Hot stamping parts and method for manufacturing the same

Abstract

The present invention relates to a hot stamping part, which includes a base material, a decarburized layer disposed on the base material, and an internal oxide layer disposed on the decarburized layer. The hot stamping part has a tensile strength (TS) of 1,350 MPa to 1,680 MPa. From the surface of the hot stamping part in the thickness direction of the hot stamping part, a hot stamping part is provided in which the hardness within a depth of 50 μm and the average hardness of the hot stamping part satisfy the following relational expression 1: [Relational Expression 1] (A / B) ≤ 0.7 (In the relational expression 1, A is the hardness (Hv(≤50 μm)) within a depth of 50 μm in the thickness direction of the hot stamping part, and B is the average hardness (Hv(avg.)) of the hot stamping part).

Description

Technical Field

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

Background Art

[0002] Worldwide, while environmental regulations and fuel consumption regulations are being strengthened, the need for lighter vehicle materials is increasing. As a result, research and development on ultra-high strength steel and hot stamping steel are being actively conducted.

[0003] The hot stamping process can generally utilize the phase transformation of the material and the change in the microstructure in the process consisting of heating / formning / cooling / trimming. In order to improve the toughness of hot stamping steel, research is being actively conducted generally by using alloying elements to improve the toughness of the base material.

[0004] However, if the alloying elements are changed or increased, there is a problem that the economic efficiency, such as an increase in cost, is reduced.

[0005] Related technologies include Korean Patent Publication No. 10-2021-0129902 (Title of the Invention: Hot Stamping Parts and a Method for Manufacturing the Same), etc.

Summary of the Invention

Problems to be Solved by the Invention

[0006] An embodiment of the present invention improves the toughness of the manufactured hot stamping parts by appropriately forming a decarburized layer and an internal oxide layer on the surface of the base material, and at the same time, can prevent cracks from occurring during hot stamping forming.

Means for Solving the Problems

[0007] One embodiment of the present invention is a hot stamping part, which includes a base material, a decarburized layer disposed on the surface of the base material, and an internal oxide layer disposed on the surface of the decarburized layer. The hot stamping part has a tensile strength (TS) of 1,350 MPa to 1,680 MPa. On the surface of the hot stamping part, in the thickness direction of the hot stamping part, the hardness within a depth of 50 μm and the average hardness of the hot stamping part satisfy the following relational expression 1, and a hot stamping part is provided. [Relational Expression 1] (A / B) ≤ 0.7 (In the relational expression 1, A is the hardness (Hv(≤50 μm)) within a depth of 50 μm in the thickness direction of the hot stamping part, and B is the average hardness (Hv(avg.)) of the hot stamping part).

[0008] In this embodiment, the depth of the internal oxide layer may satisfy the following relational expression 2. [Relational Expression 2] C ≤ 5 μm (In the relational expression 2, C is the depth of the internal oxide layer in the thickness direction of the hot stamping part).

[0009] In this embodiment, the hot stamping part may have a VDA bending angle of 60° or more.

[0010] In this embodiment, the hot stamping part may have a yield stress (YP) of 900 MPa to 1,300 MPa and an elongation (EL) of 4% to 10%.

[0011] In this embodiment, the hot stamping part may have a microstructure including a martensite fraction of 90% or more.

[0012] In this embodiment, it further includes a plating layer disposed on the surface of the internal oxide layer.

[0013] In this embodiment, the thickness of the plating layer is also 10 μm to 30 μm.

[0014] Another embodiment of the present invention is a method for manufacturing hot stamping parts, including the steps of cutting a plated steel sheet having a plating layer formed on at least one surface of a base material to form a blank, and heating the blank in a heating furnace having a plurality of sections with different temperature ranges. The step of heating the blank includes a multi-stage heating step of heating the blank step by step, and a soaking heating step of heating the multi-stage heated blank at a temperature of Ac1 to 910 °C. A method for manufacturing a hot stamping part is provided, in which the hardness within a depth of 50 μm in the plate thickness direction of the hot stamping part from the surface of the hot stamping part and the average hardness of the hot stamping part satisfy the following relational expression 3. [Relational Expression 3] (A / B) ≤ 0.7 (In the relational expression 3, A is the hardness (Hv(≤50 μm)) within a depth of 50 μm in the plate thickness direction of the hot stamping part, and B is the average hardness (Hv(avg.)) of the hot stamping part.)

[0015] In this embodiment, the dew point temperature of the annealing furnace for the base material is also -15 °C to +15 °C.

[0016] In this embodiment, the tempering temperature of the base material is also 750 °C to 900 °C.

[0017] In this embodiment, after the step of heating the blank, the steps further include transferring the heated blank, pressing the transferred blank into a mold to form a molded body, and cooling the formed molded body.

[0018] In this embodiment, it further includes a decarburized layer formed on the surface of the base material and an internal oxide layer formed on the surface of the decarburized layer.

[0019] In this embodiment, the depth of the internal oxide layer can satisfy the following relational expression 4. [Relational Expression 4] C ≤ 5 μm (In the relational expression 4, C is the depth of the internal oxide layer in the plate thickness direction of the hot stamping part).

[0020] Other aspects, features, and advantages other than those described above will become clear from the specific content, claims, and drawings for implementing the following invention.

Effects of the Invention

[0021] According to an embodiment of the present invention made as described above, by forming a decarburized layer on the surface of the base material, the toughness of the manufactured hot stamping part can be improved.

[0022] Further, according to an embodiment of the present invention, by providing the internal oxide layer formed on the surface of the decarburized layer to be below a preset depth, cracks can be prevented from occurring during the hot stamping process.

Brief Description of the Drawings

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Embodiments for Carrying Out the Invention

[0024] 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 explanations. The effects, features, and methods for achieving them 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 be embodied in various forms.

[0025] 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.

[0026] In the following embodiments, singular expressions include plural expressions unless clearly stated otherwise in the context.

[0027] In the following embodiments, terms such as "including" or "having" mean that the features or components described in the specification exist, and do not preclude the possibility of adding one or more other features or components in advance.

[0028] In the following embodiments, when a part such as a film, region, or component is on or above another part, it includes not only the case where it is directly above the other part but also the case where still other films, regions, components, etc. are interposed in between.

[0029] In the drawings, for convenience of explanation, the sizes of the components may be exaggerated or reduced. For example, the sizes and thicknesses of each configuration shown in the drawings are arbitrarily shown for convenience of explanation, so the present invention is not necessarily limited to what is shown in the drawings.

[0030] In this specification, "A and / or B" indicates the case where it is A, B, or both A and B. Also, in this specification, "at least one of A and B" indicates the case where it is A, B, or both A and B.

[0031] In the following embodiments, when referring to "on a plane", it means when looking at the target part from above, and when referring to "in a cross-section", it means when looking at the cross-section obtained by vertically cutting the target part from the side. In the following embodiments, when referring to "superposition", it includes superposition "on a plane" and "in a cross-section".

[0032] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. When describing with reference to the drawings, the same or corresponding components are denoted by the same reference numerals.

[0033] FIG. 1 is a cross-sectional view schematically showing a hot stamping part according to an embodiment of the present invention.

