JP4591020B2 - Tailored blank press forming method - Google Patents

Description

  The present invention relates to a press forming method of a tailored blank material in which metal plates having different plate thicknesses and materials (hereinafter also simply referred to as different metal plates) are joined.

  2. Description of the Related Art In recent years, the weight of automobile bodies has been reduced due to increasing demands for ensuring safety and environmental performance of automobiles. As one of the means, conventionally, for example, BH (Bake Hardening) steel plate, which appears in Patent Document 1 and whose strength increases after baking coating, and high-tensile steel plate, etc., which appears in Patent Document 2, are materials for various automotive parts. The method used as a metal plate has been taken. In any of these, a thin metal plate is used as much as the strength of the material metal plate is increased, thereby reducing the weight.

On the other hand, a metal plate having a uniform thickness and material is heated to a temperature exceeding, for example, 300 ° C. and then press-molded, so that the metal plate is also cooled. In some cases, methods such as Patent Document 3 and Patent Document 4 for increasing the strength of parts are also employed.
In Patent Document 3, the heating temperature of the metal plate when the metal plate is press-molded is set to a high temperature of 850 ° C. or higher. In Patent Document 4, the heating temperature of the metal plate is further set to a temperature exceeding 300 ° C. The applied strain amount during press molding is 0.1 or more, the applied strain rate is 0.1 or more per second, and the cooling rate is 20 ° C. or more per second.

Alternatively, as another example, a method such as Patent Literature 5 called a press quench method in which cooling is performed by further press-molding a molded part and the strength is increased by a quenching effect may be employed.
In Patent Document 5, the heating temperature of the part when the molded part is press-molded is set to 850 ° C. or higher, the cooling rate to the temperature range of 500 to 450 ° C. is 80 to 150 ° C. per second, and thereafter 100 ° C. The cooling rate up to the following is specified to be 20 to 100 ° C. per second.

These are the same in that they are trying to reduce the weight by using a thin metal plate as much as possible to increase the strength. Further, the problem that the temperature of the press-molding mold rises as the press molding is repeated can be solved, for example, by cooling the inside of the mold with water as in Patent Document 6.
However, in spite of the appearance of the above method, there is a demand for further reduction in the weight of the vehicle body of an automobile. To meet these demands, high-strength metal plates are used locally as parts for press molding only in parts where high strength is required, and relatively low-strength metal is used in other parts. There is a movement to use a plate as a material for press molding. In other words, only parts that require high strength among parts are locally made of thicker materials or made of high-strength materials.

In other words, a so-called tailored blank material obtained by joining metal plates having at least one of different plate thicknesses and materials is used as a material metal plate, which is press-molded and processed into a desired shape. However, the technique relating to the press forming method of such tailored blank material is now attracting attention.
Further, in Patent Document 7, when press forming, a metal plate having high strength is used for a portion where distortion is likely to be high, and a portion having relatively low strength is used for a portion where distortion is likely to be small. It is said.
JP 2004-43884 A JP 2003-73775 A JP 2002-102980 A JP 2004-25247 A Japanese Patent Laid-Open No. 10-96031 JP 52-35756 A JP 2001-1062 A JP 2004-34053 A JP 2004-58082 A

  However, when the tailored blank material is press-molded, there is a problem that wrinkles R are generated at the joint portion of the tailored blank material 5 as shown in FIG. Incidentally, FIG. 7A shows a state in which the state of the wrinkle is seen in a cross section in the thickness direction of the metal plate. As shown in FIG. 7 (b), the cause of such wrinkles is that even if the tailored blank material 5 slides between the punch 1 and the die 2, Since the clearance 6 called the escape portion is provided so as not to be caught and broken, the tailored blank material 5 easily enters the clearance 6 when it slides.

  However, in order to solve this problem, this gap 6 is eliminated by using a punch 1 or die 2 having a step corresponding to the plate thickness difference when there is a plate thickness step at the joint. Then, when the tailored blank material 5 slides between the punch 1 and the die 2, as shown in FIG. 7 (c), as described above, a portion having a plate thickness step is caught. Because it breaks, it is practically impossible. In addition, even when a tailored blank material in which materials having no difference in thickness and having only different materials are joined is used, this wrinkle may occur.

