Patent Description:
Press forming is a method of processing a metal sheet, such as a steel sheet, by clamping the metal sheet with a die of press forming and transferring the shape of the die. In particular, many automotive parts are manufactured by press forming. Nowadays, there has been a strong tendency to use high-strength steel sheets for body parts in view of weight reduction of automotive bodies. However, as the strength as a property of steel sheets and other metal materials increases, elongation tends to decrease, and forming defects such as fractures and wrinkles often occur in the press forming of high-strength steel sheets, causing problems.

Among the frame parts of an automotive body, curved parts having a steeply curved shape, such as front side members and rear side members, tend to have fractures and wrinkles when manufactured by press forming. These members are thus considered to be parts difficult to form. Recently, automobile and parts manufacturers have been studying the application of high-strength steel sheets in the manufacture of such curved parts in order to further reduce the weight of automotive bodies, and an issue is how to perform press forming while preventing fractures and wrinkles.

Several techniques have been developed for press forming of curved parts while suppressing fractures and wrinkles. For example, Patent Literature <NUM> discloses a technique for avoiding wrinkles in a punch bottom and fractures in a flange in press forming of an L-shaped part that is curved in top view, by using a forming load to form the flange and a side wall and sliding the material at the punch bottom.

Patent Literature <NUM> provides a method of preventing out-of-plane deformation and suppressing wrinkles of parts that are curved in the vertical direction by performing drawing forming while applying pressure to the punch bottom of a blank in the thickness direction with a pad.

Patent Literature <NUM> discloses a technique for suppressing the occurrence of wrinkles in a flange portion in press forming of a curved press part with a hat-shaped cross section curved in the longitudinal direction, by preforming a folding portion at an end portion of a blank material in the width direction and then press-forming the curved press part while leaving the folding portion. It is described that, with this technique, the stiffness of the end portion of the blank material in the width direction increases due to the folding portion added to the end portion of the blank material in the preforming process, and the resistance against force of shrinking the blank in the longitudinal direction increases, and thus the occurrence of wrinkles in the flange portion can be suppressed even when the force of shrinking the blank in the longitudinal direction is applied due to excess metal resulted from the curved shape.

In addition, several techniques have been developed to press-form a curved part by adding a bead, with the aim of suppressing the occurrence of fractures and wrinkles. Patent Literature <NUM> discloses a technique for suppressing the occurrence of wrinkles in a material formed section when press-forming, in one process, a material into a shape that has a curvature when an end portion of the material is viewed in plan view and that has a flange surface below a side wall surface in side view, by adding a convex bead to the side wall surface and a concave bead to the flange surface directly below the concave bead. Further, <CIT> discloses a method for producing a press-formed product having a hat-shaped cross section and being curved convexly and concavely curved in the longitudinal direction. The method involves producing an intermediate product with a twisted side wall portion wherein the angle between a side wall portion and a web portion is larger at an end portion than at a center portion in the longitudinal direction.

However, the technique disclosed in Patent Literature <NUM> has limited applicability to parts that have a shape such as a mounted surface at the punch bottom or that have a closed shape such as a bag shape, because the material cannot be moved significantly.

In the technique disclosed in Patent Literature <NUM>, a blank holder and pad are used at the same time for forming, and when a formed product is removed from a die, if the blank holder or pad remains under pressure, it will crush the formed product. Thus, a locking structure is necessary to stop movement. However, since press machines equipped with this mechanism are not common, the technique lacks versatility.

In the technique disclosed in Patent Literature <NUM>, the bending shape of the flange portion needs to be flattened in the next process, but there is a risk that curling may remain. Particularly in the case of automotive parts, a flange is often a surface for joining with another part, and high surface accuracy is required. Thus, care needs to be taken in applying this forming method.

The technique disclosed in Patent Literature <NUM> is press forming in one process, and has the problem that the bead added to prevent wrinkle occurrence and fracturing remains as it is.

The present invention has been made in order to solve the above problems, and an object of the present invention is to provide a press forming method that can press-form, into a favorable shape, a press-formed product that has a hat-shaped cross section with a web portion, side wall portions, and flange portions, and that is convexly curved in the height direction along the longitudinal direction, while suppressing fractures and wrinkles.

To solve the above problems, the invention provides a press forming method as defined in claim <NUM>.

According to the present invention, shear deformation can be generated in the twisted side wall portion, and the occurrence of fractures and wrinkles is suppressed, enabling the press-formed product to be press-formed into a favorable shape.

Before explaining a press forming method according to an embodiment of the present invention, the following explains a press-formed product to be formed in the present invention, the reason that fractures and wrinkles occur when the press-formed product is press-formed, and the background that led to the present invention. In the present embodiment, the height direction of the press-formed product coincides with the press-forming direction of the press-formed product.

