MANUFACTURING METHOD FOR HOT-STAMPED PARTS

The present disclosure provides a method of manufacturing hot-stamped parts, the method including: a heating operation of heating a blank; a transferring operation of transferring the heated blank to a press die including a punch; and a forming and piercing operation of hot-forming the transferred blank into a shape of a hot-stamped part and hot-piercing the transferred blank to form a pierced portion in the hot-stamped part.

FIELD

The present disclosure relates to a method of manufacturing hot-stamped parts.

BACKGROUND

As environmental and fuel efficiency regulations have been strengthened worldwide, the need for lighter vehicular materials is increasing. Accordingly, ultra-high-strength steel and hot-stamping steel have been actively researched and developed.

Hot stamping includes a process in which steel sheets are heated to a high temperature in a heating furnace and then rapidly cooled while being formed in a press to manufacture high-strength parts. In addition, a piercing process may be additionally performed to form holes by cutting/processing the high-strength parts.

The piercing process may be performed using laser equipment or press dies. However, when laser equipment is used, a processing time may increase, and when press dies are used, the quality of sheared surfaces may deteriorate.

SUMMARY

In one aspect, embodiments of the present disclosure are characterized by deriving a maximum delay time for hot piercing by considering factors such as the thickness of a blank, the start temperature of forming, etc.

An embodiment of the present disclosure provides a method of manufacturing hot-stamped parts, the method including: a heating operation of heating a blank; a transferring operation of transferring the heated blank to a press die including a punch; and a forming and piercing operation of hot-forming the transferred blank into a shape of a hot-stamped part and hot-piercing the transferred blank to form a pierced portion in the hot-stamped part, wherein a hot piercing delay time in the forming and piercing operation is within a range of 0 second to λmax from a time point at which the press die reaches a bottom dead point, and λmax is derived by Equation below:

λ
      max
     
     =
     
      
       (
       
        
         
          a
          p
         
         ⁢
         T
        
        +
        
         b
         p
        
       
       )
      
      *
      
       t
       
        c
        p
       
      
     
    
   
   
    
     〈
     Equation
     〉

(In the Equation above, λmax refers to a maximum delay time (s) of hot piercing, ap refers to a correction coefficient considering a forming start temperature and hot piercing conditions, T refers to the forming start temperature (° C.), bp refers to a correction coefficient considering a pressing force of the press die, cp refers to a correction coefficient considering thickness sensitivity of a material, and t refers to a thickness (mm) of the material).

According to the embodiment, in the Equation above, ap may may have a value greater than 0 and less than or equal to 0.001, bp may may have a value greater than 0 and less than or equal to 0.65, and cp may may have a value ranging from about 1 to about 1.2.

According to the embodiment, in the transferring operation of transferring the heated blank, the heated blank suitably may be air cooled at room temperature.

According to the embodiment, the heating operation of heating the blank may include: a multi-stage heating operation in which the blank may be heated step by step; and a soaking operation in which the blank may be heated to a temperature of Ac3 to 1000° C. Heating step by step can include applying a first temperature for at least 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 seconds or more and then applying a second heating temperature (e.g. a second temperature may be 3, 5, 10 degrees centigrade or more higher than the first temperature) and that second temperature for at least 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 seconds or more, and then optionally applying subsequent higher or lower temperatures for such time periods.

According to the embodiment, the heating operation of heating the blank suitably may be performed in a heating furnace, and the heating furnace suitably may include a plurality of zones having different temperature ranges.

According to the embodiment, in the forming and piercing operation, the transferred blank suitably may be cooled in the press die.

According to the embodiment, the pierced portion suitably may include at least two pierced portions.

Other aspects, features, and advantages, in addition to those described above, will become apparent from the following detailed description for carrying out the disclosure, the claims, and the accompanying drawings.

As described above, according to an embodiment of the present disclosure, a maximum delay time for hot piercing is derived by considering factors such as the thickness of a blank or the start temperature of forming, thereby enabling flexible process design and facilitating the quality control of manufactured hot-stamped parts. However, the scope of the present disclosure is not limited by these effects.

DETAILED DESCRIPTION

The present disclosure may have various different forms and various embodiments, and specific embodiments are illustrated in the accompanying drawings and are described herein in detail. Effects and features of the present disclosure, and methods of achieving the effects and features will become apparent with reference to the accompanying drawings and the embodiments described below in detail. However, the present disclosure is not limited to the embodiments described below and may be implemented in various forms

In the following embodiments, terms such as “first” and “second” are used not in a limiting sense, but for the purpose of distinguishing one element from another element.

In the following embodiments, the terms of a singular form may include plural forms unless referred to the contrary.

In the following embodiments, terms such as “comprise,” “include,” and “have” specify the presence of features or elements stated in the specification, but do not preclude the presence or addition of one or more other features or elements.

In the drawings, the sizes of elements may be exaggerated or reduced for ease of illustration. For example, in the drawings, the size or thickness of each element may be arbitrarily shown for illustrative purposes, and thus the present disclosure should not be construed as being limited thereto.

The order of processes explained in one embodiment may be changed in a modification of the embodiment or another embodiment. For example, two processes sequentially explained may be performed substantially at the same time or in the reverse of the explained order.

In the present specification, the expression “A and/or B” indicates A, B, or A and B. In addition, the expression “at least one of A and B” indicates A, B, or A and B.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the drawings, like reference numerals denote like elements, and overlapping descriptions thereof will be omitted.