[0034] Referring to FIG. 1, a hot stamping part 1 according to an embodiment includes a base material 100, a decarburized layer 200, an internal oxide layer 300, and a plating layer 400. The base material 100, the decarburized layer 200, the internal oxide layer 300, and the plating layer 400 can be sequentially laminated in the thickness direction of the hot stamping part 1.

[0035] In one embodiment, the base material 100 also contains carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), boron (B), the balance iron (Fe), and other inevitable impurities. For example, the base material 100 contains, by weight, carbon (C) of about 0.15 wt% or more and about 0.3 wt% or less, silicon (Si) of about 0.05 wt% or more and about 0.8 wt% or less, manganese (Mn) of about 0.8 wt% or more and about 3.0 wt% or less, phosphorus (P) in excess of 0 and about 0.1 wt% or less, sulfur (S) in excess of 0 and about 0.1 wt% or less, chromium (Cr) of about 0.1 wt% or more and about 0.9 wt% or less, boron (B) of about 0.001 wt% or more and about 0.005 wt% or less, the balance iron (Fe), and other inevitable impurities.

[0036] Also, the base material 100 may further contain one or more components among titanium (Ti), niobium (Nb), and vanadium (V). Also, the base material 100 may further contain calcium (Ca). For example, the base material 100 may further contain one or more components among titanium (Ti) of about 0.01 wt% or more and about 0.1 wt% or less, niobium (Nb) of about 0.01 wt% or more and about 0.1 wt% or less, vanadium (V) of about 0.01 wt% or more and about 0.1 wt% or less, and calcium (Ca) of about 0.0001 wt% or more and about 0.01 wt% or less.

[0037] Carbon (C) is a main element that determines the strength and hardness of steel and can be added for the purpose of ensuring the tensile strength of the steel material after the hot stamping (or hot pressing) process. Also, carbon (C) can be added for the purpose of ensuring the hardenability characteristics of the steel material. In one embodiment, carbon (C) can be contained in an amount of about 0.15 wt% or more and about 0.3 wt% or less based on the total weight of the base material 100. If carbon (C) is contained in an amount less than about 0.15 wt% based on the total weight of the base material 100, it is difficult to achieve the desired mechanical strength. On the other hand, if carbon (C) is contained in an amount exceeding about 0.3 wt% based on the total weight of the base material 100, problems such as a decrease in the toughness of the steel material or problems in controlling the brittleness of the steel can be caused.

[0038] Silicon (Si) can act as a ferrite stabilizing element within the base material 100. Silicon (Si) can perform the function of improving ductility by cleaning the ferrite and improving the carbon concentration in austenite by suppressing carbide formation in the low temperature range. Furthermore, silicon (Si) is also a core element for hot rolling, cold rolling, hot stamping microstructure homogenization (control of pearlite and manganese segregation bands), and fine dispersion of ferrite. In one embodiment, silicon (Si) can be included in an amount of about 0.05 wt% or more and about 0.8 wt% or less based on the total weight of the base material 100. If silicon (Si) is included in an amount of less than about 0.05 wt% based on the total weight of the base material 100, the above-described functions cannot be fully exerted. On the other hand, if silicon (Si) is included in an amount exceeding about 0.8 wt% based on the total weight of the base material 100, the hot rolling load and the cold rolling load increase, excessive hot rolling red scale occurs, and the weldability may be reduced.

[0039] Manganese (Mn) can be added for the purpose of hardenability and strength increase during heat treatment. In one embodiment, manganese (Mn) can be included in an amount of about 0.8 wt% or more and about 3.0 wt% or less based on the total weight of the base material 100. If manganese (Mn) is included in an amount of less than about 0.8 wt% based on the total weight of the base material 100, the hardenability is insufficient, and the material may become insufficient (for example, the fraction of hard phases is insufficient) after hot stamping. On the other hand, if manganese (Mn) is included in an amount exceeding about 3.0 wt% based on the total weight of the base material 100, the ductility and toughness are reduced due to manganese segregation or pearlite bands, which can cause a decrease in bending performance and the formation of a non-uniform fine microstructure.

[0040] Phosphorus (P) is an element that segregates and also an element that inhibits the toughness of steel. In one embodiment, phosphorus (P) can be included in an amount greater than 0 and about 0.1 wt% or less based on the total weight of the base material 100. When phosphorus (P) is included within the above-described range based on the total weight of the base material 100, a decrease in the toughness of the steel can be prevented. On the other hand, if phosphorus (P) is included in an amount exceeding about 0.1 wt% based on the total weight of the base material 100, cracks may occur during the process, iron phosphide compounds are formed, and the toughness of the steel may be reduced.

[0041] Sulfur (S) is also an element that inhibits workability and physical properties. In one embodiment, sulfur (S) may be contained in an amount of more than 0 and about 0.1 wt% or less based on the total weight of the base material 100. When sulfur (S) is contained in an amount of more than about 0.1 wt% based on the total weight of the base material 100, hot workability is reduced, and surface defects such as cracks may occur due to the formation of large inclusions.

[0042] Chromium (Cr) may be added for the purpose of improving the hardenability and strength of steel. In one embodiment, chromium (Cr) is contained in an amount of about 0.1 wt% or more and about 0.9 wt% or less based on the total weight of the base material 100. When chromium (Cr) is contained within the aforementioned range based on the total weight of the base material 100, the hardenability and strength of the steel can be improved, an increase in production cost can be prevented, and a decrease in the toughness of the steel material can be prevented.

[0043] Boron (B) is added for the purpose of ensuring the hardenability and strength of the steel material by ensuring a martensite structure, and may have a grain refinement effect due to an increase in the austenite grain growth temperature. In one embodiment, boron (B) may be contained in an amount of about 0.001 wt% or more and about 0.005 wt% or less based on the total weight of the base material 100. When boron (B) is contained within the aforementioned range based on the total weight of the base material 100, the occurrence of hard phase grain boundary brittleness can be prevented, and high toughness and bendability can be ensured.

[0044] Titanium (Ti) may be added for the purpose of strengthening hardenability and improving material properties by forming precipitates after hot stamping heat treatment. Also, titanium (Ti) can effectively contribute to the refinement of austenite grain size by forming a precipitate phase such as Ti(C,N) at high temperatures. In one embodiment, titanium (Ti) is contained in an amount of about 0.01 wt% or more and about 0.1 wt% or less based on the total weight of the base material 100. When titanium (Ti) is contained within the aforementioned range based on the total weight of the base material 100, continuous casting defects can be prevented, coarsening of precipitates can be prevented, the physical properties of the steel material can be easily ensured, and the occurrence of cracks on the surface of the steel material can be prevented or minimized.

[0045] Niobium (Nb) can be added for the purpose of increasing strength and toughness by reducing the martensite packet size. In one embodiment, niobium (Nb) is also included in an amount of about 0.01 wt% or more and about 0.1 wt% or less based on the total weight of the base material 100. When niobium (Nb) is included within the aforementioned range based on the total weight of the base material 100, it has excellent grain refinement effect of the steel material in the hot rolling and cold rolling processes, prevents the occurrence of slab cracks and brittle fracture of the product during steelmaking / continuous casting, and can minimize the formation of coarse precipitates during steelmaking.

[0046] Vanadium (V) can be added for the purpose of increasing the strength of the steel material through the precipitation strengthening effect by precipitate formation. In one embodiment, vanadium (V) is also included in an amount of about 0.01 wt% or more and about 0.1 wt% or less based on the total weight of the base material 100. When vanadium (V) is included within the aforementioned range based on the total weight of the base material 100, the strength of the steel material can be improved.