As illustrated in <FIG> and <FIG> as an example, a press-formed product <NUM> to be formed in the present invention has a hat-shaped cross section with a web portion <NUM>, side wall portions <NUM> continuous from the web portion <NUM>, and flange portions <NUM> continuous from the respective side wall portions <NUM>, and includes a convex curved portion <NUM> in which the web portion <NUM> and the flange portions <NUM> are convexly curved in the height direction along the longitudinal direction in side view (<FIG>). Straight portions <NUM> extending in a straight shape are provided on both sides of the convex curved portion <NUM> in the longitudinal direction. Here, the convex curved portion <NUM> being convexly curved in the height direction along the longitudinal direction means that the center of the convexly curved arc is located on the flange portion <NUM> side in side view.

<FIG> illustrates the movement of a material during press forming when the press-formed product <NUM> is viewed in side view. In a process of press forming a blank (metal sheet), the blank is bent at a punch corner portion <NUM> between the web portion <NUM> and each of the side wall portions <NUM>, and the material moves in a direction (direction of the arrows in <FIG>) orthogonal to the ridgeline of the punch corner portion <NUM>.

Thus, in the convex curved portion <NUM>, the longitudinal length of the web portion <NUM> becomes longer while the longitudinal length of each of the flange portions <NUM> becomes shorter due to the concentration of the material, resulting in a line length difference in the longitudinal direction between the web portion <NUM> and the flange portion <NUM>. As a result, tensile deformation acts on the web portion <NUM>, causing it to fracture easily, and compressive deformation acts on the flange portion <NUM>, causing it to wrinkle easily.

Therefore, in order to suppress the occurrence of fractures and wrinkles during press forming of the press-formed product <NUM>, it is considered important to change the movement of the material during the press forming process so that tensile and compressive deformations do not occur in the web portion <NUM> or the flange portion <NUM> in the convex curved portion <NUM>, thereby reducing the line length difference in the longitudinal direction between the web portion <NUM> and the flange portion <NUM>.

Thus, consider the ideal state of press forming that does not cause a line length difference in the longitudinal direction between the web portion <NUM> and the flange portion <NUM> in the convex curved portion <NUM>. <FIG> illustrates the movement of the material in the ideal state. In order to avoid a line length difference in the longitudinal direction between the web portion <NUM> and the flange portion <NUM>, as illustrated in <FIG>, it is necessary to generate shear deformation in the blank at a portion corresponding to the side wall portion <NUM> (hereinafter referred to as a "portion corresponding to side wall") to move the material in the same direction as the press-forming direction. However, in press forming, a die basically moves only in the vertical direction, and it is not easy to generate in-plane shear deformation in the material at the portion corresponding to side wall by this limited movement of the die.

The inventors have studied a method to induce in-plane shear deformation in the material. As a result, it has been found that in-plane shear deformation can be generated in the portion corresponding to side wall of the blank by press-forming it into a curved surface shape with an out-of-plane twist along the longitudinal direction. The present invention has been made based on such studies, and the press forming method according to the embodiment of the present invention is described below.

The press forming method according to the present embodiment is to press-form the press-formed product <NUM> illustrated in <FIG> and <FIG> as a target shape, and includes a first forming process to preform a blank into a preformed part <NUM> (<FIG>(a-<NUM>) and <NUM>(a-<NUM>)), and a second forming process to press-form the preformed part <NUM> into the press-formed product <NUM> having the target shape. Note that the blank used for the press forming method according to the present invention is not limited to a steel sheet and may be a sheet made of a plastic material such as an aluminum alloy sheet, a magnesium alloy sheet, a titanium alloy sheet, and a plastic sheet, for example. In addition, the material strength of the blank is not specifically limited.

The first forming process is a process to preform the blank into the preformed part <NUM> (<FIG>(a-<NUM>) and <NUM>(a-<NUM>)).

As illustrated in <FIG>(a-<NUM>), the preformed part <NUM> has a hat-shaped cross section with a portion <NUM> corresponding to web corresponding to the web portion <NUM> of the press-formed product <NUM>, portions <NUM> corresponding to side wall corresponding to the side wall portions <NUM> of the press-formed product <NUM> and including respective twisted side wall portions 25a of a curved surface shape twisted along the longitudinal direction compared with the side wall portion <NUM>, and portions <NUM> corresponding to flange corresponding to the flange portions <NUM> of the press-formed product <NUM>, and includes a portion <NUM> corresponding to convex curve corresponding to the convex curved portion <NUM> of the press-formed product <NUM>, and portions <NUM> corresponding to straight portion corresponding to the straight portions <NUM>.

