Patent Grant
9914163
This is the U.S. National Phase application of PCT International Application No. PCT/JP2014/071230, filed Aug. 11, 2014, and claims priority to Japanese Patent Application No. 2013-224855, filed Oct. 30, 2013, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.
The present invention relates to a method of sheet forming that is used for sheet-forming a metal sheet by a sheet forming tool to manufacture a sheet-formed part including a top portion, a side wall portion formed on each of both sides of the top portion, and a flange portion contiguously formed with the side wall portion, the sheet-formed part having an L-shape as viewed in a plan view.
When generally manufacturing a product having an L-shaped portion, such as a front pillar reinforcement that is one of vehicle body frame parts in an automobile, by sheet-forming a metal sheet, a sheet forming tool constituted of a punch, a die, and a blank holder is used. In this case, the punch and the die are brought closer to each other in a state that the whole periphery of the metal sheet is held by the die and the blank holder to apply drawing to the metal sheet.
In such sheet forming, when sheet-forming an L-shaped part having a bent portion being bent steeply, wrinkles easily occur in a top portion of the L-shaped part. When the holding force by the die and the blank holder is increased, the wrinkles are reduced, while cracks easily occur in a shoulder portion or a side wall portion of the L-shaped part. In recent years particularly, in order to achieve the enhanced safety and the weight reduction of an automotive body, the strength of the metal sheet used as a blank has been increasingly enhanced and hence, it is impossible to expect a blank ductility equivalent to that of the conventional soft steel sheet. Accordingly, it is important to prevent cracks or wrinkles at the time of sheet forming.
As a technique of sheet-forming an L-shaped part, patent literature 1 proposes a method for draw-forming a blank shallowly first and thereafter, further bending-forming the blank by using another sheet forming tool to form the blank into a final product shape. Furthermore, as another technique with respect to a frame member having an L-shape, patent literature 2 proposes a method of sheet forming that uses a sheet forming tool unit constituted of a die, a pad, and a bending tool.
Patent Literature 1: WO 2012/070623
Patent Literature 2: WO 2011/145679
In the techniques described in patent literatures 1 and 2, only the part whose L-shaped bending portion to be joined to other parts has a top portion, one side wall extending from the top portion, and one flange portion connected to the side wall can be formed, and an L-shaped part having a hat-shaped cross section extending over the entire length of the L-shaped part is incapable of being formed.
The present invention has been proposed to overcome such problems and it is an object of the present invention to provide a method of sheet forming capable of forming an L-shaped part having a hat-shaped cross section extending over the entire length of the L-shaped part even when a metal sheet composed of a high-strength steel sheet having ultrahigh strength of at least 980 MPa class and low ductility is used as a blank.
A method of sheet forming according to the present invention includes a method of sheet forming for manufacturing a sheet-formed part that is formed in an L-shape as viewed in a plan view and includes a top portion, side wall portions formed on the respective both sides of the top portion, and flange portions contiguously formed with the respective side wall portions, by sheet-forming a metal sheet with the a sheet forming tool, and includes: a first sheet forming process in which the metal sheet is arranged on a first sheet forming tool including a first punch, a first die, a first blank holder, and a pad so that a part corresponding to an end portion of a short side of the L-shape in the metal sheet does not overlap with the first die and the first blank holder, a part of the top portion is held by the pad, and the metal sheet is sheet-formed into a first sheet-formed part having an intermediate shape; and a second sheet forming process in which the first sheet-formed part formed in the first sheet forming process is sheet-formed into a second sheet-formed part having a top portion and a side wall portion whose shapes are identical with those of the sheet-formed part, with a second sheet forming tool having a second punch and a second die, wherein the side wall portion on an outer side that includes an L-shaped bent portion in the first sheet-formed part formed in the first sheet forming process is outwardly stretched in an arcuate shape as viewed in a plan view from the side wall portion of the sheet-formed part, a curvature radius of a circular arc of the side wall portion stretched in an arcuate shape is larger than a curvature radius of the L-shaped bent portion of the sheet-formed part, an outside of the top portion of the L-shaped bent portion is stretched from an outside of the top portion of the sheet-formed part, and a curvature radius in a cross section of a shoulder portion contiguously formed with the top portion stretched, as viewed in a side view, is larger than a curvature radius in a cross section of a shoulder portion contiguously formed with the outside of the top portion of the L-shaped bent portion of the sheet-formed part, as viewed in a side view.
