Patent Description:
In order to increase collision safety, an automobile structural member (in particular, continuous length member) is required to have high characteristics in a three-point bending test. Accordingly, there have been a variety of proposals conventionally.

Drawings of Patent Literature <NUM> (<CIT>) and Patent Literature <NUM> (<CIT>) disclose an impact absorbing member which includes a portion where a steel sheet is folded over into three layers.

Patent Literature <NUM> (<CIT>) discloses a method for forming a recessed portion on a wall portion of a member having a substantially hat shape in cross section. In this method, the wall portion is pressed by a power supply roller, thus forming the recessed portion. Accordingly, a portion which projects from the wall portion is not formed with this method before the recessed portion is formed.

Patent Literature <NUM> (<CIT>), Patent Literature <NUM> (<CIT>), and Patent Literature <NUM> (<CIT>) state that although an application of a high tensile strength material has been contemplated in order to improve safety of an automobile, the high tensile strength material has a problem in terms of workability (paragraph [<NUM>] of each Literature). Accordingly, as an example of a component having high collision safety even without using a high tensile strength material, Patent Literatures <NUM> and <NUM> take a component having a hat shape in cross section which has a large number of ridge lines in cross section (paragraph [<NUM>] of Patent Literatures <NUM> and <NUM>). Further, as an example of a component having high collision safety even without using a high tensile strength material, Patent Literatures <NUM> to <NUM> take a component where recessed portions (bead portions) are formed along the longitudinal direction (paragraph [<NUM>] of Patent Literatures <NUM> and <NUM>, and paragraph [<NUM>] of Patent Literature <NUM>).

Patent Literature <NUM> discloses, as a component having high safety even without using a high tensile strength material, a component having a hollow columnar shape where connection regions between vertical wall portions and a top wall portion bulge outward. In order to increase the number of ridge lines in cross section, the bulged portion is not folded.

Patent Literature <NUM> discloses, as a method for producing a component having high safety even without using a high tensile strength material, a method for producing a component having a hat shape in cross section where groove-shaped bead portions are formed on vertical wall portions along the longitudinal direction.

Patent Literature <NUM> discloses, as a component having high safety even without using a high tensile strength material, a frame component which includes reinforcing portions each of which is formed at a connection portion between a top wall portion and a vertical wall portion. This reinforcing portion is formed of an overlapping portion rolled into a cylindrical shape ([<NUM>] of Patent Literature <NUM>).

Patent Literature <NUM> discloses that, in manufacturing, a medium formed article <NUM> is first made through rolling a band plate-like metal material. This medium formed article <NUM> is provided with a belt-like connection part <NUM> to be made into exactly same trim panels 11A, 11B through cutting it into two equal parts. The short width of a hand L2 of this connection part <NUM> is a width of 11A, 11B placed in opposite direction to each other. Subsequently, the connection part <NUM> is cut into two equal parts along a cutting line l2 drawn slantingly in the longitudinal direction. This forms trim panels 11A, 11B on the base parts 10A, 10B with fitting grooves 9A, 9B for door glasses on the medium formed article, two exactly same sash components 12A, 12B being obtained. Later, the slanting end <NUM> of each component <NUM> is folded back to the base part <NUM> side.

Currently, there is a demand for a structural member which can increase collision safety. In other words, there is a demand for an automobile structural member with a press formed product having higher characteristics in a three-point bending test. Under such circumstances, it is one of the objectives of the present invention to provide an automobile structural member with a press formed product having higher characteristics in a three-point bending test, and a method for producing an automobile structural member.

The present invention is as described in the appended claims.

According to the present invention, it is possible to acquire a press formed product having higher characteristics in a three-point bending test, and an automobile structural member with the press formed product. Further, according to the production method of this embodiment, the press formed product can be easily produced.

Inventors of the present invention have made extensive studies and, as a result, have newly found that a specific structure can improve characteristics with respect to collision. The present invention is based on this new finding.

Hereinafter, embodiments of the present invention are described. In the description made hereinafter, the embodiments of the present invention are described by giving examples. However, the present invention is not limited to the examples described hereinafter. In the description made hereinafter, there may be the case where a specific numerical value or a specific material is exemplified. However, another numerical value or another material may be used provided that the advantageous effects of the present invention can be acquired.

