KR102366284B1 - Hot stamping component and method of manufacturing the same - Google Patents

Abstract

The present invention provides a hot stamping component and a manufacturing method thereof, which can secure excellent mechanical properties and hydrogen embrittlement by controlling the residual stress of a hot stamping component. According to one aspect of the present invention, the method of manufacturing a hot stamping component of which a residual stress analysis value satisfies a preset condition comprises: a step of heating a blank; a step of hot-stamping the blank to form a formed body; and a step of cooling the formed body to form a hot stamping component. The residual stress analysis value is the product of the magnitude of an X-ray diffraction (XRD) value resulting from digitizing the residual stress by XRD analysis and the magnitude of an electron backscatter diffraction (EBSD) value resulting from digitizing the bearing by EBSD analysis. The preset condition is higher than or equal to 2.85*10^-4 (degree*MPa/μm^2) and lower than or equal to 0.05 (degree*MPa/μm^2).

Description

Hot stamping component and method of manufacturing the same

Embodiments of the present invention relate to hot stamping parts and methods of manufacturing the same.

High-strength steel for weight reduction and stability is applied to parts used in automobiles. On the other hand, high-strength steel can secure high-strength properties compared to its weight, but as the strength increases, press formability deteriorates, causing material breakage during processing or springback phenomenon, making it difficult to form products with complex and precise shapes. There are difficulties.

As a method to improve this problem, there is a representative hot stamping method, and as interest in it increases, research on materials for hot stamping is also being actively conducted. For example, as disclosed in Korean Patent Application Laid-Open No. 10-2017-0076009, the hot stamping method is a molding technology for manufacturing high-strength parts by heating a boron steel sheet to an appropriate temperature, forming it in a press mold, and then rapidly cooling it. According to the invention of Korean Patent Application Laid-Open No. 10-2017-0076009, problems such as crack generation or shape freezing defect during forming, which are problems in high-strength steel sheet, are suppressed, so that it is possible to manufacture parts with good precision.

Laid-open Patent Publication No. 10-2017-0076009, 2017.07.04.

Embodiments of the present invention are to solve various problems including the above-described problems, and to provide a hot stamping part and a method for manufacturing the same, which can secure excellent mechanical properties and hydrogen embrittlement by controlling the residual stress of the hot stamping part. can However, these problems are exemplary, and the scope of the present invention is not limited thereto.

According to one aspect of the present invention, in a method for manufacturing a hot stamping part in which the residual stress analysis value satisfies a preset condition, heating a blank, hot stamping the blank to form a molded body, and the molded body Cooling to form a hot stamping part, wherein the residual stress analysis value is the magnitude of the XRD value quantifying the residual stress by X-ray diffraction (XRD) and backscattered electron diffraction pattern analysis (EBSD; electron backscatter diffraction) is the product of the magnitude of the EBSD value quantifying the orientation, and the preset condition is 2.85 * 10 ^-4 (Degree * MPa / μm ² ) or more and 0.05 (Degree * MPa / μm ² ) or less, A method of manufacturing a hot stamped part is provided.

According to this embodiment, the heating of the blank includes a multi-stage heating step of heating the blank while passing sections in which the temperature range is increased stepwise among a plurality of sections provided in the heating furnace, and heating the blank to a temperature of Ac3 or higher It may include a crack heating step of heating with a furnace.

According to this embodiment, the ratio of the length of the section for heating the blank in multiple stages in the plurality of sections to the length of the section for heating the blank by cracking may be 1:1 to 4:1.

According to the present embodiment, the temperatures of the plurality of sections may increase in a direction from the entrance of the heating furnace to the exit direction of the heating furnace.

According to this embodiment, the temperature increase rate of the blank in the multi-stage heating step may be 6 °C / s to 12 °C / s.

According to the present embodiment, a temperature of a section in which the blank is heated by cracking among the plurality of sections may be higher than a temperature of sections in which the blank is heated in multiple stages.

According to this embodiment, the blank may stay in the furnace for 180 seconds to 360 seconds.

According to this embodiment, the step of cooling the molded body to form the hot stamping part may include maintaining the molded body in a press mold at a temperature below the temperature at which martensitic transformation starts for 3 seconds to 20 seconds. there is.

According to this embodiment, the molded body may be cooled at an average cooling rate of 15 °C/s or more to a temperature at which martensitic transformation is terminated in the press mold.

According to this embodiment, the hot stamping part is located in the martensite phase having an area fraction of 80% or more and the martensite phase, and having an area fraction of less than 5% based on the martensite phase, iron-based carbide. can

According to this embodiment, the iron-based carbide may have a needle-like shape, and the needle-like shape may have a diameter of less than 0.2 μm and a length of less than 10 μm.

According to this embodiment, the martensite phase includes a lath phase, and the iron-based carbide includes ferrous carbide horizontal to the longitudinal direction of the lath phase and ferric carbide perpendicular to the longitudinal direction of the lath phase. and a reference area fraction of the iron-based carbide of the ferrous carbide may be greater than an area fraction of the iron-based carbide of the ferric carbide.

According to this embodiment, in the ferrous carbide, an angle formed with the longitudinal direction of the lath may be 0° or more and 20° or less, and the reference area fraction of the iron-based carbide may be 50% or more.

According to this embodiment, in the ferric carbide, an angle formed with the longitudinal direction of the lath may be 70° or more and 90° or less, and the reference area fraction of the iron-based carbide may be less than 50%.

According to another aspect of the present invention, in the hot stamping parts in which the residual stress analysis value satisfies a preset condition, the residual stress analysis value is an X-ray diffraction analysis (XRD; X-ray diffraction) to quantify the residual stress. It is the product of the magnitude of the XRD value and the magnitude of the EBSD value quantified by the orientation by electron backscatter diffraction (EBSD), and the preset condition is 2.85 * 10 ^-4 (Degree * MPa / μm ² ) More than 0.05 (Degree * MPa / μm ² ) or less, a hot stamping part is provided.

According to this embodiment, a martensite phase having an area fraction of 80% or more and an iron-based carbide having an area fraction of less than 5% based on the martensite phase and located inside the martensite phase may be provided.

According to this embodiment, the iron-based carbide may have a needle-like shape, and the needle-like shape may have a diameter of less than 0.2 μm and a length of less than 10 μm.

