KR102411174B1 - Hot stamping component - Google Patents

Abstract

The present invention is 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 wt% or less, chromium (Cr): 0.1 to 0.6 wt%, boron (B): 0.001 to 0.005 wt%, additives 0.1 wt% or less, and the remaining iron (Fe) and other unavoidable impurities including base steel sheet In the hot stamping part to do, in the indentation strain rate for the indentation depth of 200nm to 600nm observed during the nanoindentation test, the number of indentation dynamic strain aging is 26 to 40, We provide hot stamping parts.

Description

Hot stamping component

The present invention relates to hot stamping parts.

As environmental regulations and fuel economy regulations are tightened around the world, the need for lighter vehicle materials is increasing. Accordingly, research and development of ultra-high-strength steel and hot stamping steel are being actively conducted. Among them, the hot stamping process is generally made of heating/forming/cooling/trimming, and uses the phase transformation of the material and the change of the microstructure during the process.

Recently, studies to improve delayed fracture, corrosion resistance, and weldability occurring in hot stamping parts manufactured by a hot stamping process are being actively conducted. As a related technology, there is Korean Patent Laid-Open Publication No. 10-2018-0095757 (Title of the Invention: Method of Manufacturing Hot Stamping Part).

No. 10-2018-0095757

Embodiments of the present invention provide a hot stamped part with improved impact performance.

However, these problems are exemplary, and the scope of the present invention is not limited thereto.

According to one aspect of the present invention, 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, additives 0.1% by weight or less, and the remaining iron (Fe) and other unavoidable impurities In the hot stamping part including the base steel sheet, in the indentation strain rate for the indentation depth of 200 nm to 600 nm observed during the nano indentation test, the number of indentation dynamic strain aging is 26 From dog to 40 pieces, hot stamped parts are provided.

According to this embodiment, the base steel sheet may include a martensitic structure in which a plurality of lath structures are distributed.

According to this embodiment, the average interval of the plurality of laces may be 140nm to 300nm.

According to this embodiment, it further includes fine precipitates distributed in the base steel sheet, and the fine precipitates may include at least one nitride or carbide of titanium (Ti), niobium (Nb), and vanadium (V). .

According to this embodiment, the number of the fine precipitates distributed per unit area (100 μm ² ) may be 7,500 or more and 18,000 or less.

According to this embodiment, the average diameter of the fine precipitates may be 0.0068㎛ or less.

According to this embodiment, the ratio of the fine precipitates having a diameter of 0.01 μm or less may be 63% or more.

According to this embodiment, the ratio of the fine precipitates having a diameter of 0.005 μm or less may be 28% or more.

According to this embodiment, the V-bending angle of the hot stamping part may be 50° or more.

According to this embodiment, the tensile strength of the hot stamping part may be 1350 MPa or more.

According to this embodiment, the amount of activated hydrogen of the hot stamping part may be 0.8 ppm or less.

According to an embodiment of the present invention made as described above, it is possible to implement a hot stamping part. Of course, the scope of the present invention is not limited by these effects.

1 is a Transmission Electron Microscopy (TEM) image showing a portion of a hot stamping part according to an embodiment of the present invention.
2 is a load-displacement graph according to a nano-indentation test of a hot stamping part according to an embodiment of the present invention.
3 is an enlarged view illustrating a serration behavior of part A of FIG. 2 .
4 is a graph measuring indentation dynamic strain aging.
5 is an enlarged view illustrating an enlarged portion B of FIG. 4 .
6 is a schematic diagram illustrating a mechanism of indentation dynamic deformation aging according to movement of dislocations at lath and lath boundaries of hot stamping parts according to an embodiment of the present invention.

Since the present invention can apply various transformations and can have various embodiments, specific embodiments are illustrated in the drawings and described in detail in the detailed description. Effects and features of the present invention, and a method for achieving them will become apparent with reference to the embodiments described below in detail in conjunction with the drawings. However, the present invention is not limited to the embodiments disclosed below and may be implemented in various forms.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, and when described with reference to the drawings, the same or corresponding components are given the same reference numerals, and the overlapping description thereof will be omitted. .

In the present specification, terms such as first, second, etc. are used for the purpose of distinguishing one component from other components without limiting meaning.

In this specification, the singular expression includes the plural expression unless the context clearly dictates otherwise.

In the present specification, the terms include or have means that the features or components described in the specification are present, and the possibility that one or more other features or components may be added is not excluded in advance.

In the present specification, when it is said that a part such as a film, region, or component is on or on another part, not only when it is directly on the other part, but also when another film, region, component, etc. is interposed therebetween. include

In this specification, when a membrane, a region, or a component is connected, when the membrane, a region, or the components are directly connected, or/and another membrane, a region, or a component is interposed between the membranes, the region, and the components. Indirect connection is also included. For example, in the present specification, when it is said that a film, region, component, etc. are electrically connected, when the film, region, component, etc. are directly electrically connected, and/or another film, region, component, etc. is interposed therebetween. This indicates an indirect electrical connection.