A press formed product of this embodiment is a press formed product formed from a single steel sheet. Hereinafter, this press formed product may be referred to as "press formed product (P)". The press formed product (P) includes two vertical wall portions, a top plate portion which connects the two vertical wall portions with each other, and at least one projecting portion which projects from at least one boundary portion of two boundary portions each of which connects the vertical wall portion and the top plate portion with each other. In the projecting portion, the steel sheet extending from the vertical wall portion (the steel sheet which is contiguous from the vertical wall portion) and the steel sheet extending from the top plate portion (the steel sheet which is contiguous from the top plate portion) project from the boundary portion so as to overlap at an overlapping portion located at least at the distal end of the projecting portion. The projecting portion is present at least at a portion of the press formed product (P) in the longitudinal direction. An angle formed between the top plate portion and the overlapping portion is larger than <NUM>°. The angle formed between the top plate portion and the overlapping portion may be referred to as "angle X" hereinafter. The angle X is described in detail in a first embodiment. There may be the case where minute unevenness or the like is formed on the top plate portion so that a portion of the top plate portion does not have a flat plate shape. In such a case, an angle acquired by assuming the entire top plate portion as a flat plate is assumed as the angle of the top plate portion.

The press formed product (P) of this embodiment may include two flange portions which extend from edge portions of the two vertical wall portions (edge portions on a side opposite to the top plate portion), respectively.

At least at the distal end portion of the projecting portion, the steel sheet extending from the top plate portion and the steel sheet extending from the vertical wall portion are made to overlap with each other, thus forming two layers. In this specification, a portion of the projecting portion where the steel sheets are made to overlap with each other so as to form two layers may be referred to as "overlapping portion". The steel sheet is bent at the distal end portion of the projecting portion.

The press formed product (P) of this embodiment can be formed by deforming a single steel sheet (blank steel sheet). Specifically, the press formed product (P) of this embodiment can be produced by performing press forming on a single blank steel sheet by a production method of this embodiment. The blank steel sheet which is used as a material is described later.

The press formed product (P) of this embodiment has a long and thin shape as a whole. All of the vertical wall portions, the top plate portion, the flange portions, and the projecting portions extend along the longitudinal direction of the press formed product (P). The projecting portion may be formed over the entire press formed product (P) in the longitudinal direction, or may be formed only at a portion of the press formed product (P) in the longitudinal direction.

Hereinafter, a region surrounded by the two vertical wall portions, an imaginary surface which connects the edge portions of the two vertical wall portions with each other, and the top plate portion may be referred to as "the inside of the press formed product (P)". Further, a region on a side opposite to the inside with respect to the vertical wall portions and the top plate portion may be referred to as "the outside of the press formed product (P)".

The top plate portion connects the two vertical wall portions with each other. To be more specific, the top plate portion connects the two vertical wall portions with each other via projecting portions. In another aspect, the top plate portion is a lateral wall portion which connects the two vertical wall portions with each other. Accordingly, in this specification, the top plate portion may be alternatively referred to as a lateral wall portion. In the case where the press formed product (P) is disposed with the lateral wall portion (top plate portion) facing downward, the lateral wall portion may be also referred to as bottom plate portion. In this specification, however, the case where the lateral wall portion is disposed on the upper side is used as a reference so that the lateral wall portion is referred to as top plate portion.

Each angle Y formed between the top plate portion and the vertical wall portion is usually <NUM>° or an angle around <NUM>°. The angle Y is described in the first embodiment. The angle Y may be less than <NUM>°. However, the angle Y is usually <NUM>° or more, and may fall within a range from <NUM>° to <NUM>°. Two angles Y may differ from each other. However, it is preferable that the two angles Y be substantially equal to each other (the difference between the two angles Y be equal to or less than <NUM>°). The two angles Y may be equal to each other.

It is preferable that the press formed product (P) of this embodiment include two projecting portions which respectively project from the two boundary portions. In this case, one projecting portion projects from each of the two boundary portions. It is preferable that the angles X at the two projecting portions be substantially equal to each other (the difference between the angles X be equal to or less than <NUM>°). The angles X at the two projecting portions may be equal to each other. It is preferable that the two projecting portions be formed so as to have shapes which are line-symmetrical with each other in cross section perpendicular to the longitudinal direction. However, the two projecting portions may not be formed so as to be line-symmetrical with each other.

The angle X formed between the top plate portion and the overlapping portion may be larger than <NUM>° and <NUM>° or less.

In the press formed product (P) of this embodiment, the length of the projecting portion in cross section perpendicular to the longitudinal direction may be <NUM> or more (<NUM> or more, <NUM> or more, or <NUM> or more, for example). The upper limit of the length is not particularly limited. However, the length may be <NUM> or less, for example.

