Hot stamped component

Abstract

The present invention provides a hot-stamped part including a base steel sheet containing 0.19 to 0.25% by weight of carbon (C), 0.1 to 0.6% by weight of silicon (Si), 0.8 to 1.6% by weight of manganese (Mn), 0.03% by weight or less of phosphorus (P), 0.015% by weight or less of sulfur (S), 0.1 to 0.6% by weight of chromium (Cr), 0.001 to 0.005% by weight of boron (B), 0.1% by weight or less of an additive, the remainder of iron (Fe), and other inevitable impurities, wherein the number of indentation dynamic strain aging in the indentation strain rate observed with respect to an indentation depth of 200 to 600 nm during nanoindentation testing is 26 to 40.

Description

TECHNICAL FIELD

[0001] The present application relates to a hot stamped part. BACKGROUND

[0002] As environmental and fuel economy regulations are strengthened around the world, the demand for lightweight vehicle materials is also increasing. Accordingly, research and development of ultra-high strength steel and hot stamped steel are actively being conducted. In this case, the hot stamping process generally consists of heating / forming / cooling / fettling, and includes phase transformation and microstructure change of the material during the process.

[0003] In recent years, research has been actively conducted to improve the delayed fracture, corrosion resistance, and weldability of hot stamped parts manufactured through the hot stamping process. As prior art, there are Korean Patent Publication No. 10-2018-0095757 (Invention title: Method for manufacturing hot stamped part), etc. SUMMARY

[0004] Technical problem

[0005] The present application provides a hot stamped part having improved crashworthiness.

[0006] However, this problem is only an example, and the scope of the present application is not limited thereto.

[0007] Technical scheme

[0008] According to one aspect of the present application, a hot stamped part includes a base steel sheet including: carbon (C) in an amount of about 0.19 wt% to about 0.25 wt%, silicon (Si) in an amount of about 0.1 wt% to about 0.6 wt%, manganese (Mn) in an amount of about 0.8 wt% to about 1.6 wt%, phosphorus (P) in an amount of about 0.03 wt% or less, sulfur (S) in an amount of about 0.015 wt% or less, chromium (Cr) in an amount of about 0.1 wt% to about 0.6 wt%, boron (B) in an amount of about 0.001 wt% to about 0.005 wt%, an additive in an amount of about 0.1 wt% or less, the remainder being iron (Fe) and other unavoidable impurities. In the indentation strain rate observed in the nanoindentation test with respect to an indentation depth of about 200 nm to about 600 nm, the number of indentation dynamic strain aging (DSA) is about 26 to about 40.

[0009] According to an exemplary embodiment, the base steel sheet can include a martensite structure in which a plurality of lath structures are distributed.

[0010] According to an exemplary embodiment, the average interval between the plurality of laths can be about 140 nm to about 300 nm.

[0011] According to an exemplary embodiment, the hot-stamped part can further include fine precipitates distributed in the base steel sheet, wherein the fine precipitates include nitrides or carbides of at least any one of titanium (Ti), niobium (Nb), and vanadium (V).

[0012] According to an exemplary embodiment, the number of the fine precipitates distributed per unit area (100 μm 2 ) can be about 7500 to about 18000.

[0013] According to an exemplary embodiment, the average diameter of the fine precipitates can be about 0.0068 μm or less.

[0014] According to an exemplary embodiment, the proportion of the fine precipitates having a diameter of about 0.01 μm or less among the fine precipitates can be about 63% or more.

[0015] According to an exemplary embodiment, the proportion of the fine precipitates having a diameter of about 0.005 μm or less among the fine precipitates can be about 28% or more.

[0016] According to an exemplary embodiment, the V-bend angle of the hot-stamped part can be equal to or greater than about 50°.

[0017] According to an exemplary embodiment, the tensile strength of the hot-stamped part can be equal to or greater than about 1350 MPa.

[0018] According to an exemplary embodiment, the amount of active hydrogen of the hot-stamped part can be about 0.8 wppm or less.

[0019] Advantageous effects

[0020] According to the embodiments of the present application as described above, a hot-stamped part can be implemented. However, the scope of the present application is not limited by the effects. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 shows a transmission electron microscope (TEM) image of a portion of a hot-stamped part according to an exemplary embodiment of the present application.

[0022] Figure 2 is a load-displacement graph regarding nanoindentation testing of a hot-stamped part according to an exemplary embodiment of the present application.

[0023] Figure 3 is a magnified view showing serrations in portion A of Figure 2

[0024] Figure 4 is a graph obtained by measuring indentation dynamic strain aging. ​

[0025] Figure 5 is an enlarged view of the portion B in FIG. 1. Figure 4 is an enlarged view of the portion B in FIG. 1.

