CN116783013A

CN116783013A - Hot stamping part and manufacturing method thereof

Info

Publication number: CN116783013A
Application number: CN202180086821.XA
Authority: CN
Inventors: 金惠真; 黄奎然; 郑贤泳; 李晋昊; 郑胜必
Original assignee: Hyundai Steel Co
Current assignee: Hyundai Steel Co
Priority date: 2020-12-28
Filing date: 2021-12-27
Publication date: 2023-09-19
Also published as: EP4268986A1; JP7453424B2; KR102366284B9; KR102366284B1; WO2022145924A1; US20230107817A1; JP2023537810A

Abstract

According to an aspect of the present application, there is provided a method of manufacturing a hot stamped component having a residual stress analysis value satisfying a preset condition, the method comprising the steps of: heating the blank; hot stamping the blank to form a shaped body; and cooling the molded body to form a hot stamped part, wherein the residual stress analysis value is a product of a value of an XRD value, which is a residual stress quantified by X-ray diffraction (XRD), and a value of an EBSD value, which is an azimuth quantified by Electron Back Scattering Diffraction (EBSD), and the preset condition is 2.85X 10 or more ^‑4 (degree. Times. MPa/. Mu.m) ² ) And less than or equal to 0.05 (DEG. Times. MPa/. Mu.m) ² )。

Description

Hot stamping part and manufacturing method thereof

Technical Field

The present application relates to a hot stamped component and a method of manufacturing the hot stamped component.

Background

High strength steels are used to make lightweight and stable automotive parts. On the other hand, high strength steel can provide high strength properties as compared with its weight, but as strength increases, press formability decreases, which causes a breakage or rebound phenomenon of a material during processing, and thus, it is difficult to form a product having a complicated and precise shape.

As a method for improving these problems, a hot stamping method has been used, and therefore, researches on materials for hot stamping have been actively conducted. For example, as disclosed in korean patent application laid-open No. 10-2017-007409, the hot stamping method is a molding technique for manufacturing a high-strength member by heating a steel plate for hot stamping at a high temperature and then rapidly cooling while molding in a press mold. According to korean patent application laid-open No. 10-2017-007409, it is possible to manufacture a component with high accuracy by suppressing problems such as occurrence of cracks or shape freezing defects during forming, which are problems in high-strength steel plates.

Disclosure of Invention

Technical problem

Embodiments of the present application are directed to solving various problems including the above-described problems, and provide a hot stamped part capable of ensuring high mechanical properties and hydrogen embrittlement by controlling residual stress of the hot stamped part, and a method of manufacturing the same. However, these problems are exemplary, and the scope of the present application is not limited thereto.

Technical proposal

According to an aspect of the present application, there is provided a method of manufacturing a hot stamped component having a residual stress analysis value satisfying a preset condition. The method comprises heating a blank, forming a molded body by hot stamping the blank, and cooling the molded body to form a hot stamped part, wherein the residual stress analysis value may be a product of a value of an X-ray diffraction analysis (XRD) value obtained by quantifying residual stress by XRD analysis and a value of an Electron Back Scattering Diffraction (EBSD) value obtained by quantifying orientation by EBSD analysis, with a preset condition of about 2.85×10 ^-4 Degree of freedom MPa/μm ² Or greater and about 0.05 degrees MPa/μm ² Or smaller.

According to an exemplary embodiment, heating the blank may include multi-stage heating the blank while passing through multiple zones in the heating furnace (wherein the temperature ranges are elevated in the multiple zones of the heating furnace), and soaking the blank to a temperature of about Ac3 or higher.

According to an exemplary embodiment, the ratio of the length of the section for multi-section heating the blank to the length of the section for soaking the heated blank in the plurality of sections is about 1:1 to 4:1.

According to an exemplary embodiment, the temperatures of the plurality of sections may increase in a direction from an inlet of the heating furnace to an outlet of the heating furnace.

According to an exemplary embodiment, in multi-stage heating, the heating rate of the blank may be in the range of about 6 ℃/s to about 12 ℃/s.

According to an exemplary embodiment, the temperature of the section for soaking the heated blank is higher than the temperature of the section for multi-section heating of the blank in the plurality of sections.

According to an exemplary embodiment, the blank may remain in the oven for about 180 seconds to about 360 seconds.

According to an exemplary embodiment, cooling the shaped body to form the hot stamped component may include holding the shaped body in the die at a temperature below the martensite phase transition initiation temperature for about 3 seconds to about 20 seconds.

According to an exemplary embodiment, the shaped body may be cooled to the martensite finish temperature in the compression mold at an average cooling rate of 15 ℃/s or more.

According to an exemplary embodiment, a hot stamped component may include a martensite phase having an area fraction of 80% or more and an iron-based carbide located inside the martensite phase and having an area fraction of less than 5% based on the martensite phase.

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.

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

According to an exemplary embodiment, the first iron-based carbide may be formed at an angle of 0 ° or more and 20 ° or less to the longitudinal direction of the lath, and the iron-based carbide reference area fraction is 50% or more.

According to an exemplary embodiment, the second iron-based carbide may form an angle of 70 ° or more and 90 ° or less with the longitudinal direction of the lath, and the iron-based carbide reference area fraction is less than 50%.

According to another aspect of the present application, there is provided a hot stamped component having a residual stress analysis value satisfying a preset condition. The residual stress analysis value may be a product of a value of an X-ray diffraction analysis (XRD) value obtained by quantifying residual stress by XRD analysis and a value of an Electron Back Scattering Diffraction (EBSD) value obtained by quantifying orientation by EBSD analysis, with a preset condition of about 2.85×10 ^-4 Degree of freedom MPa/μm ² Or greater and about 0.05 degrees MPa/μm ² Or smaller.

Advantageous effects

According to the exemplary embodiments of the present application, a hot stamped part and a method of manufacturing the same, which can ensure high mechanical properties and hydrogen embrittlement by controlling residual stress of the hot stamped part, can be realized. Of course, the scope of the present application is not limited to these effects.

Drawings

Fig. 1 is a plan view showing a part of a hot stamped part according to an exemplary embodiment of the application;

fig. 2 is a plan view showing a portion of a hot stamped component according to an exemplary embodiment of the application;

fig. 3 is a flow chart schematically illustrating a method of manufacturing a hot stamped component according to an exemplary embodiment of the application;

fig. 4 is a graph showing a temperature change when heating a blank in a plurality of stages in a method of manufacturing a hot stamped component according to an exemplary embodiment of the application; and

Fig. 5 is a graph showing temperature changes when comparing a multi-stage heating of a blank with heating of the blank in a single stage.

Detailed Description

Since the application is susceptible of various modifications and alternative embodiments, specific embodiments thereof are shown in the drawings and will be described in detail herein. The advantages, features and methods of accomplishing the advantages of the present application may be apparent from the following description of embodiments with reference to the accompanying drawings. The application may, however, be embodied in different forms and should not be construed as limited to the descriptions set forth herein.

It will be understood that, although the terms "first," "second," "third," etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element.

In the following embodiments, the singular forms include the plural unless the context clearly indicates otherwise.

It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features or elements, but do not preclude the presence or addition of one or more other features or elements.

It will be understood that when an element or layer is referred to as being "on" another element or layer, it can be directly on the other element or layer or be between the elements or layers.

