JP2017140636A - Hot-press molding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-press molding method for improving moldability of a joining plate W.SOLUTION: A hot-press molding method of the present invention comprises a heating process of heating a joining plate (W) composed of two or more of metal plates (W1 and W2) joined by a plurality of separately arranged welding parts, an arrangement process of arranging the joining plate between a die (1) having a molding recessed part (11) and a punch (2) having a molding projection part (21) corresponding to the molding recessed part and a molding process of forming a molding part (F) by pressing the joining plate by the die and the punch. The hot-press molding method of the present invention also comprises a cooling process of cooling a specific area (Wc) of the joining plate of including at least a welding part (Wn) in the specific timing being at least one timing before finishing the molding process after finishing the heating process. Since hot-press molding is executed in a state of cooling and hardening at least the welding part, even when executing deep drawing molding by the joining plate, separation and a crack in the welding part can be suppressed.SELECTED DRAWING: Figure 1A

Description

  The present invention relates to a hot press forming method for a bonded plate in which a metal plate is partially welded.

  Press-molded products (referred to as “press-formed products”) are widely used in various fields such as automobiles, home appliances, furniture and general merchandise. In a press-molded product, a metal plate sandwiched between a peripheral edge portion of a die and a blank holder (also referred to as “wrinkle presser”) is usually stretched or stretched between a molding concave portion of a die and a molding convex portion of a punch. The metal plate is obtained by plastic deformation into a desired shape.

  According to such press molding, members having various shapes can be efficiently mass-produced. However, when the amount of plastic deformation increases as in a deep-drawn molded product, cracks and breakage are likely to occur in a specific local area, and the moldability is lowered. In particular, when a bonded plate in which a plurality of metal plates are bonded in advance is press-molded, peeling or cracking is likely to occur at the bonded portion. For this reason, various proposals for improving the moldability have been made, and for example, there are descriptions related to the following patent documents.

JP 2014-208374 A JP 2014-193712 A

The Japan Welding Society Proceedings Vol.33 No.2, p144-152 (2015)

  Patent Document 1 discloses hot press forming that can significantly improve formability by cooling a partial region of a heated steel sheet (region that becomes a side wall portion of a press-formed product) at a specific time before the end of forming. Proposed methods. However, Patent Document 1 does not specifically describe the case where the joining plate is so-called hot stamped. Hot stamping is a kind of hot press forming method in which heat treatment (particularly quenching by martensitic transformation) is also performed when a steel plate heated to an austenite temperature range is press formed with a mold.

  Patent Document 2 proposes hot stamping a blank material obtained by spot welding a plurality of steel plates having different carbon contents in one step. Thus, it is described that press molding can be performed in one step, and the degree of freedom of spot welding is increased. However, in Patent Document 2, it is assumed that the spot welded portion is in a flat region of each member ([0014]), and it is substantially assumed that the spot welded portion is bent or stretched during molding. Not.

  Non-Patent Document 1 discloses that a spot welded joint obtained by hot stamping a pre-spot welded steel sheet is more static strength characteristics (shear strength, tensile strength, etc.) than a spot welded joint obtained by spot welding a hot stamped steel sheet later. ) Is shown to be superior. However, nothing is mentioned about the formability when hot stamping the bonded steel sheets.

  This invention is made | formed in view of such a situation, and it aims at providing the hot press molding method which can obtain high moldability using the joining board which welded the several metal plate. .

  As a result of diligent research to solve this problem, the present inventor has conceived that the joining plate is formed while cooling the periphery of the welded portion, and thereby confirms that the formability of the joining plate can be greatly improved. By developing this result, the present invention described below has been completed.

<Hot press molding method>
(1) The hot press molding method of the present invention includes a heating step of heating a joining plate composed of two or more metal plates joined by a plurality of welded portions arranged apart from each other, and a die having a molding recess. And a step of arranging the bonding plate between the punch having a molding convex portion corresponding to the molding concave portion, and pressing the bonding plate with the die and the punch to form the molding concave portion and the molding convex portion. A hot press forming method comprising: a forming step for forming a corresponding forming portion, and at least the weld portion at a specific time after the end of the heating step and before the end of the forming step. A cooling step for cooling a specific region of the joining plate is included.

