Hot stamped parts and their manufacturing methods

Abstract

According to one aspect of the present invention, a method for manufacturing a hot-stamped part with residual stress analysis values ​​satisfying preset conditions is provided. The method includes the steps of: heating a blank; hot-stamping the blank to form a molded body; and cooling the molded body to form the hot-stamped part, wherein the residual stress analysis value is the product of an XRD value (which is the residual stress quantified by X-ray diffraction (XRD)) and an EBSD value (which is the orientation quantified by electron backscatter diffraction (EBSD)), and the preset condition is greater than or equal to 2.85 * 10⁻⁶. ‑4 (degrees*MPa / μm) 2 And less than or equal to 0.05 (degrees * MPa / μm) 2 ).

Description

Technical Field

[0001] This application relates to hot-stamped parts and methods for manufacturing said hot-stamped parts. Background Technology

[0002] High-strength steel is used to manufacture lightweight and stable automotive parts. On the other hand, while high-strength steel offers high strength relative to its weight, its formability decreases with increasing strength. This can lead to breakage or springback during processing, making it difficult to form products with complex and precise shapes.

[0003] Hot stamping has been used as a method to improve these problems, and therefore, research on materials for hot stamping has been actively conducted. For example, as disclosed in Korean Patent Application Publication No. 10-2017-0076009, hot stamping is a molding technique that manufactures high-strength parts by heating a steel sheet for hot stamping at a high temperature, then rapidly cooling it while molding it in a die. According to Korean Patent Application Publication No. 10-2017-0076009, it is possible to manufacture parts with high precision by suppressing problems such as cracks or shape freeze defects that occur during the forming process (these are problems in high-strength steel sheets). Summary of the Invention

[0004] Technical issues

[0005] The embodiments of this application aim to solve various problems including those described above, and provide hot-stamped parts that can ensure high mechanical properties and hydrogen embrittlement by controlling the residual stress of the hot-stamped parts, and a method for manufacturing said hot-stamped parts. However, these problems are exemplary, and the scope of this application is not limited thereto.

[0006] Technical solution

[0007] According to one aspect of this application, a method for manufacturing a hot-stamped part with residual stress analysis values ​​meeting preset conditions is provided. The method includes heating a blank, hot-stamping the blank to form a molded body, and cooling the molded body to form the hot-stamped part. The residual stress analysis value can be the product of an X-ray diffraction (XRD) value obtained by quantifying residual stress through XRD analysis and an electron backscatter diffraction (EBSD) value obtained by quantifying orientation through EBSD analysis, with the preset condition being approximately 2.85 × 10⁻⁶. -4 Degree * MPa / μm 2 Or larger and approximately 0.05 degrees*MPa / μm 2 Or smaller.

[0008] According to an exemplary embodiment, heating the billet may include multi-stage heating of the billet as it passes through multiple sections in a heating furnace (where the temperature range increases in the multiple sections of the heating furnace), and homogenizing the billet to a temperature of approximately Ac3 or higher.

[0009] According to an exemplary embodiment, in a plurality of sections, the ratio of the length of the section used for heating the multiple sections of the blank to the length of the section used for homogenizing the blank is approximately 1:1 to 4:1.

[0010] According to an exemplary embodiment, the temperature of multiple sections can be increased in the direction from the furnace inlet to the furnace outlet.

[0011] According to an exemplary embodiment, in multi-stage heating, the heating rate of the blank can be in the range of about 6°C / s to about 12°C / s.

[0012] According to an exemplary embodiment, in a plurality of sections, the temperature of the section used for homogenizing the heating blank is higher than the temperature of the section used for multi-segment heating of the blank.

[0013] According to an exemplary embodiment, the blank can remain in the heating furnace for about 180 seconds to about 360 seconds.

[0014] According to an exemplary embodiment, cooling the molded body to form a hot stamped part may include holding the molded body in a die at a temperature below the martensitic transformation initiation temperature for about 3 seconds to about 20 seconds.

[0015] According to an exemplary embodiment, the molded body can be cooled to the martensitic transformation termination temperature in the mold at an average cooling rate of 15°C / s or greater.

[0016] According to an exemplary embodiment, the hot-stamped component may comprise a martensitic phase and iron-based carbides, wherein the martensitic phase has an area fraction of 80% or higher, the iron-based carbides are located within the martensitic phase, and the martensitic phase has an area fraction of less than 5%.

[0017] According to an exemplary embodiment, the iron-based carbide may have a needle-like form, and the needle-like form may have a diameter of less than 0.2 μm and a length of less than 10 μm.

[0018] According to an exemplary embodiment, the martensitic phase may include a lath phase, and the iron-based carbide may include a first iron-based carbide parallel to the longitudinal direction of the lath and a second iron-based carbide perpendicular to the longitudinal direction of the lath, and the iron-based carbide reference area fraction of the first iron-based carbide may be greater than the iron-based carbide reference area fraction of the second iron-based carbide.

[0019] According to an exemplary embodiment, the first iron-based carbide may be formed at an angle of 0° or greater and 20° or less with respect to the longitudinal direction of the slat, and the iron-based carbide reference area fraction is 50% or greater.

[0020] According to an exemplary embodiment, the second iron-based carbide may be formed at an angle of 70° or greater and 90° or less with respect to the longitudinal direction of the slat, and the iron-based carbide reference area fraction is less than 50%.

[0021] According to another aspect of this application, a hot-stamped component whose residual stress analysis value meets a preset condition is provided. The residual stress analysis value can be the product of an X-ray diffraction (XRD) value obtained by quantifying residual stress through XRD analysis and an electron backscatter diffraction (EBSD) value obtained by quantifying orientation through EBSD analysis, with the preset condition being approximately 2.85 × 10⁻⁶. -4 Degree * MPa / μm 2 Or larger and approximately 0.05 degrees*MPa / μm 2 Or smaller.

[0022] According to an exemplary embodiment, the hot-stamped component may comprise a martensitic phase and iron-based carbides, wherein the martensitic phase has an area fraction of 80% or greater, the iron-based carbides are located within the martensitic phase, and the martensitic phase has an area fraction of less than 5%.

[0023] According to an exemplary embodiment, the iron-based carbide may have a needle-like form, and the needle-like form may have a diameter of less than 0.2 μm and a length of less than 10 μm.

[0024] According to an exemplary embodiment, the martensitic phase may include a lath phase, and the iron-based carbide may include a first iron-based carbide parallel to the longitudinal direction of the lath and a second iron-based carbide perpendicular to the longitudinal direction of the lath, and the iron-based carbide reference area fraction of the first iron-based carbide may be greater than the iron-based carbide reference area fraction of the second iron-based carbide.

[0025] According to an exemplary embodiment, the first iron-based carbide may be formed at an angle of 0° or greater and 20° or less with respect to the longitudinal direction of the slat, and the iron-based carbide reference area fraction is 50% or greater.

[0026] According to an exemplary embodiment, the second iron-based carbide may be formed at an angle of 70° or greater and 90° or less with respect to the longitudinal direction of the slat, and the iron-based carbide reference area fraction is less than 50%.

[0027] Beneficial effects

[0028] According to exemplary embodiments of this application, it is possible to achieve hot-stamped parts that ensure high mechanical properties and hydrogen embrittlement by controlling the residual stress of the hot-stamped parts, and a method for manufacturing said hot-stamped parts. Of course, the scope of this application is not limited to these effects. Attached Figure Description

[0029] Figure 1 A plan view illustrating a portion of a hot-stamped component according to an exemplary embodiment of this application;

[0030] Figure 2 A plan view illustrating a portion of a hot-stamped component according to an exemplary embodiment of this application;

[0031] Figure 3 A flowchart illustrating a method for manufacturing a hot-stamped part according to an exemplary embodiment of this application;

[0032] Figure 4 A graph illustrating temperature variations during multi-stage heating of a blank in a method for manufacturing a hot-stamped part according to an exemplary embodiment of this application; and

[0033] Figure 5 A graph showing the temperature changes when heating a blank in multiple stages versus heating a blank in a single stage. Detailed Implementation

[0034] Because this application allows for various variations and multiple embodiments, specific embodiments will be shown in the accompanying drawings and described in detail in the specification. The advantages, features, and methods for achieving the advantages of this application will become clear when referring to the embodiments described below in conjunction with the accompanying drawings. However, this application may take different forms and should not be construed as limited to the descriptions set forth herein.

