100). The gamma phase may contain, in wt%, 19 wt% to 32 wt% iron (Fe), the remainder zinc (Zn), and other unavoidable impurities. The gamma phase is the phase with the highest Fe content among the phases of the plating layer (200), and the hardness of the gamma phase may be higher than the hardness of the other phases of the plating layer (200). The thickness of the gamma phase may be 4 μm or more and 15 μm or less. The thickness of the gamma phase may be 4 μm or more and 10 μm or less. The thickness of the gamma phase may be the average thickness after measuring at least 10 points. If the thickness of the gamma phase is less than 4 μm, the thickness of the eta phase, which will be described later, increases, and the hydrogen embrittlement resistance of the hot stamping part may be inferior. On the other hand, if the thickness of the gamma phase is greater than 15 μm, a powdering phenomenon may occur in which the plating layer peels off from the base material during press forming (e.g., hot forming).

[0056] The delta phase and zeta phase may be zinc (Zn)-iron (Fe) alloy phases formed on the gamma phase. The delta phase may be formed on the gamma phase, and the zeta phase may be formed on the delta phase. However, the delta phase and / or zeta phase may be omitted. The delta phase may contain, in wt%, 8 wt% to 12 wt% iron (Fe), the remainder zinc (Zn), and other unavoidable impurities. The zeta phase may contain, in wt%, 3 wt% to 8 wt% iron (Fe), the remainder zinc (Zn), and other unavoidable impurities. The sum of the thicknesses of the delta phase and the zeta phase may be 14 μm or less. The sum of the thicknesses of the delta phase and the zeta phase may be 0 μm or more and 10 μm or less. The sum of the thicknesses of the delta phase and the zeta phase may be the average thickness after measuring at least 10 points. If the thickness of the delta phase and zeta phase exceeds 14㎛, a powdering phenomenon may occur during press forming (e.g., hot forming) in which the plating layer peels off from the base material.

[0057] The delta phase may have a lower Fe content and a higher Zn content compared to the gamma phase. Since Fe has a smaller atomic size than Zn, a higher Fe content can lead to a denser crystal lattice; this is advantageous for hindering hydrogen diffusion, which can improve hydrogen embrittlement characteristics (e.g., hydrogen embrittlement resistance).

[0058] The eta phase is a phase composed of pure zinc (or, mostly zinc) and may be a phase formed on the zeta phase. The eta phase may contain, in wt%, more than 0 wt% iron (Fe), the remainder zinc (Zn), and other unavoidable impurities. The thickness of the eta phase may be 16 μm or less. Preferably, the thickness of the eta phase may be 10 μm or less. More preferably, the thickness of the eta phase may be 5 μm or less. The thickness of the eta phase may be the average thickness after measuring at least 10 points. Since the eta phase degrades paintability and weldability, it may be desirable for the thickness of the eta phase to be 0 μm. If the thickness of the eta phase exceeds 16 μm, the hydrogen embrittlement resistance of the manufactured hot stamping part may be inferior. This will be explained in more detail below.

[0059] In one embodiment, the hot stamping part (10) may have a tensile strength (TS) of 1680 MPa to 2300 MPa and a yield strength (YP) of 1150 MPa to 1300 MPa. Additionally, the hot stamping part (10) may have an elongation of 4% to 15%.

[0060] In one embodiment, the diffusible hydrogen content of the hot stamping part (10) may be 0.3 wppm or less. Preferably, the diffusible hydrogen content of the hot stamping part (10) may be 0.2 wppm or less.

[0061] The following describes the manufacturing method of hot stamping parts.

[0062] FIG. 2 is a flowchart schematically illustrating a method for manufacturing a hot stamping part according to one embodiment, and FIG. 3 is a flowchart schematically illustrating a step for manufacturing a steel plate for hot stamping according to one embodiment.

[0063] Referring to FIG. 2, a method for manufacturing a hot stamping part according to one embodiment may include a step of manufacturing a steel plate for hot stamping (S100), a heating step (S200), a transfer step (S300), and a forming / cooling step (S400).

[0064] Referring to FIG. 3, the hot stamping steel sheet manufacturing step (S100) may include a hot rolling step (S110), a cold rolling step (S120), an annealing step (S130), a plating step (S140), and an alloying step (S150). At this time, the hot stamping steel sheet may be an alloy and a hot-dip galvanized steel sheet.

