KR102389359B1 - Hot stamping component and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a method for manufacturing a hot stamping component, which comprises: a step of preparing a blank in which a plating layer is formed on at least one surface of a base material; a step of injecting the blank into a heating furnace; a heating step of heating the blank; and a cooling step of cooling the heated blank. The heating step includes: a first heating step of heating the blank formed with the plating layer to a first temperature; a second heating step of heating the blank formed with the plating layer to a second temperature; and a third heating step of heating the blank formed with the plating layer to a third temperature. The blank before the heating step has a first specific gravity, and the hot stamping component after the cooling step has a second specific gravity. The second specific gravity is smaller than the first specific gravity. Therefore, a chemical reaction of the plating layer can be controlled.

Description

Hot stamping component and method of manufacturing the same

FIELD OF THE INVENTION The present invention relates to hot stamping parts and methods 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 are being 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 Part).

No. 10-2018-0095757

Embodiments of the present invention are hot stamping parts capable of controlling the chemical reaction of the plating layer formed on the blank by controlling the heating time of the blank while controlling the blank temperature increase rate in a certain temperature section during the heating process, and manufacturing thereof provide a way

One embodiment of the present invention provides a method of manufacturing a hot stamping part, comprising the steps of: preparing a blank in which a plating layer is formed on at least one surface of a base material; putting the blank into a heating furnace; a heating step of heating the blank; and a cooling step of cooling the heated blank, wherein the heating step includes a first heating step of heating the blank on which the plating layer is formed to a first temperature: heating the blank on which the plating layer is formed to a second temperature a second heating step; and a third heating step of heating the blank on which the plating layer is formed to a third temperature, wherein the blank before the heating step has a first specific gravity, and the hot stamping part after the cooling step has a second specific gravity, , wherein the second specific gravity is less than the first specific gravity.

In this embodiment, the change (decrease value) of the second specific gravity relative to the first specific gravity may be 1.5% or less.

In this embodiment, the surface shape scale (R.L.T.) of the hot stamping part is greater than 1 and less than or equal to 2, and the surface shape scale (R.L.T.) may be derived by Equation 1 below.

<Equation 1>

(Where n is the number of measurements, d is the measurement length, and f(x) is the surface shape equation)

In this embodiment, the average temperature increase rate of the blank in the second heating step may satisfy the following Equation (2).

<Equation 2>

Average temperature increase rate ≤ 7.841 / t ℃/s

(In this case, t is the thickness of the blank)

In this embodiment, the total heating time during which the first heating step, the second heating step, and the third heating step are performed may satisfy Equation 3 below.

<Equation 3>

Total heating time ≤ 300 x t + 60s

(In this case, t is the thickness of the blank)

In this embodiment, the alloying of the plating layer and the phase transformation of the base material proceed in the second heating step, and the homogenization treatment of the base material may be performed in the third heating step.

In this embodiment, the base material may include Fe, and the plating layer may include Al and Si.

In this embodiment, the heating furnace may include a heating section and a cracking section, wherein the blank is heated for a first time in the heating section, and the blank is heated for a second time in the cracking section.

In this embodiment, the ratio of the first time to the second time may satisfy 4:1 to 1:1.

In this embodiment, the first heating step and the second heating step may be performed in the heating section, and the third heating step may be performed in the cracking section.

In this embodiment, the temperature for heating the blank in the heating section may rise stepwise.

In this embodiment, the temperature of the cracking section may be greater than Ac3 and less than or equal to 1000 °C.

In this embodiment, between the heating step and the cooling step, transferring the heated blank from the heating furnace to the press mold; and hot stamping the transferred blank to form a molded body.

In another embodiment of the present invention, the surface shape scale (R.L.T.) of the hot stamping part is greater than 1 and less than or equal to 2, and the surface shape scale (R.L.T.) may be derived by Equation 4 below.

<Equation 4>

(Where n is the number of measurements, d is the measurement length, and f(x) is the surface shape equation)

According to the embodiments of the present invention, by controlling the chemical reaction of the plating layer formed on the blank, delayed rupture performance of the manufactured part can be improved, and excellent spot weldability can be secured by reducing the dynamic resistance of the plating layer.

In addition, by controlling the surface shape of the plating layer (or alloying layer) formed on the blank, it is possible to prevent or minimize the temperature drop due to radiation during the transfer process.

1 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment.
2 is a plan view schematically illustrating a blank used in a method of manufacturing a hot stamping part according to an embodiment.
3 is a plan view schematically illustrating a blank input into a heating furnace in a method of manufacturing a hot stamping part according to an embodiment.
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.
6 is a flowchart schematically illustrating a heating step according to an embodiment.
7 is a graph illustrating a temperature change of a blank according to time according to an embodiment.
8A to 8C are graphs illustrating surface profiles of first to third parts manufactured using a method of manufacturing a hot stamping part according to an exemplary embodiment, respectively.
9 is a graph showing a temperature change in a conveying step of blanks manufactured from first to third parts.

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 that one or more other features or components will be added 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 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.

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.

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 are 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. Hereinafter, a method of manufacturing a hot stamping part will be described with reference to FIG. 1 .

In one embodiment, the method of manufacturing a hot stamping part includes a blank preparation step (S100), a blank input step (S200), a heating step (S300), a transfer step (S400), a forming step (S500), and a cooling step (S600) may include

First, the blank preparation step (S100) may be a step of preparing the blank to be input into the heating furnace. The blank input into the heating furnace may be formed by cutting a plate (or a base material) for forming a hot stamping part. The plate material (or base 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 (or base material).

In one embodiment, an aluminum-silicon (Al-Si) plating layer or a zinc (Zn) plating layer may be formed on at least one surface of the annealed heat-treated plate (or base material). However, the type of the plating layer formed on one surface of the plate material (or the base material) is not limited thereto.

In one embodiment, the annealing heat-treated plate (or base material) may be immersed in a hot-dip plating bath containing 8 wt% to 12 wt% of silicon (Si), and excess aluminum (Al). At this time, the hot-dip plating bath can maintain a temperature of 400 ℃ to 700 ℃. The plating layer may be formed by plating at 55 g/m ² to 200 g/m ² on one surface of the plate (or base material). Through the above process, an aluminum-silicon (Al-Si) plating layer having a plating amount of 55 g/m ² to 200 g/m ² may be formed on the blank.

Thereafter, the blank input step ( S200 ) may be performed. The blank input step (S200) may be a step of inputting a blank having an aluminum-silicon (Al-Si) plating layer formed on one surface of the base material into the heating furnace. In this case, the heating furnace may have a plurality of sections having different temperature ranges. For example, the furnace may include a heating section, and a cracking section.

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

Referring to FIG. 2 , a 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 different types of plate materials having different thicknesses into a required shape (Tailor Welded) Blank, TWB), a Taylor 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 a vehicle's collision member, is a form in which plates of different strengths are combined in an upper collision support part and a lower shock absorber, and may be 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 a plate with good shock absorption performance is connected to the lower part of the B-pillar where stress is concentrated to increase the shock absorption ability in the event of a vehicle crash. can be improved

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 . In one embodiment, the thickness profile may be performed in a conventional manner. For example, when the cold-rolled steel material is cold-rolled, a first region 231 having a first thickness, a second region 232 having a second thickness, and a third region having a third thickness ( 233 ), 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. Taylor rolled blank 230 has 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. In one embodiment, after the patch 243 having a second size smaller than the first size is welded to the base material 241 having a first size, it may be simultaneously molded.