In the present specification, "A and/or B" refers to A, B, or A and B. And, "at least one of A and B" represents the case of A, B, or A and B.

In cases where certain embodiments are otherwise practicable in this specification, a specific process sequence may be performed differently from the described sequence. For example, two processes described in succession may be performed substantially simultaneously, or may be performed in an order opposite to the order described.

In the drawings, the size of the components may be exaggerated or reduced for convenience of description. For example, since the size and thickness of each component shown in the drawings are arbitrarily indicated for convenience of description, the present invention is not necessarily limited to the illustrated bar.

1 is a Transmission Electron Microscopy (TEM) image showing a portion of a hot stamping part according to an embodiment of the present invention.

Referring to FIG. 1 , the hot stamping part may include a base steel plate. The base steel sheet may be a steel sheet manufactured by performing a hot rolling process and/or a cold rolling process on a slab cast to include a predetermined alloying element in a predetermined content. In one embodiment, the base steel sheet includes carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), boron (B), and the remainder iron (Fe) and It may contain other unavoidable impurities. Also, in one embodiment, the base steel sheet may further include at least one of titanium (Ti), niobium (Nb), and vanadium (V) as an additive. In another embodiment, the base steel sheet may further include a predetermined amount of calcium (Ca).

Carbon (C) acts as an austenite stabilizing element in the base steel sheet. Carbon is a major element that determines the strength and hardness of the base steel sheet, and after the hot stamping process, it is added for the purpose of securing the tensile strength of the base steel sheet (eg, tensile strength of 1,350 MPa or more) and securing the hardenability. Such carbon may be included in an amount of 0.19 wt% to 0.25 wt% based on the total weight of the base steel sheet. When the carbon content is less than 0.19 wt%, it is difficult to secure a hard phase (martensite, etc.), so it is difficult to satisfy the mechanical strength of the base steel sheet. Conversely, when the carbon content exceeds 0.25 wt%, brittleness of the base steel sheet or a problem of reducing bending performance may occur.

Silicon (Si) acts as a ferrite stabilizing element in the base steel sheet. Silicon (Si) as a solid solution strengthening element improves the strength of the base steel sheet and improves the carbon concentration in austenite by suppressing the formation of carbides in the low-temperature region. In addition, silicon is a key element in hot rolling, cold rolling, hot pressing, homogenizing the structure (perlite, manganese segregation zone control), and fine dispersion of ferrite. Silicon serves as a martensitic strength heterogeneity control element to improve collision performance. Such silicon may be included in 0.1wt% to 0.6wt% based on the total weight of the base steel sheet. If the content of silicon is less than 0.1wt%, it is difficult to obtain the above-described effect, and cementite formation and coarsening may occur in the final hot stamping martensite structure, and the uniforming effect of the base steel sheet is insignificant and the V-bending angle cannot be secured. do. Conversely, when the content of silicon exceeds 0.6wt%, hot-rolling and cold-rolling loads increase, hot-rolled red scale may become excessive, and plating properties of the base steel sheet may be deteriorated.

Manganese (Mn) acts as an austenite stabilizing element in the base steel sheet. Manganese is added to increase hardenability and strength during heat treatment. Such manganese may be included in 0.8wt% to 1.6wt% based on the total weight of the base steel sheet. When the manganese content is less than 0.8 wt%, the hardenability effect is not sufficient, and the hard phase fraction in the molded article after hot stamping may be insufficient due to insufficient hardenability. On the other hand, when the content of manganese exceeds 1.6 wt%, ductility and toughness due to manganese segregation or pearlite bands may be reduced, and it may cause deterioration of bending performance and may generate a heterogeneous microstructure.

Phosphorus (P) may be included in an amount of greater than 0 and 0.03 wt% or less based on the total weight of the base steel sheet in order to prevent deterioration of the toughness of the base steel sheet. When the phosphorus content exceeds 0.03 wt%, an iron phosphide compound is formed to deteriorate toughness and weldability, and cracks may be induced in the base steel sheet during the manufacturing process.

Sulfur (S) may be included in an amount greater than 0 and 0.015 wt% or less based on the total weight of the base steel sheet. If the sulfur content exceeds 0.015 wt%, hot workability, weldability, and impact properties are deteriorated, and surface defects such as cracks may occur due to the formation of large inclusions.

Chromium (Cr) is added for the purpose of improving the hardenability and strength of the base steel sheet. Chromium makes it possible to refine grains and secure strength through precipitation hardening. Such chromium may be included in 0.1 wt% to 0.6 wt% based on the total weight of the base steel sheet. When the content of chromium is less than 0.1wt%, the precipitation hardening effect is low, and on the contrary, when the content of chromium exceeds 0.6wt%, the Cr-based precipitates and matrix solid solution increase to decrease toughness, and production cost due to increased cost may increase.