According to this embodiment, the martensite phase includes a lath phase, and the iron-based carbide includes ferrous carbide horizontal to the longitudinal direction of the lath phase and ferric carbide perpendicular to the longitudinal direction of the lath phase. and a reference area fraction of the iron-based carbide of the ferrous carbide may be greater than an area fraction of the iron-based carbide of the ferric carbide.

According to this embodiment, in the ferrous carbide, an angle formed with the longitudinal direction of the lath may be 0° or more and 20° or less, and the reference area fraction of the iron-based carbide may be 50% or more.

According to this embodiment, in the ferric carbide, an angle formed with the longitudinal direction of the lath may be 70° or more and 90° or less, and the reference area fraction of the iron-based carbide may be less than 50%.

According to the embodiments of the present invention, it is possible to implement a hot stamping part and a method for manufacturing the same, which can secure excellent mechanical properties and hydrogen embrittlement by controlling the residual stress of the hot stamping part. Of course, the scope of the present invention is not limited by these effects.

1 is a plan view illustrating a portion of a hot stamping part according to an embodiment of the present invention.
2 is a plan view illustrating a portion of a hot stamping part according to an embodiment of the present invention.
3 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment of the present invention.
4 is a graph illustrating a temperature change when a blank is heated in multiple stages in a method of manufacturing a hot stamping part according to an embodiment of the present invention.
5 is a graph showing a comparison of the temperature change when the blank is heated in multiple stages and when the blank is heated in single stages.

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

In the following embodiments, terms such as first, second, etc. are used for the purpose of distinguishing one component from another, not in a limiting sense.

In the following examples, the singular expression includes the plural expression unless the context clearly dictates otherwise.

In the following embodiments, terms such as include or have means that the features or components described in the specification are present, and the possibility of adding one or more other features or components is not excluded in advance.

In the following embodiments, when it is said that a part such as a film, region, or component is on or on another part, not only when it is directly on the other part, but also another film, region, component, etc. is interposed therebetween. Including cases where

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

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

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

In the following embodiments, when a film, region, or component is connected, when the film, region, or component is directly connected, or/and in the middle of another film, region, or component It includes cases where they are interposed and indirectly connected. For example, in the present specification, when it is said that a film, region, component, etc. are electrically connected, when the film, region, component, etc. are directly electrically connected, and/or another film, region, component, etc. is interposed therebetween. This indicates an indirect electrical connection.

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

1 is a plan view illustrating a portion of a hot stamping part according to an embodiment of the present invention.

Referring to FIG. 1 , a hot stamping part according to an embodiment of the present invention includes a steel plate 10 .

The steel sheet 10 may be a steel sheet manufactured by performing a hot rolling process and/or a cold rolling process on a slab cast to include a predetermined alloying element in a predetermined content. 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 upon cooling.

In one embodiment, the steel sheet 10 is carbon (C), manganese (Mn), boron (B), phosphorus (P), sulfur (S), silicon (Si), chromium (Cr), and the remainder 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 plate 10 may further include a predetermined amount of calcium (Ca).

Carbon (C) acts as an austenite stabilizing element in the steel sheet 10 . Carbon is a main element that determines the strength and hardness of the steel sheet 10, and after the hot stamping process, the purpose of securing the tensile strength (eg, tensile strength of 1,350 MPa or more) of the steel sheet 10, and securing the hardenability characteristics is added as Such carbon may be included in an amount of 0.19 wt% to 0.38 wt% based on the total weight of the steel sheet 10 . When the carbon content is less than 0.19 wt%, it is difficult to secure a hard phase (martensite, etc.), so it is difficult to satisfy the mechanical strength of the steel sheet 10 . Conversely, when the carbon content exceeds 0.38 wt%, brittleness of the steel sheet 10 or a reduction in bending performance may occur.

Manganese (Mn) acts as an austenite stabilizing element in the steel sheet 10 . Manganese is added to increase hardenability and strength during heat treatment. Such manganese may be included in 0.5wt% to 2.0wt% based on the total weight of the steel sheet 10 . When the manganese content is less than 0.5 wt%, the hardenability effect is not sufficient, and the hard phase fraction in the molded article after hot stamping may be insufficient due to insufficient hardenability. On the other hand, when the content of manganese exceeds 2.0 wt%, ductility and toughness due to manganese segregation or pearlite bands may be reduced, which may cause deterioration in bending performance and may cause a heterogeneous microstructure.

Boron (B) is added for the purpose of securing the hardenability and strength of the steel sheet 10 by suppressing the transformation of ferrite, pearlite, and bainite to secure a martensitic structure. In addition, boron segregates at grain boundaries to increase hardenability by lowering grain boundary energy, and has an effect of grain refinement due to an increase in austenite grain growth temperature. Such boron may be included in an amount of 0.001 wt % to 0.005 wt % based on the total weight of the steel sheet 10 . When boron is included in the above range, it is possible to prevent the occurrence of brittleness at the hard phase grain boundary, and secure high toughness and bendability. When the content of boron is less than 0.001 wt%, the hardenability effect is insufficient, and on the contrary, when the content of boron exceeds 0.005 wt%, the solid solution is low and easily precipitated at the grain boundary depending on the heat treatment conditions to deteriorate the hardenability or It may cause embrittlement at high temperatures, and toughness and bendability may be reduced due to the occurrence of hard phase intergranular embrittlement.

Phosphorus (P) may be included in an amount greater than 0 and 0.03 wt% or less based on the total weight of the steel sheet 10 in order to prevent deterioration of the toughness of the steel sheet 10 . When the phosphorus content exceeds 0.03 wt%, a phosphide compound is formed to deteriorate toughness and weldability, and cracks may be induced in the steel sheet 10 during the manufacturing process.

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

Silicon (Si) acts 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. Such silicon may be included in an amount of 0.1 wt% to 0.6 wt% based on the total weight of the steel sheet 10 . When the content of silicon is less than 0.1wt%, it is difficult to obtain the above-described effect, and cementite formation and coarsening may occur in the final hot stamping martensitic structure. Conversely, when the content of silicon exceeds 0.6 wt%, hot-rolling and cold-rolling loads may increase, and the plating properties 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 makes it possible to refine grains and secure strength through precipitation hardening. Such chromium may be included in 0.05wt% to 0.6wt% based on the total weight of the steel sheet 10 . When the content of chromium is less than 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 increase to decrease toughness, and production cost due to increased cost may increase.

