Material for hot stamping and method for producing the same

Abstract

The present invention provides a hot stamping material and a manufacturing method thereof that can ensure excellent mechanical properties and hydrogen delayed fracture characteristics of hot stamped parts. The material contains carbon (C): 0.19 to 0.25 wt%, silicon (Si): 0.1 to 0.6 wt%, manganese (Mn): 0.8 to 1.6 wt%, phosphorus (P): 0.03 wt% or less, sulfur (S): 0.015 wt% or less, chromium (Cr): 0.1 to 0.6 wt%, and boron (B): 0.001 to 0.005 wt%. The present invention provides a material for hot stamping that traps hydrogen, the material comprising a steel sheet containing 0.1% by weight or less of an alloying element, 0.1% by weight or less of an additive, and the remainder being iron (Fe), and other unavoidable impurities, and fine precipitates distributed within the steel sheet, the additive including at least one of titanium (Ti), niobium (Nb), and vanadium (V), and the fine precipitates including at least one nitride or carbide of titanium (Ti), niobium (Nb), and vanadium (V).

Description

Technical Field

[0001] The present invention relates to a material for hot stamping and a method for manufacturing the same, and more particularly, to a material for hot stamping capable of ensuring excellent mechanical properties and hydrogen delayed fracture characteristics of hot stamping parts and a method for manufacturing the same.

Background Art

[0002] High-strength steel is applied to parts used in automobiles and the like for weight reduction and stability. On the other hand, high-strength steel can ensure high-strength characteristics in terms of weight ratio, but 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 typical solution to such problems, there is a hot stamping process, and as the interest in it is increasing, research on materials for hot stamping is also actively carried out. For example, as disclosed in the invention of Korean Patent Publication No. 10-2017-0076009, the hot stamping process is a forming technique for manufacturing high-strength parts by heating a boron steel sheet at an appropriate temperature, forming it in a press die, and then rapidly cooling it. According to the invention of Korean Patent Publication No. 10-2017-0076009, problems such as crack generation or poor shape freezing during the forming of high-strength steel sheets can be suppressed, and it is possible to manufacture parts with good accuracy.

[0004] However, in the case of hot stamping steel sheets, there is a problem that hydrogen delayed fracture occurs due to hydrogen and residual stress introduced in the hot stamping process. In this regard, Korean Patent Publication No. 10-2020-0061922 discloses that before heating a hot stamping blank at a high temperature, preheating is performed to form a thin oxide layer on the surface of the blank, thereby blocking the inflow of hydrogen in the high-temperature heating process and minimizing hydrogen delayed fracture. However, since it is impossible to completely block the inflow of hydrogen, the introduced hydrogen cannot be controlled, which may lead to hydrogen delayed fracture.

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention is for solving many problems including the above-mentioned problems, and provides a material for hot stamping and a method for manufacturing the same, which can ensure excellent mechanical properties and hydrogen delayed fracture characteristics of hot stamping parts. However, such problems are exemplary and do not limit the scope of the present invention thereby.

Means for Solving the Problems

[0006] According to one aspect of the present invention, there is provided a steel sheet containing carbon (C): 0.19 to 0.25% by weight, silicon (Si): 0.1 to 0.6% by weight, manganese (Mn): 0.8 to 1.6% by weight, phosphorus (P): 0.03% by weight or less, sulfur (S): 0.015% by weight or less, chromium (Cr): 0.1 to 0.6% by weight, boron (B): 0.001 to 0.005% by weight, additive: 0.1% by weight or less, and the balance iron (Fe) and other inevitable impurities, and fine precipitates distributed in the steel sheet, wherein the additive contains at least one of titanium (Ti), niobium (Nb), and vanadium (V), and the fine precipitates contain at least one nitride or carbide of titanium (Ti), niobium (Nb), and vanadium (V) and trap hydrogen, and a material for hot stamping is provided.

[0007] According to this embodiment, the fine precipitates can be formed at 7,000 or more and 16,500 or less per unit area (100 μm 2 )).

[0008] According to this embodiment, 60% or more of the fine precipitates can be formed to have a diameter of 0.01 μm or less.

[0009] According to this embodiment, among the fine precipitates, the number of fine precipitates having a diameter of 0.01 μm or less is per unit area (100 μm 2)It is also 4,500 or more and 16,000 or less per hit.

[0010] According to this embodiment, 25% or more of the fine precipitates can be formed to have a diameter of 0.005 μm or less.

[0011] According to this embodiment, the average distance between the fine precipitates is also 0.4 μm or more and 0.8 μm or less.

[0012] According to another aspect of the present invention, a step of reheating the slab in a slab reheating temperature range of 1,200 °C to 1,250 °C, a step of hot rolling the reheated slab in a finish rolling temperature range of 840 °C to 920 °C to produce a steel sheet, and a step of winding the steel sheet in a coiling temperature range of 700 °C to 780 °C to form fine precipitates in the steel sheet, wherein the slab contains carbon (C): 0.19 to 0.25% by weight, silicon (Si): 0.1 to 0.6% by weight, manganese (Mn): 0.8 to 1.6% by weight, phosphorus (P): 0.03% by weight or less, sulfur (S): 0.015% by weight or less, chromium (Cr): 0.1 to 0.6% by weight, boron (B): 0.001 to 0.005% by weight, additive: 0.1% by weight or less, and the balance of iron (Fe) and other inevitable impurities, the additive contains at least one of titanium (Ti), niobium (Nb), and vanadium (V), and the fine precipitates contain at least one nitride or carbide of titanium (Ti), niobium (Nb), and vanadium (V) and trap hydrogen, and a method for manufacturing a material for hot stamping is provided.

[0013] According to this embodiment, the fine precipitates can be formed to be 7,000 or more and 16,500 or less per unit area (100 μm 2 ) per hit.

[0014] According to this embodiment, 60% or more of the fine precipitates can be formed to have a diameter of 0.01 μm or less.

