JP7477810B2 - Manufacturing method of press-molded products - Google Patents

Description

The present disclosure relates to a method for manufacturing press-molded products.

Conventionally, press-formed products have been widely used as components that make up the body of an automobile. Press-formed products are manufactured, for example, by subjecting a steel plate, which is the raw material, to hot pressing (hot stamping). In recent years, in order to reduce the weight of the car body and improve collision safety, differential thickness steel plates have been adopted as the raw material for press-formed products. Differential thickness steel plates are, for example, patchwork blanks made by overlapping multiple steel plates. In patchwork blanks, the position and range of the thick-walled regions, and the difference in plate thickness between the thick-walled and thin-walled regions, etc. can be freely designed.

In a patchwork blank, the overlapping steel sheets are joined together, for example, by spot welding. In this case, the integration effect between the steel sheets is likely to be insufficient, and it is difficult to make the overlapping steel sheets behave as a single thick material. Therefore, in a press-formed product manufactured from such a patchwork blank, it is difficult to significantly improve the rigidity of the overlapping surfaces between the steel sheets. In order to improve the integration effect between the steel sheets, it is possible to increase the joint area between the steel sheets by employing, for example, laser welding or brazing. However, the larger the joint area, the more time and effort is required to join the steel sheets together.

In response to this, Patent Document 1 discloses a technique for joining steel sheets together using the plating layer between the steel sheets. In Patent Document 1, when manufacturing a press-formed product, steel sheets having zinc or aluminum plating layers on both sides are placed one on top of the other and carried into a heating furnace and heated. This heating melts the plating layers. Next, the stacked steel sheets are removed from the heating furnace, and these steel sheets are formed into a predetermined shape using a metal mold and cooled. Patent Document 1 describes that the molten plating layers solidify due to cooling using the metal mold, and the steel sheets are welded together.

JP 2014-124673 A

In Patent Document 1, the overlapping steel sheets are simply heated to melt the plating layer, which is then solidified. However, simply overlapping steel sheets results in microscopic gaps between the steel sheets. When the air that has entered these gaps becomes hot due to the heating of the steel sheets, the air expands, causing the overlapping steel sheets to peel off. Therefore, with the technology in Patent Document 1, the overlapping steel sheets are difficult to bond together due to the re-solidification of the plating layer, resulting in insufficient bonding between the steel sheets.

The objective of the present disclosure is to easily and effectively join overlapping steel plates together in a method for manufacturing a press-formed product using multiple steel plates.

The manufacturing method according to the present disclosure is a method for manufacturing a press-molded product. The manufacturing method includes the following steps (a) to (c).
(a) overlapping a first steel sheet having a plating layer on its surface with a second steel sheet such that the plating layer is disposed between the first steel sheet and the second steel sheet; (b) applying pressure to the overlapped first steel sheet and second steel sheet in the sheet thickness direction to bring them into close contact; (c) heating the closely contacted first steel sheet and second steel sheet to a temperature equal to or higher than the melting point of the plating layer.

According to the present disclosure, in a method for manufacturing a press-molded product using multiple steel plates, overlapping steel plates can be joined together easily and effectively.

FIG. 1 is a flowchart of a method for manufacturing a press-molded product according to an embodiment. FIG. 2 is a schematic diagram for explaining a preparation step included in the manufacturing method according to the embodiment. FIG. 3 is a schematic diagram for explaining a pressurizing step included in the manufacturing method according to the embodiment. FIG. 4 is another schematic diagram for explaining the pressurizing step included in the manufacturing method according to the embodiment. FIG. 5 is a schematic diagram for explaining a molding step included in the manufacturing method according to the embodiment. FIG. 6 is another schematic diagram for explaining the molding step included in the manufacturing method according to the embodiment. FIG. 7 is a perspective view of a press-formed product manufactured by the manufacturing method according to the embodiment. FIG. 8 is a photograph showing the surface of the peeled steel plate for each of the examples and the comparative examples. FIG. 9 is an observation image of the surface of the steel plate shown in FIG.

