Patent Description:
There is a technique of forming a substantially cylindrical burring processed portion by performing burring processing on a prepared hole provided in a metal component or a metal plate that is a workpiece. In this burring processing, a circumferential portion of the prepared hole is extruded and a part thereof is formed into a cylindrical shape to form the burring processed portion. In the burring processed portion, a cylindrical flange (an upright portion) is connected to a part of the metal component or the metal plate at a circumferential portion thereof via a curved portion. Fatigue characteristics and dimensional accuracy are required for the burring processed portion. For example, Patent Document <NUM> discloses a technique in which a compressive stress is applied to an end portion of a burring processed portion by coining to relax a tensile residual stress, and a compressive stress is locally concentrated on an inner surface of a bend of a curved portion constituting a root of the burring processed portion to inhibit wrinkles or cracks that occur on the inner surface. Also, as a burring processing technique, there is an ironing burring processing method as described in Patent Document <NUM>.

Incidentally, a burring processed portion is also used for undercarriage components of a vehicle. In particular, undercarriage components of a vehicle such as a lower arm and a trailing arm are required to have fatigue characteristics, but with some burring processing methods, a tensile residual stress is likely to occur inside a curved portion of a burring processed portion. When a fatigue load is applied to a component in which a tensile residual stress occurs inside a curved portion of a burring processed portion, the burring processed portion may be deformed. In addition, depending on a burring processing method, minute cracks (in-bend cracks) may occur inside the curved portion, and it may be necessary to change a shape such as increasing the radius of curvature of the curved portion.

The present invention has been made in view of above problems, and an object of the present invention is to provide a method for manufacturing a burring processed product, and a burring processed product in which generation of cracks in a curved portion of a burring processed portion can be inhibited.

The invention is defined in the appended independent claims <NUM> and <NUM>. Preferred embodiments of the invention are defined in the appended dependent claims.

According to the present invention, it is possible to provide a method for manufacturing a burring processed product, and a burring processed product in which generation of cracks in a burring processed portion can be inhibited.

The present inventors have found that, in a forming process of burring processing, unevenness is generated due to compressive strain generated on an inner surface of a curved portion, and thus the above-mentioned in-bend cracks are generated. <FIG> is a schematic cross-sectional view of a burring processed portion for showing a state of compressive strain and in-bend cracks in a curved portion of a burring processed portion. <FIG> shows a cross-section of a burring processed product <NUM> in a plane that passes through an axis cb of a burring processed portion <NUM> and is parallel to the axis cb and shows only one side of the burring processed portion <NUM> centered on the axis cb. As shown in <FIG>, the burring processed portion <NUM> of the burring processed product <NUM> has a curved portion <NUM> and an upright portion <NUM>. In the forming process, compressive strain is generated on an inner surface 12a of the curved portion <NUM> in directions of arrows in the figure, and in-bend cracks CR are generated starting from unevenness caused by the compression strain.

In addition, the present inventors have also found that such unevenness on the surface is likely to occur in a case in which a workpiece and a mold do not come into contact with each other during forming. In particular, in a case in which a radius of curvature of the curved portion is small, generation of the in-bend cracks tend to be remarkable.

Here, as a technique of burring processing, in an ironing burring processing method as described in Patent Document <NUM>, when burring processing is performed, a clearance between a cylindrical punch for increasing a diameter of a prepared hole and a die having a shape corresponding to the punch is made smaller than a plate thickness around the prepared hole. For that reason, forming proceeds while a workpiece and a mold are in good contact with each other. Thus, the present inventors have focused on the fact that in the ironing burring processing method, in-bend cracks are relatively unlikely to be generated.

On the other hand, the present inventors have found that, even if an ironing burring processing method is used, in a case in which a radius of curvature of a curved portion of a burring processed portion is extremely small (about <NUM>) with respect to a plate thickness of a workpiece and the workpiece is a high-strength material, in-bend cracks may be generated, and these in-bend cracks are generated due to a portion at which a material rises on an inner surface of the curved portion in a forming process.

<FIG> shows a schematic cross-sectional view of the burring processed portion for describing a state in which a workpiece rises in the forming process of the burring processing. <FIG> shows a state in which a workpiece M is being sandwiched between a die <NUM> and a holder <NUM> in the forming process of the burring processing, and the workpiece M is being deformed by a punch <NUM> to form the curved portion <NUM>. As illustrated in <FIG>, in a case in which a radius of curvature of a die shoulder <NUM> corresponding to a radius of curvature of the curved portion of the burring processed portion is small, particularly in an initial stage of the forming process, a raised portion BP is generated on the inner surface 12a of the curved portion <NUM> that comes into contact with the die shoulder <NUM>. Since compressive strain is generated in such a raised portion BP, it may cause the in-bend cracks as described above. Thus, the present inventors have sought to investigate a method for inhibiting such rising of the material, which causes the compression strain.

Further, the technique of Patent Document <NUM> is a technique of inhibiting concentration of the compressive stress locally on the inner surface of the bend of the curved portion of the burring processed portion during a coining process. On the other hand, the present invention has focused on inhibiting in-bend cracks in the forming process of the burring processing and is different from the technique of Patent Document <NUM> in a forming method of the burring processed portion.

Although embodiments of the present invention will be described below with reference to examples, it is obvious that the present invention is not limited to the examples described below. In the following description, specific numerical values and materials are exemplary examples, but other numerical values and materials may be applied as long as effects of the present invention can be obtained.

The burring processing method according to the present embodiment is a method of increasing a diameter of a prepared hole provided in a metal component and bending a part of the metal component to form a burring processed portion including an upright portion and a curved portion. In the burring processing method, in a cross-section that passes through an axis of the burring processed portion and is parallel to the axis of the burring processed portion, a cross-sectional shape of the curved portion on an inner surface thereof consists of a curve, and a length of the curved portion in a direction perpendicular to the axis of the burring processed portion is at least <NUM> times a height thereof in the direction parallel to the axis of the burring processed portion.

By using the burring processing method having the above configuration, generation of the in-bend cracks in the curved portion of the burring processed portion can be inhibited.