Boron (B) is added for the purpose of securing the hardenability and strength of the base steel sheet by suppressing the transformation of ferrite, pearlite and bainite to secure a martensitic structure. In addition, boron segregates at grain boundaries to increase hardenability by lowering grain boundary energy, and has an effect of grain refinement due to an increase in austenite grain growth temperature. Such boron may be included in an amount of 0.001 wt% to 0.005 wt% based on the total weight of the base steel sheet. When boron is included in the above range, it is possible to prevent the occurrence of brittleness at the hard phase grain boundary, and secure high toughness and bendability. When the content of boron is less than 0.001 wt%, the hardenability effect is insufficient, and on the contrary, when the content of boron exceeds 0.005 wt%, the solid solution is low and easily precipitates at the grain boundary depending on the heat treatment conditions, resulting in deterioration of hardenability or It may cause embrittlement at high temperatures, and toughness and bendability may be reduced due to the occurrence of hard phase intergranular embrittlement.

Meanwhile, in the base steel sheet according to an embodiment of the present invention, fine precipitates may be included. The additive constituting some of the elements included in the base steel sheet may be a nitride or carbide generating element contributing to the formation of fine precipitates.

Specifically, the additive may include at least one of titanium (Ti), niobium (Nb), and vanadium (V). Titanium (Ti), niobium (Nb), and vanadium (V) form fine precipitates in the form of nitrides or carbides, thereby securing the strength of the hot stamped or quenched member. In addition, they are contained in the Fe-Mn-based composite oxide, function as an effective hydrogen trap site for improving the delayed fracture resistance, and are elements necessary for improving the delayed fracture resistance. These additives may be included in an amount of 0.1 wt% or less based on the total weight of the base steel sheet in total. If the content of the additive exceeds 0.1 wt%, the yield strength may be excessively increased.

Titanium (Ti) may be added for the purpose of strengthening grain refinement and upgrading the material by forming precipitates after hot press heat treatment. In addition, by forming a precipitated phase such as TiC and/or TiN at a high temperature, it effectively contributes to the refinement of austenite grains. Such titanium may be included in an amount of 0.018 wt% to 0.045 wt% based on the total weight of the base steel sheet. When titanium is included in the content range, poor performance and coarsening of precipitates can be prevented, and physical properties of the steel can be easily secured, and defects such as cracks can be prevented on the steel surface. On the other hand, when the content of titanium exceeds 0.045wt%, the precipitates may be coarsened, and elongation and bendability may decrease.

Niobium (Nb) and vanadium (V) are added for the purpose of increasing strength and toughness according to a decrease in martensite packet size. Each of niobium and vanadium may be included in 0.025 wt% to 0.050 wt% based on the total weight of the base steel sheet. When niobium and vanadium are included in the above range, the crystal grain refining effect of steel is excellent in the hot rolling and cold rolling process, cracks in the slab during steel making/playing, and brittle fracture of the product are prevented, and the generation of coarse precipitates in steelmaking is reduced. can be minimized

Calcium (Ca) may be added to control the shape of the inclusions. Such calcium may be included in an amount of 0.003 wt% or less based on the total weight of the base steel sheet.

As described above, the hot stamping part according to an embodiment of the present invention may include fine precipitates including nitride or carbide of at least one of titanium (Ti), niobium (Nb) and vanadium (V) in the base steel sheet. can These fine precipitates may be distributed in the base steel sheet and serve to trap hydrogen. That is, the fine precipitates can improve hydrogen embrittlement of the hot stamping part by providing a trap site for hydrogen introduced into the hot stamping part during or after manufacturing.

In one embodiment, the number of fine precipitates formed in the base steel sheet may be controlled to satisfy a preset range. In one embodiment, the fine precipitates may be included in the base steel sheet in an amount of 6,000 pieces/100 μm ² or more and 21,000 pieces/100 μm ² or less per unit area (100 μm ² ). In addition, in one embodiment, the average diameter of the fine precipitates distributed in the base steel sheet may be about 0.0075㎛ or less, preferably about 0.004㎛ to 0.0075㎛. The hot stamping part including the above-mentioned fine precipitates has excellent V-bending properties, so that bendability and collision performance may be improved.

More specifically, the fine precipitates may be included in an amount of 7,500 or more/100 μm ² or more and 18,000/100 μm ² or less per unit area (100 μm ² ) in the base steel sheet. Also, in one embodiment, the average diameter of the fine precipitates distributed in the base steel sheet may be about 0.0068㎛ or less. Among these fine precipitates, the proportion of the fine precipitates having a diameter of 10 nm or less is about 63% or more, and the ratio of the fine precipitates having a diameter of 5 nm or less may be about 28% or more. A hot stamping part including fine precipitates within the above-described conditions may have excellent bendability and impact performance, as well as improved hydrogen-delayed fracture characteristics.