The manufacturing method according to the embodiment is a method for manufacturing a press-molded product. This manufacturing method includes the following steps (a) to (c) (first configuration).
(a) overlapping a first steel sheet having a plating layer on its surface with a second steel sheet such that the plating layer is disposed between the first steel sheet and the second steel sheet; (b) applying pressure to the overlapped first steel sheet and second steel sheet in the sheet thickness direction to bring them into close contact; (c) heating the closely contacted first steel sheet and second steel sheet to a temperature equal to or higher than the melting point of the plating layer.

In the manufacturing method of the first configuration, the overlapped first and second steel sheets are pre-pressurized. This allows the first and second steel sheets to adhere to each other, and the microscopic gap between the first and second steel sheets can be reduced or substantially eliminated. That is, since air can be prevented from entering between the overlapped first and second steel sheets, when the adhered first and second steel sheets are heated to the melting point of the coating layer or higher, it is possible to suppress the air expanding between the first and second steel sheets and pushing aside the molten coating layer. As a result, the adhesion caused by resolidification of the coating layer between the first and second steel sheets is less likely to be hindered. Therefore, the first and second steel sheets can be well joined over a wide area by utilizing the adhesion caused by resolidification of the coating layer. In addition, the first and second steel sheets can be easily joined without performing a joining operation such as brazing by utilizing the adhesion caused by resolidification of the coating layer.

In the above (b), the first steel plate and the second steel plate are preferably bonded by pressure (second configuration). This makes it possible to prevent the first steel plate and the second steel plate from peeling off after pressure is applied.

The plating layer may be a zinc-based plating layer or an aluminum-based plating layer (third configuration).

The second steel sheet may have a plating layer on its surface. In this case, the first steel sheet and the second steel sheet may be overlapped so that the plating layers face each other (fourth configuration).

The plating layer of the second steel sheet may be the same type of plating layer as the plating layer of the first steel sheet (fifth configuration).

Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[Method of manufacturing press-molded products]
The manufacturing method according to the present embodiment is a method for manufacturing a press-formed product by hot stamping. The press-formed product is used, for example, as a structural member constituting a part of an automobile body. Examples of the structural member include an A-pillar reinforcement, a B-pillar reinforcement, a bumper reinforcement, a tunnel reinforcement, a side sill reinforcement, a roof reinforcement, and a floor cross member.

Figure 1 is a flowchart of the method for manufacturing a press-molded product according to this embodiment. As shown in Figure 1, the manufacturing method according to this embodiment includes a preparation step S1, a pressurizing step S2, a heating step S3, and a molding step S4. Each step will be described in detail below.

(Preparation process)
2 is a schematic diagram for explaining the preparation step S1. Referring to FIG. 2, in the preparation step S1, a plurality of steel plates 10, 20 that are materials for the press-formed product are prepared. In this embodiment, two steel plates 10, 20 are prepared. In the example shown in FIG. 2, the area of the surface (surface substantially perpendicular to the plate thickness direction) of the steel plate 20 is smaller than the area of the surface (surface substantially perpendicular to the plate thickness direction) of the steel plate 10. The plate thickness of the steel plate 20 may be the same as or different from the plate thickness of the steel plate 10.

Steel sheet 10 or steel sheet 20 has a plating layer on at least one surface. Alternatively, each of steel sheets 10, 20 has a plating layer on at least one surface. That is, at least one of steel sheets 10, 20 is a plated steel sheet. When steel sheet 10 is a plated steel sheet, steel sheet 20 may be a plated steel sheet or may be a so-called bare material (steel sheet without a plating layer). Similarly, when steel sheet 20 is a plated steel sheet, steel sheet 10 may be a plated steel sheet or may be a bare material.

The materials of the steel sheets 10 and 20 as the base material may be the same or different. The materials of the steel sheets 10 and 20 are not particularly limited and can be selected appropriately. The plating layer may be a zinc-based plating layer or an aluminum-based plating layer.

The zinc-based plating layer is, for example, a zinc plating layer or a zinc alloy plating layer. The zinc plating layer is a plating layer whose main component is zinc (Zn). The zinc alloy plating layer is a plating layer whose main component is a zinc alloy, and is, for example, a Zn-Fe-based plating layer, a Zn-Al-based plating layer, a Zn-Mg-based plating layer, a Zn-Ni-based plating layer, and a Zn-Al-Mg-based plating layer. More specifically, examples of the zinc-based plating layer include hot-dip Zn plating, alloyed hot-dip Zn (e.g., Zn-10%Fe) plating, hot-dip Zn-55%Al-1.6%Si plating, hot-dip Zn-11%Al plating, hot-dip Zn-11%Al-3%Mg plating, hot-dip Zn-6%Al-3%Mg plating, hot-dip Zn-11%Al-3%Mg-0.2%Si plating, electric Zn plating, electric Zn-Ni plating, and electric Zn-Co plating (% means mass %). The zinc-based plating layer may be formed of a vapor deposition plating having the same components as any of these platings.