[0015] According to this embodiment, among the fine precipitates, the number of fine precipitates having a diameter of 0.01 μm or less per unit area (100 μm2 )It is also 4,500 or more and 16,000 or less per hit.

[0016] According to this embodiment, 25% or more of the fine precipitates can be formed to have a diameter of 0.005 μm or less.

[0017] According to this embodiment, the average distance between the fine precipitates is also 0.4 μm or more and 0.8 μm or less.

[0018] Other aspects, features, and advantages will become clear from the specific content, claims, and drawings for carrying out the following invention.

Effect of the Invention

[0019] According to an embodiment of the present invention, a material for hot stamping and a method for manufacturing the same that ensure excellent mechanical properties and hydrogen delayed fracture characteristics of hot stamping parts can be realized. Of course, the scope of the present invention is not limited by such effects.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2A

Figure 2B

Figure 3

Figure 4

Figure 5A

Figure 5B

Mode for Carrying Out the Invention

[0021] The present invention can be subjected to various conversions and can 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 of the present invention will become clear by referring to the embodiments described in detail hereinafter together with the drawings. However, the present invention is not limited to the embodiments disclosed below and can also be embodied in various forms.

[0022] 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 given the same reference numerals, and redundant descriptions related thereto are omitted.

[0023] In this specification, terms such as first and second are not used in a limiting sense and are used for the purpose of distinguishing one component from another component.

[0024] In this specification, singular expressions include plural expressions unless the context clearly indicates a different meaning.

[0025] In this specification, 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.

[0026] In this specification, 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 other films, regions, components, etc. are interposed in between.

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

[0028] When a certain embodiment can be implemented differently, a specific process order can be carried out differently from the order described. For example, two processes described continuously may be carried out substantially simultaneously, or may be carried out in the reverse order from the order described.

[0029] In this specification, "A and / or B" indicates the case where it is A, B, or both A and B. And "at least one of A and B" indicates the case where it is A, B, or both A and B.

[0030] In this specification, when it is assumed that a film, a region, a component, etc. are connected, it includes the case where the film, the region, the component are directly connected, and / or the case where other films, regions, components are interposed between the film, the region, the component and are indirectly connected. For example, in this specification, when it is assumed that a film, a region, a component, etc. are electrically connected, it indicates the case where the film, the region, the component, etc. are directly electrically connected, and / or the case where other films, regions, components, etc. are interposed therebetween and are indirectly electrically connected.

[0031] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

[0032] FIG. 1 is a TEM (Transmission Electron Microscopy) image showing a part of a material for hot stamping according to an embodiment of the present invention.

[0033] As shown in FIG. 1, a material 1 for hot stamping according to an embodiment of the present invention includes a steel sheet 10 and fine precipitates 20 distributed in the steel sheet 10.

[0034] The steel sheet 10 is also a steel sheet manufactured by subjecting a slab cast to contain a predetermined alloy element in a predetermined content to a hot rolling process and / or a cold rolling process. The steel sheet 10 contains carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), boron (B), and the balance of iron (Fe), as well as other inevitable impurities. Also, in one embodiment, the steel sheet 10 further contains at least any one of titanium (Ti), niobium (Nb), and vanadium (V) as an additive. In another embodiment, the steel sheet 10 may further contain a predetermined content of calcium (Ca).

[0035] Carbon (C) acts as an austenite stabilizing element in the steel sheet 10. Carbon is a major element that determines the strength and hardness of the steel sheet 10, and is added for the purpose of ensuring the tensile strength of the steel sheet 10 (for example, a tensile strength of 1,350 MPa or more) and ensuring hardenability characteristics after the hot stamping process. Such carbon is contained in an amount of 0.19 wt% to 0.25 wt% based on the total weight of the steel sheet 10. When the content of carbon is less than 0.19 wt%, it is difficult to ensure a hard phase (such as martensite), and it is difficult to satisfy the mechanical strength of the steel sheet 10. Conversely, when the content of carbon exceeds 0.25 wt%, problems such as brittleness generation or bending performance deterioration of the steel sheet 10 may be brought about.

[0036] Silicon (Si) acts as a ferrite stabilizing element in the steel sheet 10. Silicon (Si) improves the ductility of the steel sheet 10 as a solid solution strengthening element 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 microstructure homogenization (perlite, manganese segregation band control) and fine ferrite dispersion. Silicon acts as a martensite strength inhomogeneity control element and plays a role in improving the collision performance. Such silicon is contained in an amount of 0.1 wt% to 0.6 wt% based on the total weight of the steel sheet 10. When the content of silicon is less than 0.1 wt%, the above-described effects cannot be obtained, cementite formation and coarsening occur in the final hot stamping martensite structure, the homogenization effect of the steel sheet 10 is negligible, and the V bending angle cannot be ensured. Conversely, when the content of silicon exceeds 0.6 wt%, the hot rolling and cold rolling loads increase, the hot rolling red scale becomes excessive, and the plating characteristics of the steel sheet 10 may deteriorate.

[0037] Manganese (Mn) acts as an austenite stabilizing element in the steel sheet 10. Manganese is added for the purpose of hardenability and strength increase during heat treatment. Such manganese may be contained in an amount of 0.8 wt% to 1.6 wt% based on the total weight of the steel sheet 10. When the content of manganese is less than 0.8 wt%, the grain refinement effect is insufficient, the hardenability is insufficient, and the fraction of hard phases in the molded product after hot stamping may be insufficient. On the other hand, when the content of manganese exceeds 1.6 wt%, the ductility and toughness are reduced due to manganese segregation or pearlite banding, which causes a decrease in bending performance and may result in an inhomogeneous fine structure.

[0038] Phosphorus (P) may be contained in an amount exceeding 0 and not exceeding 0.03 wt% based on the total weight of the steel sheet 10 in order to prevent a decrease in the toughness of the steel sheet 10. When the content of phosphorus exceeds 0.03 wt%, iron phosphide compounds are formed, the toughness and weldability are reduced, and cracks may be induced in the steel sheet 10 during the manufacturing process.