The diameter of such fine precipitates can have a great influence on the improvement of the hydrogen delayed fracture characteristics. When the number, size, and ratio of the fine precipitates are formed within the above-described range, the required tensile strength (eg, 1,350 MPa) after hot stamping may be secured and formability or bendability may be improved. For example, if the number of fine precipitates per unit area (100 μm ² ) is less than 7,500/100 μm ² , the strength of the hot stamping part may be reduced, and when it exceeds 18,000/100 μm ² , formability of the hot stamping part or the bendability may be reduced.

In addition, in one embodiment, the amount of activated hydrogen in the base steel sheet may be about 0.8ppm or less. The activated hydrogen amount refers to an amount of hydrogen excluding hydrogen trapped in fine precipitates among hydrogen introduced into the base steel sheet. Such an amount of activated hydrogen may be measured using a thermal desorption spectroscopy method. Specifically, while heating the specimen at a preset heating rate to increase the temperature, it is possible to measure the amount of hydrogen emitted from the specimen at a specific temperature or less. In this case, hydrogen emitted from the specimen at a temperature below a certain temperature is not trapped among the hydrogen introduced into the specimen and may be understood as activated hydrogen that affects delayed hydrogen destruction. For example, as a comparative example, when the hot stamping part contains more than 0.8 ppm of activated hydrogen in the base steel sheet, the hydrogen delayed fracture characteristic is lowered, and compared to the hot stamping part according to this embodiment in the bending test under the same conditions can be easily broken.

Meanwhile, the base steel sheet according to the present embodiment may include a martensitic structure in which a microstructure is distributed. The martensitic structure is the result of diffusion-free transformation of austenite γ under the onset temperature (Ms) of martensitic transformation during cooling. The microstructure in the martensitic structure is a diffusion-free transformation structure created during rapid cooling within a grain called a prior austenite grain boundary (PAGB), and may include a plurality of lath (L) structures. The plurality of class (L) structures may further constitute a unit such as a block and a packet. In more detail, a plurality of lath (L) structures form a block, a plurality of blocks form a packet, and a plurality of packets form an initial austenite grain boundary (PAGB). can be formed

As described above, martensite may have a lath (L) structure in the form of a long and thin rod oriented in one direction within each initial grain of austenite. The plurality of lath (L) structures may have a property of resisting external deformation at a boundary between them, that is, a lath boundary (LB). This will be described in detail later.

Meanwhile, the V-bending angle of the hot stamping part according to the present embodiment may be 50° or more. 'V-bending' is a parameter that evaluates the bending deformation properties in the maximum load sections among deformations in the bending performance of hot stamping parts. In other words, looking at the tensile deformation region during bending in macroscopic and microscopic sizes according to the load-displacement evaluation of hot stamping parts, if a microcrack occurs and propagates in the local tensile region, the bending performance called V-bending angle can be evaluated. have.

As described above, the hot stamping part according to the present embodiment may include a martensitic structure having a plurality of lath (L) structures, and the crack generated during bending deformation is a one-dimensional defect called dislocation. It can occur as it moves through interactions within the site organization. In this case, it can be understood that the higher the local strain rate among the given plastic deformations, the higher the energy absorption for the plastic deformation of martensite, and thus the collision performance increases.

In the hot stamping part according to an embodiment of the present invention, as the martensitic structure has a plurality of lath (L) structures, the dislocation during bending deformation repeatedly moves the lath (L) and the lath boundary (LB) in the process Dynamic strain aging (DSA), that is, indentation dynamic strain aging due to the difference in strain rate, may appear. Indentation dynamic deformation aging is a concept of plastic deformation absorption energy, and since it means resistance to deformation, the more frequent the indentation dynamic deformation aging phenomenon, the better the resistance performance to deformation.

In the hot stamping part according to an embodiment of the present invention, as the martensitic structure has a plurality of lath (L) structures in a dense form, the indentation dynamic deformation aging phenomenon may occur frequently, and through this, the V-bending angle is reduced to 50 ° or higher to improve bendability and impact performance.

In one embodiment, the average distance of the plurality of laths L included in the martensitic structure of the hot stamping part according to the present embodiment may be about 140 nm to 300 nm. As a comparative example, it is assumed that a hot stamping part including a base steel sheet having a composition deviating from the composition of the elements described above includes a lath structure. The average spacing between the lath structures of the hot stamping part of the comparative example may be larger than the average spacing of the lath (L) structures of the hot stamping part according to the present embodiment. That is, the hot stamping part according to the present embodiment has a more dense lath (L) structure than the comparative example, and as the lath (L) structure in the hot stamping part becomes dense, the number of indentation dynamic deformation aging is more can increase

2 is a load-displacement graph according to a nano-indentation test of a hot stamping part according to an embodiment of the present invention, and FIG. 3 is an enlarged view showing the serration behavior of part A of FIG. 2 .