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

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

The additive is a carbide generating element that contributes 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. Such titanium may be included in an amount of 0.001 wt% to 0.050 wt% based on the total weight of the steel plate 10 . When titanium is included in the content range, poor performance and coarsening of precipitates can be prevented, and physical properties of the steel can be easily secured, and defects such as cracks can be prevented on the steel surface. On the other hand, when the content of titanium exceeds 0.050 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 0.01 wt% to 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 excellent in the hot rolling and cold rolling process, and it prevents cracks in the slab and brittle fracture of the product during steelmaking/playing, It is possible to minimize the formation of precipitates.

Calcium (Ca) may be added to control the inclusion shape. Such calcium may be included in an amount of 0.003 wt% or less with respect to the total weight of the steel plate 10 .

After the hot rolling process and/or the cold rolling process and cooling to room temperature, residual stress exists in the steel sheet 10 of the hot stamping part manufactured through the hot stamping process. Here, the 'residual stress' means a stress that exists in the hot stamping part in a state where no external force acts on the steel sheet 10 .

Residual stresses can result from defects in the material. For example, point defects such as vacancy, interstitials, impurities, etc., line defects such as dislocations, etc. and external surfaces, grain boundaries, etc. , twin boundaries, stacking faults, and interfacial defects such as phase boundaries may be due to the generation of residual stress. That is, it can 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 (eg, tensile strength) and hydrogen embrittlement of the steel sheet 10 .

Specifically, the tensile strength of the hot stamping part is, when defects inside the steel sheet 10 exist at an appropriate level, the more defects (or the greater the residual stress), the higher the tensile strength, and the fewer the defects (or the smaller the residual stress). more), the tensile strength may be lowered. This is because the more defects there are, the more irregular the elements are, making it difficult to move dislocations that cause material deformation.

However, the hydrogen embrittlement of the steel sheet 10 may be decreased as the number of internal defects (or the larger residual stress) decreases, and the smaller the number of internal defects (or the smaller the residual stress) is, the better. In general, as there are more effective hydrogen trap sites therein, the amount of activated hydrogen is reduced, and hydrogen embrittlement of the product may be improved. For example, fine precipitates present therein (eg, nitrides or carbides of titanium (Ti), niobium (Nb), and vanadium (V)) function as effective hydrogen trap sites and serve to improve hydrogen embrittlement. On the other hand, defects existing therein may also serve as hydrogen trap sites. However, since 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. Accordingly, the defect does not function as an effective hydrogen trap site, but rather, the hydrogen embrittlement may be lowered by locally concentrating activated hydrogen in a portion having many defects (or a portion having a large residual stress). In particular, the hot stamping part may include at least one bent portion according to an applied position in the structure of the vehicle, and the bent portion is a portion that is formed excessively compared to a flat area during the hot stamping process. That is, since the stress caused by the press is relatively concentrated during the hot stamping process and the residual stress may increase, it may act as a hydrogen embrittlement weakness.

Accordingly, defects existing in the steel sheet 10 and residual stresses resulting therefrom need to be controlled to an appropriate level.

According to the embodiments of the present invention, by controlling the residual stress analysis value quantifying the residual stress present in the steel plate 10 to satisfy a preset condition, defects existing in the steel plate 10 and the residual stress resulting therefrom are appropriately adjusted. level can be controlled.

In one embodiment, the residual stress analysis value is the magnitude of the XRD value quantifying the residual stress by X-ray diffraction (XRD) (or the absolute value of the XRD value), and the backscattered electron diffraction pattern analysis (EBSD). ; It can be the product of the magnitude of the EBSD value (or the absolute value of the EBSD value) that quantifies the orientation with electron backscatter diffraction. In addition, the preset condition may be 2.85*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.05 (Degree*MPa/㎛ ² ) or less. More preferably, when the size of the XRD value is 5 MPa or more and less than 15 MPa, the residual stress analysis value satisfies the range of 2.95*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.01 (Degree*MPa/㎛ ² ) or less. is controlled, and when the size of the XRD value is 15 MPa or more and less than 55 MPa, the residual stress analysis value is controlled to satisfy the range of 9.31*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.035(Degree*MPa/㎛ ² ) or less. , when the size of the XRD value is 55 MPa or more and 70 MPa or less, the residual stress analysis value can be controlled to satisfy the range of 3.96*10 ^-3 (Degree*MPa/㎛ ² ) or more and 0.043(Degree*MPa/㎛ ² ) or less. .

'X-ray diffraction analysis (XRD)' is an analysis method for measuring residual stress 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 the crystal lattice. Specifically, the residual stress can be measured by the sin ² φ method. The sin ² φ method irradiates an X-ray to a measurement target to obtain the peak position of the diffraction line. At this time, the peak position of the changed diffraction line is taken as the vertical axis, and sin ² φ of the incident angle of the X-ray is taken as the horizontal axis, the slope is obtained by linear regression by the least squares method, and the obtained slope is multiplied by the stress constant obtained from the Young's modulus and Poisson's ratio. The stress value (XRD value) can be calculated|required by Formula 1.

[Equation 1]

σ=-E/2(1+v)*cotθ*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-sin ^2θ

2θ: Diffraction angle of no strain (°)

K: Stress constant (MPa)

Such X-ray diffraction analysis (XRD) has excellent representativeness because it targets a relatively wide range, but has a disadvantage in that the deviation is large and the uniformity is not good. In addition, the deviation of these 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 X-ray diffraction analysis (XRD).

On the other hand, 'backscattered electron diffraction pattern analysis (EBSD)' uses the diffraction pattern of a certain specimen to determine the crystallographic phase and crystallographic orientation, and based on this, the shape of the specimen microstructure ( It is a method to analyze by combining morphologic information and crystallographic information.

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 the direction of the specimen surface. This is called an electron back scattered diffraction pattern (EBSP), and this pattern responds to the crystal orientation of the area irradiated with the electron beam, and can measure the crystal orientation of a material with an accuracy of less than 1°.

Since such backscattered electron diffraction pattern analysis (EBSD) targets a relatively narrow range, it has an advantage of small deviation and good uniformity compared to X-ray diffraction analysis (XRD). However, the EBSD value, which quantifies the residual stress by backscattered electron diffraction pattern analysis (EBSD), also has a disadvantage in that it is not representative.