(2) In the hot press molding method of the present invention (referred to as “molding method” as appropriate), at least one period (specific period) after the end of the heating process and before the end of the molding process (also simply referred to as “in the molding process”). ) At least the welded portion is cooled and press-molded. As a result, even when a deep drawn product or curved product is press-molded using a joining plate (particularly when the welded portion is present in the side wall surface or curved surface to be extended). Favorable moldability can be ensured while preventing the occurrence of peeling and cracking at the welded portion.

  Moreover, when press-molding a joining plate in which a plurality of metal plates are previously welded as in the present invention (this case is appropriately referred to as “pre-joining / post-molding”), a plurality of assembled press-formed products are later welded. In this case, the number of press processes is reduced, the degree of freedom of placement of the welded portion, or a press-formed product (also simply referred to as “molded product”). It is easy to improve the degree of freedom of shape.

  Furthermore, in the case of pre-molding and post-joining, it is necessary to determine the shape of each molded product assembled in anticipation of dimensional errors and the like, so the number of molded products (number of metal plates) before joining tends to increase. On the other hand, in the case of pre-joining / post-molding, the number of parts (number of metal plates) to be used can be reduced more easily than in the case of pre-molding / post-joining because the molding is performed integrally with the joining plate.

  In addition, in the case of pre-joining and post-molding, gaps are less likely to occur between the joined metal plates than in the case of pre-molding and post-joining. It is also easy to reduce the thickness or the number of sheets used.

(3) The reason why the moldability can be improved without causing peeling or cracking in the welded portion by the molding method of the present invention is considered as follows. When the specific region including the welded portion is cooled during the molding process, the specific region is hardened and has higher strength than the peripheral region. When the molding step is performed in this state, the deformation force acting during molding is easily dispersed outside the specific region, and excessive shear stress or tensile stress is prevented from acting on the welded portion or its periphery. Thus, according to the molding method of the present invention, it is considered that high moldability can be obtained without causing peeling or cracking in the welded portion.

  In particular, in the molding method of the present invention, press molding is performed while applying tension to the joining plate so that wrinkles do not occur in the molded part. In the conventional press molding method, a large shear stress or tensile stress is applied to the welded part and its surroundings. This is effective when is likely to act. For example, it is a case where a welding part is arrange | positioned on the curved surface of a shaping | molding part, or the side wall surface of a deep drawing. More specifically, for example, the opening peripheral region (also referred to as a corner-curved region or the corner R region) of the die forming concave portion and the parietal peripheral region (corner-curved region, corner R portion) of the punch forming convex portion. It is also a case where there is a welded portion in a bridging region (border region) formed between the region and the shoulder region.

  Such a cross-linked region is a non-contact region that is not in direct or substantial contact (intimate contact) with either the die or the punch at least at the initial stage (at the start) of the molding process. The non-contact region is maintained in a high temperature softened state at least in the initial stage of the molding process, and usually has a low deformation resistance. However, even when there is a welded portion in such a non-contact region, high formability can be ensured by intentionally cooling the specific region including the welded portion during the molding process.

  In addition, even when the welded portion is in the region (contact region) where the welded portion is in contact with the opening peripheral area of the die forming concave portion or the top peripheral edge region of the punch forming convex portion, the region including the welded portion is formed. The moldability may be improved by actively cooling at the beginning of the process.

  In addition, it is preferable that a cooling process is a temperature control process which makes the specific area | region containing a welding part in a desired temperature range. By adjusting the specific region during the molding process to an appropriate temperature (within range), the deformability (deformation resistance) in the peripheral region of the specific region is adjusted without causing peeling or cracking at or around the welded portion. You can also. As a result, the deformation (particularly stretched) region (part) during molding is distributed over a wide range (flange, head, etc.) without being concentrated locally, and adjusted so that cracks, breaks, thinning, etc. do not occur. It is also possible to do.

