Hot stamping parts

Abstract

According to one aspect of the present invention, there is provided a hot stamping part including a steel sheet containing carbon (C): 0.26 to 0.40 wt%, silicon (Si): 0.02 to 2.0 wt%, manganese (Mn): 0.3 to 1.60 wt%, phosphorus (P): 0.03 wt% or less, sulfur (S): 0.008 wt% or less, chromium (Cr): 0.05 to 0.90 wt%, boron (B): 0.0005 to 0.01 wt%, molybdenum (Mo): 0.05 to 0.2 wt%, titanium (Ti): 0.001 to 0.095 wt%, niobium (Nb): 0.001 to 0.095 wt%, vanadium (V): 0.001 to 0.095 wt%, and the balance of iron (Fe) and other inevitable impurities, having a tensile strength of 1,700 MPa or more and a yield strength of 1,150 MPa or more, wherein the hot stamping part includes a microstructure containing austenite crystal grains and carbon-based precipitates containing at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V), and the average size of the austenite crystal grains is 15 μm or less.

Description

Technical Field

[0001] The present invention relates to hot stamping parts, and more particularly, to hot stamping parts in which the formed parts after hot stamping have excellent mechanical properties of high strength and high toughness.

Background Art

[0002] For automotive parts, high-strength steel is applied for weight reduction and stability. Although the high-strength steel can ensure high-strength characteristics in terms of weight ratio, as the strength increases, the press formability decreases, and during processing, the material may break or the springback phenomenon may occur, making it difficult to form products with complex and precise shapes.

[0003] As a solution to such problems, there is a hot stamping method, and while the interest in it is increasing, research on hot stamping materials is also actively carried out. For example, as disclosed in the invention of Korean Patent Publication No. 10-2017-0076009, the hot stamping method is a forming technique that heats a hot stamping steel sheet to a high temperature and then forms it in a press die while rapidly cooling to manufacture high-strength parts.

[0004] Also, as disclosed in the invention of Korean Patent Publication No. 10-2019-0095858, as a typical example of a hot stamping steel sheet, so-called boron steel (22MnB5) containing carbon (C) and elements such as manganese (Mn) and boron (B) for improving heat treatment performance is used.

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in such conventional hot stamping parts, due to the components contained in the hot stamping steel sheet and the strength difference by region caused by the microstructure, there is a problem that the mechanical properties such as the tensile strength and bending characteristics of the formed parts after hot stamping are deteriorated.

[0006] Embodiments of the present invention are for solving various problems including the above-described problems, and provide hot stamping parts in which the formed parts after hot stamping have excellent mechanical properties of high strength and high toughness. However, such problems are exemplary, and thereby, the scope of the present invention is not limited.

Means for Solving the Problems

[0007] According to one aspect of the present invention, in a hot stamping part including a steel sheet containing carbon (C): 0.26 to 0.40 wt%, silicon (Si): 0.02 to 2.0 wt%, manganese (Mn): 0.3 to 1.60 wt%, phosphorus (P): 0.03 wt% or less, sulfur (S): 0.008 wt% or less, chromium (Cr): 0.05 to 0.90 wt%, boron (B): 0.0005 to 0.01 wt%, molybdenum (Mo): 0.05 to 0.2 wt%, titanium (Ti): 0.001 to 0.095 wt%, niobium (Nb): 0.001 to 0.095 wt%, vanadium (V): 0.001 to 0.095 wt%, and the balance iron (Fe) and other inevitable impurities, and having a tensile strength of 1,700 MPa or more and a yield strength of 1,150 MPa or more, the hot stamping part includes a microstructure containing austenite crystal grains and a carbon-based precipitate containing at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V), and the average size of the austenite crystal grains is 15 μm or less.

Effects of the Invention

[0008] According to one embodiment of the present invention made as described above, the formed part after hot stamping can embody a hot stamping part having excellent mechanical properties of high strength and high toughness.

[0009] Specifically, by controlling the properties of the components, microstructure, and precipitates contained in the hot stamping steel sheet, the strength difference by region of the hot stamping steel sheet is adjusted, and a hot stamping steel sheet excellent in mechanical properties such as the tensile strength and bending properties of the formed part after hot stamping and a manufacturing method thereof can be embodied. According to one embodiment of the present invention, it goes without saying that the scope of the present invention is not limited by such effects.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0011] According to the present embodiment, in the austenite grains, the fraction having a size of 10 μm or more is also 67% or less.

[0012] According to the present embodiment, in the austenite grains, the fraction having a size of 20 μm or more is also 10% or less.