In the drawings, the thickness of layers and regions may be exaggerated or reduced for convenience of explanation. For example, the dimensions and thicknesses of elements in the drawings are arbitrarily shown for convenience of explanation, and thus, the inventive concept is not limited to the drawings.

While certain embodiments may be practiced differently, the specific process sequence may be performed in a different manner than that described. For example, two consecutively described processes may be performed substantially simultaneously, or in reverse order from the order described.

In the present application, "a and/or B" as used herein means A, B or a and B. Further, "at least one of a and B" means A, B or the case of a and B.

In the following embodiments, when connecting a film, a region, a constituent element, or the like, it may include a case where a film, a region, a constituent element are directly connected or/and a case where a film, a region, a constituent element are indirectly connected by interposing another film, a region, a constituent element therebetween. For example, in this specification, when a film, a region, a constituent element, or the like is electrically connected, it may mean directly electrically connecting the film, the region, the constituent element, or the like, and/or indirectly electrically connecting another film, the region, the member, or the like by interposing another film, the region, the constituent element, or the like therebetween.

The inventive concept will be described in more detail below with reference to the attached drawings showing embodiments of the inventive concept. In describing the inventive concept with reference to the drawings, the same reference numerals are used for elements substantially identical or corresponding to each other, and the description thereof will not be repeated.

Fig. 1 is a plan view showing a part of a hot stamped part according to an embodiment of the application.

Referring to fig. 1, a hot stamped component according to an exemplary embodiment of the application includes a steel plate 10.

The steel sheet 10 may be manufactured by performing a hot rolling process and/or a cold rolling process on a cast slab to include a predetermined alloy element in a predetermined content. Such a steel sheet 10 may exist as a fully austenitic structure at the hot stamping heating temperature and then may be transformed into a martensitic structure upon cooling.

In one embodiment, the steel sheet 10 may include carbon (C), manganese (Mn), boron (B), phosphorus (P), sulfur (S), silicon (Si), chromium (Cr), the balance of iron (Fe), and other unavoidable impurities. In addition, the steel sheet 10 may further include at least one alloy element of titanium (Ti), niobium (Nb), and vanadium (V) as an additive. Further, the steel sheet 10 may further include a predetermined amount of calcium (Ca).

Carbon (C) is used as an austenite stabilizing element in the steel sheet 10. Carbon is a main element determining the strength and hardness of the steel sheet 10, and the purpose of adding carbon after the hot stamping process is to ensure the tensile strength of the steel sheet 10 (e.g., about 1350MPa or higher) and to ensure the hardenability property of the steel sheet 10. Such carbon may be included in an amount of about 0.19 wt% to about 0.38 wt% based on the total weight of the steel sheet 10. When the carbon content is less than about 0.19 wt%, it is difficult to secure a hard phase (martensite or the like), and thus it is difficult to satisfy the mechanical strength of the steel sheet 10. Conversely, when the carbon content exceeds about 0.38 wt%, a problem of brittleness or a problem of reduced bending performance of the steel sheet 10 may occur.

Manganese (Mn) is used as an austenite stabilizing element in the steel sheet 10. The purpose of the manganese addition is to improve hardenability and strength during heat treatment. Such manganese may be included in an amount of about 0.5 wt% to about 2.0 wt% based on the total weight of the steel sheet 10. When the manganese content is less than about 0.5 wt%, the hardenability effect is insufficient, and the hard phase fraction in the molded body after hot stamping may be insufficient due to the insufficient hardenability. On the other hand, when the content of manganese exceeds about 2.0 wt%, ductility and toughness may be reduced due to manganese segregation or pearlite bands, which may lead to deterioration of bending properties, and a heterogeneous microstructure may be generated.

The purpose of the boron (B) is to ensure a martensitic structure by suppressing transformation of ferrite, pearlite and bainite, thereby ensuring hardenability and strength of the steel sheet 10. Further, boron segregates at grain boundaries and improves hardenability by lowering grain boundary energy, and has an effect of refining grains by increasing austenite grain growth temperature. Boron may be included in an amount of about 0.001 wt% to about 0.005 wt% based on the total weight of the steel sheet 10. When boron is contained in the above range, it is possible to prevent brittleness from occurring at hard phase grain boundaries and ensure high toughness and bendability. When the content of boron is less than about 0.001 wt%, the effect of hardenability is insufficient, and conversely, when the content of boron exceeds about 0.005 wt%, since the solid solubility is reduced, it is liable 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 be reduced due to occurrence of hard-phase intergranular brittleness.

Phosphorus (P) may be included in an amount of more than 0 wt% and about 0.03 wt% or less based on the total weight of the steel sheet 10, thereby preventing deterioration of toughness of the steel sheet 10. When the phosphorus content exceeds about 0.03 wt%, a phosphide compound is formed, which deteriorates toughness and weldability, and cracks may be generated in the steel sheet 10 during the manufacturing method.

Sulfur (S) may be included in an amount of greater than 0 wt% and about 0.003 wt% or less based on the total weight of the steel sheet 10. If the sulfur content exceeds about 0.003 wt%, hot workability, weldability, and impact properties are deteriorated, and surface defects (e.g., cracks) may occur due to the generation of large inclusions.

Silicon (Si) is used as a ferrite stabilizing element in the steel sheet 10. Silicon improves the strength of the steel sheet 10 as a solid solution strengthening element, and increases 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, structural homogenization (pearlite, manganese segregation zone control) and fine dispersion of ferrite. Silicon is used as a martensite strength non-uniformity controlling element to improve collision performance. Silicon may be included in an amount of about 0.1 wt% to about 0.6 wt% based on the total weight of the steel sheet 10. When the content of silicon is less than about 0.1 wt%, it is difficult to obtain the above effect, and cementite formation and coarsening may occur in the final hot-stamped martensitic structure. In contrast, when the content of silicon exceeds about 0.6 wt%, the load of hot rolling and cold rolling may increase, and the plating characteristics of the steel sheet 10 may deteriorate.

Chromium (Cr) is added for the purpose of improving hardenability and strength of the steel sheet 10. Chromium contributes to grain refinement and ensures strength of the steel sheet 10 through precipitation hardening. Chromium may be included in an amount of about 0.05 wt% to about 0.6 wt% based on the total weight of the steel sheet 10. When the content of chromium is less than about 0.05 wt%, the precipitation hardening effect is low, and conversely, when the content of chromium exceeds 0.6 wt%, cr-based precipitates and matrix solid solutions increase, resulting in a decrease in toughness, and production costs may increase.

Meanwhile, other unavoidable impurities may include nitrogen (N) or the like.

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

The additive is an element that forms carbide to contribute to the formation of precipitates in the steel sheet 10. Specifically, the additive may include at least one of titanium (Ti), niobium (Nb), and vanadium (V).

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

Niobium (Nb) and vanadium (V) can increase strength and toughness as the size of the martensitic packet decreases. Each of niobium and vanadium may be included in an amount of about 0.01 wt% to about 0.1 wt% based on the total weight of the steel sheet 10. When niobium and vanadium are contained in the above-mentioned ranges, the grain refining effect of the steel sheet 10 is high in the hot and cold rolling processes, cracks in the slab and brittle fracture of the product are prevented from occurring in the steelmaking/casting process, and the generation of coarsened precipitates in the steelmaking is minimized.

Calcium (Ca) may be added to control the inclusion shape. The calcium may be included in an amount of about 0.003 wt% or less based on the total weight of the steel sheet 10.