[0047] Calcium (Ca) can be added for the purpose of forming CaS, reducing the sulfur content in the steel, and preventing the formation of MnS inclusions that induce defects such as hook cracks when being stretched and gas resistance welded during rolling.

[0048] In one embodiment, the base material 100 can have a microstructure containing a martensite fraction of about 90% or more. Specifically, the base material 100 can have a microstructure containing about 90% or more of martensite, the balance being other inevitable tissues and less than 10% of other precipitates.

[0049] In one embodiment, a decarburized layer 200 can be disposed on the base material 100. Specifically, the decarburized layer 200 can be disposed on the surface of the base material 100. When the decarburized layer 200 is disposed on the base material 100, since the decarburized layer 200 is a softer layer than the base material 100, the toughness of the hot stamping part 1 can be improved.

[0050] A layer with a hardness of about 70% or less in terms of the average hardness ratio at about 1 / 4 point in the thickness direction of the hot stamping part 1 from the surface of the base material 100 can be defined as the decarburized layer 200. Or, a layer with a hardness of about 70% or less in terms of the average hardness ratio at about 1 / 4 point in the thickness direction of the hot stamping part 1 from the surface of the hot stamping part 1 can be defined as the decarburized layer 200. That is, the average hardness of the decarburized layer 200 is also about 70% or less in terms of the average hardness ratio at about 1 / 4 point in the thickness direction of the hot stamping part 1 from the surface of the base material 100 or the surface of the hot stamping part 1. In other words, the average hardness of the decarburized layer 200 is also about 70% or less of the average hardness at about 1 / 4 point in the thickness direction of the hot stamping part 1 from the surface of the base material 100 or the surface of the hot stamping part 1.

[0051] In one embodiment, an internal oxide layer 300 can be disposed on the decarburized layer 200. Specifically, the internal oxide layer 300 can be disposed on the surface of the decarburized layer 200. The internal oxide layer 300 contains silicon (Si), manganese (Mn), chromium (Cr), etc.

[0052] In one embodiment, the depth (or thickness (t1)) of the internal oxide layer 300 can be provided to be about 5 μm or less in the thickness direction of the hot stamping part 1. This will be described in more detail below.

[0053] In one embodiment, a plating layer 400 can be disposed on the internal oxide layer 300. Specifically, the plating layer 400 can be disposed on the surface of the internal oxide layer 300. The plating layer 400 is also a zinc (Zn)-based plating layer or an aluminum (Al)-based plating layer. For example, the plating layer 400 contains zinc (Zn) and / or aluminum (Al).

[0054] In one embodiment, when the plating layer 400 is provided as a zinc (Zn)-based plating layer, the plating layer 400 contains iron (Fe), aluminum (Al), manganese (Mn), silicon (Si), the balance zinc (Zn), and other inevitable impurities. For example, the plating layer 400 contains about 10 wt% or more and about 70 wt% or less of iron (Fe), more than 0 and about 5 wt% or less of aluminum (Al), more than 0 and about 1 wt% or less of manganese (Mn), more than 0 and about 1 wt% or less of silicon (Si), the balance zinc (Zn), and other inevitable impurities.

[0055] The plating layer 400 can be provided at a depth (or thickness (t2)) of about 10 μm to about 30 μm in the thickness direction of the hot stamping part 1. When the thickness (t2) of the plating layer 400 is less than about 10 μm, the sacrificial effect peculiar to zinc is reduced. When the thickness (t2) of the plating layer 400 exceeds about 30 μm, the thickness (t2) of the plating layer 400 is excessively thick, and the toughness of the hot stamping part 1 including the plating layer 400 can be reduced. Therefore, when the plating layer 400 is provided with a thickness (t2) of about 10 μm to about 30 μm, the surface of the base material (or steel material) can be protected, and at the same time, the reduction in the toughness of the hot stamping part 1 can be prevented or minimized.

[0056] The decarburized layer 200 is a soft layer compared to the base material 100. When the hot stamping part 1 includes the decarburized layer 200, the toughness of the hot stamping part 1 including the same can be improved.

[0057] However, as described above, when the depth (or thickness (t1)) of the internal oxide layer 300 is excessively large, the internal oxide layer 300 allows the liquid zinc to penetrate more easily into the base material 100, increasing the possibility of cracks occurring during hot stamping forming, thereby reducing the bendability of the manufactured hot stamping part 1. Specifically, when the depth (or thickness (t1)) of the internal oxide layer 300 exceeds about 5 μm, the internal oxide layer 300 allows the liquid zinc to penetrate more easily toward the base material 100 side, increasing the possibility of cracks occurring during hot stamping forming, thereby reducing the bendability of the manufactured hot stamping part 1.

[0058] As will be described later in the method for manufacturing a hot stamping part, a decarburized layer 200 can be formed on the base material 100 at the annealing stage. At this time, an internal oxide layer 300 can be simultaneously formed on the decarburized layer 200. Specifically, at the annealing stage, a decarburized layer 200 is formed on the surface of the base material 100, and at the same time, an internal oxide layer 300 can be formed on the surface of the decarburized layer 200.

[0059] If the dew point temperature of the annealing furnace in which the annealing stage is performed is increased, the depth (or thickness) of the decarburized layer 200 can be increased. However, when the depth (or thickness) of the decarburized layer 200 is increased, the depth (or thickness) of the internal oxide layer 300 can also be increased. That is, in order to increase the toughness of the hot stamping part 1, the depth (or thickness) of the decarburized layer 200 must be increased. However, when the depth (or thickness) of the decarburized layer 200 is increased, the depth (or thickness) of the internal oxide layer 300 also increases, increasing the possibility of cracks occurring during hot stamping forming, thereby reducing the bendability of the manufactured hot stamping part 1. Therefore, it is necessary to appropriately adjust the depth (or thickness) of the decarburized layer 200 and the depth (or thickness) of the internal oxide layer 300.

[0060] Accordingly, through excessive repeated experiments, the inventor of the present invention derived Relational Expression 1 and Relational Expression 2 for making the hot stamping part 1 have a VDA bending angle of about 60° or more. In one embodiment, the hot stamping part 1 may satisfy Relational Expression 1 and Relational Expression 2. Specifically, on the surface of the hot stamping part 1, in the thickness direction of the hot stamping part 1, the hardness within a depth of about 50 μm and the average hardness of the hot stamping part 1 satisfy the following Relational Expression 1, and the depth (or thickness) of the internal oxide layer 300 may satisfy the following Relational Expression 2. For example, the hot stamping part 1 may satisfy both of the following Relational Expression 1 and Relational Expression 2.

[0061] [Relational Expression 1] (A / B)≦0.7 In Relational Expression 1, A is the hardness (Hv(≦50μm)) within a depth (or thickness) of about 50 μm in the thickness direction of the hot stamping part 1, and B is the average hardness (Hv(avg.)) of the hot stamping part 1.

