JP2007283341A - Press-forming die for butt-welded metal plate, and press-forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance the forming limit to the defective press-forming of a tailored blank, in particular, the breakage in the "stress rate-controlling mode" having no effective countermeasures therefore by using the friction between the tailored blank and a die. <P>SOLUTION: In a press-forming die for press-forming a plate formed by butt-welding metal plates different in one or both of the thickness and the strength, the roughness of a corner R part of a punch surface of the press die is ≥ 0.9 μm in terms of Ra. In a press-forming method, a portion having the roughness of the corner R part of the punch surface of the press die being ≥ 0.9 μm in terms of Ra is brought into contact with a high-strength metal plate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a metal mold for press working and a brazing method for a metal plate obtained by welding a plurality of metal plates together, and more specifically, among metal plates produced by joining metal plates, a press or the like. The present invention relates to a press working die or a press working method for so-called tailored blank materials used for plastic working.

  For press parts for automobiles, a technique for integrally molding two or more kinds of parts is widely adopted in order to simplify the process and reduce the number of dies. However, when producing an integrally molded part from a base plate (metal plate), there are many scrap parts, so that thin metal plates of the same and the same material are laser welded or mash seamed to improve the base plate yield. A technique has been developed in which continuous welding is performed by welding, electron (laser) beam welding, TIG welding, arc welding, and the like, and integrated press molding is performed on a welded metal plate. Furthermore, recently, from the viewpoint of collision safety, when it is required to give some parts strength, continuous welding is performed on base plates with different material strength and thickness required for some parts. The so-called dissimilar tailored blank material is often used.

  FIG. 1 is an overhead view of a laser butt welding process of a tailored blank material.

  Tailored blanks are manufactured by laser butt welding of steel plates with different thicknesses and strengths. For example, as shown in FIG. 1, a laser beam 5 is applied to the welding zone 7 from the laser torch 3 to the gap between the butt steel plates 1 and 2 and the laser torch is moved in the welding progress direction 6 along the butt line. Alternatively, the weld bead 4 is formed while the steel plate is moved in the welding progress direction 6 with respect to the fixed laser torch to obtain a tailored blank material.

  These tailored blanks joined by welding have economic effects such as simplifying the process and reducing the number of dies when manufacturing parts with different strengths, etc. The molding defect is a problem. Breaking that may cause molding failure during press molding includes a ductile rate limiting mode in which when the base plate is extended in parallel with the weld bead, the weld bead part that has deteriorated in material will break, and the weld bead is sandwiched between When the base plate is stretched, it is divided into “stress-controlled mode” which leads to the fracture of the base material of the base plate on the low strength side.

  In order to prevent such molding defects, for example, in the press molding of different thickness and different material tailored blank materials, for the purpose of avoiding breakage on the low strength (or low plate thickness) material side due to strength rate limiting The ratio (n1 / n2) of the work hardening characteristic value (n1) of the high tensile strength side material to the work hardening characteristic value (n2) of the low tensile strength side material is set to 0.75 or more and 3.8 or less. Good molding when press molding is performed after welding with high density energy beam such as laser beam, electron beam, plasma arc, etc. Steel sheet satisfying 2.6 <f (C, Si, Mn, P, B) <12.5 as an ultra-low carbon cold-rolled steel sheet exhibiting properties (provided that, when B ≦ 0.0005%, f ( C, Si, Mn, P B) = 100 [% C] + [% Si] +2 [% Mn] +50 [% P] +9000 [% B], and when B> 0.0005%, f (C, Si, Mn, P, B) = 100 [% C] + [% Si] +2 [% Mn] +50 [% P] +1000 ([% B] −0.0005) +4.5) (for example, patent literature) 2).

  However, the invention of such a material alone is not sufficient to effectively prevent the material from being broken during the press die molding. For example, ultra-low carbon steel sheets may not be able to satisfy the required strength for members that require high strength in recent years, and the effect on ductile rate-limiting mode breakage due to improved weld bead properties. However, it has been found that the effect on the breakage of the “stress limited mode” is low.