In the press formed product (P) of this embodiment, the steel sheet extending from the vertical wall portion and the steel sheet extending from the top plate portion may be welded to each other at the projecting portion. For example, the steel sheets which are formed into two layers at the overlapping portion may be welded by spot welding or laser welding. Further, the steel sheet extending from the vertical wall portion and the steel sheet extending from the top plate portion may be joined with each other by arc welding (fillet welding) at the root portions of the projecting portion (a boundary between the top plate portion and the projecting portion, and a boundary between the vertical wall portion and the projecting portion). The lengths of the two projecting portions may or may not be equal to each other.

In the press formed product (P) of this embodiment, the tensile strength of the steel sheet which forms the press formed product may be <NUM> MPa or more (for example, <NUM> MPa or more, <NUM> MPa or more, <NUM> MPa or more, <NUM> MPa or more, or <NUM> MPa or more). In the case where a second step in the production method described later is performed by hot stamping, the tensile strength of a press formed product can be made higher than the tensile strength of the steel sheet (blank) which is used as a material.

The press formed product (P) of this embodiment can be used for various applications. For example, the press formed product (P) of this embodiment can be used for a structural member of various transportation means (an automobile, a motorcycle, a railway vehicle, a ship, an aircraft) or for a structural member of various machines. An example of an automobile structural member may be a side sill, a pillar (a front pillar, a front pillar lower, a center pillar or the like), a roof rail, a roof arch, a bumper beam, a belt line reinforcement, and a door impact beam. The automobile structural member may be a structural member other than the above-mentioned structural members.

The press formed product (P) of this embodiment may be directly used as any of various structural members. That is, an automobile structural member of this embodiment includes the press formed product (P) of this embodiment. The automobile structural member of this embodiment may be referred to as "structural member (S)" hereinafter. The structural member described hereinafter can be used as a structural member for a product other than an automobile.

The structural member (S) of this embodiment may include the press formed product (P) and another member. Another member may be referred to as "another member (M)" or "member (M)" hereinafter. Another member (M) may be fixed to the press formed product (P) such that the press formed product (P) and another member (M) form a closed cross section. In the case where the press formed product (P) includes the above-mentioned two flange portions, another member (M) may be fixed to the two flange portions such that the press formed product (P) and another member (M) form a closed cross section.

The member (M) is a member (steel sheet member) formed from a steel sheet, for example. A steel sheet of the same kind as a steel sheet for forming the press formed product (P) may be used as a steel sheet for forming the member (M). One example of the member (M) is the press formed product (P) of this embodiment. In this case, two press formed products (P) are fixed to each other.

A method for fixing the press formed product (P) and another member (M) with each other is not limited. The press formed product (P) and another member (M) may be fixed with each other by welding, or by another fixing method. Examples of the welding include the above-mentioned examples.

A production method of this embodiment is a method for producing the press formed product (P) of this embodiment. The description of the press formed product (P) of this embodiment is applicable to the production method of this embodiment and hence, a repeated description may be omitted. Further, the description of the production method of this embodiment is applicable to the press formed product (P) of this embodiment.

The production method of this embodiment includes a first step and a second step. In the first step, a preformed product, which includes two first portions to be formed into two vertical wall portions and a second portion to be formed into the top plate portion, is formed by deforming a blank steel sheet. In the second step, the preformed product is subjected to press forming, thus forming a press formed product (P).

The preformed product includes surplus portions for forming the projecting portions. In the second step, at least portions of a blank steel sheet (deformed blank steel sheet) constituting the surplus portion are made to overlap with each other, thus forming an overlapping portion. Typically, in the preformed product, there is no clear boundary between the surplus portion and the remaining portions. However, there may be a boundary between the surplus portion and the remaining portions.

The preformed product may include a U-shaped portion having a U shape in cross section perpendicular to the longitudinal direction. This U-shaped portion forms the two vertical wall portions, the top plate portion, and the projecting portion. In the description made hereinafter, the term "cross section" means a cross section in a direction perpendicular to the longitudinal direction in principle.

The first step is not particularly limited, and may be performed by known press forming. The second step is described later. A press formed product acquired from the second step may be further subjected to post processing. The press formed product acquired from the second step (or acquired from post processing performed thereafter) may be directly used, or may be used in combination with another member.