[0062] At this time, the hardness within a depth of about 50 μm in the thickness direction of the hot stamping part 1 from the surface of the hot stamping part 1 is also the hardness value measured by a Vickers hardness tester at a point within a depth of about 50 μm or less in the thickness direction of the hot stamping part 1 from the surface of the hot stamping part 1, and the average hardness (Hv(avg.)) of the hot stamping part 1 is also the hardness value measured by a Vickers hardness tester at about 1 / 4 point in the thickness direction of the hot stamping part 1.

[0063] [Relational Expression 2] C≦5μm In Relational Expression 2, C is the depth (or thickness) of the internal oxide layer 300 in the thickness direction of the hot stamping part 1.

[0064] Since the decarburized layer 200 is disposed on the surface of the base material 100, the decarburized layer 200 can be disposed in a portion adjacent to the surface of the hot stamping part 1. Since the decarburized layer 200 corresponds to a softer layer than the base material 100, in the hot stamping part 1, the hardness of the portion adjacent to the surface of the hot stamping part 1 is lower than the average hardness of the hot stamping part 1. At this time, when the depth (or thickness) of the decarburized layer 200 increases, in the hot stamping part 1, the difference between the hardness of the portion adjacent to the surface of the hot stamping part 1 and the average hardness of the hot stamping part 1 can increase. On the other hand, when the depth (or thickness) of the decarburized layer 200 decreases, in the hot stamping part 1, the difference between the hardness of the portion adjacent to the surface of the hot stamping part 1 and the average hardness of the hot stamping part 1 can be reduced.

[0065] When the ratio of the hardness (Hv(≤50μm)) within a depth (or thickness) of about 50μm in the plate thickness direction of the hot stamping part 1 to the average hardness (Hv(avg.)) of the hot stamping part 1 exceeds about 0.7, the decarburized layer 200 is not formed (or provided) to a sufficient depth (or thickness), and the toughness of the hot stamping part 1 including the same is low. In particular, the VDA bending angle of the hot stamping part 1 is also less than about 60°.

[0066] When the ratio of the hardness (Hv(≤50μm)) within a depth (or thickness) of about 50μm in the plate thickness direction of the hot stamping part 1 to the average hardness (Hv(avg.)) of the hot stamping part 1 is about 0.7 or less, it may mean that the decarburized layer 200 is formed (or provided) to a sufficient depth (or thickness). Therefore, when the ratio of the hardness (Hv(≤50μm)) within a depth (or thickness) of about 50μm in the plate thickness direction of the hot stamping part 1 to the average hardness (Hv(avg.)) of the hot stamping part 1 satisfies about 0.7 or less, the decarburized layer 200 is formed (or provided) to a sufficient depth (or thickness), and the toughness of the hot stamping part 1 including the same can be improved. In particular, the hot stamping part 1 can have a VDA bending angle of about 60° or more.

[0067] Also, as described above, the depth (or thickness) of the internal oxide layer 300 included in the hot stamping part 1 is also about 5 μm or less.

[0068] When the plating layer 400 is provided as a zinc (Zn)-based plating layer, due to the low melting point of zinc, a liquid metal embrittlement (LME) phenomenon occurs, thereby causing cracks inside, and the bendability of the hot stamping part 1 may be reduced. At this time, when the depth (or thickness) of the internal oxide layer 300 is large, the internal oxide layer 300 allows liquid zinc to penetrate more easily inside, and during hot stamping forming, the probability of cracks occurring increases, thereby reducing the bendability of the manufactured hot stamping part 1.

[0069] When the depth (or thickness) of the internal oxide layer 300 exceeds about 5 μm, the internal oxide layer 300 allows liquid zinc to penetrate more easily inside, and during hot stamping forming, the probability of cracks occurring increases, thereby reducing the bendability of the manufactured hot stamping part 1.

[0070] Therefore, when the depth (or thickness) of the internal oxide layer 300 is provided to be about 5 μm or less in the plate thickness direction of the hot stamping part 1, cracks can be prevented from occurring during hot stamping, and through this, the hot formability of the hot stamping part (or blank) can be improved.

[0071] In one embodiment, the hot stamping part 1 can simultaneously satisfy the aforementioned relational expressions 1 and 2. When the hot stamping part 1 simultaneously satisfies the aforementioned relational expressions 1 and 2, the hot stamping part 1 has high toughness while also having excellent hot formability. Specifically, when the hardness within a depth (or thickness) of 50 μm in the thickness direction of the hot stamping part 1 from the surface of the hot stamping part 1 and the average hardness of the hot stamping part 1 satisfy the aforementioned relational expression 1, and the depth (or thickness) of the internal oxide layer 300 satisfies the aforementioned relational expression 2, the hot stamping part 1 has high toughness while simultaneously having excellent hot formability of the hot stamping part.

[0072] In one embodiment, when the hot stamping part 1 satisfies both of the aforementioned relational expressions 1 and 2, the hot stamping part 1 may have a tensile strength (TS) of about 1,350 MPa to about 1,680 MPa, a yield stress (YP) of about 900 MPa to about 1,300 MPa, and an elongation (EL) of about 4% to about 10%. Also, the hot stamping part 1 may have a VDA bending angle of about 60° or more. At this time, the VDA bending angle can be measured in accordance with the VDA standard (VDA238 - 100).

[0073] FIG. 2 is a flowchart schematically showing a method for manufacturing a hot stamping part according to an embodiment of the present invention, and FIGS. 3 to 5 are cross-sectional views schematically showing a method for manufacturing a hot stamping part according to an embodiment of the present invention.

[0074] Referring to FIGS. 2 to 5, a method for manufacturing the hot stamping part 1 (FIG. 1) according to an embodiment includes a hot rolling step (S100), a cooling / winding step (S200), a cold rolling step (S300), a annealing step (S400), a plating step (S500), and a hot stamping step (S600).

[0075] First, the reheating stage of the base material 100 (e.g., steel slab) provided by the composition described in FIG. 1 can proceed. In the steel slab reheating stage, the steel slab secured through the continuous casting process is reheated to a predetermined temperature, so that the components segregated during casting can be redissolved. In one embodiment, the slab reheating temperature (SRT) is also about 1,200°C to about 1,400°C. When the slab reheating temperature (SRT) is lower than about 1,200°C, the components segregated during casting cannot be sufficiently redissolved, it is difficult to see a great homogenization effect of alloying elements, and it is difficult to see a great solution effect of titanium (Ti). Although the higher the slab reheating temperature (SRT), the more advantageous for homogenization, when the slab reheating temperature (SRT) exceeds about 1,400°C, not only does the austenite crystal grain size increase and it becomes difficult to ensure strength, but also the manufacturing cost of the steel sheet can increase due to an excessive heating process.

[0076] In the hot rolling stage (S100), the reheated base material 100 can be hot rolled at a predetermined finishing delivery temperature (FDT). Through the hot rolling stage (S100), a hot rolled steel sheet can be manufactured. In one embodiment, the finishing delivery temperature (FDT) is also about 880°C to about 950°C. At this time, if the finishing delivery temperature (FDT) is lower than about 880°C, not only is it difficult to ensure the workability of the steel sheet due to the occurrence of a mixed grain structure caused by abnormal area rolling, and the workability is reduced due to non-uniform fine structure, but also problems with the sheet passing property can occur during hot rolling due to a rapid phase change. When the finishing delivery temperature (FDT) exceeds about 950°C, the austenite crystal grains become coarser, the TiC precipitates become coarser, and the performance of the hot stamping parts can be degraded.