[0035] It will be understood that although the terms “first,” “second,” “third,” etc., may be used in this document to describe the various elements, these elements should not be limited by these terms. These terms are used only to distinguish one element from another.

[0036] In the following implementations, unless the context clearly indicates otherwise, the singular form includes the plural form.

[0037] It will also be understood that when the terms “comprising” and / or “including” are used in this specification, they indicate the presence of the said feature or component, but do not exclude the presence or addition of one or more other features or combinations thereof.

[0038] It will be understood that when an element or layer is referred to as being "on" another element or layer, the element or layer may be located directly on the other element or layer, or between elements or layers.

[0039] In the accompanying drawings, the thickness of layers and regions may be exaggerated or reduced for ease of explanation. For example, the dimensions and thicknesses of elements in the drawings are arbitrarily shown for ease of explanation; therefore, the concept of the invention is not limited to the drawings.

[0040] When a particular implementation scheme can be carried out differently, a specific process sequence can be performed in a manner different from the stated sequence. For example, two consecutively described processes can be performed substantially simultaneously, or in the reverse order of the stated sequence.

[0041] In this application, as used herein, "A and / or B" means A, B, or A and B. Furthermore, "at least one of A and B" means A, B, or A and B.

[0042] In the following embodiments, when connecting films, regions, components, etc., it can include directly connecting films, regions, components, etc., and / or indirectly connecting films, regions, and components by inserting another film, region, component, etc., therebetween. For example, in this specification, when electrically connecting films, regions, components, etc., it can mean directly electrically connecting films, regions, components, etc., and / or indirectly electrically connecting another film, region, component, etc., by inserting another film, region, component, etc., therebetween.

[0043] The present invention will now be described in more detail with reference to the accompanying drawings, which illustrate embodiments of the invention. In describing the invention with reference to the drawings, the same reference numerals are used for substantially the same or corresponding elements, and their descriptions will not be repeated.

[0044] Figure 1 A plan view showing a portion of a hot-stamped component according to an embodiment of this application.

[0045] refer to Figure 1 The hot-stamped component according to an exemplary embodiment of this application includes a steel plate 10.

[0046] The steel plate 10 can be manufactured by hot rolling and / or cold rolling on a cast billet to contain a predetermined amount of predetermined alloying elements. This steel plate 10 can exist as a fully austenitic structure at the hot stamping heating temperature and then transform into a martensitic structure upon cooling.

[0047] In one embodiment, the steel plate 10 may contain carbon (C), manganese (Mn), boron (B), phosphorus (P), sulfur (S), silicon (Si), chromium (Cr), the balance iron (Fe), and other unavoidable impurities. Furthermore, the steel plate 10 may also contain at least one alloying element selected from titanium (Ti), niobium (Nb), and vanadium (V) as an additive. Additionally, the steel plate 10 may contain a predetermined amount of calcium (Ca).

[0048] Carbon (C) acts as an austenite stabilizing element in steel sheet 10. Carbon is a major element determining the strength and hardness of steel sheet 10, and its addition after the hot stamping process aims to ensure the tensile strength of steel sheet 10 (e.g., approximately 1350 MPa or higher) and its hardenability. Based on the total weight of steel sheet 10, this carbon can be included in an amount from approximately 0.19 wt% to approximately 0.38 wt%. When the carbon content is less than approximately 0.19 wt%, it is difficult to ensure the presence of a hard phase (martensite, etc.), thus making it difficult to meet the required mechanical strength of steel sheet 10. Conversely, when the carbon content exceeds approximately 0.38 wt%, brittleness or reduced bending properties of steel sheet 10 may occur.

[0049] Manganese (Mn) acts as an austenite stabilizing element in steel sheet 10. The purpose of adding manganese is to improve hardenability and strength during heat treatment. Based on the total weight of steel sheet 10, this manganese can be included in an amount from about 0.5 wt% to about 2.0 wt%. When the manganese content is less than about 0.5 wt%, the hardenability effect is insufficient, and due to insufficient hardenability, the fraction of hard phase in the formed body after hot stamping may be insufficient. On the other hand, when the manganese content exceeds about 2.0 wt%, ductility and toughness may decrease due to manganese segregation or pearlite banding, which may lead to deterioration of bending properties and may produce a heterogeneous microstructure.

[0050] The purpose of adding boron (B) is to ensure the martensitic structure by suppressing the transformation of ferrite, pearlite, and bainite, thereby ensuring the hardenability and strength of the steel sheet 10. Furthermore, boron segregates at grain boundaries and improves hardenability by lowering grain boundary energy, and also has the effect of refining grains by increasing the austenite grain growth temperature. Based on the total weight of the steel sheet 10, boron can be included in an amount from about 0.001 wt% to about 0.005 wt%. When boron is included within the above range, brittleness at the hard phase grain boundaries can be prevented, and high toughness and bendability are ensured. When the boron content is less than about 0.001 wt%, the hardenability effect is insufficient, and conversely, when the boron content exceeds about 0.005 wt%, due to reduced solid solubility, it is prone to precipitate at grain boundaries depending on the heat treatment conditions, which may lead to deterioration of hardenability or high-temperature embrittlement, and toughness and bendability may decrease due to the presence of hard phase intergranular brittleness.

[0051] Based on the total weight of the steel plate 10, phosphorus (P) may be included in an amount greater than 0% by weight and about 0.03% by weight or less, thereby preventing the deterioration of the toughness of the steel plate 10. When the phosphorus content exceeds about 0.03% by weight, phosphide compounds are formed, which deteriorate the toughness and weldability, and may cause cracks in the steel plate 10 during the manufacturing process.

[0052] Based on the total weight of the steel plate 10, sulfur (S) may be included in an amount greater than 0% by weight and about 0.003% by weight or less. If the sulfur content exceeds about 0.003% by weight, hot workability, weldability and impact properties deteriorate, and surface defects (such as cracks) may occur due to the formation of larger inclusions.

[0053] Silicon (Si) acts as a ferrite stabilizing element in steel sheet 10. As a solid solution reinforcing element, silicon improves the strength of steel sheet 10 and increases the carbon concentration in austenite by suppressing carbide formation in low-temperature regions. Furthermore, silicon is a key element in hot rolling, cold rolling, hot pressing, structural homogenization (control of pearlite and manganese segregation regions), and fine dispersion of ferrite. Silicon also acts as a martensitic strength inhomogeneity control element to improve impact resistance. Based on the total weight of steel sheet 10, silicon can be included in an amount from about 0.1 wt% to about 0.6 wt%. When the silicon content is less than about 0.1 wt%, the aforementioned effects are difficult to achieve, and cementite formation and coarsening may occur in the final hot-stamped martensitic structure. Conversely, when the silicon content exceeds about 0.6 wt%, the loads of hot rolling and cold rolling may increase, and the coating properties of steel sheet 10 may deteriorate.

[0054] The purpose of adding chromium (Cr) is to improve the hardenability and strength of the steel sheet 10. Chromium helps refine the grains and ensure the strength of the steel sheet 10 through precipitation hardening. Based on the total weight of the steel sheet 10, chromium can be included in an amount from about 0.05 wt% to about 0.6 wt%. When the chromium content is less than about 0.05 wt%, the precipitation hardening effect is lower, and conversely, when the chromium content exceeds 0.6 wt%, Cr-based precipitates and matrix solid solutions increase, resulting in reduced toughness and potentially increased production costs.

[0055] Meanwhile, other unavoidable impurities may include nitrogen (N).