Hereinafter, a steel sheet (blank steel sheet) which is a starting material may be referred to as "blank". The blank is usually a steel sheet having a flat plate shape, and has a plane shape which corresponds to the shape of the press formed product (P) to be produced. The thickness and physical properties of the blank are selected according to characteristics which the press formed product (P) is required to possess. For example, in the case where a press formed product (P) is an automobile structural member, a blank which is suitable for the automobile structural member is selected. The thickness of the blank may fall within a range from <NUM> to <NUM>, or a range from <NUM> to <NUM>, for example. The wall thickness of the press formed product (P) of this embodiment is determined by the thickness of the blank and processing steps, and may fall within a range of the thickness of the blank exemplified in this paragraph.

It is preferable that the blank be formed of a high tensile strength steel sheet (high tensile strength material) having tensile strength of <NUM> MPa or more (for example, <NUM> MPa or more, <NUM> MPa or more, <NUM> MPa or more, <NUM> MPa or more, or <NUM> MPa or more). To reduce the weight of a structural member, it is preferable that the blank have high tensile strength. It is more preferable that the blank have tensile strength of <NUM> MPa or more (<NUM> MPa or more, or <NUM> MPa or more, for example). There is no upper limit of the tensile strength of a blank. In one example, the tensile strength of a blank is <NUM> MPa or less. The tensile strength of the press formed product (P) of this embodiment is usually equal to or higher than the tensile strength of the blank. The tensile strength of the press formed product (P) of this embodiment may fall within a range exemplified in this paragraph.

In the case where the tensile strength of a blank steel sheet (blank) is <NUM> MPa or more, the second step may be performed by hot stamping (hot pressing). In the case where a blank has high tensile strength, cold pressing easily causes cracks at the distal end portion of a projecting portion. Accordingly, in the case of using a blank having tensile strength of <NUM> MPa or more (<NUM> MPa or more, for example), it is preferable to perform the second step by hot stamping. It is needless to say that the second step may be performed by hot stamping even in the case of using a blank having tensile strength of less than <NUM> MPa. In the case of performing hot stamping, a blank may be used which has a known composition suitable for hot stamping.

In the case where a blank has tensile strength of <NUM> MPa or more and a wall thickness of <NUM> or more, it is particularly preferable to perform the second step by hot stamping in order to suppress the occurrence of cracks at the projecting portion. For the similar reason, in the case where a blank has tensile strength of <NUM> MPa or more and a wall thickness of <NUM> or more, it is particularly preferable to perform the second step by hot stamping. Heated steel sheet has high ductility. Accordingly, in the case of performing the second step by hot stamping, there is a low occurrence of cracks even if the wall thickness of a blank is <NUM>.

None of Patent Literatures <NUM>, <NUM>, nor <NUM> discloses a production method which uses hot stamping. However, as described above, it is preferable to perform the second step by hot stamping in the case of using a high tensile strength material.

Usually, the amount of deformation in the first step is not especially large. Accordingly, usually, the first step can be performed by cold working (cold pressing, for example) regardless of tensile strength of a blank. However, the first step may be performed by hot working (hot pressing, for example) when necessary. In a preferred example, the first step is performed by cold working, and the second step is performed by hot stamping.

One example of hot stamping is described hereinafter. In the case of performing hot stamping, first, a workpiece (a blank or a preformed product) is heated to a predetermined quenching temperature. The quenching temperature is a temperature higher than the A3 transformation point (more specifically, the Ac3 transformation point) at which the workpiece is austenitized. The quenching temperature may be <NUM> or above, for example. Next, the heated workpiece is pressed by a pressing device. The workpiece is heated already and hence, cracks do not easily occur even if the workpiece is significantly deformed. The workpiece is rapidly cooled in pressing the workpiece. With such rapid cooling, the workpiece is quenched at the time of pressing. The workpiece can be rapidly cooled by cooling a press tooling, or by injecting water to the workpiece from the press tooling. The procedure (heating and pressing and the like) of the hot stamping and a device used for the hot stamping is not particularly limited. A known procedure and a known device may be used.

The second step may be performed using a press die including a lower die, an upper die, and slide dies which are movable in the horizontal direction toward the lower die. In this case, the second step may include the following step (i) and step (ii). The step (i) is a step where the two first portions (portions to be formed into vertical wall portions) are constrained by the lower die and the slide die. The step (ii) is a step where, in a state where the two first portions are constrained, the second portion (a portion to be formed into a top plate portion) is pressed by the lower die and the upper die, and the surplus portions are pressed by the upper die and the slide dies, thus forming a press formed product.