FIG. 1 is a perspective view schematically illustrating a hot-stamped part according to an embodiment of the present disclosure.

Referring to FIG. 1, according to an embodiment of the present disclosure, a hot-stamped part 10 may include a pierced portion 110. In an embodiment, the hot-stamped part 10 may include two pierced portions 110. The pierced portion 110 may include a first pierced portion 110a and a second pierced portion 110b. However, the present disclosure is not limited thereto. The hot-stamped part 10 may include one pierced portion 110 or three or more pierced portions 110.

Although not illustrated, the hot-stamped part 10 may include an additional pierced portion in addition to the pierced portion 110. In this case, the pierced portion 110 may be formed by hot piercing, and the additional pierced portion may be formed by cold piercing or laser piercing. The hot-stamped part 10 may include one or more additional pierced portions.

In one embodiment, an additional pierced portion may be formed using the pierced portion 110. This will be described in more detail below.

In addition, although not illustrated, the hot-stamped part 10 may include an edge portion. In this case, the edge portion of the hot-stamped part 10 may refer to a side extending along the length of the hot-stamped part 10.

FIG. 2 is a flowchart schematically illustrating a method of manufacturing hot-stamped parts according to an embodiment of the present disclosure.

Referring to FIG. 2, according to the embodiment, the method of manufacturing hot-stamped parts may include a preparing operation S100, a heating operation S200, a transferring operation S300, and a forming/piercing operation S400.

FIG. 3 is a flowchart schematically illustrating the preparing operation S100 in the method of manufacturing hot-stamped parts, according to an embodiment of the present disclosure, and FIG. 4 is a plan view schematically illustrating a blank 100 according to an embodiment of the present disclosure.

Referring to FIGS. 3 and 4, the preparing operation S100 may be an operation of preparing a blank 100 for hot stamping. In one embodiment, the preparing operation S100 may include a hot rolling operation S110, a cooling/coiling operation S120, a cold rolling operation S130, an annealing operation S140, a plating operation S150, and a cutting operation S160.

First, an operation of reheating a steel slab may be performed. In the slab reheating operation, a steel slab obtained through a continuous casting process is reheated to a predetermined temperature, and thus, components that have segregated during casting may be redissolved. In one embodiment, a slab reheating temperature (SRT) may range from about 1,200° C. to about 1,400° C. When the slab reheating temperature (SRT) is lower than about 1,200° C., components that have segregated during casting may not be sufficiently redissolved, making it difficult to achieve significant homogenization of alloy elements and a significant solid solution effect of titanium (Ti). Although a higher slab reheating temperature (SRT) is advantageous for homogenization, a slab reheating temperature (SRT) exceeding about 1,400° C. may lead to an increase in the grain size of austenite, which may reduce strength, and may also increase manufacturing costs due to excessive heating.

In the hot rolling operation S110, a reheated steel sheet may be hot rolled at a predetermined finishing delivery temperature. A hot-rolled steel sheet may be produced through the hot rolling operation S110. In one embodiment, the finishing delivery temperature (FDT) may range from about 880° C. to about 950° C. Here, when the finishing delivery temperature (FDT) is lower than about 880° C., it may be difficult to secure workability of the steel sheet due to the occurrence of mixed microstructures caused by rolling in an abnormal region, and the workability of the steel sheet may deteriorate due to microstructural non-uniformity. Furthermore, during hot rolling, it may be difficult to transfer the steel sheet due to abrupt phase transitions. When the finishing delivery temperature (FDT) exceeds about 950° C., austenite grain coarsening may occur, and TiC precipitate coarsening may also occur, leading to deterioration in the quality of hot-stamped parts.

In the cooling/coiling operation S120, the hot-rolled steel sheet may be cooled to a predetermined coiling temperature (CT) and then coiled. In one embodiment, the coiling temperature of the cooling/coiling operation S120 may range from about 550° C. to about 800° C. The coiling temperature affects the redistribution of carbon. When the coiling temperature is lower than about 550° C., the fraction of low-temperature phases may increase due to overcooling, which may lead to an increase in strength, an increase in rolling load during cold rolling, and a significant decrease in ductility. Conversely, when the coiling temperature exceeds about 800° C., abnormal grain growth or excessive grain growth may occur, potentially lowering formability and strength.

In the cold rolling operation S130, the coiled hot-rolled steel sheet may be uncoiled, pickled, and then cold rolled. Here, the pickling may be performed to remove scale from the coiled hot-rolled steel sheet, that is, to remove scale from a hot-rolled coil produced by hot rolling. A cold-rolled steel sheet may be produced through the cold rolling operation S130.

In the annealing operation S140, the cold-rolled steel sheet may be annealed at a temperature of about 700° C. or higher. For example, the annealing operation S140 may include heating the cold-rolled steel sheet and cooling the heated cold-rolled steel sheet at a predetermined cooling rate. In the annealing operation S140, the cold-rolled steel sheet may be annealed. The annealing operation S140 may be performed in an annealing furnace.

In one embodiment, the annealing temperature of the cold-rolled steel sheet may range from about 750° C. to about 900° C. When the annealing temperature of the cold-rolled steel sheet is lower than about 750° C., a desired microstructure may not be obtained, and recrystallization may not be sufficiently completed. On the other hand, when the annealing temperature of the cold-rolled steel sheet exceeds about 900° C., manufacturing process efficiency may decrease because the annealing temperature is excessively high. Therefore, when the annealing temperature of the cold-rolled steel sheet is in the range of about 750° C. to about 900° C., a desired microstructure, sufficient recrystallization, and improved manufacturing process efficiency may be achieved.