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

Referring to FIG. 3 , in one embodiment, in the blank input step ( S200 ), at least two blanks 200 different in at least one of a thickness and a size may be simultaneously introduced into the heating furnace. 3 shows two first blanks 250 and two second blanks 260 that are simultaneously fed into the furnace. In this case, the first blank 250 and the second blank 260 may have different sizes and/or 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/or one second blank 260 may be 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 one embodiment, in the blank input step (S200), one blank 200 having a single thickness may be input into the heating furnace. Alternatively, in the blank input step ( S200 ), the above-described Taylor welded blank 220 ( FIG. 2 ), the Taylor rolled blank 230 ( FIG. 2 ), and the patchwork 240 ( FIG. 2 ) may be introduced into the heating furnace.

In one embodiment, in the blank input step (S200), 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 one embodiment, the blank introduced into the heating furnace may be transported along a transport direction after being mounted on a roller.

Again, referring to FIG. 1 , after the blank input step ( S200 ), the heating step ( S300 ) may be performed. In one embodiment, the heating step (S300) may be a step of heating the blank in which an aluminum-silicon (Al-Si) plating layer is formed on at least one surface of the base material.

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 single heating (310, 320) at the same time, it is a graph showing the temperature change of the blanks with time.

In this case, the target temperature (T _t ) of the blank may be equal to or higher than Ac3 (the temperature at which the transformation from ferrite to austenite is completed). Preferably, the target temperature (T _t ) of the blank may be about 930° C. or higher. More preferably, the target temperature (T _t ) of the blank may be about 950° C. or higher. However, the present invention is not limited thereto. In addition, the 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 in the heating furnace, but setting the temperature of the heating furnace to a single temperature and then setting the temperature of the heating furnace to a single temperature of 1.2 mm in the heating furnace It refers to a case where a blank having a thickness of , 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 (T _t ) 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 , and the blank with a thickness of 1.2 mm is crack-heated 310 for a first time S ₁ , and with a thickness of 1.6 mm The blank may be crack-heated for a second time (S ₂ ) shorter than the first time (S ₁ ) ( 320 ). Since the crack heating time is adjusted based on the blank reaching the target temperature (T _t ) late, the blank with a thickness of 1.2 mm that has reached the target temperature (T _t ) first is overheated and the thickness of the blank with a thickness of 1.2 mm is overheated. Hydrogen delayed fracture may increase, and weldability may deteriorate.

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. 5 is a graph showing the temperature change with time when a blank having a thickness of 1.2 mm is heated in multiple stages (330), and a blank having a thickness of 1.6 mm is heated in multiple stages (340).

Referring to FIG. 5 , the heating furnace according to an embodiment may include a plurality of sections having different temperature ranges. Specifically, the heating furnace is a first section (P ₁ ) having a first temperature range (T ₁ ), a second section (P ₂ ) having a second temperature range (T ₂ ), a third temperature range (T ₃ ) A third section (P ₃ ) having a fourth section (P ₄ ) having a fourth temperature range (T ₄ ), a fifth section (P ₅ ) having a fifth temperature range (T ₅ ), a sixth temperature range A sixth section (P ₆ ) having (T ₆ ), and a seventh section (P ₇ ) having a seventh temperature range (T ₇ ) may be provided.

In one embodiment, the furnace may include a heating section ( _{Pa ), and a cracking section (P b )} . In one embodiment, the blank may be heated (or multistage heated) in the heating section P _a , and the blank may be crack heated (or heated, and crack maintained) in the cracking section P _b . In one embodiment, the heating section (P _a ) may include a first section (P ₁ ) to a fourth section (P ₄ ), and the cracking section (P _b ) is a fifth section (P ₅ ) to a seventh section It may include a section P ₇ .

The first section (P ₁ ) to the seventh section (P ₇ ) may be sequentially disposed in the heating furnace. The first section (P ₁ ) having the first temperature range (T ₁ ) is closest to the inlet of the heating furnace into which the blank is input, and the seventh section (P ₇ ) having the seventh temperature range (T ₇ ) is the blank may be closest to the outlet of the heating furnace from which is taken out. Accordingly, the first section (P ₁ ) having the first temperature range (T ₁ ) may be the first section of the heating furnace, and the seventh section (P ₇ ) having the seventh temperature range (T ₇ ) is the heating furnace may be the last section of 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 crack heating (eg, heating and crack maintenance) that is not a section in which multi-stage heating is performed. ) may be a section in which it is performed.

The temperature of a 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 or similar. In addition, 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 an embodiment, the temperature for heating the blank in the heating step (eg, the heating section Pa ) may be increased in _a stepwise manner. In an 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 915°C to 945°C. However, the present invention is not limited thereto.

In addition, 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 to 1,000 °C. More preferably, the fifth temperature range (T ₅ ) of the fifth section (P ₅ ) may be 950 °C to 1,000 °C. The sixth temperature range (T ₆ ) of the sixth section (P ₆ ), and the seventh temperature range (T ₇ ) of the seventh section (P ₇ ) are the fifth temperature range (T ₅ ) of the fifth section (P ₅ ) ) may be the same as or similar to However, the present invention is not limited thereto.

In one embodiment, the heating furnace includes _{a heating section (Pa ), and a crack section (P b )} , but the temperature of the heating section (Pa) may be provided (or set) as a single temperature, or, The temperature of the section Pa may be provided (set) to increase (or to increase gradually) along the direction in which the blank is transported. For example, the temperature of the first section (P ₁ ) to the fourth section ( _{P 4} ) provided in the heating section (Pa) may be provided (or, set) all the same or similar, or, the heating section The temperature of the first section (P ₁ ) to the fourth section (P ₄ ) provided in (P _a ) increases from the first section (P ₁ ) to the fourth section (P ₄ ) (or gradually to be increased) may be provided (or set).

In one embodiment, the temperature of the cracking section (P _b ) may be provided (or set) to a uniform temperature. That is, the temperature of the cracking section (P _b ) may be provided (or set) to a temperature greater than Ac3 and less than or equal to 1000 °C. For example, the fifth section (P ₅ ) to the seventh section (P ₇ ) provided in the cracking section (P _b ) may be provided (or set) at a temperature greater than Ac3 and 1000° C. or less.

5, the heating furnace is illustrated as having seven sections having different temperature ranges, but the present invention is not limited thereto. Various modifications are possible, such as five, six, or eight sections having different temperature ranges in the heating furnace.

In one embodiment, in the heating step (S300), the blank may be heated (or heated in stages) while passing through _a plurality of sections defined in the heating furnace (eg, the heating section (Pa )).