[0013] According to this embodiment, when the contents of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) contained in the steel sheet are represented by [Ti], [Nb], [V], and [Mo] in terms of weight %, the following formula (1) can be satisfied. [Formula (1)] 0.015 ≦ 0.25([Ti] + [Nb] + [V] + 0.25[Mo]) ≦ 0.060 (unit: wt%)

[0014] According to this embodiment, the amount of active hydrogen in the hot stamping part is also 0.6 wppm or less.

[0015] According to this embodiment, the average particle size of the precipitate is also 10 nm or less.

[0016] According to this embodiment, the precipitate contains 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo).

[0017] According to this embodiment, the average number of precipitates per unit area is 10,000 pieces / 100 μm 2 to 35,000 pieces / 100 μm 2 or less.

[0018] According to this embodiment, the average interval between the precipitates is also 0.1 nm to 100 nm.

[0019] According to this embodiment, the hot stamping part can satisfy a bending angle of 50° or more.

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

[0021] The present invention can be subjected to various transformations and may have various embodiments. Specific embodiments are illustrated in the drawings and will be described in detail by a detailed description. 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.

[0022] 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.

[0023] In the following embodiments, singular expressions include plural expressions unless the context clearly dictates otherwise.

[0024] 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.

[0025] In the drawings, for convenience of explanation, the sizes of components may be exaggerated or reduced. For example, since the sizes and thicknesses of each configuration shown in the drawings are arbitrarily shown for convenience of explanation, the present invention is not necessarily limited to what is shown.

[0026] When a certain embodiment can be embodied otherwise, a specific process order may be performed differently from the order described. For example, two processes described continuously may be performed substantially simultaneously or may be advanced in an order opposite to the order described.

[0027] In this specification, "A and / or B" indicates the case of being A, being B, or being both A and B. And "at least one of A and B" indicates the case of being A, being B, or being both A and B.

[0028] 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, and redundant descriptions thereof are omitted.

[0029] FIG. 1 is an image showing a part of the microstructure of a steel sheet before hot stamping according to an embodiment of the present invention.

[0030] Specifically, FIG. 1 shows a hot stamping steel sheet manufactured by controlling so as to satisfy the conditions in which the content of substances constituting the hot stamping steel sheet, the structure of the microstructure of the hot stamping steel sheet, and the process conditions for manufacturing the hot stamping steel sheet are preset.

[0031] The steel sheet of this embodiment is also a steel sheet manufactured by performing a hot rolling process and / or a cold rolling process on a slab cast to contain a predetermined alloy element in a predetermined content.

[0032] The steel sheet contains carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), boron (B), calcium (Ca), molybdenum (Mo), titanium (Ti), niobium (Nb), vanadium (V), and the remaining iron (Fe), as well as other inevitable impurities. As one embodiment, the hot stamping steel sheet contains carbon (C): 0.26 to 0.40 wt%, silicon (Si): 0.02 to 2.0 wt%, manganese (Mn): 0.3 to 1.60 wt%, phosphorus (P): 0.03 wt% or less, sulfur (S): 0.008 wt% or less, chromium (Cr): 0.05 to 0.90 wt%, boron (B): 0.0005 to 0.01 wt%, molybdenum (Mo): 0.05 to 0.2 wt%, titanium (Ti): 0.001 to 0.095 wt%, niobium (Nb): 0.001 to 0.095 wt%, vanadium (V): 0.001 to 0.095 wt%, and the remaining iron (Fe), as well as other inevitable impurities. Optionally, it may further contain calcium (Ca): 0.00001 to 0.0060 wt%.

[0033] Carbon (C) acts as an austenite stabilizing element in the steel sheet. Carbon is a major element that determines the strength and hardness of the steel sheet and is added during heat treatment for the purpose of increasing hardenability and strength. Such carbon can be contained in an amount of 0.26 wt% to 0.40 wt% based on the total weight of the steel sheet. When the carbon content is less than 0.26 wt%, it is difficult to ensure a hard phase (e.g., martensite, etc.), and it is difficult to satisfy the mechanical strength of the formed parts after hot stamping. On the contrary, when the carbon content exceeds 0.40 wt%, it may cause a decrease in the workability of the steel sheet or a decrease in the bending performance of the formed parts after hot stamping.