First, the burring processing method will be described. <FIG> are schematic plan views for describing the forming process of the burring processing and are plan views seen in a direction intersecting a plate surface of a metal component <NUM>. <FIG> shows the metal component <NUM> having the prepared hole <NUM>. <FIG> shows a state in which a circumferential portion of the prepared hole <NUM> is deformed and a diameter of the prepared hole <NUM> is increased. <FIG> shows a burring processed product <NUM> in which the burring processing has been completed. <FIG> are schematic cross-sectional views for respectively describing the forming process of the burring processing in <FIG> and show a cross-section that passes through a center of the prepared hole <NUM>, passes through an axis ca orthogonal to a plate surface of the metal component <NUM> or an axis cb of a formed burring processed portion <NUM>, and is parallel to these axes. Also, in general, the axis ca that passes through the center of the prepared hole <NUM> and is orthogonal to the plate surface of the metal component <NUM> and the axis cb of the burring processed portion <NUM> are aligned with each other. In the present embodiment, the metal component <NUM> will be described as a metal plate.

In the burring processing method, as shown in <FIG> and <FIG>, the diameter of the prepared hole <NUM> provided in the metal component <NUM> is increased, and a part of the metal component <NUM> is bent to form the burring processed portion <NUM> including an upright portion <NUM> and a curved portion <NUM>. There is a circumferential region <NUM> around the curved portion <NUM>.

<FIG> is a diagram for describing the burring processed product according to the present embodiment and is a cross-sectional view of the burring processed product <NUM> in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>. <FIG> shows only one side of the burring processed portion <NUM> centered on the axis cb. As shown in <FIG>, the burring processed portion <NUM> according to the present embodiment includes the cylindrical upright portion <NUM> and the curved portion <NUM>. The upright portion <NUM> is connected to the curved portion <NUM> at a connecting end portion <NUM> on a side opposite to an opening end portion <NUM> of the upright portion <NUM>.

The curved portion <NUM> is connected to the connecting end portion <NUM> of the upright portion <NUM> at a tip portion <NUM> thereof and is connected to a circumferential region <NUM> of the burring processed product <NUM> via a base end portion <NUM> on a side opposite to the tip portion <NUM>. The connecting end portion <NUM> and the tip portion <NUM> may be at the same place. A diameter of the curved portion <NUM> is increased from the tip portion <NUM> toward the base end portion <NUM>. The curved portion <NUM> is smoothly bent in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>. The axis cb of the burring processed portion <NUM> is an axis that passes through an axis of the cylindrical upright portion <NUM> in a length direction thereof.

The circumferential region <NUM> is a region surrounding the curved portion <NUM> in the burring processed product <NUM> and is a region connected to the base end portion <NUM> of the curved portion <NUM>. Although it also depends on a shape of the burring processed product <NUM>, the circumferential region <NUM> preferably has a width of about <NUM> to <NUM> in a radial direction of the burring processed portion <NUM> in a plane orthogonal to the axis cb of the burring processed portion <NUM>.

In the burring processed product <NUM> in which the burring processed portion <NUM> according to the present embodiment is formed, in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>, a cross-sectional shape of the curved portion <NUM> on an inner surface 130a thereof consists of a curve, and a length l of the curved portion <NUM> on the inner surface 130a in a direction perpendicular to the axis cb of the burring processed portion <NUM> is at least <NUM> times a height h thereof in a direction parallel to the axis cb of the burring processed portion <NUM>. With such a configuration, generation of the in-bend cracks on the inner surface of the curved portion can be inhibited. More specifically, by setting the length l at least <NUM> times the height h, cracking depths of the in-bend cracks can be reduced.

In the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>, the length l of the curved portion <NUM> on the inner surface 130a in the direction perpendicular to the axis cb of the burring processed portion <NUM> is preferably at least <NUM> times, and more preferably at least <NUM> times the height h in the direction parallel to the axis cb of the burring processed portion <NUM>. With such a configuration, compressive strain can be reduced, generation of the in-bend cracks on the inner surface 130a of the curved portion <NUM> can be inhibited, and the cracking depths can be significantly reduced.

The length l is a distance from a contact point (an origin O, which will be described later) between an outer circumferential surface 120a of the upright portion <NUM> and the inner surface 130a of the curved portion <NUM> to the base end portion <NUM> of the curved portion <NUM> and is a distance in the direction perpendicular to the axis cb in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>. The length l of the curved portion <NUM> is preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>.

The height h is a distance from the contact point (origin O, which will be described later) between the outer circumferential surface 120a of the upright portion <NUM> and the inner surface 130a of the curved portion <NUM> to the inner surface 130a of the base end portion <NUM> of the curved portion <NUM> and is a distance in the direction parallel to the axis cb in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>. The height h of the curved portion <NUM> is preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>.

In addition, in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>, the length l of the curved portion <NUM> on the inner surface 130a in the direction perpendicular to the axis cb of the burring processed portion <NUM> is preferably at most <NUM> times the height h in the direction parallel to the axis cb of the burring processed portion <NUM>. By setting the length l at most <NUM> times the height h, it is possible to avoid interference with other portions of the burring processed portion and efficiently obtain the effect of inhibiting generation of cracks.

The curve of the cross-sectional shape of the curved portion <NUM> on the inner surface 130a can be defined as follows. As shown in <FIG>, in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>, a direction parallel to the axis cb is defined as an x axis, and a direction perpendicular to the axis cb in the cross-section is defined as a y axis. In addition, the contact point between the outer circumferential surface 120a of the upright portion <NUM> and the inner surface 130a of the curved portion <NUM> is defined as the origin O, and a side on which the curved portion <NUM> is located is defined as a positive region. A curve in xy coordinates can be defined as a curve drawn by the cross-sectional shape of the curved portion <NUM> on the inner surface 130a.

The cross-sectional shape of the inner surface 130a consists of a curve in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM> and does not include a straight line. Here, in a case in which there is a straight line portion in the cross-sectional shape, a value obtained by differentiating x twice (a second derivative) becomes <NUM> in the straight line portion when an expression representing the cross-sectional shape of the inner surface 130a is expressed by y=f(x) in the above xy coordinates.

Further, the cross-sectional shape of the inner surface 130a may be a cross-sectional shape in which the value obtained by differentiating x twice (second derivative) is larger than <NUM> when an expression representing the cross-sectional shape of the inner surface 130a is expressed by y=f(x) in the above xy coordinates. That is, in the above curve drawn by the cross-sectional shape of the inner surface 130a, the value obtained by differentiating the expression representing the curve twice in the above xy coordinates may be larger than <NUM>. The fact that the value obtained by differentiating the expression representing the curve twice with x becomes larger than <NUM> means that an overall shape of the curve is convex toward the axis cb side of the burring processed portion <NUM> in the range of h≥x≥<NUM>.

