KR20210116386A - Hot stamping component and method of manufacturing the same - Google Patents

Abstract

One embodiment of the present invention discloses a manufacturing method of a hot stamping component, comprising the following steps of: inserting a blank into a furnace which has a plurality of spaces with different temperature ranges; heating the blank by stages in multi-stage heating steps; and heating the blank with temperature of Ac3 to 1,000℃ in a soaking step. In the multi-stage heating steps, a temperature condition inside the furnace satisfies the following equation. In the equation, 0 < (T_g - T_i) / L_t < 0.025 ℃/mm, wherein T_g is a soaking temperature (℃), T_i is an entrance temperature (℃) of the furnace, and L_t is a multi-stage heating length (mm). The present invention prevents or minimizes quality difference between the blanks.

Description

Hot stamping component and method of manufacturing the same

The present invention relates to a hot stamped part and a method of manufacturing the same.

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

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

No. 10-2018-0095757

In the embodiments of the present invention, even when at least two blanks, tailor welded blanks, or Taylor rolled blanks having different thicknesses and at least one size are simultaneously heated in a furnace, a quality difference between the blanks occurs. To provide a hot stamping part capable of preventing or minimizing the occurrence, and a method for manufacturing the same.

An embodiment of the present invention, the step of introducing a blank into a heating furnace having a plurality of sections having different temperature ranges; a multi-stage heating step of heating the blank step by step; and a crack heating step of heating the blank to a temperature of Ac3 to 1,000° C., wherein the temperature condition in the heating furnace in the multi-stage heating step satisfies the following equation. Discloses a method of manufacturing a hot stamping part.

<Equation>

0 < (Tg - Ti) / Lt < 0.025℃/mm

(In the above formula, Tg is the crack heating temperature (°C), Ti is the furnace initial temperature (°C), and Lt is the multi-stage heating length (mm))

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

In this embodiment, at least two blanks having different thicknesses may be simultaneously transferred into the heating furnace.

In this embodiment, the blank may include a first portion having a first thickness and a second portion having a second thickness different from the first thickness.

In this embodiment, the temperature of the plurality of sections may increase from the inlet of the heating furnace to the exit direction of the heating furnace.

In this embodiment, the temperature difference between two adjacent sections of the section heating the blank in multiple stages may be greater than 0°C and less than or equal to 100°C.

In this embodiment, the temperature of the section heating the blank by cracking among the plurality of sections may be higher than the temperature of the section heating the blank in multiple stages.

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

In this embodiment, after the crack heating step, transferring the crack-heated blank from the heating furnace to the press mold; forming a molded body by hot stamping the transferred blank; and cooling the formed body.

In this embodiment, in the step of transferring the crack-heated blank from the heating furnace to the press mold, the crack-heated blank may be air-cooled for 10 seconds to 15 seconds.

Another embodiment of the present invention discloses a hot stamping part having a diffusible hydrogen content of less than 0.45 ppm and a corrosion rate measured through an isostatic polarization test of 3 x 10 ^{-6 A or less.}

In this embodiment, the hot stamping part may have a tensile strength of 500 MPa or more and less than 800 MPa, and may have a composite structure of ferrite and martensite.

In this embodiment, the hot stamping part may have a tensile strength of 800 MPa or more and less than 1,200 MPa, and may have a composite structure of bainite and martensite.

In this embodiment, the hot stamping part may have a tensile strength of 1,200 MPa or more and less than 2,000 MPa, and may have a full martensitic structure.

According to embodiments of the present invention, by heating the blanks in multiple stages in a heating furnace having a plurality of sections having different temperature ranges, it is possible to more precisely control the time to reach the cracking temperature of the blanks.

In addition, by more precisely controlling the time to reach the cracking temperature of blanks having different thicknesses, it is possible to improve hydrogen embrittlement, corrosion resistance, and weldability of parts manufactured by the method for manufacturing hot stamping parts.