[0034] Silicon (Si) acts as a ferrite stabilizing element in the steel sheet. Silicon is a solid solution strengthening element that improves the strength of the steel sheet and improves the carbon concentration in austenite by suppressing the formation of carbides in the low-temperature range. Also, silicon is a core element for hot rolling, cold rolling, hot press structure homogenization, and fine dispersion of ferrite. Silicon acts as an element for controlling the non-uniformity of martensite strength and plays a role in improving the collision performance. Such silicon can be contained in an amount of 0.02 wt% to 2.0 wt% based on the total weight of the steel sheet. When the content of silicon is less than 0.02 wt%, it is difficult to obtain the above-mentioned effects, and in the martensite structure of the formed parts after hot stamping, cementite formation and coarsening will occur. On the contrary, when the content of silicon exceeds 2.0 wt%, the hot rolling load and cold rolling load increase, and the plating characteristics of the steel sheet may be deteriorated.

[0035] Note that when silicon (Si) is preferably added in an amount of 0.3 wt% or more, it suppresses the excessive formation of pearlite regions where pearlite is accumulated in the hot stamping steel sheet, so that the pearlite regions are formed in the hot stamping steel sheet with a minimum content.

[0036] Manganese (Mn) acts as an austenite stabilizing element in the steel sheet. Manganese is added for the purpose of increasing hardenability and strength during heat treatment. Such manganese may be contained in an amount of 0.3 wt% to 1.60 wt% based on the total weight of the steel sheet. When the manganese content is less than 0.3 wt%, the hardening effect is insufficient, and due to insufficient hardenability, the fraction of hard phases in the formed parts after hot stamping is also insufficient. On the other hand, when the manganese content exceeds 1.60 wt%, regions concentrated with pearlite in which manganese is segregated are formed, ductility and toughness are reduced, which causes a reduction in the bending performance of the formed parts after hot stamping, and a non-uniform fine microstructure is formed.

[0037] Phosphorus (P) is an element that contributes to strength improvement. Such phosphorus may be contained in an amount exceeding 0 and not exceeding 0.03 wt% based on the total weight of the steel sheet in order to prevent a reduction in the toughness of the steel sheet. When the phosphorus content exceeds 0.03 wt%, iron phosphide compounds are formed, toughness and weldability are reduced, and cracks may be induced in the steel sheet during the manufacturing process.

[0038] Sulfur (S) is an element that contributes to improvement of workability. Such sulfur may be contained in an amount exceeding 0 and not exceeding 0.008 wt% based on the total weight of the steel sheet. If the sulfur content exceeds 0.008 wt%, hot workability, weldability and impact properties are reduced, and surface defects such as cracks are formed due to the formation of large inclusions.

[0039] Chromium (Cr) is added for the purpose of improving hardenability and strength during heat treatment. Chromium enables grain refinement and strength retention through precipitation hardening. Such chromium may be contained in an amount of 0.05 wt% to 0.9 wt% based on the total weight of the steel sheet. When the chromium content is less than 0.05 wt%, the precipitation hardening effect is low. On the contrary, when the chromium content exceeds 0.9 wt%, the amount of Cr-based precipitates and matrix solid solution increases, toughness is reduced, and production costs may increase due to cost increase.

[0040] Boron (B) is added for the purpose of ensuring hardenability and strength during heat treatment by suppressing the transformation of ferrite, pearlite, and bainite and ensuring a martensite structure. Also, boron segregates at the grain boundaries, lowers the grain boundary energy, increases the hardenability, and has a grain refinement effect due to the increase in the austenite grain growth temperature. Such boron can be contained in an amount of 0.0005 wt% to 0.01 wt% based on the total weight of the steel sheet. When boron is contained within the above range, the occurrence of hard phase grain boundary brittleness can be prevented, and high toughness and bendability can be ensured. When the boron content is less than 0.0005 wt%, the hardenability effect is insufficient. On the contrary, when the boron content exceeds 0.01 wt%, the solubility is low, and it is easily precipitated at the grain boundaries depending on the heat treatment conditions, resulting in deteriorated hardenability or causing high-temperature embrittlement, and the toughness and bendability may be reduced due to the occurrence of hard phase grain boundary brittleness.

[0041] Calcium (Ca) can be added for the control of precipitates. Calcium has a high binding force with sulfur and can suppress the formation of MnS, which inhibits weldability, by forming CaS precipitates. Such calcium can be contained in an amount of 0.00001 wt% to 0.006 wt% based on the total weight of the steel sheet. When the calcium content is less than 0.00001 wt%, the MnS control effect is reduced. When the calcium content exceeds 0.006 wt%, the continuous casting property may be reduced.