In the preformed part <NUM>, the twisted side wall portions 25a are formed over the entire length of the portions <NUM> corresponding to side wall in the longitudinal direction. <FIG>(a-<NUM>) illustrates the shapes of the sections of the preformed part <NUM>, the sections being orthogonal to the longitudinal direction of the preformed part <NUM>, at the center of the portion <NUM> corresponding to convex curve in the longitudinal direction (hereinafter referred to as a "convex curve center") and at an end portion of the twisted side wall portion 25a in the longitudinal direction (hereinafter referred to as a "longitudinal end portion"). The shapes of the sections illustrated in <FIG>(a-<NUM>) are depicted with the positions of the portions <NUM> corresponding to flange in the height direction aligned with each other for convenience of explanation.

Each of the twisted side wall portions 25a is twisted such that an angle θ<NUM> at the longitudinal end portion is larger than an angle θ<NUM> at the convex curve center where an angle between the twisted side wall portion 25a and the portion <NUM> corresponding to web is θ, as illustrated in <FIG>(a-<NUM>). With this twisting, the angle θ between the twisted side wall portion 25a and the portion <NUM> corresponding to web changes continuously along the longitudinal direction.

In the present embodiment, the portion <NUM> corresponding to web of the preformed part <NUM> has the same shape as that of the web portion <NUM> (<FIG>) of the press-formed product <NUM>, as illustrated in <FIG>. In contrast, each of the portions <NUM> corresponding to flange of the preformed part <NUM> is continuous with the portion <NUM> corresponding to side wall including the twisted side wall portion 25a, which has a different shape from that of the side wall portion <NUM>, as illustrated in <FIG>. Thus, the portion <NUM> corresponding to flange has a different shape in plan view and side view from that of the flange portion <NUM> of the press-formed product <NUM> (<FIG>), as illustrated in <FIG>.

The angle between the portion <NUM> corresponding to web and the twisted side wall portion 25a of the preformed part <NUM> is larger at the longitudinal end portion (θ<NUM>) than at the convex curve center (θ<NUM>) (refer to <FIG>(a-<NUM>)). As a result, the formed height of the preformed part <NUM> in the height direction is not constant along the longitudinal direction and differs from the formed height of the press-formed product <NUM> in the height direction.

Furthermore, the ridgeline length of a punch corner portion <NUM> (<FIG>) between the portion <NUM> corresponding to web and each of the portions <NUM> corresponding to side wall of the preformed part <NUM> is different from the ridgeline length of the punch corner portion <NUM> (<FIG>) of the press-formed product <NUM>, or the ridgeline length of a die corner portion <NUM> (<FIG>) between the portion <NUM> corresponding to side wall and each of the portions <NUM> corresponding to flange is different from the ridgeline length of a die corner portion <NUM> (<FIG>) of the press-formed product <NUM>.

For example, if the portion <NUM> corresponding to web is formed into the same shape as that of the web portion <NUM> of the press-formed product <NUM> having the target shape, the ridgeline length of the punch corner portion <NUM> is the same as that of the press-formed product <NUM>, but the ridgeline length of the die corner portion <NUM> is different from that of the press-formed product <NUM>. If the portion <NUM> corresponding to flange is formed into the same shape as that of the flange portion <NUM> of the press-formed product <NUM> having the target shape, the ridgeline length of the die corner portion <NUM> is the same as that of the press-formed product <NUM>, but the ridgeline length of the punch corner portion <NUM> is different from that of the press-formed product <NUM>.

The second forming process is a process to press-form the preformed part <NUM> (<FIG>(a-<NUM>) and <NUM>(a-<NUM>)) into the press-formed product <NUM> having the target shape (<FIG>). By the second forming process, the portion <NUM> corresponding to side wall, including the twisted side wall portion 25a having the angle between the twisted side wall portion 25a and the portion <NUM> corresponding to web changing along the longitudinal direction, is formed into the side wall portion <NUM> of the target shape. Furthermore, the portion <NUM> corresponding to flange is formed into the flange portion <NUM> of the target shape.

Next, the following explains the reason that the press forming method according to the present embodiment can press-form a press-formed product that is convexly curved in the height direction along the longitudinal direction in side view, while suppressing fractures and wrinkles.

At the first forming process, as illustrated in <FIG>, each of the twisted side wall portions 25a with a curved surface shape with an out-of-plane twist along the longitudinal direction are formed on the portion <NUM> corresponding to side wall corresponding to the side wall portion <NUM> of the press-formed product <NUM>. When the material (blank) is formed into a curved surface shape with an out-of-plane twist, the material undergoes in-plane shear deformation in addition to out-of-plane shear deformation, as illustrated in <FIG>.