[0039] Sulfur (S) can be contained in an amount exceeding 0 and not exceeding 0.015 wt% based on the total weight of the steel sheet 10. If the sulfur content exceeds 0.015 wt%, the hot workability, weldability, and impact properties will deteriorate, and surface defects such as cracks may occur due to the formation of large inclusions.

[0040] Chromium (Cr) is added for the purpose of improving the hardenability and strength of the steel sheet 10. Chromium enables grain refinement and strength retention through precipitation hardening. Such chromium can be contained in an amount of 0.1 wt% to 0.6 wt% based on the total weight of the steel sheet 10. When the chromium content is less than 0.1 wt%, the precipitation hardening effect is low. Conversely, when the chromium content exceeds 0.6 wt%, the amount of Cr-based precipitates and matrix solid solution increases, toughness decreases, and production costs increase due to cost increases.

[0041] Boron (B) is added for the purpose of suppressing ferrite, pearlite, and bainite transformations and ensuring a martensite structure to secure the hardenability and strength of the steel sheet 10. 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 is contained in an amount of 0.001 wt% to 0.005 wt% based on the total weight of the steel sheet 10. 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 0.001 wt%, the hardenability effect is insufficient. Conversely, when the boron content exceeds 0.005 wt%, the solubility is low, it is easily precipitated at the grain boundaries depending on the heat treatment conditions, the hardenability deteriorates, or it may cause high-temperature embrittlement, and the toughness and bendability may decrease due to the occurrence of hard phase grain boundary brittleness.

[0042] The additive is a nitride or carbide-forming element that contributes to the formation of the fine precipitate 20. Specifically, the additive contains at least any one of titanium (Ti), niobium (Nb), and vanadium (V). Titanium (Ti), niobium (Nb), and vanadium (V) ensure the strength of hot stamping and quenched members by forming fine precipitates 20 in the form of nitrides or carbides. Also, they are contained in the Fe-Mn-based composite oxide and function as hydrogen trap sites effective for improving the hydrogen embrittlement resistance characteristics, and are elements necessary for improving the hydrogen embrittlement resistance. Such an additive can be contained in a total amount of 0.1 wt% or less based on the total weight of the steel sheet 10. If the content of the additive exceeds 0.1 wt%, the drop in strength may become excessively large.

[0043] Titanium (Ti) can be added for the purpose of strengthening hardenability and improving material properties by the formation of precipitates after hot press heat treatment. Also, it forms precipitate phases such as Ti(C, N) at high temperatures and effectively contributes to the refinement of austenite grain size. Such titanium can be contained in an amount of 0.025 wt% to 0.050 wt% based on the total weight of the steel sheet 10. 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. On the other hand, if the content of titanium exceeds 0.050 wt%, the precipitates become coarsened, and a decrease in elongation and bendability may occur.

[0044] Niobium (Nb) and vanadium (V) are added for the purpose of increasing strength and toughness by reducing the martensite packet size. Each of niobium and vanadium can be contained in an amount of 0.025 wt% to 0.050 wt% based on the total weight of the steel sheet 10. When niobium and vanadium are contained within the above range, they have an excellent effect on refining the crystal grains of the steel material in the hot rolling and cold rolling processes, prevent the occurrence of cracks in the slab and brittle fracture of the product during steelmaking / continuous casting, and can minimize the generation of coarse precipitates in steelmaking.

[0045] Calcium (Ca) can be added for inclusion shape control. Such calcium can be contained at 0.003 wt% or less based on the total weight of the steel sheet 10.

[0046] The fine precipitates 20 are distributed in the steel sheet 10 and can play a role in trapping hydrogen. That is, the fine precipitates 20 can improve the hydrogen delayed fracture characteristics of the product processed by hot stamping by providing trap sites for hydrogen that has flowed into the interior during the manufacturing process or after manufacturing of the material 1 for hot stamping. In one embodiment, the fine precipitates 20 also contain nitrides or carbides of additives. Specifically, the fine precipitates 20 also contain nitrides or carbides of at least any one of titanium (Ti), niobium (Nb), and vanadium (V).

[0047] The precipitation behavior of such fine precipitates 20 can be controlled by adjusting the process conditions. For example, by adjusting the range of the coiling temperature (CT) among the process conditions, precipitation behaviors such as the number of fine precipitates 20, the average distance between the fine precipitates 20, and the diameter of the fine precipitates 20 can be controlled. A detailed description regarding the process conditions will be described later with reference to FIG. 3.

[0048] In one embodiment, it can be controlled so that the number of fine precipitates 20 formed in the steel sheet 10 satisfies a preset range. Specifically, the fine precipitates 20 can be formed in the steel sheet 10 at 7,000 pieces / 100 μm 2 or more and 16,500 pieces / 100 μm 2 or less. In particular, among the fine precipitates 20 distributed in the steel sheet 10, the fine precipitates having a diameter of 0.01 μm or less can be formed in the steel sheet 10 at 4,500 pieces / 100 μm 2 or more and 16,000 pieces / 100 μm 2 or less.

[0049] If the number of the fine precipitates 20 is formed within the above-described range, the required tensile strength (for example, 1,350 MPa) can be ensured after hot stamping, and the formability or bendability can be improved. For example, the number of the fine precipitates 20 having a diameter of 0.01 μm or less is 4,500 pieces / 100 μm 2 If it is less than, the strength decreases. On the other hand, if it exceeds 16,000 pieces / 100 μm 2 the formability or bendability may decrease.

[0050] In other embodiments, the average distance between adjacent fine precipitates 20 can be controlled so as to satisfy a preset range. Here, the "average distance" means the mean free path of the fine precipitates 20, and the detailed content regarding the method for measuring it will be described later.