[0042] Titanium (Ti) forms precipitates at high temperatures and can effectively contribute to grain refinement. Such titanium can be contained in an amount of 0.001 wt% to 0.095 wt%, preferably 0.005 wt% to 0.06 wt% based on the total weight of the steel sheet. If titanium is contained within the above content range, continuous casting defects and coarsening of precipitates can be prevented, the physical properties of the steel material can be easily ensured, and defects such as crack generation on the surface of the steel material can be prevented. When the titanium content does not reach the above lower limit, the above effects cannot be correctly exerted. On the other hand, when the titanium content exceeds the above upper limit, the precipitates are coarsened, resulting in a decrease in elongation and bendability.

[0043] Niobium (Nb) and vanadium (V) can increase the strength and toughness by reducing the martensite packet size. Such niobium and vanadium can be contained in an amount of 0.005 wt% to 0.06 wt% respectively based on the total weight of the steel plate. When niobium is contained within the above range, in the processes of hot rolling and cold rolling, it has an excellent effect of refining the crystal grains of the steel plate, preventing the occurrence of slab cracks and brittle fractures of the product during steelmaking / continuous casting, and minimizing the formation of coarse precipitates during steelmaking. If the content of niobium is less than 0.005 wt%, the above effects cannot be exerted correctly. On the other hand, if the content of niobium exceeds 0.06 wt%, the strength and toughness due to the increase in niobium content will not be further improved, and it exists in a solid solution state in ferrite, and instead, there is a risk of reducing the impact toughness. Vanadium can also show a similar tendency to the above-mentioned niobium.

[0044] Molybdenum (Mo), as a substitutional element, improves the strength of steel by the solid solution strengthening effect. Molybdenum is added for the purpose of suppressing the coarsening of precipitates and improving the hardenability. In addition, molybdenum (Mo) can play a role in improving the hardening ability of steel. Such molybdenum can be contained in an amount of 0.05 wt% to 0.2 wt% based on the total weight of the steel plate. If the content of molybdenum is less than 0.05 wt%, the above effects cannot be exerted correctly. On the other hand, if the content of molybdenum exceeds 0.2 wt%, there is a risk of a decrease in rolling productivity and elongation, and there is a problem of only increasing the manufacturing cost without further effects.

[0045] The aforementioned titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) can be utilized as elements for controlling the formation of precipitates in the formed parts after hot stamping. As one embodiment, the steel sheet preferably contains titanium (Ti), niobium (Nb), and vanadium (V) in amounts of 0.005 to 0.06 wt% each. Therefore, when titanium (Ti), niobium (Nb), and vanadium (V) are each contained in an amount of 0.005 to 0.06 wt% and molybdenum (Mo) is contained in an amount of 0.05 to 0.2 wt%, it is possible to easily control the microstructure region in the steel sheet before hot stamping, and further, after hot stamping, it can satisfy the conditions for easily controlling the precipitates in the hot-stamped parts.

[0046] As one embodiment, when the content of each of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) contained in the steel sheet is expressed as [Ti], [Nb], [V], and [Mo] in terms of weight%, the following Equation 1 can be satisfied. [Equation 1] 0.015 ≦ 0.25([Ti] + [Nb] + [V] + 0.25[Mo]) ≦ 0.060 (unit: wt%)

[0047] Thereby, the shape of the microstructure formed in the steel sheet can be controlled. The microstructure may include, for example, a region where pearlite structures are locally accumulated (hereinafter, "pearlite region"). The region where pearlite structures are accumulated affects the grain size and fraction coarsening after hot stamping, and also becomes a factor in reducing the hydrogen embrittlement and bending angle (e.g., V-bending angle) performance of the hot-stamped parts after hot stamping. Therefore, when the contents of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) in the steel sheet satisfy Equation 1 before hot stamping, it is possible to easily control the grain refinement of the hot-stamped parts after hot stamping, and through this, ensure the performance of hydrogen embrittlement and bending angle.

[0048] Before hot stamping, the microstructure of the steel sheet also includes ferrite and pearlite. As one embodiment, the steel sheet may include, in terms of area fraction, 50 to 99% ferrite and 0.1 to 50% pearlite. Further, the steel sheet may also include other inevitable microstructures. For example, the steel sheet may include other inevitable microstructures in an amount of 0% or more and less than 5%. Note that, as one embodiment, before hot stamping, the average grain size of the ferrite included in the steel sheet can be controlled to satisfy 2 μm or more and 10 μm or less.

[0049] Carbon (C) and / or manganese (Mn) may be segregated in pearlite, but the microstructure of the hot stamping steel sheet also includes pearlite with a relatively high carbon content and / or manganese content. Further, pearlite with a relatively high carbon content and / or manganese content may be locally accumulated in the steel sheet to form a pearlite region.