After the hot rolling process and/or the cold rolling process, when the steel sheet 10 is cooled to room temperature, residual stress exists in the steel sheet 10 of the hot stamped component manufactured through the hot stamping process. Here, the "residual stress" means a stress existing in the hot stamped member in a state where no external force acts on the steel sheet 10.

Residual stresses may be caused by defects in the material. For example, point defects (e.g., vacancies, interstitials, impurities, etc.), line defects (e.g., dislocations), and interface defects (e.g., external surfaces, grain boundaries, twin boundaries, stacking faults, phase boundaries, etc.) may be the cause of the generation of residual stresses. That is, it is understood that the more defects are present in the steel sheet 10, the greater the internal residual stress.

These defects of the steel sheet 10 and the residual stress generated therefrom affect the mechanical properties (e.g., tensile strength) and hydrogen embrittlement of the steel sheet 10.

Specifically, the tensile strength of the hot-stamped component is determined such that when defects inside the steel sheet 10 exist at an appropriate level, the more defects (or the greater the residual stress), the greater the tensile strength, and the fewer defects (or the less residual stress), the lower the tensile strength. This is because the more defects inside the steel sheet 10, the more irregular the arrangement of elements, which makes it difficult to move dislocations that cause deformation of the material.

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 an decrease in residual stress). In general, since more effective hydrogen trapping sites exist inside the steel sheet 10, the amount of active hydrogen is reduced, and thus, hydrogen embrittlement of the product can be improved. For example, fine precipitates (e.g., nitrides or carbides of titanium (Ti), niobium (Nb), and vanadium (V)) present therein act as effective hydrogen trapping sites and improve hydrogen embrittlement. Meanwhile, defects present therein may also act as hydrogen trapping sites. However, since the defect has a relatively low binding energy with hydrogen, hydrogen trapped and deactivated by the defect is highly likely to be converted back to active hydrogen. Therefore, the defect cannot serve as an effective hydrogen trapping site, but hydrogen embrittlement can be reduced by locally concentrating active hydrogen at a portion where the defect is large (or a portion where residual stress is large). In particular, the hot stamped component may include at least one curved portion depending on the application location in the vehicle structure, and the curved portion is a portion that is excessively formed compared to the flat region in the hot stamping process. That is, the bent portion may act as a hydrogen embrittlement weak portion because stress generated by pressing during the hot stamping process is relatively concentrated, and thus, residual stress may increase.

Therefore, it is necessary to control defects existing in the steel sheet 10 and residual stress generated thereby to an appropriate level.

According to the exemplary embodiment of the present application, defects present in the steel sheet 10 and residual stresses generated therefrom may be appropriately adjusted by controlling the residual stress analysis value, which quantifies the residual stresses present in the steel sheet 10, to satisfy preset conditions.

In an exemplary embodiment, the residual stress analysis value may be a product of a value of an XRD value (or an absolute value of an XRD value) quantifying the residual stress by X-ray diffraction and a value of an Electron Back Scattering Diffraction (EBSD) value (or an absolute value of an EBSD value) quantifying the orientation by EBSD. In addition, the preset condition may be about 2.85×10 ^-4 Degree of freedom MPa/μm ² Or greater and about 0.05 degrees MPa/μm ² Or smaller. More preferably, the residual stress analysis value may be controlled to satisfy about 2.95×10 when the XRD value has a value of about 5MPa or more and less than about 15MPa ^-4 Degree of freedom MPa/μm ² Or greater and about 0.01 degrees MPa/μm ² Or less, the residual stress analysis value may be controlled to satisfy about 9.31 x 10 when the XRD value has a value of about 15MPa or more and less than about 55MPa ^-4 Degree of freedom MPa/μm ² Or greater and about 0.035 degrees MPa/μm ² Or less, and when the XRD value is about 55MPa or more and about 70MPa or less, the residual stress analysis value can be controlled to be about 3.96 x 10 ^-3 Degree of freedom MPa/μm ² Or greater and about 0.043 degrees MPa/μm ² Or a smaller range.

"X-ray diffraction (XRD) analysis" is an analysis method for measuring residual stress by using X-ray diffraction, in which incident X-rays irradiated onto a measurement sample are reflected in a specific direction due to the regularity of crystal lattice. In particular, the residual stress may be determined byThe method performs the measurement. />The method obtains the peak position of the diffraction line by irradiating an X-ray to a portion to be measured. By varying the angle of incidence of X-rays when residual stress is present +.>To change the peak position of the diffraction lines. At this time, the peak position of the changed diffraction line is taken as the vertical axis, and the angle of incidence of the X-ray is +.>As the horizontal axis, a slope is obtained by a linear regression by a least square method, and the obtained slope is multiplied by a stress constant obtained by young's modulus and poisson's ratio, and then, the obtained stress value (XRD value) is obtained by the following equation 1.

[ equation 1]

σ＝-E/2(1+v)*cotθ*π/180*M＝K*M

Sigma: stress or XRD values (MPa)

E: young's modulus (MPa)

v: poisson's ratio

M: slope 2 theta-sin of regression line ² θ

2 theta: diffraction angle (degree) in the absence of strain

K: stress constant (MPa)

XRD analysis is highly representative because it is directed to a relatively wide range, but has drawbacks in that the deviation is large and uniformity is poor. Furthermore, the deviation of XRD values tends to increase with an increase in residual stress inside the product. Therefore, there is a problem in that it is difficult to accurately analyze and control the residual stress of the material using only XRD values obtained by quantifying the residual stress by XRD analysis.

On the other hand, "EBSD" determines a crystal phase and a crystal orientation using a diffraction pattern of a certain sample, and based on this, "EBSD" is a method of analyzing a sample by combining morphological information and crystallographic information of the microstructure of the sample.

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, and a diffraction pattern appears in the surface direction of the sample. This is called an electron back scattering diffraction pattern (EBSP), and this pattern corresponds to the crystal orientation of the region irradiated with the electron beam, and the crystal orientation of the material can be measured with an accuracy of less than 1 °.

Since EBSD is directed to a relatively narrow range, it has advantages of small deviation and good uniformity compared to X-ray diffraction (XRD) analysis. However, the EBSD value, which quantifies the residual stress by EBSD, also has a disadvantage in that it is not representative, and it is difficult to accurately analyze and control the residual stress of the material using only the EBSD value.

In an exemplary embodiment of the present application, in order to compensate for the above-described drawbacks of X-ray diffraction (XRD) analysis and EBSD, respectively, differential residual stress analysis values are applied. Specifically, as the residual stress analysis value, a product of a numerical value of XRD value (or absolute value of XRD value) of the digitized residual stress by X-ray diffraction (XRD) analysis and a numerical value of EBSD value (or absolute value of EBSD value) of the orientation by EBSD quantification may be applied. Therefore, the deviation (which is a disadvantage of the XRD value) is compensated for by the EBSD value, and the low representativeness (which is a disadvantage of the EBSD value) is compensated for by the XRD value, and thus, it has an effect of analyzing and controlling the residual stress more accurately.

For example, the residual stress analysis value may be expressed as the following equation 2.