The lower die may include a lower die body, and a pad connected to the lower die body via an extension and contraction mechanism. In this case, the production method of this embodiment may include a step (iii) and a step (iv). The step (iii) is a step where, after the step (ii), constraint (constraint of the vertical wall portions) by the lower die and the slide dies is released, and the upper die and the pad are moved upward, thus moving the press formed product upward. The step (iv) is a step where, after the step (iii), the slide dies are separated from the lower die.

In the case where the press formed product (P) includes two flange portions which extend from edge portions of the two vertical wall portions, the production method of this embodiment may include a third step of forming the flange portions after the second step. One example of a method for forming the flange portions is described in a third embodiment.

Hereinafter, embodiments of the present invention are described with reference to drawings. The embodiments described hereinafter merely form examples, and the above-mentioned various variations are applicable. In the description made hereinafter, the identical reference symbols are given to the similar components, and repeated description may be omitted. Further, to facilitate the understanding, in the following drawings, there may be the case where a gap is illustrated between steel sheets which are made to overlap with each other at the overlapping portion. However, usually, one steel sheet and another steel sheet which are made to overlap with each other at the overlapping portion are brought into close contact with each other.

In a first embodiment, an example of a press formed product (P) of this embodiment is described. <FIG> is a perspective view schematically showing a press formed product <NUM> of the first embodiment. <FIG> is a cross-sectional view schematically showing the cross section perpendicular to the longitudinal direction of the press formed product <NUM>. Further, <FIG> are cross-sectional views schematically showing a projecting portion <NUM> and an area around the projecting portion <NUM>. Hereinafter, the upper side in <FIG> (top plate portion <NUM> side) may be referred to as the upper side of the press formed product (P) <NUM> of this embodiment. The lower side in <FIG> (flange portion <NUM> side) may be referred to as the lower side of the press formed product (P) of this embodiment.

The press formed product <NUM> is formed from a single steel sheet <NUM>. Referring to <FIG>, the press formed product <NUM> includes two vertical wall portions <NUM>, top plate portion <NUM>, two flange portions <NUM>, and two projecting portions <NUM>. Each of the vertical wall portion <NUM>, the top plate portion <NUM>, and the flange portion <NUM> has a flat plate shape. The top plate portion <NUM> connects the two vertical wall portions <NUM> with each other via the two projecting portions <NUM>. In one example shown in <FIG>, the two flange portions <NUM> extend substantially horizontally toward the outside from lower edge portions of the two vertical wall portions <NUM>. That is, the flange portions <NUM> are substantially parallel to the top plate portion <NUM>.

Referring to <FIG> and <FIG>, each projecting portion <NUM> projects outward from a boundary portion which connects the vertical wall portion <NUM> and the top plate portion <NUM> with each other (see portions indicated by dotted lines in <FIG>). In this embodiment, a portion which is bent toward the projecting portion <NUM> from the upper edge side of the vertical wall portion <NUM> is assumed as a corner portion <NUM>. That is, the corner portion <NUM> is formed of a portion ranging from a position where the vertical wall portion <NUM> starts to bend toward the projecting portion <NUM> to the upper edge portion of an overlapping portion 115d of the projecting portion <NUM>.

Each projecting portion <NUM> is formed of a steel sheet 101a extending from the top plate portion <NUM>, and a steel sheet 101b extending from the vertical wall portion <NUM>. A portion extending outward from a bent portion at the edge portion of the top plate portion <NUM> is assumed as the steel sheet 101a. A portion extending from the vertical wall portion <NUM> and which is bent at the corner portion <NUM> so as to extend outward is assumed as the steel sheet 101b. The steel sheet 101a is bent at a distal end portion 115t of the projecting portion <NUM>, and is connected to the steel sheet 101b. The surface of the steel sheet 101a on the lower side and the surface of the steel sheet 101b on the upper side are made to overlap and are brought into close contact with each other within a range from the corner portion <NUM> to the projecting portion <NUM>. Each of the steel sheet 101a and the steel sheet 101b forms a portion of the steel sheet <NUM>. The press formed product <NUM> excluding the projecting portions <NUM> has a substantially hat shape in cross section (cross section perpendicular to the longitudinal direction).