This configuration suppresses the movement of the material toward the center of the portion <NUM> corresponding to flange in the longitudinal direction in the portion <NUM> corresponding to convex curve, and also suppresses the movement of the material toward the end portion side in the longitudinal direction in the portion <NUM> corresponding to web. Thus, in the twisted side wall portion 25a, the line length difference between the line length of the portion <NUM> corresponding to web in the longitudinal direction and the line length of the portion <NUM> corresponding to flange in the longitudinal direction is reduced, as illustrated in <FIG>. As a result, fractures in the web portion <NUM> and wrinkles in the flange portion <NUM> are suppressed in the press-formed product <NUM>, which is obtained by press-forming the preformed part <NUM> into the target shape at the second forming process.

As illustrated in <FIG> described above, the press forming method according to the present invention suppresses fractures in the web portion <NUM> and wrinkles in the flange portion <NUM> of the press-formed product <NUM> (<FIG>) having the target shape, by causing in-plane shear deformation in the twisted side wall portion 25a at the first forming process.

Here, the magnitude of in-plane shear deformation in the twisted side wall portion 25a depends on the degree of torsion of the twisted side wall portion 25a. In the present invention, the degree of torsion of the twisted side wall portion 25a can be expressed using an angle change and an aspect ratio of the twisted side wall portion 25a.

The angle change of the twisted side wall portion 25a is given by the angle difference Δθ between the angle θ<NUM> at the convex curve center (the center of the portion <NUM> corresponding to convex curve in the longitudinal direction) and the angle θ<NUM> at the longitudinal end portion (the end portion of the twisted side wall portion 25a in the longitudinal direction), which are the angles between the twisted side wall portion 25a and the portion <NUM> corresponding to web (refer to <FIG>(a-<NUM>)).

The aspect ratio of the twisted side wall portion 25a is given by a ratio H/L, which is the ratio of a side wall height H to a longitudinal length L of the twisted side wall portion 25a, as illustrated in <FIG>. Here, the side wall height H and the longitudinal length L of the twisted side wall portion are the height in the direction orthogonal to the longitudinal direction and the length in the longitudinal direction, both in the plane of the twisted side wall portion 25a.

The torsion amount T (°) is then given by the following Equation (<NUM>).

It is understood from Equation (<NUM>) that the torsion amount T can be changed by changing (<NUM>) the angle θ<NUM> between the twisted side wall portion 25a and the portion <NUM> corresponding to web at the convex curve center, (<NUM>) the angle θ<NUM> between the twisted side wall portion 25a and the portion <NUM> corresponding to web at the longitudinal end portion, (<NUM>) the side wall height H of the twisted side wall portion 25a, and (<NUM>) the longitudinal length L of the twisted side wall portion 25a.

<FIG> illustrates an example of a preformed part <NUM> in which the height H of the twisted side wall portion 25a is changed, and <FIG> illustrates an example of a preformed part <NUM> in which the longitudinal length L of the twisted side wall portion 25a is changed.

The preformed part <NUM> illustrated in <FIG> and <FIG> described above includes the twisted side wall portion 25a formed over the entire length of the preformed part <NUM> in the longitudinal direction. On the other hand, the preformed part <NUM> illustrated in <FIG> includes twisted side wall portions 65a each having a longitudinal length L shorter than the longitudinal length of the side wall portion <NUM> of the press-formed product <NUM>. In the preformed part <NUM>, the angle θ<NUM> between the twisted side wall portion 65a and a portion <NUM> corresponding to web at an end portion in the longitudinal direction is not an angle at an end portion of the entire preformed part <NUM> in the longitudinal direction, but an angle at an end portion of only the twisted side wall portion 65a in the longitudinal direction.

Regarding the angle between the twisted side wall portion 25a and the portion <NUM> corresponding to web of the preformed part <NUM> press-formed at the first forming process, the angle θ<NUM> at the longitudinal end portion needs to be larger than the angle θ<NUM> at the convex curve center, as described above. For example, if the angle θ<NUM> at the longitudinal end portion is smaller than the angle θ<NUM> at the convex curve center as illustrated in <FIG>, the in-plane shear deformation in the twisted side wall portion 85a will be in the opposite direction to that in the twisted side wall portion 25a illustrated in <FIG>, as illustrated in <FIG>. Thus, when a preformed part <NUM> is press-formed into the press-formed product <NUM> having the target shape, the line length difference in the longitudinal direction between the web portion <NUM> and the flange portion <NUM> cannot be reduced, and the effect of suppressing fractures and wrinkles cannot be obtained.