The aluminum-based plating layer is, for example, an aluminum plating layer or an aluminum alloy plating layer. The aluminum plating layer is a plating layer whose main component is aluminum (Al), and the aluminum alloy plating layer is a plating layer whose main component is an aluminum alloy. The aluminum-based plating layer can have the following chemical composition. That is, the aluminum-based plating layer contains, for example, by mass%, Si: 0.05 to 15.00%, Zn: 0 to 30.00%, Mg: 0 to 5.00%, Fe: 0 to 10.00%, and Ca: 0 to 3.00%, with the balance being Al and 1% or less of impurities. In this chemical composition, the Mg content is preferably 3.00% or less, and the Si content is preferably 5.00% or more.

The aluminum-based plating layer having the above basic chemical composition may further optionally contain, by mass%, one or more of the following: Sb: 0-0.50%, Pb: 0-0.50%, Cu: 0-1.00%, Sn: 0-1.00%, Ti: 0-1.00%, Sr: 0-0.50%, Cr: 0-1.00%, Ni: 0-1.00%, and Mn: 0-1.00%. The total content of these optional added elements is preferably 5.00% or less, and more preferably 2.00% or less.

Of the above optional added elements, the respective contents of Sb and Pb are preferably 0.20% or less. The respective contents of Cu, Sn, and Ti are preferably 0.80% or less, more preferably 0.50% or less. The respective contents of Sb, Pb, Cu, Sn, and Ti may be 0.01% or more. The respective contents of Sr are preferably 0.30% or less, more preferably 0.10% or less. The respective contents of Sr may be 0.01% or more. The respective contents of Cr, Ni, and Mn are preferably 0.50% or less, more preferably 0.10% or less. The respective contents of Cr, Ni, and Mn are preferably 0.01% or more.

The chemical composition (average composition) of the plating layer can be obtained, for example, by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel base material, and measuring the resulting solution using ICP (inductively coupled plasma) optical emission spectroscopy.

(Pressing process)
3 and 4 are schematic diagrams for explaining the pressing step S2. In the pressing step S2, the overlapped steel plates 10, 20 are pressed in the plate thickness direction to be in close contact with each other. Pressing of the plate set 30 can be performed using, for example, a known press machine 40.

With reference to Figure 3, when performing the pressing step S2, first, the steel plates 10, 20 are overlapped. For example, the steel plates 10, 20 are overlapped so that the entire surface 21 of the steel plate 20 is in contact with the surface 11 of the steel plate 10. In the following description, the steel plates 10, 20 in the overlapped state may be referred to as a plate set 30.

The steel sheets 10 and 20 are overlapped such that a plating layer is disposed between the steel sheets 10 and 20. When the steel sheet 10 has a plating layer, the surface 11 of the steel sheet 10 facing the steel sheet 20 is preferably covered with the plating layer. On the other hand, the surface 12 of the steel sheet 10 disposed on the side opposite the steel sheet 20 may or may not be covered with a plating layer.

When the steel sheet 20 has a plating layer in addition to the steel sheet 10, it is preferable that the steel sheets 10 and 20 are overlapped so that the plating layers face each other. That is, when the surface 11 of the steel sheet 10 is coated with a plating layer, it is preferable that the surface 21 of the steel sheet 20 facing the steel sheet 10 is also coated with a plating layer. In this case, the plating layer of the steel sheet 20 may be the same type of plating layer as the plating layer of the steel sheet 10. The same type of plating layer means that the main metal in one plating layer is the same as the main metal in the other plating layer. For example, when the plating layer of the steel sheet 10 is a zinc-based plating layer, the plating layer of the steel sheet 20 can also be a zinc-based plating layer. Also, for example, when the plating layer of the steel sheet 10 is an aluminum-based plating layer, the plating layer of the steel sheet 20 can also be an aluminum-based plating layer. However, the plating layer of the steel sheet 20 may be a plating layer of a different type from the plating layer of the steel sheet 10.