[0077] In the cooling / winding step (S200), the hot-rolled base material 100 can be cooled to a predetermined winding temperature (CT: coiling temperature) and then wound. In one embodiment, the winding temperature in the cooling / winding step (S300) is also about 550°C to about 800°C. This winding temperature affects the redistribution of carbon (C). When the winding temperature is less than about 550°C, the low-temperature phase fraction due to supercooling increases and the strength increases. During cold rolling, there is a concern that the rolling load will deepen, and the ductility may decrease rapidly. On the contrary, when the winding temperature exceeds about 800°C, deterioration of formability and strength may occur due to abnormal crystal grain growth or excessive crystal grain growth.

[0078] In the cold rolling step (S300), the wound base material 100 can be uncoiled, pickled, and then cold rolled. At this time, the pickling can be carried out for the purpose of removing the scale of the wound steel sheet (or the base material), that is, the hot-rolled coil manufactured through the above-mentioned hot rolling process. Through the cold rolling step (S300), cold-rolled steel sheets can be manufactured.

[0079] In the annealing step (S400), annealing can be performed on the cold-rolled base material 100 at a temperature of about 700°C or higher. For example, the annealing step (S400) also includes a step of heating the cold-rolled base material 100 and cooling the heated base material 100 at a predetermined cooling rate.

[0080] In one embodiment, annealing can be performed on the base material in the annealing step (S400). The annealing step (S400) can be carried out in an annealing furnace.

[0081] The annealing of the base material 100 can be carried out in an atmosphere composed of about 0.5 vol% to about 25 vol% hydrogen and the balance nitrogen. At this time, water can be injected into the annealing furnace together with hydrogen gas and nitrogen gas. When water is injected into the annealing furnace, the dew point temperature of the annealing furnace can be increased. Therefore, the amount of water injected into the annealing furnace can be adjusted to adjust the dew point temperature of the annealing furnace.

[0082] In one embodiment, when the dew point temperature of the annealing furnace rises, a decarburized layer 200 can be formed on the base material 100. For example, carbon can disappear from the surface of the base material 100, and the decarburized layer 200 can be formed. Also, an internal oxide layer 300 can be simultaneously formed on the decarburized layer 200. That is, the decarburized layer 200 can be formed on the surface of the base material 100, and the internal oxide layer 300 can be formed on the surface of the decarburized layer 200. At this time, the decarburized layer 200 and the internal oxide layer 300 are also layers in which a part of the base material 100 has changed.

[0083] At this time, a layer in which the hardness of the average hardness ratio at about 1 / 4 point from the surface of the base material 100 is about 80% or less can be defined as the decarburized layer 200. That is, the average hardness of the decarburized layer 200 is also about 80% or less in terms of the average hardness ratio at about 1 / 4 point from the surface of the base material 100.

[0084] In one embodiment, the dew point temperature of the annealing furnace in which the annealing of the base material 100 is performed is also about -15°C to about +15°C. For the purpose of improving the toughness of the manufactured hot stamping part 1, a decarburized layer 200 is formed on the base material 100. However, when the dew point temperature of the annealing furnace is about -15°C or lower, the depth (or thickness) of the formed decarburized layer 200 is excessively thin, and the effect of improving the toughness of the manufactured hot stamping part is negligible. On the other hand, when the dew point temperature of the annealing furnace is about +15°C or higher, the depth (or thickness (t3)) of the formed internal oxide layer 300 is excessively large, LME cracks are induced, and the operability due to equipment oxidation can be reduced. For example, in order to increase the dew point temperature of the annealing furnace, a large amount of water must be supplied to the annealing furnace. However, when a large amount of water is supplied to the annealing furnace, the equipment of the annealing furnace is oxidized, and it takes a long time to clean it, and the operability can be reduced. Also, if the dew point temperature of the annealing furnace is high, the depth (or thickness) of the formed decarburized layer 200 and the depth (or thickness (t3)) of the internal oxide layer 300 increase, and cracks can occur inside due to the internal oxide layer 300 during hot forming. Therefore, when the dew point temperature of the annealing furnace in which the annealing of the base material 100 is performed satisfies about -15°C to about +15°C, the efficiency of the manufacturing process can be improved while improving the toughness of the manufactured hot stamping part 1.

[0085] In one embodiment, the line speed of the annealing furnace in which the annealing of the base material 100 is performed is also about 30 mpm (meters per minute) to about 200 mpm. When the line speed of the annealing furnace is about 30 mpm or lower, the moving speed of the base material 100 is excessively slow, and the productivity decreases sharply. When the line speed of the annealing furnace is about 200 mpm or higher, the residence time of the base material 100 in the annealing furnace is excessively short, the depth (or thickness) of the decarburized layer 200 is reduced, and thereby the effect of improving the toughness of the manufactured hot stamping part is negligible. Therefore, when the line speed of the annealing furnace in which the annealing of the base material 100 is performed satisfies about 30 mpm to about 200 mpm, the productivity of the hot stamping part can be improved, and at the same time, the toughness of the manufactured hot stamping part can be improved.

[0086] In one embodiment, the annealing temperature of the base material 100 is also about 750°C to about 900°C. When the annealing temperature of the base material 100 is less than about 750°C, the desired structure cannot be obtained and recrystallization is not sufficiently completed. On the other hand, when the annealing temperature of the base material 100 exceeds about 900°C, the annealing temperature is excessively high and the efficiency of the manufacturing process may be reduced. Therefore, when the annealing temperature of the base material 100 satisfies about 750°C to about 900°C, the desired structure can be obtained, recrystallization is sufficiently completed, and the efficiency of the manufacturing process can be improved.

[0087] The plating step (S500) is also a step of forming a plating layer 400 on the annealed base material 100. In one embodiment, a plating layer 400 can be formed on the annealed base material 100 through the plating step (S500). Specifically, through the plating step (S500), a plating layer 400 can be formed on the surface of the internal oxide layer 300. At this time, the plating layer 400 includes a zinc (Zn)-based plating layer or an aluminum (Al)-based plating layer.

[0088] Specifically, in the plating step (S500), the annealed base material 100 can be immersed in a plating bath. At this time, the plating bath can maintain a temperature of about 400°C to about 700°C. The plating adhesion amount is about 40 g / m 2 to about 200 g / m 2 also.

[0089] In one embodiment, the depth (or thickness (t4)) of the plating layer 400 formed on the base material 100 or the decarburized layer 200 is also about 5 μm to about 20 μm in the thickness direction of the base material 100. When the depth (or thickness) of the plating layer 400 is about 5 μm or less, the sacrificial function of the plating layer 400 is insufficient. When the depth (or thickness) of the plating layer 400 is about 20 μm or more, the cost of forming the plating layer 400 increases, and the economic efficiency may be reduced. Therefore, when the depth (or thickness (t4)) of the plating layer 400 satisfies about 5 μm to about 20 μm, corrosion of the base material 100 of the hot stamping part 1 can be prevented or minimized.