Regarding the strain distribution at the time of rupture in the stress-controlled mode, it is possible to obtain the strain ratio applied to two or more types of base plates by the primary analysis based on the conventional knowledge (for example, see Non-Patent Document 1). Are known. That is, if the stress-strain relational expressions of the two types of materials are subscript 1: high strength material and subscript 2: low high strength material, σ1 = K1ε1n1 and σ2 = K2ε2n2. Since forces are balanced at the joint, σ1t1 = σ2t2 holds. Accordingly, when these equations are solved, the strain (ε _1max ) on the high strength material side when the low strength material side reaches the fracture limit is given by the following (1) when _calculated from the values of TS ₁ and TS ₂ .
ε _1max = n ₁ {(t ₂ / t ₁ ) (TS ₂ / TS ₁ )} ^{1 / n} 1 (1)
Here, σ: tensile stress [MPa], K: plastic coefficient [MPa], ε: logarithmic plastic strain, n: work hardening index, TS: maximum tensile strength [MPa].

  However, although the maximum strain on the high-strength material side can be calculated, there is no disclosure about a method for improving the fracture during press molding in the “stress-controlled mode”. Therefore, at the press site, when the “stress rate-limiting mode” occurs in the different tailored blank material, the plate thickness ratio has to be reduced or the strength ratio has to be lowered in order to reduce the strength ratio of the base plate.

  For example, the press molding method of tailored blank materials with different plate thicknesses is shown in FIG. 2 and FIG. 3 as cross-sectional views at the time of hat press molding of tailored blank materials. As shown in FIG. 2, in order to perform hat press molding of tailored blank materials having different plate thicknesses, a hat press molding apparatus including a punch 8, a die 9 and a plate presser 10 is used. The tailored blank material 12 is set by the plate presser 10 so that the welding position 11 is matched, and the punch 8 is raised in the direction of the arrow to perform molding. At the time of molding, as shown in FIG. 3, strain due to pulling in a direction perpendicular to the weld bead direction becomes the rate-limiting factor for molding. For this reason, in high-capacity hat press molding, a thin steel plate portion with a low tensile strength extends more than a thick steel plate with a high tensile strength, and a crack occurs in the thin steel plate portion near the welding position 11. 13 is produced. In other words, a so-called “stress-controlled mode” breakage occurs during the press, and the moldability is hindered. Therefore, conventionally, tailored blank materials could only be applied to products with a low processing amount.

JP 2000-309843 A JP-A-7-26346 Koichi Ikemoto et al., “Plasticity and Processing”, Vol. 32, no. 370 (1991) 1383-1390.

  An object of the present invention is to solve the above-described drawbacks of press-formability of the conventional tailored blank material, and to provide a tailored blank material press-working die and a press-working method having excellent press-molding performance.

  The present inventors have made use of friction with a press die for press molding defects related to tailored blank materials, especially for the break in the “stress rate-controlled mode” that has not been able to take effective measures until now. The present invention has been completed by finding that the improvement of the limit is effective for the fracture in the “stress-controlled mode”.

  The gist of the present invention is as follows.

  (1) In a press mold for press forming a plate material in which metal plates having different thicknesses or strengths or both are butt welded, the roughness of the corner R portion of the press die punch surface is 0.9 μm or more in Ra A die for press working, characterized by having a part that is

  (2) In a press working method of press-molding a plate material obtained by butt-welding steel plates having different plate thicknesses or strengths, the roughness of the corner R portion of the press die punch surface on the high-strength side plate material is 0. A press working method, wherein a portion that is 9 μm or more is brought into contact.

(3) In a press working method in which a plate material obtained by butt welding steel plates having one or both of different plate thicknesses or strengths is press-molded, the roughness of the corner R portion on the surface of the press die punch on the high-strength side plate material is 0. A press working method, characterized in that an oil having a coating viscosity of 15 mm ² / S or less is applied to at least a part of the corner R portion on the surface of the press die punch so that a portion of 9 μm or more is brought into contact.