Referring to Figure 2, it shows a graph showing the results of the nano-indentation test for the hot stamping part according to an embodiment of the present invention. The 'nano indentation test' is a test that measures the force deformation with depth by pressing an indenter vertically on the surface of a hot stamping part. In FIG. 2 , the x-axis represents the depth at which the indenter is pressed, and the y-axis represents the force according to the indentation depth. As an example, in FIG. 2 , a cube-corner tip (centerline-to-face angle = 35.3˚, indentation strain rate = 0.22) was used as an indenter as an indenter, but this The invention is not limited thereto, and a Berkovich tip (centerline-to-face angle = 65.3˚, indentation strain rate = 0.072) may be used.

Referring to FIG. 3, which is an enlarged view of part A of FIG. 2, it can be seen that a characteristic behavior called serration is observed during indentation and plastic deformation occurring during the nanoindentation test. The serration behavior may appear repeatedly at approximately regular intervals, and the serration behavior is indicated by a down arrow (↓) in FIG. 3 .

The serration behavior may be caused by diffusion-free transformation structures in the initial austenite grain boundary (PAGB) included in the indentation test of hot stamping parts. More specifically, the serration behavior shown in the load-displacement curve as shown in FIG. 2 is shown by the interaction between dislocations and solute atoms diffusing in the material, and includes a plurality of laths distributed in the initial austenite grain boundary (PAGB) and , can be understood as originating from the difference in resistance to external pressure at the lath boundary formed between them. This serration behavior can be recognized as the main evidence of the dynamic strain aging (DSA), that is, the indentation dynamic strain aging (indentation dynamic strain aging) phenomenon of FIG. 4 to be described later.

FIG. 4 is a graph of measuring indentation dynamic strain aging, and FIG. 5 is an enlarged view illustrating an enlarged portion B of FIG. 4 .

4 is a graph analyzing the nanoindentation strain rate ([dh/dt]/h, h: indentation depth, t: unit time) based on the load-displacement curve of FIG. 3 .

In one embodiment, in the hot stamping part, in the indentation strain rate for the indentation depth of about 200 nm to 600 nm observed during the nano indentation test, the number of indentation dynamic strain aging is about 26 can be from 40. Indentation dynamic strain aging may appear as a behavior in which indentation strain repeatedly forms a plurality of peaks.

The number of indentation dynamic strain aging can be calculated based on the peak passing through the reference line (C) as the center. That is, the number of indentation dynamic strain aging may be calculated based on the peak formed through the reference line (C) without calculating the peaks formed above or below the reference line (C) with respect to the reference line (C). The reference line (C) is a line assuming that indentation dynamic deformation aging due to lath and lath boundary structures is removed when measuring indentation strain.

Referring to the indentation strain graph of FIG. 5 , it can be seen that the number and size of indentation dynamic strain aging gradually decrease as the indentation depth increases. This is because as the indentation depth increases, the indentation properties of the initial austenite crystal are mixed, so that indentation dynamic strain aging hardly appears. Referring to FIG. 4 , it can be seen that virtually no indentation dynamic strain aging occurs at an indentation depth of 600 nm or more. In the graph of FIG. 4 , an indentation depth of 700 nm or more was not measured, but if the indentation strain for an indentation depth of 700 nm or more is continuously measured, a curve in which dynamic strain aging is removed in the corresponding section can be obtained. The reference line C can be derived by estimating the indentation strain curve at the indentation depth from which the indentation dynamic strain aging is removed as described above.

As described above, the number of indentation dynamic strain aging of the hot stamping part according to the present embodiment may be in the range of 26 to 40, which is based on a measurement in the range of about 200 nm to 600 nm in indentation depth. In FIG. 4, the indentation depth was measured from 0 nm to about 700 nm, but when the indentation depth is less than about 200 nm, the accuracy of the indentation strain is low due to the effect of the dull indenter, and when the indentation depth exceeds about 600 nm, the indentation properties of the initial austenite crystal itself are mixed. This is because the evaluation of dynamic strain aging is not easy.

As shown in FIG. 4 , when viewed macroscopically, the indentation strain gradually decreases in a quadratic function according to the indentation depth. In this case, the indentation dynamic strain aging may appear as a behavior in which the indentation strain repeatedly forms a plurality of peaks. In order to observe this in detail, FIG. 5 shows an enlarged indentation strain with respect to an indentation depth of 350 nm to 400 nm in FIG. 4 .

Referring to FIG. 5 , the indentation strain may appear in the form of repeating the rising section and the falling section. Section a is a section in which the indentation strain increases during the indentation test and may mean a section in which resistance is absorbed. That is, section a can be understood as a section in which dislocations gliding within the lath distributed in the initial austenite grain boundary (PAGB) when dislocations are moved in the tensile generating portion during bending deformation. As described above, while the dislocation moves in the lath, the hot stamping part exhibits a property of absorbing external resistance, which may appear as a section in which the indentation strain increases as shown in FIG. 5 . The dislocation rises to the lath boundary, and the moment it passes the lath boundary, the indentation strain falls as in section b, which can be interpreted as a phenomenon caused by interaction with fine precipitates distributed at the lath boundary.