Referring to FIGS. 3 and 5 , when blanks having different thicknesses pass through a plurality of sections defined in a heating furnace (eg, a heating section P _a ) and are heated in stages, the blank at a single temperature Compared to the case in which is heated, the temperature change of the blanks may exhibit similar behavior to each other. For example, when the same time elapses after inserting the blank into the heating furnace, a blank having a thickness of 1.2 mm is single heated 310, and a blank having a thickness of 1.6 mm is single heated 320 The temperature difference between the blanks when a blank having a thickness of 1.2 mm is multi-staged 330 and a blank having a thickness of 1.6 mm is multi-stage heated 340 than the temperature difference between the blanks may be smaller. Therefore, when the blanks are heated in multiple stages (heating in stages), it is possible to similarly control the heating rate of the blanks having different thicknesses. When the blanks are heated in multiple stages (heating in stages), by similarly controlling the heating rate of the blanks having different thicknesses, the time difference for each blank to reach the target temperature can be reduced, so that the thin blank is overheated can be prevented from becoming

After the blank is heated in multiple stages in the heating section P _a , the multi-stage heated blank may pass through the cracking section P _b . At this time, the multi-stage heated blank may be crack-heated (or heated, and crack-maintained) while passing through the cracking section (P _b ).

In one embodiment, the cracking section (P _b ) may be a section in which the blank is crack-heated (or heated, and crack-maintained) to a temperature greater than Ac3 and less than or equal to 1,000°C. Preferably, in the cracking section (P _b ), the multi-stage heated blank may be crack-heated (or crack-maintained) at a temperature of 930°C to 1,000°C. More preferably, in the cracking section (P _b ), the multi-stage heated blank may be crack-heated (or crack-maintained) at a temperature of 950°C to 1,000°C.

The cracking section (P _b ) may be the last part of the plurality of sections of the heating furnace. In one embodiment, the cracking section (P _b ) may include a fifth section ( P ₅ ), a sixth section ( P ₆ ), and a 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 crack section (P _b ) is divided into a fifth section (P ₅ ), a sixth section (P ₆ ), and a seventh section (P ₇ ), the fifth section (P ₅ ), the sixth section ( P ₆ ), and the seventh section P ₇ may be provided at the same or similar temperature.

In one embodiment, the blank may be heated in the heating section Pa, heated in the cracking section Pb, and held cracked. In an embodiment, the blank may be heated to a second temperature in the heating section P _a , and the blank may be heated to a third temperature in the cracking section P _b and maintained cracked at the third temperature. Specifically, the blank may be heated to Ac3 in the heating section P _a , and the blank may be heated to 930° C., 950° C., or 1000° C. in the cracking section P _b , and crack maintained at this temperature.

In one embodiment, the blank may remain in the furnace for between about 180 seconds and 360 seconds. That is, the time for which the blank is heated in the heating furnace may be about 180 seconds to 360 seconds. Specifically, the time during which the blank is heated and cracked in the heating section ( _{Pa ) and the cracking section (P b )} may be about 180 seconds to 360 seconds. When the residence time of the blank in the furnace is less than 180 seconds, it may be difficult to sufficiently crack at the desired cracking temperature because the residence time in the furnace is short. On the other hand, 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 decrease.

In one embodiment, the blank may be heated for a third time in the heating section P _a , and the blank may be heated for a fourth time in the cracking section P _b . In an embodiment, the ratio of the third time to the fourth time may satisfy 4:1 to 1:1.

When the ratio of the third time to the fourth time is less than 4:1 (for example, when the ratio of the fourth time is less than 20%), the crack heating time is not sufficiently secured, so a hot stamping part manufactured by the method for manufacturing a hot stamping part strength may be non-uniform. On the other hand, when the ratio of the third time to the fourth time exceeds 1:1 (eg, when the ratio of the fourth time is more than 50%), an austenite structure is generated in the cracking section P _b and is introduced into the blank. Delayed rupture may increase due to increased hydrogen permeation. Therefore, when the ratio of the third time to the fourth time satisfies 4:1 to 1:1 (for example, when the ratio of the fourth time satisfies 20% to 50%), the occurrence of delayed rupture is prevented or It can be minimized, and the manufactured hot stamping part can have uniform strength.

While the blank having an aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material is heated in a heating furnace, the chemical reaction of the aluminum-silicon (Al-Si) plating layer is performed in parallel with the austenite phase transformation reaction of the base material. will occur At this time, the amount of chemical reaction of the aluminum-silicon (Al-Si) plating layer varies according to heating conditions, which causes a change in the volume of the plating layer, and the change in the volume of the plating layer may lead to a change in the density of the entire blank. For example, as the chemical reaction of the aluminum-silicon (Al-Si) plating layer is active, the amount of hydrogen mixed from the plating layer to the base material increases, which causes delayed fracture, and the dynamic resistance of the plating layer increases, thereby reducing weldability. In addition, as the amount of chemical reaction of the aluminum-silicon (Al-Si) plating layer increases, the surface irregularities become more severe. increases, the molding start temperature is lowered, and necking or cracks may occur during press molding.

6 is a flowchart schematically illustrating a heating step (S300) according to an embodiment, and FIG. 7 is a graph showing a change in the temperature of the blank over time according to an embodiment. Specifically, FIG. 7 is a graph showing the temperature change of the blank over time in the manufacturing process of the hot stamping part.

6 and 7 , the heating step S300 may include a first heating step S310 , a second heating step S320 , and a third heating step S330 . Hereinafter, the first heating step (S310), the second heating step (S320), and the third heating step (S330) will be sequentially described.

In one embodiment, the first heating step (S310) may be a step of heating the blank to a first temperature. Specifically, the first heating step ( S310 ) may be a step of heating the blank having an aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material to a first temperature. In this case, the first temperature may be about 600 °C.

In one embodiment, the first heating step (S310) may be performed in the above-described heating section ( _Pa , FIG. 5). Specifically, the first heating step (S310) may be performed in at least _a portion of the first section (P ₁ ) of the heating section (Pa, FIG. 5). Alternatively, the first heating step (S310) may be performed in at least a portion of the first section ( _{P 1} , Fig. 5) and the second section (P ₂ , Fig. 5) of the heating section (Pa, Fig. 5). Alternatively, the first heating step (S310) is _a heating section (Pa, Figure 5) of the first section (P ₁ , Fig. 5), the second section (P ₂ , Fig. 5) and the third section (P ₃ , Fig. 5) may also be performed at least in part.

In one embodiment, in the first heating step (S310), the blank having an aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material may be heated to a first temperature (eg, about 600° C.), and the first heating Step S310 may be performed in the heating section _Pa ( FIG. 5 ). At this time, the temperature of the heating section ( _Pa , Fig. 5) may be provided (or set) as a single temperature (or, a constant temperature), or the temperature of the heating section ( _Pa , Fig. 5) is the blank is transferred It may be provided (or set) to increase (or increase gradually) along the direction.

In one embodiment, after the first heating step (S310), a second heating step (S320) may be performed. In one embodiment, the second heating step (S320) may be a step of heating the blank to a second temperature. Specifically, the second heating step ( S320 ) may be a step of heating the blank having an aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material to a second temperature. In this case, the second temperature may be about Ac3.

In one embodiment, the second heating step (S320) may be performed in the aforementioned heating section ( _Pa , FIG. 5). Specifically, the second heating step (S320) is at least a portion of the _{second section (P 2 ,} Fig. 5) of the heating section (Pa, Fig. 5), the third section (P ₃ , Fig. 5) and the fourth section ( P ₄ , may be performed in at least a part of FIG. 5 ). Alternatively, the second heating step (S320) is at least a portion of the first section (P ₁ , Fig. 5), the second section (P ₂ , Fig. 5), the third section (P ₃ , Fig. 5) and the fourth section ( P ₄ , may be performed in at least a part of FIG. 5 ).