In the method of sheet forming according to embodiments of the present invention, a part of the top portion held by the pad in the first sheet forming process includes at least the L-shaped bent portion, and is a part with which a stretching portion of the top portion in the first sheet-formed part does not interfere.
In the method of sheet forming according to embodiments of the present invention, a timing at which the pad is started to hold the metal sheet in the first sheet forming process is the timing at which the pad is positioned in a range from 0% position to 50% position with respect to a forming depth from a position where the first punch initiates a contact with the metal sheet to a bottom dead center position of the first punch.
In the method of sheet forming according to embodiments of the present invention, amounts of stretching of the side wall portion and the top portion in the first sheet forming process are each an amount that an area ratio of an area of a stretching portion of the L-shaped bent portion of the first sheet-formed part to an area of the L-shaped bent portion of the sheet-formed part corresponding to the stretching portion is increased by 0.3% to 1.2%.
According to embodiments of the present invention, the L-shaped part having a hat cross section extending over the entire length of the L-shaped part can be formed even when the metal sheet composed of the high-strength steel sheet having ultrahigh strength of at least 980 MPa class and low ductility is used as a blank.
Hereinafter, one embodiment of the present invention is specifically explained with reference to drawings. Here, the present invention is not limited to this embodiment. In the drawings, parts having identical functions are given same numerals.
A method of sheet forming according to the one embodiment of the present invention includes a first sheet forming process illustrated in
First of all, the sheet-formed part 19 that is a finished product, the first sheet-formed part 27 having an intermediate shape, and the second sheet-formed part having a product shape are explained. Next, the first sheet forming tool 1 and the second sheet forming tool 13 are explained. Thereafter, the method of sheet forming (the first sheet forming process, and the second sheet forming process) is explained.
Sheet-Formed Part
The sheet-formed part 19 is, as illustrated in
The top portion 21 is formed in a L-shape constituted of long sides and short sides as viewed in a plan view. The top portion 21 has a plurality of convex portions 21a. The end portion of the short side forms a stepped portion thereon in such a manner that the end portion of the short side is shrunk, and the stepped portion constitutes a connection portion 21b for connection with another part. The side wall portion 23 and the flange portion 25 are not formed on the end portion of the short side. Here, the side wall portion 23 is inclined.
In this manner, the sheet-formed part 19 is a part that has a bent portion being bent steeply, and has a hat cross-sectional shape extending over the entire length thereof. Therefore, when the sheet-formed part 19 is manufactured by using a high-strength steel sheet having ultrahigh strength and low ductility in general sheet forming processes, wrinkles easily occur in the top portion 21 of an L-shaped bent portion, or cracks easily occur in a shoulder portion of the side wall portion 23 of the bent portion. Accordingly, in the present embodiment, in order to manufacture the sheet-formed part 19, as explained hereinafter, the first sheet-formed part 27 having an intermediate shape is first formed and thereafter, the second sheet-formed part having a product shape is formed from the first sheet-formed part 27.