[ equation 2]

Residual stress analysis value (degree. Times. MPa/. Mu.m) ² ) = |xrd value (MPa) ||ebsd value (degree/μm) ² )|

The residual stress analysis may be substantially proportional to defects in the hot stamped component and residual stresses resulting therefrom. Specifically, it can be understood that the larger the residual stress analysis value, the more defects exist inside the product, the larger the residual stress, and the smaller the residual stress analysis value, the fewer defects exist inside the product, and the smaller the residual stress. Further, it can be understood that the higher the residual stress analysis value, the greater the tensile strength of the product, but the hydrogen embrittlement is poor, and the smaller the residual stress analysis value, the lower the tensile strength of the product, but the better the hydrogen embrittlement is. Therefore, it is possible to appropriately secure the mechanical properties and hydrogen embrittlement of the product by controlling the residual stress analysis value to satisfy the preset conditions.

On the other hand, due to the characteristics of the method of manufacturing a product from a high temperature material by rolling and cooling, defects and residual stresses generated therefrom may be caused by temperature differences existing in the width direction or the length direction of the steel sheet 10 during the manufacturing method. According to an exemplary embodiment of the present application, the above-described residual stress analysis value may be controlled to satisfy a preset condition by applying a differentiation process condition (e.g., a heating condition and/or a cooling condition) in the manufacturing method. A detailed description of these differentiated process conditions will be described below with reference to fig. 3 to 5.

Fig. 2 is a plan view showing a part of a hot stamped part according to an exemplary embodiment of the present application.

The steel sheet 10 may include a component system having a microstructure including a martensite phase having an area fraction of about 80% or more. In addition, the steel plate 10 may include a bainite phase with an area fraction of less than about 20%.

The martensite phase is the result of the non-diffusion phase transformation of austenite gamma below the martensite start temperature (Ms) during cooling. The martensite may have a rod-shaped lath phase oriented in one direction d within each of the initial grains of austenite.

In addition, the steel sheet 10 may include iron-based carbides located inside the martensite phase. The iron-based carbide may have a needle-like form. In exemplary embodiments, the iron-based carbide may have a diameter of less than about 0.2 μm and a length of less than about 10 μm. Here, the "diameter of the iron-based carbide" may represent the short axis length of the iron-based carbide, and the "length of the iron-based carbide" may represent the long axis length of the iron-based carbide.

If the iron-based carbide has a diameter of about 0.2 μm or more, or a length of about 10 μm or more, the iron-based carbide may remain without melting even at a temperature of Ac3 or more in the annealing heat treatment process, and the bendability and yield ratio of the steel sheet 10 may be lowered. 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 of strength and formability of the steel sheet 10 may be improved.

The area fraction of iron-based carbides may be less than about 5% based on the martensite phase. When the area fraction of the iron-based carbide based on the martensite phase is about 5% or more, it may be difficult to secure the strength and the bendability of the steel sheet 10.

In an exemplary embodiment, as shown in fig. 2, the iron-based carbide may include 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 an angle formed at about 0 ° or more and about 20 ° or less with the longitudinal direction d of the slat phase, and "perpendicular" includes an angle formed at about 70 ° or more and about 90 ° or less with the longitudinal direction d of the slat phase. For example, the first iron-based carbide C1 may form an angle of about 0 ° or more and about 20 ° or less with the longitudinal direction d of the lath phase, and the second iron-based carbide C2 may form an angle of about 70 ° or more and about 90 ° or less with the longitudinal direction d of the lath phase.

The iron-based carbide reference area fraction of the first iron-based carbide C1 may be greater than the iron-based carbide reference area fraction of the second iron-based carbide. Thereby, the bendability of the steel sheet 10 can be improved. As a specific example, the iron-based carbide reference area fraction of the first iron-based carbide C1 forming an angle of about 0 ° or more and about 20 ° or less with the longitudinal direction d of the lath phase may be about 50% or more, preferably about 60% or more. Furthermore, the iron-based carbide reference area fraction of the second iron-based carbide C2 forming an angle of about 70 ° or more and about 90 ° or less with the longitudinal direction d of the lath phase may be less than about 50%, preferably less than about 40%.

Cracks occurring during bending deformation may occur with the movement of dislocations in the martensite phase. At this time, it is understood that, in a given plastic deformation, the higher the local strain rate, the greater the energy absorption of the plastic deformation of the martensite, and thus, the collision performance is improved.

On the other hand, when the iron-based carbide reference area fraction of the first iron-based carbide C1 parallel to the longitudinal direction d of the lath phase is formed to be greater than that of the second iron-based carbide C2 perpendicular to the longitudinal direction d of the lath phase, dynamic Strain Aging (DSA) (i.e., indentation dynamic strain aging) may occur due to a local strain rate difference during dislocation movement inside the lath phase during bending deformation. As a concept of plastic deformation absorption energy, indentation dynamic strain aging means resistance to deformation, because the higher the frequency at which indentation dynamic strain aging occurs, the better the deformation resistance can be evaluated.

That is, according to the exemplary embodiment, since the iron-based carbide reference area fraction of the first iron-based carbide C1 forming an angle of about 20 ° or less with the longitudinal direction d of the lath phase is formed to be about 50% or more and the iron-based carbide reference area fraction of the second iron-based carbide C2 forming an angle of about 70 ° or more and about 90 ° or less with the longitudinal direction d of the lath phase is formed to be less than about 50%, the indentation dynamic strain aging phenomenon may frequently occur, so that a V-shaped bending angle of about 50 ° or more may be ensured, and thus, the bendability and impact performance may be improved.

Since the bainite phase having a surface fraction of less than about 20% in the steel plate 10 has a uniform hardness distribution, it is a structure having an excellent balance between strength and ductility. However, since bainite is softer than martensite, it is preferable that the area fraction of bainite be less than about 20% in order to secure the strength and bendability of the steel sheet 10.

On the other hand, the above-mentioned iron-based carbide having a needle-like form may be precipitated inside the bainite phase. Since the iron-based carbide inside the bainite increases the strength of the bainite and reduces the strength difference between the bainite and the martensite, the yield ratio and the bendability of the steel sheet 10 can be improved. In this case, the iron-based carbide may be present in the bainite phase in an amount of less than about 20% based on the bainite phase. If the iron-based carbide based on the bainite phase is about 20% or more, voids may be generated, which may result in reduced bendability.

Fig. 3 is a flowchart schematically showing a method of manufacturing a hot stamped part according to an embodiment of the application, fig. 4 is a graph showing a temperature change when heating a blank in multiple stages in a method of manufacturing a hot stamped part according to an embodiment of the application, and fig. 5 is a graph showing a comparison of the temperature changes when heating a blank in multiple stages and heating the blank in a single stage.

Referring to fig. 3, a method of manufacturing a hot stamped component according to an exemplary embodiment of the present application may include inserting a blank (S110), multi-stage heating (S120), and soaking heating (S130). Further, after soaking heating (S130), the method of manufacturing the hot stamped component may further include transferring (S140), molding (S150), and cooling (S160).

First, inserting the blank (S110) may be an operation of placing the blank into a heating furnace having a plurality of sections.

The blank introduced into the heating furnace may be formed by cutting a sheet material for forming the hot stamped component. The sheet material may be manufactured by hot rolling or cold rolling on a billet followed by an annealing heat treatment. Further, after the annealing heat treatment, a plating layer may be formed on at least one surface of the plate material subjected to the annealing heat treatment. For example, the plating layer may be an Al-Si based plating layer or a Zn plating layer.