[0056] When a large amount of nitrogen (N) is added, the amount of dissolved nitrogen may increase, thereby reducing the impact properties and elongation of the steel plate 10. Based on the total weight of the steel plate 10, nitrogen may be included in an amount greater than 0 wt% and about 0.001 wt% or less. When the nitrogen content exceeds about 0.001 wt%, the impact properties and elongation of the steel plate 10 may deteriorate.

[0057] An additive is an element that forms carbides to facilitate the formation of precipitates in the steel plate 10. Specifically, the additive may include at least one of titanium (Ti), niobium (Nb), and vanadium (V).

[0058] Titanium (Ti) forms precipitates (e.g., TiC and / or TiN) at high temperatures, effectively promoting austenite grain refinement. Based on the total weight of the steel plate 10, titanium can be included in an amount from about 0.001 wt% to about 0.050 wt%. When titanium is included within the above content range, continuous casting defects and coarsening of the precipitates can be prevented, the physical properties of the steel can be easily ensured, and defects (e.g., cracks) on the steel surface can be prevented. On the other hand, when the titanium content exceeds about 0.050 wt%, coarsening of the precipitates occurs, and elongation and bendability may decrease.

[0059] Niobium (Nb) and vanadium (V) can improve strength and toughness by reducing the size of the martensite ladle. Based on the total weight of the steel plate 10, each of niobium and vanadium can be included in an amount from about 0.01 wt% to about 0.1 wt%. When niobium and vanadium are included within the above ranges, the grain refinement effect of the steel plate 10 is high in hot and cold rolling processes, preventing cracks in the slab and brittle fracture of the product during steelmaking / casting, and minimizing the formation of coarse precipitates during steelmaking.

[0060] Calcium (Ca) can be added to control the shape of inclusions. Based on the total weight of the steel plate 10, calcium can be included in an amount of about 0.003% by weight or less.

[0061] After hot rolling and / or cold rolling processes, when the steel sheet 10 is cooled to room temperature, residual stress exists in the steel sheet 10 of the hot-stamped part manufactured by the hot stamping process. Here, "residual stress" refers to the stress present in the hot-stamped part when no external force is applied to the steel sheet 10.

[0062] Residual stress can be caused by defects in the material. For example, point defects (such as vacancies, gaps, impurities, etc.), line defects (such as dislocations), and interface defects (such as outer surfaces, grain boundaries, twin boundaries, stacking faults, phase boundaries, etc.) can be the cause of residual stress. That is to say, it can be understood that the more defects present in the steel plate 10, the greater the internal residual stress.

[0063] These defects in the steel plate 10 and the resulting residual stress affect the mechanical properties (e.g., tensile strength) and hydrogen embrittlement of the steel plate 10.

[0064] Specifically, the tensile strength of the hot-stamped component is determined as follows: when defects exist at an appropriate level within the steel plate 10, the more defects (or the greater the residual stress), the greater the tensile strength; and the fewer defects (or the smaller the residual stress), the smaller the tensile strength. This is because the more defects within the steel plate 10, the more irregular the arrangement of the elements, making it difficult to move dislocations that cause material deformation.

[0065] However, the hydrogen embrittlement of the steel sheet 10 may decrease with an increase in defects (or an increase in residual stress) and may improve with a decrease in defects (or a decrease in residual stress). Generally, the amount of active hydrogen is reduced due to the presence of more effective hydrogen-capturing sites within the steel sheet 10, thus improving the product's hydrogen embrittlement. For example, fine precipitates present therein (e.g., nitrides or carbides of titanium (Ti), niobium (Nb), and vanadium (V)) serve as effective hydrogen-capturing sites and improve hydrogen embrittlement. Defects present therein can also serve as hydrogen-capturing sites. However, since defects have a relatively low binding energy with hydrogen, hydrogen captured and deactivated by defects is highly likely to revert to active hydrogen. Therefore, defects cannot serve as effective hydrogen-capturing sites; instead, hydrogen embrittlement can be reduced by locally concentrating active hydrogen in areas with more defects (or areas with higher residual stress). In particular, depending on its application location in the vehicle structure, the hot-stamped component may include at least one curved portion, and the curved portion is a portion excessively formed during the hot-stamping process compared to a flat area. In other words, the bent portion can be considered a weak point in hydrogen embrittlement because the stress generated by pressing during the hot stamping process is relatively concentrated, and therefore, the residual stress may increase.

[0066] Therefore, it is necessary to control the defects present in the steel plate 10 and the resulting residual stress at an appropriate level.

[0067] According to an exemplary embodiment of this application, defects and resulting residual stress in the steel plate 10 can be appropriately adjusted by controlling the residual stress analysis value (which quantifies the residual stress present in the steel plate 10) to meet preset conditions.

[0068] In an exemplary embodiment, the residual stress analysis value can be the product of the numerical value (or absolute value) of the XRD value quantified by X-ray diffraction of residual stress and the numerical value (or absolute value) of the electron backscatter diffraction (EBSD) value quantified by EBSD of orientation. Furthermore, the preset condition can be approximately 2.85 * 10⁻⁶. -4 Degree * MPa / μm 2 Or larger and approximately 0.05 degrees*MPa / μm 2 Or smaller. More preferably, when the XRD value is about 5 MPa or greater and less than about 15 MPa, the residual stress analysis value can be controlled to meet about 2.95 × 10⁻⁶ MPa. -4 Degree * MPa / μm 2 Or larger and approximately 0.01 degrees*MPa / μm 2 Or a smaller range, when the XRD value is approximately 15 MPa or greater but less than approximately 55 MPa, the residual stress analysis value can be controlled to meet approximately 9.31 × 10⁻⁶. -4 Degree * MPa / μm2 Or larger and approximately 0.035 degrees*MPa / μm 2 Or a smaller range, and when the XRD value is about 55 MPa or greater and about 70 MPa or less, the residual stress analysis value can be controlled to meet about 3.96 * 10. -3 Degree * MPa / μm 2 Or larger and approximately 0.043 degrees*MPa / μm 2 Or a smaller range.

[0069] X-ray diffraction (XRD) analysis is an analytical method that measures residual stress by using X-ray diffraction, where incident X-rays irradiating the sample are reflected in specific directions due to the regularity of the crystal lattice. Specifically, residual stress can be measured through... The method is used for measurement. The method obtains the peak positions of diffraction lines by irradiating the part to be measured with X-rays. When residual stress is present, the incident angle of the X-rays is changed. This is used to change the peak position of the diffraction lines. At this point, the changed peak position of the diffraction lines is used as the vertical axis, and the angle of incidence of the X-rays is used as the horizontal axis. Using the horizontal axis, the slope is obtained through linear regression using the least squares method. The obtained slope is then multiplied by the stress constant obtained from Young's modulus and Poisson's ratio. Finally, the obtained stress value (XRD value) is obtained through the following Equation 1.

[0070] [Equation 1]

[0071] σ=-E / 2(1+v)*cotθ*π / 180*M=K*M

[0072] σ: Stress value or XRD value (MPa)

[0073] E: Young's modulus (MPa)

[0074] v: Poisson's ratio

[0075] M: The slope of the regression line 2θ-sin 2 θ

[0076] 2θ: Diffraction angle (°) without strain

[0077] K: Stress constant (MPa)

[0078] XRD analysis is highly representative because it covers a relatively wide range, but its drawbacks include significant bias and poor uniformity. Furthermore, the bias in XRD values ​​tends to increase with increasing residual stress within the product. Therefore, the problem is that relying solely on XRD values ​​obtained by quantifying residual stress through XRD analysis makes it difficult to accurately analyze and control the residual stress in materials.

[0079] On the other hand, "EBSD" uses the diffraction pattern of a sample to determine the crystal phase and crystal orientation, and based on this, "EBSD" is a method to analyze a sample by combining the morphological information of the sample's microstructure and crystallographic information.

