EP4268986A1

EP4268986A1 - Hot-stamped component and method for manufacturing same

Info

Publication number: EP4268986A1
Application number: EP21915722.9A
Authority: EP
Inventors: Hye Jin Kim; Kyu Yeon Hwang; Hyun Yeong Jung; Jin Ho Lee; Seung Pill JUNG
Original assignee: Hyundai Steel Co
Current assignee: Hyundai Steel Co
Priority date: 2020-12-28
Filing date: 2021-12-27
Publication date: 2023-11-01
Also published as: JP7453424B2; KR102366284B9; KR102366284B1; WO2022145924A1; US20230107817A1; JP2023537810A; CN116783013A

Abstract

According to an aspect of the present disclosure, provided is a method of manufacturing a hot stamping component in which a residual stress analysis value satisfies a preset condition. The method includes heating a blank; forming a molded body by hot stamping the blank; and cooling the molded body to form a hot stamped component. The residual stress analysis value may be a product of a magnitude of an X-ray diffraction analysis (XRD) value obtained by quantifying residual stress by XRD analysis and a magnitude of an electron backscatter diffraction (EBSD) value obtained by quantifying an orientation by EBSD analysis, and the preset condition is about 2.85* 10-4 Degree∗MPa/µm2 or greater and about 0.05 Degree∗MPa/µm2 or less.

Description

TECHNICAL FIELD

The present disclosure relates to a hot stamping component and a method of manufacturing the hot stamping component.

BACKGROUND

A high-strength steel is used to manufacture light-weight and stable automobile parts. On the other hand, the high-strength steel may offer high-strength properties compared to its weight, but as the strength increases, press formability is lowered, which results in breakage of the material during processing or a spring-back phenomenon, and thus, it is difficult to form products with complex and precise shapes.
As a method to improve these problems, a hot stamping method has been used, and thus, research on materials for hot stamping has been actively conducted. For example, as disclosed in Korean Patent Application Laid-Open No. 10-2017-0076009 , a hot stamping method is a molding technology for manufacturing a high-strength component by heating a steel sheet for hot stamping at a high temperature and then rapidly cooling it at the same time while molding in a press mold. According to Korean Patent Application Laid-Open No. 10-2017-0076009 , it is possible to manufacture parts with high precision by suppressing problems, such as crack occurrence or shape freezing defect during forming, which are problems in high-strength steel sheet.

SUMMARY

Technical Problem

Embodiments of the present disclosure are intended to solve various problems including the above problems, and to provide a hot stamping component capable of securing high mechanical properties and hydrogen embrittlement by controlling residual stress of the hot stamping component, and a method of manufacturing the hot stamping component. However, these problems are exemplary, and the scope of the present disclosure is not limited thereto.

Technical Solution

According to an aspect of the present disclosure, a hot stamping component in which a residual stress analysis value satisfies a preset condition. The method includes heating a blank, forming a molded body by hot stamping the blank, and cooling the molded body to form a hot stamped component, and the residual stress analysis value may be a product of a magnitude of an X-ray diffraction analysis (XRD) value obtained by quantifying residual stress by XRD analysis and a magnitude of an electron backscatter diffraction (EBSD) value obtained by quantifying an orientation by EBSD analysis, and the preset condition is about 2.85^∗10^-4 Degree^∗MPa/µm² or greater and about 0.05 Degree^∗MPa/µm² or less.
According to an exemplary embodiment, the heating of the blank may include step-heating the blank while passing a plurality sections in a heating furnace in which a temperature range increases in a heating furnace in the plurality of sections, and soaking the blank to a temperature of about Ac3 or higher.
According to an exemplary embodiment, in the plurality of sections, a ratio of a length of sections for step-heating the blank to a length of a section for soaking the blank is about 1:1 to 4:1.
According to an exemplary embodiment, the temperature 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 the step heating, the temperature increase rate of the blank may be in a range from about 6 °C/s to about 12 °C/s.
According to an exemplary embodiment, in the plurality of sections, a temperature of a section for soaking the blank is higher than a temperature of sections for step-heating the blank.
According to an exemplary embodiment, the blank may be present in the heating furnace for a range from about 180 seconds to about 360 seconds.
According to an exemplary embodiment, the cooling of the molded body to form a hot stamping component may include maintaining the molded body for about 3 seconds to about 20 seconds in a press mold at a temperature below a temperature at which martensitic transformation starts.
According to an exemplary embodiment, the molded body may be cooled in the press mold at an average cooling rate of 15 °C/s or greater to a temperature at which martensitic transformation is terminated.
According to an exemplary embodiment, the hot stamping component may include a martensite phase having an area fraction of 80% or greater, 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 an acicular form, and the acicular 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 horizontal 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 have an angle formed with the longitudinal direction of the lath of 0° or greater and 20° or less and the iron-based carbide reference area fraction of 50% or greater.
According to an exemplary embodiment, the second iron-based carbide may have an angle with the longitudinal direction of the lath of 70° or greater and 90° or less and the iron-based carbide reference area fraction of less than 50%.
According to another aspect of the present disclosure, provided is a hot stamping component in which a residual stress analysis value satisfies a preset condition. The residual stress analysis value may be a product of a magnitude of an X-ray diffraction analysis (XRD) value obtained by quantifying residual stress by XRD analysis and a magnitude of an electron backscatter diffraction (EBSD) value obtained by quantifying an orientation by EBSD analysis, and the preset condition is about 2.85^∗10^-4 Degree^∗MPa/µm² or greater and about 0.05 Degree^∗MPa/µm² or less.
According to an exemplary embodiment, the hot stamping component may include a martensite phase having an area fraction of 80% or greater, 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 an acicular form, and the acicular 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 horizontal 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 have an angle with the longitudinal direction of the lath of 0° or greater and 20° or less, and the iron-based carbide reference area fraction of 50% or greater.
According to an exemplary embodiment, the second iron-based carbide may have an angle with the longitudinal direction of the lath of 0° or greater and 20° or less and the iron-based carbide reference area fraction of 50% or greater.

Advantageous Effects

According to the exemplary embodiments of the present disclosure, it is possible to implement a hot stamping component capable of securing high mechanical properties and hydrogen embrittlement by controlling residual stress of the hot stamping component, and a method of manufacturing the hot stamping component. Of course, the scope of the present disclosure is not limited by these effects.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a plan view illustrating a portion of a hot stamping component according to an exemplary embodiment of the present disclosure;
FIG. 2 shows a plan view illustrating a portion of a hot stamping component according to an exemplary embodiment of the present disclosure;
FIG. 3 shows a flowchart schematically illustrating a method of manufacturing a hot stamping component according to an exemplary embodiment of the present disclosure;
FIG. 4 shows a graph illustrating a temperature change when a blank is step-heated in a method of manufacturing a hot stamping component according to an exemplary embodiment of the present disclosure; and
FIG. 5 shows a graph showing a comparison of a temperature change when the blank is step-heated and when the blank is heated in a single stages