[0065] In a method for manufacturing a steel plate for hot stamping according to one embodiment, the semi-finished product subject to hot rolling may be a slab. The slab in the semi-finished product state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0066] In one embodiment, the slab may comprise, in wt%, carbon (C): 0.25 wt% to 0.50 wt%, silicon (Si): 0.1 wt% to 0.8 wt%, manganese (Mn): 0.3 wt% to 3.0 wt%, phosphorus (P): greater than 0 and less than or equal to 0.05 wt%, sulfur (S): greater than 0 and less than or equal to 0.01 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): 0.01 wt% to 1.00 wt%, boron (B): greater than 0 and less than or equal to 0.006 wt%, and the remainder being iron (Fe) and other unavoidable impurities. Additionally, the slab may further include titanium (Ti), molybdenum (Mo), and niobium (Nb), and the sum of the contents of titanium (Ti), molybdenum (Mo), and niobium (Nb) may be 0.01 wt% to 0.10 wt%. The reason for limiting the numerical range of the components included in the slab may be the same as the reason for limiting the numerical range of the components included in the base material (100) of the hot stamping part.

[0067] In the hot rolling step (S110), the aforementioned slab can be hot-rolled to produce a hot-rolled steel sheet.

[0068] The hot rolling step (S110) may include a step of reheating the slab, a step of rolling the reheated slab, and a step of cooling and coiling the rolled slab.

[0069] First, in the hot rolling step (S110), the slab may be reheated. In one embodiment, the slab reheating temperature may be 1200°C to 1400°C. If the slab reheating temperature is below 1200°C, the segregated components during casting may not be sufficiently re-dissolved, making it difficult to achieve the homogenization effect of alloying elements and difficult to achieve the solid solution effect of titanium (Ti). On the other hand, while a higher slab reheating temperature is advantageous for homogenization, if the slab reheating temperature exceeds 1400°C, the austenite crystal grain size increases, making it difficult to secure strength, and the manufacturing cost of the steel sheet may increase due to the excessive heating process.

[0070] After reheating the slab, the reheated slab may be hot-rolled at a predetermined finish rolling temperature. At this time, the finish rolling temperature may be 880°C to 950°C. If the finish rolling temperature is below 880°C, a mixed grain structure may occur due to abnormal region rolling, making it difficult to secure workability of the steel sheet, workability may be reduced due to microstructural non-uniformity, and problems with sheet throughability may occur during hot rolling due to rapid phase changes. In addition, if the finish rolling temperature exceeds 950°C, the austenite grains may coarsen, and the TiC precipitates may coarsen, which may degrade the performance of the manufactured hot-stamped parts.

[0071] After hot rolling a slab at a predetermined finishing rolling temperature, it may be cooled to a predetermined coiling temperature and then coiled at the coiling temperature. At this time, the coiling temperature may be 550°C to 800°C. The coiling temperature affects the redistribution of carbon (C). If the coiling temperature is below 550°C, the low-temperature phase fraction increases due to overcooling, which may increase strength, and there is a risk that the rolling load during cold rolling will be intensified, and ductility may decrease rapidly. Conversely, if the coiling temperature exceeds 800°C, deterioration in formability and strength may occur due to abnormal or excessive grain growth.

[0072] A cold rolling step (S120) may be performed after a hot rolling step (S110). A hot-rolled steel sheet that has undergone the cold rolling step (S120) may be referred to as a cold-rolled steel sheet. In the cold rolling step (S120), the coiled hot-rolled steel sheet may be uncoiled, pickled, and then cold rolling may be performed. At this time, pickling may be performed for the purpose of removing scale from the hot-rolled coil (e.g., hot-rolled steel sheet) manufactured through the above hot rolling process. In the cold rolling step (S120), cold rolling may be performed with a reduction rate of 40% to 70%.

[0073] An annealing step (S130) may be performed after the cold rolling step (S120). In the annealing step (S130), the cold-rolled steel sheet may be annealed in an annealing furnace. In the annealing step (S130), the cold-rolled steel sheet may be annealed at an annealing temperature of 750°C to 900°C. If the annealing temperature is less than 750°C, the target structure may not be obtained, and recrystallization may not be sufficiently completed. On the other hand, if the annealing temperature exceeds 900°C, the annealing temperature is too high, and the efficiency of the manufacturing process may decrease. Therefore, when the annealing temperature satisfies 750°C to 900°C, the target structure can be obtained, recrystallization can be sufficiently completed, and the efficiency of the manufacturing process can be improved.

[0074] In one embodiment, in the annealing step (S130), after performing annealing at the aforementioned annealing temperature, the annealed steel sheet (e.g., cold-rolled steel sheet) can be cooled to a temperature range of 400°C to 600°C.