[0026] Figure 6 is a schematic diagram showing a mechanism of indentation dynamic strain aging regarding a banding and dislocation movement in a band boundary of a hot stamped part according to an exemplary embodiment of the present application. DETAILED DESCRIPTION

[0027] The present application can have various modifications and various embodiments, and thus specific embodiments will be shown in the drawings and described in detail in the detailed description. The effects and features of the present application and methods of achieving the same will become apparent by referring to the embodiments described in detail in the detailed description taken in conjunction with the accompanying drawings. However, the present application is not limited to the embodiments disclosed herein but can be implemented in various forms.

[0028] Hereinafter, embodiments of the present application will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or similar components are designated by the same reference numerals, and repetitive description thereof will be omitted.

[0029] In the present document, terms such as first, second, etc. are used to distinguish one component from another component, but are not limiting.

[0030] In the present document, the singular form includes the plural form, unless the context clearly indicates otherwise.

[0031] In the present document, the terms "include", "have", etc. are intended to mean that there are the features or components described in the present document, but do not exclude the possibility of additional one or more features or components.

[0032] In the present document, when a part such as a film, a region, a component, etc. is present on or above another part, this case can include not only a case where it is directly on another part, but also a case where another film, region, component, etc. is disposed between the part and the other part.

[0033] In the present document, when films, regions, components, etc. are connected, this case can include a case where they are directly connected, or / and a case where they are indirectly connected with another film, region, and component therebetween. For example, in the present document, when films, regions, components, etc. are electrically connected, this case can include a case where they are directly electrically connected, and / or a case where they are indirectly electrically connected with another film, region, and component therebetween.

[0034] In the present document, "A and / or B" can mean A, B, or A and B. "At least one of A and B" can mean A, B, or A and B.

[0035] In this document, a particular process sequence can be performed in a different order than described. For example, two processes described sequentially can be performed at substantially the same time, or in the reverse order of the order described.

[0036] In the drawings, the size of the components can be exaggerated or reduced for the sake of convenience or clarity. For example, the size and thickness of each component shown in the drawings are chosen to facilitate the understanding of the present application, and therefore, the present application is not necessarily limited to the size and thickness shown.

[0037] Figure 1 A transmission electron microscope (TEM) image of a portion of a hot stamped part according to an exemplary embodiment of the present application is shown.

[0038] Referring to Figure 1 The hot stamped part can include a base steel sheet. The base steel sheet can be a steel sheet prepared by performing a hot rolling process and / or a cold rolling process on a cast slab to contain predetermined alloying elements in predetermined amounts. In an exemplary embodiment, the base steel sheet can contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), boron (B), the remainder of iron (Fe), and other unavoidable impurities. In an exemplary embodiment, the base steel sheet can further contain at least any one of titanium (Ti), niobium (Nb), and vanadium (V) as additives. In another embodiment, the base steel sheet can further contain calcium (Ca) in a predetermined amount.

[0039] Carbon (C) can be used as an austenite stabilizing element in the base steel sheet. Carbon is a main element that determines the strength and hardness of the base steel sheet, and the addition of carbon can ensure the tensile strength (e.g., a tensile strength of 1350 MPa or more) of the base steel sheet and ensure the hardenability after the hot stamping process. The amount of carbon contained can be about 0.19 wt% to about 0.25 wt% based on the total weight of the base steel sheet. When the content of carbon is less than 0.19 wt%, it is difficult to satisfy the mechanical strength of the base steel sheet because it is difficult to secure a hard phase (martensite, etc.). On the other hand, when the content of carbon exceeds 0.25 wt%, brittleness can occur in the base steel sheet, or the bending property of the base steel sheet can be reduced.

[0040] Silicon (Si) can be used as a ferrite stabilizing element in the base steel sheet. Silicon (Si) improves the strength of the base steel sheet as a solid solution strengthening element, and improves the concentration of carbon in austenite by suppressing the formation of low-temperature carbides. Silicon is a key element in hot rolling, cold rolling, hot pressing, microstructure homogenization (pearlite, manganese segregation zone control), and fine dispersion of ferrite. Silicon can be used as a martensite strength unevenness control element to improve crashworthiness. The amount of silicon contained can be about 0.1 wt% to about 0.6 wt% based on the total weight of the base steel sheet. When the content of silicon is less than 0.1 wt%, it is difficult to obtain the above effects, the formation and coarsening of cementite can occur in the final hot-stamped martensite structure, the balance effect of the base steel sheet is not obvious, and it can not be possible to ensure the V-bend angle. On the other hand, when the content of silicon exceeds 0.6 wt%, the load of hot rolling and cold rolling increases, the red scale of hot rolling becomes excessive, and the plating performance of the base steel sheet can be poor.

[0041] Manganese (Mn) can be used as an austenite stabilizing element in the base steel sheet. Manganese can be added to increase the hardenability and strength in heat treatment. The amount of manganese contained can be about 0.8 wt% to about 1.6 wt% based on the total weight of the base steel sheet. When the content of manganese is less than 0.8 wt%, the hardenability is insufficient due to insufficient hardenability effect, and the fraction of hard phases in the formed product after hot stamping can be insufficient. On the other hand, when the content of manganese exceeds 1.6 wt%, the ductility and toughness can be reduced due to manganese segregation or pearlite bands, resulting in a decrease in bending performance and causing uneven microstructure.