The plating operation S150 may be an operation of forming a plating layer on the annealed cold-rolled steel sheet. In one embodiment, a plating layer may be formed on the annealed cold-rolled steel sheet through the plating operation S150. In this case, the plating layer may include a zinc (Zn)-based plating layer or an aluminum (Al)-based plating layer.

Specifically, in the plating operation S150, the annealed cold-rolled steel sheet may be immersed in a plating bath. At this time, the plating bath may be maintained at a temperature of about 400° C. to about 700° C. The density of the plating layer may range from about 40 g/m2 to about 200 g/m2, based on both surfaces of a base material of the cold-rolled steel sheet. After the plating operation S150, the cold-rolled steel sheet on which the plating layer is formed may be wound into a coil form.

Although FIG. 3 shows that the cold rolling operation S130, the annealing operation S140, and the plating operation S150 are performed after the cooling/coiling operation S120, the present disclosure is not limited thereto. At least one of the cold rolling operation S130, the annealing operation S140, and the plating operation S150 may be omitted. For example, the cold rolling operation S130 and the annealing operation S140 may be omitted. In this case, the hot-rolled steel sheet on which the plating layer is formed after the plating operation S150 may be wound into a coil form.

Thereafter, in the cutting operation S160, the steel sheet (for example, a cold-rolled steel sheet or a hot-rolled steel sheet) wound into the shape of a coil may be uncoiled and may then be cut to form a blank 100 using a laser or a cold press die. At this time, the blank 100 may include an outer portion (or edge) of the coil. For example, the blank 100 may include an outer portion (or edge) of the steel sheet.

Referring to FIG. 2, after the operation S100 of preparing a blank 100, the heating operation S200 may be performed to heat the blank 100. The type of heating in the heating operation S200 may include direct heating and indirect heating. The type of heating in the heating operation S200 may be one of direct heating and indirect heating, or a combination thereof.

In one embodiment, the heating operation S200 may be performed by heating the blank 100 in a heating furnace. The heating furnace may include a single zone with a single temperature range (or a single temperature), or a plurality of zones with different temperature ranges. When the heating furnace includes a single zone with one temperature range, the blank 100 may be heated to a target temperature within the temperature range. At this time, the target temperature may range from Ac3 to about 1,000° C. That is, the blank 100 may be heated in the heating furnace having a temperature range of Ac3 to about 1,000° C. until the temperature of the blank 100 reaches a temperature ranging from Ac3 to about 1,000° C.

On the other hand, when the heating furnace includes a plurality of zones having different temperature ranges, the blank 100 may be heated in the heating furnace to the target temperature through the different temperature ranges.

FIG. 5 is a flowchart schematically illustrating the heating operation S200 in the method of manufacturing hot-stamped parts, according to an embodiment of the present disclosure. FIG. 6 is a view illustrating a heating furnace having a plurality of zones for the heating operation S200 in the method of manufacturing hot-stamped parts, according to an embodiment of the present disclosure.

Referring to FIGS. 5 and 6, in the heating operation S200, the blank 100 (refer to FIG. 4) may be heated in the heating furnace having a plurality of zones with different temperature ranges. As shown in FIG. 5, the heating operation S200 may include a multi-stage heating operation S210 and a soaking operation S220. In the multi-stage heating operation S210 and the soaking operation S220, the blank 100 may be heated while being transferred through the plurality of zones provided in the heating furnace.

In one embodiment, the overall temperature of the heating furnace may range from about 680° C. to about 1,000° C. Specifically, the overall temperature of the heating furnace in which the multi-stage heating operation S210 and the soaking operation S220 are performed may range from about 680° C. to about 1,000° C. In this case, the temperature of the heating furnace in which the multi-stage heating operation S210 is performed may range from about 680° C. to about Ac3, and the temperature of the heating furnace in which the soaking operation S220 is performed may range from about Ac3 to about 1,000° C.

In the multi-stage heating operation S210, the blank 100 may be heated (or the temperature of the blank 100 may be raised) step by step while being transferred through the plurality of zones provided in the heating furnace. The multi-stage heating operation S210 may be performed in two or more of the plurality of zones provided in the heating furnace, and the temperature of each zone may be set to increase in a direction from an entrance of the heating furnace, through which the blank 100 is loaded, toward an exit of the heating furnace, through which the blank 100 is discharged, such that the blank 100 may be heated (or the temperature of the blank 100 may be raised) step by step.

After the multi-stage heating operation S210, the soaking operation S220 may be performed. In the soaking operation S220, the multi-stage heated blank may be heated (or soaked) while being transferred through zones of the heating furnace that are set to a temperature of about Ac3 to about 1,000° C. At least one of the plurality of zones of the heating furnace may be used to perform the soaking operation S220.

In one embodiment, the heating furnace may include a plurality of zones having different temperature ranges. Specifically, the heating furnace may include a first zone P1 having a first temperature range T1, a second zone P2 having a second temperature range T2, a third zone P3 having a third temperature range T3, a fourth zone P4 having a fourth temperature range T4, a fifth zone P5 having a fifth temperature range T5, a sixth zone P6 having a sixth temperature range T6, and a seventh zone P7 having a seventh temperature range T7.

According to one embodiment, in the multi-stage heating operation S210, the blank may be heated step by step while being transferred through the first zone P1 to the fourth zone P4 that are defined in the heating furnace. Furthermore, in the soaking operation S220, the blank that has been multi-stage heated in the first zone P1 to the fourth zone P4 may be soaked while being transferred through the fifth zone P5 to the seventh zone P7.