That is, as shown in FIG. 4 later, when a butt weld bead portion is present in the joint portion, a relief portion is similarly provided, wrinkles are generated on the weak side, and the bead portion is also provided. Even when no escape portion is provided, wrinkles may occur.
To explain it, even if the frictional force and pressing force acting on different metal plates constituting the tailored blank material 5 are the same, if the yield point, work hardening index, strength coefficient, etc. are different, they are originally Wrinkles are likely to occur on the side having a small value, and there is a tendency that wrinkles are more likely to occur at the joint. Further, as shown in FIG. When viewed, the processing associated with the press forming of the tailored blank material 5 is performed on both sides in the thickness dimension direction of the punch 1, but the tailored blank material 5 on one side is stretched and thinned. On one side, the distance C between the punch 1 and the die 2 is enlarged, and the tailored blank material 5 sometimes enters the wrinkle R while generating the wrinkle R there.

  In order to solve such a problem, a method such as Patent Document 8 in which a bead portion is formed so as to straddle a joint portion of the tailored blank material 5 may be employed. As can be seen from the above, if such a bead portion remains in a part after press molding, there is a big problem that it is very unsightly and cannot be sold. However, it is not easy to eliminate this bead portion without any trace by press molding.

  On the other hand, although the story changes, using a tailored blank material as a raw metal plate, combined with a method of press molding after heating to a temperature exceeding 300 ° C. as in Patent Document 3, Patent Document 4 or Patent Document 5, A method such as Patent Document 9 (example of heating temperature 930 ° C.) can also be assumed separately, and there is a movement to further increase the strength by doing so.

However, in such a case, there is a problem that the tendency of this wrinkle is promoted. Furthermore, BH steel sheets and high-tensile steel sheets such as Patent Document 1 and Patent Document 2 are altered when heated to temperatures exceeding 300 ° C., and when used as part of a tailored blank material, the parts after press forming However, there is a problem that a desired strength may not be obtained at a necessary portion. From this point, improvement has been demanded.
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a press forming method for a tailored blank material in which wrinkles are unlikely to occur at the joint portions of different metal plates.

To solve the above problems, the press molding method of tailored blank according to 請 Motomeko 1 of the present invention, press forming of tailored blank having a joint portion at least one has joined the different steel sheet thicknesses and material in the method, during the press molding, the joining the tailored blank with a temperature of 50 ° C. to 300 ° C., the temperature of the tailored blank as a mold 10 ° C. to 200 DEG ° C. higher temperature with respect to the temperature of the press-molding The part including the part is press-molded.

  Thus, according to the press molding method of the tailored blank material of the present invention, the tailored blank material is press molded at a temperature of 50 ° C. to 300 ° C., or the temperature of the tailored blank material is set to the temperature of the mold for press molding. On the other hand, by press forming at a temperature higher by 10 ° C. to 200 ° C., wrinkles are unlikely to occur at the joints of different metal plates.

It is effective for the inventor to apply a thermal expansion to the metal plate in advance as a method of reducing wrinkles generated at the joints of different metal plates, and to cancel the wrinkles by the thermal shrinkage generated by cooling after press forming. The present invention has been conceived. In this regard, when a method such as Patent Document 3, Patent Document 4, or Patent Document 5 is used in combination with press forming of a tailored blank material, which is press-molded after heating the material metal plate to a temperature exceeding 300 ° C., It is estimated that the temperature of the material metal plate during press forming is too high, and the strain introduced into the material metal plate disappears due to stress relaxation, which in turn contributes to the generation of wrinkles. .
According to the present invention, it is not necessary to heat the BH steel sheet or the high-tensile steel sheet as in Patent Document 1 or Patent Document 2 to a temperature that is high enough to change in quality, so that this problem can also be solved.

Next, an embodiment of the press molding method of the tailored blank material of the present invention will be described with reference to the drawings.
FIG. 1 shows a press forming apparatus for a tailored blank material according to the present embodiment. FIG. 1 (a) shows a top dead center state, and FIG. 1 (b) shows a bottom dead center state. In the figure, reference numeral 1 is a punch, reference numeral 2 is a die, reference numerals 3 and 4 are holders, and reference numeral 5 is a tailored blank material. When performing press molding called single action with this press molding apparatus, the upper holder 4 is lowered together with the die 2 to sandwich the tailored blank material 5 between the lower holder 3, and then the die 2, The holders 3 and 4 and the tailored blank material 5 are lowered and brought into contact with the punch 1, and the die 2, the holders 3 and 4 and the tailored blank material 5 are further lowered to press the tailored blank material 5 into a desired shape. According to the apparatus, the upper holder 4 is lowered and the tailored blank material 5 is sandwiched between the lower holder 3 and then press molding called double action for lowering the die 2 can be performed. In the press molding apparatus of the present embodiment, the coolant (cooling medium) passage 10 is formed on the side of the punch 1, die 2, holder 3, 4 that contacts the tailored blank material 5, and the coolant supplied to each coolant passage 10. By controlling the temperature of the punch 1, the punch 1, the die 2, and the holders 3 and 4 can be cooled. As the refrigerant, a gas such as air or a liquid such as water can be used. In addition, when gas is used as a refrigerant, it is also possible to locally cool only a desired portion using the principle of adiabatic expansion.