6 is a schematic diagram illustrating a mechanism of indentation dynamic deformation aging according to dislocation movement during bending deformation of a hot stamping part according to an embodiment of the present invention.

6, the movement of dislocations according to the indentation dynamic strain aging of FIG. 5 while showing the lath (L) and the lath boundary (LB) distributed in the initial austenite grain boundary (PAGB) in the tensile generating portion during bending deformation. is schematically shown. As described above, dislocations during bending deformation may move along adjacent laths L. The arrow in FIG. 6 indicates the direction of movement of the dislocation.

As such, it can be interpreted that the indentation strain is different according to the degree of energy absorption within the lath L and at the lath boundary LB during dislocation movement. Referring to FIGS. 5 and 6 together, while the dislocation moves along the arrow of FIG. 6 within the lath L, it may correspond to section a of FIG. 5 . That is, while the dislocation moves in the lath L, the indentation strain may increase. The indentation strain rises until the dislocation is adjacent to the lath boundary LB, and then falls when passing the lath boundary LB, which may correspond to section b of FIG. 5 . In this way, the indentation dynamic strain aging as shown in FIG. 5 may occur due to the interaction between the dislocation and the lath boundary LB during dislocation movement. As described above, the fine precipitates (P) are distributed at the lath boundary (LB) to exhibit the characteristic of delaying deformation, and as such, the increase and decrease of the deformation rate are repeatedly formed while passing through the plurality of laths (L), and the indentation dynamic deformation aging may occur.

The hot stamping part according to an embodiment of the present invention reduces the average spacing between a plurality of laths by controlling the fine precipitates included in the base steel sheet, so that when dislocations slide during bending deformation, indentation dynamic deformation aging occurs more frequently can have characteristics. As the indentation dynamic deformation aging phenomenon increases through the densification of the lath structure as described above, the hot stamping part according to an embodiment of the present invention does not break during bending deformation and can secure a V-bending angle of 50° or more. Through this, bendability and impact performance can be improved.

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

Hot stamping parts according to an embodiment of the present invention may be formed through a hot stamping process on a base steel sheet having a composition as shown in Table 1 below.

Ingredients (wt%) C Si Mn P S Cr B additive 0.19~0.25 0.1~0.6 0.8~1.6 0.03 or less 0.015 or less 0.1~0.6 0.001~0.005 0.1 or less

As described above, the hot stamping part according to an embodiment of the present invention may include fine precipitates including nitrides and/or carbides of additives in the base steel sheet, and the fine precipitates in the hot stamping part have a unit area in the base steel sheet. (100 μm ² ) 6,000 pieces/100 μm ² or more and 21,000 pieces/100 μm ² or less per (100 μm 2 ) may be included. In addition, in one embodiment, the average diameter of the fine precipitates distributed in the base steel sheet may be about 0.004 to 0.0075㎛. In the case of a hot stamping part satisfying the above-mentioned conditions, the V-bending angle may represent 50° or more.

[Table 2] below shows the precipitation behavior of the fine precipitates of Examples and Comparative Examples according to the present invention according to the titanium content, and the number of indentation dynamic deformation aging and the values measured by quantifying the V-bending angle.

division Ti (wt.%) Lath spacing
(nm) TiC system
Precipitation Density (/100㎛ ² ) precipitate size
(μm) indentation dynamic strain aging
(dog) V-bending
(°) Average total number Average Example 1 0.018 300 6,032
0.0040 26 50 Example 2 0.025 200 9,954 0.0051 29 53 Example 3 0.030 180 14,266
0.0054 32 55 Example 4 0.042 160 18,920
0.0060 37 56 Example 5 0.045 140 20,990 0.0075 40 55 Example 6 0.030 190 14,443
0.0053 33 54 Example 7 0.035 168 16,599 0.0061 35 56 Comparative Example 1 0.047 135 21,063
0.0078 24 43 Comparative Example 2 0.017 320 5,911
0.0035 25 45

In [Table 2], Examples 1 to 7 are examples satisfying the precipitation behavior conditions of fine precipitates according to the titanium content and the conditions for forming a plurality of laths as described above. Specifically, in Examples 1 to 7, titanium may be included in about 0.018 wt% to 0.045 wt%, and accordingly, the average spacing of the plurality of laths may be about 140 nm to 300 nm, and fine precipitates containing titanium , for example, the number of titanium carbide (TiC) per unit area may be 6,000 pieces/100 μm ² or more and 21,000 pieces/100 μm ² or less, and the average diameter of all fine precipitates may be 0.004 μm to 0.0075 μm. In this case, the number of indentation dynamic strain aging satisfies the condition of 26 to 40.