[0042] The amount of phosphorus (P) contained can be 0 to about 0.03 wt% based on the total weight of the base steel sheet to prevent a decrease in toughness of the base steel sheet. When the content of phosphorus exceeds about 0.03 wt%, iron phosphide compounds are formed, resulting in a decrease in toughness and weldability, and causing cracking of the base steel sheet during manufacturing.

[0043] The amount of sulfur (S) contained can be 0 to about 0.015 wt% based on the total weight of the base steel sheet. When the content of sulfur exceeds 0.015 wt%, hot workability, weldability, and impact properties can decrease, and surface defects such as cracks can occur due to the generation of large inclusions.

[0044] Chromium (Cr) can be added to improve the hardenability and strength of the base steel sheet. Chromium enables grain refinement and ensures strength through precipitation hardening. The amount of chromium contained can be about 0.1 wt% to about 0.6 wt% based on the total weight of the base steel sheet. When the content of chromium is less than 0.1 wt%, the precipitation hardening effect is low, on the other hand, when the content of chromium exceeds 0.6 wt%, the amount of chromium-based precipitates and matrix solid solution increases, resulting in a decrease in toughness, and the cost price increases, resulting in an increase in production costs.

[0045] Boron (B) can be added to ensure a martensitic structure by suppressing the transformation of ferrite, pearlite and bainite, thereby ensuring the hardenability and strength of the base steel sheet. Boron can be segregated in the grain boundaries to lower the grain boundary energy to increase the hardenability, and can produce a grain refinement effect by increasing the austenite grain growth temperature. The amount of boron contained can be about 0.001 wt% to about 0.005 wt% based on the total weight of the base steel sheet. When the boron contained is within the above range, the generation of hard phase intergranular brittleness can be prevented, and high toughness and bendability can be ensured. When the content of boron is less than 0.001 wt%, the hardenability effect can be insufficient, on the other hand, when the content of boron exceeds 0.005 wt%, boron can easily precipitate in the grain boundaries due to low solid solubility depending on the heat treatment conditions, causing a decrease in hardenability or leading to thermal embrittlement, and the toughness and bendability can decrease due to the generation of hard phase intergranular brittleness.

[0046] On the other hand, according to an exemplary embodiment of the present application, fine precipitates can be included in the base steel sheet. The additives that constitute some of the elements contained in the base steel sheet can be nitride or carbide forming elements that help form fine precipitates.

[0047] More specifically, the additives can include at least any one of titanium (Ti), niobium (Nb) and vanadium (V). Titanium (Ti), niobium (Nb) and vanadium (V) can form fine precipitates in the form of nitrides or carbides, thereby ensuring the strength of the hot-stamped and quenched member. In addition, they can be contained in the Fe-Mn-based complex oxide, can act as effective hydrogen trapping sites for improving the delayed fracture resistance, and can be elements required to improve the delayed fracture resistance. The amount of the additives contained can total about 0.1 wt% or less based on the total weight of the base steel sheet. When the content of the additives exceeds 0.1 wt%, the yield strength can excessively increase.

[0048] Titanium (Ti) can be added to strengthen the grain refinement and to upgrade the material by forming precipitates after hot press heat treatment. In addition, titanium can form precipitates such as TiC and / or TiN at high temperatures, thereby effectively promoting the austenite grain refinement. The amount of titanium contained can be about 0.018 wt% to about 0.045 wt% based on the total weight of the base steel sheet. When the titanium contained is within the content range, continuous casting defects and precipitate coarsening can be prevented, the physical properties of the steel material can be easily ensured, and defects such as cracks on the surface of the steel material can be prevented. On the other hand, when the content of titanium exceeds 0.045 wt%, the precipitates can be coarsened, causing a decrease in elongation and bendability.

[0049] Niobium (Nb) and vanadium (V) can be added to increase strength and toughness according to a decrease in the size of the martensite lath. The amount of each of the niobium and vanadium contained can be about 0.025 wt% to about 0.050 wt% based on the total weight of the base steel sheet. When the niobium and vanadium contained are within the above range, excellent grain refinement effects can be produced on the steel material during the hot rolling process and the cold rolling process, cracks in the slab during steelmaking / continuous casting are prevented, and brittle fracture of the product is prevented, and the production of coarse precipitates in the steelmaking is minimized.

[0050] Calcium (Ca) can be added to control the shape of inclusions. The calcium contained can be about 0.003 wt% or less based on the total weight of the base steel sheet.