DETAILED DESCRIPTION

As the present disclosure allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description The advantages, features, and methods of achieving the advantages of the present disclosure may be clear when referring to the embodiments described below together with the drawings. However, the present disclosure may have different forms and should not be construed as being 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.
In the following embodiments, the singular forms include the plural forms 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 constituent elements but do not preclude the presence or addition of one or more other features or constituent elements.
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 directly on another element or layer or intervening elements or layers.
In the drawings, thicknesses of layers and regions may be exaggerated or reduced for convenience of explanation. For example, the sizes and thicknesses of elements in the drawings are arbitrarily expressed for convenience of explanation, and thus, the current inventive concept is not limited to the drawings.
When a certain embodiment may be implemented differently, a specific process order may be performed differently from the described order. For example, two consecutively described processes may be performed substantially at the same time or performed in an order opposite to the described order.
In the present disclosure, as used herein, "A and/or B" refers to A, B, or A and B. Also, "at least one of A and B" represents the case of A, B, or A and B.
In the following embodiments, when a film, a region, a constituent element, etc. are connected, it may include a case when a film, a region, a constituent element is directly connected or/and a case when the film, the region, and the components are indirectly connected by intervening another film, a region, a constituent element therebetween. For example, in the specification, when a film, a region, a constituent element, etc. are electrically connected, it may represent when a film, region, constituent element, etc. are directly electrically connected, and/or another film, region, component, etc. are indirectly electrically connected by intervening another film, region, constituent element, etc. there between.
Hereafter, the inventive concept will be described more fully with reference to the accompanying drawings, in which embodiments of the inventive concept are shown. In describing the inventive concept with reference to drawings, like reference numerals are used for elements that are substantially identical or correspond to each other, and the descriptions thereof will not be repeated.
FIG. 1 shows a plan view illustrating a portion of a hot stamping component according to an embodiment of the present disclosure.
Referring to FIG. 1, a hot stamping component according to an exemplary embodiment of the present disclosure includes a steel sheet 10.
The steel sheet 10 may be manufactured by performing a hot rolling process and/or a cold rolling process on a slab cast to include a predetermined alloying element in a predetermined content. Such a steel sheet 10 may exist as a full austenite structure at a hot stamping heating temperature, and then may be transformed into a martensitic structure when 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 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. In addition, the steel sheet 10 may further include a predetermined amount of calcium (Ca).
Carbon (C) functions as an austenite stabilizing element in the steel sheet 10. Carbon is a major element that determines the strength and hardness of the steel sheet 10, and, after a hot stamping process, is added for the purpose of securing tensile strength (e.g., tensile strength of about 1,350 MPa or greater) and securing 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, etc.), 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 reduction in bending performance of the steel sheet 10 may occur.
Manganese (Mn) functions as an austenite stabilizing element in the steel sheet 10. Manganese is added for the purpose of increasing 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 not sufficient, and the hard phase fraction in the molded body after hot stamping may be insufficient due to 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 cause deterioration in bending performance and may generate a heterogeneous microstructure.
Boron (B) is added for the purpose of securing a martensitic structure by suppressing the transformation of ferrite, pearlite, and bainite, and thus, to secure the hardenability and strength of the steel sheet 10. In addition, boron segregates at grain boundaries and increases 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 included in the above range, it is possible to prevent the occurrence of brittleness at a hard phase grain boundary, and secure high toughness and bendability. When the content of boron is less than about 0.001 wt%, the hardenability effect is insufficient, and, on the contrary, when the content of boron exceeds about 0.005 wt%, because the solid solubility is reduced, it is easily precipitated at grain boundaries depending on heat treatment conditions, which may lead to deterioration of hardenability or high-temperature embrittlement, and toughness and bendability may be reduced due to the occurrence of hard phase intergranular embrittlement.
Phosphorus (P) may be included in an amount greater than 0 and about 0.03 wt% or less based on the total weight of the steel sheet 10 in order to prevent 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 induced in the steel sheet 10 during a manufacturing process.
Sulfur (S) may be included in an amount greater than 0 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, such as cracks may occur due to the generation of large inclusions.
Silicon (Si) functions 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 improves the carbon concentration in austenite by suppressing the formation of carbides in the low-temperature region. In addition, silicon is a key element in hot rolling, cold rolling, hot pressing, homogenizing the structure (perlite, manganese segregation zone control), and fine dispersion of ferrite. Silicon serves as a martensitic strength heterogeneity control element to improve collision performance. 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 0.1wt%, it is difficult to obtain the above-described effect, and cementite formation and coarsening may occur in the final hot stamping martensitic structure. Conversely, when the content of silicon exceeds about 0.6 wt%, the load of hot rolling and cold rolling may increase, and the plating characteristic of the steel sheet 10 may be deteriorated.
Chromium (Cr) is added for the purpose of improving the hardenability and strength of the steel sheet 10. Chromium helps refine grains and secure 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 0.05wt%, the precipitation hardening effect is low, and on the contrary, when the content of chromium exceeds 0.6wt%, the Cr-based precipitates and matrix solid solution are increased resulting in the decrease in toughness, and production cost may be increased.