Further, a part of the above curve drawn by the cross-sectional shape of the inner surface 130a may be a part of a curve represented by a quadratic function or a curve represented by a trigonometric function. By forming a part of the above curve drawn by the cross-sectional shape of the inner surface 130a as a part of the curve represented by a quadratic function or a trigonometric function, generation of the in-bend cracks can be further inhibited. Examples of curves represented by quadratic or trigonometric functions include curves such as ellipses, parabolas, sine curves, and cosine curves.

In a case in which a part of the above curve drawn by the cross-sectional shape of the inner surface 130a is a part of an ellipse, the curve representing the ellipse can be represented by x<NUM>/a<NUM>+(y-b)<NUM>/b<NUM>=<NUM> in the above xy coordinates. Here, a is an axis of the ellipse in the x axis direction, and b is an axis of the ellipse in the y axis direction. The axis a in the x direction of the ellipse may be equal to the height h of the curved portion, and the axis b in the y direction of the ellipse may be equal to the length l of the curved portion, and in that case, this curve is conditioned on satisfying b/a><NUM>.

<FIG> shows an example of a form in which a part of the curve drawn by the cross-sectional shape of the inner surface 130a is a part of an ellipse ov in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>.

When a part of the above curve drawn by the cross-sectional shape of the inner surface 130a is a part of a parabola, the curve representing the parabola can be represented by y=ax<NUM> in the above xy coordinates. In this case, a is conditioned on satisfying a<<NUM>/h.

<FIG> shows an example of a form in which a part of the above curve drawn by the cross-sectional shape of the inner surface 130a is a part of a parabola pb in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>.

In a case in which a part of the above curve drawn by the cross-sectional shape of the inner surface 130a is a part of a sine curve, the curve representing the sine curve can be represented by y=sin(ax-π/<NUM>)+<NUM> in the above xy coordinates. In this case, a is conditioned on satisfying h>sin(ah-π/<NUM>)+<NUM>.

<FIG> shows an example of a form in which a part of the above curve drawn by the cross-sectional shape of the inner surface 130a is a part of a sine curve sn in the cross-section that passes through the axis cb of the burring processed portion <NUM> and is parallel to the axis cb of the burring processed portion <NUM>.

The cross-sectional shape of the inner surface 130a from the origin O serving as a starting point to the base end portion <NUM> of the curved portion <NUM> (the entire region of the curved portion <NUM>) may have a curve represented by a quadratic function or a trigonometric function as described above. Alternatively, the cross-sectional shape of the inner surface 130a may be a shape obtained by combining a curve represented by a quadratic function or trigonometric function starting from the origin O serving as a starting point and a smooth curve that is connected to the above curve and extends to the base end portion <NUM>.

A shape of the burring processed product <NUM> is measured using a contact type shape measuring machine. For example, in the case of measuring the inner surface 130a of the curved portion <NUM> of the burring processed portion <NUM>, shape data is measured and acquired in the direction perpendicular to the axis cb of the burring processed portion <NUM> using a contact type shape measuring machine having a probe diameter of <NUM>. A data collection interval is set to <NUM> pitch in the direction parallel to the axis cb of the burring processed portion <NUM>.

The curve of the cross-sectional shape on the inner surface 130a of the curved portion <NUM> is obtained by approximating the above shape data to the curve using the least squares method. The approximated curve is a quadratic function or trigonometric function as described above. For obtaining the expression of the curve, a function with a coefficient of determination of at least <NUM> is used. Specifically, the shape data measured in the process of obtaining the expression of the curve is approximated to several curves, and among them, one having a coefficient of determination of at least <NUM> and having the largest coefficient of determination is defined as the curve of the cross-sectional shape. Here, in a case in which the coefficient of determination does not exceed <NUM>, the curve may be configured of a plurality of curves. For that reason, an approximating section may be narrowed so that the coefficient of determination is at least <NUM>. In addition, in determining whether or not the cross-sectional shape includes a straight line, in a case in which change rates of inclinations of adjacent straight lines are within plus or minus <NUM>% for <NUM> or more consecutive sections when straight lines determined by the least squares method are sequentially created using five adjacent measurement points, the sections are determined to be a straight line.

Further, in a circumferential direction of the burring processed portion, a range in which the cross-sectional shape of the inner surface of the curved portion has the above curve may be all range in a circumferential direction of the curved portion, or may be a part thereof. For example, such a curved shape may be formed only in a range in which a load is intensely applied when the burring processed product is deformed by applying the load. Also, the length l of the curved portion <NUM> may be at most <NUM> times or at most <NUM> times the height h.

The method for manufacturing a burring processed product according to the present embodiment includes a burring processed portion forming process of increasing a diameter of a prepared hole provided in a metal component and bending a part of the metal component to form a burring processed portion including an upright portion and a curved portion. In the method for manufacturing a burring processed product, in a cross-section that passes through an axis of the burring processed portion and is parallel to the axis of the burring processed portion, a cross-sectional shape of the curved portion on an inner surface thereof consists of a curve, and a length of the curved portion in a direction perpendicular to the axis of the burring processed portion is at least <NUM> times a height thereof in a direction parallel to the axis of the burring processed portion.

By using the method for manufacturing a burring processed product having the above configuration, generation of in-bend cracks in the curved portion of the burring processed portion of the burring processed product can be inhibited.

In the burring processed portion forming process of the method for manufacturing a burring processed product according to the present embodiment, the burring processing method described in the first embodiment can be preferably used. Also, in the method for manufacturing a burring processed product according to the present embodiment, since each configuration of the first embodiment can be adopted, descriptions thereof will be omitted.

In the method for manufacturing a burring processed product according to the present embodiment, before the burring processed portion forming process, a prepared hole forming process for forming the prepared hole in the metal component may be further provided.

Next, an embodiment of the burring processing method using a mold will be described.