In one embodiment, the second heating step (S320) may be performed in the heating section ( _{Pa, Fig. 5) and the cracking section (P b ,} Fig. 5). Specifically, the second heating step (S320) is at least a portion of the second section (P ₂ , Fig. 5), the third section (P ₃ , Fig. 5), the fourth section (P ₄ , Fig. 5) and the fifth section (P ₅ , may be performed in at least a part of FIG. 5 ).

In one embodiment, in the second heating step (S320), the blank having an aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material may be heated to a second temperature (eg, about Ac3), and the second heating step (S320) may be performed in the heating section ( _Pa , FIG. 5). At this time, the temperature of the heating section (Pa, Fig. 5) may be provided (or set) as _a single temperature (or, a constant temperature), or the temperature of the heating section (Pa, Fig. 5) is the direction in which the blank is transported It may be provided (or set) to increase (or to increase gradually) according to the

In one embodiment, after the second heating step (S320), a third heating step (S330) may be performed. In one embodiment, the third heating step (S330) may be a step of heating the blank to a third temperature. Specifically, the third heating step ( S330 ) may be a step of heating the blank having an aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material to a third temperature. In this case, the third temperature may be greater than about Ac3 and less than or equal to 1000°C.

In one embodiment, the third heating step (S330) may be performed in the aforementioned cracking section (P _b , FIG. 5 ). Specifically, the third heating step (S330) is at least a part of the fifth section (P ₅ , Fig. 5) of the cracking section (P _b , Fig. 5), the sixth section (P ₆ , Fig. 5) and the seventh section ( P ₇ , may be performed in FIG. 5 ).

In one embodiment, the third heating step (S330) may be performed in the heating section ( _{Pa, Fig. 5) and the cracking section (P b ,} Fig. 5). Specifically, the third heating step (S330) is at least a portion of the fourth section (P ₄ , Fig. 5), the fifth section (P ₅ , Fig. 5), the sixth section (P ₆ , Fig. 5) and the seventh section (P ₇ , FIG. 5 ).

In one embodiment, in the third heating step (S330), the blank having an aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material may be heated to a third temperature (eg, greater than about Ac3 and less than or equal to 1000°C), The third heating step (S330) may be performed in the cracking section (P _b , FIG. 5 ). At this time, the temperature of the cracking section (P _b , FIG. 5 ) may be provided (or set) to a uniform temperature. For example, the temperature of the cracking section (P _b , FIG. 5 ) may be provided (or set) to a temperature greater than Ac3 and less than or equal to 1000°C.

In one embodiment, the first heating step (S310) and the second heating step (S320) may be performed in the heating section ( _{Pa, Fig. 5), the third heating step (S330) is the cracking section (P b ,} 5) can be performed. At this time, the temperature of the heating section (Pa, Fig. 5) may be provided (or set) as _a single temperature (or, a constant temperature), or the temperature of the heating section (Pa, Fig. 5) is the direction in which the blank is transported It may be provided (or set) to increase (or to increase gradually) according to the In addition, the temperature of the cracking section (P _b , FIG. 5 ) may be provided at a uniform temperature. For example, the temperature of the cracking section (P _b , FIG. 5 ) may be provided (or set) to a temperature greater than Ac3 and less than or equal to 1000°C.

In one embodiment, the first heating step (S310) is performed in the first section ( _P ¹ ) of the heating section (Pa, Fig. 5), the second heating step (S320) is the heating section ( _Pa , Fig. 5) ) of the second section (P ₂ , Fig. 5) to the fourth section (P ₄ , Fig. 5), the third heating step (S300) is the fifth section (P) of the cracking section (P _b , Fig. 5) ₅ , FIG. 5 ), and may be performed in a to seventh section ( P ₇ , FIG. 5 ).

In one embodiment, the blank may be heated (or multi-stage heating) in the first heating step (S310), and the second heating step (S320), and the blank is heated by cracking in the third heating step (S330) (or, heating, and crack retention). In one embodiment, the temperature inside the heating furnace for heating the blank in the first heating step (S310), and the second heating step (S320) may rise (or rise in a stepwise manner, gradually rise), 3 The temperature inside the heating furnace for heating the blank in the heating step (S330) may be maintained constant.

In one embodiment, the alloying of the aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material in the second heating step (S320), and the phase transformation of the base material may proceed, and the phase transformation in the third heating step (S330) Homogenization treatment of the advanced base material can be made.

In one embodiment, the average temperature increase rate of the blank in the second heating step (S320) may satisfy Equation 1 below.

<Equation 1>

Average temperature increase rate ≤ 7.841 / t ℃/s

where t is the thickness of the blank.

When the average temperature increase rate of the blank in the second heating step (S320) is faster than Equation 1, that is, when the average temperature increase rate > 7.841 / t ℃ / s, the chemical reaction of the aluminum-silicon (Al-Si) plating layer is As it accelerates, the difference (change) between the specific gravity of the blank and the specific gravity of the manufactured hot stamping part before the heating step (S300) increases, and the surface shape scale (R.L.T.) of the manufactured hot stamping part may increase. Specifically, when the average temperature increase rate of the blank is greater than Equation 1 in the second heating step (S320), the chemical reaction of the aluminum-silicon (Al-Si) plating layer is accelerated and the heated blank is transferred to the press mold. In , the air cooling rate by radiation may be increased, and when the air cooling rate is increased, the forming start temperature at which forming is started may be lowered, and thus the formability of the blank may be deteriorated. In addition, when the air cooling rate of the blank is increased and the forming start temperature is lowered to lower the formability of the blank, the difference (change) between the specific gravity of the blank before the heating step (S300) and the specific gravity of the manufactured hot stamping part may increase. and the surface shape scale (R.L.T.) of the manufactured hot stamping part can be increased.

When the average temperature increase rate of the blank in the second heating step (S320) satisfies Equation 1 above, it is possible to control the average temperature increase rate of the blank from 600° C. to Ac3 at which the chemical reaction of the plating layer is accelerated. By controlling the chemical reaction of the plating layer by controlling the average temperature increase rate of the blank, it is possible to prevent or minimize the lowering of the molding start temperature by reducing the rate at which the blank is air-cooled in the process of transferring the heated blank to the press mold, and the blank moldability can be improved. In the process of transferring the heated blank to the press mold, by reducing the speed at which the blank is air-cooled to increase the molding start temperature and improve the formability of the blank, the specific gravity of the blank and the manufactured hot stamping part before the heating step (S300) The difference (change) in specific gravity can be reduced, and at the same time, the surface shape scale (R.L.T.) of the manufactured hot stamping part can be reduced.

In one embodiment, the total heating time during which the blank is heated in the heating step (S300) may vary depending on the thickness of the blank. For example, when the thickness of the blank is thin, the temperature increase rate of the blank is faster and the target temperature is reached faster than when the thickness of the blank is thick. It can be small compared to the total heating time.

Accordingly, the total heating time during which the first heating step ( S310 ), the second heating step ( S320 ), and the third heating step ( S330 ) are performed in the heating step ( S300 ) may satisfy Equation 2 below.

<Equation 2>

Total heating time ≤ 300 x t + 60s

where t is the thickness of the blank.