  FIG. 2 shows a tailored blank material 5 that is press-molded as, for example, an automobile part. Line A in the figure is a joint between different metal plates, for example, cold-rolled steel plates having the same material and different thicknesses are joined together, or one of the cold-rolled steel plates having a higher strength and different thickness than the other is joined. Or cold-rolled steel sheets of different materials and thicknesses can be formed by press molding. A hot-rolled steel sheet may be used instead of the cold-rolled steel sheet, or stainless steel or other types of metal plates may be used, and the type of metal plate is not limited. Here, FIG. 3 shows a tailored blank material 5 in which metal plates having the same material but different thicknesses are joined together. As can be seen from the figure, when metal plates having different thicknesses are joined together, one surface (the lower surface in the figure) is so-called flush with each other, rather than aligning the centers in the thickness direction. Are often joined together. In this case, a gap 6 is formed on the die 2 side to absorb the step at the joint. This gap is provided so that even if the tailored blank material 5 slides between the punch 1 and the die 2 at the time of press molding, a portion having a plate thickness step at the joint portion is not caught and broken. belongs to. For example, when metal plates having the same thickness and made of different materials are joined together and there is a weld bead at the joint, a gap 6 as shown in FIG. 4 is often formed in the die 2.

  However, if these gaps 6 exist, wrinkles are likely to occur at the joints of different metal plates of the tailored blank material 5 after press forming. In order to avoid this problem, in this embodiment, the tailored blank material is press-molded after the temperature is set to 50 to 300 ° C. By doing so, as a result, the press-molding die, that is, the press-molding is performed after the tailored blank material 5 is raised by 10 ° C. to 200 ° C. higher than the punch 1, the die 2 or the holders 3 and 4. Become. Considering that the temperature of the punch 1 or die 2 or the holders 3 and 4 rises from room temperature to a maximum of 100 ° C. when the press molding is repeated several times, the temperature of the tailored blank 5 is 50 ° C. to 300 ° C. It is reasonable to do. The inventor has obtained the above knowledge as described below.

  First, when the tailored blank material was press-molded, the occurrence of wrinkles was observed after the press molding at the joint portion of different metal plates. Since the tailored blank material in which wrinkles were observed was a so-called dissimilar material with different materials joined, initially, the inventor left the distortion generated when the dissimilar materials were joined, It was thought that this was manifested as deformation along with press molding, and was considered to be a problem peculiar to tailored blank materials joined with dissimilar materials. However, this wrinkle is the same material, and then noticed that it also occurs in the tailored blank material in which the metal plates with different thicknesses are joined, and thereafter, the thermal strain generated when joining remains. It came to be thought that it became manifest as deformation with press molding.

  Therefore, an attempt was made to reduce the generation of wrinkles by annealing the tailored blank material and removing the distortion. However, the generation of wrinkles could not be reduced by annealing. Therefore, it was considered that the wrinkles at the joints of different metal plates were caused not by the material but by the press-molding die, and the press-molding die, that is, the die punch holder, was observed in detail. As a result, for example, it was noticed that a thin metal plate entered and slackened into the gap 6 formed in the die 2, and that it remained after press forming and wrinkles were generated.

  Next, in order to suppress the tailored blank material from flowing into the gap 6, an attempt was made to apply a tension by providing a draw bead at the same portion. In the example shown in FIG. 2 described above, the draw bead is intended to form the bead portion D at a location other than the joint portion, as can be seen from FIG. 5 as seen in the cross section in the thickness direction of the metal plate. The bead portion D is press-molded while being sandwiched between the holders 3 and 4, and the tailored blank material 5 is prevented from sliding between the punch 1 and the die 2 so that tension is applied to the tailored blank material 5. That's it. However, when a draw bead was installed, the cracking rate increased, and it was necessary to review the mold design itself. Therefore, in a draw bead, when a tailored blank material is press-molded, it cannot suppress that a wrinkle generate | occur | produces in the junction part of a different metal plate.

  Therefore, next, for example, optimization of the shape of the gap 6 formed in the die 2 was attempted. Specifically, when the gap was made as small as possible and the tailored blank material was press-formed, the joints of different metal plates and the tightening in the vicinity thereof were made as strong as possible. However, although it should have been possible to suppress the generation of wrinkles in the calculation, wrinkles were actually generated, and galling was performed between the tailored blank material and the press forming die such as a punch, a die, and a holder. Has occurred and did not become a product.