When steel sheet 10 has a plating layer, steel sheet 20 does not necessarily have to have a plating layer. In other words, when surface 11 of steel sheet 10 is coated with a plating layer, surface 21 of steel sheet 20 does not necessarily have to be coated with a plating layer. On the other hand, when surface 11 of steel sheet 10 is not coated with a plating layer, surface 21 of steel sheet 20 is coated with a plating layer. Surface 22 of steel sheet 20 arranged on the side opposite steel sheet 10 may or may not be coated with a plating layer.

The total basis weight of the plating layer between the steel sheet 10 and the steel sheet 20 can be determined appropriately, and may be, for example, 30 g/ ^m2 or more. When a plating layer is formed on both the surface 11 of the steel sheet 10 and the surface 21 of the steel sheet 20, the sum of the basis weight of the plating layer on the surface 11 of the steel sheet 10 and the basis weight of the plating layer on the surface 21 of the steel sheet 20 can be 30 g/ ^m2 or more. When a plating layer is formed on only one of the surface 11 of the steel sheet 10 and the surface 21 of the steel sheet 20, the basis weight of the plating layer on the surface 11 or the surface 21 can be ³⁰ g/m2 or more.

As shown in Figure 3, a mold 41 including an upper mold 411 and a lower mold 412 is installed in the press machine 40 for performing the pressurizing process S2. The upper mold 411 is fixed to a slide 42 configured to be able to rise and fall relative to a main body frame (not shown) of the press machine 40. The lower mold 412 is disposed below the upper mold 411. Each of the upper mold 411 and the lower mold 412 has, for example, a solid rectangular parallelepiped shape. The plate assembly 30 is placed on the lower mold 412 when performing the pressurizing process S2.

When performing the pressing step S2, the steel plates 10, 20 in a state of being stacked to form the plate set 30 may be transported to the press machine 40, or the steel plates 10, 20 may be transported individually to the press machine 40. When the steel plates 10, 20 are transported individually to the press machine 40, the steel plates 10, 20 are stacked on the lower die 412 to form the plate set 30. In the example shown in FIG. 3, the steel plate 10 is placed on the lower die 412 side, and the steel plate 20 is stacked on the steel plate 10. However, the steel plate 20 may be placed on the lower die 412 side, and the steel plate 10 may be stacked on the steel plate 20.

As shown in Figure 4, by lowering the slide 42 of the press 40, the upper die 411 descends and contacts the plate set 30 on the lower die 412. As the upper die 411 descends, the plate set 30 is pressed in the thickness direction of the steel sheets 10, 20 between the upper die 411 and the lower die 412. It is preferable that the plate set 30 is pressed in its entirety by the upper die 411 and the lower die 412. At this time, the average pressure applied to the plate set 30 is, for example, 20 MPa or more and 200 MPa or less. The time for which the plate set 30 is pressed can be, for example, 1 second or more and 60 seconds or less.

Such a pressing step S2 can be carried out in the cold. That is, the steel sheets 10, 20 or the sheet assembly 30 do not have to be actively heated to a high temperature before or during the pressing step S2. In the pressing step S2, for example, pressure can be applied to the sheet assembly 30 at room temperature from both sides in the sheet thickness direction. By pressing the sheet assembly 30 in the sheet thickness direction, the minute gaps between the steel sheets 10, 20 are reduced or eliminated, and the steel sheets 10, 20 are tightly attached to each other.

After the plate set 30 is pressurized to an extent that the steel sheets 10, 20 can be brought into close contact, this pressurization is terminated. That is, the upper die 411 is raised together with the slide 42 to release the pressure on the plate set 30. The plate set 30, in which the steel sheets 10, 20 are in close contact, is removed from between the upper die 411 and the lower die 412 and subjected to the subsequent heating step S3. When the plate set 30 is transported to perform the heating step S3 after the pressurization step S2, in order to prevent the steel sheets 10, 20 from separating due to vibration, tilt, etc., the location where the heating step S3 is performed is preferably near the location where the pressurization step S2 is performed.