[0051] As described above, the hot-stamped part according to the embodiment of the present application can include fine precipitates containing nitrides or carbides of at least one of titanium (Ti), niobium (Nb), and vanadium (V) in the base steel sheet. These fine precipitates can be distributed in the base steel sheet to trap hydrogen. That is, the fine precipitates can provide a trapping site for hydrogen introduced during or after the manufacture of the hot-stamped part, thereby improving the hydrogen embrittlement of the hot-stamped part.

[0052] In an exemplary embodiment, the number of fine precipitates formed in the base steel sheet can be controlled to satisfy a preset range. In an exemplary embodiment, the number of fine precipitates included in the base steel sheet can be about 6000 per unit area (100 μm 2 ) to about 21000 per 100 μm 2 . Further, in an exemplary embodiment, the average diameter of the fine precipitates distributed in the base steel sheet can be less than or equal to about 0.0075 μm, preferably about 0.004 μm to about 0.0075 μm. The hot-stamped part including the above fine precipitates can have improved bendability and crashworthiness due to excellent V-bending characteristics.

[0053] More specifically, the number of fine precipitates included in the base steel sheet can be about 7500 per unit area (100 μm 2 ) to about 18000 per 100 μm 2 . In an exemplary embodiment, the average diameter of the fine precipitates distributed in the base steel sheet can be about 0.0068 μm or less. Among these fine precipitates, the proportion of fine precipitates having a diameter of about 10 nm or less can be about 63% or more, and the proportion of fine precipitates having a diameter of about 5 nm or less can be about 28% or more. Under the above conditions, the hot-stamped part including the fine precipitates can not only have excellent bendability and crashworthiness, but also improved delayed fracture characteristics due to hydrogen.

[0054] The diameter of the fine precipitates can have a great influence on the improvement of the hydrogen delayed fracture characteristics. When the number, size, proportion, etc. of the fine precipitates are formed within the above-described ranges, the tensile strength (e.g., 1350 MPa) required after hot stamping can be secured, and the formability and the bendability can be improved. For example, when the number of the fine precipitates per unit area (100 μm 2 ) is less than 7500 / 100 μm 2 , the strength of the hot stamped part can be decreased, and when the number of the fine precipitates per unit area exceeds 18000 / 100 μm 2 , the formability or the bendability of the hot stamped part can be decreased.

[0055] In an exemplary embodiment, the amount of active hydrogen in the base steel sheet can be about 0.8 wppm or less. The amount of active hydrogen can mean the amount of hydrogen other than the hydrogen captured by the fine precipitates among the hydrogen introduced in the base steel sheet. The amount of active hydrogen can be measured using a thermal desorption spectroscopy. More specifically, while the temperature is increased by heating a test sample at a predetermined heating rate, the amount of hydrogen released from the test sample below a certain temperature can be measured. In this case, the hydrogen released from the test sample below a certain temperature can be understood as active hydrogen, which is not captured among the hydrogen introduced into the test sample, affecting the hydrogen delayed fracture. For example, as a comparative example, when the hot stamped part includes the amount of active hydrogen in the base steel sheet exceeding 0.8 wppm, the hydrogen delayed fracture characteristics can be decreased, and the hot stamped part according to the comparative example can be more easily fractured than the hot stamped part according to the current embodiment in a bend test under the same conditions.

[0056] On the other hand, the base steel sheet according to the current embodiment can include a martensite structure in which fine structures are distributed. The martensite structure is a result of a non-diffusional transformation of austenite γ at a start temperature Ms of martensite transformation during cooling. The fine structures in the martensite structure can be non-diffusional transformation structures formed within the grains during rapid cooling, referred to as prior austenite grain boundaries (PAGB), and can include a plurality of lath L structures. The plurality of lath L structures can constitute a single body, such as a lath block or a lath bundle. More specifically, the plurality of lath L structures can form a lath block, the plurality of lath blocks can form a lath bundle, and the plurality of lath bundles can form a PAGB.

[0057] As described above, the martensite can have a lath L structure, which is in the form of long and thin strips arranged in one direction in each prior grain of the austenite. The plurality of lath L structures can have a property of resisting external strain at the boundaries therebetween, i.e., lath boundaries LB. This will be described in detail below.

[0058] On the other hand, the V-bending angle of the hot stamped part according to the present embodiment can be 50° or more. The 'V-bending' can be a parameter to evaluate the bending strain property in which the maximum load portion appears when a strain occurs. That is, according to the load-displacement evaluation of the hot stamped part, the tensile strain region in the bending process is observed in macro and micro sizes, and when a fine crack is generated and propagated in the local tensile region, the bending property called the V-bending angle can be evaluated.

[0059] As described above, the hot stamped part according to the exemplary embodiment can include a martensite structure having a plurality of lath L structures, and when one-dimensional defects called dislocations move by interaction in the martensite structure, a crack can occur in the bending strain. It can be understood that, in a given plastic strain, a higher local strain rate makes the energy absorption degree of the martensite against the plastic strain higher, and thus the crashworthiness increases.