The first zone P1 to the seventh zone P7 may be arranged sequentially within the heating furnace. The first zone P1 having the first temperature range T1 may be adjacent to the entrance of the heating furnace through which the blank is loaded, and the seventh zone P7 having the seventh temperature range T7 may be adjacent to the exit of the heating furnace through which the blank is discharged. Therefore, the first zone P1 having the first temperature range T1 may be positioned first in the heating furnace, and the seventh zone P7 having the seventh temperature range T7 may be the last zone of the heating furnace.

The temperatures of the plurality of zones provided inside the heating furnace, for example, the temperatures of the first zone P1 to the seventh zone P7, may increase in the direction from the entrance of the heating furnace, through which the blank is loaded, toward the exit of the heating furnace through which the blank is discharged. However, the temperatures of the fifth zone P5, sixth zone P6, and seventh zone P7 may be the same. In addition, a temperature difference between any two adjacent zones among the plurality of zones provided inside the heating furnace may be greater than 0° C. but less than or equal to 100° C. For example, a temperature difference between the first zone P1 and the second zone P2 may be greater than 0° C. but less than or equal to 100° C.

During the soaking operation S220, the temperature of the heating furnace may range from Ac3 to about 1,000° C. When the temperature of the heating furnace is below Ac3 during the soaking operation S220, a hot-stamped part may not have desired material properties. On the other hand, when the temperature of the heating furnace exceeds about 1,000° C. during the soaking operation S220, it may be difficult to suppress grain coarsening because carbide-forming elements or nitride-forming elements such as Ti, V, Nb, or Mo included in the blank 100 are dissolved into a base material.

Although FIG. 6 illustrates that the heating furnace of the embodiment includes seven zones having different temperature ranges, the present disclosure is not limited thereto. The heating furnace may include five, six, eight, or another number of zones having different temperature ranges.

In one embodiment, the heating operation S200 includes the multi-stage heating operation S210 and the soaking operation S220, and thus, the temperature of the heating furnace may be set step by step to improve the energy efficiency of the heating furnace.

In one embodiment, the heating furnace may have a length of about 20 m to about 40 m along a transfer path of the blank 100. The heating furnace may include a plurality of zones having different temperature ranges, and the ratio of the length of the zones in which the blank is multi-stage heated to the length of the zones in which the blank is soaked may be from about 1:1 to about 4:1. When the length of the soaking zones in the heating furnace increases such that the ratio of the length of the multi-stage heating zones to the length of the soaking zones falls below about 1:1, hydrogen penetration into the blank may increase in the soaking zones, potentially leading to a rise in delayed fracture. On the other hand, when the length of the soaking zones decreases such that the ratio of the length of the multi-stage heating zones to the length of the soaking zones exceeds about 4:1, the soaking zones (or soaking time) may be insufficient, and the strength of a manufactured hot-stamped part may not be uniform. For example, the length of the soaking zones among the plurality of zones provided in the heating furnace may be from about 20% to about 50% of the total length of the heating furnace.

In one embodiment, a total heating time of the heating operation S200 may be from about 2 minutes to about 20 minutes. That is, a total residence time of the blank in the heating furnace may be from about 2 minutes to about 20 minutes. When the total heating time of the heating operation S200 is less than about 2 minutes, the total heating time may be insufficient, and a manufactured hot-stamped part may not have desired material properties. On the other hand, when the total heating time of the heating operation S200 exceeds about 20 minutes, the total heating time may be excessively long, thereby reducing the rate of production and lowering economic feasibility. Therefore, when the total heating time of the heating operation S200 is from about 2 minutes to about 20 minutes, a manufactured hot-stamped part may have desired material properties, and at the same time, degradation in the economic feasibility of manufacturing processes may be prevented or minimized.

Referring to FIG. 2, after the heating operation S200, the transferring operation S300 may be performed. In the transferring operation S300, the heated blank 100 may be transferred to a press die 400 (refer to FIG. 7). For example, after being discharged from the heating furnace, the heated blank 100 may be transferred to the press die 400.

In the transferring operation S300, the heated blank 100 may be cooled at ambient temperature (or room temperature). That is, the heated blank 100 may be air cooled at ambient temperature during transfer. When the heated blank 100 is not air cooled, a die entry temperature (for example, a forming start temperature) may be excessively high, potentially causing wrinkles (or warping) on the surface of a manufactured hot-stamped part. In addition, when a coolant is used, the coolant may affect subsequent processes (hot stamping), and thus, it may be preferable that the heated blank 100 is air cooled during transfer.

FIGS. 7 and 8 are cross-sectional views illustrating the forming/piercing operation in a process of manufacturing a hot-stamped part, according to an embodiment of the present disclosure. Specifically, FIG. 7 is a cross-sectional view illustrating the blank and the press die before the forming/piercing operation is performed, and FIG. 8 is a cross-sectional view illustrating the blank and the press die during the forming/piercing operation.

Referring to FIGS. 2, 7, and 8, after the transferring operation S300, the forming/piercing operation S400 may be performed. The forming/piercing operation S400 may be an operation of hot forming the transferred blank 100 into the shape of a hot-stamped part and forming a pierced portion 110 in the transferred blank 100 by hot piercing.