First Sheet-Formed Part
The first sheet-formed part 27 is a part formed in the first sheet forming process. The first sheet-formed part 27 is explained based on
The first sheet-formed part 27 forms a side-wall stretching portion 29, which is outwardly stretched in an arcuate manner from the side wall portion 23 of the sheet-formed part 19 as viewed in a plan view, on the side wall portion 23 on an outer side that includes the L-shaped bent portion. Furthermore, as illustrated in
The first sheet-formed part 27 forms a top-portion stretching portion 31, which is stretched from the outside of the top portion 21 of the L-shaped bent portion of the sheet-formed part 19 in an arcuate manner as viewed in a side view, on the outside of the top portion 21 of the L-shaped bent portion. Furthermore, as illustrated in
As for the amounts of stretching of the side wall portion 23 and the top portion 21, it is desirable that the area of a stretching portion of the L-shaped bent portion of the first sheet-formed part 27 be increased by 0.3% to 1.2% by area ratio compared with the area of the L-shaped bent portion of the sheet-formed part 19 corresponding to the stretching portion. Here, the stretching portions mean the side-wall stretching portion 29 and the top-portion stretching portion 31, and the area of the stretching portion (in a side view and/or a plan view) means the sum total of the areas of the side-wall stretching portion 29 and the top-portion stretching portion 31. When the increase of the area ratio is less than 0.3%, it is impossible to avoid cracks. To consider a case where the increase of the area ratio exceeds 1.2%, when the first sheet-formed part 27 is formed into the sheet-formed part 19 in the second sheet forming process, there is a risk that the excessive material of the stretching portion causes the sheet-formed part 19 to be incomplete.
The shape of the first sheet-formed part 27 is substantially the same as the shape of the sheet-formed part 19 except for the side wall stretching portion 29 and the top-portion stretching portion 31.
Second Sheet-Formed Part
The second sheet-formed part is a part formed in the second sheet forming process, and has a product shape with respect to a top portion and a side wall portion thereof. (The shapes of the top portion and the side wall portion are identical with the respective shapes of the top portion 21 and the side wall portion 23 of the sheet-formed part 19.) Here, the shape of the second sheet-formed part is substantially the same as that of the sheet-formed part 19 illustrated in
Next, the first sheet forming tool 1 and the second sheet forming tool 13 for forming the first sheet-formed part 27 and the second sheet-formed part, respectively, are explained.
First Sheet Forming Tool
The first sheet forming tool 1 is a sheet forming tool used for forming the metal sheet 11 into the first sheet-formed part 27 in the first sheet forming process. The first sheet forming tool 1 has, as illustrated in
First Punch
The first punch 3 has, as illustrated in
The top-portion forming surface portion 41 includes a top-portion stretching forming portion 41a for forming the top-portion stretching portion 31 of the first sheet-formed part 27, and the side-wall forming surface portion 43 includes a side-wall stretching forming surface portion 43a for forming the side wall stretching portion 29. The first punch 3 is formed into the above-mentioned shape and hence, in forming the first sheet-formed part 27 in the first sheet forming process, cracks of the shoulder portion contiguously formed with the outside of the top portion 21 in the L-shaped bent portion of the first sheet-formed part 27 can be avoided.
First Die
As illustrated in
Blank Holder
As illustrated in
Pad
As illustrated in
Second Sheet Forming Tool
The second sheet forming tool 13 is a sheet forming tool that forms the first sheet-formed part 27 into the second sheet-formed part in the second sheet forming process, and has the second punch 15 and the second die 17 as illustrated in
Second Punch
The second punch 15 has a forming surface 15a that forms the second sheet-formed part having a product shape, and presses and shrinks the stretching portion (the side-wall stretching portion 29 and the top-portion stretching portion 31) (see
Second Die
The second die 17 has a forming surface 17a corresponding to the forming surface 15a of the second punch 15, and forms the first sheet-formed part 27 into the second sheet-formed part in cooperation with the second punch 15. Here, in the second die 17, a knock-out pin that easily ejects a product may be, as required, arranged when the second sheet-formed part adheres to the forming surface 17a after forming.
Method of Sheet Forming
Next, the method of sheet forming (the first sheet forming process, and the second sheet forming process) according to embodiments of the present invention is explained.