Meanwhile, other unavoidable impurities may include nitrogen (N) and 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. Nitrogen may be included in an amount greater than 0 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.
An additive is an element that generates 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, such as TiC and/or TiN at a high temperature, thereby effectively contributing to 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 included in the content range described above, it is possible to prevent continuous casting defects and coarsening of precipitates, to easily secure the physical properties of the steel, and to prevent defects, such as cracks on a surface of the steel. On the other hand, when the content of titanium exceeds about 0.050 wt%, the precipitates are coarsened, and elongation and bendability may decrease.
Niobium (Nb) and vanadium (V) may increase strength and toughness according to a decrease in martensite packet size. 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 included in the above range, the crystal grain refinement effect of the steel sheet 10 is high in hot rolling and cold rolling processes, the generation of cracks in a slab during steelmaking/casting and brittle fracture of the product are prevented, and the generation of coarse precipitates in steelmaking is minimized.
Calcium (Ca) may be added to control an inclusion shape. Calcium may be included in an amount of about 0.003 wt% or less based on the total weight of the steel plate 10.
After a hot rolling process and/or a cold rolling process, when the steel sheet 10 is cooled to room temperature, residual stress exists in the steel sheet 10 of the hot stamping component manufactured through a hot stamping process. Here, the 'residual stress' refers to stress present in the hot stamping component in a state that no external force acts on the steel sheet 10.
Residual stress may result from defects in a material. For example, a point defect, such as vacancy, interstitials, impurity, etc., a line defect, such as dislocation, and an interfacial defect, such as external surface, grain boundary, twin boundary, stacking fault, phase boundary, etc. may be causes of the generation of residual stress. That is, it may be understood that the more defects are present in the steel sheet 10, the greater the internal residual stress.
These defects and the resulting residual stress of the steel sheet 10 affect the mechanical properties (e.g., tensile strength) and hydrogen embrittlement of the steel sheet 10.
Specifically, the tensile strength of a hot stamping component is determined such that when defects inside the steel sheet 10 are present at an appropriate level, the more defects (or the greater the residual stress), the higher the tensile strength, and the lower defects (or the lower the residual stress) the less the tensile strength. This is because the more defects inside the steel sheet 10, the more irregular arrangement the elements are, which makes difficult to move dislocations that cause material deformation.
However, the hydrogen embrittlement of the steel sheet 10 may be reduced as there are more defects (or the larger residual stress), and may be improved as there are less defects (or the smaller residual stress). In general, as there are more effective hydrogen trap sites inside the steel sheet 10, an amount of activated hydrogen is reduced, and thus, hydrogen embrittlement of the product may be improved. For example, fine precipitates present therein (e.g., nitrides or carbides of titanium (Ti), niobium (Nb) and vanadium (V), etc.) serve as effective hydrogen trap sites and improve hydrogen embrittlement. Meanwhile, defects existing therein may also serve as hydrogen trap sites. However, because the defect has a relatively low binding energy with hydrogen, hydrogen trapped by the defect and deactivated is highly likely to return to activated hydrogen. Therefore, the defect does not function as an effective hydrogen trap site, but rather, the hydrogen embrittlement may be reduced by locally concentrating activated hydrogen in a portion having many defects (or a portion having a large residual stress). In particular, the hot stamping component may include at least one bent portion according to an applied position in a structure of a vehicle, and the bent portion is a portion that is formed excessively compared to a flat area during a hot stamping process. That is, the bent portion may act as a hydrogen embrittlement weakness portion because stress caused by a press is relatively concentrated during the hot stamping process, and thus, residual stress may increase.
Accordingly, defects existing in the steel sheet 10 and residual stresses resulting therefrom need to be controlled to an appropriate level.
According to exemplary embodiments of the present disclosure, defects existing in the steel plate 10 and residual stress resulting therefrom may be appropriately adjusted by controlling a residual stress analysis value, which is quantified the residual stress present in the steel plate 10, to satisfy a preset condition.
In an exemplary embodiment, the residual stress analysis value may be a product of a magnitude of an XRD value (or an absolute value of the XRD value) quantifying the residual stress by X-ray diffraction and a magnitude of an electron backscatter diffraction (EBSD) value (or an absolute value of the EBSD value) quantifying an orientation by EBSD. In addition, the preset condition may be about 2.85^∗10^-4 Degree^∗MPa/µm² or greater and about 0.05 Degree^∗MPa/µm² or less. More preferably, when the magnitude of the XRD value is about 5 MPa or greater and less than about 15 MPa, the residual stress analysis value may be controlled to satisfy a range of about 2.95^∗10^-4 Degree^∗MPa/µm² or greater and about 0.01 Degree^∗MPa/µm² or less, when the magnitude of the XRD value is about 15 MPa or greater and less than about 55 MPa, the residual stress analysis value may be controlled to satisfy a range of about 9.31^∗10^-4 Degree^∗MPa/µm² or greater and about 0.035 Degree^∗MPa/µm² or less, and when the magnitude of the XRD value is about 55 MPa or greater and about 70 MPa or less, the residual stress analysis value may be controlled to satisfy a range of about 3.96^∗10^-3 Degree^∗MPa/µm² or greater and about 0.043 Degree^∗MPa/µm² or less.
'X-ray diffraction (XRD) analysis' is an analysis method of measuring residual stress by using X-ray diffraction in which incident X-rays irradiated to a measurement sample are reflected in a specific direction due to the regularity of a crystal lattice. Specifically, the residual stress may be measured by the sin²ϕ method. The sin²ϕ method is to obtain a peak position of a diffraction line by irradiating X-rays to a portion to be measured. When residual stress is present, the peak position of the diffraction line is changed by changing an angle of incidence (ϕ) of the X-ray. At this time, the peak position of the changed diffraction line is taken as the vertical axis, and sin²ϕ of the incident angle of X-rays is taken as the horizontal axis, a slope is obtained by linear regression by the least-squares method, and the obtained slope is multiplied by a stress constant obtained from the Young's modulus and Poisson's ratio, and then, the stress value (XRD value) obtained be obtained by Equation 1 below. $σ = - E / 2 (1 + v) * \cot θ * π / 180 * M = K * M$