1 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment of the present invention.
2 is a plan view schematically illustrating a blank used in a method of manufacturing a hot stamping part according to an embodiment of the present invention.
3 is a plan view schematically illustrating a blank inserted into a heating furnace in a method of manufacturing a hot stamping part according to an embodiment of the present invention.
4 is a graph showing the temperature change of the blank when the blank is single-heated by the conventional method.
5 is a graph illustrating a temperature change when a blank is heated in multiple stages and heated by cracking in the method of manufacturing a hot stamping part according to an embodiment of the present invention.
6 is a graph showing the high-temperature tensile properties of the heated blank according to the molding start temperature.
7 is a graph illustrating a temperature change when a blank is heated in multiple stages and heated by cracking in the method for manufacturing a hot stamping part according to an embodiment of the present invention.
8 is a graph showing the hydrogen release rate from parts manufactured according to the conditions of Example, Comparative Example 1, and Comparative Example 2;
9 is a graph showing the evaluation results of corrosion resistance of parts manufactured according to the conditions of Examples, Comparative Example 1, and Comparative Example 2;
10 is a graph showing resistance values for parts manufactured according to the conditions of Example, Comparative Example 1, and Comparative Example 2;

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 methods 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, it is not only when it is directly on the other part, but also another film, region, component, etc. is interposed therebetween. Including cases where there is

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.

Where certain embodiments are otherwise feasible, 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, and the same or corresponding components will be given the same reference numerals when described with reference to the drawings.

1 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment of the present invention. Hereinafter, a method of manufacturing a hot stamping part will be described with reference to FIG. 1 .

The method of 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), and after the crack heating step (S130) , a transfer step (S140), a forming step (S150), and a cooling step (S160) may be further included.

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

The blank input into the heating furnace may be formed by cutting a plate for forming a hot stamping part. 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, an Al-Si-based plating layer or a Zn plating layer may be formed on at least one surface of the sheet material subjected to the annealing heat treatment.

2 is a plan view schematically illustrating a blank used in a method of manufacturing a hot stamping part according to an embodiment of the present invention.

Referring to FIG. 2 , the blank 200 according to an embodiment is a blank 210 having a single thickness, and a Taylor welded blank 220 obtained by cutting and welding heterogeneous plates having different thicknesses into a required shape. Blank, TWB), a tailor rolled blank 230 (Tailor Rolled Blank, TRB) having different thicknesses by rolling a single-thickness plate, and a patchwork 240 manufactured by welding a small patch blank to a large blank (Patchwork) ) may include at least one of.

The Taylor welded blank 220 may be manufactured by welding the first plate 221 and the second plate 223 having different thicknesses. The B-pillar, which is an important component for collision members of a vehicle, is a form in which plates of different strengths are combined in an upper collision support part and a lower shock absorber, and is manufactured by welding the two plates and then forming them. The Taylor welded blank method, which is mainly used at this time, refers to a series of processes for manufacturing parts by cutting and welding different types of sheets with different thicknesses, strengths, and materials to the required shape, and then press forming to manufacture parts. By welding blanks having different thicknesses, it is possible to have different properties for each part of the blank. For example, 120~200K grade ultra-high-strength plates are used for the collision support part of the upper part of the B-pillar, and the lower part of the B-pillar, where stress is concentrated, is connected to a plate with good shock absorption performance to improve the shock absorption ability in the event of a vehicle collision. can do it

The Taylor rolled blank 230 may be manufactured by rolling a cold-rolled steel material to have a specific thickness profile, and the weight reduction effect is excellent when manufacturing hot stamping parts using the Taylor rolled blank 230 . For example, the thickness profile may be performed by a conventional method. For example, when cold-rolling a steel material in a cold-rolled state, a first region 231 having a first thickness, a second region 232 having a second thickness, and a third region 233 having a third thickness by adjusting the reduction ratio ), and a Taylor rolled blank 230 including a fourth region 234 having a fourth thickness may be formed. In this case, the first thickness, the second thickness, the third thickness, and the fourth thickness may be different from each other, and between the first region 231 and the second region 232 , the second region 232 and the third region A transition period 235 may exist between 233 and between the third region 233 and the fourth region 234 . However, although FIG. 2 illustrates that the Taylor rolled blank 230 includes the first region 231 to the fourth region 234 , the present invention is not limited thereto. The Taylor rolled blank 230 includes a first area 231 , a second area 232 , . . . , may be formed to include an n-th region.

The patchwork 240 is a method of partially reinforcing the base material using at least two or more plate materials, and the patch is bonded to the base material before the forming process so that the base material and the patch can be formed at the same time. For example, after the patch 243 having a second size smaller than the first size is welded to the base material 241 having the first size, it may be simultaneously molded.

3 is a plan view schematically illustrating a blank inserted into a heating furnace in a method of manufacturing a hot stamping part according to an embodiment of the present invention.