(4) In a press working method of press-molding a plate material obtained by butt-welding steel plates having different plate thicknesses or strengths, the roughness of the corner R portion of the press die punch surface on the high-strength side plate material is 0. A press characterized in that a part having a size of 9 μm or more is brought into contact, and at least a part of the corner R part on the surface of the press die punch is coated with oil having an oil coating amount of 0.3 g / m ² or less. Processing method.

(5) In a press working method of press-molding a plate material obtained by butt-welding steel plates having one or both of different plate thicknesses or strengths, the roughness of the corner R portion of the press die punch surface on the high-strength side plate material is 0. A portion having a thickness of 9 μm or more is brought into contact, and an oil having a coating viscosity of 15 mm ² / S or less and a coating amount of 0.3 g / m ² or less is applied to at least a part of the corner R portion of the press die punch surface. A press working method characterized by oiling.

  According to the present invention, it is possible to improve the molding limit by utilizing friction with the press die against breakage of the “stress-controlled mode” at the time of press processing of the tailored blank material. There is a remarkable effect that it is possible to improve the moldability of the tailored blank material that can only be solved by a design change.

Hereinafter, the present invention will be described in detail with reference to the drawings.
An example in which a tailored blank material is press-molded using a press die will be described.

  4A to 4C are cross-sectional views of the tailored blank material during pressing. In order to perform press-molding of tailored blank materials having different plate thicknesses, a low strength of the tailored blank material 12 is formed on the top of the punch 8 by using a hat press molding apparatus including a punch 8, a die 9 and a plate presser 10. Setting is performed by the plate retainer 10 so that the welding position 11 where the side member 14 and the high-strength side member 13 are welded comes, and the punch 8 is raised in the direction of the arrow to perform molding.

  At the time of press molding, in such a case, distortion caused by pulling in a direction perpendicular to the weld bead direction becomes the rate-limiting factor of the molding. When the amount of press molding increases, the so-called “stress-controlled mode” breaks during pressing, which impairs moldability. Conventionally, tailored blanks have only been applied to the production of press-molded products with a low processing amount. I could not do it. For example, when a steel sheet having a thickness of 1.4 mm, a work hardening index n value of 0.25, and a maximum tensile strength of 400 MPa is pulled, this material allows an elongation of approximately 25%. Therefore, for example, when both ends of a steel plate having a width of 100 mm are pulled, an elongation of 25 mm is allowed. Next, the plate thickness is 1.0 mm, but when a steel plate having an n value of 0.25 and a maximum tensile strength of 400 MPa is pulled, this material also allows an elongation of approximately 25%. Therefore, for example, when both ends of a steel plate having a width of 100 mm are pulled, an elongation of 25 mm is allowed.

  However, when such a material is butt welded and pulled in a state in which a place with a width of 50 mm is constrained on both sides around the weld line, the material with a plate thickness of 1.0 mm is stretched to 25% of the limit. The thickness of the 1.4 mm material is only 6-7%, that is, the 1.0 mm material is 12.5 mm longer than the 50 mm length, but the 1.4 mm material is 50 mm long. Since only 3.3 mm extends, that is, the total length of 100 mm extends only 16 mm. At this time, the material with a thickness of 1.4 mm still leaves room for deformation, but the material with a thickness of 1.0 mm has already reached the limit of elongation. appear. In this way, tailored blanks that combine dissimilar materials with different strengths and plate thicknesses have significantly reduced elongation compared to the base material, and therefore can exhibit sufficient formability during press molding. There wasn't.

  The present invention pays attention to the fact that the material with higher thickness or strength still leaves room for deformation even near the press molding limit of the tailored blank material, and the deformation of the material with higher thickness or strength is simplified. The moldability is enhanced by prompting with a method. Specifically, the friction force between the mold and the material is applied to promote the deformation of the material having the higher plate thickness or strength.