As shown in <FIG>, an angle formed between the top plate portion <NUM> and the overlapping portion 115d in cross section is assumed as an angle X. To be more specific, the angle X is an angle formed between a surface <NUM> of the top plate portion <NUM> on the upper side of the surfaces of the top plate portion <NUM> and a surface <NUM> of the overlapping portion 115d on the upper side of the surfaces of the overlapping portion 115d. In this embodiment, in the case where the overlapping portion 115d of the projecting portion <NUM> includes a straight line shape in cross section (see <FIG>, <FIG>, for example), an angle formed between the surface <NUM> of the straight line portion of the overlapping portion 115d and the surface <NUM> of the top plate portion <NUM> is assumed as the angle X. In the case where the cross sectional shape of the overlapping portion 115d does not include a straight line shape as shown in <FIG>, of the surface of the steel sheet 101a on the upper side, the edge portion of the overlapping portion 115d on the distal end portion 115t side is assumed as an edge 101at. An angle formed between an imaginary tangential line (a dotted line extending in the vertical direction in <FIG>) at the edge 101at and the surface <NUM> of the top plate portion <NUM> is assumed as the angle X.

In the case where the angle X is larger than <NUM>°, when the press formed product <NUM> is viewed from above the top plate portion <NUM>, the steel sheet 101b which forms the projecting portion <NUM> cannot be observed due to the steel sheet 101a. Such a portion may be referred to as negative angle portion. In another aspect, the negative angle portion is a portion which has an inverse gradient when press forming is performed only by an upper die and a lower die.

It is preferable that the angle X formed between the top plate portion <NUM> and the overlapping portion 115d is larger than <NUM>° and <NUM>° or less. When the angle X falls within such a range, there is no possibility that the overlapping portion 115d of the projecting portion <NUM> (the steel sheet 101b on the inner side) is brought into close contact with the vertical wall portion <NUM> and hence, a clearance is ensured between the projecting portion <NUM> and the vertical wall portion <NUM>. With such a configuration, when a collision load is applied to the top plate portion <NUM>, stress applied to the top plate portion <NUM> is dispersed at the corner portions <NUM> and the projecting portions <NUM> so that the stress is applied to the vertical wall portions <NUM> while the shapes of the corner portions <NUM> are maintained. Accordingly, the stress is received by the entire vertical wall portions <NUM>, thus improving collision characteristics. Further, it is possible to prevent that the corner portion <NUM> is locally deformed, or the vertical wall portion <NUM> is inclined outward using the corner portion <NUM> as a fulcrum. Accordingly, even in the case where the vertical wall portion <NUM> is deformed, the vertical wall portion <NUM> is deformed so as to be inclined inward.

On the other hand, in the case where the overlapping portion 115d of each projecting portion <NUM> is brought into close contact with the vertical wall portion <NUM>, when a collision load is applied to the top plate portion <NUM>, stress of the collision load is applied to the portion of each vertical wall portion <NUM> which is brought into close contact with the overlapping portion 115d via the overlapping portion 115d in a concentrated manner. Accordingly, the vertical wall portion <NUM> is deformed so as to be locally inclined inward from the portion of the vertical wall portion <NUM> which is brought into close contact with the overlapping portion 115d. In this case, collision characteristics are reduced.

<FIG> shows one example of the case where an angle Y formed between the vertical wall portion <NUM> and the top plate portion <NUM> is <NUM>°. In this embodiment, the angle Y is an angle shown in <FIG>, that is, an angle formed between the vertical wall portion <NUM> and the top plate portion <NUM> on the inside of the press formed product <NUM>.

As shown in <FIG>, it is preferable that a corner portion <NUM> which connects the vertical wall portion <NUM> and the flange portion <NUM> have a round shape in cross section. Causing the corner portion <NUM> to have a round shape can suppress occurrence of buckling at the corner portion <NUM>.

As shown in <FIG> and <FIG>, it is preferable that the corner portion <NUM> (corresponding to "Ra" in <FIG>) at the boundary between the steel sheet 101b forming the projecting portion <NUM> and the vertical wall portion <NUM> have a curved surface. Causing the corner portion <NUM> to have a curved surface allows stress applied to the top plate portion <NUM> from above to be dispersed at the corner portion <NUM> and hence, it is possible to suppress buckling of the corner portion <NUM>. In the cross section perpendicular to the longitudinal direction of the press formed product <NUM>, the radius of curvature r of the corner portion <NUM> may fall within a range from <NUM> to <NUM> (range from <NUM> to <NUM>, for example). However, the radius of curvature r is shorter than the length of the projecting portion <NUM> in cross section.