Here, the adhesion of the steel sheets 10, 20 includes the adhesion of the steel sheets 10, 20 and the pressure bonding of the steel sheets 10, 20. The steel sheets 10, 20 are adhered to each other by overlapping the steel sheets 10, 20 and applying pressure to remove air between the steel sheets 10, 20. In the manufacturing method according to the present embodiment, the adhered steel sheets 10, 20 may be subjected to hot stamping. However, when the steel sheets 10, 20 are adhered to each other, if at least one of the steel sheets 10, 20 is deformed, air may enter between the steel sheets 10, 20, causing the steel sheets 10, 20 to peel off. In addition, if the shape and surface roughness of the steel sheets 10, 20 are not properly managed, the steel sheets 10, 20 may not adhere well. Therefore, in the pressure application step S2, it is preferable that the steel sheets 10, 20 are pressure bonded to each other. The pressure bonding of the steel sheets 10, 20 means that the steel sheets 10, 20 are fixed as a result of being subjected to pressure. The plating layer does not melt or resolidify simply by being pressure bonded.

(Heating process)
The heating step S3 is a step of heating the plate set 30 after the pressing of the plate set 30 is completed. In the heating step S3, the plate set 30 can be heated using a heating device (not shown). The heating device is, for example, a walking beam type or batch type heating furnace. The heating method of the heating furnace may be a combustion type or an electric type. However, the heating device used in the heating step S3 is not limited to a heating furnace. Instead of a heating furnace, an electric heating device, a high-frequency heating device, or the like may be used.

When the plate set 30 is heated by the heating furnace, the plate set 30 is carried into the heating furnace after the pressing step S2 is completed and the pressing is released. The steel sheets 10 and 20 that are in close contact with each other by pressing are heated by the heating furnace to a temperature equal to or higher than the melting point of the plating layer between the steel sheets 10 and 20. The plate set 30 may be heated by the heating furnace to a temperature suitable for hot stamping. The plate set 30 is heated to, for example, 750°C or higher. The heating temperature of the plate set 30 is preferably equal to or higher than the A _c3 point of the steel sheets 10 and 20. The heating temperature of the plate set 30 is, for example, equal to or lower than the A _c3 point of the steel sheets 10 and 20 + 200°C. The holding time of the plate set 30 in the heating furnace can be 60 seconds or more and 360 seconds or less. It is preferable that such heating of the plate set 30 is performed in a reducing atmosphere. That is, when a heating furnace is used in the heating step S3, it is preferable to heat the plate set 30 in a heating furnace in which a reducing atmosphere is created using, for example, hydrogen gas or the like.

The melting point of the plating layer is equal to or lower than the heating temperature of the plate assembly 30. The plating layer may have a melting point that is, for example, 50°C or more lower than the heating temperature of the plate assembly 30. Therefore, the plating layer between the steel sheets 10 and 20 melts due to the heating in the heating step S3.

After being heated to a predetermined heating temperature and holding time, the plate assembly 30 is removed from the heating furnace. The plate assembly 30 is then subjected to the forming process S4.

(Molding process)
5 and 6 are schematic diagrams for explaining the forming step S4. In the forming step S4, hot stamping is performed on the sheet set 30 heated to a temperature equal to or higher than the melting point of the plating layer in the heating step S3. The hot stamping can be performed using, for example, a known press machine 50. The heated sheet set 30 is transported from the heating furnace to the press machine 50 by, for example, a conveyor or the like.

A mold 51 is installed in the press machine 50. The mold 51 includes an upper mold 511 and a lower mold 512. The upper mold 511 and the lower mold 512 have molding surfaces having the shape of the desired press molded product. In the example of this embodiment, the upper mold 511 is a die and has a concave molding surface. In the example of this embodiment, the lower mold 512 is a punch that pairs with the die and has a convex molding surface corresponding to the concave molding surface of the upper mold 511. The upper mold 511 is fixed to a slide 52 that is configured to be able to rise and fall relative to the main body frame (not shown) of the press machine 50. The lower mold 512 is arranged below the upper mold 511. As shown in the example in FIG. 5, holders 513 that are configured to be able to rise and fall may be arranged on both sides of the lower mold 512.

