HOT PRESSING LINE AND METHOD OF MANUFACTURING THE HOT PRESSED PRODUCT

Abstract

A hot pressing line (100) includes: a heating device (30); a first pressing device (10) having first die parts (1A, 1B); a second pressing device (20) having second die parts (2A, 2B); a first conveying device (41) conveying a sheet metal (B) to the first pressing device; and a second conveying device (42) conveying the sheet metal (B) to the second pressing device. One of the pair of first die parts and the pair of second die parts includes an inwardly recessed clearance portion (1Ac), while the other pair of die parts includes a butt contact surface (2At) on at least part of the portion corresponding to the clearance portion (1Ac) of one pair of die parts, wherein the butt contact surface makes butt contact with the sheet metal (B) when the die is at bottom dead center.

Description

HOT PRESSING LINE AND METHOD OF MANUFACTURING THE HOT PRESSED PRODUCT TECHNICAL FIELD

[0001] The present invention relates to a hot pressing line and to a method of manufacturing a product shaped by hot pressing. STATE OF THE ART

[0002] In some metallic structural members, properties such as strength can vary locally. For example, when a high-strength member is used as a vehicle frame member, some low-strength portions may be provided in the member, rather than providing high strength throughout. There are several reasons for doing this. For example, machining such as drilling can be performed on low-strength portions. In other applications, the deformation behavior of a member can be controlled by providing low-strength portions that are intended to deform initially during the member's deformation.

[0003] A method for manufacturing a member with low-strength portions involves welding steels with different properties to provide a custom-made, welded blank, followed by hot working (i.e., hot stamping). For example, Japanese Patent No. 5864414 describes a hot-pressing method that forms a steel blank composed of separate sheets that have been welded together. In this method, a steel blank is heated and then hot-pressed into shape within a pair of cooled tooling. While the blank is still within the tooling pair, the formed product is hardened. The welded portions of the two sheets are cooled at lower rates than the portions on either side of each welded portion. This forms portions with low martensite content along the welded portions.The cooling rate is reduced by maintaining a space between the pair of tools and the final product.

[0004] Document JP 2015-226936 A describes a manufacturing method that allows for local adjustment of the construction of a metal structural component. In this manufacturing method, a steel member is hot-formed and then at least several sections are hardened through contact with the tool surface. At least one of the two surface sections of the 1 The CZfrZ Ln / Zznz / E / YIAI tool has a surface coating that decreases or increases thermal conductivity. Sections of the tool surface with different thermal conductivities lead to different cooling rates. Subregions of the steel member with different cooling rates have different microscopic structures after hardening. STATE OF THE ART DOCUMENTS PATENT DOCUMENTS

[0005] Patent Document 1: Japanese Patent No. 5864414 Patent Document 2: JP 2015-226936 A SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] The conventional techniques described above achieve a local decrease in the cooling rate of a sheet metal by means of a gap (clearance) between the formed product and the die, or by a thermal conductivity distribution on the die surface. However, when the formed product has been removed from the die, the temperatures in the portions with the lowest cooling rates remain high. Then, as these portions undergo thermal contraction during cooling, the formed product may develop form defects. Furthermore, if there are large temperature differences within the formed product, when the formed product is removed from the die, it may deform due to thermal contraction, leading to form defects.To reduce the temperature of the formed product upon removal from the die, as well as the temperature differences within the formed product, it must be held in the die until a uniform temperature is reached. On the other hand, from a manufacturing cost perspective, for example, it is preferable to minimize the time the formed product remains in the die (i.e., the bottom dead center holding time). In other words, it is difficult to achieve both productivity and shape accuracy using conventional methods.

[0007] Taking this into consideration, the present invention provides a hot pressing line and a method of manufacturing a hot pressing shaped product that guarantee the accuracy of the shape of a shaped product provided with a distribution of 2 czfrz Ln / zznz / E / YiAi properties, without prolonging the bottom dead center clamping time of the shaped product in the die during hot pressing. MEANS TO SOLVE PROBLEMS

[0008] A hot pressing line according to one embodiment of the present invention includes: a heating device adapted for heating a sheet of metal; a first pressing device including a pair of first die parts movable relative to each other in a pressing direction and adapted to press-form the heated sheet of metal by moving the first die parts closer together in the pressing direction and, at a lower dead center, clamping the sheet of metal; a second pressing device including a pair of second die parts movable relative to each other in the pressing direction and adapted, at a lower dead center of the second die parts, to clamp the sheet of metal formed by pressing by the first pressing device;a first transport device adapted to transport the sheet metal from the heating device to the first pressing device; and a second transport device adapted to transport the sheet metal from the first pressing device to the second pressing device. At least one of the pair of first die parts and the pair of second die parts includes an inwardly facing clearance portion to create clearance with the sheet metal while the die parts are at bottom dead center, and the other pair of die parts includes a butt contact surface located on at least part of a portion corresponding to the clearance portion of one pair of die parts and adapted to make butt contact with the sheet metal while the die parts are at bottom dead center. EFFECTS OF THE INVENTION

[0009] The present invention guarantees the accuracy of the shape of a formed product provided with a distribution of properties without prolonging the bottom dead center clamping time for the formed product in the die during hot pressing.

[0010] BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows an exemplary construction of a hot pressing line according to a modality. Figure 2 is a cross-sectional view of a first pressing device according to a modality, illustrating its construction. Figure 3 illustrates the first pressing device shown in Figure 2 at its bottom dead center. Figure 4 is a cross-sectional view of a second pressing device according to one modality, illustrating its construction. Figure 5 shows the second pressing device shown in Figure 4 at its bottom dead center. Figure 6 shows a variation of the first and second parts of the matrix, modified in construction. Figure 7 is a graph illustrating an application in which a butt contact period is provided in the first bottom dead center clamping period and a non-contact period is provided in the second bottom dead center clamping period. Figure 8 is a graph illustrating an application in which a no-contact clamping period is provided in the first bottom dead center clamping period and a butt contact period is provided in the second bottom dead center clamping period. Figure 9 shows a variation of the first and second parts of the matrix, modified in construction. Figure 10 shows a variation of the first and second parts of the matrix, modified in construction. Figure 11 shows a variation of the first and second parts of the matrix, modified in construction. Figure 12 shows a variation of the first and second parts of the matrix, modified in construction. Figure 13 shows a variation of the first and second parts of the matrix, modified in construction. Figure 14 shows the position in the shaped product of a modality in which the accuracy of the shape is evaluated. Figure 15 is a graph showing the results of the measurement of the hardness distributions of the shaped product. Figure 16 is a graph showing the results of the torsion angle measurements of CZfrZ Ln / Zznz / E / YIAI the shaped products. Figure 17 is a graph showing the results of the out-of-plane deformation measurement of the shaped products. MODALITIES FOR CARRYING OUT THE INVENTION

[0011] (Arrangement 1) A hot pressing line according to one embodiment of the present invention includes: a heating device adapted for heating a sheet of metal; a first pressing device comprising a pair of first die parts movable relative to each other in a pressing direction and adapted to press-form the heated sheet of metal by moving the first die parts closer together in the pressing direction and, at a lower dead center, clamping the sheet of metal; a second pressing device comprising a pair of second die parts movable relative to each other in the pressing direction and adapted, at a lower dead center of the second die parts, to clamp the sheet of metal formed by pressing by the first pressing device;a first transport device adapted to transport the sheet metal from the heating device to the first pressing device; and a second transport device adapted to transport the sheet metal from the first pressing device to the second pressing device. At least one of the pair of first die parts and the pair of second die parts includes an inwardly recessed clearance portion to create clearance with the sheet metal while the die parts are at bottom dead center, and the other pair of die parts includes a butt contact surface located on at least part of a portion corresponding to the clearance portion of one pair of die parts and adapted to make butt contact with the sheet metal while the die parts are at bottom dead center.