A burring processing method according to the present embodiment is a method for forming a burring processed portion on a metal component provided with a prepared hole using a mold including a die having a die hole, a holder, and a punch. In the burring processing method, the metal component is sandwiched between the die and the holder, and the punch is inserted into the die hole along an axis of the die hole to increase a diameter of the prepared hole and to bend a part of the metal component, thereby forming the burring processed portion including an upright portion and a curved portion. In the burring processing method, in a cross-section that passes through the axis of the die hole and is parallel to the axis of the die hole, a cross-sectional shape of a die shoulder consists of a curve, and a length thereof in a direction perpendicular to the axis of the die hole is at least <NUM> times a height thereof in a direction parallel to the axis of the die hole.

By using the burring processing method having the above configuration, generation of in-bend cracks in the curved portion of the burring processed portion.

<FIG> is a schematic cross-sectional view for describing the mold used in the burring processing method of the present embodiment. This cross-sectional view is a cross-sectional view that passes through the axis of the die hole and is parallel to the axis of the die hole. As shown in <FIG>, a mold <NUM> includes a die <NUM>, a holder <NUM>, and a punch <NUM>.

The die <NUM> has a die hole <NUM> having a predetermined depth. The die hole <NUM> has a substantially cylindrical inner circumferential surface shape, and a cylindrical inner circumferential surface <NUM> of the die hole <NUM> and a die pressing surface <NUM> are connected to each other via a die shoulder <NUM>. The die <NUM> may be movable relative to the holder <NUM> by a drive mechanism (not shown).

A surface 1120a of the die shoulder <NUM> is connected to the cylindrical inner circumferential surface <NUM> at a tip portion <NUM> and is connected to the die pressing surface <NUM> via a base end portion <NUM> on a side opposite to the tip portion <NUM>. A diameter of the surface 1120a of the die shoulder <NUM> is increased from the tip portion <NUM> toward the base end portion <NUM>. The surface 1120a of the die shoulder <NUM> is smoothly curved in a cross-section that passes through an axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>. A cross-sectional shape of the die shoulder <NUM> means a shape of the surface 1120a of the die shoulder <NUM> in the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>. The axis cd of the die hole <NUM> is an axis that passes through an axis line of the cylindrical inner circumferential surface <NUM> in a height direction thereof.

The holder <NUM> has a holder pressing surface <NUM>, and a workpiece is sandwiched between the holder pressing surface <NUM> and the die pressing surface <NUM>. The holder <NUM> has a holder hole <NUM> corresponding to an outer circumferential shape of the punch <NUM>, and the punch <NUM> is disposed to be movable relative to the holder <NUM> along an axis of the holder hole <NUM>. The die <NUM> and the holder <NUM> may be disposed such that the axis of the die hole <NUM> and the axis of the holder hole <NUM> coincide with each other. The holder <NUM> may be movable relative to the die <NUM> by a drive mechanism (not shown).

The punch <NUM> has a substantially cylindrical shape, and has a side circumferential surface <NUM> and a punch surface <NUM>. The side circumferential surface <NUM> and the punch surface <NUM> are connected to each other via a ridge line portion <NUM>. The ridge line portion <NUM> of the punch <NUM> may have a radius of curvature (a punch shoulder R) of, for example, <NUM> to <NUM> not to cause local contact with the workpiece in a cross-sectional view orthogonal to an extending direction of the ridge line portion <NUM> at each position along the extending direction of the ridge line portion <NUM>.

The punch <NUM> is movable relative to the die <NUM> and the holder <NUM> by a drive mechanism <NUM>. Further, the punch <NUM> is movable along the axis of the die hole <NUM> inside the die hole <NUM>.

In order to make a plate thickness of the upright portion of the burring processed portion uniform in the circumferential direction, an axis of the punch <NUM> and the axis of the die hole <NUM> are preferably disposed to coincide with each other.

<FIG> shows a state in which a workpiece <NUM> (metal component) is sandwiched between the die <NUM> and the holder <NUM>. In this state, as shown in <FIG>, the punch <NUM> may be housed inside the holder hole <NUM> of the holder <NUM>.

<FIG> shows a state in which the punch <NUM> is moved relative to the die <NUM> in a direction from the holder <NUM> to the die <NUM> along the axis of the die hole <NUM>. As shown in <FIG>, the workpiece <NUM> is pressed into the die hole <NUM> while being deformed by the punch <NUM>. Further, a diameter of the prepared hole <NUM> of the workpiece <NUM> is increased due to this deformation.

<FIG> shows a state in which the punch <NUM> reaches a predetermined position inside the die hole <NUM>, thereby completing burring processing.

After the state shown in <FIG>, the punch <NUM> is pulled out from the die hole <NUM>, and the die <NUM> and the holder <NUM> are moved relative to each other in a direction in which they are separated from each other.

<FIG> is a diagram for describing the mold according to the present embodiment and is a cross-sectional view in the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>.

In the mold <NUM> according to the present embodiment, in the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>, the cross-sectional shape of the die shoulder <NUM> consists of a curve, and a length ld of the die shoulder <NUM> in the direction perpendicular to the axis cd of the die hole <NUM> is at least <NUM> times a height hd thereof in the direction parallel to the axis cd of the die hole <NUM>.

With such a configuration, a contact area between the workpiece and the die in the forming process can be increased, and thus swelling generated on the inner surface of the curved portion as described above can be inhibited. Further, with such a configuration, a contact between the workpiece and the mold at a high surface pressure can be prevented. For that reason, generation of in-bend cracks in the curved portion of the burring processed portion of the burring processed product can be inhibited. More specifically, by setting the length ld at least <NUM> times the height hd, cracking depths of the in-bend cracks can be reduced.

In the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>, the length ld of the die shoulder <NUM> in the direction perpendicular to the axis cd of the die hole <NUM> is more preferably at least <NUM> times, and further preferably at least <NUM> times the height hd in the direction parallel to the axis cd of the die hole <NUM>. With such a configuration, compression strain can be reduced, the generation of in-bend cracks on the inner surface of the curved portion of the burring processed portion of the burring processed product can be inhibited, and the cracking depths can be significantly reduced.

The length ld is a distance from the tip portion <NUM>, which is the contact point between the cylindrical inner circumferential surface <NUM> and the surface 1120a of the die shoulder <NUM>, to the base end portion <NUM> of the die shoulder <NUM> and is a distance in the direction perpendicular to the axis cd in the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>. The length ld of the die shoulder <NUM> is preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>.