《Hot press forming device》
The present invention can be grasped not only as a molding method but also as a hot press molding apparatus (referred to as a “molding apparatus” as appropriate) suitable for its implementation. That is, the present invention includes a die having a molding recess, a punch having a molding projection corresponding to the molding recess, a blank holder for inserting the punch, and driving the die or the punch to form the molding recess and the punch. A hot press molding apparatus comprising a driving means for bringing the molding convex portion close,
After completion of heating, at least a specific region including a welded portion of a joining plate made of two or more metal plates held by the die and the blank holder and joined by a plurality of welded portions disposed apart from each other. Thus, it can also be grasped as a hot press forming apparatus characterized by including a cooling means capable of cooling at a specific time which is at least one time before the end of forming.

<Press-formed product>
Further, the present invention can be grasped not only as the above-described hot press molding method and hot press molding apparatus, but also as a press molded product (referred to as “molded product” as appropriate) obtained by using them. For example, the molded product is joined at a plurality of welded portions spaced apart by a die having a molding recess, a punch having a molding projection corresponding to the molding recess, and a blank holder for interposing the punch. It is obtained by hot press-molding a bonded plate made of two or more metal plates.

<Others>
(1) “Hot” press molding in the present specification means press molding of a heated joining plate, and the heating temperature does not matter. Therefore, not only hot press molding in which the bonding plate before molding is heated to a temperature higher than the recrystallization temperature of the metal plate, but also warm press molding in which the bonding plate is heated to a temperature lower than the recrystallization temperature may be used.

However, it is preferable that heat treatment is performed in parallel with forming by press forming according to the present invention. Therefore, it is preferable that the joining plate before molding is heated to a temperature suitable for the heat treatment. For example, when the metal plate is a steel plate, the heating step is a step of heating the joining plate to a temperature higher than the austenite transformation temperature (Ac ₃ points) of the steel plate, and the forming step starts the martensitic transformation at the end of the forming step. It is preferable that it is a process which makes it the end temperature lower than temperature (Ms point). As a result, quenching by so-called hot stamping is possible, and a high-strength molded product in which at least a part of the structure becomes a martensite structure is obtained.

Incidentally, the Ac ₃ point and the Ms point of each of the plurality of steel plates to be joined may vary depending on the alloy element amount (particularly the C amount), but the variation range is small if the steel plate is used for press forming. If it dares to say, what is necessary is just to employ | adopt the lowest Ac ₃ point in the some steel plate joined as a minimum temperature (Ac ₃ point) at the time of a heating process as used in this specification. Conversely, the highest Ms point among the plurality of steel plates to be joined may be adopted as the upper limit temperature (Ms point) during the forming step referred to in this specification.

  If the metal plate is other than a steel plate, it is preferable to heat the joining plate to a temperature suitable for solution treatment or the like in the heating step.

(2) The joining plate referred to in this specification is at least a welded portion, and it is sufficient that the plurality of metal plates are in a laminated state, and the plurality of metal plates are not necessarily laminated in the whole joining plate. Further, the number of metal plates may be different depending on the part (region). Such a thing is also referred to as a “joining plate” in the present specification.

(3) Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. A range such as “a to b” can be newly established with any numerical value included in various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value.

It is a schematic cross section which shows the mode before shape | molding a joining board with the hot press molding apparatus which is an example. It is a schematic cross section which shows the mode in the shaping | molding process. It is a figure which shows the analysis model before shaping | molding. It is a figure which shows the analysis model after shaping | molding. It is a figure which shows the temperature distribution of each sample (joining board) used for the analysis. It is a figure which shows the mode after shaping | molding of each analyzed sample.

  One or more components arbitrarily selected from the present specification may be added to the components of the present invention. Constituent elements related to the molding method are fixed (when it is impossible or impractical (impossible / unpractical circumstances), etc.) to directly identify the “object” by structure or characteristics As a product-by-process, it can also be a component related to “things” (press-molded products). Which embodiment is the best depends on the target, required performance, and the like.

《Joint plate》
(1) Metal plate The metal plate which concerns on this invention is an iron-type board (a steel plate is included), an aluminum-type board, a magnesium-type board, a titanium-type board etc., for example. The “˜system” here means the pure metal or the alloy thereof. The metal plates to be joined may be the same or different. In the case of the same kind of metal plates, their compositions may be the same or different. The metal plates to be joined may have the same form (shape, size, etc.) or different forms. By using a joining plate made of metal plates having different compositions or forms, it becomes easy to obtain a press-molded product suitable for the required specifications for each part.