In one embodiment, the forming/piercing operation S400 may be performed in the press die 400. The press die 400 may include a lower die 410, an upper die 420, and a punch 430. The lower die 410 may have a shape corresponding to a lower end surface of a hot-stamped part. The upper die 420 may face the lower die 410 and may have a shape corresponding to an upper end surface of the hot-stamped part. The upper die 420 may be moved up and down between a top dead center and a bottom dead center. Here, the top dead center may refer to an uppermost position of the upper die 420, and the bottom dead center may refer to a lowest position of the upper die 420 when the upper die 420 is moved toward the lower die 410 for forming of the blank 100. Although not illustrated, the upper die 420 may be moved up and down between the top dead center and the bottom dead center by a hydraulic cylinder that is separately provided. However, the present disclosure is not limited thereto.

The press die 400 may include at least one punch 430, and preferably at least two punches 430. In one embodiment, the punch 430 may include a first punch and a second punch. However, the present disclosure is not limited thereto. For example, various modifications are possible, such as configuring the press die 400 to include only one punch 430 or three or more punches 430.

Although not illustrated, when the press die 400 includes only one punch 430, a form bead may be formed at an edge of the blank 100 during forming of the blank 100.

For example, as described below, a laser process or a cold piercing process may be performed using the pierced portion 110 formed by hot piercing. Specifically, the pierced portion 110 formed by hot piercing may be used as a guide pattern (or a reference point) to position the blank 100 on a jig, and then, a subsequent process may be performed. Therefore, at least two pierced portions 110 may be formed in the blank 100 by hot piercing.

In one embodiment, a clearance between the punch 430 and a die (for example, the lower die 410 and/or the upper die 420) may be from about 2% to about 30%. When the clearance is less than about 2%, a problem may occur in which the punch 430 becomes stuck in the die due to thermal expansion. On the other hand, when the clearance exceeds about 30%, the punch 430 may rattle excessively, potentially resulting in non-uniform sheared surface quality. For example, a decrease in the uniformity of sheared surface quality, such as grain flow lines being dense in only a portion of a sheared surface, may occur. Therefore, when the clearance between the punch 430 and the die (for example, the lower die 410 and/or the upper die 420) is from about 2% to about 30%, the punch 430 may be prevented from being stuck in the die due to thermal expansion, and grain flow lines may be uniformly formed around a sheared surface.

According to one embodiment, in the forming/piercing operation S400, the blank 100 may be hot formed (or hot pressed). The transferred (or heated) blank 100 may be hot formed into the shape of a hot-stamped part by using the press die 400 including the lower die 410 and the upper die 420. Specifically, the upper die 420, which has the shape of an upper end surface of a hot-stamped part, may press (for example, hot press) the blank 100 and the lower die 410, which has the shape of a lower end surface of the hot-stamped part, to form the blank 100 into the shape of the hot-stamped part. For example, the press die may reach the bottom dead center at which the blank 100 may be formed into the shape of the hot-stamped part 10. Specifically, the upper die 420 may be moved downward to the bottom dead center, and after the upper die 420 is moved to the bottom dead center, the blank 100 may be formed into the shape of the hot-stamped part 10.

According to one embodiment, in the forming/piercing operation S400, a forming start temperature of the blank 100 for hot forming may be greater than or equal to an Ms temperature. Specifically, the forming start temperature refers to a temperature at which forming of the blank 100 begins, and in the forming/piercing operation S400, the forming start temperature of the blank 100 for hot forming may range from the Ms temperature to a heating furnace discharge temperature of the blank 100. When the blank 100 is hot formed below the Ms temperature, a hot forming load may be large, potentially causing damage to the press die 400, degrading the formability of the blank 100, and preventing a manufactured hot-stamped part from achieving a desired microstructure and physical properties. On the other hand, because the heated blank 100 is air cooled while being transferred from the heating furnace to the press die 400, the temperature of the blank 100 during the hot forming is inevitably lower than the temperature of the blank 100 at the time when the blank 100 is discharged from the heating furnace. Therefore, when the forming start temperature is between the Ms temperature and the heating furnace discharge temperature of the blank 100, the formability of the blank 100 may be improved, and a manufactured hot-stamped part may have a desired microstructure and physical properties.

According to one embodiment, in the forming/piercing operation S400, hot piercing may be performed on the blank 100 after completion of the hot forming of the blank 100. Specifically, after completion of the hot forming of the blank 100, the punch 430 included in the press die 400 may be moved downward to hot pierce the blank 100, thereby forming the pierced portion 110 in the hot-stamped part 10 (or the blank 100). For example, after the upper die 420 reaches the bottom dead center and the hot forming of the blank 100 is completed, the punch 430 may be moved downward to hot pierce the blank 100, thereby forming the pierced portion 110 in the hot-stamped part 10 (or the blank 100). In other words, the hot piercing may be performed after the press die (or the upper die) reaches the bottom dead center. Although not illustrated, the punch 430 may be moved up and down by a hydraulic cylinder. However, the present disclosure is not limited thereto.

In one embodiment, after the hot forming of the blank 100 is completed using the press die 400, the first punch and the second punch included in the press die 400 may be moved downward to form at least two pierced portions 110a and 110b in the blank 100. Specifically, after the hot forming of the blank 100 is completed by pressing the lower die 410 and the blank 100 with the upper die 420, the first punch and the second punch may be moved downward to form a first pierced portion 110a and a second pierced portion 110b in the hot-stamped part 10.

In one embodiment, the hot piercing may be performed after the hot forming is completed.

Although FIGS. 7 and 8 illustrate that the punch 430 included in the press die 400 is moved downward to form the pierced portion 110 in the blank 100, the present disclosure is not limited thereto. Although not illustrated, the pierced portion 110 may be formed in the blank 100 by using a separately provided external device.