[0090] In one embodiment, the annealing step (S400) and the plating step (S500) can be performed on the same line. Therefore, the line speed at which the plating step (S500) is performed is also about 30 mpm to about 200 mpm. When the line speed is about 30 mpm or less, the line speed is excessively slow, and the productivity may be reduced. Although an air knife is used to control the plating amount, when the line speed is about 200 mpm or more, the line speed is excessively fast, and it becomes difficult to control the plating amount using the air knife. Therefore, when the line speed at which the plating step (S500) is performed satisfies about 30 mpm to about 200 mpm, the productivity can be improved, and at the same time, the plating amount can be easily controlled.

[0091] In one embodiment, a plated steel sheet in which a plating layer 400 is formed on at least one surface of the base material 100 can be manufactured through the plating step (S500). At this time, the plated steel sheet includes the base material 100, the decarburized layer 200 formed on the base material 100, the internal oxide layer 300 formed on the decarburized layer 200, and the plating layer 400 formed on the internal oxide layer 300. Specifically, the plated steel sheet includes the base material 100, the decarburized layer 200 formed on the surface of the base material 100, the internal oxide layer 300 formed on the surface of the decarburized layer 200, and the plating layer 400 formed on the surface of the internal oxide layer 300.

[0092] FIG. 6 is a flowchart schematically showing a hot stamping step according to an embodiment of the present invention, and FIG. 7 is a flowchart schematically showing a heating step according to an embodiment of the present invention.

[0093] Referring to FIGS. 6 and 7, after the plating step (S500) (FIG. 2), the hot stamping step (S600) can be performed. The hot stamping step (S600) includes a heating step (S610), a transfer step (S620), a forming step (S630), and a cooling step (S640).

[0094] First, a plated steel sheet having a plating layer 400 (FIG. 5) formed on at least one surface of a base material 100 (FIG. 5) can be cut to form a blank. At this time, a decarburized layer 200 (FIG. 5) and an internal oxide layer 300 (FIG. 5) may exist between the base material 100 and the plating layer 400.

[0095] In the heating step (S610), the blank can be heated in a heating furnace having a plurality of sections with different temperature ranges. As shown in FIG. 7, the heating step (S610) includes a multi-stage heating step (S611) and a soaking heating step (S612). The multi-stage heating step (S611) and the soaking heating step (S612) are also steps in which the blank is heated by passing through a plurality of sections provided in the heating furnace.

[0096] In one embodiment, the overall temperature of the heating furnace is also about 680°C to about 910°C. Specifically, the overall temperature of the heating furnace in which the multi-stage heating step (S611) and the soaking heating step (S612) are performed is also about 680°C to about 910°C. At this time, the temperature of the heating furnace in which the multi-stage heating step (S611) is performed is also about 680°C to about Ac1, and the temperature of the heating furnace in which the soaking heating step (S612) is performed is also about Ac1 to about 910°C.

[0097] In the multi-stage heating step (S611), the blank can pass through a plurality of sections provided in the heating furnace and be heated (or the temperature can be raised) step by step. In the plurality of sections provided in the heating furnace, there are a plurality of sections where the multi-stage heating step (S611) is performed. From the inlet of the heating furnace where the blank is loaded to the outlet direction of the heating furnace where the blank is taken out, the temperature is set for each section so that the temperature increases, and the blank can be heated (or the temperature can be raised) step by step.

[0098] After the multi-stage heating step (S611), a soaking heating step (S612) can be performed. In the soaking heating step (S612), the blank that has been multi-stage heated can pass through a section of the heating furnace set at a temperature of about Ac1 to about 910 °C and be heated (or soaked and heated). In the plurality of sections provided in the heating furnace, there is at least one section where the soaking heating step (S612) is performed.

[0099] FIG. 8 is a drawing illustrated to explain a heating furnace having a plurality of sections in the heating step of the method for manufacturing a hot stamping part according to an embodiment of the present invention.

[0100] Referring to FIG. 8, a heating furnace according to an embodiment can include a plurality of sections having different temperature ranges. 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.

[0101] In one embodiment, in the multi-stage heating step (S611), the blank can pass through the first section P1 to the fourth section P4 defined in the heating furnace and be heated step by step. Also, in the soaking heating step (S612), the blank that has been multi-stage heated in the first section P1 to the fourth section P4 can pass through the fifth section P5 to the seventh section P7 and be soaked and heated.

[0102] 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 is adjacent to the inlet of the heating furnace into which the blank is charged, and the seventh section P7 having the seventh temperature range T7 can be adjacent to the outlet of the heating furnace from which 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.

[0103] The temperatures of a plurality of sections provided in the heating furnace, for example, the temperatures of the first section P1 to the seventh section P7, can rise from the inlet of the heating furnace into which the blank is charged in the direction of the outlet of the heating furnace from which the blank is taken out. However, the temperatures of the fifth section P5, the sixth section P6, and the seventh section P7 are the same. Also, in a plurality of sections provided in the heating furnace, the temperature difference between two adjacent sections is greater than 0°C and at most about 100°C. For example, the temperature difference between the first section P1 and the second section P2 is greater than 0°C and at most about 100°C.

[0104] The heating furnace temperature in the soaking heating stage (S612) is also about Ac1 to about 910°C. When the heating furnace temperature in the soaking heating stage (S612) is at most about Ac1, the manufactured hot stamping part does not have the desired material. On the other hand, when the heating furnace temperature in the soaking heating stage (S612) is at least about 910°C, the zinc (Zn) contained in the plating layer 400 is vaporized, and loss of the plating layer 400 can occur. Therefore, when the heating furnace temperature in the soaking heating stage (S612) satisfies about Ac1 to about 910°C, the manufactured hot stamping part can be formed of the desired material, and loss of the plating layer 400 can be prevented.

[0105] In FIG. 8, the heating furnace according to one embodiment is illustrated as including seven sections having different temperature ranges, but the present invention is not limited thereto. The heating furnace may also include five, six, or eight such sections having different temperature ranges.

[0106] In one embodiment, since the heating stage (S610) is provided by a multi-stage heating stage (S611) and an isothermal heating stage (S612), the temperature of the heating furnace can be set step by step, and the energy efficiency of the heating furnace can be improved.

[0107] In one embodiment, the heating furnace may have a length of about 20 m to about 40 m along the transfer path of the blank. The heating furnace includes a plurality of sections having different temperature ranges. In the plurality of sections, the ratio of the length of the section where the blank is multi-stage heated to the length of the section where the blank is isothermally heated may satisfy about 1:1 to about 4:1. In the heating furnace, when the length of the section where the blank is isothermally heated increases and the ratio of the length of the section where the blank is multi-stage heated to the length of the section where the blank is isothermally heated exceeds about 1:1, the amount of hydrogen permeating into the blank increases in the isothermal heating section, and delayed fracture may increase. On the other hand, when the length of the section where the blank is isothermally heated decreases and the ratio of the length of the section where the blank is multi-stage heated to the length of the section where the blank is isothermally heated is less than about 4:1, the isothermal heating section (or time) is not sufficiently ensured, and the strength of the hot stamping parts manufactured by the manufacturing process of the hot stamping parts becomes non-uniform. For example, in the plurality of sections provided in the heating furnace, the length of the isothermal heating section is also about 20% to about 50% of the total length of the heating furnace.