[0060] In the hot stamped part according to the exemplary embodiment of the present application, since the martensite structure has a plurality of lath L structures, dynamic strain aging (DSA), i.e., indentation DSA, can occur because of the difference in strain rate between the lath L and the lath boundary LB during the repeated movement of dislocations in the bending strain. The indentation DSA is a concept of plastic strain absorbing energy, and can mean a resistance property with respect to strain, and thus as the indentation DSA phenomenon is frequent, the resistance property with respect to strain can be evaluated as excellent.

[0061] In the hot stamped part according to the exemplary embodiment of the present application, the martensite structure has a plurality of lath L structures in a coarse form, and thus the indentation DSA phenomenon can frequently occur, so that a V-bending angle of 50° or more can be secured, thereby improving the bending property and the crashworthiness.

[0062] In the exemplary embodiment, the average interval between a plurality of lath L included in the martensite structure of the hot stamped part according to the exemplary embodiment can be about 140 nm to about 300 nm. As a comparative example, it is assumed that a hot stamped part including a base steel sheet composed of the above-described elements includes a lath structure. The average interval between the lath structures of the hot stamped part of the comparative example can be greater than the average interval of the lath L structure of the hot stamped part according to the present embodiment. That is, the hot stamped part according to the present embodiment can have a lath L structure coarser than that of the comparative example, and as the lath L structure in the hot stamped part becomes coarser, the number of indentation DSAs can further increase.

[0063] Figure 2 is a load-displacement graph regarding the nanoindentation test of the hot stamped part according to the exemplary embodiment of the present application, Figure 3 is a graph showing Figure 2 is a magnified view of the sawtooth behavior of part A in FIG. 8.

[0064] Referring to Figure 2 , a graph showing the results of nanoindentation testing of the hot stamped part according to the embodiments of the present application is shown. The 'nanoindentation testing' is a test in which the strain of force with respect to the depth is measured by vertically pressing an indenter on the surface of the hot stamped part. In Figure 2 , the x-axis indicates the depth of the indenter pressed, and the y-axis indicates the force with respect to the depth of the press. For example, although a cube corner indenter (included angle of centerline to face = 35.3°, indentation strain rate = 0.22) is used as the indenter in Figure 2 , the present application is not limited thereto, and a Berkovich indenter (included angle of centerline to face = 65.3°, indentation strain rate = 0.072) can also be used.

[0065] Referring to the Figure 2 enlarged view of part A, Figure 3 , it can be seen that, among the indentation and plastic strain occurring in the nanoindentation test, a sawtooth-like strain, i.e., a characteristic behavior called a sawtooth, is observed. The sawtooth behavior can repeatedly occur at nearly fixed intervals, and in Figure 3 , the sawtooth behavior is indicated by downward arrows (↓).

[0066] The sawtooth behavior can be caused by the non-diffusional transformation structure in the PAGB contained in the hot stamped part in the indentation test of the hot stamped part. More specifically, the sawtooth behavior in the load-displacement curve as shown in Figure 2 occurs due to the interaction between dislocations and diffused solute atoms in the material, and can be understood to be caused by the difference in resistance to external pressure between a plurality of laths distributed in the PAGB and the lath boundary portions formed therebetween. The sawtooth behavior can be considered as Figure 4 evidence of the DSA in the indentation (i.e., the indentation DSA phenomenon).

[0067] Figure 4 is a graph measuring the indentation DSA, Figure 5 is an enlarged view of part B in Figure 4 .

[0068] Figure 4 is a graph analyzing the nanoindentation strain rate ([dh / dt] / h, where h indicates the indentation depth and t indicates the unit time) based on the load-displacement curve in Figure 3 .

[0069] In an exemplary embodiment, in the hot stamped part, the number of indentation DSAs can be about 26 to about 40 for the indentation strain rate observed in the nanoindentation test with respect to the indentation depth of about 200 nm to about 600 nm. The indentation DSA can show a behavior in which the indentation strain rate repeatedly forms a plurality of peaks.

[0070] The number of indentation DSAs can be calculated based on peaks passing through the reference line C as the center. That is, the number of indentation DSAs can be calculated based on peaks formed passing through the reference line C, rather than peaks formed above or below the reference line C. The reference line C can be a line assuming that indentation DSAs caused by the slugs and slug boundary structure are removed in the indentation strain rate measurement.

[0071] Referring to Figure 5 the indentation strain rate graph, it can be seen that the number and size of indentation DSAs gradually decrease as the indentation depth gradually increases. This is because the indentation physical properties of the prior austenite grains are mixed as the indentation depth gradually increases, so that the indentation DSAs rarely occur. Referring to Figure 4 , it can be seen that the indentation DSAs are substantially rarely present at an indentation depth of 600 nm or more. In Figure 4 the graph, indentation depths of 700 nm or more are not measured, but a curve removing DSAs from the corresponding portion can be obtained by continuously measuring the indentation strain rate with respect to the indentation depth of 700 nm or more. The reference line C can be derived by inversely estimating the indentation strain rate curve at the indentation depth at which the indentation DSAs are removed.