In addition, the torsion amount T suitable for suppressing fractures and wrinkles was investigated by finite element method (FEM) simulation. As a result, it was found that setting the torsion amount T to be in the range of <NUM>° or larger and <NUM>° or smaller is desirable to suppress both fractures and wrinkles. When the torsion amount T is smaller than <NUM>°, the in-plane shear deformation of the twisted side wall portion 25a may be insufficient. When the torsion amount T is larger than <NUM>°, the twisted side wall portion 25a may undergo excessive shear deformation at the first forming process, resulting in wrinkles by shear deformation in the portion corresponding to side wall.

The aspect ratio H/L of the twisted side wall portion 25a may be given by using the side wall height H at the center of the longitudinal length (a middle position between the convex curve center and the longitudinal end portion) and the longitudinal length L at the center in the side wall height direction, of the twisted side wall portion 25a.

In the above explanation, the preformed part <NUM> (<FIG> and <FIG>), the preformed part <NUM> (<FIG>), and the preformed part <NUM> (<FIG>) each have the portion corresponding to web having the same shape as that of the web portion of the target shape, and have the portions corresponding to flange having a different shape from that of the flange portions <NUM> of the target shape.

However, the present invention may form portions <NUM> corresponding to flange having the same shape as that of the flange portions <NUM> of the target shape (<FIG>), and may form a portion <NUM> corresponding to web having a different shape from that of the web portion <NUM> of the target shape, as in a preformed part <NUM> illustrated in <FIG>.

In the preformed part <NUM> as described above, when the angle θ<NUM> between each of twisted side wall portions 105a and the portion <NUM> corresponding to web at the longitudinal end portion is larger than the angle θ<NUM> between the twisted side wall portion 105a and the portion <NUM> corresponding to web at the convex curve center, the twisted side wall portion 105a formed in each of portions <NUM> corresponding to side wall is press-formed while undergoing in-plane shear deformation as illustrated in <FIG>. Thus, both fractures in the web portion <NUM> and wrinkles in the flange portion <NUM> can be suppressed in the press-formed product <NUM>, which is obtained by press-forming the preformed part <NUM> into the target shape.

However, as illustrated in <FIG>, when the portion <NUM> corresponding to web of the preformed part <NUM> has the same shape as that of the web portion <NUM> of the target shape, press forming can be performed stably without any wobble when the preformed part <NUM> is placed on a punch of the die used at the second forming process. Thus, it is preferable to form the preformed part <NUM> including the portion <NUM> corresponding to web having the same shape as that of the web portion <NUM> of the target shape.

Furthermore, the press-formed product <NUM> to be formed as described above includes the web portion <NUM> and the flange portion <NUM> both being convexly curved in the height direction along the longitudinal direction. However, as illustrated in <FIG>, the present invention may be used to form a press-formed product <NUM> in which a web portion <NUM> alone is convexly curved or a press-formed product <NUM> in which a flange portion <NUM> alone is convexly curved.

The above explanation is for forming a press-formed product such as the press-formed product <NUM> illustrated in <FIG> in which the radius of curvature of the convex curved portion <NUM> is constant in the longitudinal direction. However, the present invention may be used to form a press-formed product that has a plurality of consecutive convex curved portions with different radii of curvature.

In such a case, for each convex curved portion with a constant radius of curvature, the twisted side wall portion in the convex curved portion may have a curved surface shape twisted from the center toward the end portion of the convex curved portion in the longitudinal direction. Then, for each convex curved portion, the angle between the portion corresponding to web and the twisted side wall portion at the longitudinal end portion of the twisted side wall portion of the convex curved portion may be larger than that at the center of the convex curved portion in the longitudinal direction.

In addition, the press-formed product <NUM> to be formed in the present embodiment includes the straight portions <NUM> on both sides of the convex curved portion <NUM> in the longitudinal direction. However, the present invention may be used to form a press-formed product including a straight portion on one side of the convex curved portion in the longitudinal direction or a press-formed product including the convex curved portion alone.

Furthermore, in the press-formed product <NUM> illustrated in <FIG>, the angle between the web portion <NUM> and the side wall portion <NUM> is constant along the longitudinal direction, that is, as illustrated in <FIG>, the angle θ<NUM>,<NUM> at the convex curve center of the press-formed product <NUM> (the center of the convex curved portion <NUM> in the longitudinal direction) and the angle θ<NUM>,<NUM> at the longitudinal end portion (the end portion of the side wall portion <NUM> in the longitudinal direction) are equal. However, the present invention may be used to form a press-formed product in which the angle between the web portion and the side wall portion changes along the longitudinal direction, that is, the side wall portion has a curved surface shape twisted along the longitudinal direction.