[0080] Specifically, when an electron beam is irradiated onto a sample in a scanning electron microscope (SEM), the incident electron beam is scattered within the sample, resulting in a diffraction pattern along the surface of the sample. This is called an electron backscattered diffraction pattern (EBSP), and the pattern corresponds to the crystal orientation of the region irradiated by the electron beam, allowing for the measurement of the material's crystal orientation with an accuracy of less than 1°.

[0081] Because EBSD covers a relatively narrow range, it has the advantages of smaller bias and better uniformity compared to X-ray diffraction (XRD) analysis. However, a drawback of EBSD values ​​(which quantify residual stress) is their lack of representativeness; it is difficult to accurately analyze and control the residual stress of materials using only EBSD values.

[0082] In an exemplary embodiment of this application, differentiated residual stress analysis values ​​are applied to compensate for the shortcomings of X-ray diffraction (XRD) analysis and EBSD, respectively. Specifically, as the residual stress analysis value, the product of the numerical value (or absolute value) of the XRD value digitized by X-ray diffraction (XRD) analysis and the numerical value (or absolute value) of the EBSD value quantified by EBSD is applied. Therefore, the bias (a shortcoming of XRD values) is compensated for by the EBSD value, and the low representativeness (a shortcoming of EBSD values) is compensated for by the XRD value, thus achieving the effect of more accurate analysis and control of residual stress.

[0083] For example, the residual stress analysis value can be expressed as Equation 2 below.

[0084] [Equation 2]

[0085] Residual stress analysis values ​​(degrees * MPa / μm) 2 = |XRD value (MPa)| * |EBSD value (degrees / μm) 2 )|

[0086] The residual stress analysis value is substantially proportional to the defects in the hot-stamped component and the resulting residual stress. Specifically, it can be understood that a higher residual stress analysis value indicates more internal defects and greater residual stress, while a lower value indicates fewer internal defects and lower residual stress. Furthermore, it can be understood that a higher residual stress analysis value results in greater tensile strength but poorer hydrogen embrittlement, while a lower value results in lower tensile strength but better hydrogen embrittlement. Therefore, the mechanical properties and hydrogen embrittlement resistance of the product can be appropriately ensured by controlling the residual stress analysis value to meet preset conditions.

[0087] On the other hand, due to the characteristics of the method of manufacturing products from high-temperature materials through rolling and cooling, defects and resulting residual stresses may occur during the manufacturing process due to temperature differences in the width or length direction of the steel plate 10. According to an exemplary embodiment of this application, the aforementioned residual stress analysis values ​​can be controlled to meet preset conditions by applying differentiated process conditions (e.g., heating and / or cooling conditions) in the manufacturing process. Reference will be made below. Figures 3 to 5 Detailed description of these differentiated process conditions.

[0088] Figure 2 A plan view illustrating a portion of a hot-stamped component according to an exemplary embodiment of this application.

[0089] The steel plate 10 may comprise a component system having a microstructure comprising a martensitic phase with an area fraction of about 80% or greater. Furthermore, the steel plate 10 may comprise a bainitic phase with an area fraction of less than about 20%.

[0090] The martensitic phase is the result of a diffusionless transformation of austenite γ below the martensitic transformation initiation temperature (Ms) during cooling. Martensite can have rod-like lath phases oriented in one direction d within each initial grain of austenite.

[0091] Furthermore, the steel plate 10 may contain iron-based carbides located within the martensitic phase. The iron-based carbides may be in a needle-like form. In an exemplary embodiment, the iron-based carbides may have a diameter of less than about 0.2 μm and a length of less than about 10 μm. Here, "diameter of the iron-based carbides" may refer to the minor axis length of the iron-based carbides, and "length of the iron-based carbides" may refer to the major axis length of the iron-based carbides.

[0092] If the diameter of the iron-based carbide is about 0.2 μm or greater, or the length is about 10 μm or greater, the iron-based carbide can remain unmelted even at Ac3 or higher temperatures during annealing heat treatment, and the bendability and yield ratio of the steel sheet 10 may decrease. On the other hand, when the diameter of the iron-based carbide is less than about 0.2 μm and the length is less than about 10 μm, the balance between the strength and formability of the steel sheet 10 can be improved.

[0093] Based on the martensitic phase, the area fraction of iron-based carbides can be less than about 5%. When the area fraction of iron-based carbides based on the martensitic phase is about 5% or greater, it may be difficult to ensure the strength and flexibility of the steel plate 10.

[0094] In an exemplary implementation, such as Figure 2 As shown, the iron-based carbide may comprise a first iron-based carbide C1 and a second iron-based carbide C2. The first iron-based carbide C1 may be an iron-based carbide parallel to the longitudinal direction d of the lath phase, and the second iron-based carbide C2 may be an iron-based carbide perpendicular to the longitudinal direction d of the lath phase. Here, "parallel" includes forming an angle of about 0° or greater and about 20° or less with respect to the longitudinal direction d of the lath phase, and "perpendicular" includes forming an angle of about 70° or greater and about 90° or less with respect to the longitudinal direction d of the lath phase. For example, the first iron-based carbide C1 may form an angle of about 0° or greater and about 20° or less with respect to the longitudinal direction d of the lath phase, and the second iron-based carbide C2 may form an angle of about 70° or greater and about 90° or less with respect to the longitudinal direction d of the lath phase.

[0095] The iron-based carbide reference area fraction of the first iron-based carbide C1 can be greater than that of the second iron-based carbide. This improves the bendability of the steel plate 10. Specifically, the iron-based carbide reference area fraction of the first iron-based carbide C1, which forms an angle of about 0° or greater and about 20° or less with respect to the longitudinal direction d of the lath phase, can be about 50% or greater, preferably about 60% or greater. Furthermore, the iron-based carbide reference area fraction of the second iron-based carbide C2, which forms an angle of about 70° or greater and about 90° or less with respect to the longitudinal direction d of the lath phase, can be less than about 50%, preferably less than about 40%.

[0096] Cracks that appear during bending deformation may arise from the movement of dislocations in the martensitic phase. In this case, it can be understood that, for a given plastic deformation, the higher the local strain rate, the greater the energy absorption of the martensite during plastic deformation, thus improving impact performance.

[0097] On the other hand, when the iron-based carbide reference area fraction of the first iron-based carbide C1 in the longitudinal direction d parallel to the lath phase is greater than that of the second iron-based carbide C2 in the longitudinal direction d perpendicular to the lath phase, dynamic strain aging (DSA) (i.e., indentation dynamic strain aging) may occur due to the difference in local strain rate during dislocation movement within the lath phase during bending deformation. As a concept of plastic deformation absorption energy, indentation dynamic strain aging represents the resistance to deformation; the higher the frequency of indentation dynamic strain aging, the better the resistance to deformation.

[0098] In other words, according to the exemplary embodiment, since the iron-based carbide reference area fraction of the first iron-based carbide C1, which forms an angle of about 20° or less with the longitudinal direction d of the lath phase, is about 50% or more, and the iron-based carbide reference area fraction of the second iron-based carbide C2, which forms an angle of about 70° or more and about 90° or less with the longitudinal direction d of the lath phase, is less than about 50%, the dynamic strain aging phenomenon of indentation may occur frequently, thereby ensuring a V-shaped bending angle of about 50° or more. Therefore, bendability and impact performance can be improved.

[0099] Since the bainitic phase with an area fraction of less than about 20% in the steel plate 10 has a uniform hardness distribution, it is a structure with an excellent balance between strength and ductility. However, since bainite is softer than martensite, it is preferable that the area fraction of bainite is less than about 20% in order to ensure the strength and flexibility of the steel plate 10.

[0100] On the other hand, the aforementioned iron-based carbides in the form of needles can precipitate within the bainitic phase. Since the iron-based carbides within the bainitic phase increase the strength of bainite and reduce the strength difference between bainite and martensite, the yield ratio and bendability of the steel plate 10 can be improved. In this case, based on the bainitic phase, the iron-based carbides can exist in the bainitic phase in an amount of less than about 20%. If the amount of iron-based carbides based on the bainitic phase is about 20% or higher, voids may be generated, which could lead to a decrease in bendability.