[0050] According to one embodiment of the present invention, the steel sheet before hot stamping contains carbon and manganese in amounts as optimized above, but the size, density, and area fraction of the pearlite region of the hot stamping steel sheet can be controlled to satisfy pre-set conditions. Through this, mechanical properties such as the tensile strength, yield strength, bending properties, and elongation of the formed part after hot stamping can be controlled.

[0051] The tensile strength of the hot stamping part formed by hot stamping such a steel sheet satisfies 1,700 MPa or more, and desirably, can satisfy 1,760 MPa or more and 1,950 MPa or less. Further, the yield strength of the hot stamping part satisfies 1,150 MPa or more, and desirably, can satisfy 1,200 MPa or more and 1,350 MPa or less. Further, the hot stamping part satisfies a bending angle of 50° or more and may have an elongation of 5% or more. Here, the "bending angle" may mean the V-bending angle in the rolling direction (RD).

[0052] Before hot stamping, in the steel sheet, depending on the carbon (C) content and manganese (Mn) content contained in the pearlite accumulated in the pearlite region, the degree of influence on the mechanical properties of the hot-stamped parts after hot stamping may vary. Specifically, what affects the mechanical properties of the hot-stamped parts is the region where pearlite containing 0.27 wt% or more of carbon and 1.0 wt% or more of manganese is locally concentrated. On the other hand, the region where pearlite with a carbon content of less than 0.27 wt% and a manganese content of less than 1.0 wt% is locally concentrated has a negligible effect on the mechanical properties of the hot-stamped parts. Therefore, the hot-stamping steel sheet according to an embodiment of the present invention controls the size, density, and area fraction of the region where pearlite containing 0.27 wt% or more of carbon and 1.0 wt% or more of manganese is locally concentrated so as to satisfy the preset conditions.

[0053] The hot-stamping steel sheet according to an embodiment of the present invention also includes a pearlite region in which pearlite containing 0.27 to 0.70 wt% of carbon (C) and / or pearlite containing 1.0 to 5.0 wt% of manganese (Mn) is locally accumulated. The size, shape, and area fraction of such a pearlite region can be controlled so as to satisfy the preset conditions.

[0054] As one embodiment, the average length of the pearlite region can be controlled to satisfy 0.01 μm or more and 500 μm or less, preferably 0.1 μm or more and 100 μm or less. Also, the average thickness of the pearlite region can be controlled to satisfy 0.01 μm or more and 30 μm or less. Further, the average interval between the pearlite regions can be controlled to 0.01 μm or more and 10 μm or less.

[0055] As one embodiment, the area fraction of the pearlite region in the hot-stamping steel sheet can be controlled to satisfy 0.1% or more and 15% or less.

[0056] Such a pearlite region also includes, as an area fraction, 50% or more of pearlite and 5% or less of ferrite. Optionally, it also includes 5% or less of low-temperature phase structures such as precipitates, martensite, and / or bainite.

[0057] Figure 2 is an image showing the microstructure of a hot stamping part formed by hot stamping the steel sheet shown in Figure 1.

[0058] The hot stamping part may also contain martensite, bainite, ferrite, and / or austenite. The ratio of the microstructure of the hot stamping part and the average grain size of the microstructure can be controlled to meet pre-set conditions. Through this, mechanical properties such as the tensile strength, yield strength, bending characteristics, and elongation rate of the hot stamping part can be controlled. For example, the tensile strength of the hot stamping part satisfies 1,700 MPa or more, and desirably, it can satisfy 1,760 MPa or more and 1,950 MPa or less. Also, the yield strength of the hot stamping part satisfies 1,150 MPa or more, and desirably, it can satisfy 1,200 MPa or more and 1,350 MPa or less. Further, the hot stamping part satisfies a bending angle of 50° or more and can have an elongation rate of 5% or more.

[0059] As one embodiment, the microstructure of the hot stamping part also includes 70% or more of martensite, 30% or less of bainite and ferrite, and 5% or less of the remaining carbides and retained austenite.

[0060] As one embodiment, the microstructure contained in the hot stamping part can be refined. Specifically, the average grain size of the microstructure contained in the hot stamping part can be controlled to satisfy 15 μm or less, and desirably, 2 μm or more and 15 μm or less.

[0061] Referring to FIG. 2, the hot stamping part according to an embodiment of the present invention also includes a microstructure containing austenite grains. The steel sheet also contains a martensite phase of 70% or more as an area fraction, and the austenite grains can generally be distributed in such a martensite phase.