As described above, in Examples 1 to 7, which satisfy the precipitation behavior condition and the plurality of lath formation conditions of the present invention, the V-bending angle can be secured by 50° or more, thereby confirming that the tensile strength and bendability are improved.

On the other hand, Comparative Examples 1 and 2 did not satisfy at least some of the precipitation behavior conditions and the plurality of lath formation conditions described above, so it was confirmed that the tensile strength and bendability were lower than those of Examples 1 to 7 can

In Comparative Example 1, as the titanium content was 0.047 wt%, the size of the fine precipitates was coarsened, the average interval of the plurality of laths was reduced to about 135 nm, and the indentation dynamic strain aging was 24, which does not satisfy the above-mentioned conditions . Accordingly, it can be confirmed that the V-bending angle of Comparative Example 1 is only 43°.

In the case of Comparative Example 2, the size and density of the fine precipitates decreased as the titanium content was 0.017 wt%, the average spacing of the plurality of laths increased to about 320 nm, and the indentation dynamic strain aging was 25, which also satisfies the above-mentioned conditions. can't make it Accordingly, it can be confirmed that the V-bending angle of Comparative Example 2 is only 45°.

More specifically, the fine precipitates in the hot stamping part according to an embodiment of the present invention may be included in an amount of 7,500 pieces/100 μm ² or more and 18,000 pieces/100 μm ² or less per unit area (100 μm ² ) in the base steel sheet. Also, in one embodiment, the average diameter of the fine precipitates distributed in the base steel sheet may be about 0.0068㎛ or less. Among these fine precipitates, the proportion of the fine precipitates having a diameter of 0.01 μm or less is about 63% or more, and the ratio of the fine precipitates having a diameter of 0.005 μm or less may be 28% or more. In addition, in one embodiment, the amount of activated hydrogen in the base steel sheet may be about 0.8ppm or less. A hot stamping part having such a characteristic has excellent bendability, and hydrogen embrittlement resistance can be improved.

The following [Table 3] shows the values measured by quantifying the precipitation behavior of the fine precipitates of Examples and Comparative Examples according to the present invention.

The precipitation behavior of fine precipitates can be measured by analyzing a TEM (Transmission Electron Microscopy) image. Specifically, TEM images of arbitrary regions are acquired as many as a preset number for the specimen. It is possible to extract fine precipitates from the acquired images through an image analysis program, etc., and measure the number of fine precipitates for the extracted fine precipitates, the average distance between the fine precipitates, and the diameter of the fine precipitates.

In one embodiment, in order to measure the precipitation behavior of fine precipitates, a surface replication method may be applied as a pretreatment to a measurement target specimen. For example, a one-step replica method, a two-step replica method, an extraction replica method, etc. may be applied, but the exemplary embodiment is not limited thereto.

In another embodiment, when the diameter of the fine precipitates is measured, the diameter of the fine precipitates may be calculated by converting the shape of the fine precipitates into a circle in consideration of the non-uniformity of the shape of the fine precipitates. Specifically, the diameter of the fine precipitates can be calculated by measuring the area of the extracted fine precipitates using a unit pixel having a specific area, and converting the fine precipitates into a circle having the same area as the measured area.

Psalter Total fine precipitates
Count
(pcs/100㎛ ² ) Total fine precipitates
average diameter
(μm) 10 nm or less in diameter
Fine Precipitation Ratio
(%) 5 nm or less in diameter
Fine Precipitation Ratio
(%) activate
amount of hydrogen
(ppm) A 7,512 0.0067 63.2 28.0 0.779 B 7,520 0.0040 92.4 37.7 0.764 C 8,768 0.0059 62.9 29.0 0.764 D 12,973 0.0044 93.5 62.9 0.771 E 16,316 0.0041 95.9 75.1 0.759 F 17,990 0.0057 63.1 28.2 0.752 G 17,980 0.0044 80.9 28.1 0.767 H 8,944 0.0047 71.7 41.4 0.751 I 13,173 0.0044 98.9 82.8 0.714 J 11,796 0.0068 84.7 60.4 0.739 K 14,612 0.0070 88.7 30.9 0.891 L 16,520 0.0057 62.8 26.2 0.878 M 7,318 0.0058 68.2 27.8 0.865 N 16,600 0.0061 96.1 27.9 0.859

In [Table 3], the precipitation behavior of fine precipitates (total number of fine precipitates per unit area, average diameter of total fine precipitates, ratio of fine precipitates with a diameter of 10 nm or less, amount of activated hydrogen) for specimens A to N was measured in [Table 3].