  The steel plate as an example of the metal plate is not limited to a carbon steel plate (including an alloy steel plate) but may be a stainless steel plate or the like. In the case of a steel sheet containing carbon (C), considering the hardenability, toughness, etc., the entire steel sheet is usually 100% by mass (referred to simply as “%” as appropriate), and the C content is 0.15-0.8. %, And further 0.2 to 0.7%. By forming a joining plate by combining ordinary steel plates, high-tensile steel plates, ultra-high-strength steel plates and the like within such a range, press-formed products having different strengths and the like can be obtained for each part.

(2) Welding part A plurality of metal plates are joined by the welding part. The welds are distributed (discrete) between the metal plates to be joined. As heating means for forming the welded portion, gas (plasma, laser, electron beam) can be used in addition to electricity (joule heat, arc discharge, etc.). A typical example of the welded portion is a melted and solidified portion (so-called nugget) in lap resistance welding (resistance welding performed by applying pressure from both sides of the stacked metal plates) by spot welding, projection welding, seam welding, or the like.

"Mold"
The specific form of the metal mold | die which hot-press-molds the heated joining board is not ask | required. The mold according to the present invention only needs to include a die (also referred to as an upper mold or a lower mold) and a punch (also referred to as a lower mold or an upper mold). In addition, the blank holder (also referred to as a heel presser, including a die cushion as appropriate) according to the present invention is sufficient if it can hold the outer periphery (flange portion) of the molded portion when press-molding the joining plate. It is sufficient that at least one of the die and punch is driven.

<Molding method>
(1) Heating process The heating method of the joining plate may be in-furnace heating or high-frequency heating. The heating temperature varies depending on the type of metal plate, heat treatment, and the like. When the metal plate is a steel plate, for example, the heating temperature (initial temperature) is preferably 800 ° C. or higher, 840 ° C. or higher, and further 880 ° C. or higher.

(2) Arrangement Step When the heated joining plate comes into contact with the die or the punch, heat is removed from the contact portion, the temperature of the contact portion is lowered, and the moldability can be lowered. Therefore, the arranging step performed before the cooling step is preferably a non-contact holding step for holding the bonded plate after the heating step without substantially contacting (intimately contacting) any of the die, punch and blank holder.

(3) Cooling step (cooling means)
The cooling process may be performed at least at an appropriate time (specific time) after the heating process and before the molding process, and the timing, the cooling time, the cooling temperature, etc. are not limited. However, since the shape of the specific region to be cooled and the stress state acting on the welded portion can change during the molding process, it is preferable to perform the cooling process before or near the start of the molding process.

  The cooling step may be, for example, a refrigerant supply step of supplying a refrigerant to a specific region including at least a welded portion, or a cold body contact step of bringing a cold body (for example, a contact metal) into contact with the specific region. The refrigerant supply step can be performed, for example, by supplying (jetting or outflowing) refrigerant (gas or liquid) to a specific region from a refrigerant supply pipe (cooling means) disposed on the outer peripheral side of the punch. For example, the cold body used in the cold body contact step may be disposed on the outer peripheral side of the punch. In addition, the refrigerant supply pipe and the cold body are accommodated in a mold (for example, in a die or a blank holder) in a state in which they do not contact, interfere with, etc. the joining plate except during the cooling process (for example, during the molding process) It is preferable.

  The type, supply amount, direction, temperature, supply time, or the like of the refrigerant supplied from the refrigerant supply pipe, or the type (material) of the cold body, the heat capacity, the temperature before contact, the contact time, etc. are appropriately adjusted. As a result, at least the welded portion can be controlled within a desired temperature range. Such a refrigerant is economical if it is air, for example. Further, when the cooling means is configured by injecting a fluid such as air, the position, size (opening diameter), direction, refrigerant temperature, etc. of the injection hole (hole) are also adjusted as appropriate.