[0108] In one embodiment, the total heating time during which the heating step (S610) is performed is also about 2 minutes (min) to about 20 minutes (min). That is, the total time the blank stays in the heating furnace is also about 2 minutes (min) to about 20 minutes (min). If the total heating time during which the heating step (S610) is performed is less than about 2 minutes (min), the heating time is insufficient, and the manufactured hot stamping part 1 does not have the desired material. On the other hand, if the total heating time during which the heating step (S610) is performed is about 20 minutes (min) or more, the heating time is excessively long, the production speed is reduced, and the economic efficiency may be reduced. Therefore, when the total heating time during which the heating step (S610) is performed satisfies about 2 minutes (min) to about 20 minutes (min), the manufactured hot stamping part 1 has the desired material, and at the same time, the reduction in the economic efficiency of the manufacturing process can be prevented or minimized.

[0109] After the heating step (S610), a transfer step (S620), a forming step (S630), and a cooling step (S640) may be further performed.

[0110] In one embodiment, the transfer step (S620) is also a step of transferring the heated blank from the heating furnace to the mold. At this time, in the transfer step (S620), the heated blank may be cooled to the atmospheric temperature (or normal temperature). That is, the heated blank may 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 buckles) may occur on the surface of the manufactured hot stamping part 1. 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.

[0111] In one embodiment, the forming step (S630) is also a step of hot stamping the transferred blank to form a formed body. Specifically, in the forming step (S400), the blank may be pressed with a mold to form a formed body.

[0112] In one embodiment, the forming start temperature is also about 500°C or higher and about 700°C or lower. If the forming start temperature is less than about 500°C, the forming start temperature is excessively low, the formability of the blank is reduced, and the manufactured hot stamping part 1 cannot have the target structure and physical properties. On the other hand, if the forming start temperature exceeds about 700°C, wrinkles (or buckling) may occur on the surface of the manufactured hot stamping part 1. Also, the plating layer 400 may stick to the mold. Therefore, when the forming start temperature is about 500°C or higher and about 700°C or lower, the formability of the blank is improved, the manufactured hot stamping part 1 has the target structure and physical properties, and wrinkles (or buckling) can be prevented or minimized on the surface of the manufactured hot stamping part 1.

[0113] In one embodiment, the cooling stage (S500) is also the stage of cooling the formed molded body. In the cooling stage (S500), it is also performed inside the mold in which the blank is pressurized.

[0114] Specifically, in the mold, at the same time as forming into the final part shape, the molded body can be cooled to form the final product. The mold may be provided with cooling channels through which a refrigerant circulates inside. By using the refrigerant supplied through the cooling channels provided in the mold, the molded body can be rapidly cooled by circulation. At this time, in order to prevent the spring back phenomenon of the plate material and maintain the desired shape, rapid cooling can be performed while pressurizing with the mold closed. When performing the forming operation and the cooling operation on the molded body, it can be cooled with an average cooling rate of at least about 10°C / s until the martensite finish temperature.

[0115] In one embodiment, the cooling end temperature at the end of the cooling stage (S640) is also approximately room temperature or higher and about 200°C or lower. If the cooling end temperature is less than room temperature, the productivity of the manufacturing process may be reduced. On the other hand, if the cooling end temperature exceeds about 200°C, the manufactured hot stamping part 1 is air-cooled at room temperature. At this time, however, the hot stamping part 1 is bent, and it is also difficult to ensure the target material. Therefore, when the cooling end temperature at the end of the cooling stage (S640) satisfies the range of room temperature or higher and about 200°C or lower, the productivity of the manufacturing process can be improved, and the occurrence of bending in the manufactured hot stamping part 1 can be prevented or minimized.

[0116] Thereby, the present inventor derived Relational Expression 3 and Relational Expression 4 so that the manufactured hot stamping part 1 manufactured through excessively repeated experiments has a VDA bending angle of about 60° or more. In one embodiment, the manufactured hot stamping part 1 may satisfy the following Relational Expression 3 and the following Relational Expression 4. Specifically, the hardness within a depth (or thickness) of 50 μm in the plate thickness direction of the hot stamping part 1 from the surface of the hot stamping part 1 and the average hardness of the hot stamping part 1 satisfy the following Relational Expression 3, and the depth (or thickness) of the internal oxide layer 300 may satisfy the following Relational Expression 4. For example, the hot stamping part 1 may satisfy both Relational Expression 3 and Relational Expression 4.

[0117] [Relational Expression 3] (A / B) ≦ 0.7 In Relational Expression 3, A is the hardness (Hv(≦50μm)) within a depth (or thickness) of about 50 μm in the plate thickness direction of the hot stamping part 1, and B is the average hardness (Hv(avg.)) of the hot stamping part 1.

[0118] At this time, the hardness within about 50 μm from the surface of the hot stamping part 1 in the thickness direction of the hot stamping part 1 is also the hardness value measured by a Vickers hardness tester at a point within about 50 μm or less from the surface of the hot stamping part 1 in the thickness direction of the hot stamping part 1, and the average hardness (Hv(avg.)) of the hot stamping part 1 is also the hardness value measured by a Vickers hardness tester at about 1 / 4 point in the thickness direction of the hot stamping part 1.

[0119] [Relational Expression 4] C ≦ 5 μm In Relational Expression 4, C is the depth (or thickness) of the internal oxide layer 300 in the thickness direction of the hot stamping part 1.

[0120] In one embodiment, when the manufactured hot stamping part 1 satisfies the above-mentioned Relational Expression 3 and Relational Expression 4, the toughness of the hot stamping part 1 manufactured through the manufacturing method of the hot stamping part can be improved. For example, the hot stamping part 1 manufactured through the manufacturing method of the hot stamping part can have a VDA bending angle of about 60° or more. Also, the hot stamping part 1 manufactured through the manufacturing method of the hot stamping part can have a tensile strength (TS) of about 1,350 MPa to about 1,680 MPa, a yield stress (YP) of about 900 MPa to about 1,300 MPa, and an elongation (EL) of about 4% to about 10%.

[0121] <Experimental Example> In the following, the present invention will be described in more detail through experimental examples. However, the following experimental examples are for more specifically explaining the present invention, and the scope of the present invention is not limited by the following experimental examples. The following experimental examples can be appropriately modified and changed by those skilled in the art within the scope of the present invention.

[0122]

Table 1

[0123]

Table 2

[0124] Example 1, Example 2, Comparative Example 1, and Comparative Example 2 are hot stamping parts (specimens) manufactured from slabs having the compositions described in Table 1 through the process conditions described in Table 2.

[0125]

Table 3

[0126] In Table 3, A is the hardness (Hv(≦50μm)) within a depth (or thickness) of about 50 μm in the thickness direction of the hot stamping part, B is the average hardness (Hv(avg.)) of the hot stamping part, and C is the depth (or thickness) of the internal oxide layer 300 in the thickness direction of the hot stamping part. At this time, the hardness within a depth of about 50 μm in the thickness direction of the hot stamping part 1 from the surface of the hot stamping part 1 is also the hardness value measured with a Vickers hardness tester at a point within a depth of about 50 μm or less in the thickness direction of the hot stamping part 1 from the surface of the hot stamping part 1, and the average hardness (Hv(avg.)) of the hot stamping part 1 is also the hardness value measured with a Vickers hardness tester at about the 1 / 4 point in the thickness direction of the hot stamping part 1.

[0127] In Table 3, the VDA bending angle is evaluated by utilizing the VDA standard (VDA238 - 100), and the crack depth is measured through a scanning electron microscope. At this time, the crack depth corresponds to the deepest crack depth measured through the scanning electron microscope.

