JP7713478B2 - Hot stamping part and manufacturing method thereof - Google Patents

Description

The present invention relates to hot stamping parts and their manufacturing methods.

As environmental and fuel economy regulations become stronger worldwide, the need for lighter vehicle materials is increasing. This has led to active research and development of ultra-high strength steels and hot stamping steels. Here, the hot stamping process generally consists of heating/forming/cooling/trim, and utilizes phase transformation and microstructural changes of the material during the process.

Recently, research has been actively conducted to improve delayed fracture, corrosion resistance, and weldability of hot stamped parts manufactured by the hot stamping process. Related technology is Korean Patent Publication No. 10-2018-0095757 (Invention title: Manufacturing method for hot stamped parts).

Republic of Korea Patent Publication No. 10-2018-0095757

The problem to be solved by the present invention is to provide a hot stamping part and a manufacturing method thereof that can prevent or minimize the occurrence of quality differences between at least two blanks, tailor welded blanks, or tailor rolled blanks that differ from each other in at least one of thickness and size, even when the blanks are heated simultaneously in a heating furnace.

One embodiment of the present invention discloses a method for manufacturing a hot stamped part, comprising: a step of inserting a blank into a heating furnace having a plurality of sections having different temperature ranges; a multi-stage heating step of heating the blank in stages; a soaking heating step of heating the blank at a temperature of Ac3 to 1,000°C; and a transport step of transporting the heated blank from the heating furnace to a die; wherein in the multi-stage heating step, a temperature condition in the heating furnace satisfies the following formula:

(In the above formula, Tg is the soaking heating temperature (° C.), Ti is the initial temperature at the start of the heating furnace (° C.), and Lt is the length of the section in which the multi-stage heating step is performed (mm).)
In this embodiment, in the plurality of sections, the ratio of the length of the section in which the blank is heated in multiple stages to the length of the section in which the blank is heated uniformly can satisfy 1:1 to 4:1.

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

In this embodiment, the blank includes 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 multiple sections may increase from the inlet of the furnace to the outlet of the furnace.

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

In this embodiment, among the multiple sections, the temperature of the section in which the blank is soaked is higher than the temperature of the section in which the blank is heated in multiple stages.

In this embodiment, the blank can remain in the heating furnace for 180 to 360 seconds.

In this embodiment, the air-cooling time of the blank in the transport step satisfies the following Equation 2.

(In this case, _λt is the air cooling time (s), _at is a correction coefficient considering the temperature at the time of removal from the heating furnace and the atmospheric temperature, _Tt is the heating temperature (°C), _bt is a correction coefficient considering the material composition, t is the material thickness (mm), and _ct is a correction coefficient considering the material thickness sensitivity to high temperatures.)
In this embodiment, in the formula 2, the a _t is 0.0160 or more and 0.0165 or less, the T _t is Ac3 or more and 1000° C. or less, the b _t is −10 or more and 0.5 or less, the t is 1 mm or more and 2.6 mm or less, and the c _t is 0.7 or more and 0.9 or less.

In this embodiment, in the above formula 2, λ _t is not less than 5 s and not more than 20 s.

Another embodiment of the present invention discloses a hot stamped part having a diffusible hydrogen content of less than 0.45 ppm and a corrosion rate of less than 3×10 ⁻⁶ A as measured through potentiodynamic polarization testing.

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

According to an embodiment of the present invention, the blank is heated in multiple stages in a heating furnace having multiple sections with different temperature ranges, thereby enabling more precise control of the time for the blank to reach the soaking temperature.

In addition, by more precisely controlling the time it takes for blanks with different thicknesses to reach the soaking temperature, the hydrogen embrittlement resistance, corrosion resistance, and weldability of parts manufactured by the hot stamping part manufacturing method can be improved.

1 is a flow chart that illustrates a schematic diagram of a method for manufacturing a hot stamped part according to an embodiment of the present invention. FIG. 2 is a plan view showing a schematic diagram of a blank used in a method for manufacturing a hot stamped part according to an embodiment of the present invention. FIG. 2 is a plan view showing a blank inserted into a heating furnace in a method for manufacturing a hot stamped part according to an embodiment of the present invention; 1 is a graph showing the temperature change of a blank when the blank is heated singly by a conventional method. 4 is a graph showing temperature changes when a blank is multi-stage heated and uniformly heated in a manufacturing method of a hot stamped part according to an embodiment of the present invention. 1 is a graph showing high-temperature tensile properties depending on the forming start temperature of a heated blank. 4 is a graph showing temperature changes when a blank is multi-stage heated and uniformly heated in a manufacturing method of a hot stamped part according to an embodiment of the present invention. 1 is a diagram showing air-cooling time depending on material thickness and heating temperature. 1 is a graph showing hydrogen release rates from parts manufactured under the conditions of an example, a comparative example 1, and a comparative example 2. 1 is a graph showing the results of corrosion resistance evaluation for parts manufactured under the conditions of an example, comparative example 1, and comparative example 2. 1 is a graph showing resistance values for parts manufactured under conditions of an example, comparative example 1, and comparative example 2.