Then, multi-stage heating (S120) and soaking heating (S130) may be sequentially performed. The blank inserted into the heating furnace may be heated as it passes through the plurality of sections disposed in the heating furnace. In an exemplary embodiment, the blanks introduced into the heating furnace may be mounted on rollers and conveyed along a conveying direction.

The heating furnace may include a plurality of sections arranged in sequence in the heating furnace. The plurality of sections included in the heating furnace include a section in which the temperature range is raised stepwise from the inlet of the furnace into which the blank is introduced to the outlet of the furnace from which the blank is discharged and a section in which the temperature range is uniformly maintained.

The multi-stage heating (S120) is an operation of heating the blank while passing through a section in which the temperature range is raised stepwise among a plurality of sections arranged in the heating furnace. Soaking heating (S130) is an operation of heating a blank subjected to multi-stage heating (stepwise heating or sectional heating) by a section uniformly held in a plurality of sections arranged in a heating furnace across a temperature range.

The temperature ranges of the sections arranged in the heating furnace are gradually increased from the inlet of the heating furnace into which the blank is introduced to the outlet of the heating furnace from which the blank is discharged to the target temperature Tt range, and then from the section having the target temperature Tt range to the outlet of the heating furnace, it is possible to maintain within a uniform temperature range, i.e., the target temperature Tt range. In this case, the number of sections whose temperature range is raised stepwise, the number of sections whose temperature range is uniformly maintained, and the temperature range of each section are not limited.

In an exemplary embodiment, as shown in fig. 4, 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, the furnace may include 6 or less or 8 or more sections, unlike that shown in fig. 4, and the temperature range of each section may also vary differently. Hereinafter, for convenience of description, the embodiment shown in fig. 4 will be described.

The first to seventh sections P1 to P7 may be sequentially arranged in the heating furnace. The first section P1 with the first temperature range T1 is adjacent to the inlet of the furnace inlet blank and the seventh section P7 with the seventh temperature range T7 is adjacent to the outlet of the furnace outlet blank. That is, the first section P1 having the first temperature range T1 may be a first section among a plurality of sections included in the heating furnace, and the seventh section P7 having the seventh temperature range T7 may be a last section among the plurality of sections included in the heating furnace. The blank may be heated by moving from the first section P1 to the seventh section P7 provided in the heating furnace in sequence.

In the exemplary embodiment, as shown in fig. 4, the temperature ranges of the sections from the first section P1 to the fifth section P5 are gradually increased to the target temperature Tt range, and in the sixth section P6 and the seventh section P7, the temperature ranges are maintained in the target temperature Tt range (which is the temperature range of the fifth section P5). However, the present application is not limited to the above-described embodiment, and the number of sections in which the temperature range is raised stepwise and the temperature range is uniformly maintained may be variously changed.

Meanwhile, a temperature difference between two adjacent sections among the plurality of sections arranged in the heating furnace may be about 0 ℃ or more and about 100 ℃ or less. For example, the temperature difference between the first and second sections P1 and P2 may be about 0 ℃ or more and about 100 ℃ or less.

In exemplary embodiments, the first temperature range T1 of the first section P1 may be in a range of about 840 ℃ to about 860 ℃, or in a range of about 835 ℃ to about 865 ℃. The second temperature range T2 of the second section P2 may be in the range of about 870 ℃ to about 890 ℃, or in the range of about 865 ℃ to about 895 ℃. The third temperature range T3 of the third section P3 may be in the range of about 900 ℃ to about 920 ℃, or in the range of about 895 ℃ to about 925 ℃. The fourth temperature range T4 of the fourth section P4 may be in the range of about 920 ℃ to about 940 ℃, or in the range of about 915 ℃ to about 945 ℃. The fifth temperature range T5 of the fifth section P5 may be in the range of about Ac3 to about 1000 ℃. Preferably, the fifth temperature range T5 of the fifth section P5 may be about 930 ℃ or higher and about 1000 ℃ or lower. More preferably, the fifth temperature range T5 of the fifth section P5 may be about 950 ℃ or more and about 1000 ℃ or less. The sixth temperature range T6 of the sixth section P6 and the seventh temperature range T7 of the seventh section P7 may be the same as the fifth temperature range T5 of the fifth section P5.

In this case, the multi-stage heating (S120) may be performed in the first to fourth sections P1 to P4, and the soaking heating (S130) may be performed in the fifth to seventh sections P5 to P7. In this way, by providing a section in which soaking heating (S130) is performed in a plurality of sections (e.g., fifth section P5 to seventh section P7) instead of one section, it is possible to prevent or minimize a temperature difference occurring between the sections.

Soaking heating (S130) is performed in the temperature range of the fifth section P5, and the temperature range of the fifth section P5 is the target temperature Tt range, which may be Ac3 or higher. That is, in soaking heating (S130), the blank that is multi-stage heated while passing through the first to fourth sections P1 to P4 may be soaking heated at a temperature of Ac3 or higher. Preferably, in soaking heating (S130), the multi-stage heated blank may be subjected to soaking heating at a temperature of about 930 ℃ or more and about 1000 ℃ or less. More preferably, in soaking heating (S130), the multi-stage heated blank may be subjected to soaking heating at a temperature of about 950 ℃ or more and about 1000 ℃ or less.

In an exemplary embodiment, the length of the oven along the conveying path of the blanks may be about 20m to about 40m. The heating furnace may have a plurality of sections having different temperature ranges, and a ratio of a length (D1, see fig. 4) of a section in which the blank is multi-section heated to a length (D2, see fig. 4) of a section in which the blank is soaking heated in the plurality of sections may satisfy a range of about 1:1 to about 4:1. That is, the length of the soaking heating section D2 among the plurality of sections arranged in the heating furnace may correspond to about 20% to about 50% of the total length (d1+d2) of the heating furnace.

When the ratio of the length (D1) of the sections of the multi-section heated blank to the length (D2) of the sections of the soaking heated blank exceeds 1:1 due to the increase in the length of the sections for soaking heated blank, an austenitic (FCC) structure is created in the soaking heated sections which increases the amount of hydrogen permeated into the blank and the delayed fracture may increase. Further, when the ratio of the length (D1) of the section for multi-stage heating the blank to the length (D2) of the section for soaking the blank is less than 4:1, since the soaking heating section (time) cannot be sufficiently ensured, the strength of the hot stamped part manufactured by the manufacturing method of the hot stamped part may be uneven.

In an exemplary embodiment, in the multi-stage heating (S120) and the soaking heating (S130), the heating rate of the blank may be about 6 ℃/S to about 12 ℃/S, and the soaking heating time may be in the range of about 3 minutes to about 6 minutes. More specifically, when the thickness of the blank is about 1.6mm to about 2.3mm, the temperature rise rate is about 6 ℃/s to about 9 ℃/s, and the soaking heating time may be in the range of about 3 minutes to about 4 minutes. Further, when the thickness of the blank is in the range of about 1.0mm to about 1.6mm, the temperature rise rate is about 9 ℃/s to about 12 ℃/s, and the soaking heating time may be in the range of about 4 minutes to about 6 minutes.

The temperature change when heating the blank B' singly and heating the blank B in plural pieces will be described with reference to fig. 5.