First Sheet Forming Process
In the first sheet forming process, the metal sheet 11 is set to the first sheet forming tool 1 illustrated in
As illustrated in
As illustrated in
When the metal sheet 11 is sandwiched between the first die 5 and the blank holder 7, the first die 5 is downwardly moved and hence, the first punch 3 is pressed on the metal sheet 11 to perform sheet forming. While the sheet forming is performed, a part of the top portion 21 in the metal sheet 11 is held by the pad 9. Here, the part of the top portion 21 is an area that includes at least the L-shaped bent portion, and does not interfere with the top-portion stretching portion 31 in the first sheet-formed part 27. In this manner, the metal sheet 11 is held by the pad 9 thus effectively preventing the occurrence of wrinkles in the L-shaped bent portion. As described above, the area 11a corresponding to the short-side end portion of the second sheet-formed part in the metal sheet 11 is arranged so that the area 11a does not overlap with the blank holder 7 and hence, the metal sheet 11 that is a material moves to the bent portion while the sheet forming is performed, and wrinkles easily occur. Accordingly, in embodiments of the present invention, in order to prevent the occurrence of wrinkles, the metal sheet 11 is held by the pad 9. When an area of the metal sheet 11 that is held by the pad 9 is set so that the area possesses a range within at least the L-shaped bent portion, the occurrence of wrinkles is prevented. The top portion 21 may be held in the wider range thereof. The area of the metal sheet 11 that is held by pad 9 is set so that the area does not interfere with the top-portion stretching portion 31 (see
In a forming depth to the bottom dead center position of the pad 9, a position where the first punch 3 initiates a contact with the metal sheet 11 is set to 0%, and the bottom dead center position of the pad 9 is set to 100%. In that case, as for the timing at which the pad 9 holds the metal sheet 11, it is preferable that the pad 9 start to hold the metal sheet 11 when the first punch 3 is located at a position in the range from 0% to 50% of the forming depth. Here, in the explanation made hereinafter, a pad holding start timing means the timing at which the pad 9 starts to hold the metal sheet 11.
In the case where the pad holding start timing is a timing when the first punch 3 is located at the position of 0% or later of the forming depth, the swelling of the metal sheet 11 in the top portion 21 that starts from the early stage of contact between the first punch 3 and the metal sheet 11 is thus effectively prevented the occurrence of wrinkles. Hereinafter, the position of 0% of the forming depth is merely described as “0% position”, and other positions are described in the same manner as above. On the other hand, in the case where the pad holding start timing is a timing when the first punch 3 is located at the position prior to the 0% position (−100% (minus 100%) position, for example), the pad 9 presses the metal sheet 11 in the direction toward the first punch 3 before the first die 5 and the blank holder 7 sandwich the metal sheet 11 therebetween. This case is undesirable since the displacement of the metal sheet 11 occurs and causes excessive leftover materials with respect to the shape of a formed part.
Furthermore, when the first punch 3 and the pad 9 start to hold the metal sheet 11 after the first punch 3 passes through a 50% position of the forming depth, wrinkles due to enlarged wavelike swelling in the top portion 21 are incapable of being prevented and hence, it is preferable that the pad holding start timing be set to a timing when the first punch 3 is located at the 50% position or does not pass through the 50% position.
It is desirable that the mean pressure (pad pressure) applied to the metal sheet 11 when the first punch 3 and the pad 9 hold the metal sheet 11 be set to 3 MPa or higher. Due to such constitution, the occurrence of wrinkles in the top portion 21 can be prevented.