[0101] Figure 3 A flowchart illustrating a method for manufacturing a hot-stamped part according to an embodiment of this application is provided. Figure 4 To illustrate the temperature variation during multi-stage heating of the blank in a method for manufacturing a hot-stamped part according to an embodiment of this application, and Figure 5 A graph comparing temperature changes when heating a blank in multiple stages versus heating a blank in a single stage.

[0102] refer to Figure 3A method for manufacturing a hot-stamped part according to an exemplary embodiment of this application may include inserting a blank (S110), multi-stage heating (S120), and homogenization heating (S130). Furthermore, after homogenization heating (S130), the method for manufacturing the hot-stamped part may also include transfer (S140), forming (S150), and cooling (S160).

[0103] First, inserting the blank (S110) can be an operation of placing the blank into a heating furnace having multiple sections.

[0104] A blank introduced into a heating furnace can be formed by cutting sheet metal for use in forming hot-stamped parts. Sheet metal can be manufactured by hot rolling or cold rolling a steel billet followed by annealing. Furthermore, after annealing, a coating can be formed on at least one surface of the annealed sheet metal. For example, the coating can be an Al-Si based coating or a Zn coating.

[0105] Then, multi-segment heating (S120) and homogenization heating (S130) can be performed sequentially. The blank inserted into the heating furnace can be heated as it passes through multiple sections arranged in the heating furnace. In an exemplary embodiment, the blank introduced into the heating furnace can be mounted on rollers and conveyed along the conveying direction.

[0106] The heating furnace may include multiple sections arranged sequentially within the furnace. These multiple sections include sections where the temperature gradually increases from the furnace inlet where the billet is introduced to the furnace to the furnace outlet where the billet is discharged, and sections where the temperature range is maintained uniformly.

[0107] Multi-stage heating (S120) is the operation of heating the billet by passing through sections of temperature that are gradually increased in multiple sections arranged in the heating furnace. Soaking heating (S130) is the operation of heating the billet that has undergone multi-stage heating (gradual heating or segmented heating) by passing through sections of temperature that are uniformly maintained in multiple sections arranged in the heating furnace.

[0108] The temperature range of multiple sections arranged in the heating furnace gradually increases from the inlet of the heating furnace where the billet is introduced to the outlet of the heating furnace, reaching a target temperature range Tt. Then, from the section with the target temperature range Tt to the outlet of the heating furnace, the temperature can be maintained within a uniform range, i.e., the target temperature range Tt. In this case, the number of sections with gradually increasing temperature ranges, the number of sections with uniformly maintained temperature ranges, and the temperature range of each section are unrestricted.

[0109] In an exemplary implementation, such as Figure 4As shown, the heating furnace may include a first section P1 having a first temperature range T1, a second section P2 having a second temperature range T2, a third section P3 having a third temperature range T3, a fourth section P4 having a fourth temperature range T4, a fifth section P5 having a fifth temperature range T5, a sixth section P6 having a sixth temperature range T6, and a seventh section P7 having a seventh temperature range T7. In another embodiment, different from Figure 4 As shown, the heating furnace may include six or fewer, or eight or more sections, and the temperature range of each section may vary. For ease of description, the following will describe... Figure 4 The implementation scheme shown.

[0110] The first section P1 to the seventh section P7 can be arranged sequentially in the heating furnace. The first section P1, having a first temperature range T1, is adjacent to the inlet of the heating furnace where the billet is introduced, and the seventh section P7, having a seventh temperature range T7, is adjacent to the outlet of the heating furnace where the billet is discharged. That is, the first section P1, having a first temperature range T1, can be the first section among multiple sections included in the heating furnace, and the seventh section P7, having a seventh temperature range T7, can be the last section among multiple sections included in the heating furnace. The billet can be heated by sequentially moving from the first section P1 to the seventh section P7 in the heating furnace.

[0111] In an exemplary implementation, such as Figure 4 As shown, the temperature range of the segments from the first segment P1 to the fifth segment P5 gradually increases to the target temperature range Tt, and in the sixth segment P6 and the seventh segment P7, the temperature range remains within the target temperature range Tt (which is the temperature range of the fifth segment P5). However, this application is not limited to the above embodiment, and the number of segments with a phased increase in temperature range and the number of segments with a uniformly maintained temperature range can vary.

[0112] Meanwhile, the temperature difference between two adjacent sections in a plurality of sections arranged in the heating furnace can be about 0°C or greater and about 100°C or less. For example, the temperature difference between the first section P1 and the second section P2 can be about 0°C or greater and about 100°C or less.

[0113] In an exemplary embodiment, the first temperature range T1 of the first segment P1 can be in the range of about 840°C to about 860°C, or in the range of about 835°C to about 865°C. The second temperature range T2 of the second segment P2 can be in the range of about 870°C to about 890°C, or in the range of about 865°C to about 895°C. The third temperature range T3 of the third segment P3 can be in the range of about 900°C to about 920°C, or in the range of about 895°C to about 925°C. The fourth temperature range T4 of the fourth segment P4 can be in the range of about 920°C to about 940°C, or in the range of about 915°C to about 945°C. The fifth temperature range T5 of the fifth segment P5 can be in the range of about Ac3 to about 1000°C. Preferably, the fifth temperature range T5 of the fifth segment P5 can be about 930°C or higher and about 1000°C or lower. More preferably, the fifth temperature range T5 of the fifth segment P5 can be about 950°C or higher and about 1000°C or lower. The sixth temperature range T6 of the sixth segment P6 and the seventh temperature range T7 of the seventh segment P7 can be the same as the fifth temperature range T5 of the fifth segment P5.

[0114] In this configuration, multi-segment heating (S120) can be performed in the first segment P1 to the fourth segment P4, and homogenization heating (S130) can be performed in the fifth segment P5 to the seventh segment P7. In this way, by providing segments where homogenization heating (S130) is performed in multiple segments (e.g., the fifth segment P5 to the seventh segment P7) instead of in a single segment, temperature differences between segments can be prevented or minimized.

[0115] The homogenization heating (S130) is performed within the temperature range of the fifth section P5, and the temperature range of the fifth section P5 is the target temperature range Tt, which can be Ac3 or higher. That is, in the homogenization heating (S130), the billet heated in multiple stages from the first section P1 to the fourth section P4 can be homogenized at a temperature of Ac3 or higher. Preferably, in the homogenization heating (S130), the billet heated in multiple stages can be homogenized at a temperature of about 930°C or higher and about 1000°C or lower. More preferably, in the homogenization heating (S130), the billet heated in multiple stages can be homogenized at a temperature of about 950°C or higher and about 1000°C or lower.

[0116] In an exemplary embodiment, the length of the heating furnace along the conveying path of the billet can be from about 20 m to about 40 m. The heating furnace can have multiple sections with different temperature ranges, and the length of the multiple sections heating the billet is (D1, see...) Figure 4 ) and the length of the section in which the blank is uniformly heated in multiple sections (D2, see Figure 4The ratio of (D1+D2) can be in the range of approximately 1:1 to approximately 4:1. That is, the length of the homogenizing heating section D2 in the multiple sections arranged in the heating furnace can correspond to approximately 20% to approximately 50% of the total length (D1+D2) of the heating furnace.

[0117] When the ratio of the length of the multi-segment heated blank (D1) to the length of the soaking-heated blank (D2) exceeds 1:1 due to the increased length of the section used for soaking the blank, an austenitic (FCC) structure is formed in the soaking-heating section. This increases the amount of hydrogen penetrating into the blank, and delayed fracture may increase. Furthermore, when the ratio of the length of the multi-segment heated blank (D1) to the length of the soaking-heated blank (D2) is less than 4:1, the strength of the hot-stamped part manufactured by the hot-stamping part manufacturing method may be uneven because the soaking-heating section (time) cannot be sufficiently ensured.