As a comparative example, a case of heating the blank B' once may be assumed. In the single heating, the temperature of the furnace is set so that the internal temperature of the heating furnace is kept equal to the target temperature Tt of the blank. In this case, the target temperature Tt of the blank B' may be equal to or higher than Ac3. Preferably, the target temperature Tt of the blank B' may be about 930 ℃. More preferably, the target temperature Tt of the blank B' may be about 950 ℃.

The temperature of the blank B' in the single heating may reach the target temperature Tt faster than the temperature of the blank B in the multi-stage heating. For example, the heating rate of the blank B' in the single heating may be about 2 ℃/s or more than the heating rate of the blank B in the multi-stage heating. Since the temperature in the single heating reaches the target temperature Tt faster than the temperature in the multi-stage heating, the soaking heating time ET2 of the single heating may be longer than the soaking heating time ET1 of the multi-stage heating. As in the case of the single heating, if the soaking heating time ET2 is prolonged, the size of the grain boundary is not uniform, and the above-described defect may be excessively formed unnecessarily.

Accordingly, in the method of manufacturing the hot stamping part according to the exemplary embodiment of the present application, the proper soaking heating time (ET 1) is ensured by delaying the time for the blank member to reach the target temperature (Tt) by the multi-stage heating method, and thus, uniformity of grain boundary size can be ensured and formation of a proper level of defects can be controlled. Accordingly, the hot stamped member manufactured by applying the multi-stage heating method can be controlled to have defects and residual stresses within a preset range, and whether or not it satisfies the preset range can be checked by the above-described residual stress analysis value.

Referring to fig. 3, after soaking heating (S130), transfer (S140), molding (S150), and cooling (S160) may also be performed.

The transferring (S140) may be an operation of transferring the heated blank from the heating furnace to the pressing mold. In the operation of transferring the heated blank from the heating furnace to the pressing mold, the heated blank may be air-cooled for about 10 seconds to about 15 seconds.

The forming (S150) may be an operation of hot stamping the transferred blank to form a formed body. The cooling (S160) may be an operation of cooling the molded body.

After molding into the final part shape in the compression mold, the final product may be formed by cooling the molded body. A cooling channel for the circulation of the refrigerant may be arranged in the compression mold. The heated blank may be rapidly cooled by circulating a supplied refrigerant in a cooling channel arranged in the compression mold. At this time, in order to prevent the rebound phenomenon of the plate material and maintain a desired shape, the press mold may be rapidly cooled while being pressurized in a closed state. That is, the molding process (or molding (S150)) and the cooling process (or cooling (S160)) may be performed simultaneously while the blank is placed in the press mold.

In an exemplary embodiment, the blank may be maintained in the die at a temperature below the austenite transformation starting temperature (MS temperature) for a predetermined time (e.g., about 3 seconds to about 20 seconds) while the heated blank is subjected to the forming process and the cooling process. In addition, the blank may be cooled while maintaining an average cooling rate of about 15 ℃/s or greater until the temperature at which the martensitic transformation is completed (Mf temperature). By securing the cooling time in this way, the martensite structure is self-tempered to obtain self-tempered martensite, and deformation of the molded part can be prevented, and therefore, there is an effect of reducing residual stress inside the product.

If the holding time in the press mold is less than 3 seconds, the material may not be sufficiently cooled, and thermal deformation may occur due to residual heat of the product and temperature deviation of each part. Further, when the blank is held in the press mold for more than 20 seconds, unnecessary defects and residual stress generated thereby may occur, and the holding time in the press mold may be increased, thereby decreasing productivity.

In an exemplary embodiment, the tensile strength of the hot stamped part manufactured by the method of manufacturing the hot stamped part may be about 1350MPa or more, and the amount of active hydrogen may be about 0.7wppm or less.

The present application will be described in more detail hereinafter by way of embodiments and comparative examples. However, the following embodiments and comparative examples are used to explain the present application in more detail, and the scope of the present application is not limited by the following examples and comparative examples. Those skilled in the art can appropriately modify and change the following examples and comparative examples within the scope of the present application.

TABLE 1

Table 1 shows the measurement results of XRD values, tensile strengths, and amounts of active hydrogen of each of the samples a-1 to a-9. Specifically, table 1 confirms whether the magnitude of the XRD value measured for the sample satisfies the range of 5MPa or more and 70MPa or less, and shows data for comparing and analyzing the tensile strength and the amount of active hydrogen in the case where the above range is satisfied and in the case where the above range is not satisfied.

The XRD value is a value obtained by quantifying the residual stress by the above-mentioned X-ray diffraction (XRD) analysis. XRD values were measured by removing the coating layer of the sample and irradiating X-rays to a target position (e.g., 1/4 point) after electropolishing. In addition, electropolishing is performed using an electropolishing solution comprising about 5% 2-butoxyethanol, about 20% perchloric acid, about 35% ethanol, and about 40% water.

The amount of active hydrogen can be measured using thermal desorption spectroscopy. Thermal desorption spectroscopy is a method of measuring the amount of hydrogen released from a sample below a certain temperature when the sample is heated at a predetermined heating rate to raise the temperature, and hydrogen released from the sample can be understood as active hydrogen that is not trapped and affects delayed destruction of hydrogen in hydrogen introduced into the sample. That is, if the amount of hydrogen as a measurement result of the thermal desorption spectroscopy is large, it means that a large amount of uncaptured active hydrogen that may cause delayed destruction of hydrogen is contained.

Specifically, the amount of active hydrogen in table 1 is a value obtained by measuring the amount of hydrogen discharged from the samples at about 350 ℃ or lower when the temperature of each sample is raised from room temperature to about 500 ℃ at a heating rate of about 20 ℃/min.

Samples A-1 through A-5 satisfy a range of measured XRD values of about 5MPa or more and about 70MPa or less. That is, it can be appreciated that appropriate levels of defects and residual stresses exist in samples A-1 through A-5. Therefore, it was confirmed that the tensile strengths of the samples A-1 to A-5 satisfied 1350MPa or more, and that the amounts of active hydrogen in the samples A-1 to A-5 satisfied 0.7wppm or less.

On the other hand, in the case of samples A-6 and A-7, the XRD values were measured to be less than 5MPa. That is, it can be seen that there are defects in the samples A-6 and A-7 at lower than desired levels, and the residual stress generated thereby is too small. Thus, it can be seen that the amount of active hydrogen in each of samples A-6 and A-7 satisfies 0.7wppm or less, but the tensile strength is less than 1350MPa.

In the case of the samples A-8 and A-9, moreover, the XRD values were measured to be more than 70MPa. That is, it can be seen that there are unnecessary defects inside the samples A-8 and A-9, and the residual stress generated thereby is excessive. Thus, each of samples A-8 and A-9 satisfied a tensile strength of 1350MPa or more, but the amount of active hydrogen exceeded 0.7wppm, demonstrating a decrease in hydrogen embrittlement.

Meanwhile, referring to table 1, it can be seen that as the number of XRD values increases, the deviation of XRD values also tends to increase. That is, the larger the internal stress, the larger the error range of the XRD value, and thus, the more necessary it is to correct it.

TABLE 2

Table 2 shows the measurement results of the EBSD value, tensile strength and amount of active hydrogen of each of the samples B-1 to B-8. Specifically, table 1 confirms whether the value of the EBSD value measured for the sample satisfies about 5.71×10 ^-5 Degree/. Mu.m ² Or greater and about 7.14 x 10 ^-4 Degree/. Mu.m ² Or less, and shows data for comparative analysis of tensile strength and active hydrogen amount in the case where the above range is satisfied and the above range is not satisfied, respectively.