In such a case, the angle difference between the angle at the end portion of the twisted side wall portion in the longitudinal direction and the angle at the center of the portion corresponding to convex curve in the longitudinal direction in the preformed part may be made larger than the angle difference between the angle at the center of the convex curved portion in the longitudinal direction (convex curve center) and the angle at the end portion of the side wall portion in the longitudinal direction (longitudinal end portion) in the press-formed product of the target shape, so that the twisted side wall portion of the preformed part may have a curved surface shape more twisted along the longitudinal direction than that of the side wall portion of the target shape.

For example, the angle between the portion corresponding to web and the twisted side wall portion at the convex curve center of the preformed part may be the angle between the web portion and the side wall portion at the convex curve center of the target shape, and the angle between the portion corresponding to web and the twisted side wall portion at the longitudinal end portion of the preformed part may be larger than the angle between the web portion and the side wall portion at the longitudinal end of the target shape.

Even when the twisted side wall portion 25a is formed at the first forming process to produce in-plane shear deformation as described above, the portion <NUM> corresponding to flange may undergo compressive deformation and wrinkles may occur. In such a case, drawing forming is desirable for the first forming process, in which the blank <NUM> is press-formed with its end portions being clamped with blank holders <NUM> and a die <NUM> as illustrated in <FIG>. On the other hand, at the second forming process, the preformed part <NUM> is bent at each punch corner portion <NUM> between the portion <NUM> corresponding to web and the portion <NUM> corresponding to side wall, and the twisted side wall portion of the preformed part <NUM> is formed into the side wall portion of the target shape. Thus, crash forming may be employed for the second forming process, in which the preformed part <NUM> is press-formed by being sandwiched between a die <NUM> and a punch <NUM> as illustrated in <FIG>. However, if wrinkle occurrence is also a concern for a flange portion of a press-formed product <NUM> to be press-formed at the second forming process, drawing forming may be used at the second forming process.

In the drawing forming and the crash forming, a pad (not illustrated) that is paired with a punch <NUM> (<FIG>) or the punch <NUM> (<FIG>) may be inserted on the die <NUM> side (<FIG>) or the die <NUM> side (<FIG>), and a part 201a (refer to <FIG>) in the blank <NUM> corresponding to a portion 203a corresponding to web of the preformed part <NUM> at the first forming process, or the portion 203a corresponding to web (refer to <FIG>) of the preformed part <NUM> at the second forming process, may be pressed down with the pad while being forming.

Specific press forming experiments were conducted on the operation and effect of the press forming method according to the present invention, and are described below.

In the press forming experiments, as illustrated in <FIG>, the press-formed product <NUM> having a hat-shaped cross section with the web portion <NUM>, the side wall portions <NUM>, and the flange portions, and including the convex curved portion <NUM> in which the web portion <NUM> and each of the flange portions <NUM> are convexly curved in the height direction along the longitudinal direction in side view, and the straight portions <NUM> extending on both sides of the convex curved portion <NUM> in the longitudinal direction, was to be formed.

The dimensions of the press-formed product <NUM> were as illustrated in <FIG>: the width of the web portion <NUM> was <NUM>, the side wall height of the side wall portion <NUM> was <NUM>, the width of the flange portion <NUM> was <NUM>, and the angle between the web portion <NUM> and the side wall portion <NUM> was <NUM>°. Furthermore, the longitudinal length was <NUM>, the radius of curvature of the curve in the convex curved portion <NUM> was R150 mm, and the angle θ<NUM> on the acute side of the angle between the web portion <NUM> and the press-forming direction at the portion <NUM> corresponding to straight portion in side view was <NUM>°. The material used for press forming in the experiments was a steel sheet with a thickness of <NUM> and a tensile strength of <NUM> MPa.

The pressing technique used at the first forming process was drawing forming (refer to <FIG>), and the pressing technique used at the second forming process was crash forming (refer to <FIG>). At the first process, a blank holder load was set to <NUM> tonf.

At the first forming process, as illustrated in <FIG>, the preformed part <NUM> is press-formed, in which the portion <NUM> corresponding to web, the portions <NUM> corresponding to side wall including the twisted side wall portions 25a of a twisted shape along the longitudinal direction, and the portions <NUM> corresponding to flange are formed, the preformed part <NUM> including the portion <NUM> corresponding to convex curve. Here, the twisted side wall portion 25a has the longitudinal length L of <NUM> and the side wall height H of <NUM> (refer to <FIG>).