[0075] A plating step (S140) may be performed after the annealing step (S130). The plating step (S140) may include a step of heating the annealed cold-rolled steel sheet and a step of immersing the heated cold-rolled steel sheet in a molten zinc plating bath to form a molten zinc plating layer. The steel sheet on which the plating step (S140) is performed may be called a plated steel sheet.

[0076] In the plating step (S140), the annealed cold-rolled steel sheet can be heated in a temperature range of 400°C to 480°C. By heating the annealed cold-rolled steel sheet in the above temperature range, the temperature at which it enters the plating bath can be controlled. However, the present invention is not limited thereto. Instead of heating the annealed cold-rolled steel sheet, it may be cooled to a temperature range of 400°C to 480°C after annealing and then introduced into the plating bath in the above temperature range.

[0077] Additionally, in the plating step (S140), a heated cold-rolled steel sheet may be immersed in a molten zinc plating bath to form a molten zinc plating layer. At this time, the molten zinc plating bath may contain, in wt%, less than 0.3 wt% aluminum (Al), less than 0.1 wt% iron (Fe), the remainder being zinc (Zn) and other unavoidable impurities. Additionally, the temperature of the molten zinc plating bath may be 400°C to 700°C. The plating adhesion amount in the plating step (S140) is 45 g / m² on a single side. 2 Up to 90 g / m² 2 It could be.

[0078] An alloying step (S150) may be performed after the plating step (S140). In the alloying step (S150), the plated steel sheet with the molten zinc plating layer formed thereon may be alloyed by heating it in a temperature range of 500°C to 600°C.

[0079] A steel plate for hot stamping (not shown) may include a base material (not shown) and a plating layer (not shown). The component content of the base material of the steel plate for hot stamping may be the same as the component content of the base material (100) of the hot stamping part (10) described above. The plating layer of the steel plate for hot stamping may include 10 wt% to 70 wt% of iron (Fe), the remainder being zinc (Zn) and other unavoidable impurities.

[0080] The plating layer of a steel sheet for hot stamping may include a gamma phase, a delta phase, a zeta phase, and an eta phase. The gamma phase may include 19 wt% to 32 wt% of iron (Fe), the remainder being zinc (Zn) and other unavoidable impurities. The delta phase may include 8 wt% to 12 wt% of iron (Fe), the remainder being zinc (Zn) and other unavoidable impurities. The zeta phase may include 3 wt% to 8 wt% of iron (Fe), the remainder being zinc (Zn) and other unavoidable impurities. The eta phase may include more than 0 wt% of iron (Fe), the remainder being zinc (Zn) and other unavoidable impurities.

[0081] Referring again to FIG. 2, a heating step (S200) may be performed after the step of manufacturing a steel plate for hot stamping (S100). In a method for manufacturing a hot stamping part according to one embodiment of the present invention, a steel plate for hot stamping may be cut to manufacture a blank, and then the blank may be hot stamped. For convenience, the following description will assume that a steel plate for hot stamping (e.g., an alloyed hot-dip galvanized steel plate) is hot stamped.

[0082] In one embodiment, in the heating step (S200), a steel plate for hot stamping may be heated in a temperature range of 870°C to 950°C for a time of 180 s to 360 s. If the heating temperature in the heating step (S200) is less than 870°C, the manufactured hot stamping part may not have the desired material. If the heating temperature in the heating step (S200) is greater than 950°C, zinc (Zn) contained in the plating layer may vaporize, causing loss of the plating layer. Additionally, if the heating temperature in the heating step (S200) is greater than 950°C, the thickness of the eta phase within the plating layer increases, which may result in a decrease in the hydrogen embrittlement resistance of the manufactured hot stamping part. If the heating time in the heating step (S200) is less than 180 s, the thickness of the eta phase within the plating layer increases, which may result in a decrease in the hydrogen embrittlement resistance of the manufactured hot stamping part. If the heating time in the heating step (S200) exceeds 360s, a large amount of hydrogen may diffuse into the base material, and subsequently, as the plating layer cools to room temperature, the hydrogen that has entered the base material cannot escape and becomes trapped within the base material by the plating layer, which may cause hydrogen embrittlement.