The EBSD value is a value obtained by quantifying orientation using the above EBSD. EBSD values were measured by scanning 4000 times a sample area of 25 μm by 70 μm in steps of 50 nm. In addition, these measurements were performed on 5 viewing surfaces.

The amount of active hydrogen was measured using thermal desorption spectroscopy under the same conditions as in table 1.

The measured EBSD values for samples B-1 to B-4 met about 5.71 x 10 ^-5 Degree/. Mu.m ² Or greater and about 7.14 x 10 ^-4 Degree/. Mu.m ² Or a smaller range. That is, it can be appreciated that the samples B-1 to B-4 have an appropriate level of defects and residual stresses generated thereby. Thus, it was confirmed that the tensile strengths of the samples B-1 to B-4 satisfied about 1350MPa or more, and that the amounts of active hydrogen in the samples B-1 to B-4 satisfied about 0.7wppm or less.

On the other hand, in the case of samples B-5 and B-6, the EBSD values were measured to have values less than about 5.71 x 10 ^-5 Degree/. Mu.m ² . That is, it can be seen that there are defects below the desired level in samples B-5 and B-6, and the residual stress resulting therefrom is too small. Thus, it can be seen that the amount of active hydrogen for each of samples B-5 and B-6 satisfies about 0.7wppm or less, but the tensile strength is less than about 1350MPa.

In addition, in the case of samples B-7 and B-8, EBSD was measuredValues above about 7.14 x 10 ^-4 Degree/. Mu.m ² . That is, it can be seen that there are unnecessary defects inside the samples B-7 and B-8, and the residual stress generated thereby is excessive. Thus, it was confirmed that the tensile strength of each of the samples B-7 and B-8 satisfied about 1350MPa or more, but the amount of active hydrogen exceeded about 0.7wppm, and thus the hydrogen embrittlement was reduced.

TABLE 3 Table 3

Table 3 shows XRD values, EBSD values, residual stress analysis values, tensile strengths, active hydrogen amounts, and 4-point bend test results for each of samples C-1 through C-12.

XRD values, EBSD values, tensile strengths, and amounts of active hydrogen were measured under the same conditions and methods as in tables 1 and 2. Further, the residual stress analysis value is calculated as the product of the value (or absolute value) of the XRD value and the value (or absolute value) of the EBSD value.

The 4-point bending test is a test method for checking whether a stress corrosion crack occurs by applying a stress below an elastic limit to a specific point on a test specimen manufactured by reproducing a state in which the test specimen is exposed to a corrosive environment. In this case, the stress corrosion crack means a crack generated when corrosion and continuous tensile stress are simultaneously applied.

Specifically, the 4-point bending test results in table 1 are results of checking whether or not fracture occurs by applying a stress of 1000MPa to each specimen in air for 100 hours.

According to the exemplary embodiment of the present application, by applying the product of the value of the XRD value and the value of the EBSD value as the residual stress analysis value, inaccurate information of each of the XRD value and the EBSD value can be mutually corrected, and thus, the residual stress inside the product can be accurately analyzed and controlled. Specifically, the residual stress analysis value is controlled to satisfy about 2.85×10 ^-4 Degree of freedom MPa/μm ² Or greater and about 0.05 degrees MPa/μm ² Or a smaller range.

On the other hand, referring to the XRD values in table 3, it can be seen that as the number of XRD values increases, the deviation of XRD values also tends to increase. That is, the larger the internal stress, the larger the error range of the XRD value, and the more necessary it is to correct it. Thus, the more significant the effect of the residual stress analysis value may be when the internal residual stress of the product is large (or the deviation of the XRD value is large).

In view of this, the residual stress analysis value can be controlled more accurately according to the range of XRD values. Specifically, when the XRD value is 5MPa or more and less than 15MPa, the residual stress analysis value may be controlled so as to satisfy about 2.95×10 ^-4 Degree of freedom MPa/μm ² Or greater and about 0.01 degrees MPa/μm ² In a smaller range, when the XRD value is 15MPa or more and less than 55MPa, the residual stress analysis value can be controlled to be about 9.31×10 ^-4 Degree of freedom MPa/μm ² Or greater and about 0.035 degrees MPa/μm ² Or less, and when the XRD value is 55MPa or more and 70MPa or less, the residual stress analysis value can be controlled to be about 3.96 x 10 ^-3 Degree of freedom MPa/μm ² Or greater and about 0.043 degrees MPa/μm ² Or a smaller range.

Samples C-1 to C-6 are hot stamped parts manufactured by applying the above-described process conditions through operations S110 to S160. That is, the samples C-1 to C-6 are samples produced by: applying the conditions of multi-stage heating (S120) and soaking heating (S130) described above, applying an average cooling rate of about 15 ℃/S or more in cooling (S160) to a temperature (Mf) at which martensitic transformation of the blank is completed, and holding the sample in the press mold at a temperature lower than the martensitic transformation initiation temperature (MS temperature) for about 3 seconds to about 20 seconds.

Thus, in samples C-1 to C-6, the values of the measured XRD values satisfy the range of about 5MPa or more and about 70MPa or less, and the values of the measured EBSD values satisfy about 5.71 x 10 ^-5 Degree/. Mu.m ² Or greater and about 7.14 x 10 ^-4 Degree/. Mu.m ² Or a smaller range. In addition, the residual stress analysis values (product of XRD value and EBSD value) of samples C-1 to C-6 also satisfied about 2.85×10 ^-4 Degree of freedom MPa/μm ² Or largerAnd about 0.05 degrees MPa/μm ² Is not limited in terms of the range of (a).

More specifically, in samples C-1 and C-2, the XRD values were about 5MPa or more and less than about 15MPa, and the residual stress analysis values satisfied about 2.95×10 ^-4 Degree of freedom MPa/μm ² Or greater and 0.01 degree MPa/μm ² Or a smaller range. Further, in samples C-3 and C-4, the XRD values were about 15MPa or more and less than about 55MPa, and the residual stress analysis values satisfied about 9.31×10 ^-4 Degree of freedom MPa/μm ² Or greater and about 0.035 degrees MPa/μm ² Or a smaller range. Further, in samples C-5 and C-6, the XRD values were about 55MPa or more and about 70MPa or less, and the residual stress analysis values satisfied about 3.96×10 ^-3 Degree of freedom MPa/μm ² Or greater and about 0.043 degrees MPa/μm ² Or a smaller range.

That is, in the samples C-1 to C-6, not only the values of the XRD values and the EBSD values satisfy the preset conditions, but also the residual stress analysis values satisfy the preset conditions, and therefore, it can be understood that the defects and the residual stresses at the corrected levels exist in the samples C-1 to C-6. Thus, it was confirmed that the tensile strengths of the samples C-1 to C-6 satisfied about 1350MPa or more and the active hydrogen contents satisfied about 0.7wppm or less. Furthermore, it was confirmed that the samples C-1 to C-6 were not broken as a result of the 4-point bending test. That is, since the samples C-1 to C-6 were manufactured by applying the above-described process conditions, the residual stress analysis values were controlled to satisfy the preset conditions, and thus, the tensile strength and hydrogen embrittlement at appropriate levels were ensured.