<FIG> illustrates the shapes of sections of the preformed part <NUM>. In the present examples, inventive examples were set to have the angle θ<NUM> between the portion <NUM> corresponding to web and the twisted side wall portion 25a at the longitudinal end portion (<FIG>) is larger than the angle θ<NUM> between the portion <NUM> corresponding to web and the twisted side wall portion 25a at the center of the portion <NUM> corresponding to convex curve in the longitudinal direction (<FIG>). These two angles θ<NUM> and θ<NUM>, the angle difference Δθ (= θ<NUM> - θ<NUM>) were changed to different values, and the preformed part <NUM> was press-formed at the first forming process and press-formed into the press-formed product <NUM> having the target shape at the second forming process. The press formability was then evaluated by the presence of fractures and wrinkles in the press-formed product <NUM>.

For the evaluation of fractures, "×" indicates that fractures are present, "△" indicates that fractures are not present but necking due to thickness reduction is present, and "o" indicates that no fracture or necking is present at all. For the evaluation of wrinkles, "×" indicates that remarkable wrinkles are present, and "△" indicates that minute wrinkles are present, and "o" indicates that no wrinkle is present at all.

In the present examples, conventional examples were set to include an example in which the press-formed product <NUM> is press-formed in one process of crash forming or drawing forming, and an example in which the press-formed product <NUM> is press-formed in two processes of the first forming process and the second forming process, and the portion <NUM> corresponding to side wall of the preformed part <NUM> press-formed at the first forming process does not have a curved surface shape twisted along the longitudinal direction.

Furthermore, comparative examples were set to include an example in which the press-formed product <NUM> is press-formed in two processes of the first forming process and the second forming process, and the angles θ<NUM> and θ<NUM> between the portion <NUM> corresponding to web and the twisted side wall portion 25a of the preformed part <NUM> press-formed at the first forming process are outside the scope of the present invention.

Then, the press-formed products according to the conventional examples and the comparative examples were evaluated for the presence of fractures and wrinkles in the same manner as the inventive examples. Table <NUM> presents the press forming conditions and the evaluation results of press formability.

In Table <NUM>, the convex curve center angle θ<NUM> is the angle between the portion <NUM> corresponding to web and the twisted side wall portion 25a at the center of the portion <NUM> corresponding to convex curve of the preformed part <NUM> in the longitudinal direction (<FIG>), and the longitudinal end portion angle θ<NUM> is the angle between the portion <NUM> corresponding to web and the twisted side wall portion 25a (or the portion <NUM> corresponding to side wall) at the end portion of the twisted side wall portion 25a in the longitudinal direction (<FIG>). The angle difference Δθ, the longitudinal length L, and the side wall height H are given in the same manner as in the embodiment described above, and the torsion amount T is calculated by substituting the angle difference Δθ, the longitudinal length L, and the side wall height H into Equation (<NUM>) described above.

In Table <NUM>, Conventional Examples <NUM> to <NUM>, Comparative Examples <NUM> to <NUM>, and Examples <NUM> to <NUM> are presented by being grouped by condition with the same angle difference Δθ. In Conventional Example <NUM>, the press-formed product <NUM> was press-formed in one process of crash forming. In Conventional Example <NUM>, the press-formed product <NUM> was formed in one process of drawing forming, and the angle between the web portion <NUM> and the side wall portion <NUM> is <NUM>°, which is the target shape.

In Conventional Example <NUM>, occurrence of fractures was not observed in the web portion <NUM>, but wrinkles occurred in the flange portion <NUM>. In Conventional Example <NUM>, occurrence of wrinkles was not observed in the flange portion <NUM>, but fractures occurred in the web portion <NUM>.

In Conventional Examples <NUM> to <NUM>, the angle difference Δθ between the convex curve center angle θ<NUM> and the longitudinal end portion angle θ<NUM> is zero, and thus press-forming of the preformed part <NUM> by giving in-plane shear deformation to the portion <NUM> corresponding to side wall was not possible. Thus, in the press-formed product <NUM> obtained by press-forming the preformed part <NUM> into the target shape, a line length difference in the longitudinal direction was generated between the web portion <NUM> and the flange portion <NUM>, and both fractures in the web portion <NUM> and wrinkles in the flange portion <NUM> could not be suppressed at the same time.