The present invention can be modified in various ways and can have various embodiments, and specific embodiments are illustrated in the drawings and will be described in detail in the detailed description. The advantages and features of the present invention, as well as the methods for achieving them, will become apparent from the following detailed description of the embodiments together with the drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various forms.

In the following examples, terms such as "first" and "second" are not used in a limiting sense but are used to distinguish one component from another.

In the following examples, singular expressions include plural expressions unless the context clearly indicates otherwise.

In the following examples, terms such as "comprise" or "have" mean the presence of the features or components described in the specification, but do not preclude the possibility that one or more other features or components may also be added.

In the following examples, when a film, region, component, or other part is said to be "on" or "above" another part, this does not only mean that it is directly on top of the other part, but also includes cases where there is another film, region, component, etc. in between.

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

When an embodiment can be implemented differently, the order of certain steps may be performed out of the order described. For example, two steps described as successive may be performed substantially simultaneously or may be performed in the opposite order to that described.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. When describing the embodiment with reference to the drawings, the same or corresponding components are designated by the same reference numerals.

Figure 1 is a flow chart that shows a schematic diagram of a method for manufacturing a hot stamped part according to one embodiment of the present invention. The method for manufacturing a hot stamped part will be described below with reference to Figure 1.

The method for manufacturing a hot stamping part according to one embodiment of the present invention includes a blank inserting step (S110), a multi-stage heating step (S120), and a soaking heating step (S130), and further includes a transfer step (S140), a forming step (S150), and a cooling step (S160) after the soaking heating step (S130).

First, the blank loading step (S110) is a step of loading blanks into a heating furnace having multiple sections with different temperature ranges.

The blanks fed into the heating furnace are also formed by cutting sheet material for forming hot stamping parts. The sheet material can be manufactured by hot rolling or cold rolling a steel slab and then annealing the sheet material. After the annealing, an Al-Si-based plating layer or a Zn plating layer can be formed on at least one surface of the annealed sheet material.

Figure 2 is a plan view that shows a schematic diagram of a blank used in a method for manufacturing a hot stamped part according to one embodiment of the present invention.

Referring to FIG. 2, the blank 200 according to one embodiment includes at least one of a blank 210 having a uniform thickness, a tailor welded blank 220 (TWB) in which different types of plate materials having different thicknesses are cut into a required shape and welded together, a tailor rolled blank 230 (TRB) in which plate materials of a uniform thickness are rolled to have different thicknesses, and a patchwork 240 manufactured by welding a small patch blank to a large blank.

The tailor welded blank 220 can be manufactured by welding a first plate material 221 and a second plate material 223 having different thicknesses. The B-pillar, an important part for a vehicle collision member, can be manufactured by welding two plates with different strengths connected to the upper collision support part and the lower impact absorption part, and then forming them. The tailor welded blank method mainly used here means a series of processes in which different types of plate materials with different thicknesses, strengths, and materials are cut into the required shape, welded, and then press-formed to manufacture parts. By welding plates with different thicknesses to manufacture blanks with different thicknesses, different properties can be given to different parts of the blank. For example, a 120 to 200K class ultra-high strength plate material is used for the upper impact support part of the B-pillar, and a plate material with excellent impact absorption performance is connected to the lower end of the B-pillar where stress is concentrated, so that the impact absorption capacity during a vehicle collision can be improved.

The tailor rolled blank 230 is manufactured by rolling a cold-rolled steel material to have a specific thickness profile, and when a hot stamping part is manufactured using the tailor rolled blank 230, the weight reduction effect is excellent. As an example, the thickness profile can be implemented by a conventional method. For example, when the cold-rolled steel material is cold-rolled, the rolling reduction rate can be adjusted to form a tailor rolled blank 230 including a first region 231 having a first thickness, a second region 232 having a second thickness, a third region 233 having a third thickness, and a fourth region 234 having a fourth thickness. In this case, the first thickness, the second thickness, the third thickness, and the fourth thickness are different from each other, and a transition section 235 may exist between the first region 231 and the second region 232, between the second region 232 and the third region 233, and between the third region 233 and the fourth region 234. However, although FIG. 2 illustrates the tailor rolled blank 230 including the first region 231 to the fourth region 234, the present invention is not limited thereto. The tailor rolled blank 230 may be formed to include the first region 231, the second region 232, ..., the nth region.

Patchwork 240 is a method of partially reinforcing a base material using at least two or more plate materials, and the base material and the patch can be formed simultaneously by joining the patch to the base material before the molding process. As an example, a patch 243 having a second size smaller than the first size can be welded to a base material 241 having a first size and then molded simultaneously.

Figure 3 is a plan view that shows a schematic of a blank placed in a heating furnace in a method for manufacturing a hot stamped part according to one embodiment of the present invention.