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

Referring to table 3, for samples C-7 to C-12, the values of the measured XRD values satisfied a range of about 5MPa or more and about 70MPa or less, and the values of the measured EBSD values satisfied about 5.71 x 10 ^-5 Degree/. Mu.m ² Or greater and about 7.14 x 10 ^-4 Degree/. Mu.m ² Or a smaller range. Thus, it can be seen that the tensile strengths of the samples C-7 to C-12 satisfy about 1350MPa or more, andand the amounts of active hydrogen of samples C-7 to C-12 satisfy about 0.7wppm or less.

However, the residual stress analysis values of the samples C-7 to C-12 do not satisfy the above-mentioned preset conditions.

Sample C-7 has an XRD value of about 5MPa or greater and less than about 15MPa and a residual stress analysis value of less than about 2.95 x 10 ^-4 Degree of freedom MPa/μm ² . That is, it can be appreciated that there are defects in sample C-7 that are below the desired level and that the residual stress resulting therefrom is too small. Thus, it was confirmed that the 4-point bending test of the specimen C-7 resulted in breakage.

Sample C-8 has an XRD value of about 5MPa or more and less than about 15MPa, and a residual stress analysis value exceeding about 0.01 degrees. Times. MPa/. Mu.m ² . That is, it can be understood that there are unnecessary defects inside the specimen C-8, and the residual stress generated thereby is excessive. Thus, it was confirmed that the 4-point bending test of the specimen C-8 resulted in breakage.

Sample C-9 has an XRD value of about 15MPa or greater and less than about 55MPa and a residual stress analysis value of less than about 9.31 x 10 ^-4 Degree of freedom MPa/μm ² . That is, it can be appreciated that there are defects in sample C-9 that are below the desired level and that the residual stress resulting therefrom is too small. Thus, it was confirmed that the 4-point bending test of the specimen C-9 resulted in breakage.

Sample C-10 has an XRD value of about 15MPa or more and less than about 55MPa, and a residual stress analysis value exceeding about 0.035 degrees. Times. MPa/. Mu.m ² . That is, it can be understood that there are unnecessary defects inside the specimen C-10, and the residual stress generated thereby is excessive. Thus, it was confirmed that the 4-point bending test of the specimen C-10 resulted in breakage.

Sample C-11 has an XRD value of about 55MPa or greater and about 70MPa or greater, and a residual stress analysis value of less than about 3.96 x 10 ^-3 Degree of freedom MPa/μm ² . That is, it can be appreciated that there are defects in sample C-11 that are below the desired level and that the residual stress resulting therefrom is too small. Thus, it was confirmed that the 4-point bending test of the specimen C-11 resulted in breakage.

Sample C-12 has an XRD value of about 55MPa or more and about 70MPa or more, and a residual stress analysis value exceeding about 0.043 DEG MPa/. Mu.m ² . That is, it can be understood that there are unnecessary defects inside the specimen C-12, and the residual stress generated thereby is excessive. Thus, it was confirmed that the 4-point bending test of the specimen C-12 resulted in breakage.

Since the residual stress analysis value does not satisfy the preset condition, the 4-point bending test of the samples C-7 to C-12 results in fracture, although each of the value of the XRD value and the value of the EBSD value satisfies the preset condition. It can be appreciated that it is difficult to completely control defects inside the hot stamped component and residual stresses generated thereby by XRD analysis or EBSD analysis alone.

On the other hand, as in the samples C-1 to C-6, if the residual stress analysis value satisfies the preset condition, the 4-point bending test results in no breakage, and it was confirmed that the defects inside the hot-stamped part and the residual stress generated thereby can be more accurately analyzed and controlled by the residual stress analysis value.

Although the present application has been described with reference to the exemplary embodiments shown in the drawings by way of example only, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the inventive concept. Accordingly, the scope of the application is defined not by the detailed description of the application but by the appended claims.

Claims

1. A method of manufacturing a hot stamped component having residual stress analysis values satisfying preset conditions, the method comprising:

heating the blank;

forming a shaped body by hot stamping the blank; and

cooling the shaped body to form a hot stamped part,

wherein the residual stress analysis value is a product of a value of an X-ray diffraction analysis (XRD) value obtained by quantifying the residual stress by XRD analysis and a value of an Electron Back Scattering Diffraction (EBSD) value obtained by quantifying the orientation by EBSD analysis, and

the preset condition is about 2.85 x 10 ^-4 Degree of freedom MPa/μm ² Or greater and about 0.05 degrees MPa/μm ² Or smaller.

2. The method of claim 1, wherein heating the blank comprises:

heating the blank in multiple stages while passing through multiple zones in the heating furnace, wherein the temperature range gradually increases in the multiple zones arranged; and

the blank is heated to a temperature of about Ac3 or greater by soaking.

3. The method of claim 2, wherein the ratio of the length of the section for multi-section heating the blank to the length of the section for soaking the heated blank in the plurality of sections is about 1:1 to 4:1.

4. The method of claim 2, wherein the temperatures of the plurality of zones increase in a direction from an inlet of the furnace to an outlet of the furnace.

5. The method of claim 4, wherein the heating rate of the blank is in the range of about 6 ℃/s to about 12 ℃/s during the multi-stage heating.

6. The method of claim 5, wherein the temperature of the section for soaking the heated blank is higher than the temperature of the section for multi-section heating the blank in the plurality of sections.

7. The method of claim 2, wherein the blank is held in the oven for about 180 seconds to about 360 seconds.

8. The method of claim 1, wherein cooling the shaped body to form the hot stamped component comprises holding the shaped body in the die at a temperature below a martensitic transformation start temperature for about 3 seconds to about 20 seconds.

9. The method according to claim 8, wherein the shaped body is cooled in the press mold to the martensite phase transition end temperature at an average cooling rate of 15 ℃/s or more.

10. The method of claim 1, wherein hot stamping the part comprises:

a martensite phase, the area fraction of the martensite phase being 80% or more; and

an iron-based carbide located within the martensite phase and having an area fraction of less than 5% based on the martensite phase.

11. The method of claim 10, wherein

The iron-based carbide has 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.

12. The method according to claim 10, wherein:

the martensite phase comprises a lath phase,

the iron-based carbide comprises 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 the iron-based carbide reference area fraction of the second iron-based carbide.

13. The method of claim 12, wherein the first iron-based carbide forms an angle of 0 ° or more and 20 ° or less with the longitudinal direction of the lath, and the iron-based carbide reference area fraction is 50% or more.

14. The method of claim 12, wherein the second iron-based carbide forms an angle of 70 ° or more and 90 ° or less with the longitudinal direction of the stave and the iron-based carbide reference area fraction is less than 50%.

15. A hot stamping part with residual stress analysis value meeting preset conditions,

16. The hot stamped component of claim 15 wherein

The hot stamped component comprises a martensite phase having an area fraction of 80% or greater, and

17. The hot stamped component of claim 16, wherein

The iron-based carbide has a needle-like form, and

18. The hot stamped component of claim 16, wherein

The martensite phase comprises a lath phase,

19. The hot stamped component of claim 18, wherein the first iron-based carbide forms an angle of 0 ° or more and 20 ° or less with the longitudinal direction of the lath, and the iron-based carbide reference area fraction is 50% or more.

20. The hot stamped component of claim 18, wherein the second iron-based carbide forms an angle of 70 ° or more and 90 ° or less with the longitudinal direction of the stave and the iron-based carbide reference area fraction is less than 50%.