In this manner, the first sheet-formed part 27 is formed. The side-wall stretching portion 29 and the top-portion stretching portion 31 (see
Second Sheet Forming Process
The second sheet forming process is a sheet-forming process in which the first sheet-formed part 27 formed in the first sheet forming process is formed into the second sheet-formed part with the second sheet forming tool 13. First of all, as illustrated in
As mentioned above, in the present embodiment, the metal sheet 11 is sheet-formed into the first sheet-formed part 27 having an intermediate shape in the first sheet forming process. Next, in the second sheet forming process, the first sheet-formed part 27 formed in the first sheet forming process is sheet-formed into the second sheet-formed part having the top portion 21 and the side wall portion 23 each of which has a product shape. In the first sheet-formed part 27, the side wall portion 23 on an outer side that includes the L-shaped bent portion is outwardly stretched in an arcuate shape from the side wall portion 23 of the sheet-formed part 19 as viewed in a plan view. The curvature radius RBm of the circular arc of the side wall portion 23 stretched in an arcuate shape is larger than the curvature radius RBp of the L-shaped bent portion of the sheet-formed part. Furthermore, the outside of the top portion 21 of the L-shaped bent portion is stretched from the outside of the top portion 21 of the sheet-formed part 19. The curvature radius RSm in the cross section of the shoulder portion contiguously formed with the top portion 21 stretched as viewed in a side view is larger than the curvature radius RSp in the cross section of the shoulder portion contiguously formed with the outside of the top portion 21 of the L-shaped bent portion of the sheet-formed part 19 as viewed in a side view. Accordingly, even when the metal sheet composed of a high-strength steel sheet having ultrahigh strength and low ductility is used as a blank material, the sheet-formed part 19 that is an L-shaped part having a hat cross section extending over the entire length of the L-shaped part is precisely and stably formed without the occurrence of cracks in the L-shaped bent portion.
Here, the explanation above has been made with respect to the example such that the pad 9 presses the metal sheet 11 by using the spring 39. However, as in examples described later, the pad 9 may press the metal sheet 11 by using a gas cylinder.
Furthermore, as described above, the explanation has been made with respect to the example such that the second sheet-formed part is trimmed after the second sheet forming process to form the sheet-formed part 19. However, the timing when the second sheet-formed part is trimmed is not limited to this example. For example, the first sheet-formed part 27 may be trimmed after the first sheet forming process, and again the second sheet-formed part is trimmed after the second sheet forming process as finishing.
The specific experiments were performed in order to confirm the manner of operation and advantageous effects by the method of sheet forming of the present invention. In the experiments, in order to manufacture a front pillar reinforcement (hereinafter, referred merely to as “front pillar 51”) of an automotive body that is a part formed in a L-shape illustrated in
Hereinafter, the explanation is made with respect to the front pillar 51 (see
First of all, the front pillar 51 is explained. The front pillar 51 has, as illustrated in
Next, the explanation is made with respect to a sheet forming tool (a first sheet forming tool, and a second sheet forming tool) used for sheet forming. In the first sheet forming process, the first sheet forming tool (a blank holder 61, a first punch 65, a first die 69, and a pad 73) illustrated in
The blank holder 61 is, as illustrated in
In the replaceable portion 67, as illustrated in
The first die 69 has, as illustrated in
In the second sheet forming process, the second sheet forming tool (a second punch 75, and a second die 77) illustrated in
Next, sheet forming conditions in the first sheet forming process are explained in detail. In the first sheet forming process, sheet forming was performed with the following parameters combined variously with each other. That is, the parameters to be combined with each other are a material of a metal sheet 79, an area increasing rate (%), a curvature radius ratio of the side wall portion, a curvature radius ratio of the shoulder portion, a blank holder pressure (MPa), a pad holding area, a pad pressure (MPa), a pad projecting position (%), and a blank-metal-sheet arrangement. Hereinafter, each of the above-mentioned parameters is explained in detail.
As the material of the metal sheet 79, two types of high-strength steel sheets each having ultrahigh strength and low ductility of respective cold rolled steel sheets having 980 MPa-class tensile strength and 1180 MPa-class tensile strength were used. Here, a sheet thickness was set to 1.4 mm.
The area increasing rate means an increasing rate of the area of the stretching portion of the L-shaped bent portion of the first sheet-formed part with respect to the area of an L-shaped bent portion having a product shape that corresponds to the stretching portion. The area increasing rate is an area increasing rate of the stretching portion of the side wall portion on an outer side of the L-shaped bent portion and the top portion in the first sheet-formed part with respect to the product shape, and it is preferable to set the area increasing rate in the range from 0.3% to 1.2%.