In the blank loading step (S100), at least two blanks 200 that differ from each other in at least one of thickness and size may be loaded into the heating furnace simultaneously.

3 shows two first blanks 250 and two second blanks 260 that are simultaneously inserted into a heating furnace. In this case, the first blanks 250 and the second blanks 260 may have different sizes and 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 inserted into the heating furnace. In addition, the first blanks 250 and the second blanks 260 may be formed to have the same size and different thicknesses, or may be formed to have the same thickness and different sizes, and various modifications are possible.

Alternatively, in the blank loading step (S100), at least two blanks 200 having a single thickness may be loaded simultaneously into the heating furnace. For example, at least two blanks 250 having a thickness of 1.2 mm may be loaded simultaneously, and at least two blanks 260 having a thickness of 1.6 mm may be loaded simultaneously. Also, in the blank loading step (S100), the tailor welded blank 220 (see FIG. 2) or tailor rolled blank 230 (see FIG. 2) described above may be loaded into the heating furnace.

The blanks placed in the heating furnace can be mounted on rollers and then transported along the transport direction.

After the blank insertion step (S110), a multi-stage heating step (S120) and a uniform heating step (S130) can be performed. The multi-stage heating step (S120) and the uniform heating step (S130) are also steps in which the blank is heated by passing through multiple sections provided in the heating furnace.

Specifically, in the multi-stage heating step (S120), the blank can be heated stepwise as it passes through multiple sections provided in the heating furnace. Among the multiple sections provided in the heating furnace, there are multiple sections in which the multi-stage heating step (S120) is performed, and the temperature of the blank can be raised stepwise by setting the temperature for each section so that it increases from the entrance of the heating furnace where the blank is inserted toward the exit of the heating furnace where the blank is removed.

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

4 is a graph showing the temperature change of a blank when the blank is heated by a single heating method according to the prior art. Specifically, FIG. 4 is a graph showing the temperature change of the blanks over time when a blank having a _thickness of 1.2 mm and a blank having a thickness of 1.6 mm are simultaneously heated by a single heating (320) after the temperature of the furnace is set so that the internal temperature of the furnace is kept equal to the target temperature Tt of the blank.

In this case, the target temperature _Tt of the blank is Ac3 or more. Preferably, the target temperature _Tt of the blank is 930° C. More preferably, the target temperature _Tt of the blank is 950° C. However, the present invention is not limited thereto. In addition, the single heating does not mean that a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm are placed in a heating furnace and heated, but means that the temperature of the heating furnace is set to a single temperature, and then the blank having a thickness of 1.2 mm and the blank having a thickness of 1.6 mm are placed in the heating furnace at the same time and heated.

Referring to FIG. 4, when the temperature inside the heating furnace is set to the same temperature as the target temperature _Tt of the blanks and then a blank having a thickness of 1.2 mm and a blank having a thickness of 1.6 mm are heated simultaneously, it can be seen that the blank having a thickness of 1.2 mm reaches the target temperature _Tt earlier than the blank having a thickness of 1.6 mm.

That is, the blank having a thickness of 1.2 mm reaches the target temperature _Tt first, and the blank having a thickness of 1.2 mm is soaked for a first time _S1 , and the blank having a thickness of 1.6 mm is soaked for a second time S2 that is shorter than _the first time _S1 . Since the soaking time is adjusted based on the blank that reaches the target temperature last, the blank having a thickness of 1.2 mm that reaches the target temperature first may be overheated, which may increase delayed fracture of the blank having a thickness of 1.2 mm and decrease weldability.

Figure 5 is a graph showing temperature changes when a blank is heated in multiple stages and uniformly heated in a method for manufacturing a hot stamped part according to one embodiment of the present invention. Figure 5 is a graph showing temperature changes over 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) in one embodiment of the present invention.

5, the furnace according to an embodiment may include a plurality of sections having different temperature ranges. More specifically, the furnace may include a first section P1 having a first temperature range _T1 , a second section _P2 having a second temperature range _T2 , a third section _P3 having a third temperature range _T3 , a fourth section _P4 having a fourth temperature range _T4 , a fifth section _P5 having a _fifth temperature range _T5 , a sixth section _P6 having a sixth temperature range _T6 , and a seventh section _{P7 having a seventh temperature range T7 .}

The first section _P1 to the seventh section _P7 may be arranged in the furnace in the specified order. The first section _P1 having the first temperature range _T1 may be adjacent to the entrance of the furnace where the blanks are inserted, and the seventh section _P7 having the seventh temperature range _T7 may be adjacent to the exit of the furnace where the blanks are discharged. Therefore, the first section _P1 having the first temperature range _T1 may be the first section of the furnace, and the seventh section _P7 having the seventh temperature range _T7 may be the last section of the furnace. As described below, among the sections of the furnace, the fifth section _P5 , the sixth section _P6 , and the seventh section _P7 are sections where soaking heating is performed, not sections where multi-stage heating is performed.