[0062] As an embodiment, the average size of the austenite grains in the hot stamping part is about 15 μm or less, desirably 13 μm or less. When the average size of the austenite grains exceeds 15 μm, fracture may occur during hydrogen embrittlement evaluation. By controlling the austenite grain size (AGS) to a certain level or less in the hot stamping part, the sensitivity to hydrogen embrittlement can be reduced. Such an austenite grain size can be controlled through elements that form precipitates in the steel sheet. As an example, in the case of a steel sheet containing niobium (Nb), titanium (Ti), and molybdenum (Mo), after hot stamping, the refinement of austenite grains can be easily realized in the hot stamping part. Further, the steel sheet further contains vanadium (V) in addition to niobium (Nb), titanium (Ti), and molybdenum (Mo).

[0063] FIGS. 3 and 4 are graphs measuring the austenite grain size and fraction of the hot stamping parts according to an example and a comparative example of the present invention.

[0064] Specifically, FIG. 3 is a graph measuring the fraction of the austenite grain size of the hot stamping part according to this example being about 10 μm or more, and FIG. 4 is a graph measuring the fraction of the austenite grain size of the hot stamping part according to this example being about 20 μm or more. Specimen (1) and specimen (2) are comparative examples, and specimen (3) and specimen (4) are examples of the present invention. Fracture occurred in the comparative example specimens (1) and (2) during hydrogen embrittlement evaluation, while fracture did not occur in the example specimens (3) and (4) during hydrogen embrittlement evaluation, satisfying the performance embodied by the present invention.

[0065] As one embodiment, the fraction of the austenite crystal grain size of the hot stamping part being about 10 μm or more is 67% or less, desirably 65% or less.

[0066] As shown in FIG. 3, in the specimens (1) and (2) of the comparative example, the fraction of the austenite crystal grain size being about 10 μm or more is formed to exceed 67%. It can be confirmed that such specimens (1) and (2) of the comparative example break during the hydrogen embrittlement evaluation and do not reach the performance embodied by the present invention. On the other hand, in the specimens (3) and (4) of the example, the fraction of the austenite crystal grain size being about 10 μm or more is formed to be 67% or less. It can be confirmed that such specimens (3) and (4) of the example do not break during the hydrogen embrittlement evaluation and satisfy the performance embodied by the present invention. That is, the average size of the austenite crystal grains in the hot stamping part is about 15 μm or less, and when the fraction of the austenite crystal grain size being about 10 μm or more is formed to be 67% or less, the performance of hydrogen embrittlement and bending angle can be satisfied.

[0067] As one embodiment, the fraction of the austenite crystal grain size of the hot stamping part being about 20 μm or more is 10% or less, desirably 7% or less.

[0068] As shown in FIG. 4, in the specimens (1) and (2) of the comparative example, a fraction in which the austenite grain size is 20 μm or more is formed exceeding 10%. It can be confirmed that such specimens (1) and (2) of the comparative example break during the hydrogen embrittlement evaluation and do not reach the performance embodied by the present invention. On the other hand, in the specimens (3) and (4) of the example, a fraction in which the austenite grain size is 20 μm or more is formed to be 10% or less. It can be confirmed that such specimens (3) and (4) of the example do not break during the hydrogen embrittlement evaluation and satisfy the performance embodied by the present invention. That is, the average size of the austenite grains in the hot stamping part is about 15 μm or less, and when the fraction in which the austenite grain size is 20 μm or more is formed to be 10% or less, the performance of hydrogen embrittlement and bending angle can be satisfied. Those specimens (3) and (4) of the example can be formed so that the average size of the austenite grains in the hot stamping part is 13 μm or less.

[0069] In addition, the hot stamping part according to an embodiment of the present invention also includes a precipitate containing at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V). Niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V) contained in the steel sheet are carbide-forming elements that contribute to precipitate formation. Titanium (Ti), niobium (Nb), and molybdenum (Mo) can ensure the strength, hydrogen embrittlement, and bendability of the hot stamping part by forming carbon (C)-based precipitates. They can function as hydrogen trap sites effective for improving the stress corrosion cracking resistance. That is, the precipitate can be distributed in the steel sheet and play a role of trapping hydrogen. That is, the precipitate can improve the hydrogen stress corrosion cracking resistance of the hot stamping part by providing a trap site for hydrogen that has flowed into the inside of the steel sheet before hot stamping.

[0070] As described above, it also contains niobium (Nb), titanium (Ti), and vanadium (V) in amounts of 0.005 to 0.06 wt% respectively, and molybdenum (Mo) in an amount of 0.05 to 0.2 wt%. In particular, niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V) can satisfy the aforementioned formula (1). In that case, as described above, the average size of austenite crystal grains in the hot stamping part is about 15 μm or less, the fraction of the austenite crystal grain size of the hot stamping part that is about 10 μm or more is 67% or less, and the fraction of the austenite crystal grain size of the hot stamping part that is about 20 μm or more can be formed to be 10% or less. Therefore, the hydrogen embrittlement and the performance of the bending angle of the hot stamping part can be satisfied.