  In the end, we had to conclude that the wrinkles were due to unpredictable deformation on the side of the thin metal plate, in other words, the low strength metal plate. Therefore, as a last measure, the inventor attempted to heat the tailored blank material to a temperature higher than that of a press-molding die such as a die, punch, or holder, and perform press molding in that state. As a result, it has been found that there is an effect in suppressing the generation of wrinkles. The inventor presumes the reason as follows. That is, due to heating, the tailored blank material is stretched due to thermal expansion. When the stretched tailored blank material is press-molded as it is, it heat shrinks in the process of cooling to room temperature. By this heat shrinkage, the tailored blank material is extended to the joint portion of different metal plates when press-molded, that is, even if wrinkles are generated, it can be extended, and as a result, generation of wrinkles can be suppressed. Moreover, since there is a gap between the metal plates of different thicknesses between the molds, the temperature drop of the tailored blank material during press molding is small, so the thermal shrinkage after removal from the mold is large and sufficient. It is estimated that a good effect can be obtained.

By the way, if the thermal expansion coefficient of steel is about 12 × 10 ⁻⁶ (/ K), the wrinkle height is 20 μm, and the number of wrinkles is five, the tailored blank material is 10 ° C. than the press mold. If the temperature is high, the wrinkle is extended by heat shrinkage in the process of cooling to room temperature, and the wrinkle disappears. However, this does not mean that the temperature difference between the tailored blank material and the press mold is large. In other words, in actual operation, the temperature of the press molding die also rises, so that the temperature difference between the press molding die and the tailored blank material is simply 200 ° C or higher, or the temperature of the tailored blank material alone exceeds 300 ° C. If this is done, the tailored blank material will deviate from the elastic deformation region, and press forming will be performed in the plastic deformation region. Thus, the generated wrinkles are not sufficiently extended by heat shrinkage in the process of cooling to room temperature, and wrinkles remain even after cooling to room temperature. On the contrary, the higher the temperature is, the softer it is, and the first wrinkles are likely to occur. As a result, the generation of wrinkles is promoted. Therefore, the temperature of the tailored blank material itself is 50 ° C to 300 ° C, or the temperature of the tailored blank material is 10 ° C to 200 ° C higher than the temperature of the die for press molding, that is, the die punch holder, etc. Can be pressed in the elastic deformation region of the tailored blank material, and stress relaxation can be suppressed, and even if wrinkles occur during press forming, it is sufficiently cooled by heat shrinkage in the process of cooling to room temperature. After being stretched and cooled to room temperature, no wrinkles remain and as a result no wrinkles are generated.

  In order to satisfy the temperature difference condition, that is, the temperature difference condition between the press-molding die and the tailored blank material, the die for punch-molding such as a die, punch, and holder is actively cooled. It is also preferable. When the press molding die is cooled, it is preferable to detect the temperature at an appropriate location with a thermometer and perform feedback control so that the temperature falls within a desired temperature range. In addition, the degree of cooling of the press molding die can be locally changed, and the processing heat generation of the tailored blank material during press molding is locally large, that is, a region where the temperature rise is locally large is selectively strong. It may be cooled. In addition, during press molding, both the temperature rise profile of the press mold and the temperature rise profile of the tailored blank material are detected by a thermometer, and the part where the temperature difference between the two temperature rise profiles is maximum is detected. You may make it cool the die | dye for press molding so that the temperature difference of a type | mold and tailored blank material may enter into the range of 10 to 200 degreeC.

Further, in the embodiment described above, when the tailored blank material is press-formed, the case where a gap is formed on the side of the press-molding die corresponding to the joint portion of different metal plates has been described. Even if not, wrinkles can occur. That is, in the tailored blank material in which metal plates of the same thickness and different materials are joined, no gap is required when the weld pile (bead) is removed. However, for example, when a so-called hat-shaped member as shown in FIG. 6 (a) is press-molded using a tailored blank material, the press-molding die is slightly displaced in the direction of the arrow in FIG. 6 (b), for example. In the ZZ cross section, as shown in FIG. 6C, a gap is generated between the right side standing wall and the die among the standing walls of the hat-shaped member. In some cases, the press-molding die is displaced in the direction opposite to the arrow, and a gap is formed between the left wall and the die. In this way, even when a gap is generated between the tailored blank material and the press molding die, for example, wrinkles as shown in FIG. 6D may occur. According to the press forming method of the tailored blank material of the present embodiment, the occurrence of wrinkles can be suppressed even in such a case.