[0118] In an exemplary embodiment, during the multi-stage heating (S120) and homogenization heating (S130), the heating rate of the blank can be from about 6°C / s to about 12°C / s, and the homogenization heating time can be in the range of about 3 minutes to about 6 minutes. More specifically, when the thickness of the blank is from about 1.6 mm to about 2.3 mm, the heating rate is from about 6°C / s to about 9°C / s, and the homogenization heating time can be in the range of about 3 minutes to about 4 minutes. Furthermore, when the thickness of the blank is in the range of about 1.0 mm to about 1.6 mm, the heating rate is from about 9°C / s to about 12°C / s, and the homogenization heating time can be in the range of about 4 minutes to about 6 minutes.

[0119] Reference Figure 5 Describe the temperature changes when heating blank B' in a single heating cycle and when heating blank B in multiple stages.

[0120] As a comparative example, we can assume the case of heating the billet B' only once. In this single heating, the furnace temperature is set such that the internal temperature of the furnace remains equal to the target temperature Tt of the billet. In this case, the target temperature Tt of the billet B' can be equal to or higher than Ac3. Preferably, the target temperature Tt of the billet B' can be about 930°C. More preferably, the target temperature Tt of the billet B' can be about 950°C.

[0121] In a single heating cycle, the temperature of blank B' can reach the target temperature Tt faster than in a multi-stage heating cycle. For example, the heating rate of blank B' in a single heating cycle can be approximately 2°C / s or greater than that in a multi-stage heating cycle. Because the temperature in a single heating cycle reaches the target temperature Tt faster than in a multi-stage heating cycle, the soaking time ET2 in a single heating cycle can be longer than the soaking time ET1 in a multi-stage heating cycle. Similar to the case of single heating, if the soaking time ET2 is prolonged, the grain boundary size becomes inhomogeneous, and the aforementioned defects may be unnecessarily excessively formed.

[0122] Therefore, in the method for manufacturing a hot-stamped part according to an exemplary embodiment of this application, delaying the time for the blank to reach the target temperature (Tt) using a multi-stage heating method ensures an appropriate homogenization heating time (ET1), thereby ensuring uniformity of grain boundary dimensions and controlling the formation of defects at an appropriate level. Thus, the hot-stamped part manufactured by applying the multi-stage heating method can be controlled to have defects and residual stress within a predetermined range, and its compliance with the predetermined range can be checked using the aforementioned residual stress analysis values.

[0123] refer to Figure 3 After homogenization heating (S130), transfer (S140), forming (S150) and cooling (S160) can be carried out.

[0124] The transfer (S140) can be an operation of transferring the heated blank from the heating furnace to the pressing die. In the operation of transferring the heated blank from the heating furnace to the pressing die, the heated blank can be air-cooled for about 10 seconds to about 15 seconds.

[0125] Forming (S150) can be an operation of hot stamping the transferred blank to form a molded body. Cooling (S160) can be an operation of cooling the molded body.

[0126] After the final part shape is formed in the mold, the final product can be formed by cooling the molded body. Cooling channels for refrigerant circulation can be arranged in the mold. The heated blank can be rapidly cooled by circulating the supplied refrigerant in the cooling channels arranged in the mold. At this time, in order to prevent the sheet metal from springing back and maintain the desired shape, rapid cooling can be performed while the mold is pressed in the closed state. That is, when the blank is placed in the mold, the forming process (or forming (S150)) and the cooling process (or cooling (S160)) can be performed simultaneously.

[0127] In an exemplary embodiment, during the forming and cooling processes of the heated blank, the blank can be held in the mold at a temperature below the austenitic transformation initiation temperature (MS temperature) for a predetermined time (e.g., about 3 seconds to about 20 seconds). Furthermore, the blank can be cooled while maintaining an average cooling rate of about 15°C / s or greater until the temperature at which the martensitic transformation is complete (Mf temperature). By ensuring the cooling time in this way, the martensitic structure is self-tempered to obtain self-tempered martensite, and deformation of the molded part can be prevented, thus having the effect of reducing residual stress within the product.

[0128] If the holding time in the die is less than 3 seconds, the material may not cool sufficiently, and thermal deformation may occur due to residual heat in the product and temperature variations in each part. Furthermore, when the blank is held in the die for more than 20 seconds, unwanted defects and resulting residual stress may occur, and the increased holding time in the die may reduce productivity.

[0129] In an exemplary embodiment, the tensile strength of a hot-stamped part manufactured by the method of manufacturing hot-stamped parts can be about 1350 MPa or higher, and the amount of active hydrogen can be about 0.7 wppm or lower.

[0130] The present application will be described in more detail below through embodiments and comparative examples. However, the following embodiments and comparative examples are used to explain the present invention in more detail, and the scope of the present invention is not limited to the following embodiments and comparative examples. Those skilled in the art can make appropriate modifications and changes to the following embodiments and comparative examples within the scope of this application.

[0131] Table 1

[0132]

[0133] Table 1 shows the measured XRD values, tensile strength, and amount of active hydrogen for each of specimens A-1 to A-9. Specifically, Table 1 confirms whether the XRD values ​​measured for the specimens meet the range of 5 MPa or greater and 70 MPa or less, and shows the data used for comparison and analysis of tensile strength and amount of active hydrogen when the above range is met and when the above range is not met.

[0134] The XRD value is obtained by quantifying the residual stress through the aforementioned X-ray diffraction (XRD) analysis. The XRD value is measured by removing the coating from the sample and then irradiating the target location (e.g., the 1 / 4 point) with X-rays after electropolishing. Furthermore, electropolishing is performed using an electropolishing solution containing approximately 5% 2-butoxyethanol, approximately 20% perchloric acid, approximately 35% ethanol, and approximately 40% water.

[0135] The amount of active hydrogen can be measured using thermal desorption spectroscopy. Thermal desorption spectroscopy is a method that measures the amount of hydrogen released from a sample below a certain temperature while heating the sample at a predetermined heating rate to raise its temperature. The hydrogen released from the sample can be understood as active hydrogen that has not been captured and influences delayed hydrogen destruction in the hydrogen introduced into the sample. In other words, a large amount of hydrogen as measured by thermal desorption spectroscopy indicates the presence of a significant amount of uncaptured active hydrogen that may contribute to delayed hydrogen destruction.

[0136] Specifically, the amount of active hydrogen in Table 1 is the value obtained by measuring the amount of hydrogen emitted from the sample at a temperature of about 350°C or lower when the temperature of each sample is raised from room temperature to about 500°C at a heating rate of about 20°C / min.

[0137] Samples A-1 to A-5 meet the range of measured XRD values ​​of approximately 5 MPa or greater and approximately 70 MPa or less. That is, it can be understood that appropriate levels of defects and residual stress are present in samples A-1 to A-5. Therefore, it can be confirmed that the tensile strength of samples A-1 to A-5 meets 1350 MPa or greater, and the amount of active hydrogen in samples A-1 to A-5 meets 0.7 wppm or less.

[0138] On the other hand, in the case of samples A-6 and A-7, the measured XRD values ​​are less than 5 MPa. This indicates that samples A-6 and A-7 contain defects below the required level, resulting in insufficient residual stress. Therefore, it can be seen that the amount of active hydrogen in each of samples A-6 and A-7 meets the requirement of 0.7 wppm or less, but the tensile strength is less than 1350 MPa.

[0139] Furthermore, in the case of samples A-8 and A-9, the measured XRD values ​​exceeded 70 MPa. This indicates the presence of unnecessary defects within samples A-8 and A-9, resulting in excessive residual stress. Therefore, while the tensile strength of each of samples A-8 and A-9 meets 1350 MPa or greater, the amount of active hydrogen exceeds 0.7 wppm, demonstrating reduced hydrogen embrittlement.