[0051] Specifically, the average distance between the fine precipitates 20 is also 0.4 μm or more and 0.8 μm or less. When the average distance between the fine precipitates 20 is less than 0.4 μm, the formability or bendability decreases. On the other hand, when it exceeds 0.8 μm, the strength may decrease.

[0052] In still other embodiments, the diameter of the fine precipitates 20 can be controlled so as to satisfy preset conditions. Specifically, it can be formed such that 60% or more of the fine precipitates 20 formed in the steel sheet 10 have a diameter of 0.01 μm or less. Also, among the fine precipitates 20 formed in the steel sheet 10, 25% or more can be formed to have a diameter of 0.005 μm or less. Also, in an alternative embodiment, the average diameter of all the fine precipitates 20 formed in the steel sheet 10 is also 0.007 μm or less.

[0053] The diameter of such fine precipitates 20 has a great influence on the improvement of hydrogen delayed fracture characteristics. Hereinafter, with reference to FIGS. 2A and 2B, the difference in the effect of improving hydrogen delayed fracture characteristics due to the diameter of the fine precipitates 20 will be described.

[0054] FIGS. 2A and 2B are exemplary diagrams schematically showing a part of the shape in which hydrogen is trapped in the fine precipitates 20.

[0055] Specifically, FIG. 2A shows the shape in which hydrogen is trapped in relatively large-diameter fine precipitates 20, and FIG. 2B shows the shape in which hydrogen is trapped in relatively small-diameter fine precipitates 20.

[0056] When the fine precipitates 20 are relatively large in diameter as shown in FIG. 2A, the number of hydrogen atoms trapped in one fine precipitate 20 increases. That is, the hydrogen atoms flowing into the steel sheet 10 are not uniformly dispersed, and the probability that a plurality of hydrogen atoms are trapped at one hydrogen trap site increases. A plurality of hydrogen atoms trapped at one hydrogen trap site bond to each other to form hydrogen molecules (H 2 ). The formed hydrogen molecules increase the probability of internal pressure generation, and as a result, the hydrogen delayed fracture characteristics of the hot-stamped product can be degraded.

[0057] In contrast, when the fine precipitates 20 are relatively small in diameter as shown in FIG. 2B, the probability that a plurality of hydrogen atoms are trapped in one fine precipitate 10 decreases. That is, the hydrogen atoms flowing into the steel sheet 10 can be relatively uniformly dispersed by being trapped at different hydrogen trap sites. As a result, the hydrogen atoms cannot bond to each other, the probability of internal pressure generation by hydrogen molecules decreases, and the hydrogen delayed fracture characteristics of the hot-stamped product can be improved.

[0058] On the other hand, the precipitation behavior of such fine precipitates 20 can be measured by a method of analyzing a TEM (Transmission Electron Microscopy) image. Specifically, TEM images for an arbitrary number of regions as set in advance are obtained for a specimen. The fine precipitates 20 are extracted from the obtained images through an image analysis program or the like, and the number of fine precipitates 20, the average distance between the fine precipitates 20, the diameter of the fine precipitates 20, etc. can be measured for the extracted fine precipitates 20.

[0059] In one embodiment, for measuring the precipitation behavior of the fine precipitate 20, the replication method can be applied to the test piece to be measured as a pretreatment. For example, a one-step replication method, a two-step replication method, an extraction replication method, etc. can be applied, but it is not limited to the above-mentioned examples.

[0060] In other embodiments, when measuring the diameter of the fine precipitate 20, considering the non-uniformity of the morphology of the fine precipitate 20, the shape of the fine precipitate 20 is converted into a circle to calculate the diameter of the fine precipitate 20. Specifically, using a unit pixel having a specific area, the area of the extracted fine precipitate 20 is measured, and the fine precipitate 20 is converted into a circle having the same area as the measured area to calculate the diameter of the fine precipitate 20.

[0061] In still other embodiments, the average distance between the fine precipitates 20 can be measured through the aforementioned mean free path. Specifically, the average distance between the fine precipitates 20 can be calculated using the particle area fraction and the number of particles per unit length. For example, the average distance between the fine precipitates 20 has a correlation as shown in the following Equation 1.

Equation

[0062] The method for measuring the precipitation behavior of the fine precipitate 20 is not limited to the above-mentioned examples, and various methods can be applied.

[0063] Figure 3 is a flowchart schematically showing a method for manufacturing a material for hot stamping according to an embodiment of the present invention.

[0064] As shown in Figure 3, the method for manufacturing a material for hot stamping according to an embodiment of the present invention includes a reheating step (S100), a hot rolling step (S200), a cooling / winding step (S300), a cold rolling step (S400), an annealing heat treatment step (S500), and a plating step (S600).

[0065] In FIG. 3, the steps S100 to S600 are illustrated as independent steps. However, among the steps S100 to S600, some are performed by one process and, if necessary, some may be omitted.

[0066] First, a slab in a semi-finished product state that is the target of the process of forming the hot stamping material 1 is prepared. The slab contains carbon (C): 0.19 to 0.25% by weight, silicon (Si): 0.1 to 0.6% by weight, manganese (Mn): 0.8 to 1.6% by weight, phosphorus (P): 0.03% by weight or less, sulfur (S): 0.015% by weight or less, chromium (Cr): 0.1 to 0.6% by weight, boron (B): 0.001 to 0.005% by weight, an additive: 0.1% by weight or less, and the balance of iron (Fe) and other inevitable impurities. Further, the slab may further contain an additive in a total amount of 0.1% by weight or less. At this time, the additive may contain at least any one of titanium (Ti), niobium (Nb), and vanadium (V). For example, the content of each of titanium (Ti), niobium (Nb), and / or vanadium (V) is also 0.025 wt% to 0.050 wt%.

[0067] The reheating step (S100) is a step of reheating the slab for hot rolling. In the reheating step (S100), by reheating the slab secured through the continuous casting process within a predetermined temperature range, the components segregated during casting are redissolved.