[0128] The VDA bending angle of the hot stamping parts required in the present invention is about 60° or more. Also, if the crack depth in the hot stamping parts is deep, the VDA bending angle of the hot stamping parts becomes small, and the toughness of the hot stamping parts may be reduced. Therefore, the crack depth in the hot stamping parts required in the present invention is 10 μm or less. When outside the above range, it corresponds to the case where the required conditions cannot be satisfied.

[0129] In the cases of Example 1 and Example 2, it corresponds to the case where both Relational Expression 1 ((A / B) ≤ 0.7) and Relational Expression 2 (C ≤ 5) are satisfied. Comparative Example 1 corresponds to the case where Relational Expression 2 (C ≤ 5) is not satisfied. Comparative Example 2 corresponds to the case where Relational Expression 1 ((A / B) ≤ 0.7) is not satisfied. Example 1, Example 2, Comparative Example 1, and Comparative Example 2 correspond to specimens manufactured by the manufacturing method of hot stamping parts using the base material 100 (or steel sheet) that satisfies the composition described in FIG. 1. However, Comparative Example 1 and Comparative Example 2 are specimens that cannot satisfy Relational Expression 1 ((A / B) ≤ 0.7) and / or Relational Expression 2 (C ≤ 5) due to differences in process control conditions.

[0130] When both Relational Expression 1 ((A / B) ≤ 0.7) and Relational Expression 2 (C ≤ 5) are satisfied, it can be confirmed that the VDA bending angle and the crack depth satisfy the required conditions. Specifically, when both Relational Expression 1 ((A / B) ≤ 0.7) and Relational Expression 2 (C ≤ 5) are satisfied, it can be confirmed that the VDA bending angle is 60° or more and the crack depth is 10 μm or less.

[0131] However, when Relational Expression 2 (C ≤ 5) is not satisfied, it can be confirmed that the VDA bending angle and the crack depth do not satisfy the required conditions. Specifically, when Relational Expression 2 (C ≤ 5) is not satisfied, it can be confirmed that the VDA bending angle is less than 60° and the crack depth exceeds 10 μm.

[0132] Also, when the relational expression 1 ((A / B) ≤ 0.7) is not satisfied, it can be confirmed that the VDA bending angle does not satisfy the required conditions. Specifically, when the relational expression 1 ((A / B) ≤ 0.7) is not satisfied, it can be confirmed that the VDA bending angle is less than 60°.

[0133] Therefore, when the hot stamping part 1 satisfies both the relational expression 1 ((A / B) ≤ 0.7) and the relational expression 2 (C ≤ 5), the hot stamping part 1 has the VDA bending angle required for the hot stamping part 1, and the crack depth in the hot stamping part 1 can be formed to be equal to or less than a pre-set value. That is, when the hot stamping part 1 satisfies both the relational expression 1 ((A / B) ≤ 0.7) and the relational expression 2 (C ≤ 5), the toughness and hot formability of the hot stamping part 1 are both excellent.

[0134] The present invention has been described with reference to the embodiments illustrated in the drawings, but they are merely exemplary, and those having ordinary knowledge in the relevant technical field will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention is defined by the technical idea of the appended claims.

Claims

1. These are hot stamping parts. Base material and, A decarburized layer placed on the aforementioned base material, The decarburized layer includes an internal oxide layer disposed on the decarburized layer, The aforementioned base material contains, by weight percent, carbon (C) 0.15% to 0.3%; silicon (Si) 0.05% to 0.8%; manganese (Mn) 0.8% to 3.0%; phosphorus (P) more than 0% but less than 0.1%; sulfur (S) more than 0% but less than 0.1%; chromium (Cr) 0.1% to 0.9%; boron (B) 0.001% to 0.005%; the remainder being iron (Fe); and other unavoidable impurities. The hot stamped part has a tensile strength (TS) of 1,350 MPa to 1,680 MPa. The hardness within a depth of 50 μm from the surface of the hot stamped part in the thickness direction of the hot stamped part, and the average hardness of the hot stamped part, satisfy the following relational expression 1. The depth of the internal oxide layer satisfies the following relational equation 2 for hot stamping parts: [Relationship 1] (A / B) ≤ 0.7 (In relational equation 1, A is the hardness (Hv (≤ 50 μm)) within a depth of 50 μm in the thickness direction of the hot stamped part, and B is the average hardness (Hv (avg.)) at a point 1 / 4 of the way from the surface of the hot stamped part in the thickness direction). [Relationship 2] C ≤ 5 μm (In relational equation 2, C is the depth of the internal oxide layer in the thickness direction of the hot stamped part.)

2. The hot stamping part according to claim 1, wherein the hot stamping part has a VDA bending angle of 60° or more.

3. The hot stamping part according to claim 1, wherein the hot stamping part has a yield strength (YP) of 900 MPa to 1,300 MPa and an elongation (EL) of 4% to 10%.

4. The hot stamping part according to claim 1, wherein the hot stamping part has a microstructure containing martensite at a fraction of 90% or more.

5. A method for manufacturing hot stamped parts, The steps include cutting a plated steel sheet, on which a plating layer has been formed on at least one surface of a base material that has been annealed in an annealing furnace, to form a blank, The process includes the step of heating the blank in a heating furnace having multiple sections having different temperature ranges, The dew point temperature of the annealing furnace for the base material is between -15°C and +15°C. The annealing temperature of the base material is 750°C to 900°C. The aforementioned base material contains, by weight percent, carbon (C) 0.15% to 0.3%; silicon (Si) 0.05% to 0.8%; manganese (Mn) 0.8% to 3.0%; phosphorus (P) more than 0% but less than 0.1%; sulfur (S) more than 0% but less than 0.1%; chromium (Cr) 0.1% to 0.9%; boron (B) 0.001% to 0.005%; the remainder being iron (Fe); and other unavoidable impurities. The step of heating the blank is as follows: A multi-stage heating step in which the blank is heated in stages, The process includes a soaking heating step in which the multi-stage heated blank is heated to a temperature of Ac1 to 910°C, A method for manufacturing a hot stamping part, wherein the hardness within a depth of 50 μm from the surface of the hot stamping part in the thickness direction of the hot stamping part and the average hardness of the hot stamping part satisfy the following relational expression 3: [Relationship 3] (A / B) ≤ 0.7 (In relational equation 3, A is the hardness (Hv (≤ 50 μm)) within a depth of 50 μm in the thickness direction of the hot stamped part, and B is the average hardness (Hv (avg.)) at a point 1 / 4 of the way from the surface of the hot stamped part in the thickness direction).

6. After the step of heating the blank, The steps include transporting the heated blank, The steps include: pressing the transferred blank into a mold to form a molded body, A method for manufacturing a hot stamped part according to claim 5, further comprising the step of cooling the molded body.

7. A decarburized layer formed on the base material, The method for manufacturing a hot stamping part according to claim 6, further comprising an internal oxide layer formed on the decarburized layer.

8. The depth of the internal oxide layer satisfies the following relational expression 4, as described in the method for manufacturing a hot stamped part according to claim 7: [Relationship 4] C ≤ 5 μm (In the relational equation 4 above, C is the depth of the internal oxide layer in the thickness direction of the hot stamped part.)

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