Embodiments of the present invention apply a differentiated residual stress analysis value in order to compensate for the disadvantages of the aforementioned X-ray diffraction analysis (XRD) and backscattered electron diffraction pattern analysis (EBSD), respectively. Specifically, as the residual stress analysis value, the magnitude of the XRD value (or the absolute value of the XRD value) in which the residual stress is quantified by X-ray diffraction analysis (XRD), and the orientation by the backscattered electron diffraction pattern analysis (EBSD) The product of the magnitude of the EBSD value (or the absolute value of the EBSD value) may be applied. Accordingly, the deviation, which is a disadvantage of the XRD value, is compensated for by the EBSD value, and the representativeness, which is a disadvantage of the EBSD value, is supplemented by the XRD value, so that residual stress can be analyzed and controlled more accurately.

For example, the residual stress analysis value may be expressed as in Equation 2 below.

[Equation 2]

Residual stress analysis value (Degree*MPa/㎛ ² )=|XRD value (MPa)|*|EBSD value (Degree/㎛) ² )|

Such residual stress analysis value may be substantially proportional to the defect in the hot stamping part and the resulting residual stress. Specifically, it can be understood that the larger the residual stress analysis value is, the more defects exist and the residual stress is large inside the product, and the smaller the residual stress analysis value is, the fewer defects exist inside the product and the residual stress is small. Furthermore, it can be understood that the higher the residual stress analysis value, the higher the tensile strength of the product, but the hydrogen embrittlement is not excellent, and the smaller the residual stress analysis value, the lower the tensile strength of the product but the hydrogen embrittlement is excellent. Therefore, by controlling the residual stress analysis value to satisfy the preset condition, it is possible to properly secure the mechanical properties and hydrogen embrittlement of the product.

On the other hand, due to the characteristics of the process of manufacturing a product through rolling and cooling from a high-temperature material, defects and residual stress may be caused by a temperature difference existing in the width direction or length direction of the steel sheet 10 during the manufacturing process. . According to embodiments of the present invention, by applying a differentiated process condition, for example, a heating condition and/or a cooling condition during the manufacturing process, the above-described residual stress analysis value may be controlled to satisfy a preset condition. A detailed description of these differentiated process conditions will be described later with reference to FIGS. 3 to 5 .

2 is a plan view illustrating a portion of a hot stamping part according to an embodiment of the present invention.

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

The martensitic phase is the result of diffusionless transformation of austenite γ below the onset temperature (Ms) of martensitic transformation during cooling. Martensite may have a rod-shaped lath phase oriented in one direction (d) in each initial grain of austenite.

In addition, the steel plate 10 may have iron-based carbide positioned inside the martensite phase. The iron-based carbide may be in the form of needles. In one embodiment, the diameter of the iron-based carbide may be less than 0.2㎛, and the length of the iron-based carbide may be less than 10㎛. Here, the 'diameter of the iron-based carbide' may mean a minor axis length of the iron-based carbide, and the 'length of the iron-based carbide' may mean a major axis length of the iron-based carbide.

If the diameter of the iron-based carbide is 0.2 μm or more or the length is 10 μm or more, it remains 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 reduced. On the other hand, when the diameter of the iron-based carbide is less than 0.2 μm and the length is less than 10 μm, the balance between strength and formability of the steel sheet 10 may be improved.

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

In one embodiment, as shown in FIG. 2 , the iron-based carbide may include a ferrous carbide (C1) and a ferric carbide (C2). The ferrous carbide C1 may be an iron-based carbide horizontal to the longitudinal direction d of the lath, and the ferric carbide C2 may be an iron-based carbide perpendicular to the longitudinal direction d of the lath. Here, 'horizontal' includes forming an angle of 0° or more and 20° or less with the longitudinal direction (d) of the lath, and 'vertical' refers to an angle of 70° or more and 90° or less with the longitudinal direction (d) of the lath. may include forming For example, the ferrous carbide (C1) may form an angle of 0° or more and 20° or less with the longitudinal direction (d) of the lath, and the ferric carbide (C2) is 70° or more with the longitudinal direction (d) of the lath. Angles of 90° or less can be achieved.

The iron-based carbide reference area fraction of the ferrous carbide C1 may be greater than the iron-based carbide reference area fraction of the ferric ferrous 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 ferrous carbide (C1) forming an angle of 0° or more and 20° or less with the longitudinal direction (d) of the lath may be 50% or more, preferably 60% or more. In addition, the iron-based carbide reference area fraction of the ferric carbide (C2) forming an angle of 70° or more and 90° or less with the longitudinal direction (d) of the lath may be less than 50%, preferably less than 40%.

Cracks generated during bending deformation may be generated as dislocations move in the martensite phase. At this time, it can be understood that as the local strain rate among the given plastic deformations has a larger value, the degree of energy absorption for the plastic deformation of martensite increases, so that the collision performance is improved.

On the other hand, the iron-based carbide reference area fraction of the ferrous carbide (C1) horizontal to the longitudinal direction (d) of the lath phase is higher than the iron-based carbide reference area fraction of the ferric carbide (C2) perpendicular to the longitudinal direction (d) of the lath phase. When formed large, dynamic strain aging (DSA) due to the local strain rate difference in the process of dislocation movement inside the lath phase during bending deformation, that is, indentation dynamic strain aging, may appear. . Indentation dynamic deformation aging is a concept of plastic deformation absorption energy, and since it means resistance to deformation, the more frequent the indentation dynamic deformation aging phenomenon, the better the resistance performance to deformation.

That is, according to this embodiment, the iron-based carbide reference area fraction of the ferrous carbide (C1) forming an angle of 20° or less with the longitudinal direction (d) of the lath is 50% or more, and the longitudinal direction (d) of the lath is formed. ) and the ferric carbide reference area fraction of ferric carbide (C2) forming an angle of 70° or more and 90° or less is formed to be less than 50%, so indentation dynamic deformation aging phenomenon may occur frequently, and through this, V- By securing a bending angle of 50° or more, bendability and collision performance can be improved.

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

On the other hand, the aforementioned acicular iron-based carbide may be precipitated inside the bainite phase. Since the iron-based carbide inside bainite increases the strength of bainite and reduces the strength difference between bainite and martensite, the yield ratio and bendability of the steel sheet 10 can be increased. In this case, the iron-based carbide may be present in an amount of less than 20% in the bainite phase, based on the bainite phase. If the iron-based carbide is 20% or more based on the bainite phase, voids may be generated, which may lead to a decrease in bendability.