[0012] In arrangement 1 described above, the formed sheet metal is rapidly cooled during a bottom-dead center clamping period, which is represented by the sum of the period during which the first die parts of the first pressing device clamp the sheet metal at its bottom dead center and the period during which the second die parts of the second pressing device clamp the sheet metal at its bottom dead center. At least one of the pair of first die parts and the pair of second die parts includes a clearance portion, and the portion of the other pair of die parts corresponding to the clearance portion is provided with a butt contact surface to make butt contact with the sheet metal while the die is at bottom dead center.Therefore, the bottom dead center clamping period, represented by the sum of the bottom dead center clamping periods for the first die parts and the bottom dead center clamping period for the second die parts, includes a non-contact period during which some portions of the sheet metal are not in contact with the die due to clearance, as well as a butt contact period during which the die is in contact with those portions of the sheet metal. During the non-contact period, the cooling rate can be reduced, i.e., gradual cooling can be achieved. Furthermore, during the butt contact period of the bottom dead center clamping time, when the die makes contact with the aforementioned portions of the sheet metal, the cooling rate can be increased, i.e., rapid cooling can be achieved.This achieves a nearly uniform temperature distribution across the sheet metal, while the portion of the formed sheet metal corresponding to the clearance section experiences different cooling conditions than the portions with which a die is in contact throughout the bottom dead center clamping period. This provides the formed sheet metal removed from the second die section with a property distribution resulting from the differences in cooling conditions and, at the same time, reduces the decrease in the form accuracy of the formed product due to temperature differences. This ensures the form accuracy of a formed product with a specific property distribution without extending the bottom dead center clamping time for the formed product in the die.

[0013] In conventional methods where the cooling rate is reduced by clearance or by means of the thermal conductivities of the die surface, the cooling conditions under which part of the member is gradually cooled are predetermined based on the die construction. Therefore, the composition of the metal structure obtained by gradual cooling and the temperature distribution of the member as it is removed from the die also depend on the die construction. Changing these characteristics requires adjusting the die construction or manufacturing a new die. In contrast, in Agreement 1 described above, the cooling conditions can be easily adjusted by changing the pressing or transport conditions.For example, cooling conditions can be controlled by adjusting the lengths of time during which the first die parts hold the sheet metal at bottom dead center and the time during which the second die parts hold the sheet metal at bottom dead center. Therefore, in a pressing process using a hot pressing line, the cooling conditions under which certain portions of the formed sheet metal cool gradually can be easily changed.

[0014] The construction of the clearance portions of the first die parts and the second die parts differs. A clearance portion is provided in at least one of the pair of first die parts and one of the second die parts. The geometry of the pair of surfaces of the first die parts that face each other in the pressing direction (i.e., forming surfaces) and the geometry of the pair of surfaces of the second die parts that face each other in the pressing direction (i.e., forming surfaces) may be the same, except for the clearance portion. Thus, the sheet metal formed by the first die parts may be clamped by the second die parts at bottom dead center, while the portions of the sheet metal other than the portions corresponding to the clearance portion of the first die parts may retain their shape.In some applications, the second die parts can be built, at the bottom dead center, to hold the sheet metal that has been pressure-formed by the first die parts while maintaining the shape of the sheet metal.

[0015] (Arrangement 2) Based on arrangement 1 described above, the first die parts can include the clearance portion, and the second die parts can include the butt contact surface in at least part of a section that corresponds to the clearance portion of the first die parts. Therefore, the second die parts can come into contact with the sheet metal for rapid cooling during a sub-period of the bottom dead center clamping period for the second die parts, when the sheet metal temperature is relatively low. This will make it easier to achieve a nearly uniform temperature distribution across the sheet metal. In other words, it will be easier to ensure the shape accuracy of the entire formed sheet metal. Furthermore, it will be easier to adjust the cooling conditions by controlling the butt contact period.

[0016] (Arrangement 3) Starting with arrangement 1 or 2 described above, the second die parts can include the clearance portion, and the first die parts can include the butt contact surface in at least part of a portion that corresponds to the clearance portion of the second die parts. Therefore, the first die parts can form the sheet metal during a sub-period of the bottom-dead center clamping period for the first die, during which the sheet metal is at a relatively high temperature and easily formed. This makes it easier to ensure local form accuracy—that is, the form accuracy of a portion of a formed sheet metal that corresponds to the clearance portion of the die.

[0017] (Arrangement 4) Starting with any of the Arrangements 1 through 3 described above, the clearance portion of one pair of die parts may include a pair of clearance portions facing each other with the sheet metal positioned between them. In such applications, the butt contact surface of the other pair of die parts may include a pair of butt contact surfaces facing each other with the sheet metal positioned between them, the pair of butt contact surfaces located on at least part of the portions corresponding to the pair of clearance portions of one pair of die parts. In the clearance portions of one pair of die parts, clearances are present on both sides of the sheet metal when the die is at bottom dead center, and in the other pair of die parts, both sides of the sheet metal make butt contact with the die at bottom dead center. This will increase the robustness of the quenching conditions.

[0018] (Arrangement 5) Starting from any of the Arrangements 1 to 4 described above, the butt contact surface of the other pair of die parts includes a pair of butt contact surfaces facing each other in the pressing direction, and the pair of butt contact surfaces is shaped to bend the sheet metal in the pressing direction. This will allow the portion of the sheet metal corresponding to the clearance portion of one pair of die parts to be formed into a shape corresponding to the pair of butt contact surfaces of the other pair of die parts.

[0019] For example, one of the butt contact surfaces of the other pair of die parts may have a recess or protrusion in the pressing direction. In such applications, the other butt contact surface facing the first butt contact surface may have a shape corresponding to the recess or protrusion of the first butt contact surface.

[0020] Starting from any of the arrangements 1 to 4, the butt contact surface of the other pair of die parts corresponding to the clearance portion of a pair of die parts may be a flat surface. Therefore, a flat surface portion of the formed sheet metal is provided with a property distribution.

[0021] (Arrangement 6) From any of the arrangements 1 to 5 described above, a die portion of one pair of die parts facing the clearance portion of one pair may be provided with the contact surface butted, to make butt contact with the sheet metal when the die parts are at bottom dead center, and a die portion of the other pair of die parts facing 8 CZfrZ Ln / Zznz / E / YIAI The butt contact surface of the other pair can be provided with the inwardly hollowed clearance portion to create clearance with the sheet metal when the die parts meet at bottom dead center.

[0022] The clearance portion area in a pair of dies is preferably no greater than half the area that makes contact with the sheet metal when the die is at bottom dead center, more preferably no more than 30%, and still more preferably no more than 20%. An excessively high clearance portion reduces the proportion of the area gripping the formed sheet metal when the die is at bottom dead center, making it difficult to achieve high form accuracy.

[0023] The edge of the gap formed by the clearance portion in a pair of die parts may be surrounded by the die's pressing surface. The pressing surface is the die surface that makes butt contact with the sheet metal when the die is at bottom dead center. That is, the clearance portion may be located in a region surrounded by the surface that makes butt contact and presses the sheet metal B when the die is at bottom dead center. Therefore, when the die is at bottom dead center, the portions of the shaped sheet metal B surrounding the clearance portion are gripped by the die. This makes it easier to ensure the accuracy of the shaped sheet metal B's form.

[0024] The second transport device preferably carries the sheet metal in such a way that the period from the point at which the sheet metal is removed from the first die parts to the point at which the sheet metal is placed in the second die parts is no more than 30 seconds, more preferably no more than 15 seconds, and even more preferably no more than 10 seconds. This will reduce the time from the end of the bottom dead center clamping by the first die parts to the beginning of the bottom dead center clamping by the second die parts, thereby reducing the temperature drop during this time.

[0025] Starting from any of the arrangements 1 to 6 described above, the first pressing device and the second pressing device may include a cooling mechanism adapted to cool the first die parts and the second die parts. For example, at least one of the first die parts and the pair of second die parts may include a tube or a slot to allow a cooling medium to pass through them.

[0026] Starting from any of the arrangements 1 to 6 described above, the hot pressing line may include a control unit adapted to control the first pressing device and the 9 Ln / Zznz / E / YIAI second pressing device. The control unit is capable of controlling, for example, the clamping time of the sheet metal by the first die parts in the first pressing device at bottom dead center, and the clamping time of the sheet metal by the second die parts in the second pressing device at bottom dead center. This allows adjustment of the non-contact period and the butt contact period throughout the entire bottom dead center clamping period. In other words, the cooling conditions can be adjusted for a portion of the sheet metal corresponding to the clearance portion.

[0027] For example, the control unit can control the first die parts and the second die parts, so that the butt contact period represents between 20 and 90% of the total bottom dead center clamping period. In such applications, the butt contact period is preferably not more than 70% of the total bottom dead center clamping period, and more preferably not more than 50%.

[0028] (Manufacturing Method 1) A method of manufacturing a hot-pressed product according to one embodiment of the present invention includes: heating a sheet of metal; placing the heated sheet of metal between a pair of first die parts of a first pressing device; pressing the sheet of metal together by moving the first die parts closer together in a pressing direction; a first bottom-dead center clamping step for holding the sheet of metal while the pair of first die parts are at bottom dead center; after the first bottom-dead center clamping step, transporting the press-formed sheet of metal to a pair of second die parts of a second pressing device and placing the sheet of metal between them;and a second bottom-dead center clamping step to hold the sheet metal formed by pressing by the first pressing device while the pair of second die parts are at bottom dead center. During one of the first bottom-dead center clamping steps and one of the second bottom-dead center clamping steps, a surface of the sheet metal has a butt-non-contact region that does not contact a die at bottom dead center, and at least part of the non-contact region makes contact with a die at bottom dead center during the other bottom-dead center clamping step.