In one embodiment, a thin blank may reduce the overall heating time, and a thick blank may increase the overall heating time. In one embodiment, when the total heating time is greater than Equation 2, that is, when the total heating time > 300 x t + 60s, the chemical reaction of the aluminum-silicon (Al-Si) plating layer is accelerated before the heating step (S300) The difference (change) between the specific gravity of the blank and the specific gravity of the manufactured hot stamped part may become large, and the surface shape scale (R.L.T.) of the manufactured hot stamped part may increase. Specifically, when the total heating time in the heating step (S300) is greater than Equation 2 above, the chemical reaction of the aluminum-silicon (Al-Si) plating layer is accelerated, and in the process of transferring the heated blank to the press mold, The air cooling rate may be increased by the air cooling rate, and when the air cooling rate is increased, the forming start temperature at which forming is started may be lowered, and thus the formability of the blank may be deteriorated. In addition, when the air cooling rate of the blank is increased and the forming start temperature is lowered to lower the formability of the blank, the difference (change) between the specific gravity of the blank before the heating step (S300) and the specific gravity of the manufactured hot stamping part may increase. and the surface shape scale (R.L.T.) of the manufactured hot stamping part can be increased. In this case, the tendency as described above may appear more sensitively as the thickness of the blank becomes thinner.

When the total heating time of the blank satisfies Equation 2 above, by controlling the chemical reaction of the plating layer, the speed at which the blank is air cooled in the process of transferring the heated blank to the press mold is reduced to prevent the molding start temperature from being lowered or It can be minimized, and the formability of the blank can be improved. In the process of transferring the heated blank to the press mold, by reducing the speed at which the blank is air-cooled to increase the molding start temperature and improve the formability of the blank, the specific gravity of the blank and the manufactured hot stamping part before the heating step (S300) The difference (change) in specific gravity can be reduced, and at the same time, the surface shape scale (R.L.T.) of the manufactured hot stamping part can be reduced.

1 and 7, after the heating step (S300), the transfer step (S400), the forming step (S500), and the cooling step (S600) may be further performed. Specifically, after the third heating step (S330), the transferring step (S400), the forming step (S500), and the cooling step (S600) may be further performed.

The transferring step ( S400 ) may be a step of transferring the heated blank from the heating furnace to the press mold. In one embodiment, the blank heated in the transfer step (S400) may be air-cooled for 5 seconds to 15 seconds.

The forming step (S500) may be a step of forming a compact by hot stamping the transferred blank, and the cooling step (S600) may be a step of cooling the molded article.

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. 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 may 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, which may deteriorate the dimensional quality. On the other hand, when the holding time in the press mold exceeds 20 seconds, the holding time in the press mold becomes longer and productivity may decrease.

As parameters representing the roughness (roughness) of the part surface, there are arithmetic mean roughness (Ra), maximum height (Ry), and ten-point mean roughness (Rz). However, the arithmetic mean roughness (Ra), the maximum height (Ry), and the ten-point average roughness (Rz) do not represent the actual shape of the surface, but rather the roughness (roughness) of the surface obtained by a statistical method, and the arithmetic mean roughness (Ra) , the maximum height (Ry), the ten-point average roughness (Rz), etc., it is difficult to represent the actual surface shape.

8A to 8C are graphs illustrating surface profiles of first to third parts manufactured using a method of manufacturing a hot stamping part according to an exemplary embodiment, respectively. Specifically, FIG. 8A is a graph showing the surface profile of the first part, FIG. 8B is a graph showing the surface profile of the second part, and FIG. 8C is a graph showing the surface profile of the third part.

The arithmetic mean roughness (Ra) and the ten-point mean roughness (Rz) of the parts may be derived using the surface profiles of FIGS. 8A to 8C , respectively.

In this case, the arithmetic mean roughness Ra of the first part may be 3.37 μm, and the ten-point average roughness Rz of the first part may be 286 μm. In addition, the arithmetic mean roughness Ra of the second component may be 1.56 μm, and the ten-point average roughness Rz of the second component may be 22.2 μm. Also, the arithmetic mean roughness Ra of the third component may be 1.45 μm, and the ten-point average roughness Rz of the third component may be 55.2 μm.

Comparing the arithmetic mean roughness (Ra) of the second part and the third part, the arithmetic mean roughness (Ra) of the second part is 1.56 µm, and the arithmetic mean roughness (Ra) of the third part is 1.45 µm. It can be seen that the arithmetic mean roughness Ra of the part is lower than the arithmetic mean roughness Ra of the second part.

However, comparing the ten-point average roughness (Rz) of the second part and the third part, the ten-point average roughness (Rz) of the second part is 22.2 µm, and the ten-point average roughness (Rz) of the third part is 55.2 µm, It can be seen that the ten-point average roughness Rz of the second part is lower than the ten-point average roughness Rz of the third part.

Therefore, the arithmetic mean roughness (Ra) of the second part is greater than the arithmetic mean roughness (Ra) of the third part, but the ten-point average roughness (Rz) of the second part is smaller than the ten-point average roughness (Rz) of the third part can check that Through this, it can be seen that it is difficult to represent the roughness (roughness) of the actual surface using the arithmetic mean roughness (Ra) and the ten-point average roughness (Rz).

Therefore, the present inventor derived the surface shape scale (R.L.T.), which is a parameter for representing the shape of the actual surface, through the actually measured data. The surface shape scale (R.L.T.) is a concept similar to the surface roughness, and may be a parameter for representing the shape of an actual surface.

In an embodiment, the surface shape scale (R.L.T.) may be derived by Equation 3 below.

<Equation 3>

Here, n is the number of measurements, d is the measurement length, and f(x) is the surface shape equation. In Equation 3, n ≥ 5 and d may be ≥ 4 mm.

In one embodiment, the surface shape measure (R.L.T.) may be derived by measuring the actual shape with respect to the surface of the hot stamped part using a non-contact three-dimensional shape measuring machine according to ISO 4287. For example, the shape of the surface of the hot stamping part can be measured by measuring at least 4 mm or more in one measurement by performing 5 or more times in any direction using the line measurement method, and using the measured data Through this, the surface shape scale (R.L.T.) can be derived. In this case, the surface shape scale (R.L.T.) may be a value obtained by obtaining the total length (integration) of the line through the measured data.

In one embodiment, the closer the surface shape scale (R.L.T.) to 1, the smoother the surface of the manufactured hot stamping part, the larger the surface shape scale (R.L.T.), the roughness of the surface of the manufactured hot stamping part. (roughness) can mean large.

In one embodiment, the surface shape scale (R.L.T.) of the hot stamping part manufactured through the heating step (S300), the transferring step (S400), the forming step (S500), and the cooling step (S600) may be greater than 1 and less than or equal to 2 .

9 is a graph showing a temperature change in a conveying step of blanks manufactured from first to third parts.

The surface shape scale (R.L.T.) of each part can be derived using the surface profile of FIGS. 8A to 8C . In one embodiment, the surface shape scale (R.L.T.) of the first part may be 1.81, the surface shape scale (R.L.T.) of the second part may be 1.69, and the surface shape scale (R.L.T.) of the third part may be 1.66 days can That is, the surface shape scale (R.L.T.) of the first part may be the largest, and the surface shape scale (R.L.T.) of the third part may be the smallest.