The temperatures of the sections in the heating furnace, for example, the temperatures of the first section _P1 to the seventh section _P7 , may increase from the entrance of the heating furnace where the blanks are inserted to the exit of the heating furnace where the blanks are removed. However, the temperatures of the fifth section _P5 , the sixth section _P6 , and the seventh section _P7 may be the same. Also, the temperature difference between two adjacent sections among the sections in the heating furnace may be greater than 0° C. and less than 100° C. For example, the temperature difference between the first section _P1 and the second section _P2 may be greater than 0° C. and less than 100° C.

In one embodiment, the first temperature range _T1 of the first section _P1 may be 820°C to 860°C or 835°C to 865°C. The second temperature range _T2 of the second section _P2 may be 850°C to 890°C or 865°C to 895°C. The third temperature range _T3 of the third section _P3 may be 880°C to 920°C or 895°C to 925°C. The fourth temperature range T4 of the _fourth section _P4 may be 910°C to 940°C or 915°C to 945°C. The fifth temperature range _T5 of the fifth section _P5 may be Ac3 to 1,000°C. Preferably, the fifth temperature range _T5 of the fifth section _P5 may be 930°C to 1,000°C. More preferably, the fifth temperature range _T5 of the fifth section _P5 is 950° C. to 1,000° C. The sixth temperature range _T6 of the _sixth section P6 and the seventh temperature range _T7 of the _seventh section P7 are the same as the fifth temperature range _T5 of the fifth section _P5 , but the present invention is not limited thereto.

In FIG. 5, a furnace according to an embodiment of the present invention is illustrated as having seven sections with different temperature ranges, but the present invention is not limited thereto. The furnace may have five, six, eight, etc. sections with different temperature ranges.

According to an embodiment, the blank may be heated stepwise by passing through a plurality of sections defined in a heating furnace. In an embodiment, in a multi-stage heating step in which the blank is heated stepwise by passing through a plurality of sections in a heating furnace, a temperature condition in the heating furnace may satisfy the following Equation 1:

Here, Tg is the soaking heating temperature (° C.), Ti is the initial temperature entering the heating furnace (° C.), and Lt is the length (mm) of the section where the multi-stage heating step is performed.

If the value of Equation 1 exceeds 0.025° C./mm, the initial temperature entering the heating furnace becomes low, the blank temperature rise rate decreases, and sufficient soaking time cannot be secured, and if the roller driving speed is reduced to secure the soaking time, the productivity may decrease. Also, if the value of Equation 1 is 0° C./mm, it corresponds to single heating, and as described above, there may be cases where the thin blank reaches the target temperature _Tt first, causing overheating of the thin blank.

4 and 5, when the blank is heated in multiple stages through a plurality of sections (e.g., the first section _P1 to the fourth section _P4 ) defined in the heating furnace, and the temperature conditions of the multiple heating satisfy the above formula, the temperature change graphs of the blanks having different thicknesses may show similar behaviors compared to the case where the blank is heated by single heating. For example, when the same time has elapsed after the blanks are put into the heating furnace, the temperature difference between the blanks when the blanks having a thickness of 1.2 mm are heated in multiple stages (330) and the blanks having a thickness of 1.6 mm are heated in multiple stages (340) is smaller than the temperature difference between the blanks when the blanks having a thickness of 1.2 mm are heated in single heating (310) and the blanks having a thickness of 1.6 mm are heated in single heating (320). Therefore, when the blanks are heated in multiple stages, the temperature rise rate of the blanks having different thicknesses is controlled in the same way, thereby reducing the time difference between the blanks reaching the target temperature and preventing the thin blanks from being overheated.

After the multi-stage heating step (S120), a soaking heating step (S130) may be performed. The soaking heating step (S130) is the last of the multiple sections in the heating furnace, and is a step of soaking the blank at a temperature of 950°C to 1,000°C.

The soaking step (S130) may be performed in the last of the multiple sections of the furnace. As an example, the soaking step (S130) may be performed in the fifth section _P5 , the sixth section _P6 , and the seventh section _P7 of the furnace. When multiple sections are provided in the furnace, if one section is long, there may be a problem that temperature changes occur within the section. Therefore, the sections in which the soaking step (S130) is performed are divided into the fifth section P5, the sixth section _P6 , and the seventh section _P7 , and the fifth section _P5 , the _sixth section _P6 , and the seventh section _P7 may have the same temperature range within the furnace.

In the soaking heating step (S130), the multi-stage heated blank can be soaked at a temperature of Ac3 to 1,000°C. Desirably, in the soaking heating step (S130), the multi-stage heated blank can be soaked at a temperature of 930°C to 1,000°C. More desirably, in the soaking heating step (S130), the multi-stage heated blank can be soaked at a temperature of 950°C to 1,000°C.