In another aspect, the lower limit of the radius of curvature r of the corner portion <NUM> may be half of the sheet thickness of the steel sheet <NUM>, or <NUM>, whichever is greater. The upper limit of the radius of curvature r of the corner portion <NUM> may be a value ten times greater than the sheet thickness. When a radius of curvature r is excessively small, stress is not sufficiently dispersed at the corner portion <NUM> so that the vertical wall portion <NUM> may be broken off using the corner portion <NUM> as a starting point at the time of collision. Further, the excessively small radius of curvature r causes the overlapping portion 115d of the projecting portion <NUM> to be brought into close contact with the vertical wall portion <NUM>. Accordingly, stress at the time of collision is concentrated at the portion where the overlapping portion 115d is brought into close contact with the vertical wall portion <NUM> and hence, the vertical wall portion <NUM> may be broken off. On the other hand, when the radius of curvature r is excessively large, stress applied to the top plate portion <NUM> does not easily transferred to the projecting portion <NUM> via the corner portion <NUM> and hence, an advantageous effect of improving collision characteristics obtained by providing the projecting portions <NUM> is reduced. In order to improve collision characteristics, it is desirable that a clearance between the projecting portion <NUM> and the vertical wall portion <NUM> has a value of at least half or the sheet thickness or <NUM>, whichever is greater.

To be more specific, the radius of curvature r of the corner portion <NUM> means, the radius of curvature of an outer surface <NUM> of the corner portion <NUM> in cross section perpendicular to the longitudinal direction of the press formed product <NUM>. The outer surface <NUM> of the corner portion <NUM> is a surface which is positioned between an upper edge 111sp of an outer surface <NUM> of the vertical wall portion <NUM> and an upper edge 101bsp of a surface 101bs of the steel sheet 101b forming the projecting portion <NUM>.

In a second embodiment, the description is made with respect to examples of structural members (S) each of which uses a press formed product (P) of this embodiment. The examples of the structural members (S) are shown in <FIG>. <FIG>, <FIG>, <FIG> are views each of which schematically shows a cross section perpendicular to the longitudinal direction of the structural member (S). <FIG> are perspective views each of which schematically shows the structural member (S). Each of all structural members (S) shown in <FIG> has a closed cross section.

A structural member <NUM> shown in <FIG> includes a press formed product <NUM> which includes projecting portions <NUM>, and a member <NUM> (another member (M)) having a flat plate shape. The member <NUM> is fixed to two flange portions <NUM> of the press formed product <NUM> such that the press formed product <NUM> and the member <NUM> form a closed cross section.

A structural member <NUM> shown in <FIG> includes a press formed product <NUM> which includes projecting portions <NUM>, and another member <NUM>. The member <NUM> is a member having a substantially hat shape in cross section, and includes two flange portions <NUM>. The two flange portions <NUM> of the press formed product <NUM> are fixed to the two flange portions <NUM> of the member <NUM> such that the inside of the press formed product <NUM> and the inside of the member <NUM> oppose to each other.

<FIG> is a perspective view showing one example of a structural member having the cross section shown in <FIG>, and <FIG> is a perspective view showing another example. In the structural member <NUM> shown in <FIG>, the projecting portions <NUM> are formed over the entire structural member <NUM> in the longitudinal direction. In the structural member <NUM> shown in <FIG>, the projecting portions <NUM> are only partially formed on the structural member <NUM> in the longitudinal direction.

A structural member <NUM> shown in <FIG> includes two press formed products <NUM> each of which includes projecting portions <NUM>. Flange portions <NUM> are fixed with each other such that the inside of one press formed product <NUM> and the inside of another press formed product <NUM> oppose to each other. Either one of the two press formed products <NUM> may be assumed as another member (M).

A structural member <NUM> shown in <FIG> includes a press formed product <NUM> which includes projecting portions <NUM>, and a member <NUM>. The press formed product <NUM> includes two vertical wall portions <NUM> and a top plate portion <NUM> which connects the two vertical wall portions <NUM> with each other. The member <NUM> includes two vertical wall portions <NUM> and a top plate portion <NUM> which connects the two vertical wall portions <NUM> with each other. In the structural member <NUM> shown in <FIG>, neither of the press formed product <NUM> nor the member <NUM> includes a flange portion. In one example shown in <FIG>, the vertical wall portions <NUM> of the press formed product <NUM> and the vertical wall portions <NUM> of the member <NUM> are fixed with each other such that the top plate portions have the same direction with respect to the vertical wall portions.