[0071] The precipitation behavior of such precipitates can be measured by a method of analyzing TEM (transmission electron microscope) images. Specifically, for a specimen, TEM images related to an arbitrary number of preset regions are acquired. From the acquired images, precipitates are extracted through an image analysis program or the like. For the extracted precipitates, the number of precipitates, the average distance between precipitates, the diameter of the precipitates, etc. can be measured.

[0072] Also, when measuring the diameter of the precipitate, considering the non-uniformity of the morphology of the precipitate, the shape of the precipitate can be converted into a circle, and the diameter of the precipitate can be calculated. Specifically, the area of the precipitate extracted using a unit pixel having a specific area is measured, and the precipitate is converted into a circle having the same area as the measured area, and the diameter of the precipitate can be calculated.

[0073] As one embodiment, the average particle size (size, diameter) of the precipitate can be controlled to satisfy preset conditions. Specifically, the average particle size of the precipitate formed in the hot stamping part is also 10 nm or less, and preferably, it is also 1 nm or more and 6 nm or less. Also, the amount of activated hydrogen in the hot stamping part containing such precipitates is also 0.6 wppm or less.

[0074] As a comparative example, when niobium (Nb), titanium (Ti), and vanadium (V) are each contained in an amount exceeding 0.06 wt%, or when molybdenum (Mo) is contained in an amount less than 0.05 wt%, the average particle size of the precipitates exceeds 10 nm, and the probability of hydrogen embrittlement occurring in the hot stamping parts increases.

[0075] As one embodiment, the component of the precipitate (i.e., the average component) may also contain 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo). In order for the precipitate to satisfy the average particle size as described above, the component of the precipitate must satisfy 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo). As a comparative example, when the component of the precipitate contains titanium (Ti) in an amount exceeding 50 wt% or molybdenum (Mo) in an amount less than 30 wt%, coarsening of the precipitate occurs and the designed strength cannot be ensured.

[0076] As one embodiment, the distance between adjacent precipitates, i.e., the average distance, can be controlled to satisfy a preset range. Here, the "average distance" can be measured via the mean free path of the precipitates. Specifically, the average distance between the precipitates can be calculated using the particle area fraction and the number of particles per unit length. However, the method for measuring the precipitation behavior of the precipitates is not limited to the above examples, and various methods can be applied.

[0077] Specifically, the average distance between the precipitates is also 0.1 nm or more and 100 nm or less, desirably 0.1 nm or more and 50 nm or less, and more desirably 0.1 nm or more and 10 nm or less. When the average distance between the fine precipitates is less than 0.1 nm, the formability or bendability is reduced, while when the average distance between the fine precipitates exceeds 100 nm, the strength may be reduced.

[0078] As one embodiment, the average number of precipitates per unit area can be controlled to satisfy a preset condition. Specifically, when the above-mentioned formula (1) is satisfied, the precipitation is refined. In that case, the average number of precipitates per unit area is 10,000 pieces / 100 μm 2 to 35,000 pieces / 100 μm 2 as well. As a comparative example, when the above-mentioned formula (1) cannot be satisfied, the precipitation coarsens. In that case, the average number of precipitates per unit area can be formed to be less than 10,000 pieces / 100 μm 2

[0079] Hereinafter, the present invention will be described in more detail through examples and comparative examples. However, the following examples and comparative examples are for more specifically explaining the present invention, and the scope of the present invention is not limited by the following examples and comparative examples. The following examples and comparative examples can be appropriately modified and changed by those skilled in the art within the scope of the present invention.

[0080]

Table 1

[0081]

Table 2

[0082] Table 1 shows the composition of the hot stamping steel sheet, and Table 2 shows the measured values obtained by evaluating the austenite grain size and hydrogen embrittlement of the specimens corresponding to the hot stamping parts. Comparative Examples 1 to 12 are specimens that do not satisfy the value of formula (1) in the composition of Table 1, and Examples 1 to 8 are specimens corresponding to the hot stamping parts formed by hot stamping the steel sheet having the composition as shown in Table 1.

[0083] ​The evaluation of the hydrogen embrittlement of each test piece was carried out by applying the ASTM G39-99 standard (four-point bending test) test method. Specifically, the test piece was mounted on a four-point bending test instrument, and after applying a yield strength (YP) of 100% stress, it was immersed in an aqueous solution of 0.1 N HCl for 100 hours, and it was measured whether cracks, that is, fractures occurred on the surface of the test piece.