[0029] In manufacturing method 1 described above, the bottom dead center clamping period represented by the sum of the times of the first and second bottom dead center clamping steps includes a non-contact period during which the non-contact region of the sheet metal surface does not make contact with a die at its bottom dead center, as well as a period of 10 CZfrZ Ln / Zznz / E / YIAI butt contact during which that surface makes butt contact with a die at its bottom dead center. The cooling rate can be reduced during the non-contact period of the bottom dead center clamping period. Furthermore, the cooling rate can be increased during the butt contact period of the bottom dead center clamping period. This will achieve a nearly uniform temperature distribution of the sheet metal, while the non-contact region of the formed sheet metal has different cooling conditions than the other portions. This ensures the shape accuracy of a formed product with a uniform property distribution without prolonging the bottom dead center clamping time for the formed product in the die.

[0030] (Manufacturing Method 2) Based on manufacturing method 1 described above, at least part of the non-contact region of the sheet metal during the first bottom dead center clamping step can make butt contact with at least one of the pair of second die parts during the second bottom dead center clamping step. Therefore, throughout the entire bottom dead center clamping period, represented by the sum of the times for the first and second bottom dead center clamping steps, the die can be in contact with the sheet metal for rapid cooling during a sub-period in which the sheet metal temperature is relatively low. This will make it easier to ensure the shape accuracy of the entire formed sheet metal. Furthermore, it will be easier to adjust the cooling conditions by controlling the butt contact period. [0031 ] (Manufacturing Method 3) Starting with manufacturing method 1 or 2 described above, at least part of the non-contact region of the sheet metal for the second bottom dead center clamping step can make butt contact with at least one of the first pair of die parts during the first bottom dead center clamping step. Therefore, throughout the entire bottom dead center clamping period, the die can be in contact with the sheet metal for rapid cooling during a sub-period in which the sheet metal temperature is relatively high. This will make it easier to ensure local form accuracy, i.e., the form accuracy of a portion of a formed sheet metal that corresponds to the clearance portion.

[0032] (Manufacturing Method 4) Based on manufacturing method 1 or 2 described above, the non-contact region of the sheet metal during a bottom dead center clamping step may include a pair of regions, opposite each other, on both sides of the sheet metal, and at least part of each region of the pair of regions in the non-contact region may come into contact with a die portion at bottom dead center 11 CZfrZ Ln / Zznz / E / YIAI during the other bottom dead center clamping step. This increases the robustness of the cooling conditions.

[0033] (Manufacturing Method 5) Starting from any of the manufacturing methods 1 to 4 described above, during the other bottom dead center clamping step, at least part of the non-contact region portion of the sheet metal for the bottom dead center clamping step may come into contact with a die portion at the bottom dead center and may be formed to bend in the pressing direction.

[0034] (Manufacturing Method 6) Starting from any of the manufacturing methods 1 to 5 described above, during the bottom dead center clamping step, at least part of a rear side region of the sheet metal for the non-contact region may be in butt contact by a die piece at the bottom dead center and, during the other bottom dead center clamping step, at least part of the rear side region for the non-contact region may not be in butt contact by a die portion.

[0035] Now, the embodiments of the present invention will be described in detail with reference to the figures. Similar or corresponding elements in the figures are labelled with the same reference characters and their description will not be repeated. For ease of explanation, the figures referred to below show the components in a simplified or schematic manner, or some components are omitted.

[0036] (Exemplary construction of a hot pressing line) Figure 1 shows an exemplary construction of a hot pressing line according to one modality. The hot pressing line 100 includes a heating device 30, a first transport device 41, a first pressing device 10, a second transport device 42, a second pressing device 20, and a control unit 5.

[0037] The heating device 30 heats an object to be heated. The heating device 30 may be, for example, a gas heating oven, a far-infrared heating oven, or a near-infrared heating oven. The heating device 30 is not limited to a heating oven and may be, for example, a high-frequency induction heater, a low-frequency induction heater, or an electric heater that heats the object to be heated by passing electricity through it. The heating device 30 may include a heating chamber. The heating device 30 may include, within the heating chamber, a plurality of chamber rollers 31 that are driven by a drive mechanism, not shown, to rotate.As the rollers in chamber 31 rotate, the object to be heated (in this application, the metal sheet B to be pressed) is conveyed onto the chamber rollers 31. Conveyor rollers 26 are positioned adjacent to the heating device 30. The metal sheet B, heated by the heating device 30, is conveyed by the convection rollers 26 out of the heating device 30.

[0038] The first transport device 41 transports the sheet metal B from the heating device 30 to the first pressing device 10. The first transport device 41 may be a manipulator, for example. The operations of the first transport device 41 include lifting, clamping, transporting, and placing the sheet metal B. The first transport device 41 is not limited to a manipulator. The first transport device 41 may be, for example, a forklift or a roller conveyor.

[0039] The first pressing device 10 includes a pair of first die parts 1A and IB movable relative to each other in the pressing direction. The first transport device 41 places the sheet metal B between the first die parts 1A and IB of the first pressing device 10. The first pressing device 10 forms the heated sheet metal B by pressing, moving the first die parts 1A and 2A closer to each other in the pressing direction and holding the sheet between the die parts at the bottom dead center.

[0040] The second transport device 42 transports the sheet metal B from the first pressing device 10 to the second pressing device 20. Like the first transport device 41, the second transport device 42 may consist of a manipulator, a forklift, or a roller conveyor.

[0041] The second pressing device 20 includes a pair of second die parts 2A and 2b movable relative to each other in the pressing direction. The second transport device 42 places, between the second die parts 2A and 2B, the metal sheet B that has been formed by pressing by the first pressing device 10. The second pressing device 20 holds, between the second die parts 2A and 2B at the bottom dead center, the metal sheet B that has been formed by pressing by the first pressing device 10.

[0042] At least one of the pair of die parts 1L and IB and of the pair of second die parts 2A and 2B includes a clearance portion lAc. In the application shown in Figure 1, the pair of first die parts 1A and IB includes a clearance portion. A clearance portion is provided on at least one of the two surfaces of the pair of die parts that face each other in the pressing direction. A clearance portion is a recess in a die part, cupped inward. With one pair of die parts (i.e., first die parts 1A and IB in the application of Figure 1) including a clearance portion, the other pair of die parts (i.e., second die parts 2A and 2B in the application of Figure 1) includes a butt contact surface 2At.The butt contact surface 2At is formed by at least a portion of the surface of one of the other die parts that corresponds to the clearance portion of that die part. The butt contact surface 2At makes butt contact with the sheet metal when the die is at bottom dead center. Therefore, the first die parts 1A and IB differ from the second die parts 2A and 2B in the construction of their clearance-related portions. Except for the clearance-related portions, the forming surfaces of the first die parts 1A and IB have the same construction as the forming surfaces of the second die parts 2A and 2B.

[0043] The application shown in Figure 1 is an exemplary application in which one pair of die parts with a clearance portion is constituted by the pair of first die parts 1A and IB, while the other pair of die parts is constituted by the pair of second die parts 2A and 2B. In other applications, on the contrary, the pair of second die parts 2A and 2B may constitute one pair of die parts with a clearance portion, while the pair of first die parts 1A and IB may constitute the other pair of die parts with a butt contact surface.

[0044] Control unit 5 controls the hot pressing line 100. Control unit 5 can be configured to control at least one of the heating devices 30, the first transport device 41, the first pressing device 10, the second transport device 42, and the second pressing device 20. Control unit 5 can consist of one or more computers, including a processor and memory.

[0045] The processor of control unit 5 executes a program stored in memory to perform the function of supplying control information to at least one of the heating devices 30, the first transport device 41, the first pressing device 10, the second transport device 42, and the second pressing device 20 (i.e., the device to be controlled). By way of example, based on input from the outside and / or data stored in advance in memory, control unit 5 decides when to operate the device to be controlled and the quantities of operation (or operating instructions), and determines the control information necessary for the relevant movements. Control unit 5 sends the control information to the device to be controlled.