In the blank input step (S100), at least two blanks 200 having different at least one of thickness and size may be simultaneously introduced into the heating furnace.

As an example, FIG. 3 shows two first blanks 250 and two second blanks 260 that are simultaneously introduced into the heating furnace. In this case, the first blank 250 and the second blank 260 may have different sizes and different thicknesses. For example, the first blank 250 may have a thickness of 1.2 mm, and the second blank 260 may have a thickness of 1.6 mm. However, the present invention is not limited thereto, and one first blank 250 and one second blank 260 may be simultaneously introduced into the heating furnace. In addition, the first blank 250 and the second blank 260 have the same size but different thicknesses, or are formed to have different sizes with the same thickness, and various modifications are possible.

In another embodiment, in the blank input step (S100), at least two blanks 200 having a single thickness into the heating furnace may be simultaneously input. For example, at least two or more blanks 250 having a thickness of 1.2 mm may be simultaneously fed, and at least two or more blanks 260 having a thickness of 1.6 mm may be fed simultaneously. In addition, in the blank input step (S100), the above-described Taylor welded blank (220, see FIG. 2) or the Taylor rolled blank (230, see FIG. 2) may be introduced into the heating furnace.

The blank introduced into the heating furnace may be transferred along a transfer direction after being mounted on a roller.

After the blank input step (S110), a multi-stage heating step (S120) and a crack heating step (S130) may be performed. The multi-stage heating step ( S120 ) and the crack heating step ( S130 ) may be a step in which the blank is heated while passing through a plurality of sections provided in the heating furnace.

Specifically, in the multi-stage heating step (S120), the blank may be heated step by step while passing through a plurality of sections provided in the heating furnace. A plurality of sections in which the multi-stage heating step ( S120 ) is performed among a plurality of sections provided in the heating furnace may exist, and each section increases from the entrance of the heating furnace into which the blank is input to the exit direction of the heating furnace where the blank is taken out. A temperature can be set to raise the temperature of the blank step by step.

A crack heating step (S130) may be performed after the multi-stage heating step (S120). In the crack heating step (S130), the multi-stage heated blank may be heat-treated while passing through the section of the furnace set at a temperature of Ac3 to 1,000°C. Preferably, in the crack heating step (S130), the multi-stage heated blank may be crack-heated at a temperature of 930 °C to 1,000 °C. More preferably, in the crack heating step (S130), the multi-stage heated blank may be crack-heated at a temperature of 950°C to 1,000°C. In addition, there may be at least one section in which the crack heating step ( S130 ) is performed among a plurality of sections provided in the heating furnace.

4 is a graph showing the temperature change of the blank when the blank is single-heated by the conventional method. Specifically, FIG. 4 shows a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm after setting the temperature of the furnace so _{that the internal temperature of the furnace is maintained equal to the target temperature (T t ) of the blank.} In the case of a single heating 320 at the same time, it is a graph showing the temperature change of these blanks with time.

In this case, the target temperature (T _t ) of the blank may be Ac3 or more. Preferably, the target temperature (T _t ) of the blank may be 930 °C. More preferably, the target temperature (T _t ) of the blank may be 950 °C. However, the present invention is not limited thereto. In addition, single heating is not heating by putting a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm into the furnace, but setting the temperature of the furnace to a single temperature, and then setting the temperature of the furnace to a single temperature. It means a case in which a blank having a thickness and a blank having a thickness of 1.6 mm are simultaneously input and heated.

Referring to FIG. 4 , after setting the temperature inside the heating furnace to the _{same temperature as the target temperature (T t} ) of the blank, a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm are single-heated at the same time , it can be seen that the blank with a thickness of 1.2 mm reaches the target temperature Tt first compared to the blank with a thickness of 1.6 mm.

That is, the blank with a thickness of 1.2 mm first _{reaches the target temperature (T t} ), the blank with a thickness of 1.2 mm is _{crack-heated for the first time (S 1} ), and the blank with a thickness of 1.6 mm is the first Crack heating may be performed for a second time period (S ₂ ) shorter than 1 hour (S _{1 ).} Since the crack heating time is adjusted based on the blank reaching the target temperature late, the blank with a thickness of 1.2 mm that has reached the target temperature first is overheated, and delayed fracture of the blank with a thickness of 1.2 mm increases, and weldability is improved. may be lowered.