Figure 6 is a graph showing high-temperature tensile properties depending on the forming start temperature of the heated blank. Figure 6 is a graph showing high-temperature tensile tests for blank 410 that was soaked at 950°C, removed, and exposed to air cooling for 10 seconds, and blank 420 that was soaked at 900°C, removed, and exposed to air cooling for 10 seconds. In this case, the forming start temperature of blank 410 that was soaked at 950°C, removed, and exposed to air cooling for 10 seconds is 650°C to 750°C, and the forming start temperature of blank 420 that was soaked at 900°C, removed, and exposed to air cooling for 10 seconds is 550°C to 650°C.

Referring to FIG. 6, it was confirmed that the blank 410, which was soaked at 950°C, removed, and then exposed to air cooling for 10 seconds, had a lower true stress than the blank 420, which was soaked at 900°C, removed, and then exposed to air cooling for 10 seconds. Therefore, if the soaking temperature in the heating furnace is less than 950°C, the press forming start temperature becomes excessively low due to the air cooling exposure time after the heated blank is removed from the heating furnace, and the elongation rate of the heated blank decreases, causing a thickness reduction or breakage during forming. As the heated blank cools during the air cooling exposure time, the strength of the blank increases, and a large force is required to simultaneously form multiple blanks, which overloads the press equipment. In addition, if the soaking temperature exceeds 1,000°C, carbide-forming elements and nitride-forming elements such as Ti, V, Nb, and Mo in the blank are dissolved in the base material, making it difficult to suppress grain coarsening.

In one embodiment, among the multiple sections in the heating furnace, the temperature of the section in which the blank is heated uniformly is higher than or equal to the temperature of the section in which the blank is heated in multiple stages.

In one embodiment, the blank remains in the heating furnace for 180 to 360 seconds. More specifically, the time for which the blank is multi-stage heated and soaked in the heating furnace is 180 to 360 seconds. If the blank remains in the heating furnace for less than 180 seconds, it is difficult to sufficiently soak at the desired soaking temperature. Also, if the blank remains in the heating furnace for more than 360 seconds, the amount of hydrogen that penetrates into the blank increases, increasing the risk of delayed fracture and reducing corrosion resistance after hot stamping.

Figure 7 is a graph showing temperature changes when a blank is heated in multiple stages and uniformly in a method for manufacturing a hot stamped part according to one embodiment of the present invention. The graph in Figure 7 is a graph showing the temperature of the blank according to distance, unlike the graph in Figure 5.

7, in one embodiment, the heating furnace may have a length of 20m to 40m along the blank transfer path. The heating furnace includes a plurality of sections having different temperature ranges, and the ratio of the length _D1 of the section in which the blank is heated in multiple stages to the length _D2 of the section in which the blank is soaked among the plurality of sections satisfies 1:1 to 4:1. For example, the section in which the blank is soaked among the plurality of sections may be the last part of the heating furnace (e.g., the fifth section _P5 to the seventh section _P7 ). If the length of the section in which the blank is soaked is increased so that the ratio of the length _D1 of the section in which the blank is heated in multiple stages to the length _D2 of the section in which the blank is soaked exceeds 1:1, an austenite (FCC) structure may be generated in the soaked heating section, increasing the amount of hydrogen penetration into the blank, and increasing delayed fracture. In addition, if the length of the section in which the blank is soaked is reduced and the ratio of the length _D1 of the section in which the blank is heated in multiple stages to the length _D2 of the section in which the blank is soaked is less than 4:1, the soaked heating section (time) is not sufficiently secured, and the strength of the part manufactured by the hot stamping part manufacturing process becomes non-uniform.

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

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

In one embodiment, the transfer step (S140) is also a step of transferring the heated blank from the heating furnace to the mold. In this case, in the transfer step (S140), the heated blank may be cooled to ambient temperature (or room temperature) while being transferred to the mold. The heated blank may be air-cooled during transfer. If the heated blank is not air-cooled, the mold entry temperature (e.g., molding start temperature) may become high, and wrinkles (or bends) may occur on the surface of the manufactured hot stamped part. In addition, when a refrigerant is used, it may affect the subsequent process (hot stamping), so it is preferable that the heated blank is air-cooled during transfer.

The forming step (S150) is a step of hot stamping the transferred blank to form a compact. The cooling step (S160) is a step of cooling the formed compact.

After being molded into the final part shape in the press die, the molded body can be cooled to form the final product. The press die can be provided with a cooling channel through which a refrigerant circulates. The heated blank can be quenched by circulating the refrigerant supplied through the cooling channel provided in the press die. In this case, in order to prevent the springback phenomenon of the plate material and to maintain the desired shape, quenching can be performed while pressurizing the press die in a closed state. When performing the molding and cooling operation of the heated blank, it can be cooled to the martensite finish temperature at an average cooling rate of at least 10°C/s. The blank can be held in the press die for 3 to 20 seconds. If the holding time in the press die is less than 3 seconds, the material cannot be sufficiently cooled, and thermal deformation can occur due to the residual heat of the product and the temperature deviation in each part, which can reduce the dimensional quality. In addition, if the holding time in the press die exceeds 20 seconds, the holding time in the press die is extended, which reduces productivity.