[0072] As described above, the number of indentation DSAs of the hot-stamped part according to the current embodiment can be 26 to 40, which can be based on measurements in a portion having an indentation depth of about 200 nm to about 600 nm. Although the measurements in Figure 4 are made at an indentation depth of 0 nm to about 700 nm, the accuracy of the indentation strain rate is low at an indentation depth lower than about 200 nm due to the dullness of the indenter, and at an indentation depth higher than about 600 nm, the indentation physical properties of the prior austenite grains are mixed, so that it is difficult to evaluate the DSAs.

[0073] As shown in Figure 4 , the indentation strain rate is secondarily gradually reduced according to the indentation depth, macroscopically. In this case, the indentation DSAs can be manifested as a behavior in which the indentation strain rate repeatedly forms a plurality of peaks. To observe this in detail, in Figure 5 , the indentation strain rate with respect to the indentation depth of about 350 nm to about 400 nm in Figure 4 is shown in an enlarged manner.

[0074] Referring to Figure 5The indentation strain rate can exhibit a repeating pattern of rising and falling portions. Portion 'a' can refer to the portion of the indentation strain rate that increases during the indentation test, absorbing resistance. That is, portion 'a' can be understood as a portion in which, during bending strain, dislocations slide within the laths distributed in the PAGB during dislocation movement within the tension-generating portion. Therefore, as dislocations move within the laths, the hot-stamped component exhibits the property of absorbing external resistance, which can be manifested as follows: Figure 5 The diagram shows the portion where the indentation strain rate increases. As dislocations rise to the lath boundary, the indentation strain rate decreases as the dislocations pass through the lath boundary, as in portion b. This can be explained as a phenomenon occurring due to interaction with fine precipitates distributed along the lath boundary.

[0075] Figure 6 The schematic diagram illustrates the mechanism of indentation DSA relative to dislocation movement in the bending strain of a hot-stamped component according to an embodiment of the present invention.

[0076] Reference Figure 6 The diagram shows the laths L distributed in the PAGB and the lath boundaries LB in the tensile portion during bending strain, schematically illustrating the distribution according to... Figure 5 Dislocation movement in indentation DSA. As mentioned above, in bending strain, dislocations can move along adjacent laths L. Figure 6 The arrows in the diagram indicate the direction of dislocation movement.

[0077] Therefore, it can be analyzed that during dislocation movement, the indentation strain rate varies with the degree of energy absorption in the lath L and the lath boundary LB. Simultaneously, referring to... Figure 5 and Figure 6 Dislocation along Figure 6 The movement of the arrow in slat L can correspond to Figure 5 Part a. That is, the indentation strain rate increases as the dislocation moves within the lath L. The indentation strain rate increases until the dislocation is adjacent to the lath boundary LB, and decreases as the dislocation passes through the lath boundary LB. This can correspond to... Figure 5 Part b in the text. Therefore, as... Figure 5 The indentation DSA shown may occur due to the interaction between dislocations and lath boundaries LB during dislocation movement. As described above, fine precipitates distributed within the lath boundaries LB delay strain, and repeated increases and decreases in strain rate occur during the passage through multiple laths L, causing the indentation DSA.

[0078] The hot stamped part according to the exemplary embodiment of the present application can control fine precipitates contained in the base steel sheet to reduce the average interval between a plurality of laths, so that the indentation DSA phenomenon occurs more frequently at dislocation slip in bending strain. As such, the hot stamped part according to the exemplary embodiment of the present application can secure a V-bending angle of 50° or more without being broken in bending strain as the indentation DSA phenomenon increases through coarsening of the lath structure.

[0079] Hereinafter, the present application will be described in more detail through embodiments and comparative examples. However, the following embodiments and comparative examples are to describe the present application in more detail, and the scope of the present application is not limited by the following embodiments and comparative examples. The following embodiments and comparative examples can be modified and changed appropriately by those of ordinary skill in the art within the scope of the present application.

[0080] The hot stamped part according to the exemplary embodiment of the present application can be formed by performing a hot stamping process on a base steel sheet having components shown in Table 1 below.

[0081] Table 1

[0082]

[0083] As described above, the hot stamped part according to the exemplary embodiment of the present application can include fine precipitates in the base steel sheet, the fine precipitates containing nitrides and / or carbides of an additive, wherein the fine precipitates in the hot stamped part can be distributed at 6000 / 100 μm 2 to 21000 / 100 μm 2 to 21000 / 100 μm 2 in the base steel sheet. In the exemplary embodiment, the fine precipitates distributed in the base steel sheet can have an average diameter of about 0.004 μm to about 0.0075 μm. The hot stamped part satisfying the above conditions can have a V-bending angle of 50° or more. Table 2 shows values measured by quantifying the precipitation behavior of the fine precipitates and the number of indentation DSA and the V-bending angle with respect to the titanium content of the exemplary embodiment of the present application and comparative examples.