5 is a graph illustrating a temperature change when a blank is heated in multiple stages and heated by cracking in the method of manufacturing a hot stamping part according to an embodiment of the present invention. 5 is a multi-stage heating 330 of a blank having a thickness of 1.2 mm, and a multi-stage heating 340 of a blank having a thickness of 1.6 mm according to an embodiment of the present invention. It is a graph.

Referring to FIG. 5 , a heating furnace according to an embodiment may include a plurality of sections having different temperature ranges. More specifically, the heating to the first temperature range, the first section having a _{(T 1) (P 1)} , a second temperature range, the second section having a _{(T 2) (P 2)} , the third temperature (T ₃ ) to have a third region (P _3), the fourth temperature range (the fourth section having a T _4), (P _4), a fifth temperature range (the fifth segment having a T ₅₎ (P _5), a sixth temperature A sixth section P ₆ having a range T ₆ , and a seventh section P ₇ having a seventh temperature range T ₇ may be provided.

The first section (P ₁ ) to the seventh section (P ₇ ) may be sequentially disposed in the heating furnace. A first temperature the first section having a (T ₁₎ (P ₁₎ is a seventh period (P ₇₎ having a seventh temperature range (T ₇₎ adjacent to the inlet, and in a heating furnace which the blank is introduced is blank is It may be adjacent to the outlet of the furnace to be discharged. Thus, the first temperature range, the first section having a (T ₁₎ (P ₁₎ to the first may first period, the seventh temperature range, the seventh segment having a (T ₇₎ (P ₇₎ is heated in a heating may be the last section of As will be described later, among the plurality of sections of the heating furnace, the fifth section (P ₅ ), the sixth section (P ₆ ), and the seventh section (P ₇ ) are not a section in which multi-stage heating is performed, but a crack heating is performed. It may be a section where

The temperature of the plurality of sections provided in the heating furnace, for example, the temperature of the first section (P ₁ ) to the seventh section (P ₇ ) increases in the direction of the outlet of the furnace from which the blank is taken out from the inlet of the furnace into which the blank is input. can do. However, the temperatures of the fifth section P ₅ , the sixth section P ₆ , and the seventh section P ₇ may be the same. Also, a temperature difference between two adjacent sections among a plurality of sections provided in the heating furnace may be greater than 0°C and less than or equal to 100°C. For example, the temperature difference between the first section (P ₁ ) and the second section (P ₂ ) may be greater than 0°C and less than or equal to 100°C.

In one embodiment, the first temperature range (T _{1 ) of the first section (P 1} ) may be 840 °C to 860 °C, and 835 °C to 865 °C. The second temperature range (T ₂ ) of the second section (P ₂ ) may be 870 °C to 890 °C, and 865 °C to 895 °C. The third temperature range (T ₃ ) of the third section (P ₃ ) may be 900°C to 920°C, and 895°C to 925°C. The fourth temperature range (T ₄ ) of the fourth section (P ₄ ) may be 920°C to 940°C, and may be 915°C to 945°C. The fifth temperature range (T ₅ ) of the fifth section (P ₅ ) may be Ac3 to 1,000°C. Preferably, the fifth temperature range (T ₅ ) of the fifth section (P ₅ ) may be 930°C or more and 1,000°C or less. More preferably, the fifth temperature range (T ₅ ) of the fifth section (P ₅ ) may be 950°C or more and 1,000°C or less. Sixth section fifth temperature range from the sixth temperature (T _6), and the seventh segment of claim 7, the temperature range (T ₇₎ has a fifth segment (P ₅₎ of the (P ₇₎ of the (P ₆₎ (T ₅ ) can be the same as

5, the heating furnace according to an embodiment of the present invention is illustrated as having seven sections having different temperature ranges, but the present invention is not limited thereto. Five, six, or eight sections having different temperature ranges may be provided in the heating furnace.

The blank according to an embodiment may be heated in stages while passing through a plurality of sections defined in the heating furnace. In one embodiment, the temperature condition in the heating furnace in the multi-stage heating step in which the blank passes through a plurality of sections in the heating furnace and is heated step by step may satisfy the following equation.

<Equation>

0 < (Tg - Ti) / Lt < 0.025℃/mm

Here, Tg is the crack heating temperature (°C), Ti is the furnace entry temperature (°C), and Lt is the multi-stage heating length (mm).