[0068] The slab reheating temperature (SRT) can be controlled within a preset temperature range to maximize austenite refinement and precipitation hardening effects. At this time, the slab reheating temperature (SRT) range can include the temperature range (about 1,000 °C or higher) at which additives (Ti, Nb, and / or V) are completely dissolved based on the equilibrium precipitation amount of fine precipitates 20 during slab reheating. When the slab reheating temperature (SRT) does not reach the complete solid solution temperature range of the additives (Ti, Nb, and / or V), during hot rolling, the driving force required for fine grain structure control is not fully reflected, and an excellent mechanical property ensuring effect through the required precipitation amount control cannot be obtained.

[0069] In one embodiment, the slab reheating temperature (SRT) can be controlled to 1,200 °C to 1,250 °C. When the slab reheating temperature (SRT) is less than 1,200 °C, during casting, the segregated components are not fully redissolved, the homogenization effect of alloying elements is not considered significant, and there is a problem that the solid solution effect of titanium (Ti) is not considered significant. On the other hand, the higher the slab reheating temperature (SRT), the more favorable it is for homogenization. However, when it exceeds 1,250 °C, the austenite crystal grain size increases, making it difficult to ensure strength, and only the manufacturing cost of the steel plate can increase due to an excessive heating process.

[0070] The hot rolling stage (S200) is a stage in which the reheated slab is hot rolled within a predetermined finishing delivery temperature (FDT) range to manufacture a steel plate in the S100 stage. In one embodiment, the finishing delivery temperature (FDT) range can be controlled to 840 °C to 920 °C. When the finishing delivery temperature (FDT) is less than 840 °C, it is difficult to ensure the workability of the steel plate due to the generation of a mixed grain structure by abnormal area rolling. There are not only problems with a decrease in workability due to non-uniform fine grain structures, but also problems with the passing property during hot rolling due to a rapid phase change. Conversely, when the finishing delivery temperature (FDT) exceeds 920 °C, the austenite crystal grains are coarsened. Also, there is a risk that the TiC precipitates are coarsened and the performance of the final parts deteriorates.

[0071] On the other hand, in the reheating stage (S100) and the hot rolling stage (S200), some of the fine precipitates 20 can precipitate from the grain boundaries where the energy is unstable. At this time, the fine precipitates 20 precipitated at the grain boundaries can act as elements that hinder the growth of austenite crystal grains and provide the effect of improving strength through austenite refinement. On the other hand, the fine precipitates 20 precipitated in the S100 and S200 stages are at the level of 0.007 wt% based on the equilibrium precipitation amount, but are not limited thereto.

[0072] The cooling / coiling stage (S300) is a stage in which, in the S200 stage, the hot-rolled steel sheet is cooled and coiled within a predetermined coiling temperature (CT) range to form fine precipitates 20 in the steel sheet. That is, in the S300 stage, fine precipitates 20 are formed by forming nitrides or carbides of additives (Ti, Nb, and / or V) contained in the slab. On the other hand, coiling can be carried out in the ferrite region so that the equilibrium precipitation amount of the fine precipitates 20 reaches the maximum value. After the recrystallization of the crystal grains is completed in this way, when the structure transforms into ferrite, not only at the grain boundaries but also within the grains, the particle size of the fine precipitates 20 can be uniformly precipitated.

[0073] In one embodiment, the coiling temperature (CT) is also 700°C to 780°C. The coiling temperature (CT) affects the redistribution of carbon (C). When such a coiling temperature (CT) is less than 700°C, the low-temperature phase fraction due to supercooling increases, and there is a risk that the rolling load will become intense during strength increase and cold rolling, and there is a problem that the ductility suddenly decreases. Conversely, when the coiling temperature exceeds 780°C, there are problems such as abnormal crystal grain growth and excessive crystal grain growth, resulting in deterioration of formability and strength.

[0074] Thus, according to this embodiment, by controlling the coiling temperature (CT) range, the precipitation behavior of the fine precipitates 20 can be controlled. Experimental examples of the property changes of the hot stamping material 1 according to the coiling temperature (CT) range will be described later with reference to FIGS. 4, 5A, and 5B.

[0075] The cold rolling stage (S400) is a stage where the steel sheet wound up in the S300 stage is uncoiled, pickled, and then cold rolled. At this time, pickling is carried out for the purpose of removing the scale of the wound-up steel sheet, that is, the hot-rolled coil manufactured through the above-mentioned hot rolling process. On the other hand, in one embodiment, the reduction ratio during cold rolling can be controlled to 30% - 70%, but it is not limited thereto.

[0076] The annealing heat treatment stage (S500) is a stage where the steel sheet cold rolled in the S400 stage is subjected to annealing heat treatment at a temperature of 700°C or higher. In one specific example, the annealing heat treatment includes a stage of heating the cold-rolled sheet material and cooling the heated cold-rolled sheet material at a predetermined cooling rate.

[0077] The plating stage (S600) is a stage of forming a plating layer on the steel sheet subjected to annealing heat treatment. In one embodiment, in the plating stage (S600), an Al - Si plating layer can be formed on the steel sheet annealed and heat treated in the S500 stage.

[0078] Specifically, the plating stage (S600) may include a stage of immersing the steel sheet in a plating bath having a temperature of 650°C - 700°C to form a molten plating layer on the surface of the steel sheet, and a cooling stage of cooling the steel sheet on which the molten plating layer is formed to form a plating layer. At this time, the plating bath may contain, but is not limited to, Si, Fe, Al, Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, etc. as additive elements.

[0079] By performing the hot stamping process on the hot stamping material 1 manufactured through the S100 to S600 stages in this way, hot stamping parts that satisfy the required strength and bendability can be manufactured. In one embodiment, the hot stamping material 1 manufactured to satisfy the above-mentioned content conditions and process conditions can have a tensile strength of 1,350 MPa or more and a bendability of 50° (degree) or more after going through the hot stamping process.