3 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment of the present invention, and FIG. 4 is a case in which a blank is heated in multiple stages in the manufacturing method of a hot stamping part according to an embodiment of the present invention It is a graph showing the temperature change, and FIG. 5 is a graph showing the comparison of the temperature change when the blank is heated in multiple stages and when the blank is heated in single stages.

Referring to FIG. 3 , the method for manufacturing a hot stamping part according to an embodiment of the present invention may include a blank input step ( S110 ), a multi-stage heating step ( S120 ), and a crack heating step ( S130 ). In addition, the hot stamping part manufacturing method may further include a transfer step (S140), a forming step (S150), and a cooling step (S160) performed after the crack heating step (S130).

First, the blank input step ( S110 ) may be a step of inputting the blank into a heating furnace having a plurality of sections.

The blank input into the heating furnace may be formed by cutting a plate for forming a hot stamping part. The plate 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 annealing heat treated plate material. For example, the plating layer may be an Al-Si-based plating layer or a Zn plating layer.

Subsequently, the multi-stage heating step (S120) and the crack heating step (S130) may be sequentially performed. The blank introduced into the heating furnace may be heated while passing through a plurality of sections provided in the heating furnace. In one 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 of the heating furnace include sections in which the temperature range is increased stepwise from the inlet of the furnace into which the blank is input to the outlet of the furnace in which the blank is taken out, and sections in which the temperature range is maintained uniformly. .

The multi-stage heating step ( S120 ) is a step of heating the blank while passing the sections in which the temperature range is increased step by step among a plurality of sections provided in the heating furnace. The uniform heating step (S130) is a step of heating the multi-stage heated blank 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 input to the exit direction of the furnace where the blank is taken out to the target temperature (Tt) range, and then increases the target temperature (Tt) range. It can be maintained in a uniform temperature range, that is, the target temperature (Tt) range from the section having the furnace to the exit of the heating furnace. In this case, the number of sections in which the temperature range is increased in stages, the number of sections in which the temperature range is uniformly maintained, and the temperature range of each section are not limited.

In one embodiment, as shown in FIG. 4 , the heating furnace includes a first section P1 having a first temperature range T1 , a second section P2 having a second temperature range T2 , and a third A third section P3 having a 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 temperature range A sixth section P6 having T6 and a seventh section P7 having a seventh temperature range T7 may be provided. In another embodiment, unlike shown in FIG. 4 , the heating furnace may have 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 the first temperature range (T1) is adjacent to the inlet of the furnace into which the blank is put, and the seventh section (P7) having the seventh temperature range (T7) is the furnace through which the blank is discharged. may be adjacent to the exit of That is, the first section P1 having the first temperature range T1 may be the first section among a plurality of sections included in the heating furnace, and the seventh section P7 having the seventh temperature range T7. It may be the last section among a plurality of sections provided in this 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 one 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 the sixth section P6 and the seventh section P7 may be maintained in the same temperature range as the target temperature Tt range, which is the temperature range of the fifth section P5 . However, it 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 0°C or more and 100°C or less. For example, the temperature difference between the first section P1 and the second section P2 may be 0 °C or more and 100 °C or less.

In an embodiment, the first temperature range T1 of the first section P1 may be 840 °C to 860 °C, and 835 °C to 865 °C. The second temperature range T2 of the second section P2 may be 870 °C to 890 °C, and 865 °C to 895 °C. The third temperature range T3 of the third section P3 may be 900 °C to 920 °C, and 895 °C to 925 °C. The fourth temperature range T4 of the fourth section P4 may be 920 °C to 940 °C, or 915 °C to 945 °C. The fifth temperature range T5 of the fifth section P5 may be Ac3 to 1,000 °C. Preferably, the fifth temperature range T5 of the fifth section P5 may be 930 °C or more and 1,000 °C or less. More preferably, the fifth temperature range T5 of the fifth section P5 may be 950 °C or more and 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 multi-stage heating step (S120) is performed in the first section (P1) to the fourth section (P4), and the uniform heating step (S130) is performed in the fifth section (P5) to the seventh section (P7). can By providing the section in which the crack heating step (S130) is performed as a plurality of sections, for example, the fifth section (P5) to the seventh section (P7), not a single section, the temperature difference occurring within the section can be prevented or minimized.

The uniform heating step 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, and may be a temperature of Ac3 or higher. That is, in the crack heating step ( S130 ), the blank heated in multiple stages passing through the first section ( P1 ) to the fourth section ( P4 ) can be crack-heated at a temperature of Ac3 or higher. Preferably, in the crack heating step (S130), the multi-stage heated blank may be crack-heated at a temperature of 930 °C or more and 1,000 °C or less. More preferably, in the crack heating step (S130), the multi-stage heated blank may be crack-heated at a temperature of 950 °C or more and 1,000 °C or less.

In one embodiment, the heating furnace may have a length of 20 m to 40 m along the transport path of the blank. The heating furnace may have a plurality of sections having different temperature ranges, the length of the section for multi-stage heating of the blank among the plurality of sections (D1, see FIG. 4) and the section for crack heating the blank among the plurality of sections The ratio of the length (D2, see FIG. 4 ) of may satisfy 1:1 to 4:1. That is, the length D2 of the uniform heating section among the plurality of sections provided in the heating furnace may have a length corresponding to 20% to 50% of the total length (D1+D2) of the heating furnace.

When the ratio of the length (D1) of the section heating the blank in multiple stages (D1) to the length of the section heating the blank by cracking (D2) exceeds 1:1 due to the increase in the length of the section for crack heating the blank, austenite in the crack heating section (FCC) tissue may be generated, which may increase the amount of hydrogen permeation into the blank, resulting in increased delayed rupture. In addition, when the ratio of the length (D1) of the section for heating the blank in multiple stages (D1) to the length (D2) of the section for heating the blank by cracking is less than 4:1 because the length of the section for crack heating the blank is reduced, the crack heating section (time) This is not sufficiently ensured, so the strength of the hot stamping part manufactured by the manufacturing process of the hot stamping part may be non-uniform.