When the value of the formula exceeds 0.025°C/mm, the heating furnace entry temperature is lowered and the heating rate of the blank is lowered, so that sufficient crack heating time cannot be secured, and the driving speed of the roller is lowered to ensure crack heating time In the case of driving, productivity may decrease. In addition, when the value of the equation is 0 ℃ / mm, as described above for a single heating, the thin blank first _{reaches the target temperature (T t} ) and overheating occurs in the thin blank. may exist.

4 and 5, the blank passes through a plurality of sections defined in the heating furnace (for example, the first section (P ₁ ) to the fourth section (P ₄ )) and is heated in stages in stages, When the temperature condition of the multi-stage heating satisfies the equation, compared to the case where the blank is heated by a single heating, the temperature change graphs of the blanks having different thicknesses may exhibit similar behavior to each other. For example, when the same time elapses after inserting the blank into the heating furnace, a single heating 310 of a blank having a thickness of 1.2 mm, and a single heating 320 of a blank having a thickness of 1.6 mm The temperature difference between the blanks may be smaller than the temperature difference between the blanks when the blank having a thickness of 1.2 mm is heated 330 in multiple stages and the blank having a thickness of 1.6 mm is heated 340 in multiple stages. Therefore, in the case of multi-stage heating of blanks, by similarly controlling the heating rate of blanks having different thicknesses, it is possible to reduce the time difference for each blank to reach the target temperature, thereby preventing the thin blank from being overheated can do.

A crack heating step (S130) may be performed after the multi-stage heating step (S120). The crack heating step (S130) may be a step of crack heating the blank at a temperature of 950°C to 1,000°C in the last part of a plurality of sections provided in the heating furnace.

The crack heating step ( S130 ) may be performed in the last part of the plurality of sections of the heating furnace. For example, the crack heating step ( S130 ) may be performed in the fifth section ( P ₅ ), the sixth section ( P ₆ ), and the seventh section ( P ₇ ) of the heating furnace. When a plurality of sections are provided in the heating furnace, if the length of one section is long, there may be problems such as temperature change in the section. Therefore, the section in which the crack heating step (S130) is performed is divided into a fifth section (P ₅ ), a sixth section (P ₆ ), and a seventh section (P ₇ ), a fifth section (P ₅ ), the second section Section 6 (P ₆ ), and the seventh section (P ₇ ) may have the same temperature range in the heating furnace.

In the crack heating step (S130), the multi-stage heated blank may be crack-heated at a temperature of Ac3 to 1,000°C. Preferably, in the crack heating step (S130), the multi-stage heated blank may be crack-heated at a temperature of 930 °C to 1,000 °C. More preferably, in the crack heating step (S130), the multi-stage heated blank may be crack-heated at a temperature of 950°C to 1,000°C.

6 is a graph showing the high-temperature tensile properties of the heated blank according to the molding start temperature. 6 shows a blank 410 that is crack-heated at a temperature of 950° C. and exposed to air cooling for 10 seconds after being taken out, and a blank 420 that is crack-heated at a temperature of 900° C. and exposed to air cooling for 10 seconds after being taken out. This is a high temperature tensile test graph. At this time, the molding start temperature of the blank 410 exposed to air cooling for 10 seconds after being heated by cracking at a temperature of 950° C. and taken out is 650° C. to 750° C., heated by cracking at a temperature of 900° C., and 10 seconds after being taken out. The forming initiation temperature of the blank 420 exposed to air cooling during the period may be 550° C. to 650° C.

Referring to FIG. 6 , the blank 410 exposed to crack heating at a temperature of 950° C. and air-cooled for 10 seconds after being taken out is crack-heated at a temperature of 900° C., and the blank 420 exposed to air cooling for 10 seconds after being taken out. ), it can be seen that the true stress is low. Therefore, when the crack heating temperature in the heating furnace is less than 950°C, after the heated blank is taken out from the heating furnace, the press-forming start temperature is excessively lowered by the air-cooling exposure time, so that the elongation of the heated blank decreases, thereby reducing the thickness during molding may occur or fracture may occur. As the heated blank is cooled during the air cooling exposure time, the strength of the blank is increased and a large force is required to simultaneously form a plurality of blanks, which may overload the press equipment. In addition, when the crack heating temperature exceeds 1,000° C., carbide-forming elements or nitride-forming elements such as Ti, V, Nb, and Mo in the blank are dissolved into the base material, making it difficult to suppress grain coarsening.