[0046] In the hot pressing line 100, the metal sheet B heated by the heating device 30 is formed by pressing by the first pressing device 10 and held by the 14 Ln / Zznz / E / YIAI first die parts 1A and IB at bottom dead center. Therefore, sheet metal B, while maintaining the shape resulting from press forming, is gripped by the die parts and cools rapidly. The portions of the sheet metal B surfaces that correspond to the clearance portion lAc of the first die parts 1A and IB at bottom dead center provide a non-contact region that does not come into contact with the die. The non-contact region of sheet metal B cools gradually. The non-contact region has different cooling conditions than the other regions. In the second press device 20, sheet metal B, which has been press formed by the first press device 10, is held between the second die parts 2A and 2B at bottom dead center.Therefore, the formed sheet metal B grips the second die parts 2A and 2B and cools rapidly. At this time, the second die parts 2A and 2B also make butt contact with at least part of the non-contact region. This rapidly cools the non-contact region. The formed sheet metal B cools and quenches for the sum of the bottom-dead center clamping period of the first die parts 1A and 1B and the bottom-dead center clamping period of the second die parts 2A and 2B, i.e., the total bottom-dead center clamping period.

[0047] In the application shown in Figure 1, the pair of first die parts 1A and IB of the first pressing device 10, on the one hand, and the pair of second die parts 2A and 2B of the second pressing device, on the other hand, are configured to operate independently of each other. More specifically, the first pressing device 10 includes a pair of supports (e.g., guides and braces, not shown) that support the respective first die parts 1A and IB, and an actuator (not shown) that moves at least one of these supports in the pressing direction. The second pressing device 20, independent of the first pressing device 10, includes a pair of supports that support the respective second die parts 2A and 2B and an actuator that moves at least one of these supports.

[0048] The first and second pressing devices 10 and 20 are not limited to this arrangement. For example, the first die parts 1A and IB and the second die parts 2A and 2B may share supports. More specifically, it is possible to provide a common support (for example, a guide) that supports one first die part 1A and one second die part 2A, a common support (for example, a brace) that supports the other first die part IB and the other second die part 2B, and a common actuator that moves at least one of these supports. In such applications, the first and second pressing devices 10 and 20 are constructed to share supports and an actuator. By way of example, the first and second pressing devices 10 and 20 may consist of 15 C7fr7 ίη / 77Π7 / E / YΙΛΙ a single pressing device that performs transfer pressing with first die parts 1A and 1B and second die parts 2A and 2B.

[0049] (Exemplary construction of the first pressing device) Figure 2 is a cross-sectional view of the first pressing device 10 shown in Figure 1, illustrating its construction. Figure 3 illustrates the first pressing device 10 shown in Figure 2 as being at its bottom dead center. In the application shown in Figures 2 and 3, the first pressing device 10 includes a die block IB and a punch 1A, which exemplify the first pair of die parts IA and IB. The die block IB is movable relative to the punch 1A in the pressing direction PD. That is, the die block IB and the punch 1A are movable relative to each other. The directions of such relative movement are known as the pressing direction.

[0050] The die block IB is movable by a lifting mechanism (i.e., actuator) 81 in the pressing direction relative to the punch 1A. The lifting mechanism 81 may include, for example, a hydraulic cylinder, an air cylinder, an air cushion, or a cam. In the current application, the die block IB moves relative to the punch 1A; in some arrangements, the punch 1A may move relative to the die block IB. In other arrangements, both the die block IB and the punch 1A may move.

[0051] Control unit 5 controls die block IB and punch 1A. In the application shown in Figures 2 and 3, control unit 5 controls the lifting mechanism 8 for die block IB to control the relative movement of die block IB and punch 1A. Control unit 5 supplies the lifting mechanism (i.e., actuators) 8 and 7 with control signals to control such actuation.

[0052] The first pressing device 10 forms the metal sheet B by placing the metal sheet B between the die block IB and the punch 1A and pushing the metal sheet B with both the die block IB and the punch 1A. The die block IB has, inside, a hollow shape that corresponds to the shape of the product to be pressed. The punch 1A has a protruding shape that corresponds to the hollow shape of the die block IB.

[0053] The surface of die block IB facing punch 1L includes a pressing surface IBu that contacts and presses sheet metal B. Die block IB includes recessed portions, i.e., clearance portions IBc, on its surface facing punch 1A. The clearance portions IBc do not make butt contact with sheet metal B even when the die is at bottom dead center. That is, when the die is at bottom dead center, the clearance portions IBc form a gap with sheet metal B. A portion of the surface of sheet metal B 16 CZfrZ Ln / Zznz / E / YIAI held by the matrix at the bottom dead center that corresponds to each portion of clearance IBc constitutes a non-contact region.

[0054] The surface of punch 1A facing die block IB includes a pressing surface lAu that contacts and presses the sheet metal B. Punch 1A includes clearance portions IAc positioned to face the clearance portion IBc of die block IB. Each clearance portion IBc of die block IB and the associated clearance portion IAc of punch 1A are positioned to face each other. As viewed in the pressing direction, at least part of the clearance portion IAc of punch 1A overlaps the clearance portion IBc of die block IB.

[0055] As shown in Figure 3, when the die is at bottom dead center, the surfaces of sheet metal B make butt contact with the pressing surface IBu of the die block IB and the pressing surface lAu of the punch 1A. In the clearance portions IBc and IAc, the surfaces of sheet metal B do not make butt contact with the die. A portion of a sheet metal surface B that corresponds to a clearance portion IBc, IAc constitutes a non-contact region Be. In the application shown in Figure 3, the clearance portions IBc and IAc are positioned to face each other, and therefore the non-contact regions Be occur in opposite regions on either side of sheet metal B.

[0056] Starting with the heated sheet metal B positioned between the die block IB and the punch 1A, separated from each other, control unit 5 causes the die block IB and punch 1A to move closer together in the pressing direction until bottom dead center is reached. Sheet metal B is thus formed by pressing. Control unit 5 then clamps the die block IB and punch 1A at bottom dead center. Therefore, during the bottom dead center clamping period of the first pressing device 10, the portions of the formed sheet metal B in contact with the die block IB and punch 1A cool and harden rapidly. The non-contact regions Be of sheet metal B in the clearance portions 1Be and IAc cool gradually.

[0057] In the application shown in Figure 2, each of the die parts 1A and IB of the first pressing device 10 includes a tube 11 that functions as a channel to allow a cooling medium to pass through it. The tube 11 exemplifies the cooling device. The tube 11 is constituted, for example, by a through-hole in die part 1A, IB. The amount of cooling medium flowing through the tube 11 is controlled by a valve 21, for example. The channel is not limited to a tube 11 and may be a groove in the surface of die part 1A, IB, for example. The cooling medium flowing through the channel cools the die portion 1A, IB. Such cooling 17 Ln / Zznz / E / YIAI keeps the matrix portion 1A, IB no higher than the Mf point (approximately 300 °C), for example. The cooling devices are not shown in the other drawings that show the matrix parts lA and IB.

[0058] (Exemplary construction of the second press device) Figure 4 is a cross-sectional view of the second pressing device 20 shown in Figure 1, illustrating its construction. Figure 5 illustrates the second pressing device 20 shown in Figure 4 as being at its bottom dead center. In the application shown in Figures 4 and 5, the second pressing device 20 includes a die block 2B and a punch 2A, which exemplify the pair of second die parts 2A and 2B. The die block 2B is movable relative to the punch 2A in the pressing direction PD.

[0059] The die block 2B has the same shape as the die block IB of the first pressing device 10, except for the clearance portion IBc. The punch 2A has the same shape as the punch 1A of the first pressing device 10, except for the clearance portions lAc. The lifting mechanism (i.e., the actuator) 82 that moves the die block 2B and the punch 2A relative to each other and the control unit 5 may have the same configuration as those of the first pressing device 10.

[0060] The surface of the die block 2B facing the punch 2A includes a pressing surface that contacts and presses the sheet metal B. The pressing surface of the die block 2B includes the butt contact surfaces 2Bt that correspond to the clearance portion IBc of the first pressing device 10. The butt contact surfaces 2Bt make butt contact with the sheet metal B when the die is at bottom dead center. That is, when the die is at bottom dead center, the non-contact regions Be of the sheet metal B are positioned at those locations on the die block 2B of the second pressing device 20 that correspond to the clearance portions IBc.

[0061] The surface of punch 2A facing the die block 2B includes a pressing surface that contacts and presses the sheet metal B. The pressing surface of punch 2A includes the butt contact surfaces 2At that correspond to the clearance portion lAc of the first pressing device 10. The butt contact surfaces 2At make butt contact with sheet metal B when the die is at bottom dead center. When the die is at bottom dead center, the non-contact regions Be of sheet metal B are positioned at those locations on punch 2A of the second pressing device 20 that correspond to the clearance portions lAc.