[0084] Table 2

[0085]

[0086] In Table 2, Embodiments 1 to 7 satisfy the precipitation behavior condition of fine precipitates and the formation condition of a plurality of laths with respect to the titanium content as described above. More specifically, in Embodiments 1 to 7, the amount of titanium contained can be about 0.018 wt% to about 0.045 wt%, and the respective average interval of a plurality of laths can be about 140 nm to about 300 nm, and the number of fine precipitates containing titanium, such as titanium carbide (TiC) per unit area can be about 6000 / 100 μm 2 to about 21000 / 100 μm 2 , and the average diameter of all fine precipitates can be about 0.004 μm to about 0.0075 μm. In this case, the number of indentation DSA satisfies the condition of 26 to 40. Thus, it can be seen that Embodiments 1 to 7 satisfying the precipitation behavior condition and the plurality of lath formation condition of the present application can ensure a V-bend angle of 50° or more, thereby improving the tensile strength and the bendability. On the other hand, it can be seen that Comparative Example 1 and Comparative Example 2 fail to satisfy at least some of the above-mentioned precipitation behavior condition and the plurality of lath formation condition, so that the tensile strength and the bendability are lower than Embodiments 1 to 7.

[0087] In Comparative Example 1, since the titanium content is 0.047 wt%, the size of fine precipitates is coarse, so that the average interval between a plurality of laths is reduced to about 135 nm, the number of indentation DSA is 24, and the foregoing condition is not satisfied. Thus, the V-bend angle of Comparative Example 1 is only 43°.

[0088] In Comparative Example 2, since the titanium content is 0.017 wt%, the size and density of fine precipitates are reduced, so that the average interval between a plurality of laths is increased to about 320 nm, the number of indentation DSA is 25, and the foregoing condition is not satisfied. Thus, the V-bend angle of Comparative Example 2 is only 45°.

[0089] More specifically, the fine precipitates in the hot-stamped part according to the exemplary embodiment of the present application can be about 7500 / 100 μm 2 per unit area (100 μm 2 to about 18000 / 100 μm 2The fine precipitates are included in the base steel sheet. In an exemplary embodiment, the average diameter of the fine precipitates included in the base steel sheet can be about 0.0068 μm or less. Among the fine precipitates, the proportion of fine precipitates having a diameter of about 0.01 μm or less can be about 63% or more, and the proportion of fine precipitates having a diameter of about 0.005 μm or less can be about 28% or more. In an exemplary embodiment, the amount of active hydrogen in the base steel sheet can be about 0.8 wppm or less. The hot-stamped part having the above-described characteristics has excellent bendability and improved resistance to hydrogen embrittlement.

[0090] Table 3 shows values measured by quantifying the precipitation behavior of the fine precipitates according to exemplary embodiments of the present application and comparative examples.

[0091] The precipitation behavior of the fine precipitates can be measured using a method of analyzing TEM images. More specifically, TEM images of up to a predetermined number of random regions can be obtained for a test sample. The fine precipitates can be extracted from the obtained images through an image analysis program or the like, and the number of fine precipitates, the average distance between the fine precipitates, the diameter of the fine precipitates, etc. can be measured for the extracted fine precipitates.

[0092] In an exemplary embodiment, for the measurement of the precipitation behavior of the fine precipitates, a replication method can be applied to the test sample, such as pre-treatment. For example, a one-step replication method, a two-step replication method, an extraction replication method, etc. can be applied, without being limited thereto.

[0093] In another exemplary embodiment, in measuring the diameter of the fine precipitates, the shape of the fine precipitates can be converted into a circular shape to calculate the diameter of the fine precipitates by considering the non-uniformity of the form of the fine precipitates. More specifically, the area of the fine precipitates extracted using a unit pixel having a specific area can be measured, and the fine precipitates can be converted into a circle having the same area as the measured area, thereby calculating the diameter of the fine precipitates.

[0094] Table 3

[0095]

[0096] In Table 3, the precipitation behavior of the fine precipitates (the total number of fine precipitates per unit area, the average diameter of the fine precipitates, the proportion of fine precipitates having a diameter of about 10 nm or less, the amount of active hydrogen) was measured for test samples A to N. Test samples A to J of Table 3 are test samples of hot-stamped parts manufactured using base steel sheets satisfying the above-described content conditions (see [Table 1]), as embodiments of the present application. That is, test samples A to J satisfy the above-described precipitation behavior conditions of the fine precipitates. More specifically, in test samples A to J, 7500 / 100 μm 2to 18000 / 100 μm 2 of the total fine precipitates was about 0.0068 μm or less, 63% or more of the fine precipitates formed in the steel sheet had a diameter of about 10 nm or less, and 28% or more of the fine precipitates had a diameter of 5 nm or less. It can be seen that, for the test pieces A to J satisfying the precipitation behavior conditions of the fine precipitates of the present application, the hydrogen delayed fracture characteristics were improved because the condition that the amount of active hydrogen was 0.8 wppm or less was satisfied.