[0080] Hereinafter, the present invention will be described in more detail through Examples and Comparative Examples. However, the following Examples and Comparative Examples are for further 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.

[0081] Figure 4 is a graph showing a comparison of the tensile strength and bending stress according to the winding temperature of Examples and Comparative Examples of the present invention, and Figures 5A and 5B are images showing the results of a four-point bending test according to the winding temperature of Examples and Comparative Examples.

[0082] Examples (CT700) and Comparative Examples (CT800) are test pieces manufactured by hot stamping Material 1 for hot stamping, which was manufactured by performing the above-described S100 to S600 steps on a slab having a composition as shown in Table 1 below. At this time, Examples (CT700) and Comparative Examples (CT800) apply the same content conditions and process conditions in the manufacturing process of Material 1 for hot stamping, but are test pieces manufactured by differentially applying only the winding temperature (CT) as a variable.

[0083]

Table 1

[0084] Specifically, Example (CT700) is a test piece manufactured by hot stamping Material 1 for hot stamping manufactured by applying a winding temperature (CT) of 700 °C, and Comparative Example (CT800) is a test piece manufactured by hot stamping Material 1 for hot stamping manufactured by applying a winding temperature (CT) of 800 °C.

[0085] Note that Figure 4 is a graph showing the measurement results of the tensile strength and bending stress of Example (CT700) and Comparative Example (CT800).

[0086] Referring to FIG. 4, in the case of tensile strength, the tensile strength of the example (CT700) is greater than that of the comparative example (CT800), and it was also confirmed that the bending stress affecting the impact characteristics was improved in the example (CT700) compared to the bending stress of the comparative example (CT800).

[0087] This is because, as can be confirmed in Table 2 below, in the case of the example (CT700), the precipitation amount of the fine precipitate 20 increased compared to the comparative example (CT800), and thus the hydrogen trapping ability was improved.

[0088] Table 2 below shows the measured values of the equilibrium precipitation amount and the amount of activated hydrogen, and the results of the bent-beam stress corrosion test for the example (CT700) and the comparative example (CT800). Here, the equilibrium precipitation amount means the maximum number of precipitates precipitated when in a thermodynamically equilibrium state, and the larger such an equilibrium precipitation amount is, the more the number of precipitated precipitates increases. Also, the amount of activated hydrogen means the amount of hydrogen excluding the hydrogen trapped by the fine precipitate 20 among the hydrogen flowing into the steel sheet 10.

[0089] Such an amount of activated hydrogen can be measured using a thermal desorption spectroscopy method. Specifically, while heating the specimen at a preset heating rate to raise the temperature, the amount of hydrogen released from the specimen at a specific temperature or lower can be measured. At this time, the hydrogen released from the specimen at a specific temperature or lower can be understood as the activated hydrogen that is not trapped and affects hydrogen delayed fracture among the hydrogen flowing into the specimen.

[0090]

Table 2

[0091] Table 2 shows the results of performing a four-point bending test on each of the samples with different equilibrium precipitation amounts of the fine precipitates, and the amount of activated hydrogen measured using the thermal desorption spectroscopy method.

[0092] Here, the four-point bending test is a test method in which a specimen manufactured to reproduce the state of exposing the specimen to a corrosive environment is subjected to a stress below the elastic limit at a specific point to check for the occurrence of stress corrosion cracks. At this time, stress corrosion cracks mean cracks that occur when corrosion and a continuous tensile stress act simultaneously.

[0093] Specifically, the results of the four-point bending test in Table 2 are the results of checking for the occurrence of fracture by applying a stress of 1,000 MPa in air for 100 hours to each sample. Also, the amount of activated hydrogen was measured using the thermal desorption spectroscopy method described above, and it is the value obtained by measuring the amount of hydrogen released from the specimen at 350 °C or lower while raising the temperature from room temperature to 500 °C at a heating rate of 20 °C / min for each sample.

[0094] Referring to Table 2, in the case of the equilibrium precipitation amount of the fine precipitate 20, the equilibrium precipitation amount of the example (CT700) was measured to be 0.028 wt%, and the equilibrium precipitation amount of the comparative example (CT800) was 0.009 wt%. That is, it was confirmed that by forming more fine precipitates 20 in the example (CT700) compared to the comparative example (CT800), more hydrogen trap sites can be provided.

[0095] On the one hand, in the case of the four-point bending test results, the specimen of the example (CT700) did not break, while the specimen of the comparative example (CT800) broke. Also, in the case of the amount of active hydrogen, the amount of active hydrogen in the example (CT700) was measured to be about 0.780 wppm, and the amount of active hydrogen in the comparative example (CT800) was about 0.801 wppm. In this connection, it was confirmed that the example (CT700) with a relatively lower amount of active hydrogen did not break, while the comparative example (CT800) with a relatively higher amount of active hydrogen broke. This can be understood as the hydrogen delayed fracture property of the example (CT700) being improved compared to the comparative example (CT800).

[0096] That is, in the example (CT700), the precipitation amount of the fine precipitate 20 increased compared to the comparative example (CT800), and as a result, the amount of active hydrogen decreased. This means that in the example (CT700), the amount of hydrogen trapped inside increased compared to the comparative example (CT800), and as a result, it can be understood that the hydrogen delayed fracture property was improved.

[0097] Figures 5A and 5B are images showing the results of performing a four-point bending test on the example (CT700) and the comparative example (CT800), respectively.

[0098] Specifically, Figure 5A shows the result of performing a four-point bending test on the example (CT700), and Figure 5B corresponds to the result of performing a four-point bending test on the comparative example (CT800) under the same conditions as those applied to the example (CT700).

[0099] As shown in Figures 5A and 5B, it can be confirmed that in the case of the example (CT700), the specimen in the four-point bending test result did not break, while in the case of the comparative example (CT800), the specimen broke.