[0062] As shown in Figure 5, when the die is at bottom dead center, the surfaces of sheet metal B make butt contact with the pressing surface of die block 2B and the pressing surface of punch 2A. The pressing surfaces also include the butt contact surfaces 2Bt and 2At, which correspond to the clearance portions IBc and lAc. The non-contact regions Be of sheet metal B, which do not make contact with the die in the first pressing device 10 at bottom dead center, now make contact through die block 2B and punch 2A. In the application shown in Figure 5, the die, i.e., die block 2B and punch 2A, make contact with both non-contact regions of sheet metal B on opposite sides.

[0063] Starting with the sheet metal B formed by the first pressing device 10 positioned between the die block 2B and the punch 2A, the control unit 5 brings the die block 2B and the punch 2A closer together in the pressing direction until they reach bottom dead center. Thereafter, the control unit 5 clamps the die block 2B and the punch 2A at bottom dead center. Therefore, during the bottom dead center clamping period of the second pressing device 20, the portions of the formed sheet metal B in contact with the die block 2B and the punch 2A cool and harden rapidly.

[0064] In the application shown in Figure 4, each of the die parts 2A and 2B of the second pressing device 20 includes a tube 12 that functions as a channel to allow a cooling medium to pass through them. The tube 12 exemplifies the cooling device. The tube 12 is constituted, for example, by a through-hole in the die part 2A, 2B. The amount of cooling medium flowing through the tube 12 is controlled by a valve 22, for example. The channel is not limited to a tube 22 and may be a groove in the surface of the die part 2A, 2B, for example. The cooling medium flowing through the channel cools the die part 2A, 2B. Such cooling keeps the die part 2A, 2B no higher than the Mf point (approximately 300°C), for example. The cooling devices are not shown in the other drawings that show matrix parts 2A and 2B.

[0065] In the application shown in Figures 2 to 5, the sheet metal B, having the shape as formed, is gripped by means of a die and cooled for the sum of the bottom dead center clamping period of the first pressing device 10 (hereafter referred to as the first bottom dead center clamping period) and the bottom dead center clamping period of the second pressing device 20 (hereafter referred to as the second bottom dead center clamping period), i.e., the total bottom dead center clamping period. During the total bottom dead center clamping period, the portions of the sheet metal B corresponding to the clearance portions IAc and IBc, i.e., the non-contact regions Be, experience a non-contact period and a butt-contact period.Therefore, sheet metal B cools locally in a gradual manner; that is, the portions defined by the non-contact regions Be cool gradually and have different cooling conditions than the other portions. Consequently, the properties of the portions of sheet metal B defined by the non-contact regions Be are different from the properties of the other portions. Furthermore, for the portions of sheet metal B defined by the non-contact regions Be, the total clamping period at the bottom dead center includes a non-contact period with gradual cooling and a butt-contact period with rapid cooling; thus, while these portions cool gradually, their temperature decreases to some extent as they are gripped by the die.This reduces the temperature difference between the portions defined by the non-contact Be regions and those of the other portions—that is, the portions that make butt contact with a die and cool rapidly during the total bottom-dead center clamping period. This makes it easier to ensure form accuracy.

[0066] (Exemplary manufacturing process) An exemplary manufacturing process for a hot-pressed product using hot press line 100 will now be described. First, a material, namely a sheet of metal B, is heated by the heating device 30. The sheet of metal B can be, for example, a flat sheet, or it can be a formed intermediate product that has been pressed. For example, the sheet of metal B is a sheet of steel. In the heating stage, the sheet of metal B is heated to the Ac3 temperature or higher to austenitize the metal microstructure. The heated sheet of metal B is conveyed by the first conveying device 41 and placed between the die block IB and the punch 1A of the first pressing device 10.

[0067] In the first pressing device 10, the heated sheet metal B is placed between the die block IB and the punch 1A, and at least one of the die blocks IB and punch 1A is moved to bottom dead center. The sheet metal B is thus formed by hot pressing. The formed sheet metal B is held between the die block IB and the punch 1A at bottom dead center. During this first bottom dead center clamping period, the sheet metal B in contact with the die block IB and punch 1A cools rapidly. Some die portions of the first pressing device 10 provide clearance portions, consisting of clearance portions IBc recessed in the die block IB and clearance portions IAc recessed in the punch 1A. When the die is at bottom dead center, the sheet metal B does not make contact with the clearance portions 20 CZfrZ Ln / Zznz / E / YIAI ÍAc and IBc. Thus, the portions of sheet metal B that correspond to the clearance portions ÍAc and IBc, i.e., the portions defined by the non-contact regions Be, cool at a lower rate than the portions that are in contact with the die block IB and the punch 1A. This achieves the gradual cooling of some portions of sheet metal B.

[0068] At the end of the first bottom dead center clamping period, the shaped sheet metal B is positioned by the second transport device 42 between the die block 2B and the punch 2A of the second pressing device 20. The second pressing device 20 moves at least one of the die block 2B and the punch 2A to bottom dead center. The shaped sheet metal B is held between the die block 2B and the punch 2A at bottom dead center. The die block 2B and the punch 2A have no clearance portions. Therefore, both sides of the entire sheet metal B make contact with the die. During this second bottom dead center clamping period, the sheet metal B in contact with the die block 2B and the punch 2A cools rapidly.

[0069] The non-contact regions Be of sheet metal B, which cooled gradually in the first pressing device 10, now make contact with the second die parts of the second pressing device 20, namely die block 2B and punch 2A, during the second bottom-dead center clamping period. During the second bottom-dead center clamping period, the non-contact regions Be of sheet metal B cool rapidly. A near-uniform temperature distribution of sheet metal B is achieved at the end of the second bottom-dead center clamping period.

[0070] At the end of the second bottom dead center clamping period, the formed sheet metal B (i.e., the formed product) is removed from the die (i.e., the die block 2B and the punch 2A). The formed product thus obtained has been provided with a strength distribution and has good form accuracy.

[0071] Details of the mechanism by which a resistance distribution is provided are as follows: there are three types of cooling of portions of the sheet metal B formed by hot pressing by the first pressing device 10 that correspond to the gap portions IBc and IAc of the first die parts (i.e., die block IB and punch 1L), i.e., portions defined by the non-contact regions Be, specifically: (1) heat conduction within the sheet metal B; (2) heat conduction from the sheet metal B to the atmosphere; and (3) radiation from the sheet metal B to the die. As such, the cooling rates in the gap portions are lower than those of heat conduction from the sheet metal B to the die due to the butt contact of the sheet with the die. If the cooling rate of the austenite is less than the critical cooling rate CZfrZ Ln / Zznz / E / YIAI, which depends on the steel sheet used as the material, undergoes diffusion transformation within the steel, producing a soft metallic microstructure of ferrite and / or bainite, for example. On the other hand, the portions in contact with the matrix undergo a non-diffusion transformation, producing a hard metallic microstructure composed mainly of martensite. That is, reducing the cooling rate of some portions of the metal sheet allows the manufacture of a press-formed product with some soft portions.

[0072] If large temperature differences occur within the formed sheet metal (i.e., the formed product) when the formed product is removed from the die, thermal contraction can cause the formed product to deform, leading to shape defects. In contrast, according to the present embodiment, the second die parts (i.e., the die block 2B and the punch 2A) make contact with the non-contact regions Be of the sheet metal B during the second bottom-dead center clamping period of the second pressing device 20. This results in a nearly uniform temperature, rather than temperature differences, in the formed product at the end of the second bottom-dead center clamping period. This makes it easier to ensure the shape accuracy of the entire formed product.Furthermore, during the butt contact period in the second bottom-dead center clamping period, the sheet metal cools while being gripped by the die. This makes it easier to ensure form accuracy, due to the portions gripped by the die, compared to arrangements where the sheet metal is not gripped for the entire bottom-dead center clamping period.

[0073] In the application described above, some portions of a die are caused to separate from the sheet metal B at an early stage of the total bottom dead center clamping period, i.e., during the first bottom dead center clamping period, to provide non-contact regions, and thereafter, a die is caused to make butt contact with the non-contact regions of the sheet metal B at a late stage of the total bottom dead center clamping period, i.e., during the second bottom dead center clamping period.