In one embodiment, in the multi-stage heating step (S120) and the crack heating step (S130), the blank may have a temperature increase rate of about 6 °C/s to 12 °C/s, and the cracking time is about 3 minutes to It can be 6 minutes. More specifically, when the thickness of the blank is about 1.6 mm to 2.3 mm, the temperature increase rate is about 6 °C/s to 9 °C/s, and the soaking time may be about 3 to 4 minutes. In addition, when the thickness of the blank is about 1.0 mm to 1.6 mm, the temperature increase rate is about 9 °C / s to 12 °C / s, and the soaking time may be about 4 to 6 minutes.

The temperature change when the blank (B') is single-heated and when the blank (B) is heated in multiple stages will be described with reference to FIG. 5 .

As a comparative example, it can be assumed that the blank B' is single heated. In a single heating step, the temperature of the furnace is set so that the internal temperature of the 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 930 °C. More preferably, the target temperature (Tt) of the blank (B') may be 950 °C.

The temperature of the blank B' in the single heating step may reach the target temperature Tt faster than the temperature of the blank B in the multi-step heating step. For example, the temperature increase rate of the blank (B') in a single heating step may be faster than the temperature increase rate of the blank (B) in a multi-stage heating stage by about 2°C/s or more. Since the single heating step reaches the target temperature Tt faster than the multi-stage heating step, the cracking time ET2 of the single heating step may be formed longer than the cracking time ET1 of the multi-step heating step. As in the case of a single heating step, if the cracking time ET2 is long, the size of the grain boundary is not formed uniformly, and the aforementioned defects may be excessively formed more than necessary.

Accordingly, in the method of manufacturing a hot stamping part according to an embodiment of the present invention, by delaying the time for the blank to reach the target temperature (Tt) through a multi-stage heating method to secure an appropriate cracking time (ET1), the uniformity of grain boundary size can be secured and controlled to form an appropriate level of defects. Therefore, the hot stamping parts manufactured by applying the multi-stage heating method can be controlled to have defects and residual stress within a preset range, and whether the preset range is satisfied can be checked through the residual stress analysis value described above.

Referring back to FIG. 3 , after the crack heating step ( S130 ), the transferring step ( S140 ), the forming step ( S150 ), and the cooling step ( S160 ) may be further performed.

The transferring step ( S140 ) may be a step of transferring the heated blank from the heating furnace to the press mold. In the step of transferring the heated blank from the heating furnace to the press mold, the heated blank may be air-cooled for 10 to 15 seconds.

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

After being molded into a final part shape in a 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 the plate material and maintain a desired shape, rapid cooling may be performed while pressurizing the press die in a closed state. That is, the forming process (or forming step, S150) and the cooling process (or cooling step, S160) may be simultaneously performed while the blank is disposed in the press mold.

In one embodiment, in performing the forming process and the cooling process for the heated blank, the blank is a preset time in the press mold at a temperature below the temperature at which martensitic transformation starts (MS temperature), for example, 3 seconds to 20 It can be held for seconds. In addition, the blank may be cooled while maintaining the average cooling rate at 15 °C/s or more until the temperature at which martensitic transformation is terminated (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 part can be prevented. Residual stress inside the product has the effect of reducing

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 part. In addition, if the time the blank is maintained in the press mold exceeds 20 seconds, more than necessary defects and residual stress resulting therefrom may occur, and the retention time in the press mold may be prolonged, thereby reducing productivity.

In one embodiment, the tensile strength of the hot stamping part manufactured by the manufacturing method of the hot stamping part may be 1350 MPa or more, and the amount of activated hydrogen may be 0.7 wppm or less.

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

Psalter 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 the results of measuring the XRD value, tensile strength, and amount of activated hydrogen for each of specimens A-1 to A-9. Specifically, Table 1 confirms whether the size of the XRD value measured for the specimens satisfies the range of 5 MPa or more and 70 MPa or less, and compares the tensile strength and the amount of activated hydrogen in the case of satisfying the range and the case of dissatisfaction Data for analysis are presented.

The XRD value is a value obtained by quantifying the residual stress by the aforementioned X-ray diffraction analysis (XRD). The XRD value was measured by removing the coating layer of the specimen and irradiating X-rays after electropolishing to a target position (eg, 1/4 point). In addition, the electrolytic polishing is an electrolytic polishing solution containing 5% of 2-butoxyethanol (2-Butoxyethanol), 20% of perchloric acid, 35% of ethanol (Ethanol) and 40% of water (water) was performed with

The amount of activated hydrogen can be measured using a thermal desorption spectroscopy method. The heating degassing analysis method is to measure the amount of hydrogen released from the specimen at a specific temperature or lower while heating the specimen at a preset heating rate to increase the temperature. It can be understood as activated hydrogen that affects destruction. That is, if the amount of hydrogen measured as a result of thermal degassing analysis is large, it means that a large amount of activated hydrogen that can cause delayed destruction of uncaptured hydrogen 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 350 °C or less while raising the temperature from room temperature to 500 °C at a heating rate of 20 °C/min for each of the specimens.

Specimens A-1 to A-5 satisfy the range of the measured XRD value of 5 MPa or more and 70 MPa or less. That is, it can be understood that an appropriate level of defects and residual stress are present in the specimens A-1 to A-5. Accordingly, it can be confirmed that the tensile strength of specimens A-1 to A-5 satisfies 1350 MPa or more, and the activated hydrogen amount of 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 can be seen that defects exist less than the required level inside specimens A-6 and A-7, and the resulting residual stress is too small. Accordingly, it can be seen that the amount of activated hydrogen of 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 can be seen that defects exist more than necessary inside specimens A-8 and A-9, and the resulting residual stress is too large. Accordingly, the tensile strength of each of specimens A-8 and A-9 satisfies 1350 MPa or more, while the amount of activated hydrogen exceeds 0.7 wppm, confirming that hydrogen embrittlement is reduced.

On the other hand, referring to Table 1, it can be seen that as the size of the XRD value increases, the deviation of the XRD value also tends to increase. That is, the greater the internal stress is, the greater the error range of the XRD value, and the greater the need to correct it.

Psalter EBSD value
(degree/㎛ ² ) 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 measuring the EBSD value, tensile strength, and amount of activated hydrogen for each of specimens B-1 to B-8. Specifically, Table 1 confirms whether the size of the EBSD value measured for the specimens satisfies the range of 5.71*10 ^-5 (degree/㎛2) or more and 7.14*10 ^-4 (degree/㎛2) or less, and , shows data for comparative analysis of tensile strength and activated hydrogen amount, respectively, when satisfying and dissatisfied with the range.