σ: Stress value or XRD value (MPa)
E: Young's modulus (MPa)
v: Poisson's ratio
M: slope of regression line 2θ-sin²θ
2θ: angle of diffraction in the absence of strain (°)
K: stress constant (MPa)

The XRD analysis has high representativeness because it targets a relatively wide range, but has a disadvantage in that deviation is large and uniformity is not good. In addition, the deviation of the XRD values tends to increase as the residual stress inside the product increases. Therefore, there is a problem in that it is difficult to accurately analyze and control the residual stress of a material only with the XRD value obtained by quantifying the residual stress by XRD analysis.
On the other hand, the 'EBSD' determines a crystallographic phase and crystallographic orientation using a diffraction pattern of a certain specimen, and based on this, is a method of analyzing the specimen by combining morphologic information and crystallographic information of the specimen microstructure.
Specifically, when an electron beam is irradiated to a specimen in a scanning electron microscope (SEM), the incident electron beam is scattered within the specimen, and a diffraction pattern appears in a surface direction of the specimen. This is referred to as an electron back scattered diffraction pattern (EBSP), and this pattern responds to a crystal orientation of an area to which an electron beam is irradiated, and may measure the crystal orientation of a material with an accuracy of less than 1°.
Because the EBSD targets a relatively narrow range, and thus, has an advantage of small deviation and good uniformity compared to X-ray diffraction (XRD) analysis. However, an EBSD value, which quantifies residual stress by EBSD, also has a disadvantage in that it is not highly representative, it is difficult to accurately analyze and control the residual stress of a material using only the EBSD value.
In an exemplary embodiment of the present disclosure, in order to compensate for the disadvantages of the aforementioned X-ray diffraction (XRD) analysis and EBSD, respectively, a differentiated residual stress analysis value is applied. Specifically, as the residual stress analysis value, a product of a magnitude of an XRD value (or an absolute value of the XRD value) in which the residual stress is digitized by X-ray diffraction (XRD) analysis and a magnitude of an EBSD value (or an absolute value of the EBSD value) in which an orientation is quantified by EBSD may be applied. Accordingly, a deviation, which is a disadvantage of the XRD value, is compensated by the EBSD value, and the low representativeness, which is a disadvantage of the EBSD value, is compensated by the XRD value, therefore, it has the effect of more accurately analyzing and controlling the residual stress.
For example, the residual stress analysis value may be expressed as in Equation 2 below. $Residual stress analysis value (Degree * MPa/ {μm}^{2}) = |XRD value (MPa)| * |\begin{array}{l} EBSD value \\ (degree/ {μm}^{2}) \end{array}|$
The residual stress analysis value may be substantially proportional to the defect in the hot stamping component and the resulting residual stress. Specifically, it may be understood that the larger the residual stress analysis value, the more defects exist inside the product and the greater the residual stress, and the less the residual stress analysis value, the fewer defects exist inside the product and the less the residual stress. Furthermore, it may be understood that the higher the residual stress analysis value, the greater the tensile strength of the product, but the hydrogen embrittlement is not high, and the less the residual stress analysis value, the less the tensile strength of the product, but the better the hydrogen embrittlement. Therefore, mechanical properties and hydrogen embrittlement of a product may be properly secured by controlling the residual stress analysis value to satisfy a preset condition.
On the other hand, due to the characteristics of a process of manufacturing a product through rolling and cooling from a high-temperature material, defects and a resulting residual stress may be caused by a temperature difference existing in a width direction or length direction of the steel sheet 10 during the manufacturing process. According to exemplary embodiments of the present disclosure, the above-described residual stress analysis value may be controlled to satisfy a preset condition by applying a differentiated process condition, for example, a heating condition and/or a cooling condition in the manufacturing process. A detailed description of these differentiated process conditions will be described later with reference to FIGS. 3 to 5.
FIG. 2 shows a plan view illustrating a portion of a hot stamping component according to an exemplary embodiment of the present disclosure.
The steel plate 10 may include a component system having a microstructure including a martensite phase of about 80% or greater by area fraction. In addition, the steel sheet 10 may include a bainite phase of less than about 20% by area fraction.
The martensitic phase is a result of diffusionless transformation of austenite γ below a start temperature (Ms) of martensitic transformation during cooling. Martensite may have a rod-shaped lath phase oriented in one direction d within each initial grain of austenite.
In addition, the steel sheet 10 may include an iron-based carbide positioned inside the martensite phase. The iron-based carbide may have an acicular form. In an exemplary embodiment, 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 denote a minor axis length of the iron-based carbide, and the 'length of the iron-based carbide' may denote a major axis length of the iron-based carbide.
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 may remain without melting even at a temperature of Ac3 or greater in the annealing heat treatment process, and the bendability and yield ratio of the steel sheet 10 may be reduced. 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 iron-based carbide may have an area fraction of less than about 5% based on the martensite phase. When the area fraction of the iron-based carbide is about 5% or greater based on the martensite phase, it may be difficult to secure the strength or 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 horizontal to a longitudinal direction d of a 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, 'horizontal' includes forming an angle of about 0° or greater and about 20° or less with the longitudinal direction d of the lath phase, and 'vertical' includes forming an angle of about 70° or greater and about 90° or less with 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 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 the longitudinal direction d of the lath phase.
An iron-based carbide reference area fraction of the first iron-based carbide C1 may be greater than an iron-based carbide reference area fraction of the second iron-based carbide. Through this, the bendability of the steel plate 10 may be improved. As a specific example, the iron-based carbide reference area fraction of the first iron-based carbide C 1 forming an angle of about 0° or greater and about 20° or less with the longitudinal direction d of the lath phase may be about 50% or greater, preferably about 60% or greater. In addition, the iron-based carbide reference area fraction of the second iron-based carbide C2 forming an angle of about 70° or greater 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 occurred during bending deformation may be generated as a dislocation movement in the martensite phase. At this time, it may be understood that the higher the local strain rate during given plastic deformations, the greater the energy absorption for the plastic deformation of martensite, 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 that is horizontal to the longitudinal direction d of the lath phase is formed greater than the iron-based carbide reference area fraction of the second iron-based carbide C2 that is perpendicular to the longitudinal direction d of the lath phase, a dynamic strain aging (DSA), that is, indentation dynamic strain aging may appear due to a local strain rate difference in a process of dislocation movement inside the lath phase during bending deformation. Indentation dynamic strain aging, as a concept of plastic deformation absorption energy, denotes a resistance performance with respect to deformation, as the more frequent the indentation dynamic strain aging phenomenon occurs, it may be evaluated as a better resistance performance to deformation.
That is, according to an exemplary embodiment, because 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 about 50% or greater, and the iron-based carbide reference area fraction of the second iron-based carbide C2 forming an angle of about 70° or greater and about 90° or less with the longitudinal direction d of the lath phase is formed less than about 50%, the indentation dynamic strain aging phenomenon may occur frequently, as a result, a V-bending angle of about 50° or greater may be secured, and thus, bendability and impact performance may be improved.
Because a bainite phase having an area fraction of less than about 20% in the steel sheet 10 has a uniform hardness distribution, it is a structure having an excellent balance between strength and ductility. However, because bainite is softer than martensite, in order to secure strength and bendability of the steel sheet 10, it is preferable that the bainite has an area fraction of less than about 20%.
On the other hand, the aforementioned iron-based carbide having an acicular form may be precipitated inside the bainite phase. Because the iron-based carbide inside bainite increases the strength of bainite and reduces the difference in strength between bainite and martensite, a yield ratio and bendability of the steel sheet 10 may be increased. In this case, the iron-based carbide may be present in an amount of less than about 20% in the bainite phase based on the bainite phase. If the iron-based carbide is about 20% or greater based on the bainite phase, voids may be generated, which may lead to a decrease in bendability.
FIG. 3 shows a flowchart schematically illustrating a method of manufacturing a hot stamping component according to an embodiment of the present disclosure, and FIG. 4 is a graph illustrating a temperature change when a blank is step-heated in a method of manufacturing a hot stamping component according to an embodiment of the present disclosure, and FIG. 5 is a graph showing a comparison of a temperature change when the blank is step-heated and when the blank is heated in single stages.
Referring to FIG. 3, the method of manufacturing a hot stamping component according to an exemplary embodiment of the present disclosure may include inserting a blank (S110), step heating (S120), and soaking (S130). In addition, the method of manufacturing a hot stamping component may further include transferring (S140), forming (S150), and cooling (S160) after the soaking (S130).
First, the inserting a blank (S110) may be an operation of placing a 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 a hot stamping component. The sheet material may be manufactured by performing hot rolling or cold rolling on a steel slab, followed by annealing heat treatment. In addition, after the annealing heat treatment, a plating layer may be formed on at least one surface of the sheet 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.
Next, the step heating (S120) and the soaking (S130) may be sequentially performed. The blank inserted into the heating furnace may be heated while passing through a plurality of sections provided in the heating furnace. In an exemplary embodiment, the blank introduced into the heating furnace may be mounted on a roller and transported along a transport direction.