[0074] In the application described above, the non-contact regions of sheet metal B do not make contact with a die at an early stage of the total bottom dead center clamping period, and make butt contact with a die at a later stage. In other applications, by contrast, the non-contact regions of sheet metal B may make butt contact with a die at an early stage of the total bottom dead center clamping period and not with a die at a later stage. Figure 6 shows such a variation of the first and second die parts, modified in construction. In the application shown in Figure 6, the first die parts (i.e., die block IB and punch 1A) of the first pressing device 10 have no clearance portions.The second die parts (i.e., die block 2B and punch 2A) of the second pressing device 20 include clearance portions 2Bc and 2Ac. The portions of the sheet metal surfaces B that do not make butt contact with a die when the second pressing device 20 is at bottom dead center, i.e., the non-contact regions Be, make butt contact with the butt contact surfaces lAt and IBt of the die when the first pressing direction 10 is at bottom dead center. That is, during the total bottom dead center clamping period, the non-contact regions Be of the sheet metal B make butt contact with a die and cool rapidly during the first bottom dead center clamping period, and the non-contact regions Be of the sheet metal B do not make contact with a die and cool gradually during the second bottom dead center clamping period.In such applications, the properties of the portions of the formed sheet metal B defined by the non-contact regions Be are also different from those of the other portions. Furthermore, a nearly uniform temperature, rather than temperature variations, in the formed product results in the completion of the second bottom-dead center clamping period.

[0075] Figure 7 is a graph illustrating an application in which a butt-contact period is provided during the first bottom-dead center clamping period and a non-contact period is provided during the second bottom-dead center clamping period. Figure 7 illustrates an exemplary application in which press forming is performed by the first die parts and then by the second die parts, both shown in Figure 6. In Figure 7, line L1 indicates the temperature of the portions of sheet metal B being press-formed that correspond to the clearance portions. Line L2 indicates the temperature of the portions of sheet metal B that make butt contact with a die throughout the bottom-dead center clamping period (i.e., other portions).

[0076] In the application shown in Figure 7, from the beginning of the first bottom-dead center clamping period, the first die parts, which do not have a clearance portion, make butt contact with the sheet metal B. At this time, the clearance CL between the die and the sheet metal B is 0 mm. During the first bottom-dead center clamping period, the sheet metal B cools rapidly. At the end of the first bottom-dead center clamping period, the sheet metal B is removed from the first die, conveyed, and then placed between the second die parts. The second die parts press-form the sheet metal B and, after reaching bottom dead center, clamp the sheet metal B while it is at bottom dead center. The second die parts include clearance portions.When the die is at bottom dead center, the portions of sheet metal B that correspond to the clearance portions of the second die parts do not make contact with the die. That is, the portions of sheet metal B that correspond to the clearance portions separate from the second die parts during the second bottom dead center clamping period and remain separated even at the end of the second bottom dead center clamping period. Therefore, the portions of sheet metal B corresponding to the clearance portions—that is, the portions defined by the non-contact region—do not make contact with the die after the completion of the first bottom dead center clamping period, i.e., the butt contact period.The period after the butt-contact period until the second die parts are no longer at bottom dead center constitutes the non-contact period. During the second bottom dead center clamping period, the portions of sheet metal B defined by the non-contact regions experience a lower cooling rate than the other portions and cool gradually.

[0077] Therefore, the first and second bottom-dead center clamping periods include a butt-contact period with rapid cooling and a non-contact period with gradual cooling of some portions of the sheet metal. This reduces the temperature difference between the gradually cooling portions of sheet metal B and the other portions. This makes it easier to ensure the shape accuracy of the entire formed product. Furthermore, since the gradually cooled portions are gripped by a die during the first bottom-dead center clamping period, it is easier to ensure the shape accuracy of the gradually cooled portions.

[0078] In the application shown in Figure 7, during the first bottom-dead center clamping period, i.e., the butt-contact period, the portions of sheet metal B defined by the non-contact regions cool rapidly, as indicated by line L1, and then separate from the first die parts before the Ms point (the starting point of martensitic transformation) is reached and gradual cooling begins. Thereafter, the portions of sheet metal B defined by the non-contact regions do not make contact with the die and therefore cool gradually during transport, forming by pressing through the second die parts and the second bottom-dead center clamping. This produces a soft metallic microstructure.On the other hand, as indicated by line L2, the other portions of sheet metal B, excluding the non-contact regions Be, make butt contact and therefore cool rapidly during the first and second bottom-dead center clamping periods. These other portions cool to and below the Mf point (the endpoint of martensitic transformation) during the second bottom-dead center clamping period. This results in a hard metallic microstructure composed primarily of martensite. czt?? ίη / ζζηζ / Β / γίΛΐ Thus, the portions of the shaped sheet metal B that correspond to the clearance portions, on the one hand, and the other portions, on the other hand, have different properties (i.e., resistances in the present application).

[0079] In the application of Figure 7, the portions of interest of sheet metal B are in the butt contact period and therefore cool rapidly during the first bottom-dead center clamping period, i.e., the period in which the temperature is relatively high. Since sheet metal B is gripped by a die during a period when it is at a high temperature and soft, it is easier to ensure shape accuracy due to the gripped portions.

[0080] Figure 8 is a graph illustrating an application in which a non-contact clamping period is provided during the first bottom-dead center clamping period and a dwell period during the second bottom-dead center clamping period. Figure 8 illustrates an application in which press forming is performed by the first die parts shown in Figure 2 and then the second die parts shown in Figure 4. In Figure 8, line L3 indicates the temperature of the portions of sheet metal B being press-formed that correspond to the clearance portions. Line L2 indicates the temperature of the portions of sheet metal B that make butt contact with a die throughout the bottom-dead center clamping period (i.e., other portions). In the application shown in Figure 8, the first die parts include clearance portions.At the beginning of the first bottom-dead center clamping period, the clearance portions of the first die parts separate from the sheet metal B. During this first bottom-dead center clamping period, the portions of sheet metal B corresponding to the clearance portions—that is, the portions defined by the non-contact regions Be—do not touch the first die parts and separate from them. At the end of the first bottom-dead center clamping period, the formed sheet metal B is removed from the first die parts, transported, and placed between the second die parts. The second die parts press-form the sheet metal B and, at bottom dead center, clamp the sheet. During the second bottom-dead center clamping period, the portions of sheet metal B corresponding to the clearance portions make butt contact with the second die parts.Thus, in the application shown in Figure 8, there is a non-contact period from the beginning of the first bottom-dead center clamping period. The transport period is also part of the non-contact period. The period from the beginning of the second bottom-dead center clamping period to the end of the second bottom-dead center clamping period constitutes the butt contact period.

[0081] Therefore, the first and second bottom-dead center clamping periods include a non-contact period with gradual cooling and a butt-contact period with rapid cooling of some portions of the formed sheet metal B. This reduces the temperature difference between the gradually cooling portions of sheet metal B and the other portions. This makes it easier to ensure the shape accuracy of the entire formed product. Furthermore, since the gradually cooled portions are gripped by a die during the second bottom-dead center clamping period, it is easier to ensure the shape accuracy of the gradually cooled portions.

[0082] In the application shown in Figure 8, for the portions of sheet metal B in the non-contact regions Be, the butt contact period, i.e., the second bottom-dead center clamping period, ends before the temperature drops to point Ms, as indicated by line L3. This produces a soft metallic microstructure. On the other hand, as indicated by line L2, the other portions of sheet metal B, other than the non-contact regions Be, cool rapidly during the first and second bottom-dead center clamping periods, and thus cool to such an extent that the temperature drops to or below point Mf at the end of the second bottom-dead center clamping period. This produces a hard metallic microstructure composed mainly of martensite.Thus, the portions of the shaped sheet metal B that correspond to the clearance portions, on the one hand, and the other portions, on the other hand, have different properties (i.e., resistances).

[0083] In the application shown in Figure 8, the portions of sheet metal B corresponding to the clearance portions are in the butt contact period and cool rapidly during the second bottom-dead center clamping period—a period when the temperature is relatively low and the cooling rate has slowed. In this application, the temperature difference caused by rapid cooling is small, making temperature control easier. Furthermore, sheet metal B grips and cools rapidly when its temperature has decreased and the sheet has hardened somewhat, further simplifying shape accuracy.

[0084] The butt contact period and the non-contact period of the bottom dead center clamping period are not limited to these exemplary applications. For example, there may be two or more separate butt contact periods in the bottom dead center clamping period. By way of example, a butt contact period may be provided in each of the early and late stages of the bottom dead center clamping period, and a non-contact period may be provided in an intermediate period between the early and late stages.For example, after the first die parts that do not have clearance portions press-form the blade and the 26 czfrz Ln / zznz / E / YiAi hold it while it is at bottom dead center, the second die parts that have clearance portions can hold the blade while it is at bottom dead center and then again the first die parts (or the third die parts without clearance portions) can hold the blade while it is at bottom dead center.