Figure 8 is a diagram showing the air cooling time according to material thickness and heating temperature. Specifically, Figure 8 is a graph shown to explain the maximum allowable air cooling time according to material thickness and heating temperature. In Figure 8, a high heating temperature can be understood to mean that the heating furnace removal temperature is high.

Referring to Figures 1 and 8, it can be seen that for the same material thickness, the maximum allowable air cooling time increases as the heating temperature decreases. It can also be seen that for the same heating temperature, the maximum allowable air cooling time increases as the material thickness increases.

If the heated blank is exposed to room temperature too much, not only will productivity decrease, but the blank will undergo phase transformation during air cooling, reducing formability and making it difficult to obtain the desired material. On the other hand, if the heated blank is exposed to room temperature for a short time, wrinkles (or bending) may occur in the hot stamping part manufactured by starting forming at an excessively high temperature. In addition, the plating layer of the blank may be burned onto the die. Therefore, it is necessary to adjust the air cooling time in the transfer step (S140). However, in order to adjust the air cooling time in the transfer step (S140), various variables must be considered, including not only the heating temperature and the thickness of the blank (e.g., the thickness of the material), but also the thermal conductivity, thermal conductivity and heat transfer rate of the blank due to the amount of plating and surface emissivity, and the temperature at which the blank is removed from the heating furnace and the air temperature.

Therefore, the present inventors conducted extensive repetitive experiments and derived Equation 2, which allows easy control of the air-cooling time. In one embodiment, the air-cooling time of the blank in the transfer step (S140) may satisfy the following Equation 2.

In Equation 2, _λt is the cooling time (s), _at is a correction coefficient considering the temperature at the time of removal from the heating furnace and the atmospheric temperature, _Tn is the heating temperature (°C), _bt is a correction coefficient considering the material composition, _cn is a correction coefficient considering the material thickness sensitivity to high temperatures, and t is the material thickness (mm). In this case, the material refers to a blank, and the unit of the cooling time, s, refers to seconds.

a _t is a correction coefficient that takes into account the temperature at which the heated blank is removed from the heating furnace and the atmospheric temperature, and may have a value of about 0.0160 to about 0.0165 inclusive, where a _t may have a unit of s/(° C.×mm).

_bt is a correction coefficient that takes into account the case where each material has different components, and may have a value of about -10.0 _to about 0.5. _bt may have a unit of s _/ mm.

In addition, the amount of heat transfer from inside the material varies depending on the thickness of the material. c _t is a correction coefficient that takes into account the difference in heat transfer amount depending on the thickness of the material at high temperatures and may have a value of about 0.7 to about 0.9. In this case, high temperature means 600°C or higher. However, high temperature means 500°C or higher or 700°C or higher.

The heating temperature _Tt refers to the soaking temperature in the soaking step (S130), _and may have a value of about Ac3 or more and about 1000° C. or less. In this case, the heating temperature _Tt may refer to the temperature of removing from the heating furnace. In addition, the material thickness (t) may have a value of about 1 mm or more and about 2.6 mm or less.

In one embodiment, the air cooling time λ _t according to Equation 2 is about 5 s to about 20 s. If the air cooling time λ _t is less than 5 s, the forming start temperature at which the blank starts to be formed is excessively high, and the blank is formed at a high temperature, causing wrinkles (or bending) in the manufactured hot stamped part, and it is difficult to realize an air cooling time λ _t of less than 5 s due to equipment. On the other hand, if the air cooling time λ _t exceeds 20 s, not only the productivity decreases, but also phase transformation occurs in the blank during the process of transferring the blank, reducing the formability of the blank, and the manufactured hot stamped part does not have the desired material. Therefore, if the air cooling time λ _t satisfies the range of about 5 s to about 20 s, the blank formability and the process productivity can be improved, and the manufactured hot stamped part can have the desired material.

In one embodiment, the hot stamped part manufactured by the above-mentioned manufacturing method for a hot stamped 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 stamped part manufactured by the manufacturing method for a hot stamped 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 stamped part manufactured by the manufacturing method for a hot stamped part may have a tensile strength of 1,200 MPa or more and less than 2,000 MPa, and may have a full martensite structure.