[0140] Meanwhile, referring to Table 1, it can be seen that as the XRD value increases, the deviation of the XRD value also tends to increase. In other words, the greater the internal stress, the larger the error range of the XRD value, and therefore, the more necessary it is to correct it.

[0141] Table 2

[0142]

[0143] Table 2 shows the measured EBSD values, tensile strengths, and amounts of active hydrogen for each of samples B-1 to B-8. Specifically, Table 1 confirms that the EBSD values ​​measured for the samples meet the requirement of approximately 5.71 × 10⁻⁶. -5 degrees / μm 2 Or larger and approximately 7.14*10 -4 degrees / μm 2 Or a smaller range, and shows comparative analysis data for tensile strength and active hydrogen content under conditions that meet the above range and those that do not.

[0144] The EBSD value was obtained by quantifying the orientation using the EBSD method described above. The EBSD value was measured by scanning a 25μm x 70μm sample area 4000 times in 50nm increments. Furthermore, these measurements were performed on five observation surfaces.

[0145] The amount of active hydrogen was measured using thermal desorption spectroscopy under the same conditions as in Table 1.

[0146] The measured EBSD values ​​for samples B-1 to B-4 meet the requirement of approximately 5.71 × 10⁻⁶. -5 degrees / μm 2 Or larger and approximately 7.14*10 -4 degrees / μm 2 Or a smaller range. That is, it can be understood that there are appropriate levels of defects and resulting residual stress in specimens B-1 to B-4. Therefore, it can be confirmed that the tensile strength of specimens B-1 to B-4 meets about 1350 MPa or greater, and the amount of active hydrogen in specimens B-1 to B-4 meets about 0.7 wppm or less.

[0147] On the other hand, in the cases of samples B-5 and B-6, the measured EBSD values ​​were less than approximately 5.71 × 10⁻⁶. -5 degrees / μm 2 In other words, it can be seen that there are defects below the required level in samples B-5 and B-6, and the resulting residual stress is too small. Therefore, it can be seen that the amount of active hydrogen in each of samples B-5 and B-6 meets the requirement of about 0.7 wppm or less, but the tensile strength is less than about 1350 MPa.

[0148] Furthermore, in the cases of samples B-7 and B-8, the measured EBSD values ​​exceeded approximately 7.14 × 10⁻⁶. -4 degrees / μm 2In other words, it can be seen that there are unnecessary defects inside samples B-7 and B-8, and the resulting residual stress is too large. Therefore, it can be confirmed that the tensile strength of each of samples B-7 and B-8 meets the requirement of about 1350 MPa or greater, but the amount of active hydrogen exceeds about 0.7 wppm, thus reducing hydrogen embrittlement.

[0149] Table 3

[0150]

[0151] Table 3 shows the XRD value, EBSD value, residual stress analysis value, tensile strength, active hydrogen content, and 4-point bending test results for each of the specimens C-1 to C-12.

[0152] XRD values, EBSD values, tensile strength, and the amount of active hydrogen were measured under the same conditions and methods as in Tables 1 and 2. Furthermore, the residual stress analysis values ​​were calculated as the product of the numerical (or absolute) XRD value and the numerical (or absolute) EBSD value.

[0153] The four-point bending test is a method for checking for stress corrosion cracking by applying stress below the elastic limit to specific points on a specimen manufactured to reproduce the state of the specimen being exposed to a corrosive environment. In this context, stress corrosion cracking refers to cracks that occur when corrosive and continuous tensile stresses act simultaneously.

[0154] Specifically, the four-point bending test results in Table 1 are the results of checking whether fracture occurs by applying a stress of 1000 MPa to each specimen in air for 100 hours.

[0155] According to an exemplary embodiment of this application, by applying the product of the XRD value and the EBSD value as residual stress analysis values, inaccuracies in each of the XRD and EBSD values ​​can be mutually corrected. Therefore, residual stress within the product can be accurately analyzed and controlled. Specifically, the residual stress analysis value is controlled to meet approximately 2.85 * 10-1 -4 Degree * MPa / μm 2 Or larger and approximately 0.05 degrees*MPa / μm 2 Or a smaller range.

[0156] On the other hand, referring to the XRD values ​​in Table 3, it can be seen that as the XRD value increases, the deviation of the XRD value also tends to increase. That is, the greater the internal stress, the larger the error range of the XRD value, and the more necessary it is to correct for it. Therefore, when the internal residual stress of the product is large (or the deviation of the XRD value is large), the role of residual stress analysis values ​​may be more significant.

[0157] With this in mind, the residual stress analysis value can be controlled more accurately based on the range of XRD values. Specifically, when the XRD value is 5 MPa or greater but less than 15 MPa, the residual stress analysis value can be controlled to meet approximately 2.95 × 10⁻⁶. -4 Degree * MPa / μm 2 Or larger and approximately 0.01 degrees*MPa / μm 2 Within a smaller range, when the XRD value is 15 MPa or greater but less than 55 MPa, the residual stress analysis value can be controlled to meet approximately 9.31 × 10⁻⁶. -4 Degree * MPa / μm 2 Or larger and approximately 0.035 degrees*MPa / μm 2 Or a smaller range, and when the XRD value is 55 MPa or greater and 70 MPa or less, the residual stress analysis value can be controlled to meet approximately 3.96 * 10. -3 Degree * MPa / μm 2 Or larger and approximately 0.043 degrees*MPa / μm 2 Or a smaller range.

[0158] Samples C-1 to C-6 are hot-stamped parts manufactured by operating S110 to S160 under the above-described process conditions. That is, samples C-1 to C-6 are samples manufactured by applying the above-described multi-stage heating (S120) and homogenization heating (S130), applying an average cooling rate of about 15°C / s or greater in cooling (S160) to the temperature (Mf) at which the martensitic phase transformation of the blank is completed, and holding the sample in a die at a temperature below the martensitic phase transformation initiation temperature (MS temperature) for about 3 seconds to about 20 seconds.

[0159] Therefore, in samples C-1 to C-6, the measured XRD values ​​meet the range of approximately 5 MPa or greater and approximately 70 MPa or less, and the measured EBSD values ​​meet the range of approximately 5.71 × 10⁻⁶. -5 degrees / μm 2 Or larger and approximately 7.14*10 -4 degrees / μm 2 Or a smaller range. Furthermore, the residual stress analysis values ​​(the product of the XRD value and the EBSD value) for specimens C-1 to C-6 also satisfy approximately 2.85 × 10⁻⁶. -4 Degree * MPa / μm 2 Or larger and approximately 0.05 degrees*MPa / μm 2 The range.

[0160] More specifically, in specimens C-1 and C-2, the XRD values ​​are approximately 5 MPa or greater and less than approximately 15 MPa, and the residual stress analysis values ​​meet the requirement of approximately 2.95 × 10⁻⁶.-4 Degree * MPa / μm 2 Or larger and 0.01 degrees * MPa / μm 2 Or a smaller range. Furthermore, in specimens C-3 and C-4, the XRD values ​​were approximately 15 MPa or greater and less than approximately 55 MPa, and the residual stress analysis values ​​met the requirement of approximately 9.31 × 10⁻⁶. -4 Degree * MPa / μm 2 Or larger and approximately 0.035 degrees*MPa / μm 2 Or a smaller range. Furthermore, in specimens C-5 and C-6, the XRD values ​​are approximately 55 MPa or greater and approximately 70 MPa or less, and the residual stress analysis values ​​meet the requirement of approximately 3.96 × 10⁻⁶. -3 Degree * MPa / μm 2 Or larger and approximately 0.043 degrees*MPa / μm 2 Or a smaller range.