  In press molding, friction between a material and a mold is generally a big technical problem. Until now, in the world of press working, friction between material and mold is treated as harmful because it concentrates processing strain locally, and it is molded in strict areas due to workability of materials such as deep drawing press molding. In order to improve the property, a method of reducing the friction as much as possible has been taken. For example, techniques such as finishing the mold surface as smooth as possible, applying sufficient lubricating oil, and applying a special baking coating to the material in some cases have been taken. However, even if the friction coefficient during press processing of the tailored blank material is made uniform by reducing the friction coefficient, the above-mentioned “stress rate limiting” is applied to different material tailored blank materials with different strength and thickness. The failure in the “mode” could not be prevented, and no essential measures were taken.

  Description will be made based on the situation in the press shown in FIGS. In this case, if the mold surface is sufficiently adjusted and oiled to improve the moldability, and the friction coefficient between the material and the mold is made small enough to be ignored, the total elongation of the material will be as described above. However, even if it allowed 25% elongation deformation, it was only able to stretch by only 14% as a whole, and it was possible to mold only with a protruding amount corresponding to that. In that case, the fracture occurred near the weld line of the thin or low-strength side material.

In addition, when there is friction between the material and the mold, a crack is generated at the R stop portion of the low-strength side material. When the coefficients are defined as shown in FIG. 5, the winding angle 15 of each material is θ, and the coefficient of friction 16 between the material and the mold is μ, the low-strength side material is molded at the R-stop portion 17 of the mold. The ultimate strain at the R stop portion 17 of the high-strength side material when reaching the limit is ε _1max = n ₁ {(t ₂ / t ₁ ) (TS ₂ / TS ₁ ) exp (μ × θ) ^{1 / n1} (1)
I understood that it was expressed. Here, t ₁ , t ₂ : plate thickness, TS ₁ , TS ₂ : tensile strength, μ: friction coefficient, θ: winding angle.

In order to promote the molding of the plate with higher thickness or strength and ensure the moldability of the tailored blank material, the ultimate strain of the high-strength side material when the low-strength side material reaches the molding limit, that is, the above formula The larger the ε _1max value, the better. That is, the larger μ (coefficient of friction between the material and the mold), the larger ε _{1max and the} better the moldability. Therefore, according to the present invention, it is possible to improve the moldability, which cannot be realized by the prior art, by simply reducing the coefficient of friction with the mold, without changing the combination of materials. Thus, by increasing the friction coefficient between the base material and the mold, it is possible to improve the moldability of the tailored blank material without changing the combination of the thickness ratio and strength ratio of the base material. In order to improve the moldability of the tailored blank material, it is necessary to have a portion where the roughness of the corner R portion on the punch surface is 0.9 μm or more in Ra. When the roughness of the corner R portion on the punch surface is less than 0.9 μm in Ra, it does not contribute to the improvement of the moldability of the tailored blank material. Further, although the upper limit of the roughness Ra of the corner R portion of the punch surface is not particularly limited, if the surface roughness Ra becomes too large, the roughness of the mold is transferred to the plate material (material) and causes surface flaws. Therefore, it is preferable that the upper limit is about Ra 2.0 μm. Ra is based on JIS.

  As described above, a method for controlling the friction coefficient between the metal plate surface and the mold can be achieved by changing the surface roughness of the mold. Such a mold can be manufactured by appropriately selecting a tool for finish cutting and grinding. In some cases, it can be performed by partial spraying after die grinding, shot dull processing, or the like. In general, the higher the surface roughness, the greater the friction coefficient.