In one embodiment, the temperature of the section heating the blank by cracking among the plurality of sections in the heating furnace may be higher than or equal to the temperature of sections for heating the blank in multiple stages.

In one embodiment, the blank may stay in the furnace for 180 seconds to 360 seconds. More specifically, the time for multi-stage heating and crack heating of the blank in the furnace may be 180 seconds to 360 seconds. If the residence time of the blank in the furnace is less than 180 seconds, it may be difficult to crack sufficiently at the desired cracking temperature. In addition, when the residence time of the blank in the furnace exceeds 360 seconds, the amount of hydrogen penetrating into the blank increases, thereby increasing the risk of delayed fracture, and corrosion resistance after hot stamping may be reduced.

7 is a graph illustrating a temperature change when a blank is heated in multiple stages and heated by cracking in the method for manufacturing a hot stamping part according to an embodiment of the present invention. Unlike the graph of FIG. 5 , the graph of FIG. 7 is a graph illustrating temperatures of blanks according to distance.

Referring to FIG. 7 , 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, and a length (D _{1 ) of a section for multi-stage heating of a blank among a plurality of sections (D 1 ) and a length (D 2} ) of a section for crack heating a blank among a plurality of sections A ratio of 1:1 to 4:1 may be satisfied. For example, a section for crack heating the blank among the plurality of sections may be the last portion of the heating furnace (eg, a fifth section (P ₅ ), to a seventh section (P ₇ )). When the ratio of the length of the section heating the blank in multiple stages (D ₁ ) to the length of the section heating the blank by cracking (D ₂ ) exceeds 1:1 by increasing the length of the section for crack heating the blank, in the crack heating section An austenite (FCC) structure may be generated to increase the amount of hydrogen permeation into the blank, which may increase the delayed fracture. In addition, since the length of the section for crack heating the blank is reduced _{, the ratio of the length (D 1 ) of the section heating the blank in multiple stages to the length (D 2} ) of the section heating the blank by cracking is less than 4:1, the crack heating section ( time) is not sufficiently secured, so the strength of the parts manufactured by the manufacturing process of hot stamping parts may be non-uniform.

In one embodiment, the length of the uniform heating section among the plurality of sections provided in the heating furnace may have a length of 20% to 50% of the total length of the heating furnace.

After the crack heating step (S130), the transfer step (S140), the forming step (S150); And a cooling step (S160) may be further performed.

The transferring step ( S140 ) may be a step of transferring the crack-heated blank from the heating furnace to the press mold. In the step of transferring the crack-heated blank from the heating furnace to the press mold, the crack-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 pressing the press die in a closed state. In forming and cooling the heated blank, the average cooling rate can be cooled to at least 10° C./s or more to the martensite end temperature. The blank can be held in the press mold for 3 to 20 seconds. If the holding time in the press mold is less than 3 seconds, sufficient cooling of the material is not achieved, and thermal deformation may occur due to the residual heat of the product and the temperature deviation for each part, resulting in deterioration of dimensional quality. In addition, when the holding time in the press mold exceeds 20 seconds, the holding time in the press mold becomes longer and productivity may decrease.

In one embodiment, the hot stamping part manufactured by the above-described method for manufacturing the hot stamping part may have a tensile strength of 500 MPa or more and less than 800 MPa, and may have a composite structure of ferrite and martensite. The hot stamping part manufactured by the manufacturing method of the hot stamping part may have a tensile strength of 800 MPa or more and less than 1,200 MPa, and may have a composite structure of bainite and martensite. The hot stamping part manufactured by the manufacturing method of the hot stamping part may have a tensile strength of 1,200 MPa or more and less than 2,000 MPa, and may have a full martensitic structure.

By heating the blanks having different thicknesses in multiple stages at the same time in the furnace, it is possible to more precisely control the time to reach the target temperature (eg, the cracking temperature) of the blanks. By more precisely controlling the time to reach a target temperature (eg, cracking temperature) of blanks having different thicknesses, it is possible to improve hydrogen embrittlement, corrosion resistance, and weldability of parts manufactured by the manufacturing method of hot stamping parts. More specifically, when the thin material and the thick material are simultaneously single-heated in the heating furnace, the thin material may first reach the target temperature compared to the thick material, resulting in overheating of the thin material. According to an embodiment of the present invention, even when the thin material and the thick material are heated at the same time in the heating furnace, by heating the thin material and the thick material in multiple stages, the target temperature (eg, the cracking temperature) of the thin material and the thick material is reached can be controlled similarly. Therefore, by similarly controlling the time to reach the target temperature (eg, the cracking temperature) of the thin material and the thick material, the hydrogen embrittlement, corrosion resistance, and weldability of the parts manufactured by the hot stamping manufacturing method can be improved.