Referring to FIG. 9 , the air cooling rate (or cooling rate) of the third part having a small surface shape measure (R.L.T.) is the slowest, and the air cooling rate (or cooling rate) of the first part having a large surface shape measure (R.L.T.) It can be seen that is the fastest. Through this, it can be confirmed that the smaller the surface shape scale (R.L.T.) of the part, the slower the air cooling rate (or cooling rate) in the process of transferring the heated blank to the press mold. That is, it can be seen that as the surface shape scale (R.L.T.) of the part is smaller, the air cooling rate (or cooling rate) in the process of transferring the heated blank to the press mold decreases.

In an embodiment, a large surface shape scale (R.L.T.) may mean that the manufactured hot stamped part has a large surface roughness, and a large roughness may indicate a large surface area. Therefore, the larger the surface area is, the more active cooling by radiation is, the larger the surface shape scale (R.L.T.), the faster the air cooling rate (or cooling rate) in the process of transferring the heated blank to the press mold. That is, the larger the surface shape scale (R.L.T.), the higher the air cooling rate (or cooling rate) in the process of transferring the heated blank to the press mold.

The surface shape scale R.L.T. may be affected by the temperature and time conditions in the heating step S300 . For example, when the average temperature increase rate of the blank in the second heating step (S320) is too fast or the total heating time in the heating step (S300) is large, the surface shape scale (R.L.T.) may increase.

In addition, the cooling behavior in the process of transferring the heated blank to the press mold, and the formability of the blank may be affected by the temperature and time conditions in the heating step (S300).

Accordingly, the cooling behavior of the heated blank and the formability of the blank may vary depending on the temperature and time conditions in the heating step ( S300 ), which may be indicated through the surface shape scale (R.L.T.).

When the average temperature increase rate of the blank in the second heating step (S320) is greater than the above-mentioned Equation 1, or the total heating time of the blank in the heating step (S300) is greater than the above-mentioned Equation 2, aluminum-silicon ( The chemical reaction of the Al-Si) plating layer may be accelerated to increase the roughness of the surface of the plating layer (or alloying layer if the plating layer is alloyed), and due to the increased surface roughness, the surface area of the plating layer (or alloying layer) can increase By increasing the surface area of the plating layer (or alloying layer), cooling by radiation becomes active, and the air cooling rate (or cooling rate) in the process of transferring the heated blank to the press mold increases, so that the molding start temperature can be lowered. Cracks or necking may be induced during molding, and the surface shape scale (R.L.T.) may increase. Therefore, when the average temperature increase rate of the blank in the second heating step (S320) is greater than the above-mentioned Equation 1, or the total heating time of the blank in the heating step (S300) is greater than the above-mentioned Equation 2, the manufactured The surface shape scale (R.L.T.) of hot stamped parts can be increased.

In one embodiment, the manufactured hot stamped part may have a surface shape scale (R.L.T.) greater than 1 and less than or equal to 2. The fact that the manufactured hot stamping part has a surface shape scale (R.L.T.) of greater than 1 and less than or equal to 2 means that the temperature and time for heating the blank in the heating step S300 are controlled so that the plating layer of the heated blank has a flat top surface, and the transfer It may mean that the air cooling rate (or cooling rate) of the heated blank in the process is reduced, thereby increasing the forming start temperature of the blank and improving the formability of the blank.

In one embodiment, the blank before the heating step (S300) may have a first specific gravity, and the hot stamping part after the cooling step (S600) may have a second specific gravity. Specifically, the blank having an aluminum-silicon (Al-Si) plating layer formed on at least one surface of the base material before the heating step (S300) has a first specific gravity, and the hot stamping part after the cooling step (S600) may have a second specific gravity. . At this time, the first specific gravity of the blank before the heating step (S300) and the second specific gravity of the hot stamping part after the cooling step (S600) can be measured by a buoyancy method using the Archimedes principle based on the ISO 2781 standard.

In one embodiment, the second specific gravity of the hot stamped part may be less than the first specific gravity of the blank. For example, the decrease (or change, difference) of the second specific gravity compared to the first specific gravity of the blank may be 1.5% or less. When the decrease (or change, difference) of the second specific gravity of the blank is greater than 1.5%, it may mean that the chemical reaction of the aluminum-silicon (Al-Si) plating layer is accelerated. As the chemical reaction of the aluminum-silicon (Al-Si) plating layer accelerates, the air cooling rate (or cooling rate) by radiation increases when the blank is air-cooled as it is exposed to atmospheric atmosphere in the process of transferring the heated blank to the press mold. As the temperature increases, the molding start temperature is lowered, and necking or cracks may occur during press molding. Therefore, when the decrease (or change, difference) of the second specific gravity of the blank is greater than 1.5%, the blank is air cooled as it is exposed to atmospheric atmosphere in the process of transferring the heated blank to the press mold, As the air cooling rate (or cooling rate) increases due to radiation, the molding start temperature is lowered, and necking or cracks may occur during press molding.

Therefore, that the decrease (or change, difference) of the second specific gravity of the manufactured hot stamping part relative to the first specific gravity of the blank is provided to be 1.5% or less, the aluminum-silicon (Al-Si) plating layer in the heating step The chemical reaction is controlled so that the plating layer of the heated blank has a flat top surface, and the air cooling rate (or cooling rate) of the heated blank during the transfer process is slowed, so that the forming start temperature of the blank increases and the formability of the blank is improved. can mean that

The average temperature increase rate of the blank in the second heating step (S300) is faster than the above-mentioned Equation 1 (eg, average temperature increase rate >7.841 / t ℃ / s), or the total heating time in the heating step (S300) is the above When larger than one Equation 2 (eg, total heating time > 300 x t + 60s), the difference between the specific gravity of the blank and the manufactured hot stamping part before the heating step (S300) becomes large, and the surface shape of the manufactured hot stamping part The scale (R.L.T.) may increase.

Specifically, the average temperature increase rate of the blank in the second heating step (S300) is faster than the above-mentioned Equation 1 (eg, average temperature increase rate >7.841 / t ℃ / s), or the entire heating in the heating step (S300) When the time is greater than the above Equation 2 (eg, total heating time > 300 x t + 60 s), the chemical reaction of the plating layer in the heating step is accelerated to increase the roughness and surface area of the plating layer, and the increased roughness, and Due to the surface area, the air cooling rate (or cooling rate) increases in the process of transferring the heated blank to the press mold, thereby lowering the molding start temperature, and necking or cracking may be induced during press molding. This may be indicated through an increase in the difference between the specific gravity of the blank and the specific gravity of the manufactured hot stamping part before the heating step (S300) or an increase in the surface shape scale (R.L.T.) of the manufactured hot stamping part.

In one embodiment, by controlling the average temperature increase rate of the blank in the second heating step (S320) to satisfy Equation 1 described above (or controlling) or controlling the total heating time to satisfy Equation 2 described above , it is possible to prevent or minimize the difference between the specific gravity of the blank and the specific gravity of the manufactured hot stamping part before the heating step (S300), and at the same time reduce the surface shape scale (R.L.T.) of the manufactured hot stamping part.