By simultaneously heating blanks having different thicknesses in a heating furnace in multiple stages, the time required for the blanks to reach a target temperature (e.g., soaking temperature) can be controlled more precisely. By simultaneously controlling the time required for the blanks to reach a target temperature (e.g., soaking temperature) in a heating furnace in multiple stages, the hydrogen embrittlement, corrosion resistance, and weldability of the parts manufactured by the hot stamping part manufacturing method can be improved. More specifically, when a thin material and a thick material are simultaneously heated in a heating furnace, the thin material may reach the target temperature earlier than the thick material, causing overheating of the thin material. According to one embodiment of the present invention, even when a thin material and a thick material are simultaneously heated in a heating furnace, the time required for the thin material and the thick material to reach a target temperature (e.g., soaking temperature) can be controlled in the same way by heating the thin material and the thick material in multiple stages. Therefore, by similarly controlling the time required for the thin material and the thick material to reach a target temperature (e.g., soaking temperature), the hydrogen embrittlement, corrosion resistance, and weldability of the parts manufactured by the hot stamping manufacturing method can be improved.

Experimental Example After preparing a blank having the alloy composition of Table 1, a hot stamping part was manufactured according to the conditions of Comparative Examples 1 and 2 and the Example after setting the temperature setting for each section of Table 3 in a heating furnace set according to the specifications of Table 2. Meanwhile, the total length of the heating furnace is 22000 mm.

Referring to Table 3, a hot stamping part (Example) was manufactured using the manufacturing method for a hot stamping part according to one embodiment, and in Comparative Example 1 and Comparative Example 2, the hot stamping parts were manufactured by single heating at temperatures of 950°C and 930°C, respectively.

Hydrogen embrittlement evaluation, corrosion resistance evaluation, and welding evaluation were performed on the parts manufactured under the conditions of the embodiment, comparative example 1, and comparative example 2.

1. Hydrogen Embrittlement Evaluation Hydrogen embrittlement was evaluated using a thermal desorption spectroscopy (TDS) device according to ISO 16573-2015 for the parts manufactured under the conditions of Example, Comparative Example 1, and Comparative Example 2. That is, the parts manufactured under the conditions of Example, Comparative Example 1, and Comparative Example 2 were heated in a vacuum atmosphere at 300°C or less to measure the amount of diffusible hydrogen released from the parts.

FIG. 9 is a graph showing the hydrogen release rates released from the parts manufactured under the conditions of the example, comparative example 1, and comparative example 2. Table 4 is a table showing the results of calculating the amount of diffusible hydrogen at 300° C. or less based on the hydrogen release rate results of the example, comparative example 1, and comparative example 2 in FIG. 9 , and the results of the delayed fracture experiment.

9 and Table 4, it can be seen that the amount of diffusible hydrogen at 300° C. or less in the example is 0.412 ppm, the amount of diffusible hydrogen at 300° C. or less in the comparative example 1 is 0.531 ppm, and the amount of diffusible hydrogen at 300° C. or less in the comparative example 2 is 0.475 ppm. In addition, as a result of the delayed fracture experiment, it can be seen that delayed fracture occurs in the comparative examples 1 and 2, but does not occur in the example. The amount of diffusible hydrogen in the hot stamped part manufactured through multi-stage heating is the smallest and delayed fracture does not occur, so when multi-stage heating is used, hydrogen embrittlement of the hot stamped part may be reduced.

2. Corrosion Resistance Evaluation The corrosion resistance evaluation test was performed on the hot stamping parts manufactured under the conditions of the embodiment, comparative example 1, and comparative example 2 according to the ASTM G59-97 (2014) standard. More specifically, for the corrosion resistance evaluation test, a three-electrode electrochemical cell was constructed using the specimen as the working electrode, a high-purity carbon rod as the counter electrode, and a saturated calomel electrode as the reference electrode, and a potentiodynamic polarization test was performed. The potentiodynamic polarization test was performed after confirming electrochemical stabilization by measuring the open-circuit potential (OCP) in a 3.5% NaCl solution for 10 hours, and the corrosion resistance evaluation test was performed by applying a potential at a scanning rate of 0.166 mV/s from the corrosion potential (Ecorr) standard of -250 mV to 0 mVSCE.

Fig. 10 is a graph showing the results of corrosion resistance evaluation for parts manufactured under the conditions of Example, Comparative Example 1, and Comparative Example 2, and Table 5 is a table in which the corrosion rates of the parts manufactured under the conditions of Example, Comparative Example 1, and Comparative Example 2 are calculated based on the polarization curves of Fig. 10. Here, the corrosion rates in Table 5 are values corresponding to the current density at which a branch occurs in the stably maintained potential from the polarization curves of Example, Comparative Example 1, and Comparative Example 2.

Referring to FIG. 10 and Table 5, in the case of Comparative Example 1 and Comparative Example 2, the lower the single heating temperature, the lower the corrosion rate and the better the corrosion resistance. However, when multi-stage heating is used as in the example, it is possible to ensure even better corrosion resistance than when single heating is used.

3. Weldability Evaluation Weldability evaluation was performed on the parts manufactured under the conditions of Example, Comparative Example 1, and Comparative Example 2. In the weldability evaluation, a pair of parts manufactured under the conditions of Example, Comparative Example 1, and Comparative Example 2 was prepared, and they were spot-welded for 300 ms with an electrode rod made of a chromium-copper alloy having a diameter of 6 mm, with a pressure of 350 kgf and a current of 5.5 kA applied. The resistance was measured while performing the spot welding.