The height hd is a distance from the tip portion <NUM>, which is the contact point between the cylindrical inner circumferential surface <NUM> and the surface 1120a of the die shoulder <NUM>, to the base end portion <NUM> of the die shoulder <NUM> and is a distance in the direction parallel to the axis cd in the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>. The height hd of the die shoulder <NUM> is preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>.

Further, in the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>, the length ld of the die shoulder <NUM> in the direction perpendicular to the axis cd of the die hole <NUM> is more preferable at most <NUM> times the height hd in the direction parallel to the axis cd of the die hole <NUM>. By setting the length ld at most <NUM> times the height hd, it is possible to avoid interference with other portions of the burring processed portion of the burring processed product and efficiently obtain the effect of inhibiting generation of the in-bend cracks.

A curve of the surface 1120a of the die shoulder <NUM> can be defined as follows. As shown in <FIG>, in the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM>, a direction parallel to the axis cd is defined as the x axis, and a direction perpendicular to the axis cd in the cross-section is defined as the y axis. In addition, a contact point between the cylindrical inner circumferential surface <NUM> and the surface 1120a of the die shoulder <NUM> is defined as the origin O, and a side on which the die shoulder <NUM> is located is defined as the positive region. A curve in the xy coordinates can be the curve drawn by the cross-sectional shape of the die shoulder <NUM>.

The cross-sectional shape of the die shoulder <NUM> consists of a curve in the cross-section that passes through the axis cd of the die hole <NUM> and is parallel to the axis cd of the die hole <NUM> and does not include a straight line. Here, in a case in which there is a straight line, a value obtained by differentiating x twice (second derivative) becomes <NUM> in a straight line portion when an expression representing the cross-sectional shape of the die shoulder <NUM> is expressed by y=f(x) in the above xy coordinates.

Further, the cross-sectional shape of the die shoulder <NUM> may be a cross-sectional shape in which the value obtained by differentiating x twice (second derivative) is larger than <NUM> when an expression representing the cross-sectional shape of the die shoulder <NUM> is expressed by y=f(x) in the above xy coordinates. That is, in the above curve drawn by the cross-sectional shape of the die shoulder <NUM> (the shape of the surface 1120a), the value obtained by differentiating the expression representing the curve twice in the above xy coordinates may be larger than <NUM>. The fact that the value obtained by differentiating the expression representing the curve twice with x becomes larger than <NUM> means that an overall shape of the curve is convex toward the axis cb side of the die hole <NUM> in the range of hd≥x≥<NUM>.

A part of the above curve drawn by the cross-sectional shape of the die shoulder <NUM> may be a part of a curve represented by a quadratic function or a curve represented by a trigonometric function. By forming a part of the above curve drawn by the cross-sectional shape of the die shoulder <NUM> as a part of the curve represented by a quadratic function or a trigonometric function, generation of the in-bend cracks can be further inhibited. Examples of curves represented by quadratic or trigonometric functions include curves such as ellipses, parabolas, sine curves, and cosine curves.

The definition and conditions of the curve represented by a quadratic function or a trigonometric function are the same as those in the first embodiment described above, but the length l of the curved portion <NUM> is replaced with the length ld of the die shoulder <NUM>, and the height h of the curved portion <NUM> is replaced with the height hd of the die shoulder <NUM>.

The cross-sectional shape of the die shoulder <NUM> may have a curve represented by a quadratic function or a trigonometric function over the entire region of the die shoulder <NUM> starting from the origin O. Alternatively, the cross-sectional shape of the die shoulder <NUM> may be a combination of a curve represented by a quadratic function or trigonometric function starting from the origin O and a smooth curve connected to the above curve and extending to the base end portion <NUM>.

A shape of the mold such as the die <NUM> is measured using a contact type shape measuring machine. For example, in the case of measuring the die shoulder <NUM>, shape data is measured and acquired in the direction perpendicular to the axis cd of the die hole <NUM> using a contact type shape measuring machine having a probe diameter of <NUM>. A data collection interval is set to <NUM> pitch in the direction parallel to the axis cd of the die hole <NUM>. Further, the curve of the cross-sectional shape of the die shoulder <NUM> is obtained by approximating the above shape data to the curve using the least squares method. The approximated curve is a quadratic function or trigonometric function as described above. For obtaining the expression of the curve, a function with a coefficient of determination of at least <NUM> is used. Specifically, the shape data measured in the process of obtaining the expression of the curve is approximated to several curves, and among them, one having a coefficient of determination of at least <NUM> and having the largest coefficient of determination is defined as the curve of the cross-sectional shape. Here, in a case in which the coefficient of determination does not exceed <NUM>, the curve may be configured of a plurality of curves. For that reason, an approximating section may be narrowed so that the coefficient of determination is at least <NUM>. In addition, in determining whether or not the cross-sectional shape includes a straight line, in a case in which change rates of inclinations of adjacent straight lines are within plus or minus <NUM>% for <NUM> or more consecutive sections when straight lines determined by the least squares method are sequentially created using five adjacent measurement points, the sections are determined to be a straight line.

In the mold <NUM> according to the present embodiment, the punch <NUM> has a cylindrical portion having a diameter smaller than a diameter of the die hole <NUM>, the punch <NUM> can be inserted into the die hole <NUM> along the axis of the die hole <NUM>, and when a clearance between the die hole <NUM> and the cylindrical portion (the cylindrical inner circumferential surface <NUM> of the die hole <NUM> and the side circumferential surface <NUM> of the punch <NUM>) is defined as cl, and a height of the cross-sectional shape of the die shoulder <NUM> in the direction parallel to the axis of the die hole <NUM> is defined as hd, the following Expression <NUM> may be satisfied.

More specifically, as shown in <FIG>, the clearance cl between the die hole <NUM> and the cylindrical portion is a difference between a radius rd of the die hole <NUM> and a radius pd of the punch <NUM>. In the example of <FIG>, since the axis of the punch <NUM> is located on the axis cd of the die hole <NUM>, the clearance cl can be expressed by cl=rq-pd. The cylindrical portion may be the entire punch <NUM> or a part of the punch <NUM> including the side circumferential surface <NUM>.

Also, in a circumferential direction of the die shoulder, the range in which the cross-sectional shape of the die shoulder has the above curve may be the entire circumferential direction of the die shoulder, or may be a part thereof. For example, in the case of the burring processed product, only a range of the die shoulder corresponding to the range in which the load is intensely applied may be such a curved shape. The length ld of the die shoulder <NUM> may be at most <NUM> times or at most <NUM> times the height hd.