In one embodiment, during the hot piercing in the forming/piercing operation S400, the temperature of the blank 100 may be equal to or higher than an Mf temperature. When the temperature of the blank 100 is below the Mf temperature during the hot piercing, the blank is hot pierced at an excessively low temperature, and thus, the phase transformation of the blank 100 (or a hot-stamped part) may be completed to increase shear stress. As a result, shear quality may deteriorate, and hydrogen embrittlement may occur. In addition, a large hot piercing load may occur, and thus, the press die 400 and/or the punch 430 may be damaged.

In one embodiment, the blank 100 may be cooled at the same time the blank 100 is formed into the shape of a final part in the press die 400 (or may be cooled during the hot forming). The press die 400 may include a cooling channel 440 through which a coolant circulates. For example, channels 440 may be respectively provided in the lower die 410 and the upper die 420. Specifically, the cooling channels 440 may be arranged in the lower die 410 and the upper die 420 along surfaces of the lower die 410 and the upper die 420. The heated blank 100 may be rapidly cooled by the circulation of a coolant supplied through the cooling channels 440 provided in the press die 400. At this time, the rapid cooling may be performed while the press die 400 remains in a closed and pressurized state, thereby prevent a spring-back phenomenon of a sheet material and maintaining a desired shape. During the forming and cooling of the heated blank 100, the heated blank 100 may be cooled at an average cooling rate of at least 10° C./s until the heated blank 100 reaches a martensite finish temperature. Preferably, the average cooling rate may be at least 20° C./s.

The blank 100 may be held in the press die 400 for 3 seconds to 20 seconds. For example, the press die 400 may be maintained in an engaged state for 3 seconds to 20 seconds. When the blank 100 is held in the press die 400 for less than 3 seconds, the blank 100 may be insufficiently cooled, and thus, thermal deformation may occur due to residual heat and regional temperature deviation, potentially causing deterioration in dimensional quality. On the other hand, when the blank 100 is held in the press die 400 for more than 20 seconds, productivity may decrease due to a long holding time in the press die 400.

In one embodiment, the hot forming, the hot piercing, and the cooling of the heated blank 100 may be performed within the press die 400. For example, the hot forming of the blank 100 and the cooling of the blank 100 may be performed from the moment when forming of the blank 100 begins, and the hot piercing of the blank 100 may be performed thereafter. That is, the hot forming of the blank 100 and the cooling of the blank 100 may be performed while the press die 400 is in a closed (or engaged) state, and the hot piercing of the blank 100 may be performed after a predetermined period of time has elapsed. However, the present disclosure is not limited thereto.

As described above, the hot piercing may be a process of punching the blank 100 during the cooling of the blank 100 that is heated to a high temperature. The hot piercing of the blank 100 may be performed between the time point at which the press die (or the upper die) reaches the bottom dead center and the time point at which the cooling (or rapid cooling) of the blank 100 is completed. In other words, the hot piercing may be performed between the time point at which the hot forming is completed and the time point at which the cooling (or rapid cooling) of the blank 100 is completed.

When the hot piercing of the blank 100 is performed while the blank 100 is cooled (or rapidly cooled), the temperature of the blank 100 may affect the quality of a manufactured hot-stamped part, and thus, it is necessary to control the timing of the hot piercing. For example, when the temperature of the blank 100 is excessively low during the hot piercing, problems related to sheared surface quality and shearing load may occur, potentially causing hydrogen embrittlement. In addition, because the cooling behavior of the blank 100 varies depending on the thickness of the blank 100, even when the hot piercing is performed after the same period of time has passed from the moment the press die 400 reaches the bottom dead center, the quality and dimensions of sheared surfaces of hot-stamped parts 10 may vary. Therefore, it may be difficult to ensure desired sheared surface quality and dimensional accuracy.

In addition, because the temperature behavior (or cooling behavior) of the blank 100 differs depending on the thickness (or material) of the blank 100 and the forming start temperature of the blank 100, it is necessary to individually control the timing of hot piercing for each blank by considering these factors.

Accordingly, it is necessary to adjust the timing of hot piercing in the forming/piercing operation S400. In addition, when the timing of hot piercing is adjusted in the forming/piercing operation S400, various factors must be considered, including not only the forming start temperature and the thickness of a material, but also hot piercing conditions, the pressing force of the press die, and material's thickness sensitivity. Here, the material may refer to a blank.

Accordingly, the inventors derived an equation that allows easy control of the hot piercing timing in the forming/piercing operation S400 through extensively repeated experimentation. In one embodiment, the maximum delay time of hot piercing in the forming/piercing operation S400 may satisfy the following equation.

λ
      max
     
     =
     
      
       (
       
        
         
          a
          p
         
         ⁢
         T
        
        +
        
         b
         p
        
       
       )
      
      *
      
       t
       
        c
        p
       
      
     
    
   
   
    
     〈
     Equation
     〉

In the equation, λmax refers to the maximum delay time (s) of hot piercing, ap refers to a correction coefficient considering a forming start temperature and hot piercing conditions, T refers to the forming start temperature (° C.), bp refers to a correction coefficient considering the pressing force of a press die, cp refers to a correction coefficient considering the thickness sensitivity of a material, and t refers to the thickness (mm) of the material. Here, the material may be a blank, and the maximum delay time (s) of hot piercing may be given in seconds.