[0083] A transfer step (S300) may be performed after the heating step (S200). The transfer step (S300) may be a step of transferring the heated hot stamping steel plate from the heating furnace to the mold. At this time, during the transfer step (S300), the heated hot stamping steel plate may be cooled at ambient temperature (or room temperature). That is, the heated hot stamping steel plate may be air-cooled during transfer. If the heated hot stamping steel plate is not air-cooled, the mold entry temperature (e.g., forming start temperature) increases, and wrinkles (or bends) may occur on the surface of the manufactured hot stamping part (10). In addition, since the use of a refrigerant may affect the subsequent process (hot stamping), it may be desirable for the heated hot stamping steel plate to be air-cooled during transfer.

[0084] A forming / cooling step (S400) may be performed after the transfer step (S300). In the forming / cooling step (S400), the transferred steel plate for hot stamping may be hot stamped to form a formed body, and at the same time, the steel plate for hot stamping (or the formed body) may be cooled.

[0085] In the forming / cooling step (S400), a steel sheet for hot stamping can be pressed with a mold to form a formed body. In one embodiment, the forming start temperature may be 500°C to 700°C. If the forming start temperature is less than 500°C, the forming start temperature is too low, which may reduce the formability of the hot stamping part, and the manufactured hot stamping part may not have the target structure and physical properties. On the other hand, if the forming start temperature is greater than 700°C, wrinkles (or bends) may occur on the surface of the manufactured hot stamping part. In addition, the plating layer may adhere to the mold. Therefore, when the forming start temperature is 500°C to 700°C, the formability of the steel sheet for hot stamping can be improved, the manufactured hot stamping part may have the target structure and physical properties, and the occurrence of wrinkles (or bends) on the surface of the manufactured hot stamping part can be prevented or minimized.

[0086] In addition, in the forming / cooling step (S400), a steel plate for hot stamping can be pressed with a mold to form a formed body, and at the same time, the steel plate for hot stamping (or the formed body) can be cooled. Specifically, a hot stamping part can be formed by forming the steel plate for hot stamping into the shape of a hot stamping part in the mold and simultaneously cooling the formed body. The mold may be provided with a cooling channel through which a refrigerant circulates. The steel plate for hot stamping (or the formed body) can be rapidly cooled by the circulation of the refrigerant supplied through the cooling channel provided in the mold. At this time, in order to prevent the springback phenomenon of the sheet metal and maintain the desired shape, rapid cooling can be performed while applying pressure with the mold closed. When performing forming and cooling operations on the formed body, the average cooling rate can be 10°C / s or more to the martensite end temperature.

[0087] In one embodiment, the cooling end temperature at which the cooling of the hot stamping part (or, molded body) ends may be 300°C or lower. The cooling end temperature may be higher than room temperature. On the other hand, if the cooling end temperature exceeds 300°C, warping may occur in the hot stamping part as the manufactured hot stamping part (10) is air-cooled at room temperature, and it may be difficult to secure the target material. Therefore, if the cooling end temperature satisfies 300°C or lower, warping in the manufactured hot stamping part can be prevented or minimized.

[0088] FIG. 4 is a schematic cross-sectional view of an aluminum-silicon (Al-Si) plated steel sheet, and FIG. 5 is a phase diagram of an aluminum-iron (Al-Fe).

[0089] Referring to FIGS. 4 and 5, in the case of aluminum-silicon (Al-Si) plated steel sheets, the plating layer may exist as an intermetallic compound (Fe2Al5) and solid Al (FCC). In particular, in the case of aluminum-silicon (Al-Si) plated steel sheets, most of the plating layer may exist as solid Al (FCC).

[0090] The hydrogen diffusion rate of liquid aluminum (Liquid Al) at room temperature is 1.82 x 10⁻⁶ -7 m 2 It can be / s, and the hydrogen diffusion rate of solid aluminum (Solid Al) is 1x10 -11 m 2 / s. In other words, the hydrogen diffusion rate of liquid aluminum (Liquid Al) can be faster than that of solid aluminum (Solid Al). In this case, the hydrogen diffusion rates of liquid aluminum (Liquid Al) and solid aluminum (Solid Al) can be measured through electrochemical hydrogen permeation experiments. Specifically, a specimen is placed in the center of two cells, hydrogen is electrically generated in one cell to allow it to permeate to the other cell, and the time taken is measured. The hydrogen diffusion rate can then be derived using the measured time and the thickness of the specimen.

[0091] In the heating step of the hot stamping process, the aluminum-silicon (Al-Si) plated steel sheet can be heated to a temperature of 870°C to 950°C. Since the melting point of solid aluminum (Solid Al) is approximately 660°C, when the aluminum-silicon (Al-Si) plated steel sheet is heated to a temperature range of 870°C to 950°C, a large amount of solid aluminum (Solid Al) can be transformed into liquid aluminum (Liquid Al).