Normally, the change in resistance value during the first 30 ms determines the occurrence of spatter and the characteristics of weldability, and the lower the resistance, the better the weldability.

Figure 11 is a graph showing the resistance values of parts manufactured under the conditions of the Example, Comparative Example 1, and Comparative Example 2. Referring to Figure 11, it can be seen that the hot stamping part manufactured through multi-stage heating (Example) has a lower resistance than the hot stamping part manufactured through a single heating at a temperature of 950 ° C (Comparative Example 1) and the hot stamping part manufactured through a single heating at a temperature of 930 ° C (Comparative Example 2). Therefore, it can be seen that the weldability of the hot stamping part manufactured through multi-stage heating (Example) is relatively superior to the weldability of the hot stamping part manufactured through a single heating at a temperature of 950 ° C (Comparative Example 1) and the hot stamping part manufactured through a single heating at a temperature of 930 ° C (Comparative Example 2).

Although the present invention has been described based on one embodiment shown in the drawings, this is merely illustrative, and a person having ordinary skill in the art would understand that various modifications and variations of the embodiment are possible. Therefore, the true technical scope of protection of the present invention must be determined by the technical ideas of the claims.

200 Blank 210 Single thickness blank 220 Tailor welded blank 221 First plate 223 Second plate 230 Tailor rolled blank 231 First region 232 Second region 233 Third region 234 Fourth region 240 Patchwork 241 Base material 243 Patch

Claims (15)

placing at least two blanks in a heating furnace having a plurality of sections with different temperature ranges;
a multi-stage heating step in which the blank is heated in stages by passing the blank through the multiple sections in the heating furnace;
a soaking heating step of heating the blank at a temperature of Ac3 to 1,000°C;
a transferring step of transferring the heated blank from the heating furnace to a die;
Including,
The heated blank is air-cooled during the transfer step;
In the multi-stage heating step, the temperature condition in the heating furnace satisfies the following Equation 1:
(In the above formula 1, Tg is the soaking heating temperature (° C.), Ti is the initial temperature at which the heating path enters the heating furnace (° C.), and Lt is the length of the section in which the multi-stage heating step is performed (mm). The method for manufacturing a hot stamped part according to claim 1, wherein in the multiple sections, the ratio of the length of the section in which the blank is heated in multiple stages to the length of the section in which the blank is heated uniformly satisfies 1:1 to 4:1. The method for manufacturing a hot stamped part according to claim 1, wherein at least two blanks having different thicknesses are simultaneously transferred into the heating furnace. The method for producing a hot stamped part 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. The method for manufacturing a hot stamped part according to claim 2, wherein the temperature of the plurality of sections increases from the entrance of the heating furnace toward the exit of the heating furnace. The method for manufacturing a hot stamped part according to claim 5, wherein the temperature difference between two adjacent sections in the section where the blank is heated in multiple stages is greater than 0°C and less than or equal to 100°C. The method for manufacturing a hot stamped part according to claim 2, wherein the temperature of the section in which the blank is soaked and heated is higher than the temperature of the section in which the blank is heated in multiple stages. The method for manufacturing a hot stamped part according to claim 1, wherein the blank remains in the heating furnace for 180 to 360 seconds. 2. The method for manufacturing a hot stamped part according to claim 1, wherein the cooling time of the blank in the transferring step satisfies the following Equation 2:
(In this case, _λt is the air cooling time (s), _{at is} a correction coefficient taking into consideration the temperature at the time of removal from the heating furnace and the atmospheric temperature, _Tt is the heating temperature (°C), _bt is a correction coefficient taking into consideration the material components, t is the material thickness (mm), and _ct is a correction coefficient taking into consideration the material thickness sensitivity to high temperatures). In the above formula 2,
The method for producing a hot stamped part according to claim 9, wherein a _t is 0.0160 or more and 0.0165 or less, T _t is Ac3 or more and 1000 ° C. or less, b _t is −10 or more and 0.5 or less, t is 1 mm or more and 2.6 mm or less, and c _t is 0.7 or more and 0.9 or less. 10. The method for producing a hot stamped part according to claim 9, wherein in the formula 2, λ _t is 5 s or more and 20 s or less. A hot stamped part manufactured according to any one of claims 1 to 11,
A hot stamped part having a diffusible hydrogen content of less than 0.45 ppm and a corrosion rate of 3×10 ⁻⁶ A or less as measured through an isopotential polarization test. The hot stamped part according to claim 12, having a tensile strength of 500 MPa or more and less than 800 MPa and a composite structure of ferrite and martensite. The hot stamped part according to claim 12, having a tensile strength of 800 MPa or more and less than 1,200 MPa and a composite structure of bainite and martensite. The hot stamped part according to claim 12, having a tensile strength of 1,200 MPa or more and less than 2,000 MPa and a full martensite structure.