It is also possible to change the amount of the lubricating oil applied to the surface of the mold, and to set the amount of at least a portion of the corner R portion on the surface of the press mold punch to 0.8 g / m ² or less. For example, in the oil coating process before the pressing process, the friction coefficient can be changed by changing the spray amount for spraying oil onto the mold. Of course, it can be changed with or without oiling. The lower limit of the oil coating amount may be about 0.1 g / m ² . If the oil-free state in which oil is not actively applied is increased, the effect is further increased, but seizure may occur due to an increase in the mold temperature during continuous pressing.

The friction coefficient can also be changed by changing the viscosity of the lubricating oil applied to the mold and setting the oil viscosity of at least a part of the corner R portion of the press mold punch surface to 15 mm ² / S or less. As the viscosity decreases, the coefficient of friction increases. The lower limit of the viscosity is not particularly limited, but is preferably about 5 mm ² / S. The viscosity should be low, and if it is 8 mm ² / s, the moldability will be further improved. On the other hand, if it is 3 mm ² / s or less, seizure may occur due to the oil curtain running out. Although it does not specifically limit as oil, Mineral oil, synthetic oil, etc. can be used.

  As described above, the present invention is a completely new idea of completely increasing the friction coefficient between the mold and the tailored blank material in molding of the tailored blank material. It is possible to improve the formability of tailored blanks that cannot be compromised by design changes without taking any measures.

  Hereinafter, the present invention will be described in detail based on examples.

  A test was conducted on a tailored blank material which was a steel plate made of the same material having a tensile strength TS of 400 MPa and a work hardening index n value of 0.25, but was welded in combination with different thickness materials of 1.4 mm and 1 mm.

  As a press test, a press test as shown in FIG. 4 was performed. As the press progresses, as shown in FIGS. 4A to 4C, the welding position (welding line) of the tailored blank is changed from the material on the low strength side (steel plate having a thickness of 1.1 mm) to the material on the high strength side. It moved relative to the punch R portion of the mold in the direction of (steel plate with a thickness of 1.4 mm) (in this case, from a thickness of 1 mm to a material with a thickness of 1.4 mm). This is a phenomenon that occurs because the elongation of the high-strength material is smaller than the elongation of the low-strength material. The angle of the punch R portion of the mold was 90 degrees.

(Comparative example)
First, the surface roughness of the punch R portion on the mold surface is set to 0.1 μm Ra, and an oil with a viscosity of 200 mm ² / s is applied to the surface in an amount of 10 mg / m ² , and the mold is applied to the low-strength side material. Press molding was performed such that the R portion was brought into contact. When such pressing was performed, cracks occurred in the vicinity of the welded portion of the material having a plate thickness of 1 mm when the punching amount was increased. From the welded portion of the tailored blank material, marking is performed before pressing at a location of 100 mm in the direction of the plate thickness of 1.4 mm and the plate thickness of 1.0 mm, and the length is measured when a press crack occurs. Then, the elongation of the material by press working was measured. At this time, the material with a thickness of 1 mm grew to 25% of the original length. At that time, the material on the high strength side with a thickness of 1.4 mm grew only 9% of the original length. As a tailored blank, it grew only 17% of its original length.

(Invention Example 1)
Next, according to the present invention, when the surface roughness Ra of the mold was set to 1.0 μm, the friction coefficient was increased, and a similar experiment was performed in which press molding was performed so that the R portion of the mold was brought into contact with the high-strength side material. At the crack limit of the 1 mm thick material, the 1.4 mm thick material stretched to 12%, and as a tailored blank, it was able to stretch to 19% of the original length. This made it possible to improve moldability.

(Invention Example 2)
Next, when the surface roughness Ra of the mold was set to 0.1 μm and the amount of oil applied to the mold was set to 0.2 g / m ² and the friction coefficient was increased according to the present invention, the same experiment was conducted. At the crack limit of 1 mm material, the material with a thickness of 1.4 mm stretched to 13%, and as a tailored blank, it was able to stretch to 19% of the original length. This made it possible to improve moldability.