[0092] Since the hydrogen diffusion rate in liquid aluminum (Liquid Al) is faster than in solid aluminum (Solid Al), a large amount of hydrogen may be introduced into the base material during hot stamping heating in the case of aluminum-silicon (Al-Si) plated steel sheets. Subsequently, as the iron (Fe) and aluminum (Al) in the base material are alloyed and the plating layer cools to room temperature, the hydrogen introduced into the base material cannot escape and becomes trapped within the base material by the plating layer, which can lead to hydrogen embrittlement.

[0093] Figure 6 is a diagram showing the phase diagram of zinc-iron (Zn-Fe).

[0094] Referring to FIGS. 1 and 6, when a hot stamping part is manufactured by hot stamping a steel plate on which an alloyed molten zinc plating layer is formed, the plating layer of the hot stamping part may include several alloy phases and a pure zinc (e.g., mostly zinc) phase. Specifically, the plating layer (200) may be composed of a gamma phase, a delta phase, a zeta phase, and an eta phase.

[0095] The hydrogen diffusion rate in the gamma phase at room temperature is 2.59 x 10⁻⁶ -14 m 2 It can be / s, and the hydrogen diffusion rate in the delta phase is 2.69 x 10⁻⁶ -13 m 2 It can be / s, and the hydrogen diffusion rate in the zeta phase is 1.30 x 10⁻⁶ -13 m 2 It can be / s. Also, the hydrogen diffusion rate of liquid zinc (Liquid Zn) is 1.0 x 10⁻⁶ -7 m 2 / s. That is, the hydrogen diffusion rate in liquid zinc (Zn) can be faster than the hydrogen diffusion rate in the alloy phases. In this case, the hydrogen diffusion rate of each phase can be measured through electrochemical hydrogen permeation experiments.

[0096] The melting point of solid zinc (Solid Zn) is approximately 419.5°C, so when a steel plate with an alloyed molten zinc plating layer formed thereon is heated in a temperature range of 870°C to 950°C, most of the eta phase can transform from solid zinc (Solid Zn) into liquid zinc (Liquid Zn). However, since the melting point of the zinc (Zn)-iron (Fe) alloy phases is 780°C or higher, the gamma phase, delta phase, and zeta phase of the plating layer (200) have high melting points and may exist in a large amount as a solid phase.

[0097] Therefore, since the hydrogen diffusion rate in the gamma, delta, and zeta phases is relatively slow, when the gamma, delta, and zeta phases are formed in the plating layer with an appropriate thickness, the amount of hydrogen flowing into the base material is significantly reduced, and the hydrogen embrittlement resistance of the manufactured hot stamping part can be improved.

[0098] Accordingly, the inventors utilize the fact that hydrogen inflow from the outside into the base material during the hot stamping heating step is prevented when gamma, delta, zeta, and eta phases are formed in the plating layer of a hot stamping part to an appropriate thickness, and the hydrogen diffusion distance (L) and the thickness of the plating layer (T total Relationship 1 regarding ) was derived.

[0099] <Relationship 1>

[0100] L < T total

[0101] In Equation 1, L is the hydrogen diffusion distance (in meters), and T total is the thickness of the plating layer of the hot stamping part (in meters).

[0102] In addition, the hydrogen diffusion distance (L) of Equation 1 can be derived through Equation 2.

[0103] <Relation Equation 2>

[0104]

[0105] In Equation 2, L is the hydrogen diffusion distance (in meters), and T totalε is the thickness of the plating layer of the hot stamping part (in meters), T1 is the thickness of the gamma phase (in meters), and D1 is the hydrogen diffusion rate in the gamma phase (m 2 (in units of / s), T2 is the thickness of the delta phase (in units of m), and D2 is the hydrogen diffusion rate in the delta phase (m 2 (in units of / s), T3 is the thickness of the zeta phase (in units of m), and D3 is the hydrogen diffusion rate in the zeta phase (m 2 (in units of / s), T4 is the thickness of the eta phase (in units of m), and D4 is the hydrogen diffusion rate in liquid zinc (m 2 / s unit) and t is the heating time of the heating step (s unit).

[0106] At this time, T1 may be the thickness of the gamma phase of the hot stamping part (in meters), T2 may be the thickness of the delta phase of the hot stamping part (in meters), T3 may be the thickness of the zeta phase of the hot stamping part (in meters), and T4 may be the thickness of the eta phase of the hot stamping part (in meters).