[0100] This is a specimen produced by hot stamping the hot stamping material 1 manufactured by applying a coiling temperature (CT) of 700 °C in the case of the example (CT700) in Figure 5A, and fine precipitates 20 having a diameter of 0.01 μm or less are present per unit area (100 μm2 )It is formed in an amount of 4,500 or more and 16,000 or less per hit, and the average distance between the fine precipitates 20 satisfies 0.4 μm or more and 0.8 μm or less. Therefore, it can be confirmed that in Example (CT700), the hydrogen flowing into the steel sheet 10 is efficiently dispersed and trapped, the hydrogen delayed fracture property is improved, and the tensile strength and bending property are improved.

[0101] Conversely, in the case of the comparative example (CT800) in FIG. 5B, it is a specimen manufactured by hot stamping the hot stamping material 1 manufactured by applying a coiling temperature (CT) of 800 °C. The precipitation amount of the fine precipitates 20 is insufficient, the diameter of the fine precipitates 20 is coarsened, and the probability of generating pressure resistance due to hydrogen bonding increases. Therefore, it can be confirmed that in the comparative example (CT800), the hydrogen flowing into the steel sheet 10 cannot be efficiently dispersed and trapped, and the tensile strength, bending property, and hydrogen delayed fracture property are deteriorated.

[0102] That is, even if they are composed of the same components, differences occur in the strength, bendability, hydrogen delayed fracture property, etc. that the hot stamping material 1 has after passing through the hot stamping process due to the difference in the coiling temperature (CT). This is because a difference occurs in the precipitation behavior of the fine precipitates 20 depending on the coiling temperature (CT). Therefore, by applying the content conditions and process conditions according to the above-described embodiments of the present invention, high strength can be ensured, and the bendability and hydrogen delayed fracture property can be improved.

[0103] Table 3 below quantifies the tensile strength, bendability, and hydrogen delayed fracture property due to the difference in the precipitation behavior of the fine precipitates 20 for a plurality of specimens. Specifically, in Table 3, for a plurality of specimens, the measured values of the precipitation behavior (the number of fine precipitates, the average distance between the fine precipitates, the diameter of the fine precipitates, etc.) and the measured values of the properties (tensile strength, bendability, and amount of activated hydrogen) after hot stamping are described.

[0104] On the one hand, a plurality of specimens were each heated to a temperature of Ac3 (the temperature at which the transformation from ferrite to austenite is completed) or higher, cooled at a cooling rate of 30 °C / s or higher to 300 °C or lower, and then the tensile strength, bendability, and amount of activated hydrogen were measured.

[0105] At this time, the tensile strength and the amount of activated hydrogen were measured based on the above-described four-point bending test and thermal desorption spectroscopy methods, and the bendability was measured by measuring the V-bending angle according to VDA238-100, which is a standard of the German Automobile Industry Association (VDA: Verband Der Automobilindustrie).

[0106] In addition, the precipitation behavior of fine precipitates (the number of fine precipitates, the average distance between fine precipitates, the diameter of fine precipitates, etc.) was measured through the above-described TEM image analysis. Also, the precipitation behavior of fine precipitates was measured for an arbitrary region having an area of 0.5 μm * 0.5 μm and converted based on a unit area (100 μm 2 ) for measurement.

[0107]

Table 3

[0108] Table 3 shows the measured values of the precipitation behavior of fine precipitates (the number of fine precipitates, the average distance between fine precipitates, the diameter of fine precipitates, etc.) for specimens A to T, and the measured values of the properties (tensile strength, bendability, and amount of activated hydrogen) after hot stamping.

[0109] Specimens A to J in Table 3 are specimens produced by hot stamping a hot stamping material manufactured through the S100 to S600 stages by applying the above-described process conditions to a slab satisfying the above-described content conditions (see Table 1). That is, specimens A to J are specimens satisfying the above-described precipitation behavior conditions of the fine precipitates. Specifically, in specimens A to J, the fine precipitates are formed in the steel sheet at 7,000 pieces / 100 μm 2 or more and 16,500 pieces / 100 μm 2 or less, the average diameter of the overall fine precipitates is 0.007 μm or less, and the average distance between the overall fine precipitates satisfies 0.4 μm or more and 0.8 μm or less. Further, among the fine precipitates formed in the steel sheet, 60% or more have a diameter of 0.01 μm or less, and the number of fine precipitates having a diameter of 0.01 μm or less is 4,500 pieces / 100 μm 2 or more and 16,000 pieces / 100 μm 2 or less. Further, among the fine precipitates formed in the steel sheet, 25% or more have a diameter of 0.005 μm or less.

[0110] It is confirmed that specimens A to J satisfying the precipitation behavior conditions of the present invention have improved tensile strength, bendability, and hydrogen delayed fracture characteristics. Specifically, specimens A to J satisfy a tensile strength of 1,350 MPa or more after hot stamping, a bendability of 50 degrees or more after hot stamping, and an activated hydrogen amount of 0.8 wppm or less after hot stamping.

[0111] On the other hand, specimens K to T are specimens that cannot satisfy at least a part of the above-described precipitation behavior conditions of the fine precipitates, and it can be confirmed that the tensile strength, bendability, and / or hydrogen delayed fracture characteristics are inferior to those of specimens A to J.

[0112] In the case of specimen K, the number of fine precipitates having a diameter of 0.01 μm or less is 4,494. This does not reach the lower limit of the number condition of the fine precipitates having a diameter of 0.01 μm or less. Thus, it can be confirmed that the tensile strength of specimen K is only relatively low, 1,331 MPa.

[0113] In the case of test piece L, the total number of fine precipitates is 6,991. This does not reach the lower limit of the total number of fine precipitate conditions. Thus, it can be confirmed that the tensile strength of test piece L is only relatively low at 1,322 MPa.