[0085] Although not a limiting factor, the total bottom dead center holding period, i.e., the sum of the first and second bottom dead center holding periods, can range from 2 to 90 seconds, for example. A longer total bottom dead center holding period is better in terms of the uniformity of the temperature distribution of the formed product at the end of the holding period; conversely, a shorter holding period is better in terms of manufacturing efficiency. Therefore, a lower limit for the total bottom dead center holding period is preferably 10 seconds, and more preferably 15 seconds. An upper limit for the total bottom dead center holding period is preferably 90 seconds, and more preferably 30 seconds.In the current mode, the total bottom dead center clamping period includes a butt contact period and a non-contact period; as such, a uniform temperature distribution in the formed product after completing the total bottom dead center clamping period can be easily achieved even when the total bottom dead center clamping period is no more than 30 seconds, for example.

[0086] In the application shown in Figures 7 and 8, the portions of sheet B other than the non-contact regions Be are cooled to and below point Mf during the total bottom-dead center clamping period. This allows for tempering. All die parts 1A and IB of the first pressing device 10 and die parts 2A and 2B of the second pressing device 20 can be maintained at a temperature no higher than point Mf by the cooling devices.

[0087] The clearance of the clearance portions lAc and IBc of first die parts 1A and IB, or the clearance portions 2Ac and 2Bc of second die parts 2A and 2B, i.e., the distance between a die and the sheet metal, is not limited to any particular value; for example, it cannot be less than 2 mm, preferably not less than 4 mm, and more preferably not less than 6 mm.

[0088] (Variations of matrix parts) Figure 9 shows a variation of the first and second die parts, modified in construction. In the application shown in Figure 9, both the first and second die parts include clearance portions and butt contact surfaces. A non-contact region Be of the sheet metal B corresponding to the pair of clearance portions IBc and lAc of 27 The die block IB and punch 1A, which constitute the first die parts, make butt contact with the butt contact surfaces 2Bt and 2At of the die block 2B and punch 2A, which constitute the second die parts, at the bottom dead center. A non-contact region Be of the sheet metal B, corresponding to the pair of clearance portions 2Ac and 2Bc, which constitute the second die parts, makes butt contact through the butt contact surfaces IBt and IAt of the die block IB and punch 1A, which constitute the first die parts.

[0089] As shown in Figure 9, at least some of the portions of the second die parts that correspond to the clearance portions of the first die parts may include butt contact surfaces that are in butt contact with the sheet metal while at bottom dead center, and at least some of the first die parts that correspond to the clearance portions of the second die parts may include butt contact surfaces that make butt contact with the sheet metal while at bottom dead center.In this application, sheet metal B includes both a portion that makes butt contact with a die during the first bottom dead center clamping period and does not make butt contact with a die during the second bottom dead center clamping period, and a portion that does not make butt contact with a die during the first bottom dead center clamping period and makes butt contact with a die during a second bottom dead center clamping period.

[0090] Figure 10 shows another variation of the first and second die parts, modified in construction. In the application shown in Figure 10, apart from a pair of first die parts 1A and IB, one die part IB includes a clearance portion IBc consisting of a gap. The portion of the other die part 1A facing the clearance portion IBc has no clearance portion, i.e., no gap, and is part of the pressing surface lAu that makes contact with the sheet metal B. In this application, a non-contact region Be is created on the sheet metal B on the side associated with the clearance portion IBc, and the rear portion, i.e., the side opposite the non-contact region Be, has no non-contact region Be.

[0091] In the application shown in Figure 10, the portion of the pair of second die parts 2A and 2B that corresponds to the clearance portion IBc of the first die part IB—that is, the portion where the non-contact region Be of the sheet metal B is located—represents a butt contact surface 2Bt with which the second die part 2B makes butt contact with the sheet metal. Therefore, a clearance portion, even if provided on only one side of the sheet metal B, has the effect of ensuring the shape accuracy of a formed product with a given property distribution. From the application shown in Figure 10, the first die parts and the second die parts can be reversed.That is, the second die parts may include a clearance portion on one side of the sheet metal B, while the first die parts may include a butt contact surface, in the position corresponding to the clearance portion of the second die parts.

[0092] Figure 11 shows another variation of the first and second die parts, modified in construction. In the application shown in Figure 11, of a pair of first die parts 1A and IB, one die part IB includes a clearance portion IBc consisting of a gap. The portion of the other die part 1A facing the clearance portion IBc has no clearance, i.e., a gap, and makes butt contact with the sheet metal B. The portion of the second die part 2B corresponding to the clearance portion IBc of the first die part IB represents a butt contact surface 2Bt that makes butt contact with the sheet metal B. The portion of the second die part 2A facing the butt contact surface 2Bt provides a clearance portion 2Ac consisting of a gap.The portion of the first die part 1A that corresponds to the clearance portion 2Ac of the second die part 2A represents a butt contact surface 1At that makes contact with the sheet metal B. This construction also produces the effect of ensuring the accuracy of the shape of a formed product provided with a property distribution.

[0093] Figure 12 shows another variation of the first and second die parts, modified in construction. In the application shown in Figure 12, the first die parts 1A and IB include clearance portions lAc and IBc. In the second die parts 2A and 2B, part of each of the portions corresponding to the clearance portions lAc and IBc of the first die parts 1A and IB provides a clearance portion 2Ac, 2Bc, and the other portions make butt contact with the sheet metal B. In other words, in the second die parts 2A and 2B, part of each of the portions where the non-contact region Be of the sheet metal B is placed represents a butt contact surface, and the other portions provide clearance portions 2Ac and 2Bc. In this application, both the first and second die parts include clearance portions.The clearance portions of the first die parts correspond in position to the clearance portions of the second die, but have a different surface area. Therefore, cooling conditions can be established by modifying the areas of the clearance portions, for example.

[0094] Figure 13 shows another variation of the first and second die parts, modified in construction. In the application shown in Figure 13, the first die parts 1A and IB include clearance portions lAc and IBc. The portions of the second die parts 2A and 2B that correspond to the clearance portions lAc and IBc of the first die parts 1A and IB represent a pair of butt contact surfaces 2Ap and 2Bp that face each other and make butt contact on both sides of the sheet metal B. The pair of butt contact surfaces 2Ap and 2Bp are shaped to bend the sheet metal B in the pressing direction. One of the butt contact surfaces, 2Ap, has a protruding shape, and the other has a recessed shape that corresponds to the protruding shape.Therefore, if each of the butt contact surfaces 2Ap and 2Bp of the second die parts 2A and 2B has a recessed / protruding, protruding, or recessed shape in the pressing direction, it is possible to shape the associated portion of sheet metal B into a shape that corresponds to that recessed / protruding shape.

[0095] Although not limiting, the hot press line and the method of manufacturing the hot-pressed product according to the modalities can be applied, for example, to the manufacture of structural members for vehicles. Structural members for vehicles are often required to have a strength distribution and provide dimensional accuracy. The modalities can be appropriately applied to such structural elements for vehicles. For example, the hot press line according to one modality can manufacture a structural member for a vehicle consisting of a hot-pressed product (i.e., a hot-stamped member) that has some portions, within a single piece, that have been smoothed in order to reduce the vehicle's weight or achieve high performance, for example.Examples of such structural members for vehicles include high-strength center pillars that have soft flanges or rear side members or bumper beams in which smoothed portions are placed to control the steep bending mode in an impact.

[0096] (Examples) B-pillar dies with slack portions (hereafter referred to as slack dies) and dies without slack portions, i.e., no-slack dies, were manufactured and tested. A no-slack die is an example of a pair of first die parts and has the same construction as the first die parts 1A and IB shown in Figure 6. A slack die is an example of a pair of second die parts and has the same construction as the second die parts 2L and 2B shown in Figure 6. The first die parts have no slack portions. The second die parts have slack portions in positions corresponding to the B-pillar tabs. One slack portion of the second die parts included a recessed portion (i.e., space) 2Bc in the die block 2B and a recessed portion 2Ac in the punch 2A, which faces that recessed portion.The portions of sheet metal B that correspond to the clearance portion of the second die parts were not cooled by means of the 30. CZfrZ Ln / Zznz / E / YIAI matrix in the lower dead center and, therefore, cooled gradually, producing a soft metallic microstructure.