[0107] Since the heating temperature range in the heating stage is 870°C to 950°C, the hydrogen diffusion rate in the gamma phase is 1x10 when the temperature range is 870°C to 950°C. -14 Up to 1x10 -13 m 2 It can be / s, and the hydrogen diffusion rate in the delta phase is 1x10 -13 Up to 1x10 -12 m 2 It can be / s, and the hydrogen diffusion rate in the zeta phase is 1x10 -13 Up to 1x10 -12 m 2 It can be / s, and the hydrogen diffusion rate in the eta phase (e.g., liquid zinc) is 1x10 -7 Up to 1x10 -6 m 2 / s can be used. At this time, the hydrogen diffusion rate in each phase at 870℃ to 950℃ can be derived using the hydrogen diffusion rate at room temperature.

[0108] Relationship 2 can be derived through the following process.

[0109] First, the hydrogen diffusion distance (L) can be expressed by the following relationship 3.

[0110] <Relation Equation 3>

[0111]

[0112] In addition, the diffusion coefficient relationship for the layers within the plating layer can be expressed by the following Equation 4.

[0113] <Relation Equation 4>

[0114]

[0115] D in relational expression 4 total If we organize it into an equation regarding D total It can be expressed by the relationship 5 below.

[0116] <Relation Equation 5>

[0117]

[0118] D of relation 5 total If substituted into Equation 3, Equation 2 can be derived.

[0119] Therefore, the thickness of the plating layer (T) of the hot stamping part total When ) is greater than the hydrogen diffusion distance (L) in the plating layer, the inflow of hydrogen into the base material during the hot stamping heating step can be prevented or minimized, so the hydrogen embrittlement resistance of the hot stamping part can be excellent.

[0120] Experimental Example

[0121] The present invention will be explained in more detail below through experimental examples. However, the following experimental examples are intended to explain the present invention more specifically, and the scope of the present invention is not limited by the following experimental examples. The following experimental examples may be appropriately modified or changed by those skilled in the art within the scope of the present invention.

[0122] Table 1 below shows the component content of the slabs used in the experimental examples of the present invention. The specimens of the examples and comparative examples were prepared by the following method.

[0123] A slab was prepared having the composition shown in Table 1 and the remainder being iron (Fe) and unavoidable impurities. Subsequently, the slab was reheated at a temperature of 1200°C, finished rolled at a temperature of 950°C, cooled, and coiled at a temperature of 600°C. Afterward, the coiled steel sheet was pickled and cold-rolled with a reduction rate of 50%. After cold-rolling, it was annealed at a temperature of 850°C, cooled to a temperature of 500°C, reheated to a temperature of 470°C before immersion in the plating bath, immersed in a molten zinc plating bath, and passed through the plating bath at a speed of 120 mpm so that molten zinc adhered to both the upper and lower surfaces of the steel sheet. It was then charged into an alloying furnace at 530°C to produce an alloyed hot-dip galvanized steel sheet. At this time, the position of the alloying furnace was adjusted so that the temperature of the steel plate immediately before being charged into the alloying furnace was 420°C, and the temperature difference between the steel plate and the alloying furnace was 80°C or more. The time for maintaining the alloying temperature, i.e., the alloying time, was set to 30 seconds.

[0124] In addition, heating was performed at the heating temperature and heating time shown in Table 2, and the heated steel plate for hot stamping was transferred to a mold and formed into a molded body, while simultaneously rapidly cooling it down to 300°C or lower at an average cooling rate of 10°C / s or more to manufacture a specimen (e.g., a hot stamping part).

[0125] Thickness measurement

[0126] The plating layer of the specimen was photographed using a scanning electron microscope (SEM), and the thickness of the plating layer and the thickness of each phase were determined using the captured results. At this time, the average value of the plating layer thickness and the thickness of each phase was calculated after measuring at least 10 points.

[0127] Hydrogen embrittlement resistance evaluation

[0128] Hydrogen embrittlement resistance was evaluated on the specimens using Thermal Desorption Spectroscopy (TDS). Specifically, the amount of hydrogen released from the specimens at temperatures below 350°C was measured while heating the specimens from room temperature (±20°C) to 500°C at a heating rate of 20°C / min. At this time, if the amount of diffusible hydrogen is 0.3 wppm or less, the hydrogen embrittlement resistance can be judged to be excellent.