<Example>

A blank having an alloy composition of Table 1 was prepared. In the heating furnace set by the standard of Table 2, after setting the temperature setting for each section in Table 3, hot stamping parts were manufactured according to the conditions of Comparative Examples 1 and 2 and Examples. On the other hand, the total length of the heating furnace is 22400 mm.

Referring to Table 3, a hot stamping part (Example) was manufactured using the manufacturing method of the hot stamping part according to an embodiment, and Comparative Example 1 and Comparative Example 2 were respectively 950 ° C. and 930 ° C. at a temperature of 930 ° C. Hot stamped parts were made with a single heating.

Hydrogen embrittlement evaluation, corrosion resistance evaluation, and welding evaluation were performed on parts manufactured according to the conditions of Examples, Comparative Example 1, and Comparative Example 2.

1. Hydrogen embrittlement evaluation

For parts manufactured according to the conditions of Examples, Comparative Example 1, and Comparative Example 2, hydrogen embrittlement was evaluated using TDS (Thermal Desorption Spectroscopy) equipment according to ISO16573-2015 regulations. That is, under a vacuum atmosphere, each of the parts manufactured according to the conditions of Examples, Comparative Example 1, and Comparative Example 2 were heated to measure the amount of diffusible hydrogen emitted from the parts at 300° C. or less.

8 is a graph showing the hydrogen release rate from parts manufactured according to the conditions of Example, Comparative Example 1, and Comparative Example 2, and Table 4 is the Example, Comparative Example 1, and Comparative Example 2 of FIG. Based on the hydrogen release rate results, the table shows the results of calculating the amount of diffusible hydrogen at 300° C. or less, and the delayed rupture test results.

Referring to FIG. 8 and Table 4, in the case of Example, the amount of diffusible hydrogen at 300° C. or less is 0.412 ppm, and in Comparative Example 1, the diffusible hydrogen amount at 300° C. or less is 0.531 ppm, and in Comparative Example 2 In this case, it can be seen that the amount of diffusible hydrogen at 300° C. or less is 0.475 ppm. In addition, as a result of the delayed rupture experiment, it can be confirmed that, although delayed rupture occurs in Comparative Examples 1 and 2, delayed rupture does not occur in Examples. Since the amount of diffusible hydrogen of the hot stamping part manufactured through multi-stage heating is the smallest and delayed rupture does not occur, when multi-stage heating is used, hydrogen embrittlement of the hot stamping part may be reduced.

2. Corrosion resistance evaluation

For the hot stamping parts manufactured under the conditions of Examples, Comparative Example 1, and Comparative Example 2, corrosion resistance evaluation tests were performed according to ASTM G59-97 (2014) standard. More specifically, for the corrosion resistance evaluation experiment, three electrodes were used using a specimen as a working electrode, a high-purity carbon rod as a counter electrode, and a saturated calomel electrode as a reference electrode. An electrochemical cell was constructed and an isostatic polarization test was performed. Isopotential polarization test was conducted after confirming electrochemical stabilization by measuring open-circuit potential (OCP) in 3.5% NaCl solution for 10 hours, and from -250 mV to 0 mVSCE based on corrosion potential (Ecorr) 0.166 An electric potential was applied at a scanning rate of mV/s to conduct a corrosion resistance evaluation experiment.

9 is a graph showing the corrosion resistance evaluation results for parts manufactured according to the conditions of Examples, Comparative Example 1, and Comparative Example 2, and Table 5 is an Example, Comparative Example 1, and a table in which corrosion rates of parts manufactured according to the conditions of Comparative Example 2 are calculated. In this case, the corrosion rate in Table 5 is a value corresponding to the current density at the point in time when the stably maintained potential branching occurs in the polarization curves of Examples, Comparative Examples 1 and 2, respectively.

9 and 5, in the case of Comparative Examples 1 and 2, the lower the single heating temperature, the lower the corrosion rate, the better the corrosion resistance. It can be seen that better corrosion resistance can be secured.