The heating furnace may include a plurality of sections sequentially arranged in the heating furnace. The plurality of sections included in the heating furnace include sections in which the temperature range is increased step by step from an inlet of the furnace in which the blank is introduced to an outlet of the furnace in which the blank is discharged, and sections in which the temperature range is maintained uniformly.
The step heating (S120) is an operation of heating the blank while passing through the sections in which the temperature range is increased step by step among a plurality of sections provided in the heating furnace. The soaking (S130) is an operation of heating the step-heated(stepwise heated or step heated) blank by passing through sections in which the temperature range is maintained uniformly among a plurality of sections provided in the heating furnace.
The temperature range of a plurality of sections provided in the heating furnace increases stepwise from the entrance of the heating furnace into which the blank is introduced to an outlet direction of the heating furnace where the blank is discharged to a target temperature Tt range, and then may be maintained in a uniform temperature range, that is, the target temperature Tt range from the section having the target temperature Tt range to the outlet of the heating furnace. In this case, the number of sections in which the temperature range is increased stepwise, the number of sections in which the temperature range is maintained uniformly, 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, unlike shown in FIG. 4, the heating furnace may include 6 or less or 8 or more sections, and the temperature range of each of the sections may also be variously changed. Hereinafter, for convenience of description, the embodiment shown in FIG. 4 will be described.
The first section P1 to the seventh section P7 may be sequentially disposed in the heating furnace. The first section P1 having a first temperature range T1 is adjacent to the inlet of the heating furnace into which the blank is introduced, and the seventh section P7 having a seventh temperature range T7 is adjacent to the outlet of the heating furnace through which the blank is discharged. That is, the first section P1 having a first temperature range T1 may be the first section among the plurality of sections included in the heating furnace, and the seventh section P7 having a seventh temperature range T7 may be the last section among the plurality of sections included in the heating furnace. The blank may be heated by sequentially moving the first section P1 to the seventh section P7 provided in the heating furnace.
In an exemplary embodiment, as shown in FIG. 4, the temperature range of the sections from the first section P1 to the fifth section P5 increases stepwise to the target temperature Tt range, and, in the sixth section P6 and the seventh section P7, the temperature range is maintained at the target temperature Tt range, which is the temperature range of the fifth section P5. However, the present disclosure is not limited to the above-described example, and the number of sections in which the temperature range is increased in stages and sections in which the temperature range is uniformly maintained may be variously changed.
Meanwhile, a temperature difference between two adjacent sections among a plurality of sections provided in the heating furnace may be about 0 °C or higher and about 100 °C or less. For example, the temperature difference between the first section P1 and the second section P2 may be about 0 °C or higher and about 100 °C or less.
In an exemplary embodiment, the first temperature range T1 of the first section P1 may be in a range from about 840 °C to about 860 °C, or in a range from about 835 °C to about 865 °C. The second temperature range T2 of the second section P2 may be in a range from about 870 °C to about 890 °C, or in a range from about 865 °C to about 895 °C. The third temperature range T3 of the third section P3 may be in a range from about 900 °C to about 920 °C, or in a range from about 895 °C to about 925 °C. The fourth temperature range T4 of the fourth section P4 may be in a range from about 920 °C to about 940 °C, or in a range from about 915 °C to about 945 °C. The fifth temperature range T5 of the fifth section P5 may be in a range from about Ac3 to about 1,000 °C. Preferably, the fifth temperature range T5 of the fifth section P5 may be about 930 °C or higher and about 1,000 °C or less. More preferably, the fifth temperature range T5 of the fifth section P5 may be about 950 °C or higher and about 1,000 °C 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 step heating (S120) may be performed in the first section P1 to the fourth section P4, and the soaking (S130) may be performed in the fifth section P5 to the seventh section P7. In this way, the occurrence of a temperature difference between the sections may be prevented or minimized by providing the section in which the soaking (S130) is performed not as one section but as a plurality of sections, for example, the fifth section P5 to the seventh section P7.
The soaking (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 a temperature of Ac3 or greater. That is, in the soaking (S130), the blank step-heated while passing through the first section P1 to the fourth section P4 may be soaked at a temperature of Ac3 or greater. Preferably, in the soaking (S130), the step-heated blank may be soaked at a temperature of about 930 °C or higher and about 1,000 °C or less. More preferably, in the soaking (S130), the step-heated blank may be soaked at a temperature of about 950 °C or higher and about 1,000 °C or less.
In an exemplary embodiment, the heating furnace may have a length of about 20m to about 40m along a transport path of the blank. The heating furnace may have a plurality of sections having different temperature ranges, a ratio of a length (D1, see FIG. 4) of the section in which the blank is step-heated among the plurality of sections to a length (D2, see FIG. 4) of the section in which the blank is soaked among the plurality of sections may satisfy in a range from about 1:1 to about 4:1. That is, the length D2 of the soaking section among the plurality of sections provided in the heating furnace may have a length corresponding 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 section step-heating the blank (D1) to the length (D2) of the section for soaking the blank exceeds 1: 1 due to the increase in the length of the section for soaking the blank, in the soaking section, an austenite (FCC) structure is generated, which increases the amount of hydrogen permeation into the blank, and delayed fracture may increase. In addition, when the ratio of the length (D1) of the section for step-heating the blank (D1) to the length (D2) of the section for soaking the blank is less than 4: 1, because the soaking section (time) is not sufficiently secured, the strength of the hot stamping component manufactured by the manufacturing process of the hot stamping component may be non-uniform.
In an exemplary embodiment, in the step heating (S120) and the soaking (S130), the blank may have a temperature increase rate of about 6 °C/s to about 12 °C/s, and the soaking time may be in a range from about 3 minutes to about 6 minutes. More specifically, when the thickness of the blank is about 1.6 mm to about 2.3 mm, the temperature increase rate is about 6 °C/s to about 9 °C/s, and the soaking time may be in a range from about 3 minutes to about 4 minutes. In addition, when the thickness of the blank is in a range from about 1.0 mm to about 1.6 mm, the temperature increase rate is about 9 °C/s to about 12 °C/s, and the soaking time may be in a range from about 4 minutes to about 6 minutes.
The temperature change when a blank B' is single-heated and when the blank B is step-heated will be described with reference to FIG. 5.
As a comparative example, a case may be assumed that the blank B' is single heated. In a single heating, the temperature of the furnace is set so that an 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 greater than Ac3. Preferably, the target temperature Tt of the blank B' may be about 930 °C. More preferably, the target temperature Tt of the blank B' may be about 950 °C.
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 step-heating. For example, the temperature increasing rate of the blank B' in the single heating may be greater than the temperature increasing rate of the blank B in the step-heating by about 2 °C/s or greater. Because the temperature reaches the target temperature Tt faster in the single heating than in the step-heating, a soaking time ET2 of the single heating may be longer than a soaking time ET1 of the step-heating. As in the case of a single heating, if the soaking time ET2 is lengthen, the magnitude of a grain boundary is not uniform, and the aforementioned defects may be excessively formed more than necessary.
Accordingly, in the method of manufacturing a hot stamping component according to an exemplary embodiment of the present disclosure, an appropriate soaking time (ET1) is secured by delaying the time for the blank to reach the target temperature (Tt) through a step-heating method, and thus, the uniformity of the size of a grain boundary may be secured and the formation of an appropriate level of defects may be controlled. Therefore, a hot stamping component manufactured by applying the step-heating method may be controlled to have defects and residual stress within a preset range, and whether the preset range is satisfied or not may be checked through the residual stress analysis value described above.
Referring to FIG. 3, after the soaking (S130), the transferring (S140), the forming (S150), and the cooling (S160) may further be performed.
The transferring (S140) may be an operation of transferring the heated blank from the heating furnace to a press mold. In the operation of transferring the heated blank from the heating furnace to the press 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 molded body. The cooling (S160) may be an operation of cooling the molded body.
After being molded into a final component shape in the press mold, a final product may be formed by cooling the molded body. A cooling channel through which a refrigerant circulates may be provided in the press mold. It is possible to rapidly cool the heated blank by circulating the refrigerant supplied through the cooling channel provided in the press mold. At this time, in order to prevent a spring back phenomenon of a sheet material and maintain a desired shape, rapid cooling may be performed while pressing the press die in a closed state. That is, a forming process (or the forming (S 150)) and a cooling process (or the cooling (S 160)) may be simultaneously performed while the blank is disposed in the press mold.
In an exemplary embodiment, in performing the forming process and the cooling process on the heated blank, the blank may be held in the press mold at a temperature below the martensitic transformation start temperature (MS temperature) for a preset time, for example, about 3 seconds to about 20 seconds. In addition, the blank may be cooled while maintaining the average cooling rate at about 15 °C /s or greater until the temperature at which martensitic transformation is finished (Mf temperature). By securing the cooling time in this way, the martensite structure is auto-tempered to obtain auto-tempered martensite, and distortion of the molded component may be prevented, and thus, 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 the residual heat of the product and the temperature deviation for each portion. In addition, when the time for which the blank is maintained in the press mold exceeds 20 seconds, more than necessary defects and residual stress resulting therefrom may occur, and the holding time in the press mold may increase, thereby reducing productivity.
In an exemplary embodiment, the tensile strength of the hot stamping component manufactured by the method of manufacturing a hot stamping component may be about 1350 MPa or greater, and an amount of activated hydrogen may be about 0.7 wppm or less.