The ratio of the curvature radius of the side wall portion means the ratio of a curvature radius RBm to a curvature radius RBp in the side wall portion (see
The ratio of the curvature radius of the shoulder portion means the ratio of a curvature radius RSm to a curvature radius RSp in the shoulder portion (see
The blank holder pressure is a mean pressure in the area of the metal sheet 79 sandwiched between the first die 69 and the blank holder 61, it is preferable to set the blank holder pressure to 0.7 MPa or higher.
The pad holding area was changed by using two types of the pads 73, as illustrated in
The pad projecting position means the position of the pad 73 in a condition that the pad 73 is arranged so that, as illustrated in
The blank-metal-sheet arrangement was set to two patterns, as illustrated in
Table 1 collectively illustrates these sheet forming conditions.
FIG. 24B
None
1.0
1.0
In Table 1, the respective materials having the 980 MPa-class tensile strength and the 1180 MPa-class tensile strength are mentioned as 980 and 1180. The conditions described in the columns of present invention example 1 to present invention example 15 in Table 1; that is, ratio of curvature radius of side wall portion (**), ratio of curvature radius of shoulder portion (***), pad holding area, and blank-metal-sheet arrangement, are set without departing from the gist of the present invention. In the conditions described in the columns of present invention example 1 to present invention example 10, area increasing rate ((%)*), blank holder pressure (MPa), pad pressure (MPa), and pad projecting position (%) are set in the range of preferable conditions. In the conditions described in the columns of comparative example 1 to comparative example 4, the numerical values of specific parameters (see the underlined parameters in Table 1) are set to respective values departing from the gist of present invention.
The first sheet forming process was performed based on each condition in Table 1 to form the first sheet-formed part, and the first sheet-formed part was further sheet-formed in the second sheet forming process to produce the second sheet-formed part. Furthermore, unnecessary portions around the second sheet-formed part were trimmed to manufacture the front pillar 51. Table 2 illustrates the evaluation result of the front pillar 51 manufactured for each sheet forming condition.
Evaluation items are the cracks of the part corresponding to the shoulder portion of the first punch 65 (hereinafter, referred merely to as “shoulder portion”), wrinkles of the top portion 53, and cracks and wrinkles of the flange portion 57 (see
As illustrated in Table 2, in present invention example 1 to present invention example 15, there exists the case that the cracks were slightly observed in the shoulder portion (present invention example 11). However, in the other examples, cracks were not observed in the shoulder portion and the flange portion 57. In the same manner as above, wrinkles were not observed in almost all present-invention examples, and obvious wrinkles were not observed in all present-invention examples. As mentioned heretofore, in all of present invention example 1 to present invention example 15, the overall evaluations were “Good”, and excellent sheet forming was able to be performed.
On the other hand, in comparative example 1 to comparative example 4, the overall evaluations were “No good (poor)”. In comparative example 1, the metal sheet 79 was, as illustrated in
As mentioned above, in the present embodiment in which the present invention is applied, even when the high-strength steel sheet having low ductility and ultrahigh strength was used, the occurrence of cracks was avoided, and the occurrence of wrinkles was prevented thus manufacturing the front pillar 51 stably with sufficient precision.
The above-mentioned embodiment merely constitutes one embodiment of the present invention, and it is evident that the present invention is not limited to the embodiment, various modifications can be made depending on specifications or the like without departing from the gist of the present invention, and the other various embodiments are conceivable without departing from the gist of the present invention.
The present invention is applicable to a method for manufacturing a sheet-formed part that is formed in an L-shape as viewed in a plan view and includes a top portion, side wall portions formed on the respective both sides of the top portion, and the flange portions contiguously formed with the respective side wall portions, by sheet-forming a metal sheet with the use of a sheet forming tool.
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