[0129] Ingredients (wt%) CsiMnPSAlCrBTi 0.3 0.2 1.4 0.01 20.00 20.03 0.2 0.00 50.02

[0130] Classification Heating Temperature (°C) Heating Time (s) Example 18 70 360 Example 29 50 360 Example 38 70 180 Example 48 70 360 Example 59 50 360 Comparative Example 18 70 360 Comparative Example 29 70 360 Comparative Example 38 70 160 Comparative Example 48 70 400 Comparative Example 59 50 400

[0131] Classification Gamma phase thickness (T1,m) Delta phase thickness (T2,m) Zeta phase thickness (T3,m) Eta phase thickness (T4,m) Value of Equation 2 Plating layer thickness (T total ,m)Diffusible hydrogen content (wppm) Example 1 5.2x10 -6 001.8x10 -6 6.97x10 -6 7x10 -6 0.1 Example 25.0x10 -6 005.0x10 -6 8.5x10 -6 10x10 -6 0.06 Example 37.4x10 -6 002.6x10 -6 4.9x10 -6 10x10 -6 0.04 Example 45.1x10 -6 2.4x10 -6 2.4x10 -6 0.1x10 -6 8.0x10 -6 10x10 -6 0.05 Example 5 4.2x10 -6 2.8x10 -6 2.9x10-6 0.1x10 -6 8.7x10 -6 10x10 -6 0.08 Comparative Example 1 3.7 x 10 -6 001.3x10 -6 6.97x10 -6 5x10 -6 0.72 Comparative Example 23.0x10 -6 007.0x10 -6 11x10 -6 10x10 -6 0.56 Comparative Example 3 1.5 x 10 -6 008.5x10 -6 10.3x10 -6 10x10 -6 0.51 Comparative Example 4 2.9 x 10 -6 3.5x10 -6 3.5x10 -6 0.1x10 -6 10.5x10 -6 10x10 -6 0.50 Comparison Example 5 1.9 x 10 -6 4.0x10 -6 4.0x10 -6 0.1x10 -6 12x10 -6 10x10 -6 0.82

[0132] When deriving the value of Equation 2, since substituting the fastest hydrogen diffusion rates in the temperature range of 870°C to 950°C yields the largest hydrogen diffusion distance value, the fastest hydrogen diffusion rates in the range of 870°C to 950°C were substituted into Equation 2. For example, when deriving the value of Equation 2 in Table 3, the hydrogen diffusion rate (D1) in the gamma phase is 1x10 -13 m 2 It was set to / s, and the hydrogen diffusion rates in the delta and zeta phases (D2, D3) were 1x10 -12 m 2 It was set to / s, and the hydrogen diffusion rate (D4) in the eta phase (e.g., liquid zinc) is 1x10 -6 m 2 / s was used. That is, the fastest hydrogen diffusion rates at 870℃ to 950℃ were substituted into Equation 2.

[0133] As shown in Tables 2 and 3, when the heating temperature of the heating step satisfies 870°C to 950°C and the heating time satisfies 180 s to 360 s, the hydrogen diffusion distance derived by Equation 2 and the total thickness of the plating layer satisfy Equation 1, and when the hydrogen diffusion distance and the total thickness of the plating layer satisfy Equation 1, it can be confirmed that the amount of diffusible hydrogen is 0.3 wppm or less.

[0134] Referring to Comparative Example 1, when the thickness of the plating layer is less than 7 μm, it can be confirmed that the hydrogen embrittlement resistance of the manufactured specimen (e.g., hot stamping part) is inferior because the hydrogen diffusion distance is greater than the thickness of the plating layer due to the plating layer thickness being too small.

[0135] Referring to Comparative Example 2, it can be confirmed that when the heating temperature exceeds 950°C, the thickness of the gamma phase is thin and the thickness of the eta phase increases, so the hydrogen diffusion distance is greater than the plating layer thickness, and the hydrogen embrittlement resistance of the manufactured specimen is inferior.

[0136] Referring to Comparative Example 3, it can be confirmed that when the heating time is less than 180 s, the thickness of the gamma phase is thin and the thickness of the eta phase increases, so the hydrogen diffusion distance is greater than the plating layer thickness, and the hydrogen embrittlement resistance of the manufactured specimen is inferior.

[0137] Referring to Comparative Examples 4 and 5, it can be seen that when the heating time exceeds 360 s, the hydrogen diffusion distance is too long and greater than the plating layer thickness, so the hydrogen embrittlement resistance of the manufactured specimen is inferior.

[0138] The present invention has been described with reference to the embodiments illustrated in the drawings, but this is merely illustrative, and those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible therefrom. Accordingly, the true technical scope of protection of the present invention should be determined by the technical spirit of the appended claims.