Hereinafter, the present disclosure will be described in more detail through Embodiments and Comparative Examples. However, the following Embodiments and Comparative Examples are for explaining the present invention in more detail, and the scope of the present invention is not limited by the following Examples and Comparative Examples. The following Examples and Comparative Examples may be appropriately modified and changed by those skilled in the art within the scope of the present disclosure.

Table 1

specime n	XRD value (MPa)	tensile strength (MPa)	amount of activated hydrogen (wppm)
A-1	-5.1±3.2	1369	0.581
A-2	-26.2±11.3	1373	0.652
A-3	-44.8±15.1	1410	0.667
A-4	-59.1±13.7	1434	0.681
A-5	-69.1±17.5	1481	0.689
A-6	-4.8±3.8	1331	0.573
A-7	-4.2±3.1	1332	0.571
A-8	-70.3±19.6	1448	0.739
A-9	-73.9±18.1	1508	0.751

Table 1 shows results of measuring an XRD value, tensile strength, and an amount of activated hydrogen for each of specimens A-1 to A-9. Specifically, Table 1 confirms whether the magnitude of the XRD value measured for the specimens satisfies a range of 5 MPa or greater and 70 MPa or less, and shows data for comparing and analyzing the tensile strength and the amount of activated hydrogen in the case of satisfying the range and the case of unsatisfying the range.
The XRD value is a value obtained by quantifying residual stress by the aforementioned X-ray diffraction analysis (XRD). The XRD value was measured by removing a coating layer of the specimens and irradiating X-rays after electro-polishing to a target position (e.g., 1/4 point). In addition, the electro-polishing is performed with an electrolytic polishing solution including about 5% of 2-butoxyethanol, about 20% of perchloric acid, about 35% of ethanol, and about 40% of water.
The amount of activated hydrogen may be measured using a thermal desorption spectroscopy method. The thermal desorption spectroscopy method is a method of measuring an amount of hydrogen released from the specimen below a certain temperature while heating the specimen at a preset heating rate to increase the temperature, and the hydrogen released from the specimen may be understood as activated hydrogen that is not captured and affects delayed hydrogen destruction among the hydrogen introduced into the specimen. That is, if an amount of hydrogen measured as a result of the thermal desorption spectroscopy method is large, it denotes that a large amount of uncaptured activated hydrogen that may cause delayed hydrogen destruction is included.
Specifically, the amount of activated hydrogen in Table 1 is a value obtained by measuring the amount of hydrogen emitted from the specimen at about 350°C or less while raising the temperature from room temperature to about 500°C at a heating rate of about 20 °C/min for each of the specimens.
Specimens A-1 to A-5 satisfy a range of the measured XRD value of about 5 MPa or greater and about 70 MPa or less. That is, it may be understood that an appropriate level of defects and residual stress are present in the specimens A-1 to A-5. Accordingly, it may be confirmed that the tensile strength of specimens A-1 to A-5 satisfies 1350 MPa or greater, and an amount of activated hydrogen in specimens A-1 to A-5 satisfies 0.7 wppm or less.
On the other hand, in the case of specimens A-6 and A-7, the magnitude of the measured XRD value is less than 5 MPa. That is, it may be seen that defects present less than a required level inside specimens A-6 and A-7, and the resulting residual stress is excessively small. Accordingly, it may be seen that the amount of activated hydrogen in each of specimens A-6 and A-7 satisfies 0.7 wppm or less, while the tensile strength is less than 1350 MPa.
In addition, in the case of specimens A-8 and A-9, the magnitude of the measured XRD value exceeds 70 MPa. That is, it may be seen that defects exist more than necessary inside specimens A-8 and A-9, and the resulting residual stress is excessively large. Accordingly, the tensile strength of each of specimens A-8 and A-9 satisfies 1350 MPa or greater, while the amount of activated hydrogen exceeds 0.7 wppm, confirming that hydrogen embrittlement is reduced.

Meanwhile, referring to Table 1, it may be seen that as the magnitude of the XRD value increases, the deviation of the XRD value also tends to increase. That is, the greater the internal stress, the greater the error range of the XRD value, and thus, the greater the need to correct it.

Table 2

specimen	EBSD value degree/µm²	tensile strength MPa	amount of activated hydrogen wppm
B-1	5.72^∗10-5±0.001	1359	0.579
B-2	9.82^∗10-5±0.007	1391	0.591
B-3	2.33^∗10-4±0.003	1402	0.635
B-4	7.14^∗10-4±0.012	1492	0.661
B-5	5.62^∗10-5±0.006	1327	0.589
B-6	3.13^∗10-5±0.004	1321	0.581
B-7	7.28^∗10-4±0.015	1495	0.761
B-8	8.67^∗10-4±0.011	1491	0.775

Table 2 shows the results of measurement of the EBSD value, tensile strength, and an amount of activated hydrogen for each of specimens B-1 to B-8. Specifically, Table 1 confirms whether the magnitude of the EBSD value measured for the specimens satisfies a range of about 5.71^∗10^-5 degree/µm² or greater and about 7.14^∗10^-4 degree/µm² or less, and shows data for comparative analysis of tensile strength and activated hydrogen amount, respectively, in the case of satisfying and unsatisfying of the range.
The EBSD value is a value obtained by quantifying an orientation by using the above-described EBSD. The EBSD value was measured by scanning a specimen area of 4000 times and 25 µm^∗70 µm in 50 nm steps. In addition, these measurements were performed for 5 observation surfaces.
The amount of activated hydrogen was measured using a thermal desorption spectroscopy method under the same conditions as in Table 1.
The measured EBSD values of specimens B-1 to B-4 satisfy a range of about 5.71^∗10^-5 degree/µm² or greater and about 7.14^∗10^-4 degree/µm² or less. That is, it may be understood that an appropriate level of defects and resulting residual stress present within specimens B-1 to B-4. Accordingly, it may be confirmed that the tensile strength of specimens B-1 to B-4 satisfies about 1350 MPa or greater, and the amount of activated hydrogen in specimens B-1 to B-4 satisfies about 0.7 wppm or less.
On the other hand, in the case of specimens B-5 and B-6, the magnitude of the measured EBSD value is less than about 5.71^∗10^-5 degree/ µm². That is, it may be seen that there are fewer defects than the required level in specimens B-5 and B-6, and the resulting residual stress is excessively small. Accordingly, it may be seen that the amount of activated hydrogen of each of specimens B-5 and B-6 satisfies about 0.7 wppm or less, while the tensile strength is less than about 1350 MPa.

In addition, in the case of specimens B-7 and B-8, the magnitude of the measured EBSD value exceeds about 7.14^∗10^-4 degree/µm². That is, it may be seen that defects exist more than necessary inside specimens B-7 and B-8, and the resulting residual stress is excessively large. Accordingly, it may be confirmed that the tensile strength of each of specimens B-7 and B-8 satisfies about 1350 MPa or greater, while the amount of activated hydrogen exceeds about 0.7 wppm and thus, the hydrogen embrittlement is reduced.