The EBSD value is a value obtained by quantifying the orientation by the above-described backscattered electron diffraction pattern analysis (EBSD; electron backscatter diffraction). The EBSD value was measured by scanning a sample 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.

Specimens B-1 to B-4 satisfy the range of the measured EBSD value of 5.71*10 ^-5 (degree/μm2) or more and 7.14*10 ^-4 (degree/μm2) or less. That is, it can be understood that an appropriate level of defects and residual stress are present in the specimens B-1 to B-4. Accordingly, it can be confirmed that the tensile strength of specimens B-1 to B-4 satisfies 1350 MPa or more, and the activated hydrogen amount of specimens B-1 to B-4 satisfies 0.7 wppm or less.

On the other hand, in the case of specimens B-5 and B-6, the size of the measured EBSD value is less than 5.71*10 ^-5 (degree/㎛2). In other words, it can be seen that there are fewer defects than necessary inside specimens B-5 and B-6, and the resulting residual stress is too small. Accordingly, it can be seen that the amount of activated hydrogen of each of specimens B-5 and B-6 satisfies 0.7 wppm or less, while the tensile strength is less than 1350 MPa.

In addition, in the case of specimens B-7 and B-8, the size of the measured EBSD value exceeds 7.14*10 ^-4 (degree/㎛2). That is, it can be seen that defects exist more than necessary inside specimens B-7 and B-8, and the resulting residual stress is too large. Accordingly, the tensile strength of each of specimens B-7 and B-8 satisfies 1350 MPa or more, while the amount of activated hydrogen exceeds 0.7 wppm, confirming that hydrogen embrittlement is reduced.

Psalter XRD value
(MPa) EBSD value
(degree/㎛ ² ) Residual stress analysis value
(MPa * degree/㎛ ² ) Seal
burglar
(MPa) activate
amount of hydrogen
(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-breaking C-2 -13.8±9.5 7.14*10 ^-4 ±0.007 9.85*10 ^-3 1472 0.591 non-breaking C-3 -27.2±8.8 3.09*10 ^-4 ±0.003 8.40*10 ^-3 1398 0.635 non-breaking C-4 -48.5±17.3 7.14*10 ^-4 ±0.012 3.46*10 ^-2 1495 0.661 non-breaking C-5 -55.1±15.7 6.70*10 ^-5 ±0.010 3.69*10 ^-3 1461 0.689 non-breaking C-6 -69.8±18.3 6.14*10 ^-4 ±0.011 4.29*10 ^-2 1480 0.692 non-breaking C-7 -5.1±3.5 5.71*10 ^-5 ±0.09 2.91*10 ^-4 1367 0.588 break 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 break C-10 -50.5±16.4 7.14*10 ^-4 ±0.09 3.61*10 ^-2 1474 0.672 break C-11 -54.9±15.7 6.61*10 ^-5 ±0.10 3.63*10 ^-3 1435 0.668 break C-12 -68.9±18.5 6.31*10 ^-4 ±0.07 4.35*10 ^-2 1513 0.696 break

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 value, EBSD value, tensile strength, and amount of activated hydrogen were measured under the same conditions and methods as in Tables 1 and 2. In addition, the residual stress analysis value was calculated as the product of the magnitude (or absolute value) of the XRD value and the magnitude (or absolute value) of the EBSD value.

The 4-point bending test is a test method to check whether stress corrosion cracking occurs by applying a stress level below the elastic limit to a specific point on a specimen manufactured by reproducing the state in which the specimen is exposed to a corrosive environment. At this time, stress corrosion cracking means a crack that occurs when corrosion and continuous tensile stress act simultaneously.

Specifically, the results of the four-point bending test in Table 1 are results of confirming whether fracture occurs by applying a stress of 1,000 MPa in air for 100 hours to each of the specimens.

According to the embodiments of the present invention, by applying the product of the magnitude of the XRD value and the magnitude of the EBSD value as the residual stress analysis value, by mutually correcting inaccurate information of each of the XRD value and the EBSD value, the residual stress inside the product is more It can be accurately analyzed and controlled. Specifically, the residual stress analysis value is controlled to satisfy the range of 2.85*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.05 (Degree*MPa/㎛ ² ) or less.

On the other hand, referring to the XRD values in Table 3, it can be seen that the deviation of the XRD values also tends to increase as the size of the XRD values increases. That is, the greater the internal stress is, 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 pronounced.

In consideration of this, the residual stress analysis value can be more precisely controlled according to the range of the XRD value. Specifically, when the size of the XRD value is 5 MPa or more and less than 15 MPa, the residual stress analysis value is controlled to satisfy the range of 2.95*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.01(Degree*MPa/㎛ ² ) or less, and , when the size of the XRD value is 15 MPa or more and less than 55 MPa, the residual stress analysis value is controlled to satisfy the range of 9.31*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.035(Degree*MPa/㎛ ² ) or less, and XRD When the magnitude of the value is 55 MPa or more and 70 MPa or less, the residual stress analysis value may be controlled to satisfy the range of 3.96*10 ^-3 (Degree*MPa/㎛ ² ) or more and 0.043(Degree*MPa/㎛ ² ) or less.

Specimens C-1 to C-6 are hot stamping parts manufactured through steps S110 to S160 by applying the above-described process conditions. That is, the specimens C-1 to C-6 are subjected to the conditions applied to the multi-stage heating step (S120) and the uniform heating step (S130) described above, and the temperature (Mf) at which the martensitic transformation of the blank is terminated in the cooling step (S160). Specimens manufactured by applying an average cooling rate of 15 °C/s or more to the temperature), and maintaining them for 3 to 20 seconds in a press mold at a temperature below the temperature at which martensite transformation starts (MS temperature).

Accordingly, in specimens C-1 to C-6, the size of the measured XRD value satisfies the range of 5 MPa or more and 70 MPa or less, and the size of the measured EBSD value is 5.71*10 ^-5 (degree/㎛2) or more and 7.14*10 It satisfies the range of ^-4 (degree/μm2) or less. In addition to this, the residual stress analysis values (the product of the size of the XRD value and the size of the EBSD value) of specimens C-1 to C-6 are also 2.85*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.05 (Degree*MPa) /㎛ ² ) satisfies the following ranges.