Specifically, the average temperature increase rate of the blank in the second heating step (S320) is controlled (or controlled) to satisfy the above-mentioned Equation 1, or the total heating time in the heating step (S300) is the above-mentioned Equation 2 By controlling to satisfy , the roughness and surface area of the plating layer may be reduced by controlling the chemical reaction of the plating layer in the heating step ( S300 ). In addition, by controlling the chemical reaction of the plating layer in the heating step (S300) to reduce the roughness and surface area of the plating layer, the air cooling rate (or cooling rate) in the process of transferring the heated blank to the press mold is reduced to form The initiation temperature can be increased, and the formability of the blank can be improved by preventing or minimizing necking or cracking during press forming. Therefore, the decrease (or change, difference) between the specific gravity of the blank and the specific gravity of the manufactured hot stamping part before the heating step (S300) may satisfy 1.5% or less, and the surface shape scale (R.L.T. ) may have a value greater than 1 and less than or equal to 2.

In one embodiment, the average temperature increase rate of the blank in the second heating step (S320) is controlled to satisfy Equation 1 described above, and the total heating time in the heating step (S300) is controlled to satisfy Equation 2 described above By doing so, the delayed rupture performance can be improved by controlling the chemical reaction of the aluminum-silicon (Al-Si) plating layer to suppress the amount of hydrogen mixed into the base material, and excellent spot weldability can be secured by reducing the dynamic resistance of the plating layer. . In addition, by making the surface shape of the plating layer (eg, alloying layer) of the heated blank more flat, the temperature drop due to radiation during the transport of the heated blank can be reduced, thereby improving the formability.

<Example>

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are provided to explain the present invention in more detail, and the scope of the present invention is not limited by the following examples. The following examples can be appropriately modified or changed by those skilled in the art within the scope of the present invention.

After the steel having the same component composition was heated according to the conditions shown in Table 1, hot stamping parts of Examples 1 to 6 and Comparative Examples 1 to 4 were manufactured. Specifically, the blanks having the thickness of Table 1 shown in Table 1 were heated according to the heating conditions of Table 1, and heated by crack heating (or crack maintenance), and then passed through a conveying step, a forming step, and a cooling step. Hot stamping parts of Examples 1 to 6 and Comparative Examples 1 to 4 were manufactured. In Table 1, the temperature increase rate means the average temperature increase rate of the blank in the second heating step (S320).

blank thickness
(mm) temperature rise rate
(℃/s) crack temperature
(℃) total heating time
(s) Example 1 1.2 5.343 950 240 Example 2 1.2 5.243 950 300 Example 3 1.2 5.217 950 360 Example 4 1.8 4.201 950 390 Example 5 1.8 4.157 950 450 Example 6 1.8 4.124 950 500 Comparative Example 1 1.2 9.612 950 390 Comparative Example 2 1.2 5.347 950 500 Comparative Example 3 1.8 7.271 950 390 Comparative Example 4 1.8 7.133 950 650

When the thickness of the blank is 1.2mm, the maximum temperature increase rate satisfying Equation 1 above is 6.534° C./s, and when the thickness of the blank is 1.8 mm, the maximum temperature increase rate satisfying Equation 1 above is 4.356° C. /s. Accordingly, Examples 1 to 3 have a temperature increase rate of 6.534° C./s or less, so Examples 1 to 3 satisfy Equation 1 described above, and Examples 4 to 6 have a temperature increase rate of 4.356° C. /s or less, Examples 4 to 6 satisfy Equation 1 described above.

On the other hand, since the temperature increase rate of Comparative Example 1 is 9.612 °C / s, and the temperature increase rate of Comparative Example 3 and Comparative Example 4 is 7.271 °C / s and 7.133 °C / s, respectively, Comparative Example 1 Comparative Example 3, and Comparative Example 4 does not satisfy Equation 1 described above.

When the thickness of the blank is 1.2 mm, the maximum heating time satisfying Equation 2 is 420 s, and when the thickness of the blank is 1.8 mm, the maximum heating time satisfying Equation 2 is 600 s. Therefore, since Examples 1 to 3 have a total heating time of 420 s or less, Examples 1 to 3 satisfy Equation 2 described above, and Examples 4 to 6 have a total heating time of 600 s or less. , Examples 4 to 6 satisfy Equation 2 described above.

On the other hand, since the total heating time of Comparative Example 2 is 500 s and the total heating time of Comparative Example 4 is 650 s, Comparative Examples 1 and 4 do not satisfy Equation 2 described above.

<Evaluation of specific gravity change>

In Examples 1 to 6, and Comparative Examples 1 to 4, the specific gravity change of the component after the cooling step (S600) compared to the blank before the heating step (S300) was measured. The specific gravity of the blank before the heating step (S300) and the part after the cooling step was measured by the buoyancy method using the Archimedes principle according to the ISO 2781 standard.

blank specific gravity
(g/cm ³ ) Part proportion
(g/cm ³ ) change in specific gravity
(%) Example 1 7.611 7.545 0.87% Example 2 7.637 7.568 0.91% Example 3 7.720 7.647 0.95% Example 4 7.702 7.629 0.95% Example 5 7.667 7.591 0.99% Example 6 7.647 7.555 1.2% Comparative Example 1 7.656 7.511 1.89% Comparative Example 2 7.783 7.609 2.23% Comparative Example 3 7.654 7.519 1.76% Comparative Example 4 7.664 7.472 2.5%

Table 2 shows the results of measuring the specific gravity of the parts after the heating step (S300) before the heating step (S300) and the cooling step (S600) of Examples 1 to 6, and Comparative Examples 1 to 4, and parts compared to the specific gravity of the blank This is a table showing the change (decrease value) of the specific gravity.

Referring to Table 2, when the above-described Equations 1 and 2 are satisfied, the change (reduction value) of the specific gravity of the component relative to the specific gravity of the blank satisfies 1.5% or less, but the above-described Equation 1 and/or Math When Equation 2 is not satisfied, it can be seen that the change (reduction value) of the specific gravity of the component to the specific gravity of the blank exceeds 1.5%.

<Evaluation of surface shape scale (R.L.T.)>

The surface shape scales (R.L.T.) of Examples 1 to 6 and Comparative Examples 1 to 4 were derived through Equation 3 described above. At this time, a non-contact three-dimensional shape measuring machine was used to measure the surface shape of the part according to ISO 4287, and it was measured 5 times or more in an arbitrary direction by a line measurement method, and 4 mm or more was measured at one time.

surface shape scale
(RLT) Example 1 1.694 Example 2 1.711 Example 3 1.732 Example 4 1.658 Example 5 1.649 Example 6 1.637 Comparative Example 1 2.07 Comparative Example 2 2.421 Comparative Example 3 2.332 Comparative Example 4 2.465

Table 3 is a table showing the surface shape scale (R.L.T.) derived results of Examples 1 to 6, and Comparative Examples 1 to 4.

Referring to Table 3, when the above-mentioned Equations 1 and 2 are satisfied, the surface shape scale (R.L.T.) of the manufactured hot stamping part satisfies more than 1 and 2 or less, and the above-mentioned Equations 1 and/or When Equation 2 is not satisfied, it can be confirmed that the surface shape scale (R.L.T.) of the manufactured hot stamping part exceeds 2.