In each of Comparative Examples <NUM> to <NUM>, the longitudinal end portion angle θ<NUM> is smaller than the convex curve center angle θ<NUM>, and the angle difference Δθ is a negative value. Thus, while the angle difference Δθ between the convex curve center angle θ<NUM> and the longitudinal end portion angle θ<NUM> is not zero, and the twisted side wall portion 25a was formed under in-plane shear deformation at the first forming process, the direction of the in-plane shear deformation is opposite to the direction of the shear deformation in the twisted side wall portion according to the present invention (refer to <FIG>), and thus the line length difference in the longitudinal direction between the portion <NUM> corresponding to web and the portion <NUM> corresponding to flange did not decrease. As a result, both fractures in the web portion <NUM> and wrinkles in the flange portion <NUM> could not be suppressed at the same time in the press-formed product <NUM> having the target shape.

In each of Examples <NUM> to <NUM>, the longitudinal end portion angle θ<NUM> is larger than the convex curve center angle θ<NUM>, resulting in the angle difference Δθ being a positive value, and the convex curve center angle θ<NUM>, the longitudinal end portion angle θ<NUM>, and the longitudinal length L and the side wall height H of the twisted side wall portion 25a are changed.

Table <NUM> presents that a press-formed product was able to be press-formed while suppressing both fractures and wrinkles at the same time in all of Examples <NUM> to <NUM> (Δθ = <NUM>°), Inventive Examples <NUM> to <NUM> (Δθ = <NUM>°), Inventive Examples <NUM> and <NUM> (Δθ = <NUM>°), Example <NUM> (Δθ = <NUM>°), Inventive Example <NUM> (θ<NUM>= <NUM>°) in which the convex curve center angle θ<NUM> is smaller than the angle (= <NUM>°) between the web portion <NUM> and the side wall portion <NUM> of the target shape, and Example <NUM> (L = <NUM>) and Inventive Example <NUM> (H = <NUM>) in which the longitudinal length L and the side wall height H of the twisted side wall portion 25a are changed.

It is considered that these results were obtained because, in the preformed part <NUM> according to each of Examples <NUM> to <NUM>, the twisted side wall portion 25a was formed under in-plane shear deformation as illustrated in <FIG> above, and thus the line length difference in the longitudinal direction between the web portion <NUM> and the flange portion <NUM> was reduced. Furthermore, in each of Inventive Examples <NUM> to <NUM> (T = <NUM>°), Inventive Examples <NUM> and <NUM> (T = <NUM>°), and Inventive Example <NUM> (T = <NUM>°) in which the torsion amount T is within the suitable range of the present invention (<NUM>° or larger and <NUM>° or smaller), no occurrence of fractures or wrinkles was observed at all in the press-formed product <NUM> and favorable results were obtained.

In summary, it has been demonstrated that the press forming method according to the present invention enables press forming of a press-formed product having a hat-shaped cross section, the press-formed product being convexly curved in the height direction along the longitudinal direction in side view, while suppressing both fractures and wrinkles.

Claim 1:
A press forming method of forming a press-formed product (<NUM>) having a hat-shaped cross section and including: a web portion (<NUM>); a side wall portion (<NUM>) continuous from the web portion (<NUM>); a flange portion (<NUM>) continuous from the side wall portion (<NUM>); and a convex curved portion (<NUM>) in which the web portion (<NUM>) and/or the flange portion (<NUM>) is convexly curved in a height direction along a longitudinal direction in side view, the press forming method comprising:
a first forming process of press-forming a preformed part (<NUM>) in which a portion corresponding to the web portion (<NUM>), and a portion corresponding to the side wall portion (<NUM>) and including a twisted side wall portion (25a) of a twisted shape along the longitudinal direction, are formed, the preformed part (<NUM>) including a portion corresponding to the convex curved portion (<NUM>); and
a second forming process of press-forming the preformed part (<NUM>) into the press-formed product (<NUM>),
wherein the twisted side wall portion (<NUM>) at the first forming process is twisted such that an angle between the twisted side wall portion (<NUM>) and the portion corresponding to the web portion (<NUM>) is larger on an end portion side than at a center of the portion corresponding to the convex curved portion (<NUM>) in the longitudinal direction,
the press forming method being characterized in that the twisted side wall portion (25a) at the first forming process has a torsion amount T given by a following equation, the torsion amount T being set to be in a range of <NUM>° or larger and <NUM>° or smaller: <MAT> where
Δθ: an angle difference (= θ<NUM> - θ<NUM>),
θ<NUM>: an angle (°) between the twisted side wall portion (25a) and the portion corresponding to the web portion (<NUM>) at the center of the portion corresponding to the convex curved portion (<NUM>) in the longitudinal direction,
θ<NUM>: an angle (°) between the twisted side wall portion (25a) and the portion corresponding to the web portion (<NUM>) at an end portion of the twisted side wall portion (25a) in the longitudinal direction,
H: a side wall height (mm) of the twisted side wall portion (25a), and
L: a longitudinal length (mm) of the twisted side wall portion (25a).