(Invention Example 3)
Next, according to the present invention, the surface roughness Ra of the mold is set to 0.1 μm, the amount of oil applied to the mold is set to 10 g / m ^2, and the viscosity of the applied oil is set to 10 mm ² / S to increase the friction coefficient. As a result of the same experiment, the material with a thickness of 1.4 mm stretched to 11% at the crack limit of the material with a thickness of 1 mm, and as a tailored blank, it was able to stretch to 19% of the original length. . This made it possible to improve moldability.

(Invention Example 4)
Next, according to the present invention, the same experiment was conducted by increasing the friction coefficient by setting the roughness Ra of the mold surface to 1.0 μm, the amount of oil to be applied to 0.2 g / m ² , and the viscosity to 10 m ² / s. As a result, at the crack limit of a material with a thickness of 1 mm, the material with a thickness of 1.4 mm was extended to 14%, and as a tailored blank, it was possible to extend to 20% of the original length.

(Invention Example 5)
Furthermore, when the surface roughness Ra of the mold was set to 1.0 μm and a similar experiment was conducted in an oil-free state, the material with a thickness of 1.4 mm was extended to 23% at the crack limit of the material with a thickness of 1 mm. As a tailored blank, it was able to extend to 24% of the original length.

  These test results are summarized in Table 1. As is apparent from Table 1, the inventive examples 1 to 3 have a moldability and improvement ratio of 12%, the inventive example 4 has a moldability and improvement ratio of 18%, and the inventive example 5 has a molding compared to the comparative example. It can be seen that the moldability is improved by 41%, and the moldability is improved.

It is an overhead view of the laser butt welding process of a tailored blank material. It is a figure which shows sectional drawing at the time of hat press molding of a tailored blank material. It is a figure which shows sectional drawing at the time of hat press molding of a tailored blank material. Sectional drawing at the time of the press work of a tailored blank material is shown to (a)-(c). It is a figure which shows the state at the time of press work.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Steel plate 3 Laser torch 4 Weld bead 5 Laser beam 6 Welding direction 7 Weld zone 8 Punch 9 Die 10 Plate retainer 11 Weld line position 12 Tailored blank material 13 High strength side material 14 Low strength side material 15 Winding angle 16 Material Coefficient of friction between mold and die 17 R stop

Claims (5)

  In a press working mold for press molding a plate material in which metal plates having different thicknesses or strengths or both are butt welded, a portion where the roughness of the corner R portion on the press die punch surface is 0.9 μm or more in Ra Die for press working, characterized by having   In a press processing method in which a plate material obtained by butt-welding steel plates having different thicknesses or strengths or both is press-molded, the roughness of the corner R portion of the press die punch surface on the high-strength side plate material is 0.9 μm or more in Ra. A press working method, characterized in that a certain part is brought into contact. In a press processing method in which a plate material obtained by butt-welding steel plates having different thicknesses or strengths or both is press-molded, the roughness of the corner R portion of the press die punch surface on the high-strength side plate material is 0.9 μm or more in Ra. A press working method characterized in that an oil having a coating viscosity of 15 mm ² / S or less is applied to at least a part of the corner R portion of the surface of the press die punch so as to contact a certain portion. In a press processing method in which a plate material obtained by butt-welding steel plates having different thicknesses or strengths or both is press-molded, the roughness of the corner R portion of the press die punch surface on the high-strength side plate material is 0.9 μm or more in Ra. A press working method characterized in that a certain part is brought into contact and at least a part of the corner R part of the surface of the press die punch is coated with oil having an oil coating amount of 0.3 g / m ² or less. In a press processing method in which a plate material obtained by butt-welding steel plates having different thicknesses or strengths or both is press-molded, the roughness of the corner R portion of the press die punch surface on the high-strength side plate material is 0.9 μm or more in Ra. Applying an oil having an oil coating viscosity of 15 mm ² / S or less and an oil coating amount of 0.3 g / m ² or less to at least a part of the corner R portion of the surface of the press die punch so as to contact a certain part. A press working method.

Images