  Hereinafter, examples when the press forming method of the tailored blank material of the present embodiment is actually used will be described. Table 1 shows the properties of the cold-rolled steel sheet and the high-strength cold-rolled steel sheet used in this example. These metal plates are butt welded with a laser to create a 300 mm x 600 mm tailored blank, and a hat-shaped member as shown in Fig. 6 is formed by press forming. Whether or not wrinkles occur at the joints of different metal plates Was judged. Trimming (removal removal molding) and re-striking (re-pressing) were not performed. Further, the press molding was the single action described above, and the pressing force at the holder was 400 kN. The determination of the presence or absence of wrinkles was made by visual inspection.

  Prepare the tailored blank material that joins A1 material and A2 material, A1 material and B1 material, and A1 material and B2 material in Table 1, respectively, and heat each tailored blank material to a predetermined temperature in a constant temperature furnace. The mold for press molding consisting of dies, punches, holders, etc. is cooled, the temperature of the tailored blank material, the temperature of the mold for press molding, and the temperature difference between them are detected by a thermometer, and then press molding is performed, The presence or absence of the occurrence of was determined. Table 2 shows the heating temperature of the tailored blank material, the temperature of the press molding die, the temperature difference between the two, the determination result of the presence or absence of wrinkle generation, and the presence or absence of cooling of the press molding die. As apparent from the table, in any tailored blank material, the temperature of the tailored blank material was press-molded at a temperature higher by 10 ° C. to 200 ° C. than the temperature of a die for press molding such as a die, punch, or holder. In this case, when the temperature of the tailored blank material is 50 ° C. to 300 ° C. and press-molded, wrinkles are not generated.

  Thus, according to the press molding method of the tailored blank material of the present embodiment, the tailored blank material is press molded at a temperature of 50 ° C. to 300 ° C., or the temperature of the tailored blank material is the temperature of the mold for press molding. On the other hand, it can suppress effectively that a wrinkle generate | occur | produces in the junction part of a different metal plate by press-molding as 10 to 200 degreeC high temperature.

  In the above embodiment, only the press forming method of the tailored blank material obtained by joining the cold-rolled steel plate and the high-strength cold-rolled steel plate has been described, but the tailored blank material used in the present invention is not limited to these, A tailored blank made of any material and any combination of thicknesses can be used. Similarly, the types of metal plates are not limited to two types, and tailored blank materials obtained by joining three or more types of metal plates can of course be used.

In addition, the action that can suppress the occurrence of wrinkles at the joints of different metal plates according to the present invention does not depend on the shape of the mold used for press molding, and when a member of any shape is press molded. The present invention is also applicable.
Also, the press molding method is not limited to the single action described above, and the present invention can be applied to any press molding method including so-called double action.
Further, the cooling method of the press molding die is not limited to the refrigerant method, and any cooling method can be applied to the present invention.

It is a schematic block diagram of the press molding apparatus which shows one Embodiment of the press molding method of the tailored blank material of this invention, (a) is a top dead center state, (b) is a figure which shows a bottom dead center state. It is a perspective view which shows the example of the tailored blank material press-molded by the press molding apparatus of FIG. It is explanatory drawing of the tailored blank material which joined the metal plates from which thickness differs by the same material. It is explanatory drawing of the clearance gap formed in the type | mold for press molding of the tailored blank material which joined the metal plates with the same thickness by different materials. It is explanatory drawing of a draw bead. It is an explanatory view of the principle that wrinkles are generated even when a gap is not formed in a die for press forming of a tailored blank material, (a) is an explanatory view of a hat-shaped member press-formed with the tailored blank material, (B) is an explanatory diagram showing the state of mold misalignment, (c) is an explanatory diagram of the gap formed between the tailored blank material and the press molding die, and (d) is a joint between different metal plates after press molding. It is explanatory drawing of the mode of the wrinkle which generate | occur | produced. It is a figure for demonstrating the problem of a prior art.

Explanation of symbols

1 is a punch (die for press molding)
2 is a die 3, 4 is a holder 5 is a tailored blank material 6 is a gap R is a wrinkle C is an interval D is a bead part (draw bead)

Claims (1)

In the press molding method tailored blank having a joint portion at least one has joined the different steel plate thickness and material, during press-forming, the tailored blank with a temperature of 50 ° C. to 300 ° C., the tailored blanks press-forming method of the tailored blank, characterized in that the temperature of the wood and the type 10 ° C. to 200 DEG ° C. higher temperature with respect to the temperature of the press-molding press molding a portion including the joint portion.

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