More specifically, in specimens C-1 and C-2, the size of the XRD value is 5 MPa or more and less than 15 MPa, and the residual stress analysis value is 2.95*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.01 (Degree*MPa/ μm ² ) satisfies the following ranges. In addition, in specimens C-3 and C-4, the size of the XRD value was 15 MPa or more and less than 55 MPa, and the residual stress analysis value was 9.31*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.035 (Degree*MPa/㎛ ² ) The following ranges are satisfied. In addition, specimens C-5 and C-6 had an XRD value of 55 MPa or more and 70 MPa or less, and a residual stress analysis value of 3.96*10 ^-3 (Degree*MPa/㎛ ² ) or more and 0.043 (Degree*MPa/㎛ ² ) The following ranges are satisfied.

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. It can be understood that there is a defect and the resulting residual stress. Accordingly, it can be confirmed that the tensile strength of specimens C-1 to C-6 satisfies 1350 MPa or more, and the activated hydrogen content satisfies 0.7 wppm or less. In addition, it can be confirmed that the specimens C-1 to C-6 did not break as a result of the four-point bending test. That is, specimens C-1 to C-6 were manufactured by applying the above-described process conditions, and by controlling the residual stress analysis value to satisfy preset conditions, appropriate levels of tensile strength and hydrogen embrittlement were secured.

Meanwhile, specimens C-7 to C-12 are hot stamping parts manufactured by applying different process conditions to at least some of the above-described process conditions.

Referring to Table 3, for specimens C-7 to C-12, the size of the measured XRD value satisfies the range of 5 MPa or more and 70 MPa or less, and the size of the measured EBSD value is 5.71*10 ^-5 (degree/㎛2) It satisfies the range of 7.14*10 ^-4 (degree/㎛2) or less. Accordingly, it can be confirmed that the tensile strength of specimens C-7 to C-12 satisfies 1350 MPa or more, and the activated hydrogen amount of specimens C-7 to C-12 satisfies 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 5 MPa or more and less than 15 MPa, and the residual stress analysis value is less than 2.95*10 ^-4 (Degree*MPa/㎛ ² ). That is, it can be understood that there are fewer defects in the specimen C-7 than necessary and the resulting residual stress is too small. Accordingly, it can be confirmed that the specimen C-7 was fractured as a result of the four-point bending test.

Specimen C-8 has an XRD value of 5 MPa or more and less than 15 MPa, and the residual stress analysis value exceeds 0.01 (Degree*MPa/㎛ ² ). That is, it can be understood that defects exist more than necessary inside specimen C-8, and the resulting residual stress is too large. Accordingly, it can be confirmed that the specimen C-8 was fractured as a result of the four-point bending test.

Specimen C-9 has an XRD value of 15 MPa or more and less than 55 MPa, and the residual stress analysis value is less than 9.31*10 ^-4 (Degree*MPa/㎛ ² ). That is, it can be understood that there are fewer defects in the inside of specimen C-9 than necessary, and the resulting residual stress is too small. Accordingly, it can be confirmed that the specimen C-9 was fractured as a result of the four-point bending test.

Specimen C-10 has an XRD value of 15 MPa or more and less than 55 MPa, and the residual stress analysis value exceeds 0.035 (Degree*MPa/㎛ ² ). That is, it can be understood that defects exist more than necessary inside specimen C-10, and the resulting residual stress is excessively large. Accordingly, it can be confirmed that the specimen C-10 was fractured as a result of the four-point bending test.

Specimen C-11 has an XRD value of 55 MPa or more and 70 MPa or more, and the residual stress analysis value is less than 3.96*10 ^-3 (Degree*MPa/㎛ ² ). That is, it can be understood that there are fewer defects in the specimen C-11 than the required level, and the resulting residual stress is too small. Accordingly, it can be confirmed that the specimen C-11 was fractured as a result of the four-point bending test.

Specimen C-12 has an XRD value of 55 MPa or more and 70 MPa or more, and the residual stress analysis value exceeds 0.043 (Degree*MPa/㎛ ² ). That is, it can be understood that defects exist more than necessary inside specimen C-12, and the resulting residual stress is too large. Accordingly, it can be confirmed that the specimen C-12 was fractured as a result of the four-point bending test.

Specimens C-7 to C-12 were fractured as a result of a four-point bending test because the residual stress analysis value did not satisfy the preset conditions, although each of the size of the XRD value and the size of the EBSD value satisfies the preset conditions. It can be understood that it is difficult to completely control defects inside hot stamping parts and the resulting residual stresses only by XRD analysis or EBSD analysis.

On the other hand, if the residual stress analysis value satisfies the preset condition as in specimens C-1 to C-6, the result of the four-point bending test did not break, and the residual stress analysis value determines the internal defects of hot stamping parts and It can be confirmed that the residual stress can be analyzed and controlled more accurately.

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

10: steel plate
C1: ferrous carbide
C2: ferric carbide

Claims (20)

In the hot stamping part whose residual stress analysis value satisfies a preset condition,
The residual stress analysis value is the size of the XRD value quantifying the residual stress by X-ray diffraction analysis (XRD), and EBSD quantifying the orientation by electron backscatter diffraction (EBSD) is the product of the magnitudes of the values,
The preset condition is 2.85*10 ^-4 (Degree*MPa/㎛ ² ) or more and 0.05 (Degree*MPa/㎛ ² ) or less, a hot stamping part. According to claim 1,
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;
A hot stamping part comprising a. 3. The method of claim 2,
The iron-based carbide is in the form of needles,
wherein the needle shape is less than 0.2 μm in diameter and less than 10 μm in length. 3. The method of claim 2,
The martensite phase includes a lath phase,
The iron-based carbide includes a ferrous carbide horizontal to the longitudinal direction of the lath, and a ferric carbide perpendicular to the longitudinal direction of the lath,
The ferrous carbide reference area fraction of the ferrous carbide is greater than the ferrous carbide reference area fraction of the ferric carbide. 5. The method of claim 4,
The ferrous carbide is
An angle formed with the longitudinal direction of the lath is 0° or more and 20° or less, and the iron-based carbide reference area fraction is 50% or more, a hot stamping part. 5. The method of claim 4,
The ferric carbide is
An angle formed with the longitudinal direction of the lath is 70° or more and 90° or less, and the iron-based carbide reference area fraction is less than 50%, a hot stamping part. delete delete delete delete delete delete delete delete delete delete delete delete delete delete

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