When performing the forming process S4, the plate set 30 is placed on the lower die 512. In the example shown in Figure 5, the plate set 30 is placed on the lower die 512 so that the steel plate 20 faces the lower die 512. As shown in Figure 6, by lowering the slide 52 of the press machine 50, the upper die 511 descends together with the slide 52 toward the plate set 30. When the upper die 511 reaches the bottom dead center, the plate set 30 is formed into the press-formed product 60 by the concave forming surface of the upper die 511 and the convex forming surface of the lower die 512.

The heated sheet assembly 30 is formed into a press-formed product 60 by the die 51, while being cooled (hardened) by contact with the die 51. The molten plating layer between the steel sheets 10 and 20 solidifies as the sheet assembly 30 is cooled by the die 51. This results in a surface bond between the steel sheets 10 and 20. After being sufficiently cooled by the die 51, the press-formed product 60 is removed from the press 50.

(Press molded products)
FIG. 7 is a perspective view of a press-formed product 60 manufactured by the manufacturing method according to the present embodiment. As shown in FIG. 7, the steel plate 10 constitutes the main body of the press-formed product 60. The steel plate 10 is formed into a generally hat-shape when viewed from the longitudinal direction of the press-formed product 60. On the other hand, the steel plate 20 constitutes a reinforcing member that partially reinforces the main body of the press-formed product 60. The steel plate 20 is formed into a generally U-shape when viewed from the longitudinal direction of the press-formed product 60. The steel plate 20 is disposed inside the hat-shaped steel plate 10. The steel plate 20 is joined to the steel plate 10 by adhesion due to resolidification of the plating layer. The press-formed product 60 is partially thickened by the steel plate 20.

[effect]
In the manufacturing method according to the present embodiment, in the pressurizing step S2, the sheet pair 30, which is the material of the press-formed product 60, is pre-pressurized, and the steel sheets 10 and 20 included in the sheet pair 30 are brought into close contact with each other. Therefore, it is possible to reduce or substantially eliminate the microscopic gap between the steel sheets 10 and 20. As a result, when the sheet pair 30 is heated in the subsequent heating step S3 and formed and cooled in the subsequent forming step S4, the adhesion between the steel sheets 10 and 20 due to resolidification of the plating layer is less likely to be hindered. Therefore, the steel sheets 10 and 20 can be well joined together over a wide area by utilizing the adhesion due to resolidification of the plating layer. In addition, by utilizing the adhesion due to resolidification of the plating layer, the steel sheets 10 and 20 can be easily joined together without performing a joining operation such as brazing.

For example, when overlapping steel plates 10 and 20 are welded, the periphery of the welded portion is deformed, so that a gap is likely to occur between the steel plates 10 and 20 around the welded portion, and the steel plates 10 and 20 do not adhere to each other. However, in this embodiment, the steel plates 10 and 20 are adhered (adsorbed or pressed) to each other by applying pressure in the plate thickness direction so that air does not enter between the steel plates 10 and 20, and the steel plates 10 and 20 are heated while maintaining the adhered state to melt the plating layer between the steel plates 10 and 20. Therefore, it is possible to prevent the air between the steel plates 10 and 20 from expanding at high temperature and pushing aside the plating layer between the steel plates 10 and 20. Therefore, the plating layer between the steel plates 10 and 20 is easily resolidified and fixed to the steel plates 10 and 20, and the steel plates 10 and 20 can be joined well.

In the manufacturing method according to the present embodiment, the plate assembly 30 is pressurized so as to reduce or eliminate the microscopic gap between the steel sheets 10 and 20, thereby effectively resolidifying and adhering the plating layer to the steel sheets 10 and 20. Therefore, the steel sheets 10 and 20 can be well surface-joined, and the integration effect between the steel sheets 10 and 20 in the press-formed product 60 can be improved. This allows the steel sheets 10 and 20 to behave substantially as a single sheet material, and the rigidity of the overlapping surfaces of the steel sheets 10 and 20 in the press-formed product 60 can be increased. In addition, the effective width of the press-formed product 60 can be expanded. The effective width is, for example, the width of a portion that can receive a load and undergo compressive deformation when a load is input in the longitudinal direction to the press-formed product 60, and is usually expressed as a ratio to the width of any of the surfaces included in the press-formed product 60 (for example, the width of the top plate or the width (height) of one vertical wall).