<Hydrogen embrittlement evaluation>

For the parts of Examples 1 to 6 and Comparative Examples 1 to 4, hydrogen embrittlement was evaluated using a Thermal Desorption Spectroscopy (TDS) equipment according to ISO16573-2015 regulations. That is, under a vacuum atmosphere, the parts of Examples 1 to 6 and Comparative Examples 1 to 4 were heated, respectively, and the amount of diffusible hydrogen emitted from the parts was measured at 300° C. or less.

diffusible amount of hydrogen
(ppm) Delayed rupture Example 1 0.432 not broken Example 2 0.444 not broken Example 3 0.457 not broken Example 4 0.468 not broken Example 5 0.471 not broken Example 6 0.481 not broken Comparative Example 1 0.574 break Comparative Example 2 0.595 break Comparative Example 3 0.513 not broken Comparative Example 4 0.611 break

Table 4 is a table showing the results of measuring the amount of diffusible hydrogen emitted from Examples 1 to 6 and Comparative Examples 1 to 4 and whether or not delayed rupture.

Referring to Table 4, it can be confirmed that the amount of diffusible hydrogen emitted from Examples 1 to 6 is 0.5 ppm or less, but the amount of diffusive hydrogen emitted from Comparative Examples 1 to 4 exceeds 0.5 ppm. there is. In addition, when Equations 1 and 2 are satisfied, delayed fracture does not occur, but when Equations 1 and/or Equation 2 are not satisfied, delayed fracture may occur.

Therefore, when the aforementioned Equations 1 and 2 are satisfied, the amount of hydrogen flowing into the base material from the plating layer is small, so hydrogen embrittlement may be improved and delayed fracture may not occur.

<Evaluation of moldability>

Examples 1 to 6, and Comparative Examples 1 to 4, the temperature was measured when 12 seconds elapsed after taking out the heating furnace, Examples 1 to 6, and Comparative Examples 1 to 4 were molded It was measured whether cracks or necking occurred afterward.

12 seconds after taking out the furnace
Elapsed temperature (℃) Whether cracks or necking occur Example 1 672 non-occurring Example 2 665 non-occurring Example 3 659 non-occurring Example 4 714 non-occurring Example 5 708 non-occurring Example 6 703 non-occurring Comparative Example 1 603 crack Comparative Example 2 589 crack Comparative Example 3 627 necking Comparative Example 4 615 crack

Table 5 is a table showing the temperature and whether cracks or necking occurs when 12 seconds have elapsed after taking out from the heating furnace of Examples 1 to 6, and Comparative Examples 1 to 4.

Referring to Table 5, if the above-described Equations 1 and 2 are satisfied, the temperature exceeds 650 ° C. when 12 seconds have elapsed after taking out the heating furnace, but the above-mentioned Equations 1 and / or 2 are not satisfied In this case, it can be confirmed that the temperature is less than 630°C when 12 seconds have elapsed after taking out the furnace. In particular, when the thickness of the blank is thin, there was a case where the temperature was lower than 600° C. when 12 seconds elapsed after taking out the furnace. Through this, when the thickness of the blank is thin, it can be confirmed that the air cooling rate (or cooling rate) of the blank becomes faster in the process of being taken out and then transported to the heating furnace.

In addition, if the temperature exceeds 650 ℃ when the temperature exceeds 650 ℃ after taking out from the heating furnace, cracks or necking did not occur when pressing, but when the temperature is less than 630 ℃ when the temperature is 12 seconds after taking out the furnace case existed.

Therefore, when the above-mentioned Equations 1 and 2 are satisfied, the air cooling rate (or cooling rate) in the process of transferring the heated blank to the press mold is reduced to prevent or minimize the lowering of the molding start temperature. This can improve the moldability. On the other hand, when the above-mentioned Equations 1 and/or 2 are not satisfied, the air cooling rate (or cooling rate) increases in the process of transferring the heated blank to the press mold, and the molding start temperature is lowered to lower the moldability. This can be reduced.

In one embodiment, when the above-mentioned Equations 1 and 2 are satisfied, the rate at which the heated blank is air cooled in the process of transferring it to the press mold by controlling the chemical reaction of the plating layer in the heating step (or cooling speed), the forming start temperature of the blank can be increased, and the formability of the blank is improved, so that the change (decrease value) of the specific gravity of the part relative to the specific gravity of the blank can be 1.5% or less, and the surface shape measure of the hot stamping part ( R.L.T.) may be provided in more than 1 and 2 or less.

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

Claims (14)

A method of manufacturing a hot stamped part, comprising:
preparing a blank in which a plating layer is formed on at least one surface of a base material;
putting the blank into a heating furnace;
a heating step of heating the blank; and
a cooling step of cooling the heated blank;
including,
The heating step is
A first heating step of heating the blank on which the plating layer is formed to a first temperature:
a second heating step of heating the blank on which the plating layer is formed to a second temperature; and
a third heating step of heating the blank on which the plating layer is formed to a third temperature;
the blank before the heating step has a first specific gravity, and the hot stamped part after the cooling step has a second specific gravity;
The second specific gravity is smaller than the first specific gravity,
wherein the first temperature is 600°C, the second temperature is Ac3, and the third temperature is greater than Ac3 and less than or equal to 1000°C. According to claim 1,
The change (reduction value) of the second specific gravity relative to the first specific gravity is greater than 0 and 1.5% or less, the method of manufacturing a hot stamping part. According to claim 1,
A method of manufacturing a hot stamping part, wherein the surface shape scale (RLT) of the hot stamping part is greater than 1 and equal to or less than 2, and the surface shape scale (RLT) is derived by Equation 1 below.
<Equation 1>

(Where n is the number of measurements, d is the measurement length (mm), and f(x) is the surface shape equation) According to claim 1,
The average temperature increase rate of the blank in the second heating step satisfies the following Equation 2, a method of manufacturing a hot stamping part.
<Equation 2>
Average temperature increase rate ≤ 7.841 / t ℃/s
(In this case, t is the thickness of the blank (mm)) The method of claim 1,
The total heating time during which the first heating step, the second heating step, and the third heating step are performed satisfies Equation 3 below.
<Equation 3>
Total heating time ≤ 300 xt + 60s
(In this case, t is the thickness of the blank (mm)) According to claim 1,
In the second heating step, the alloying of the plating layer and the phase transformation of the base material proceed,
In the third heating step, the homogenization treatment of the base material is made, a method of manufacturing a hot stamping part. 7. The method of claim 6,
The base material includes Fe, and the plating layer includes Al and Si. According to claim 1,
The heating furnace includes a heating section, and a cracking section,
wherein the blank is heated for a first time in the heating zone and the blank is heated for a second time in the cracking zone. 9. The method of claim 8,
The ratio of the first time to the second time satisfies 4:1 to 1:1. 9. The method of claim 8,
The method of claim 1, wherein the first heating step and the second heating step are performed in the heating section, and the third heating step is performed in the cracking section. 9. The method of claim 8,
The method of claim 1, wherein the temperature for heating the blank in the heating section rises stepwise. 9. The method of claim 8,
The temperature of the cracking section is more than Ac3 and less than or equal to 1000 ℃, the manufacturing method of a hot stamping part. The method of claim 1,
between the heating step and the cooling step,
transferring the heated blank from the heating furnace to a press mold; and
forming a molded body by hot stamping the transferred blank;
Further comprising, a method of manufacturing a hot stamping part. 14. A hot stamping part manufactured according to any one of claims 1 to 13, comprising:
wherein the surface shape scale (RLT) of the hot stamping part is greater than 1 and less than or equal to 2, and the surface shape scale (RLT) is derived by the following equation (4).
<Equation 4>

(Where n is the number of measurements, d is the measurement length (mm), and f(x) is the surface shape equation)

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