Claims

1. As a hot stamping part, Base material; and A plating layer formed on at least one surface of the above-mentioned base material and comprising at least one of a gamma phase, a delta phase, a zeta phase, and an eta phase; Includes, A hot stamping part having a diffusible hydrogen content of 0.3 wppm or less.

2. In Paragraph 1, A hot stamping part having a gamma phase thickness of 4㎛ or more and 15㎛ or less.

3. In Paragraph 1, A hot stamping part having a sum of the thicknesses of the delta phase and zeta phase of 14㎛ or less.

4. In Paragraph 1, A hot stamping part having a thickness of 16㎛ or less of the above eta.

5. In Paragraph 1, The above base material comprises, in wt%, carbon (C): 0.25 wt% to 0.50 wt%, silicon (Si): 0.1 wt% to 0.8 wt%, manganese (Mn): 0.3 wt% to 3.0 wt%, phosphorus (P): greater than 0 and less than or equal to 0.05 wt%, sulfur (S): greater than 0 and less than or equal to 0.01 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): 0.01 wt% to 1.00 wt%, boron (B): greater than 0 and less than or equal to 0.006 wt%, and the remainder being iron (Fe) and other unavoidable impurities, a hot stamping part.

6. In Paragraph 1, The above hot stamping part is a hot stamping part having a tensile strength (TS) of 1680 MPa to 2300 MPa.

7. As a method for manufacturing hot stamping parts, A step of heating a steel plate for hot stamping at a temperature range of 870℃ to 950℃ for a time of 180s to 360s; The step of transferring the heated steel plate for hot stamping to a mold; and A step of forming a molded body by pressing the hot stamping steel plate transferred to the mold and cooling the formed molded body; Includes, The above hot stamping part comprises a base material and a plating layer formed on at least one surface of the base material, and A method for manufacturing a hot stamping part, wherein the thickness of the plating layer and the hydrogen diffusion distance in the plating layer satisfy the following relationship 1. <Relation Equation 1> L < T total In the above Equation 1, L is the hydrogen diffusion distance in the plating layer (in meters), and T total It is the thickness of the silver plating layer (in meters).

8. In Paragraph 7, A method for manufacturing a hot stamping part, wherein the plating layer comprises at least one of a gamma phase, a delta phase, a zeta phase, and an eta phase.

9. In Paragraph 8, A method for manufacturing a hot stamping part, wherein the above hydrogen diffusion distance (L) is derived through the relationship 2 below. <Relation Equation 2> In the above Equation 2, L is the hydrogen diffusion distance (in meters), and T total It is the thickness of the silver plating layer (in meters), T1 is the thickness of the gamma phase (in meters), and D1 is the hydrogen diffusion rate in the gamma phase (m 2 (in units of / s), T2 is the thickness of the delta phase (in units of m), and D2 is the hydrogen diffusion rate in the delta phase (m 2 (in units of / s), T3 is the thickness of the zeta phase (in units of m), and D3 is the hydrogen diffusion rate in the zeta phase (m 2 (in units of / s), T4 is the thickness of the eta phase (in units of m), and D4 is the hydrogen diffusion rate in liquid zinc (m 2 / s unit) and t is the heating time of the heating step (s unit).

10. In Paragraph 9, A method for manufacturing a hot stamping part, wherein the thickness of the gamma phase is 4㎛ or more and 15㎛ or less.

11. In Paragraph 9, A method for manufacturing a hot stamping part, wherein the sum of the thicknesses of the delta phase and zeta phase is 14 μm or less.

12. In Paragraph 9, A method for manufacturing a hot stamping part, wherein the thickness of the eta phase is 16 μm or less.

13. In Paragraph 7, A method for manufacturing a hot stamping part, wherein the above base material comprises, in wt%, carbon (C): 0.25 wt% to 0.50 wt%, silicon (Si): 0.1 wt% to 0.8 wt%, manganese (Mn): 0.3 wt% to 3.0 wt%, phosphorus (P): greater than 0 and less than or equal to 0.05 wt%, sulfur (S): greater than 0 and less than or equal to 0.01 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): 0.01 wt% to 1.00 wt%, boron (B): greater than 0 and less than or equal to 0.006 wt%, and the remainder being iron (Fe) and other unavoidable impurities.

14. In Paragraph 7, A method for manufacturing a hot stamping part having a tensile strength (TS) of 1680 MPa to 2300 MPa.

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