In the manufacturing method according to this embodiment, the heating step S3 is carried out after the pressure application step S2 is completed. Therefore, with the pressure applied by the die 41 released, only the plate set 30 can be carried into the heating device (heating furnace), and only the plate set 30 can be carried out from the heating device. In other words, there is no need to move the die 41, etc. together with the plate set 30. This simplifies the manufacturing equipment for the press-molded product 60 and the work involved in manufacturing the press-molded product 60.

In the manufacturing method according to this embodiment, the steel plates 10 and 20 are pressurized prior to the heating step S3, so that the steel plates 10 and 20 are in close contact with each other. The steel plates 10 and 20 may be adhered to each other by pressure, but it is preferable to adhere them to each other by pressure. By adhering the steel plates 10 and 20 to each other by pressure, the steel plates 10 and 20 are less likely to peel off compared to adhesion.

In the manufacturing method according to this embodiment, in the heating step S3, the plate assembly 30 can be heated in a reducing atmosphere. This makes it difficult for oxides to form on the surface of the plating layer when the plate assembly 30 is heated. This makes it even less likely that adhesion caused by resolidification of the plating layer between the steel sheets 10 and 20 will be hindered. This allows the steel sheets 10 and 20 to be joined together better.

The above describes an embodiment of the present disclosure, but the present disclosure is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the present disclosure.

For example, in the above embodiment, the heating step S3 is carried out immediately after the pressurizing step S2. However, a step of performing simple joining of the steel plates 10, 20 can also be carried out between the pressurizing step S2 and the heating step S3. That is, after the pressurizing step S2 is completed and before the heating step S3 is started, the steel plates 10, 20 can be joined, for example, by spot welding. The simple joining step of the steel plates 10, 20 can also be carried out before the pressurizing step S2.

However, as described in the above embodiment, the steel sheets 10, 20 are surface-joined by adhesion caused by resolidification of the plating layer formed through the heating step S3 and the forming step S4. Therefore, when a simple joining process is performed between the pressure step S2 and the heating step S3, or before the pressure step S2, the steel sheets 10, 20 only need to be joined over a small area so as not to impose a large burden on the joining work.

In the above embodiment, in the pressing step S2, a press machine 40 is used to pressurize the plate assembly 30. However, it is not necessary to use the press machine 40 in the pressing step S2. As long as it is possible to pressurize the plate assembly 30, other machines or jigs, such as a rolling machine, can be used instead of the press machine 40.

In the above embodiment, in the press-formed product 60, the steel plate 20, which is a reinforcing member, is arranged inside the steel plate 10, which is the main body. However, conversely, the steel plate 20 may be arranged outside the steel plate 10. The position and range of the steel plate 20 relative to the steel plate 10 are not limited to those in the above embodiment and can be changed as appropriate. Furthermore, the overall shape of the press-formed product 60 is not limited to the example of the above embodiment and can be changed as appropriate.

In the above embodiment, the steel plate 20, which is the reinforcing member, is overlapped on the steel plate 10, which is the main body, so that the entire surface 21 of the steel plate 20 contacts the surface 11 of the steel plate 10. However, the overlapping manner of the steel plates 10 and 20 is not limited to this. For example, the steel plate 20 can be partially overlapped on the steel plate 10.

In the above embodiment, the steel plate 10 constituting the main body of the press-formed product 60 may be a steel plate having a uniform thickness and material throughout. The steel plate 10 may also be a steel plate having at least one of a thickness and material that varies from one portion to another. That is, the steel plate 10 may be a tailor rolled blank (TRB) in which the thickness is varied from portion to portion by rolling, or a tailor welded blank (TWB) made by butt welding a plurality of steel plates. When the steel plate 10 is a TWB, the steel plate 20 may be arranged across the weld line of the TWB.

In the above embodiment, the plate group 30, which is the material of the press-molded product 60, is composed of two steel plates 10 and 20. However, the plate group 30 may include three or more steel plates. For example, in the plate group 30, three or more steel plates may be stacked in order, or multiple steel plates may be arranged on one steel plate with their positions shifted. Even in such a case, if a plating layer is arranged between adjacent steel plates in the plate thickness direction and the pressure step S2 is performed before the heating step S3 to bring the steel plates into close contact with each other, the steel plates can be surface-joined by utilizing the adhesion caused by the resolidification of the plating layer.