Specimens A to J of [Table 3] are examples according to the present invention, and are specimens of hot stamping parts manufactured using a base steel sheet satisfying the above-described content conditions (see [Table 1]). In other words, specimens A to J are specimens satisfying the precipitation behavior conditions of the fine precipitates described above. Specifically, in specimens A to J, fine precipitates are formed in 7,500 pieces/100 μm ² or more and 18,000 pieces/100 μm ² or less in the steel sheet, and the average diameter of all fine precipitates is 0,0068 μm or less, and the fine precipitates formed in the steel sheet 63% or more of the precipitates have a diameter of 10 nm or less, and 28% or more satisfy a diameter of 5 nm or less.

Specimens A to J satisfying the precipitation behavior conditions of the present invention as described above, it can be confirmed that the hydrogen delayed fracture characteristics are improved as the activated hydrogen content satisfies the condition of 0.8 ppm or less.

On the other hand, specimens K to N are specimens that do not satisfy at least some of the precipitation behavior conditions of the above-mentioned fine precipitates, and it is confirmed that the tensile strength, bendability and/or hydrogen-delayed fracture characteristics are inferior compared to specimens A to J. can

In the case of specimen K, the average diameter of all fine precipitates was 0.0070 μm. This is less than the lower limit of the average diameter condition of all fine precipitates. Accordingly, it can be confirmed that the amount of activated hydrogen in specimen K is relatively high 0.891 ppm.

In the case of specimen L, the proportion of fine precipitates with a diameter of 10 nm or less was 62.8%. Accordingly, it can be confirmed that the amount of activated hydrogen in the specimen L is a relatively high 0.878 ppm.

In the case of specimen M and specimen N, the proportion of fine precipitates with a diameter of 5 nm or less was 27.8% and 27.9%, respectively. Accordingly, it can be confirmed that the amounts of activated hydrogen in specimen M and specimen N are relatively high 0.865 ppm and 0.859 ppm, respectively.

When the precipitation behavior conditions of the present invention are not satisfied as in specimens K to N, a relatively large amount of hydrogen is trapped in one of the fine precipitates during the hot stamping process, or the trapped hydrogen atoms are locally concentrated so that the trapped hydrogen atoms are bonded to each other. By forming hydrogen molecules (H ₂ ), an internal pressure is generated, and accordingly, it is judged that the delayed hydrogen destruction characteristics of the hot stamping product are deteriorated.

On the other hand, when the precipitation behavior conditions of the present invention are satisfied as in specimens A to J, the number of hydrogen atoms trapped in one fine precipitate during the hot stamping process is relatively small, or the trapped hydrogen atoms can be relatively evenly dispersed. have. Therefore, it is possible to reduce the generation of internal pressure due to the hydrogen molecules formed by the trapped hydrogen atoms, and accordingly, it is judged that the delayed hydrogen destruction characteristics of the hot stamping product are improved.

As a result, it was confirmed that the hot stamping part to which the content condition of the present invention is applied as described above satisfies the precipitation behavior condition of the fine precipitates after hot stamping, and thus the hydrogen delayed fracture characteristics are improved.

Although the present invention has been described with reference to the embodiments shown in the drawings, which are merely exemplary, those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

1: hot stamping parts
L: Rath
LB: Rath Boundary
P: fine precipitate

Claims (11)

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%, boron (B): 0.001 to 0.005 wt%, at least one of titanium (Ti), niobium (Nb) and vanadium (V): 0.1 wt% or less and the remainder In a hot stamping part comprising a base steel sheet containing iron (Fe) and other unavoidable impurities,
When the indentation strain rate for the indentation depth of 200 nm to 600 nm observed during the nano indentation test appears as a behavior of repeatedly forming a plurality of peaks, it is calculated as the number of peaks formed through the reference line The number of indentation dynamic strain aging ranges from 26 to 40,
The reference line is derived by inversely estimating the indentation strain curve from which indentation dynamic strain aging is removed for an indentation depth exceeding 600 nm during the nanoindentation test. According to claim 1,
The base steel sheet includes a martensitic structure in which a plurality of lath structures are distributed, hot stamping parts. 3. The method of claim 2,
The average spacing of the plurality of laths is 140 nm to 300 nm. According to claim 1,
Further comprising fine precipitates distributed in the base steel sheet,
The fine precipitates include a nitride or carbide of at least one of titanium (Ti), niobium (Nb), and vanadium (V). 5. The method of claim 4,
The number of the fine precipitates distributed per unit area (100 μm ² ) is 7,500 or more and 18,000 or less, a hot stamping part. 5. The method of claim 4,
The average diameter of the fine precipitates is 0.0068㎛ or less, hot stamping part. 5. The method of claim 4,
The proportion of the fine precipitates having a diameter of 0.01 μm or less is 63% or more. 5. The method of claim 4,
The proportion having a diameter of 0.005 μm or less among the fine precipitates is 28% or more. According to claim 1,
wherein the V-bending angle of the hot stamped part is at least 50°. According to claim 1,
The hot stamping part has a tensile strength of 1350 MPa or more. According to claim 1,
The hot stamping part has an activated hydrogen amount of 0.8 ppm or less.

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