Table 3

speci men	XRD value (MPa)	EBSD value degree/µm²	residual stress analysis value MPa^∗degree/µm²	tensile strength MPa	activated hydrogen amount wppm	4 point bending test result
C-1	-5.5±3.2	5.71^∗10^-5±0.001	3.14^∗10^-4	1359	0.579	non-fractured
C-2	-13.8±9.5	7.14^∗10^-4±0.007	9.85^∗10^-3	1472	0.591	non-fractured
C-3	-27.2±8.8	3.09^∗10^-4±0.003	8.40^∗10^-3	1398	0.635	non-fractured
C-4	-48.5±17.3	7.14^∗10^-4±0.012	3.46^∗10^-2	1495	0.661	non-fractured
C-5	-55.1±15.7	6.70^∗10^-5±0.010	3.69^∗10-3	1461	0.689	non-fractured
C-6	-69.8±18.3	6.14^∗10^-4±0.011	4.29^∗10^-2	1480	0.692	non-fractured
C-7	-5.1±3.5	5.71^∗10^-5±0.09	2.91^∗10^-4	1367	0.588	fractured
C-8	-14.7±9.1	7.14^∗10^-4±0.13	1.05^∗10^-2	1469	0.621	break
C-9	-15.4±9.9	6.03^∗10^-5±0.08	9.29^∗10^-4	1395	0.607	fractured
C-10	-50.5±16.4	7.14^∗10^-4±0.09	3.61^∗10^-2	1474	0.672	fractured
C-11	-54.9±15.7	6.61^∗10^-5±0.10	3.63^∗10^-3	1435	0.668	fractured
C-12	-68.9±18.5	6.31^∗10^-4±0.07	4.35^∗10^-2	1513	0.696	fractured

Table 3 shows XRD values, EBSD values, residual stress analysis values, tensile strength, activated hydrogen amount, and 4-point bending test results for each of specimens C-1 to C-12.
XRD values, EBSD values, tensile strength, and the amount of activated hydrogen were measured under the same conditions and methods as in Tables 1 and 2. In addition, the residual stress analysis values were calculated as a product of the magnitude (or absolute value) of the XRD values and the magnitude (or absolute value) of the EBSD values.
The 4-point bending test is a test method for checking whether stress corrosion cracking occurs or not by applying a stress below an elastic limit to a specific point on a specimen manufactured by reproducing a state in which the specimen is exposed to a corrosive environment. At this time, the stress corrosion cracking denotes a crack that occurs when corrosion and continuous tensile stress simultaneously act.
Specifically, the 4-point bending test results in Table 1 are results of checking whether fracture occurs by applying a stress of 1,000 MPa in air for 100 hours to each of the specimens.
According to exemplary embodiments of the present disclosure, by applying a product of the magnitude of the XRD value and the magnitude of the EBSD value as a residual stress analysis value, inaccurate information of each of the XRD value and the EBSD value may be mutually corrected, and thus, residual stress inside the product may be accurately analyzed and controlled. Specifically, the residual stress analysis value is controlled to satisfy the range of about 2.85^∗10^-4 Degree^∗MPa/µm² or greater and about 0.05 Degree^∗MPa/µm² or less.
On the other hand, referring to the XRD values in Table 3, it may be seen that the deviation of the XRD values also tends to increase as the magnitude of the XRD values increases. That is, the greater the internal stress, the greater the error range of the XRD value, and the greater the need to correct it. Therefore, when the internal residual stress of the product is large (or the deviation of the XRD value is large), the role of the residual stress analysis value may be more remarkable.
In consideration of this, the residual stress analysis value may be more precisely controlled according to the range of the XRD value. Specifically, when the magnitude of the XRD value is 5 MPa or greater and less than 15 MPa, the residual stress analysis value may be controlled to satisfy the range of about 2.95^∗10^-4 Degree^∗MPa/µm² or greater and about 0.01 Degree^∗MPa/µm² or less, when the magnitude of the XRD value is 15 MPa or greater and less than 55 MPa, the residual stress analysis value may be controlled to satisfy the range of about 9.31^∗10^-4 Degree^∗MPa/µm² or greater and about 0.035 Degree^∗MPa/µm² or less, and when the magnitude of the XRD value is 55 MPa or greater and 70 MPa or less, the residual stress analysis value may be controlled to satisfy the range of about 3.96^∗10^-3 Degree^∗MPa/µm² or greater and about 0.043 Degree^∗MPa/µm² or less.
Specimens C-1 to C-6 are hot stamping components manufactured through operations S110 to S160 by applying the above-described process conditions. That is, the specimens C-1 to C-6 are specimens manufactured by applying conditions applied to the step heating (S120) and the soaking (S130) described above, applying an average cooling rate of about 15 °C/s or greater to the temperature (Mf) at which the martensitic transformation of the blank is finished in the cooling (S160), and maintaining the specimens for about 3 seconds to about 20 seconds in a press mold at a temperature below a temperature at which martensite transformation starts (Ms temperature).
Accordingly, in specimens C-1 to C-6, the magnitude of the measured XRD value satisfies the range of about 5 MPa or greater and about 70 MPa or less, and the magnitude of the measured EBSD value satisfies the range of about 5.71^∗10^-5 degree/µm² or greater and about 7.14^∗10^-4 degree/µm² or less. In addition, the residual stress analysis value (the product of the magnitude of the XRD value and the magnitude of the EBSD value) of specimens C-1 to C-6 also satisfies the range of about 2.85^∗10^-4 Degree^∗MPa/µm² or greater and about 0.05 Degree^∗MPa)/µm².
More specifically, in specimens C-1 and C-2, the magnitude of the XRD value is about 5 MPa or greater and less than about 15 MPa, and the residual stress analysis value satisfies a range of about 2.95^∗10^-4 Degree^∗MPa/µm² or greater and 0.01 Degree^∗MPa/µm². In addition, in specimens C-3 and C-4, the magnitude of the XRD value was about 15 MPa or greater and less than about 55 MPa, and the residual stress analysis value satisfies the range of about 9.31^∗10^-4 Degree^∗MPa/µm² or greater and about 0.035 Degree^∗MPa/µm² or less. In addition, in specimens C-5 and C-6, the magnitude of the XRD value was about 55 MPa or greater and about 70 MPa or less, and the residual stress analysis value satisfies the range of about 3.96* 10-3 Degree^∗MPa/µm² or greater and about 0.043 Degree^∗MPa/µm² or less.
That is, in specimens C-1 to C-6, not only the magnitude of the XRD value and the magnitude of the EBSD value, but also the residual stress analysis value satisfies the preset conditions, thus, it may be understood that a corrected level of defects and residual stress are present in the specimens C-1 to C-6. Accordingly, it may be confirmed that the tensile strength of specimens C-1 to C-6 satisfies about 1350 MPa or greater, and the activated hydrogen content satisfies about 0.7 wppm or less. In addition, it may be confirmed that the specimens C-1 to C-6 are not fractured as a result of the 4-point bending test. That is, as specimens C-1 to C-6 were manufactured by applying the process conditions described above, the residual stress analysis value is controlled to satisfy preset conditions, and thus, an appropriate level of tensile strength and hydrogen embrittlement were secured.
Meanwhile, specimens C-7 to C-12 are hot stamping components manufactured by applying different process conditions among at least some of the process conditions described above.
Referring to Table 3, for specimens C-7 to C-12, the magnitude of the measured XRD value satisfies the range of about 5 MPa or greater and about 70 MPa or less, and the magnitude of the measured EBSD value satisfies the range of about 5.71^∗10^-5 degree/µm² or greater and about 7.14^∗10^-4 degree/µm² or less. Accordingly, it may be seen that the tensile strength of specimens C-7 to C-12 satisfies about 1350 MPa or greater, and the amount of activated hydrogen of specimens C-7 to C-12 satisfies about 0.7 wppm or less.
However, the residual stress analysis values of specimens C-7 to C-12 do not satisfy the aforementioned preset conditions.
Specimen C-7 has an XRD value of about 5 MPa or greater and less than about 15 MPa, and the residual stress analysis value is less than about 2.95^∗10^-4 Degree^∗MPa/µm². That is, it may be understood that there are fewer defects in the specimen C-7 than the required level, and the resulting residual stress is excessively small. Accordingly, it may be confirmed that the specimen C-7 was fractured as a result of the 4-point bending test.
Specimen C-8 has an XRD value of about 5 MPa or greater and less than about 15 MPa, and the residual stress analysis value exceeds about 0.01 Degree^∗MPa/µm². That is, it may be understood that defects exist more than necessary inside specimen C-8, and the resulting residual stress is excessively large. Accordingly, it may be confirmed that specimen C-8 was fractured as a result of the 4-point bending test.
Specimen C-9 has an XRD value of about 15 MPa or greater and less than about 55 MPa, and the residual stress analysis value is less than about 9.31^∗10^-4 Degree^∗MPa/µm². That is, it may be understood that there are fewer defects in the specimen C-9 than a required level, and the resulting residual stress is excessively small. Accordingly, it may be confirmed that the specimen C-9 was fractured as a result of the 4-point bending test.
Specimen C-10 has an XRD value of about 15 MPa or greater and less than about 55 MPa, and the residual stress analysis value exceeds about 0.035 Degree^∗MPa/µm². That is, it may be understood that defects exist more than necessary inside specimen C-10, and the resulting residual stress is excessively large. Accordingly, it may be confirmed that the specimen C-10 was fractured as a result of the 4-point bending test.
Specimen C-11 has an XRD value of about 55 MPa or greater and about 70 MPa or greater, and the residual stress analysis value is less than about 3.96^∗10^-3 Degree^∗MPa/µm². That is, it may be understood that there are fewer defects in the specimen C-11 than a required level, and the resulting residual stress is excessively small. Accordingly, it may be confirmed that the specimen C-11 was fractured as a result of the 4-point bending test.
Specimen C-12 has an XRD value of about 55 MPa or greater and about 70 MPa or greater, and the residual stress analysis value exceeds about 0.043 Degree^∗MPa/µm². That is, it may be understood that defects exist more than necessary inside the specimen C-12, and the resulting residual stress is excessively large. Accordingly, it may be confirmed that the specimen C-12 was fractured as a result of the 4-point bending test.
Specimens C-7 to C-12 were fractured as a result of a 4-point bending test because the residual stress analysis value did not satisfy the preset conditions, although each of the magnitude of the XRD value and the magnitude of the EBSD value satisfies the preset conditions. It may be understood that it is difficult to completely control defects inside hot stamping components and the resulting residual stresses only by XRD analysis or EBSD analysis.
On the other hand, as in specimens C-1 to C-6, if the residual stress analysis value satisfies the preset condition, there is no fracture as a result of the 4-point bending test, it may be confirmed that defects inside a hot stamping component and resulting residual stress may be analyzed and controlled more accurately through a residual stress analysis value.
Although the present disclosure has been described with reference to the exemplary embodiment shown in the drawings, which is merely an example, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the inventive concept. Accordingly, the scope of the invention is defined not by the detailed description of the invention but by the appended claims.