The mold of the present embodiment (burring processing mold) may be provided as a part of a burring processing device further including a drive mechanism that can move the die, the holder, and the punch relative to each other.

A method for manufacturing a burring processed product according to the present embodiment includes a burring processed portion forming process of forming a burring processed portion on a metal component provided with a prepared hole using a mold including a die having a die hole, a holder, and a punch. In the burring processed portion forming process of the method for manufacturing a burring processed product, a metal component is sandwiched between the die and the holder, and the punch is inserted into the die hole along an axis of the die hole to increase a diameter of the prepared hole and bend a part of the metal component, thereby forming the burring processed portion including an upright portion and a curved portion. In the method for manufacturing the burring processed product, in a cross-section that passes through the axis of the die hole and is parallel to the axis of the die hole, a cross-sectional shape of a die shoulder consists of a curve, and a length thereof in a direction perpendicular to the axis of the die hole is at least <NUM> times a height thereof in a direction parallel to the axis of the die hole. By using the method for manufacturing a burring processed product having the above configuration, generation of in-bend cracks in the curved portion of the burring processed portion of the burring processed product can be inhibited.

In the burring processed portion forming process of the method for manufacturing a burring processed product according to the present embodiment, a burring processing method described in the third embodiment can be preferably used. Also, in the method for manufacturing a burring processed product according to the present embodiment, since each configuration of the third embodiment can be adopted, descriptions thereof will be omitted.

In the method for manufacturing a burring processed product according to the present embodiment, before the burring processed portion forming process, a prepared hole forming process for forming the prepared hole in the metal component may be further provided. In the prepared hole forming process, the prepared hole is formed in the metal component or a metal plate, which is a workpiece, by punching, cutting with a tool, laser cutting, or the like.

In the burring processing method or the method for manufacturing a burring processed product according to the above embodiment, when a plate thickness of the metal component around the prepared hole is defined as t, and a height of the curved portion in a direction parallel to an axis of the burring processed portion is defined as h, the following Expression <NUM> may be satisfied.

In the burring processing method or the method for manufacturing a burring processed product according to the above embodiment, since generation of in-bend cracks inside the curved portion of the burring processed product can be inhibited, a height h of the curved portion <NUM> can be designed to be small. Conventionally, in a range in which the height h is smaller than the plate thickness t as in Expression <NUM>, excessive compressive strain is generated inside the curved portion during forming, but in the burring processing method or the method for manufacturing a burring processed product according to the above embodiment, by inhibiting generation of such compression strain, generation of in-bend cracks can be inhibited.

Further, by satisfying Expression <NUM>, there is an advantage that when used as a burring processed product, a contact area between an inner circumferential surface of the upright portion of the burring processed portion and a member inserted into the burring processed portion is increased. In addition, by satisfying Expression <NUM>, the center of gravity of the burring processed portion can be lowered, and stability of the member inserted into the burring processed portion can be ensured. For this reason, for example, in a case in which the burring processed product is used as an undercarriage component of a vehicle, riding comfort can be improved.

The height h of the curved portion may be the same as the height hd of the die shoulder.

The plate thickness t around the prepared hole is a plate thickness at an edge portion of the prepared hole (an edge portion 2a of the prepared hole <NUM> in <FIG> and <FIG>, and the like).

The burring processing method or the method for manufacturing a burring processed product according to the above embodiment may be an ironing burring processing method. In an ironing burring processing method, a clearance between a die and a punch is made smaller than a plate thickness of a workpiece, and the plate thickness of an upright portion is made smaller than the plate thickness of the workpiece.

In the burring processing method or the manufacturing method of the burring processed product according to the above embodiment, when the plate thickness around the prepared hole of the metal component is defined as t, and the plate thickness at an opening side end portion of the upright portion is defined as tb, the following Expression <NUM> may be satisfied.

By satisfying Expression <NUM>, the in-bend cracks generated in the curved portion <NUM> of the burring processed portion <NUM> can be further inhibited. This is because the workpiece <NUM> is easily formed while being in contact with the die shoulder <NUM> during forming, and buckling of a surface to the outside of the surface due to compressive strain that causes the in-bend cracks is inhibited.

Further, in a case in which the technique of Patent Document <NUM> is applied to an ironing burring processing method, since the plate thickness is locally reduced and there is a curved portion having a small radius of curvature, a problem due to stress concentration may occur.

In the burring processing method or the method for manufacturing a burring processed product according to the above embodiment, the metal component may be a metal plate. Also, the metal components may be plated or painted.

In addition, in the burring processing method or the method for manufacturing a burring processed product according to the above embodiment, the plate thickness around the prepared hole is preferably at least <NUM>. Also, in forming the burring processed portion having a plate thickness of at least <NUM>, since its rigidity is high, there is virtually no springback, and thus a shape of the die shoulder corresponds to a shape of a portion formed in contact with the die shoulder in the product (burring processed product) and the shape of the die shoulder may be regarded as being reflected in the product as it is.

In particular, the burring processing method or the method for manufacturing a burring product according to the above embodiment can be preferably used for a steel member having a tensile strength of at least <NUM> MPa. Also, the burring processing method or the method for manufacturing a burring product according to the above embodiment can be preferably used for a steel member having a tensile strength of at least <NUM> MPa. Also, the burring processing method or the method for manufacturing a burring product according to the above embodiment can be preferably used for a steel member having a tensile strength of at least <NUM> MPa.

A burring processed product according to the present embodiment has a burring processed portion including an upright portion and a curved portion, and in a cross-section that passes through an axis of the burring processed portion and is parallel to the axis of the burring processed portion, a cross-sectional shape of the curved portion on an inner surface thereof consists of a curve, and a length of the curved portion in a direction perpendicular to the axis of the burring processed portion is at least <NUM> times a height thereof in a direction parallel to the axis of the burring processed portion.

In the burring processed product having the above configuration, generation of cracks in the burring processed portion can be inhibited.

Since the burring processed product according to the present embodiment can adopt each configuration of the burring processed product described in the first embodiment, detailed descriptions thereof will be omitted.