Incidentally, if the specific region is supercooled in the cooling step, the moldability may be reduced. Therefore, when the metal plate is a steel plate, it is preferable that the specific region is a step of setting a specific temperature higher than the martensitic transformation start temperature (Ms point). Specifically, when the metal plate is a steel plate, the cooling step is performed at least for the temperature of the specific region (particularly the welded portion) with respect to the initial heating temperature (Ac ₃ points or more, for example, 800 to 950 ° C.). 50 to 400 ° C., 100 to 350 ° C., and preferably 200 to 300 ° C.

  In the cooling process, other regions (regions that do not include the welded portion), for example, regions that are prone to cracking or thinning when stretched (plastically deformed) in a high-temperature softened state are also cooled together. Also good.

(4) Molding process From the molding start to the molding end, the flange area of the heated joining plate may be held between the blank holder and the die. It is preferable that the material easily flows from the flange region (the outer peripheral region of the peripheral edge of the opening of the molding recess) into the molding part because the moldability is further enhanced.

  The forming step is preferably a heat removal suppression forming step in which press forming is performed while suppressing heat removal from the flange region of the joining plate that can be a flange portion to the die or the blank holder. As a method for suppressing the temperature drop in the flange region, for example, the mold and the flange region of the joining plate may be brought into a non-contact state or a non-contact state in the initial stage of molding. Also, a heater or the like may be embedded in a mold region that can come into contact with the flange region to heat or keep the flange region of the joining plate. Furthermore, the temperature reduction of the flange region may be suppressed by increasing the molding speed and substantially shortening the contact time (and hence the heat removal time) between the flange region of the joining plate and the mold.

(5) Hot stamp When the metal plate is a steel plate, a press-molded product quenched by the hot stamp is obtained. The structure is not limited to a single martensite structure as a whole, and may be a mixed structure including a bainite structure, a ferrite structure, a cementite structure, and the like as appropriate. Furthermore, heat treatment (tempering etc.) may be separately performed on the press-formed product after hot stamping.

《Hot press forming device》
(1) An example of the molding apparatus according to the present invention is shown in FIGS. 1A and 1B (both are simply referred to as “FIG. 1”). A molding apparatus P1 shown in FIG. 1 includes a die 1 and a punch 2 constituting a molding die, a blank holder 31 disposed opposite to the die 1, and a die cushion that supports the blank holder 31 so as to be movable up and down. 8 and a base 9 that supports the die cushion 8. The punch 2 is fixed to the base 9.

  The die 1 has a two-stage hat-shaped (stepped truncated cone-shaped) concave portion 11 having an edge 11a having a curved corner (angle R) in the peripheral edge region of the opening. The punch 2 has a two-step hat-shaped molding convex portion 21 having a shoulder portion 21a having a curved corner (angle R) in the peripheral region of the crown. When the die 1 and the punch 2 are relatively moved in the vertical direction, the molding concave portion 11 and the molding convex portion 21 are loosely fitted.

  Further, the molding apparatus P1 includes a holding pin 4 and an air pipe 5. A plurality of holding pins 4 are evenly arranged on the upper end flat surface 312 side of the blank holder 31, and can be projected and retracted with respect to the upper end flat surface 312. Specifically, the holding pin 4 is urged by a spring 41 (elastic body) housed in the blank holder 31, and the holding pin 4 protrudes from the upper end plane 312 when the die 1 is not lowered. It is in a state. With the protruding holding pins 4, the heated joining plate W is held without contacting the blank holder 3 (arrangement process, non-contact holding process), and the initial heating state is substantially maintained.

  When the die 1 is lowered, the holding pin 4 is pressed through the joining plate W, and the holding pin 4 is buried in the blank holder 3. Thus, the joining plate W is sandwiched between the die 1 and the blank holder 3 (see FIG. 1B).

  The air pipe 5 (refrigerant supply pipe, cooling means) is accommodated along an annular step 311 formed on the upper inner peripheral side of the blank holder 31. A plurality of small holes 51 are arranged at regular intervals on the obliquely inner upper surface of the air pipe 5. Air (refrigerant) can be pumped into the air pipe 5 from an air compressor (not shown). The timing when sending air to the air pipe 5, the amount of air, and the like are controlled by a control valve (not shown). Note that the air pipe 5 that is accommodated between the outer peripheral side of the molding convex portion 21 and the step portion 311 does not interfere with the joining plate W or the like even during molding in which the molding convex portion 21 enters the molding concave portion 11.