[0097] For the test, the sheet metal used was a hot-rolled sheet for hot stamping (hereafter referred to as HS) (thickness: 2.6 mm). The sheet metal was heated for 5 minutes in a furnace at 900 °C, formed by means of the first die parts and / or the second die parts, held while the die was at bottom dead center, removed from the die, and allowed to cool. The three sets of clearance conditions applied while the sheet was held by the first die parts and / or the second die parts at bottom dead center were used, i.e., sets of conditions (a) to (c) shown in Table 1 below. CZfrZ Ln / Zznz / E / YIAI

[0098] [Table 1] (a) Matrix without slack (one step) First matrix without slack (0 mm) (b) Matrix with slack (one step) Second matrix with slack (13 mm) (c) Matrices with slack (two steps) First and second matrices Without slack (0 mm), with slack (13 mm)

[0099] In Table 1, condition set (a) means that a zero-clearance die was used for press forming and represents typical HS conditions, where the entire surface of the sheet metal, including the tabs, is in contact with the first die parts. In condition set (a), the sheet metal was conveyed to the first die after heating, held by the die parts at bottom dead center for 10 seconds, and then removed from the die and allowed to cool. Condition set (b) means that the second die parts, which have clearance portions in the position corresponding to the tabs, were used for press forming. In condition set (b), the sheet metal was conveyed to the second die parts after heating, held by the die at bottom dead center for 10 seconds, and then removed from the die and allowed to cool.The amount of clearance was constant throughout the bottom dead center holding period. At the end of the bottom dead center holding period, the sheet was removed from the die while the portions corresponding to the tabs were still at high temperatures. Under set of conditions (c), the sheet metal was conveyed to the first die portions after heating, removed from the die immediately after bottom dead center was reached, conveyed to the second die portions, held by the die at bottom dead center for 30 seconds, and then removed from the die and allowed to cool.

[0100] The products formed after hot pressing were evaluated with respect to the hardness and shape accuracy of the tabs. Shape accuracy was evaluated as a function of the twist of the formed product and the out-of-plane deformation of the tabs. The position on the formed product of an example where shape accuracy was evaluated is shown in Figure 14. The shape accuracy for each of the condition sets (b) and (c) was evaluated with respect to the data from condition set (a).

[0101] Figure 15 is a graph showing the results of the hardness distribution measurement of the formed products. Compared to the product formed from condition set (a), the products formed from condition sets (b) and (c) had low hardness in the clearance portions. A clearance portion is a portion of the formed product corresponding to a clearance portion of a die. The results shown in Figure 15 demonstrate the partial softening effect produced by the clearance portions of the one-step and two-step clearance dies.

[0102] Figure 16 is a graph showing the results of the twist angle measurement of the formed product. The twist angle in the graph in Figure 16 indicates the degree to which the cross-section C1 of the torsion assessment twisted relative to that of the formed product from condition set (a), as found when the formed products from condition sets (a) to (c) were aligned in position with respect to the torsion alignment surface W1 shown in Figure 14.

[0103] The results shown in Figure 16 demonstrate that the product formed by the slack die of condition set (b) (with one step) had a higher twist, i.e., lower form accuracy, than the product formed by condition set (a) without slack. On the other hand, the product formed by pressing with the slack dies with two steps of condition set (c) had a twist angle no greater than half that of condition set (b), demonstrating an improvement in form accuracy.

[0104] Figure 17 is a graph showing the results of the out-of-plane strain measurement of the formed products. The amount of out-of-plane strain shown in the graph in Figure 17 indicates the amount of surface strain at the out-of-plane strain evaluation position F1 shown in Figure 14 relative to that of the formed product from condition set (a). The out-of-plane strain evaluation position F1 for condition sets (b) and (c) was a portion that included a position on a tab corresponding to a slack portion in a matrix. The examples shown in Figure 17 demonstrate that the slack matrix with two steps from condition set (c) also improved the local shape accuracy on the tabs corresponding to the slack portion.

[0105] Although embodiments of the present invention have been described, the embodiments described above are merely illustrative examples useful for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and the embodiments described above, when carried out, may be modified accordingly without departing from the essence of the invention. EXPLANATION OF THE CHARACTERS

[0106] 1A, IB: first parts of matrix lAc, IBc: clearance portions lAt, IBt: butt contact surfaces 10: first pressing device 100: hot pressing line 2A, 2B: second parts of matrix 2Ac, 2Bc: slack portions 2At, 2Bt: butt contact surfaces 20: second pressing device 30: heating device 41: first transport device 42: second transport device 5: control unit B: sheet of metal Be: contactless regions

Claims

1. A hot pressing line comprising: a heating device adapted for heating a sheet of metal; a first pressing device including a pair of first die parts movable relative to each other in a pressing direction and adapted to press-form the heated sheet of metal by moving the first die parts closer together in the pressing direction and, at a bottom dead center, clamping the sheet of metal; a second pressing device including a pair of second die parts movable relative to each other in the pressing direction and adapted, at a bottom dead center of the second die parts, to clamp the sheet of metal formed by pressing by the first pressing device;a first transport device adapted to transport the sheet metal from the heating device to the first pressing device, and a second transport device adapted to transport the sheet metal from the first pressing device to the second pressing device, wherein at least one of the pairs of first die parts and the pair of second die parts includes an inwardly recessed clearance portion to create a clearance with the sheet metal while the die parts are at bottom dead center, and the other pair of die parts includes a butt contact surface located on at least part of a portion corresponding to the clearance portion of a pair of die parts and adapted to make butt contact with the sheet metal while the die parts are at bottom dead center.

2. The hot pressing line according to claim 1, wherein the first die parts include the clearance portion and the second die parts include the butt contact surface in at least part of a portion corresponding to the clearance portion of the first die parts.

3. The hot pressing line according to claim 1, wherein the second die parts include the clearance portion and the first die parts include the butt contact surface in at least part of a portion corresponding to the clearance portion of the second die parts.

4. The hot pressing line according to any of claims 1 to 3, wherein the clearance portion of a pair of die parts includes a pair of clearance portions opposite each other with the sheet metal placed between them and the butt contact surface of the other pair of die parts includes a pair of butt contact surfaces opposite each other with the sheet metal placed between them, the pair of butt contact surfaces being placed on at least part of portions corresponding to the pair of clearance portions of the pair of die parts.

5. The hot pressing line according to any of claims 1 to 3, wherein the butt contact surface of the other pair of die parts includes a pair of butt contact surfaces facing each other in the pressing direction and the pair of butt contact surfaces are configured to bend the sheet metal in the pressing direction.

6. The hot pressing line according to any of claims 1 to 3, wherein a die portion of one pair of die parts facing the clearance portion of one pair is provided with the butt contact surface to make butt contact with the sheet metal when the die parts are at bottom dead center, and a die portion of the other pair of die parts facing the butt contact surface of the other pair is provided with the clearance portion cupped inwards to create a clearance with the sheet metal when the die parts are at bottom dead center.

7. A method of manufacturing a hot-pressed product, comprising: heating a sheet of metal; placing the heated sheet of metal between a pair of first die parts of a first pressing device; pressing-forming the sheet of metal by moving the first die parts closer together in a pressing direction; a first bottom-dead center clamping step for holding the sheet of metal while the pair of first die parts are at bottom dead center; after the first bottom-dead center clamping step, conveying the formed sheet of metal to a pair of second die parts of a second pressing device and placing the sheet of metal between them;and a second bottom dead center clamping step for holding the sheet metal formed by pressing by the first pressing device while the pair of second die parts are at a bottom dead center, wherein, during one of the first bottom dead center clamping steps and the second bottom dead center clamping step, a surface of the sheet metal has a non-contact region that does not come into contact with a die at a bottom dead center, and at least part of the non-contact region makes contact with a die at a bottom dead center during the other bottom dead center clamping step.

8. The method of manufacturing a hot-pressed shaped product according to claim 7, wherein at least part of the non-contact region of the sheet metal for the first bottom dead center clamping step makes butt contact with at least one of the pair of second die parts during the second bottom dead center clamping step.

9. The method of manufacturing a hot-pressed shaped product according to claim 7, wherein at least part of the non-contact region of the sheet metal for the second bottom dead center clamping step makes butt contact with at least one of the first pairs of die parts during the first bottom dead center clamping step.

10. The method of manufacturing a hot-pressed shaped product according to any of claims 7 to 9, wherein the non-contact region of the sheet metal during the bottom dead center clamping step includes a pair of regions, opposite each other, on both sides of the sheet metal, and at least part of each region of the pair of regions of the non-contact region makes contact with a die portion at the bottom dead center during the other bottom dead center clamping step.

11. The method of manufacturing a hot-pressed shaped product according to any of claims 7 to 9, wherein, during the other lower dead center clamping step, at least part of the non-contact region of the sheet metal for the lower dead center clamping step makes contact with a die part at the lower dead center and is shaped to bend in the pressing direction.

12. The method of manufacturing a hot-pressed shaped product according to any of claims 7 to 9, wherein, during a bottom dead center clamping step, at least part of a rear side region of the sheet metal for the non-contact region is in butt contact by means of a die portion at the bottom dead center, and during the other bottom dead center clamping step, at least part of the rear side region for the non-contact region is not making contact by means of a die portion.

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