[0161] In other words, for samples C-1 to C-6, not only do the XRD and EBSD values ​​meet the preset conditions, but the residual stress analysis values ​​also meet the preset conditions. Therefore, it can be understood that samples C-1 to C-6 contain a corrected level of defects and residual stress. Thus, it can be confirmed that the tensile strength of samples C-1 to C-6 is approximately 1350 MPa or greater, and the active hydrogen content is approximately 0.7 wppm or less. Furthermore, it can be confirmed that the four-point bending test results for samples C-1 to C-6 show no fracture. That is to say, because samples C-1 to C-6 were manufactured using the above-described process conditions, the residual stress analysis values ​​were controlled to meet the preset conditions, thus ensuring an appropriate level of tensile strength and hydrogen embrittlement.

[0162] Meanwhile, samples C-7 to C-12 are hot-stamped parts manufactured by applying process conditions that are at least a part different from the above-described process conditions.

[0163] Referring to Table 3, for samples C-7 to C-12, the measured XRD values ​​must be in the range of approximately 5 MPa or greater and approximately 70 MPa or less, and the measured EBSD values ​​must be approximately 5.71 × 10⁻⁶. -5 degrees / μm 2 Or larger and approximately 7.14*10 -4 degrees / μm 2 Or a smaller range. Therefore, it can be seen that the tensile strength of samples C-7 to C-12 meets about 1350 MPa or greater, and the amount of active hydrogen in samples C-7 to C-12 meets about 0.7 wppm or less.

[0164] However, the residual stress analysis values ​​of specimens C-7 to C-12 did not meet the above-mentioned preset conditions.

[0165] The XRD value of specimen C-7 is approximately 5 MPa or greater but less than approximately 15 MPa, and the residual stress analysis value is less than approximately 2.95 × 10⁻⁶. -4 Degree * MPa / μm 2 In other words, it is understandable that specimen C-7 contains defects below the required level, resulting in insufficient residual stress. Therefore, it can be confirmed that the result of the four-point bending test on specimen C-7 is fracture.

[0166] The XRD value of sample C-8 is approximately 5 MPa or greater but less than approximately 15 MPa, and the residual stress analysis value exceeds approximately 0.01 kJ / μm. 2 In other words, it is understandable that there are unnecessary defects inside specimen C-8, resulting in excessive residual stress. Therefore, it can be confirmed that the result of the four-point bending test of specimen C-8 is fracture.

[0167] The XRD value of specimen C-9 is approximately 15 MPa or greater but less than approximately 55 MPa, and the residual stress analysis value is less than approximately 9.31 × 10⁻⁶. -4 Degree * MPa / μm 2 In other words, it can be understood that there are defects in specimen C-9 that are below the required level, and the resulting residual stress is too small. Therefore, it can be confirmed that the result of the four-point bending test of specimen C-9 is fracture.

[0168] The XRD value of sample C-10 is approximately 15 MPa or greater but less than approximately 55 MPa, and the residual stress analysis value exceeds approximately 0.035 kJ / μm. 2 In other words, it is understandable that there are unnecessary defects inside specimen C-10, resulting in excessive residual stress. Therefore, it can be confirmed that the result of the four-point bending test of specimen C-10 is fracture.

[0169] The XRD values ​​of specimen C-11 are approximately 55 MPa or greater and approximately 70 MPa or greater, and the residual stress analysis value is less than approximately 3.96 × 10⁻⁶. -3 Degree * MPa / μm 2 In other words, it can be understood that there are defects in specimen C-11 that are below the required level, and the resulting residual stress is too small. Therefore, it can be confirmed that the result of the four-point bending test of specimen C-11 is fracture.

[0170] The XRD values ​​of specimen C-12 are approximately 55 MPa or greater and approximately 70 MPa or greater, and the residual stress analysis values ​​exceed approximately 0.043 kJ / μm. 2In other words, it is understandable that there are unnecessary defects inside specimen C-12, resulting in excessive residual stress. Therefore, it can be confirmed that the result of the four-point bending test of specimen C-12 is fracture.

[0171] Because the residual stress analysis values ​​did not meet the preset conditions, the four-point bending test results for specimens C-7 to C-12 showed fracture, even though each of the XRD and EBSD values ​​met the preset conditions. It is understandable that XRD or EBSD analysis alone cannot fully control internal defects and the resulting residual stresses in hot-stamped parts.

[0172] On the other hand, as with specimens C-1 to C-6, if the residual stress analysis value meets the preset conditions, the result of the 4-point bending test is that there will be no breakage. It can be confirmed that the residual stress analysis value can more accurately analyze and control the internal defects of hot-stamped parts and the resulting residual stress.

[0173] Although this application has been described with reference to exemplary embodiments shown in the accompanying drawings as examples only, those skilled in the art will understand that various changes in form and detail may be made therein without departing from the spirit and scope of the inventive concept. Therefore, the scope of the invention is not defined by the detailed description thereof, but by the appended claims.

Claims

1. A method for manufacturing a hot-stamped component whose residual stress analysis value meets preset conditions, the method comprising: Heating the blank; The blank is hot-stamped to form a molded body; as well as Cool the molded body to form a hot-stamped part. The heating of the blank includes: The billet is heated in multiple sections as it passes through multiple zones within the heating furnace, wherein the temperature range gradually increases in the arranged multiple zones; and The blank is uniformly heated to a temperature of Ac3 or higher; In multiple sections, the ratio of the length of the section used for multi-section heating of the blank to the length of the section used for homogenizing heating of the blank is 1:1 to 4:

1. In multi-stage heating, the heating rate of the blank is in the range of 6℃ / s to 12℃ / s; In multiple sections, the temperature of the section used for homogenizing the heating of the billet is higher than the temperature of the section used for multi-segment heating of the billet; The blank is held in the heating furnace for 180 to 360 seconds; Cooling the molded body to form a hot stamped part includes holding the molded body in a die at a temperature below the martensitic phase transformation initiation temperature for 3 to 20 seconds; The molded body is cooled to the martensitic transformation termination temperature in the mold at an average cooling rate of 15℃ / s or higher; wherein, the residual stress analysis value is the product of the XRD value obtained by quantifying residual stress through XRD analysis and the EBSD value obtained by quantifying orientation through EBSD analysis, and The residual stress analysis value meets the following preset condition: when the XRD value is 5 MPa or greater but less than 15 MPa, the residual stress analysis value is controlled to meet 2.95*10. -4 Degree * MPa / μm 2 Or larger and 0.01 degrees * MPa / μm 2 For a smaller range, when the XRD value is 15 MPa or greater but less than 55 MPa, the residual stress analysis value is controlled to meet 9.31 * 10⁻⁶. -4 Degree * MPa / μm 2 Or greater and 0.035 degrees * MPa / μm 2 Or a smaller range, and when the XRD value is 55 MPa or greater and 70 MPa or less, the residual stress analysis value is controlled to meet 3.96 * 10. -3 Degree * MPa / μm 2 Or larger and 0.043 degrees * MPa / μm 2 Or a smaller range.

2. The method according to claim 1, wherein, The temperature in multiple sections increases in the direction from the furnace inlet to the furnace outlet.

3. The method according to claim 1, wherein, Hot-stamped parts include: Martensitic phase, wherein the area fraction of the martensitic phase is 80% or greater; and Iron-based carbides, which are located within the martensitic phase and have an area fraction of less than 5% based on the martensitic phase.

4. The method according to claim 3, wherein Iron-based carbides have a needle-like form, and The needle-like form has a diameter of less than 0.2 μm and a length of less than 10 μm.

5. The method according to claim 3, wherein: Martensitic phase includes lath phase. The iron-based carbides comprise a first iron-based carbide parallel to the longitudinal direction of the lath and a second iron-based carbide perpendicular to the longitudinal direction of the lath, and The iron-based carbide reference area fraction of the first iron-based carbide is greater than that of the second iron-based carbide.

6. The method according to claim 5, wherein, The first iron-based carbide forms an angle of 0° or greater and 20° or less with the longitudinal direction of the lath, and the iron-based carbide has a reference area fraction of 50% or greater.

7. The method according to claim 5, wherein, The second iron-based carbide forms an angle of 70° or greater and 90° or less with the longitudinal direction of the lath, and the iron-based carbide reference area fraction is less than 50%.

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