[0084] Referring to Table 2, Comparative Examples 1 to 12 are test pieces of hot stamping parts made of steel plates in which the value of Formula 1, that is, 0.25(Ti + Nb + V + 0.25Mo), is 0.013 wt% in the composition of Table 1 and is outside the range of 0.015 to 0.060 wt%. Niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V) are carbide-forming elements that contribute to precipitate formation. Comparative Examples 1 to 12 cannot satisfy the precipitate control conditions and show the result that the austenite grain size in the hot stamping parts is coarsened. Therefore, it can be known that in the austenite grain size of Comparative Examples 1 to 12, a fraction of 10 μm or more is formed exceeding 67%, which is the reference value of the present invention. Also, it can be known that in the austenite grain size of Comparative Examples 1 to 12, a fraction of 20 μm or more far exceeds 10%, which is the reference value of the present invention, and is formed at about 50% or more. As a result, it can be seen that Comparative Examples 1 to 12 broke in the hydrogen embrittlement evaluation, and it can be confirmed that they do not satisfy the design conditions of the present invention.

[0085] On the other hand, Examples 1 to 8 of the embodiments of the present invention satisfy the composition of Table 1, and it can be known that in the austenite grain size, a fraction of 10 μm or more is formed at 67% or less, and in the austenite grain size, a fraction of 20 μm or more is formed at 10% or less. As a result, it can be confirmed that Examples 1 to 8 do not break in the hydrogen embrittlement evaluation and satisfy the design conditions of the present invention.

[0086] On the other hand, Comparative Examples 13 to 16 satisfied the composition in Table 1, but due to differences in process control conditions, they were specimens that could not satisfy conditions such as precipitates. In Comparative Examples 13 to 16, in terms of the austenite grain size, a fraction of 10 μm or more was formed exceeding 67%, and in terms of the austenite grain size, a fraction of 20 μm or more was formed exceeding 10%. As a result, it was found that breakage occurred in Comparative Examples 13 to 16 in the hydrogen embrittlement evaluation, and it was confirmed that they did not satisfy the design conditions of the present invention.

[0087] 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. A hot stamping part comprising a steel sheet containing carbon (C): 0.26 to 0.40 wt%, silicon (Si): 0.02 to 2.0 wt%, manganese (Mn): 0.3 to 1.60 wt%, phosphorus (P): 0.03 wt% or less, sulfur (S): 0.008 wt% or less, chromium (Cr): 0.05 to 0.90 wt%, boron (B): 0.0005 to 0.01 wt%, molybdenum (Mo): 0.05 to 0.2 wt%, titanium (Ti): 0.001 to 0.095 wt%, niobium (Nb): 0.001 to 0.095 wt%, vanadium (V): 0.001 to 0.095 wt%, and the balance iron (Fe) and other inevitable impurities, and having a tensile strength of 1,700 MPa or more and a yield strength of 1,150 MPa or more, wherein the hot stamping part includes a microstructure containing austenite crystal grains and carbon-based precipitates containing at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V); the hot stamping part, wherein an average size of the austenite crystal grains is 15 μm or less.

2. The hot stamping part according to claim 1, wherein a fraction of the austenite crystal grains having a size of 10 μm or more is 67% or less.

3. The hot stamping part according to claim 1, wherein a fraction of the austenite crystal grains having a size of 20 μm or more is 10% or less.

4. The hot stamping part according to claim 1, wherein when the contents of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) contained in the steel sheet are represented by [Ti], [Nb], [V], and [Mo] in wt%, the following formula (1) is satisfied: [Formula (1)] 0.015 ≦ 0.25([Ti] + [Nb] + [V] + 0.25[Mo]) ≦ 0.060 (unit: wt%).

5. The hot stamping part according to claim 1, wherein an amount of activated hydrogen in the hot stamping part is 0.6 wppm or less.

6. The hot stamping part according to claim 1, wherein an average particle size of the precipitates is 10 nm or less.

7. The hot stamping part according to claim 1, wherein the precipitates contain titanium (Ti) of 50 wt% or less and molybdenum (Mo) of 30 wt% or more.

8. The average number per unit area of the precipitate is 10,000 pieces / 100 μm ２ to 35,000 pieces / 100 μm ２ The hot stamping part according to claim 1, which is such

9. The average interval between the precipitates is from 0.1 nm to 100 nm, the hot stamping part according to claim 1.

10. The hot stamping part satisfies a bending angle of 50° or more, the hot stamping part according to claim 1.

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