First, depending on the forming start temperature of a blank and hot piercing conditions, the cooling behavior of a material (for example, the blank) may vary. Specifically, the cooling behavior of the material may vary depending on the forming start temperature, and may also vary depending on factors such as the ambient temperature during hot piercing, the design of a structure in which the material is placed in the press die, and the layout of the press die. ap, a correction coefficient considering a forming start temperature and hot piercing conditions, may be greater than 0 but less than or equal to 0.001. In this case, ap may be expressed in s/(° C.×mm).

Heat transfer (or the amount of heat transfer) from the material to the press die may vary depending on a pressing force applied from the press die to the material. For example, when the press die presses the material with a large pressing force, the heat transfer from the material to the press die may increase. Therefore, the cooling behavior of the material may vary depending on the pressing force applied by the press die. bp, a correction coefficient considering the pressing force of the press die, may be greater than 0 but less than or equal to 0.65. In this case, bp may be expressed in s/mm.

In addition, the thermal conductivity (or the amount of heat transfer) within the material may vary depending on the thickness of the material. cp, a correction coefficient that considers thermal conductivity (or heat transfer amount) varying depending on the thickness of the material, may be from about 1 to about 1.2.

The forming start temperature T, referring to a temperature at which forming of the material begins, may be greater than or equal to a Ms temperature. Specifically, the forming start temperature T, referring to a temperature at which forming of the material begins, may range from the Ms temperature of the material to a heating furnace discharge temperature of the material. In addition, the material thickness (t) may be from about 1 mm to about 2.3 mm.

In one embodiment, the maximum delay time for hot piercing in the forming/piercing operation S400 may be derived by the equation described above. Accordingly, the hot piercing delay time in the forming/piercing operation S400 may be within a range of 0 second to λmax from the time point when the press die (or the upper die) reaches the bottom dead center. In this case, the hot piercing delay time may indicate a time period between the time point at which the press die (or the upper die) reaches the bottom dead center and the time point at which hot piercing is performed on the blank 100.

In other words, the hot piercing delay time in the forming/piercing operation S400 may be within a range of 0 second to λmax from the point at which the upper die reaches the bottom dead center and forming is completed. When the hot piercing delay time in the forming/piercing operation S400 is 0 second, it may mean that hot piercing is performed simultaneously and immediately upon the completion of hot forming. That is, a hot piercing delay time of 0 second in the forming/piercing operation S400 may mean that hot piercing is performed immediately after hot forming is completed. When hot piercing is performed prior to hot forming, the dimensional quality of the pierced portion 110 formed by hot piercing may be poor. In addition, when the hot piercing delay time measured from the time point at which the press die reaches the bottom dead center in the forming/piercing operation S400 is greater than λmax, hot piercing may be performed at an excessively low temperature, causing an increase in shear stress due to completion of phase transformation of the blank (or a hot-stamped part). As a result, shear quality may deteriorate, and hydrogen embrittlement may occur.

Therefore, when the hot piercing delay time in the forming/piercing operation S400 is with the range of 0 second to λmax from the time point at which the press die reaches the bottom dead center, the sheard surface quality of the pierced portion 110 formed by hot piercing may be uniformly managed. For example, when the hot piercing delay time in the forming/piercing operation S400 is within the range of 0 second to λmax from the time point at which the press die reaches the bottom dead center, hydrogen embrittlement resistance of a manufactured hot-stamped part may be improved, and the dimensional quality of the manufactured hot-stamped part may be maintained uniformly.

In one embodiment, when the hot piercing delay time in the forming/piercing operation S400 is within the range of 0 second to λmax from the time point at which the press die reaches the bottom dead center, the amount of diffusible hydrogen in a manufactured hot-stamped part may be 0.5 ppm or less.

FIG. 9 is a view illustrating a process window derived based on a material thickness, a forming start temperature, and a hot piercing delay time. Here, in FIG. 9, the material refers to a blank 100, and Ta refers to a heating furnace discharge temperature of the blank 100.

Referring to FIGS. 2 and 9, the method of manufacturing hot-stamped parts may include the preparing operation S100, the heating operation S200, the transferring operation S300, and the forming/piercing operation S400. In addition, as described above, a maximum delay time λmax for hot piercing may be derived through the equation described above by considering the material thickness and the forming start temperature. A hot piercing delay time for the material may range from 0 second to λmax.

Accordingly, a process window may be derived using the forming start temperature, the material thickness, and the hot piercing delay time as parameters. Here, in the process window, the forming start temperature may range from a Ms temperature to Ta (the heating furnace discharge temperature of the blank), the material thickness may range from about 1.0 mm to about 2.3 mm, and the hot piercing delay time may range from 0 second to Ama.

In one embodiment, a parameter window (or a process window) for hot piercing in the forming/piercing operation S400 may be derived using the material thickness, the forming start temperature, and the hot piercing delay time to enable flexible process design, improve the quality of manufactured hot-stamped parts, and facilitate the quality control of manufactured hot-stamped parts.

Although not illustrated, a trimming operation may be performed after the forming/piercing operation S400. The trimming operation may be an operation of cutting an outer portion of a formed hot-stamped part 10 by using at least two pierced portions 110a and 110b formed in the hot-stamped part 10. In other words, the trimming operation may be an operation of cutting an outer portion of the hot-stamped part 10 by using two pierced portions 110a and 110b that are formed in the hot-stamped part 10 through hot piercing.