[0114] In the case of test piece M, the number of fine precipitates with a diameter of 0.01 μm or less is 16,038. This exceeds the upper limit of the number of fine precipitate conditions with a diameter of 0.01 μm or less. Thus, it can be confirmed that the bendability of test piece M is only relatively low at 43°.

[0115] In the case of test piece N, the total number of fine precipitates is 16,521. This exceeds the upper limit of the total number of fine precipitate conditions. Thus, it can be confirmed that the bendability of test piece N is only relatively low at 40°.

[0116] In the case of test piece O, the average diameter of the total fine precipitates is 0.0071 μm. This exceeds the upper limit of the average diameter condition of the total fine precipitates. Thus, it can be confirmed that the amount of activated hydrogen in test piece O is measured to be relatively high at 0.881 wppm, and the hydrogen delayed fracture characteristics have decreased relatively.

[0117] In the case of test piece P, the ratio of fine precipitates with a diameter of 0.01 μm or less is 59.8%. This does not reach the lower limit of the ratio condition of fine precipitates with a diameter of 0.005 μm or less. Thus, it can be confirmed that the amount of activated hydrogen in test piece P is measured to be relatively high at 0.828 wppm, and the hydrogen delayed fracture characteristics have decreased relatively.

[0118] In the case of test piece Q, the ratio of fine precipitates with a diameter of 0.005 μm or less is 24.9%. This does not reach the lower limit of the ratio condition of fine precipitates with a diameter of 0.005 μm or less. Thus, it can be confirmed that the amount of activated hydrogen in test piece Q is measured to be relatively high at 0.815 wppm, and the hydrogen delayed fracture characteristics have decreased relatively.

[0119] In the case of test piece R, the proportion of fine precipitates with a diameter of 0.005 μm or less is 24.8%. This does not reach the lower limit of the proportion condition of fine precipitates with a diameter of 0.005 μm or less. As a result, the amount of activated hydrogen in test piece R was measured to be a relatively high 0.845 wppm, and it can be confirmed that the hydrogen delayed fracture property decreased relatively.

[0120] In the case of test piece S, the average distance of the overall fine precipitates is 0.39 μm. This does not reach the lower limit of the average distance condition of the overall fine precipitates. As a result, it can be confirmed that the bendability of test piece S is only relatively low at 45 degrees.

[0121] In the case of test piece T, the average distance between the overall fine precipitates is 0.81 μm. This exceeds the upper limit of the average distance condition between the overall fine precipitates. As a result, it can be confirmed that the tensile strength of test piece T is only relatively low at 1,344 MPa.

[0122] As a result, the hot stamping material manufactured by the method for manufacturing a hot stamping material applying the content conditions and process conditions of the present invention described above satisfies the precipitation behavior conditions of the fine precipitates described above after hot stamping, and it was confirmed that the hot stamping products satisfying such precipitation behavior conditions of the fine precipitates had improved tensile strength, bendability, and hydrogen delayed fracture properties.

[0123] The present invention has been described based on the embodiments illustrated in the drawings, but this is merely exemplary, and those having ordinary knowledge in the technical field will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention must be determined by the technical idea of the claims.

Claims

1. A steel sheet comprising carbon (C): 0.19 to 0.25% by weight, silicon (Si): 0.1 to 0.6% by weight, manganese (Mn): 0.8 to 1.6% by weight, phosphorus (P): 0.03% by weight or less, sulfur (S): 0.015% by weight or less, chromium (Cr): 0.1 to 0.6% by weight, boron (B): 0.001 to 0.005% by weight, an additive: 0.1% by weight or less, and the balance iron (Fe) and other inevitable impurities, and fine precipitates distributed in the steel sheet, wherein the additive contains at least one of titanium (Ti), niobium (Nb) and vanadium (V), the fine precipitates contain at least one nitride or carbide of titanium (Ti), niobium (Nb) and vanadium (V), and trap hydrogen, The fine precipitates are formed in a number of 7,000 or more and 16,500 or less per unit area (100 μm ２ ), the fine precipitates are formed such that 60% or more of them have a diameter of 0.01 μm or less, among the fine precipitates, the number of fine precipitates having a diameter of 0.01 μm or less is 4,500 or more and 16,000 or less per unit area (100 μm2), the fine precipitates are formed such that 25% or more of them have a diameter of 0.005 μm or less, and the average distance of the fine precipitates is 0.4 μm or more and 0.8 μm or less, a material for hot stamping.

2. Reheating the slab in a slab reheating temperature range of 1,200°C to 1,250°C, hot rolling the reheated slab in a finish rolling temperature range of 840°C to 920°C to produce a steel sheet, and winding the steel sheet in a coiling temperature range of 700°C to 780°C to form fine precipitates in the steel sheet, wherein the slab comprises carbon (C): 0.19 to 0.25% by weight, silicon (Si): 0.1 to 0.6% by weight, manganese (Mn): 0.8 to 1.6% by weight, phosphorus (P): 0.03% by weight or less, sulfur (S): 0.015% by weight or less, chromium (Cr): 0.1 to 0.6% by weight, boron (B): 0.001 to 0.005% by weight, an additive: 0.1% by weight or less, and the balance iron (Fe) and other inevitable impurities, the additive contains at least one of titanium (Ti), niobium (Nb) and vanadium (V), and the fine precipitates contain at least one nitride or carbide of titanium (Ti), niobium (Nb) and vanadium (V), and trap hydrogen. The fine precipitates are formed in a number of 7,000 or more and 16,500 or less per unit area (100 μm ２ ), formed such that 60% or more of the fine precipitates have a diameter of 0.01 μm or less, among the fine precipitates, the number of fine precipitates having a diameter of 0.01 μm or less is 4,500 or more and 16,000 or less per unit area (100 μm 2), formed such that 25% or more of the fine precipitates have a diameter of 0.005 μm or less, A method for manufacturing a material for hot stamping, wherein an average distance between the fine precipitates is 0.4 μm or more and 0.8 μm or less.

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