Claims

A method of manufacturing a hot stamping component in which a residual stress analysis value satisfies a preset condition, the method comprising:
heating a blank;

forming a molded body by hot stamping the blank; and

cooling the molded body to form a hot stamped component, wherein the residual stress analysis value is a product of a magnitude of an X-ray diffraction analysis (XRD) value obtained by quantifying residual stress by XRD analysis and a magnitude of an electron backscatter diffraction (EBSD) value obtained by quantifying an orientation by EBSD analysis, and

the preset condition is about 2.85^∗10^-4 Degree^∗MPa/µm² or greater and about 0.05 Degree^∗MPa/µm² or less.
The method of claim 1, wherein the heating of the blank comprises:
step-heating the blank while passing a plurality of sections in a heating furnace wherein a temperature range increases gradually in the plurality of sections provided; and

soaking the blank to a temperature of about Ac3 or higher.
The method of claim 2, wherein, in the plurality of sections, a ratio of a length of sections for step-heating the blank to a length of a section for soaking the blank is about 1:1 to 4:1.
The method of claim 2, wherein the temperature in the plurality of sections increases in a direction from an inlet of the heating furnace to an outlet of the heating furnace.
The method of claim 4, wherein, in the step-heating, the temperature increase rate of the blank is in a range from about 6 °C/s to about 12 °C/s.
The method of claim 5, wherein, in the plurality of sections, a temperature of a section for soaking the blank is higher than a temperature of sections for step-heating the blank.
The method of claim 2, wherein the blank is present in the heating furnace for a range from about 180 seconds to about 360 seconds.
The method of claim 1, wherein the cooling of the molded body to form a hot stamping component comprises maintaining the molded body for about 3 seconds to about 20 seconds in a press mold at a temperature below a temperature at which martensitic transformation starts.
The method of claim 8, wherein the molded body is cooled in the press mold at an average cooling rate of 15 °C/s or greater to a temperature at which martensitic transformation is terminated.
The method of claim 1, wherein the hot stamping component comprises:
a martensite phase having an area fraction of 80% or greater; and

an iron-based carbide located inside the martensite phase and having an area fraction of less than 5% based on the martensite phase.
The method of claim 10, wherein
the iron-based carbide has an acicular form, and

the acicular form has a diameter of less than 0.2 µm and a length of less than 10 µm.
The method of claim 10, wherein:
the martensite phase comprises a lath phase,

the iron-based carbide comprises a first iron-based carbide horizontal 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 is greater than an iron-based carbide reference area fraction of the second iron-based carbide.
The method of claim 12, wherein the first iron-based carbide has an angle with the longitudinal direction of the lath of 0° or greater and 20° or less and the iron-based carbide reference area fraction of 50% or greater.
The method of claim 12, wherein the second iron-based carbide has an angle with the longitudinal direction of the lath of 70° or greater and 90° or less and the iron-based carbide reference area fraction of less than 50%.
A hot stamping component in which a residual stress analysis value satisfies a preset condition,
wherein the residual stress analysis value is a product of a magnitude of an X-ray diffraction analysis (XRD) value obtained by quantifying residual stress by XRD analysis and a magnitude of an electron backscatter diffraction (EBSD) value obtained by quantifying an orientation by EBSD analysis, and

the preset condition is about 2.85^∗10^-4 Degree^∗MPa/µm² or greater and about 0.05 Degree^∗MPa/µm² or less.
The hot stamping component of claim 15, wherein
the hot stamping component includes a martensite phase having an area fraction of 80% or greater, and

an iron-based carbide located inside the martensite phase and having an area fraction of less than 5% based on the martensite phase.
The hot stamping component of claim 16, wherein
the iron-based carbide has an acicular form, and

the acicular form has a diameter of less than 0.2 µm and a length of less than 10 µm.
The hot stamping component of claim 16, wherein
the martensite phase comprises a lath phase,

the iron-based carbide comprises a first iron-based carbide horizontal 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 is greater than an iron-based carbide reference area fraction of the second iron-based carbide.
The hot stamping component of claim 18, wherein the first iron-based carbide has an angle with the longitudinal direction of the lath of 0° or greater and 20° or less and the iron-based carbide reference area fraction of 50% or greater.
The hot stamping component of claim 18, wherein the second iron-based carbide has an angle with the longitudinal direction of the lath is 70° or greater and 90° or less and the iron-based carbide reference area fraction of less than 50%.