The burring processed product according to the present embodiment may satisfy the following Expression <NUM> when a plate thickness around the curved portion is defined as ts, and a height of the curved portion in a direction parallel to the axis of the burring processed portion is defined as h.

The burring processed product according to the present embodiment may satisfy the following Expression <NUM> when the plate thickness around the curved portion is ts, and a plate thickness at an opening side end portion of the upright portion is defined as tb.

As the plate thickness ts around the curved portion, a plate thickness at the base end portion <NUM> of the curved portion <NUM> as shown in <FIG> may be adopted.

The burring processed product according to the present embodiment may be any of a lower arm, a trailing arm, and an upper arm used in a vehicle.

Since the burring processed product according to the present embodiment can inhibit the generation of cracks in the burring processed portion, it can be preferably adopted for a lower arm, a trailing arm, an upper arm, and the like, which are components having the burring processed portion.

The burring processed product according to the present embodiment is a lower arm used for a vehicle and may further have a cylindrical portion and a cylindrical flange portion. In the lower arm, in a plane that is perpendicular to the axis of the burring processed portion and passes through a center C in a length direction of the cylindrical portion on an axis of the cylindrical portion, when an intersection between the axis of the burring processed portion and the plane is defined as an intersection A, an intersection between an axis of the cylindrical flange portion and the plane is defined as an intersection B, and a smaller one of angles between a line segment AB connecting the intersection A to the intersection B and a line segment AC connecting the intersection A to the center C is defined as θ, and in the range of <NUM> from the line segment AB to a side on which the center C is located, and in the range of <NUM>θ from the line segment AB to a side opposite to the side on which the center C is located, with the intersection A set as a center, in the cross-section that passes through the axis of the burring processed portion and is parallel to the axis of the burring processed portion, the cross-sectional shape of the curved portion on the inner surface consists of a curve, and the length of the curved portion in the direction perpendicular to the axis of the burring processed portion is at least <NUM> times the height in the direction parallel to the axis of the burring processed portion.

<FIG> shows an example of a lower arm according to the present embodiment. A lower arm <NUM> includes a burring processed portion <NUM>, a cylindrical portion <NUM>, a cylindrical flange portion <NUM>, a first element <NUM> connecting the burring processed portion <NUM> to the cylindrical portion <NUM>, and a second element <NUM> connecting the first element <NUM> to the cylindrical flange portion <NUM>. Also, depending on a shape of the lower arm <NUM>, it may include the burring processed portion <NUM>, the cylindrical portion <NUM>, the cylindrical flange portion <NUM>, the second element <NUM> connecting the burring processed portion <NUM> to the cylindrical flange portion <NUM>, and the first element <NUM> connecting the second element <NUM> to the cylindrical portion <NUM>.

In a part of a circumferential direction of the burring processed portion <NUM>, in the cross-section that passes through the axis of the burring processed portion <NUM> and is parallel to the axis of the burring processed portion <NUM>, the cross-sectional shape of the curved portion on the inner surface consists of a curve, and the length of the curved portion in the direction perpendicular to the axis of the burring processed portion <NUM> is at least <NUM> times the height in the direction parallel to the axis of the burring processed portion <NUM>. The length of the curved portion in the direction perpendicular to the axis of the burring processed portion <NUM> is more preferably at least <NUM> times, and further preferably at least <NUM> times the height in the direction parallel to the axis of the burring processed portion <NUM>. Such a range in the circumferential direction of the burring processed portion <NUM> is referred to as a curve forming range <NUM>.

Since basic configurations of the burring processed portion <NUM> according to the present embodiment in the curve forming range <NUM> is the same as that of the burring processed portion of the first embodiment, detailed descriptions thereof will be omitted. Next, the curve forming range <NUM> in the circumferential direction of the burring processed portion <NUM> will be described.

First, a plane that is perpendicular to the axis of the burring processed portion <NUM> (not shown) and passes through the center C in the length direction of the cylindrical portion <NUM> on the axis cc of the cylindrical portion <NUM> is defined. For example, the paper surface of <FIG> is parallel to this plane. Further, the axis of the burring processed portion <NUM> is orthogonal to the paper surface of <FIG>.

The intersection between the axis of the burring processed portion <NUM> and the plane is defined as the intersection A, and the intersection between the axis of the cylindrical flange portion and the plane is defined as the intersection B. Further, a smaller one of angles between the line segment AB connecting the intersection A to the intersection B and the line segment AC connecting the intersection A to the center C is defined as <NUM>.

In this case, the curve forming range <NUM> having the above curve is provided in the range of <NUM> from the line segment AB to the side on which the center C is located and the range of <NUM> from the line segment AB to the side opposite to the side on which the center C is located, with the intersection A as a center. With such a configuration, deformation characteristics in the range in which a load is particularly applied in the circumferential direction of the burring processed portion <NUM> can be improved.

In the circumferential direction of the burring processed portion <NUM>, the curved portion in a range other than the curve forming range <NUM> may have a surface shape other than the above curve. For example, the curved portion in the range other than the curve forming range <NUM> may be a curved portion having the same height as the curved portion in the curve forming range <NUM> and shorter than the length of the curved portion in the curve forming range <NUM>.

It is more preferable that the curve forming range <NUM> be a range of θ from the line segment AB to the side on which the center C is located and a range of 2θ from the line segment AB to the side opposite to the side on which the center C is located, with the intersection A as the center.

<FIG> shows another example of the lower arm according to the present embodiment. In this example, the axis of the cylindrical portion <NUM> is located in a direction intersecting the paper surface of <FIG>. As in the example of <FIG>, an orientation of the axis cc of the cylindrical portion <NUM> may be appropriately changed depending on a structure of a vehicle on which the lower arm is mounted.

The burring processed product according to the present embodiment has no place in which the radius of curvature of the curved portion suddenly changes, so that a risk of stress concentration can be inhibited.

In the above embodiment, the height of the upright portion of the burring processed portion is more preferably <NUM> to <NUM>.

The burring processed product according to the present embodiment is not applied to a "structure in which it forms a closed cross-sectional shape by being superposed with another member (for example, a paired burring processed product)", but is preferably applied to a single-sheet structure basically configured of a single member. In a case in which such a burring processed product having a single-sheet structure is particularly adopted as an undercarriage component of a vehicle, a contact area between an inner circumferential surface of the upright portion of the burring processed portion and a member inserted into the burring processed portion can be increased, stability of the member inserted in the burring processed portion can be ensured, and riding comfort can be improved.