(2) The heated joining plate W is hot press-molded by the die 1 and the punch 2 as follows (see FIG. 1B). First, the die 1 is driven and lowered by a hydraulic press (driving means) from above. As the die 1 descends, the joining plate W is sandwiched between the lower end plane 12 of the die 1 and the upper end plane 332 of the blank holder 31. In addition, the clamping part (Wa) of the joining board W becomes the flange part Fa of the press-formed product F finally obtained.

  When the die 1 is further lowered while the joining plate W is sandwiched, the blank holder 31 is pushed down by the die 1 and descends along the die cushion 8. In conjunction with the lowering of the die cushion 8, the molding convex portion 21 of the punch 2 starts to move relatively toward the molding concave portion 11 of the die 1. The shoulder portion 21a of the molding convex portion 21 connected to the top surface 21b is in contact with the joining plate W in the vicinity where the top surface 21b of the molding convex portion 21 of the punch 2 is substantially flush with the upper end flat surface 332 of the die cushion 8.

  As the molding convex portion 21 relatively enters the molding concave portion 11, the joining plate W also contacts the edge portion 11a of the molding concave portion 11, and is between the edge portion 11a and the shoulder portion 21a (inter-edge region). A cross-linked region for cross-linking is formed. The cross-linked region finally becomes the side portion (vertical wall portion) of the press-formed product F. The cross-linked region is a non-contact region that does not contact the die 1 and the punch 2 at a certain time (specific time) during molding.

(3) The joining plate W is formed by joining a metal plate W1 and a metal plate W2 at a plurality of welded portions Wn. The welded portion Wn is, for example, a plurality of spot-like welded portions (nuggets) formed by spot welding. After the joining plate W is heated so as to have a desired temperature distribution (heating process), it is disposed on the holding pins 4 (arranging process). Thereafter, air is injected from the air pipe 5 toward the specific region Wc including the heated welded portion Wn (cooling step, refrigerant supply step). As a result, at least the welded portion Wn is cooled before molding and is cured at a lower temperature than the surroundings, that is, a state having a large deformation resistance. When this joining plate W is press-molded (molding process), it is avoided that plastic deformation concentrates on the specific region Wc (particularly the welded portion Wn), and the welded portion Wn does not cause peeling or cracking. The top region Wb is easily stretched. Thus, even with the joining plate W, it is possible to ensure high formability. The molding method or molding apparatus of the present invention is particularly effective when the specific area Wc is in the non-contact area as described above.

  In addition, the intermediate region (the top region Wb) of the joining plate W that comes into contact with the shoulder 21a of the molding convex portion 21 tends to be in a non-contact state (floating state) with the top surface 21b of the punch 2. The specific region Wc including the welded portion Wn may exist in such a region.

<Press-formed product>
The press-formed product of the present invention may be appropriately subjected to heat treatment such as annealing, normalizing, aging, tempering, carburizing and nitriding, and surface treatment such as plating according to the specifications. Moreover, the form and use of a press molded product are not ask | required. The press-formed product of the present invention is suitable as a vehicle part such as a vehicle body (A pillar, B pillar, outer panel, inner panel, etc.), bumper, oil pan, wheel house, and the like.

  The effect of cooling the welded portion of the joining plate on hot press forming will be clarified by analysis (simulation), and the present invention will be described more specifically.

《Joint plate》
A simulation was performed on a case where the hot-press forming was performed on the joining plate W in which two types of steel plates W1 and W2 (metal plates) having different compositions were spot-welded. Specifically, a steel plate (equivalent to JIS SCr420) having a large size (280 × 50 × 2 mm in thickness) was assumed as the steel plate W1. In addition, a steel plate (corresponding to JIS SCr420) having a small size (130 × 50 × 2 mm in thickness) was assumed as the steel plate W2.

  Spot welding assumed a state in which the vicinity of the end of the steel plate W2 was joined to the steel plate W1 by a completely welded nugget (welded portion Wn). Spot welding was performed at intervals of 55 mm × 3 locations. The size of the welded portion Wn (nugget) was set to φ5.66 mm.