[0097] On the other hand, the test pieces K to N did not satisfy at least some of the above-mentioned precipitation behavior conditions of the fine precipitates, and it can be seen that the tensile strength, the bendability, and / or the hydrogen delayed fracture characteristics were lower than those of the test pieces A to J.

[0098] For the test piece K, the average diameter of the total fine precipitates was about 0.0070 μm. This was less than the lower limit of the average diameter of the total fine precipitates. Therefore, the amount of active hydrogen of the test piece K was relatively high, at 0.891 wppm.

[0099] For the test piece L, the proportion of the fine precipitates having a diameter of 10 nm or less was about 62.8%. Therefore, the amount of active hydrogen of the test piece L was relatively high, at 0.878 wppm.

[0100] For the test pieces M and N, the proportions of the fine precipitates having a diameter of about 5 nm or less were about 27.8% and about 27.9%, respectively. Therefore, it can be seen that the amounts of active hydrogen of the test pieces M and N were relatively high, at about 0.865 wppm and about 0.859 wppm.

[0101] When the precipitation behavior conditions of the present application are not satisfied as in the test pieces K to N, relatively more hydrogen is trapped in one fine precipitate, or the trapped hydrogen elements are locally concentrated and combined with each other to form hydrogen molecules H2, thereby generating internal pressure, in the hot stamping process, so that the hydrogen delayed fracture characteristics of the hot stamped product are reduced.

[0102] On the other hand, when the precipitation behavior conditions of the present application are satisfied as in the test pieces A to J, the number of hydrogen atoms trapped in one fine precipitate can be relatively small, or the trapped hydrogen atoms can be relatively uniformly distributed, in the hot stamping process. Therefore, the internal pressure generated by the hydrogen molecules formed from the trapped hydrogen atoms can be reduced, so that the hydrogen delayed fracture characteristics of the hot stamped product can be improved.

[0103] Therefore, it can be seen that the hydrogen delayed fracture characteristics are improved because the hot stamped parts to which the above-mentioned content conditions of the present application are applied satisfy the above-mentioned precipitation behavior conditions of the fine precipitates after hot stamping.

[0104] While the application has been described with reference to the embodiments illustrated in the drawings, it will be understood by those of ordinary skill in the art that various changes and equivalents can be made from the embodiments shown. Therefore, the true technical scope of the application should be defined by the technical spirit of the appended claims.

Claims

1. A hot-stamped component comprising a base steel plate, said base steel plate comprising: 0.19 wt% to 0.25 wt% carbon (C), 0.1 wt% to 0.6 wt% silicon (Si), 0.8 wt% to 1.6 wt% manganese (Mn), 0.03 wt% or less phosphorus (P), 0.015 wt% or less sulfur (S), 0.1 wt% to 0.6 wt% chromium (Cr), 0.001 wt% to 0.005 wt% boron (B), 0.025 wt% to 0.045 wt% titanium (Ti), the remainder iron (Fe), and other unavoidable impurities, wherein, In the indentation strain rates observed in nanoindentation tests relative to indentation depths of 200 nm to 600 nm, the number of indentation dynamic strain aging (DSA) values ​​was 29 to 40; wherein, the hot-stamped component further includes fine precipitates distributed in the base steel plate, the fine precipitates including titanium (Ti) nitrides or carbides; and wherein, per unit area (100 μm) 2 The number of fine precipitates distributed is between 9,954 and 21,000, the average diameter of the fine precipitates is 0.0051 μm or greater and 0.0075 μm or less, and the V-bending angle of the hot-stamped part is equal to or greater than 53°.

2. The hot-stamped component according to claim 1, wherein, The base steel plate comprises a martensitic structure, in which multiple lath structures are distributed.

3. The hot-stamped component according to claim 2, wherein, The average spacing between multiple slats ranges from 140 nm to 300 nm.

4. The hot-stamped component according to claim 1, wherein, The average diameter of the fine precipitates is 0.0068 μm or less.

5. The hot-stamped component according to claim 1, wherein, The proportion of fine precipitates with a diameter of 0.01 μm or smaller is 63% or greater.

6. The hot-stamped component according to claim 1, wherein, The proportion of fine precipitates with a diameter of 0.005 μm or smaller is 28% or greater.

7. The hot-stamped component according to claim 1, wherein, The tensile strength of the hot-stamped component is equal to or greater than 1350 MPa.

8. The hot-stamped component according to claim 1, wherein, The amount of active hydrogen in the hot-stamped component is 0.8 wppm or less.

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