A prepared hole with a diameter of <NUM> was provided in a steel member having a tensile strength of <NUM> MPa and a plate thickness of <NUM>, and the prepared hole was subjected to burring processing with each mold under the following conditions.

In Comparative Example <NUM>, burring processing was performed using each mold in which the height hd of the die shoulder was set to values shown in Table <NUM>. A cross-sectional shape of the die shoulder was formed as a part of a circle having a constant radius of curvature. For that reason, a die used in Comparative Example <NUM> has the same height hd and length ld in the cross-sectional shape of the die shoulder. In the following examples, the cross-sectional shape of the die shoulder is a shape of a surface of the die shoulder in the cross-section that passes through the axis of the die hole and is parallel to the axis of the die hole, as described in the above embodiments.

Table <NUM> shows results of presence or absence of generation of mold galling and presence or absence of generation of in-bend cracks in the case of the height hd of the die shoulder. The presence or absence of mold galling is visually determined, and a case in which there is no adhesion of the steel member to the mold (or the mold to the steel member) was designated as "∘ (good)," and a case in which the adhesion is observed was designated as "× (bad)". Mold galling is adhesion between a material and a mold and is a phenomenon in which a part of a surface of a material sticks to a mold. Mold galling occurs when a material and a mold slide under high surface pressure, and when mold galling occurs, a product is scratched, resulting in poor quality. The presence or absence of in-bend cracks was determined by polishing a cross-section of a cut sample and observing it with an optical microscope. A case in which the maximum crack is at most <NUM> was designated as "∘ (good)," and a case in which the maximum crack exceeds <NUM> was designated as "× (bad)".

Similarly to Comparative Example <NUM>, a prepared hole having a diameter of <NUM> was provided in a steel member having a tensile strength of <NUM> MPa and a plate thickness of <NUM>, and the prepared hole was subjected to burring processing with each mold under the following conditions.

In Comparative Example <NUM>, burring processing was performed with each mold in which the height hd of the die shoulder and the length ld of the die shoulder were set to values shown in Table <NUM>. In Comparative Example <NUM>, a cross-sectional shape of the die shoulder is a shape having a curved portion having a radius of curvature of <NUM> at a tip portion and a base end portion of the die shoulder and a straight portion between them, which is different from Comparative Example <NUM>.

Table <NUM> shows results of presence or absence of generation of mold galling and presence or absence of generation of in-bend cracks in the case of the height hd of the die shoulder and the length ld of the die shoulder. Evaluation criteria in Table <NUM> are the same as in Comparative Example <NUM>.

In Example <NUM>, burring processing was performed with each mold in which the height hd of the die shoulder and the length ld of the die shoulder were set to values shown in Table <NUM>. Example <NUM> is different from Comparative Example <NUM> and Comparative Example <NUM> in that the cross-sectional shape of the die shoulder is a shape including a part of an ellipse.

In the burring processing method according to the present invention, from the results of Experimental Example <NUM>, it has been understood that, even in the case of forming a burring processed product in which the height (height h) of the curved portion of the burring processed portion is small using a mold in which the height hd of the die shoulder is small, generation of mold galling and in-bend cracks is inhibited. On the other hand, in the conventional burring processing method as shown in Comparative Example <NUM>, in a case in which the height hd of the die shoulder is extremely small, mold galling and in-bend cracks are generated. In addition, it has been understood that, in a case in which the ironing burring processing is performed using a die having a plurality of bend portions and straight portions in the cross-sectional shape of the die shoulder as shown in Comparative Example <NUM>, when the height hd of the die shoulder becomes smaller, mold galling and in-bend cracks tend to be generated.

In the present experimental example, for each of the steel members having tensile strengths of <NUM> MPa, <NUM> MPa, and <NUM> MPa, the length ld of the die shoulder of the mold used for forming was changed, and depths of cracks generated in the burring processed portion in each steel member was measured.

A plate thicknesses of the steel member was <NUM>, and a prepared hole with a diameter of <NUM> was provided in the steel member, and the prepared hole was subjected to burring processing with each mold under the following conditions.

In the present experimental example, the height hd of the die shoulder was fixed to <NUM>, and the length ld of the die shoulder was changed from <NUM> to <NUM>. Also, the cross-sectional shape of the die shoulder was designed to be a part of the ellipse, the radius of the ellipse in a minor axis direction thereof was formed to correspond to the height hd of the die shoulder, and the radius of the ellipse in a major axis direction thereof was formed to correspond to the length ld of the die shoulder.

From the burring processed portion, eight cross-sections that pass through the axis of the burring processed portion and are parallel to the axis of the burring processed portion were cut out. These eight cross-sections were cross-sections that were evenly spaced in the circumferential direction of the burring processed portion. Cracking depths in each cross-section were measured. The cracking depths were measured by observing the cut cross-sections with an optical microscope after polishing them.

<FIG> shows a graph of the cracking depths (um) in the case of the length ld (mm) of the die shoulder for each steel member. In <FIG>, an arithmetic mean value of the cracking depths in these eight cross-sections is displayed as an "average value", and the maximum cracking depth among the cracking depths in the eight cross-sections is displayed as the "maximum value".

As can be understood from this graph, the burring processed portion is formed such that the cross-sectional shape of the curved portion on the inner surface consists of a curve, and the length of the curved portion in the direction perpendicular to the axis of the burring processed portion is at least <NUM> times the height in the direction parallel to the axis of the burring processed portion, and thus depths of the cracks generated in the burring processed portion can be inhibited.

Claim 1:
A method for manufacturing a burring processed product comprising a burring processed portion forming process of increasing a diameter of a prepared hole (<NUM>) provided in a metal component (<NUM>) and bending a part of the metal component to form a burring processed portion (<NUM>) including an upright portion (<NUM>) and a curved portion (<NUM>),
wherein, in a cross-section that passes through an axis of the burring processed portion and is parallel to the axis of the burring processed portion, a cross-sectional shape of the curved portion on an inner surface (130a) of the curved portion consists of a curve, and is characterized in that a length of the curved portion in a direction perpendicular to the axis of the burring processed portion (<NUM>) is at least <NUM> times a height of the curved portion in a direction parallel to the axis of the burring processed portion.