In the above embodiment, the pressurized and tightly adhered steel sheets 10, 20 are formed into a press-formed product 60 by hot pressing (hot stamping). However, the method of forming the steel sheets 10, 20 is not limited to this. The steel sheets 10, 20 may be heated to or above the melting point of the plating layer and then cooled while still in a tightly adhered state. The steel sheets 10, 20 are joined by fixation due to resolidification of the plating layer, as in the above embodiment. The joined steel sheets 10, 20 may be subjected to cold press forming.

The present disclosure will be described in more detail below with reference to examples. However, the present disclosure is not limited to the following examples.

(Example)
In order to confirm the effect of the present disclosure, an experiment was carried out to manufacture a press-formed product in the same manner as the method according to the above embodiment. In this experiment, before hot stamping, two steel sheets having aluminum-based plating layers (Al plating) on both sides were stacked to form a sheet assembly, and the sheet assembly was pressed in the sheet thickness direction by a known press machine to adhere (pressure-bond) (pressure step). The weight of the Al plating on each steel sheet was 80 g/ ^m2 . The sheet assembly was pressed in the cold (room temperature) with a load (pressure) of about 40 tons (about 80 MPa) and a pressing time of 1 second.

Next, after the pressing, the plate assembly was heated to 950°C in a heating furnace and held there for 240 seconds (heating process). The heated plate assembly was then molded into a press-molded product using a specified die (molding process).

Comparative Example
For comparison, an experiment was carried out to manufacture a press-molded product under the same conditions as in the above example, except that the pressing step was not carried out before the hot stamping.

(evaluation)
In the examples where the pressurizing step was performed, the two steel plates were firmly joined together to such an extent that they could not be easily separated by human power. In the examples, the two steel plates were finally separated by using a tool. On the other hand, in the comparative examples where the pressurizing step was not performed, the two steel plates were easily separated by human power alone without using a tool.

Figure 8 is a photograph showing the surface (overlapping surface) of the peeled steel plate for each of the examples and the comparative example. Figure 9 is an observation image (scanning electron microscope image) of the surface of the peeled steel plate.

As shown in Figure 8, in the examples where the pressing process was performed, shiny areas were generated on the surface of the steel sheet. The shiny areas were present and spread across the surface of the steel sheet. As shown in Figure 9, these shiny areas were brittle fracture surfaces that broke and became flat when the two steel sheets were peeled away, and were areas where the plating layer had melted and resolidified after being pressed together and had become fixed.

On the other hand, in the comparative example where the pressing process was not performed, there were no shiny areas on the surface of the steel sheet, only gray areas, as shown in Figure 8. These gray areas are uneven surfaces that arise due to Fe-Al alloying at the boundary between the steel sheet and the plating layer, as shown in Figure 9, and are areas where the plating did not adhere as a result of melting and resolidifying without being pressed.

These experiments confirmed that by carrying out a pressure process before hot stamping, the steel sheets adhere to each other and the overlapping steel sheets are fixed in a planar manner due to resolidification of the plating layer. It was also confirmed that this resulted in good bonding of the steel sheets.

10, 20: Steel plate 11, 12, 21, 22: Surface 51: Mold 60: Press-molded product

Claims (5)

overlapping a first steel sheet having a plating layer on a surface thereof with a second steel sheet such that the plating layer is disposed between the first steel sheet and the second steel sheet;
Pressing the entire first steel plate and the entire second steel plate in the plate thickness direction to adhere them to each other; and
Heating the first steel sheet and the second steel sheet in close contact with each other to a melting point of the plating layer or higher;
A method for manufacturing a press-molded product comprising the steps of: The method for manufacturing a press-molded product according to claim 1, wherein the adhesion is pressure bonding. The method for manufacturing a press-molded product according to claim 1 or 2, wherein the plating layer is a zinc-based plating layer or an aluminum-based plating layer. The second steel plate has a plating layer on a surface thereof,
The method for manufacturing a press-formed product according to any one of claims 1 to 3, wherein the first steel sheet and the second steel sheet are overlapped so that the plating layers face each other. The method for manufacturing a press-formed product according to claim 4 , wherein the plating layer of the second steel sheet is the same type of plating layer as the plating layer of the first steel sheet.

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