According to one embodiment, in the trimming operation, an edge portion of the hot-stamped part 10 may be cut using at least two pierced portions 110a and 110b that are formed through hot piercing as reference points (or guide patterns). In other words, the hot-stamped part 10 may be mounted on a jig using the at least two pierced portions 110a and 110b formed in the shaped hot-stamped part 10, and the edge portion of the hot-stamped part 10 may be cut. For example, fixing pins may be respectively inserted into the at least two pierced portions 110a and 110b formed in the hot-stamped part 10, and then an outer portion of the hot-stamped part 10 may be cut.

In one embodiment, the trimming operation may be performed using a laser or a press die (e.g., a cold press die).

In the method of manufacturing hot-stamped parts according to one embodiment of the present disclosure, after forming a pierced portion 110 in the blank 100 through hot piercing, an outer portion of the hot-stamped part 10 may be cut through additional cold trimming or laser trimming. That is, in the method of manufacturing hot-stamped parts according to one embodiment of the present disclosure, a hot piercing process may be performed on the blank 100, and after cooling the blank 100, cold trimming or laser trimming may be performed.

In one embodiment, a second piercing operation may be performed after the forming/piercing operation S400. The second piercing operation may be an operation of forming an additional pierced portion in the hot-stamped part 10 by using the at least two pierced portions 110a and 110b formed in the hot-stamped part 10. In other words, the second piercing operation may be an operation of forming an additional pierced portion (for example, a supplementary pierced portion) in the hot-stamped part 10 through hot piercing by using two pierced portions 110a and 110b formed in the hot-stamped part 10.

In one embodiment, the second piercing operation may be performed during the trimming operation. For example, the second piercing operation and the trimming operation may be performed simultaneously. Alternatively, the second piercing operation may be performed before or after the trimming operation. For example, the second piercing operation and the trimming operation may be performed sequentially.

In the method of manufacturing hot-stamped parts according to an embodiment of the present disclosure, a pierced portion 110 may be formed in the hot-stamped part 10 through hot piercing, and an additional pierced portion (for example, a supplementary pierced portion) may be formed in the hot-stamped part 10 through cold piercing or laser piercing. That is, in the method of manufacturing hot-stamped parts according to an embodiment of the present disclosure, a blank 100 may be hot pierced, the hot-pierced blank 100 may be cooled, and then, the cooled blank 100 may be cold pierced or laser pierced.

In one embodiment, an outer portion of a manufactured hot-stamped part may be an edge portion of a coil used as a material for manufacturing the hot-stamped part. That is, the outer portion of the manufactured hot-stamped part may an edge portion of a coil material that is not cut.

In one embodiment, after the trimming operation and/or the second piercing operation, an operation of removing burrs from the blank may be performed. Through this, burrs formed in processes such as hot piercing, cold piercing, and cold trimming may be removed.

Experimental Examples

Hereinafter, the present disclosure will be described in more detail through experimental examples. However, the following experimental examples are only provided to more specifically describe the present disclosure, and the scope of the present disclosure is not limited to the following experimental examples. The following experimental examples may be appropriately modified or changed within the scope of the present disclosure by those skilled in the art.

C
Si
Mn
P
S
Cr
B
Ti

Forming

Material
start

Hot piercing
hydrogen

thickness
temperature
λmax
delay time
amount

In Examples 1 to 3 and Comparative Examples 1 to 3, specimens were obtained by heating blanks having a composition shown in Table 1 for 360 seconds in a heating furnace with a maximum temperature of 950° C., and then subjecting the blanks to hot forming, cooling, and hot piercing under conditions shown in Table 2. At that time, the heating furnace was a multi-stage temperature-rising heating furnace including a plurality of heating zones having temperatures increasing step by step in a direction from an entrance of the heating furnace, through which a black (for example, a plated steel sheet) is loaded, toward an exit of the heating furnace, through which the blank is discharged

Furthermore, hot piercing maximum delay times Ama shown in Table 2 were calculated using the equation described above by inputting 0.001, 0.65, and 1.2 respectively into ap, bp, and cp.

Hydrogen embrittlement was evaluated using a thermal desorption spectroscopy (TDS) apparatus for the specimens of Examples 1 to 3 and Comparative Examples 1 to 3 fabricated under the conditions shown Tables 1 and 2. Specifically, while the specimens of Examples 1 to 3 and Comparative Examples 1 to 3 fabricated under the conditions shown in Tables 1 and 2 were heated from room temperature to 500° C. at a heating rate of 20° C./min, the amount of hydrogen released from each of the specimens of Examples 1 to 3 and Comparative Examples 1 to 3 was measured below 350° C.

Referring to Table 2, it may be confirmed that when a hot piercing delay time is in a range from 0 second to a hot piercing maximum delay time λmax, the amount of diffusible hydrogen in a fabricated hot-stamped part is 0.5 ppm or less.

However, in Comparative Examples 1 to 3 each having a material thickness and a forming start temperature within the scope of the present disclosure but having a hot piercing delay time greater than a hot piercing maximum delay time λmax, the amount of diffusible hydrogen in fabricated hot-stamped parts exceeds 0.5 ppm.

Therefore, when the hot piercing delay time in the forming/piercing operation S400 is within a range of 0 second to the maximum delay time λmax from the time point at which the press die reaches the bottom dead center, the amount of diffusible hydrogen in a manufactured hot-stamped part may be 0.5 ppm or less.

While embodiments of the present disclosure have been described with reference to the embodiments illustrated in the accompanying drawings, the embodiments are merely examples, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments may be derived therefrom. Accordingly, the scope of the present disclosure should be defined by the following claims.