3. Weldability evaluation

Weldability evaluation was performed on parts manufactured according to the conditions of Examples, Comparative Example 1, and Comparative Example 2. In the weldability evaluation, each pair of parts manufactured according to the conditions of Example, Comparative Example 1, and Comparative Example 2 were prepared, and these were chromium-copper alloy electrode rods having a diameter of 6 mm, a pressure of 350 kgf, and a current of 5.5 kA was applied, spot welding was performed for 300 ms. The resistance was measured while carrying out the occupying point.

In general, the resistance value change up to the initial 30 ms influences spatter generation and weldability characteristics, and the lower the resistance, the better the weldability.

10 is a graph showing resistance values for parts manufactured according to the conditions of Example, Comparative Example 1, and Comparative Example 2; Referring to FIG. 10 , a hot stamping part (Example) manufactured through multi-stage heating is a hot stamping part manufactured by single heating to a temperature of 950°C (Comparative Example 1), and manufactured by single heating to a temperature of 930°C It can be seen that it has a lower resistance than the hot stamping part (Comparative Example 2). Therefore, the weldability of the hot stamping part (Example) manufactured through multi-stage heating is a hot stamping part manufactured by single heating to a temperature of 950°C (Comparative Example 1), and hot stamping manufactured by single heating to a temperature of 930°C It can be seen that the weldability of the component (Comparative Example 2) is relatively superior.

As described above, the present invention has been described with reference to one embodiment shown in the drawings, but it will be understood by those skilled in the art that various modifications and variations of the embodiments are possible therefrom. Accordingly, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

Claims (14)

inserting the blank into a heating furnace having a plurality of sections having different temperature ranges;
a multi-stage heating step of heating the blank step by step; and
Crack heating step of heating the blank to a temperature of Ac3 to 1,000 ℃;
including,
The temperature condition in the heating furnace in the multi-stage heating step satisfies the following equation, a method of manufacturing a hot stamping part.
<Equation>
0 < (Tg - Ti) / Lt < 0.025℃/mm
(In the above formula, Tg is the crack heating temperature (°C), Ti is the furnace initial temperature (°C), and Lt is the multi-stage heating length (mm)) According to claim 1,
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 crack heating the blank satisfies 1:1 to 4:1, the method of manufacturing a hot stamping part. According to claim 1,
At least two blanks having different thicknesses are simultaneously transferred into the heating furnace. According to claim 1,
wherein the blank includes a first portion having a first thickness and a second portion having a second thickness different from the first thickness. 3. The method of claim 2,
The temperature of the plurality of sections increases in a direction from the inlet of the furnace to the outlet of the furnace, the method of manufacturing a hot stamping part. 6. The method of claim 5,
The temperature difference between two adjacent sections of the section heating the blank in multiple stages is greater than 0° C. and less than or equal to 100° C., a method of manufacturing a hot stamping part. 3. The method of claim 2,
A method of manufacturing a hot stamping part, wherein a temperature of a section in which the blank is heated by cracking among the plurality of sections is higher than a temperature of sections in which the blank is heated in multiple stages. According to claim 1,
wherein the blank stays in the furnace for 180 seconds to 360 seconds. According to claim 1,
After the crack heating step,
transferring the crack-heated blank from the furnace to a press mold;
forming a molded body by hot stamping the transferred blank; and
cooling the formed body;
Further comprising, a method of manufacturing a hot stamping part. 10. The method of claim 9,
In the step of transferring the crack-heated blank from the heating furnace to the press mold,
wherein the crack-heated blank is air-cooled for 10 to 15 seconds. A hot stamping part manufactured according to any one of claims 1 to 10, comprising:
A hot stamping part having a diffusible hydrogen content of less than 0.45 ppm and a corrosion rate of 3 x 10 ^{-6 A or less as measured by an isostatic polarization test.} 12. The method of claim 11,
A hot stamping part having a tensile strength of 500 MPa or more and less than 800 MPa, and having a composite structure of ferrite and martensite. 12. The method of claim 11,
A hot stamping part having a tensile strength of 800 MPa or more and less than 1,200 MPa, and having a composite structure of bainite and martensite. 12. The method of claim 11,
A hot stamping part having a tensile strength of 1,200 MPa or more and less than 2,000 MPa and a full martensitic structure.

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