<Molding mold / molding part>
The joining plate W is formed in a two-stage hat shape (stepped truncated cone) using a die 1 and a punch 2 (see FIG. 1) as shown in FIG. 2A or 2B (both are simply referred to as “FIG. 2”). A case of deep drawing in a cylindrical shape or a stepped bottomed cylindrical shape) was assumed. The first stage (opening edge side) has a substantially cylindrical shape of 40 mm × molding height 50 mm, and the second stage (bottom side opening edge side) has a truncated cone (bottomed cylindrical shape) of 62.65 mm × molding height 80 mm. . The welded portion Wn was assumed to be spaced apart and disposed on the lower peripheral wall surface (curved surface) constituting the second-stage truncated cone.

"Temperature distribution"
Samples with various temperature distributions of the bonding plate W were set. The temperature distribution set for each sample is shown in FIG. Note that FIG. 3 shows only a portion including one welded portion Wn cut out from the joining plate W. The punch shoulder Ws is a region in the vicinity of which the shoulder 21a of the punch 2 abuts when the joining plate W is formed, and the side wall Wd is the welded portion Wn and the punch shoulder when the joining plate W is formed. This is an intermediate region of the portion Ws. As shown in FIG. 3 and Table 1, for example, the sample Sa is obtained by cooling (−250 ° C.) the peripheral region (specific region Wc) including the welded portion Wn and the region serving as the punch shoulder Ws, and the sample Sd Is obtained by cooling (−250 ° C.) the region to be the specific region Wc and the side wall Wd.

<Evaluation>
About each sample mentioned above, the presence or absence of the crack of the peripheral region Wc including the welding part Wn after hot press molding was evaluated using the analysis software (LS-DYNA manufactured by LSTC). The presence or absence of the crack of Wc was judged by the case where the fractured element penetrated in the thickness direction. The results are also shown in Table 1.

  As is clear from Table 1, it was found that cracking and peeling of the welded portion Wn can be prevented by cooling and press molding only the welded portion Wn (near). Moreover, even when deep drawing into a two-stage hat shape by the partial cooling, the plate thickness is reduced at the side wall portion Wd and the punch shoulder portions Ws (Ws1, Ws2) while ensuring the joining length of the welded portion Wn. It was also confirmed that it can be suppressed.

P1 Hot press forming device W Joint plate W1, W2 Metal plate F Press-formed product 1 Die 2 Punch 31 Blank holder 4 Holding pin (arrangement means)
5 Air piping (refrigerant supply pipe, cooling means)

Claims (7)

A heating step of heating a joining plate composed of two or more metal plates joined at a plurality of welds disposed apart from each other;
An arrangement step of disposing the joining plate between a die having a molding recess and a punch having a molding projection corresponding to the molding recess;
A molding step for forming the molding concave portion and the molding portion corresponding to the molding convex portion by pressing the joining plate with the die and the punch; and
A hot press forming method comprising:
Hot press molding characterized by comprising a cooling step of cooling at least a specific region of the joining plate including the welded portion at a specific time that is at least one time after the heating step and before the forming step. Method.   The hot press forming method according to claim 1, wherein the cooling step is a temperature control step in which the specific region is within a desired temperature range.   The hot press molding method according to claim 1 or 2, wherein the cooling step is a refrigerant supply step of supplying a refrigerant to the specific region or a cold body contact step of bringing a cold body into contact with the specific region. The molded part has a curved surface,
The hot press molding method according to claim 1, wherein the welded portion is included in the curved surface. The metal plate is a steel plate,
The heating step is a step of heating the bonded plate to an austenite transformation temperature (Ac ₃ points) or more of the steel plate,
The hot press forming method according to any one of claims 1 to 4, wherein the forming step is a step of setting the forming portion to an end temperature lower than a martensite transformation start temperature (Ms point) at the end of the forming step. .   The hot press molding method according to claim 5, wherein the cooling step is a step of bringing the specific region of the bonding plate to a temperature higher than the Ms point.   The hot press molding method according to any one of claims 1 to 6, wherein the welded portion is a nugget portion made by spot welding.