A structural member <NUM> shown in <FIG> differs from the structural member <NUM> shown in <FIG> only with respect to the direction of fixing the member <NUM>. In one example shown in <FIG>, vertical wall portions <NUM> of a press formed product <NUM> and vertical wall portions <NUM> of a member <NUM> are fixed with each other such that the inside of the press formed product <NUM> and the inside of the member <NUM> oppose to each other.

A structural member <NUM> shown in <FIG> includes two press formed products <NUM> each of which includes projecting portions <NUM>. Neither of the two press formed products <NUM> includes a flange portion. Vertical wall portions <NUM> of one press formed product <NUM> and vertical wall portions <NUM> of another press formed product <NUM> are fixed with each other such that the inside of the one press formed product <NUM> and the inside of another press formed product <NUM> oppose to each other.

In a third embodiment, a method for producing the press formed product (P) according to the present invention is described. According to this production method, a preformed product is formed in a first step, and the preformed product is pressed in a second step. With such steps, the press formed product (P) <NUM> of this embodiment can be produced. In the third embodiment, one example where the second step is performed by hot stamping is described.

First, in the first step, a blank steel sheet is deformed so as to form a preformed product <NUM> which includes two portions (first portions) to be formed into two vertical wall portions <NUM>, and a portion (second portion) to be formed into a top plate portion <NUM>. The first step can be performed by the above-mentioned method (by pressing, for example). <FIG> schematically shows a cross section (cross section perpendicular to the longitudinal direction) of one example of the preformed product <NUM> formed in the first step.

The preformed product <NUM> has a substantially U shape in cross section (inverted in <FIG>). The preformed product <NUM> includes two first portions 301a to be formed into the two vertical wall portions <NUM>, and a second portion 301b to be formed into the top plate portion <NUM>. The preformed product <NUM> further includes portions (surplus portions 301c) to be formed into projecting portions <NUM>. <FIG> shows the case where the preformed product <NUM> includes third portions 301d to be formed into flange portions <NUM>. In the case of producing a press formed product (P) having no flange portion, a preformed product <NUM> includes no third portion 301d.

The second step is performed by hot stamping. First, the preformed product <NUM> is heated to a temperature of the Ac3 transformation point or above (a temperature greater than the Ac3 transformation point by <NUM> or greater, for example). This heating is performed by heating the preformed product <NUM> in a heater, for example.

Next, the heated preformed product <NUM> is pressed by a pressing device. One example of the configuration of a press die used for pressing is shown in <FIG>. The pressing device includes a pair of press dies <NUM>, a plate <NUM>, and two slide dies <NUM>.

The pair of press dies <NUM> includes an upper die <NUM> (die) and a lower die <NUM> (punch). The lower die <NUM> includes a lower die body 12a and a pad 12b. The pad 12b is connected to the lower die body 12a via an extension and contraction mechanism 12c which can be extended and contracted. A known extension and contraction mechanism, such as a spring or a hydraulic cylinder, may be used as the extension and contraction mechanism.

The slide dies <NUM> slide on the plate <NUM> in the horizontal direction. The slide dies <NUM> may be caused to slide using a cam mechanism which moves with the movement of the press die <NUM>. Alternatively, the slide dies <NUM> may be caused to slide using an actuator, such as a hydraulic cylinder.

One example of a process of performing press forming using the device shown in <FIG> is described. First, as shown in <FIG>, the preformed product <NUM> is set between the upper die <NUM> and the lower die <NUM>. Next, as shown in <FIG>, the slide dies <NUM> are caused to slide toward the lower die <NUM>, thus constraining the two first portions 301a by the lower die <NUM> (lower die body 12a) and the slide dies <NUM> (step (i)). With such an operation, the first portions 301a are formed into the vertical wall portions <NUM>. In this state, the second portion 301b and the surplus portions 301c can be freely deformed.

Next, in a state where the first portions 301a are constrained, as shown in <FIG>, the upper die <NUM> is moved downward so as to press the second portion 301b by the lower die <NUM> and the upper die <NUM>, and so as to press the surplus portions 301c by the upper die <NUM> and the slide dies <NUM>. With such operations, the press formed product <NUM> is formed (step (ii)). At this point of operation, the second portion 301b is sandwiched between the pad 12b and the upper die <NUM>, and is moved downward while maintaining such a state and, then, reaches the upper surface of the lower die body 12a. With such operations, the top plate portion <NUM> is formed. Each surplus portion 301c comes into contact with the upper die <NUM> and the slide die <NUM> with the downward movement of the upper die <NUM>, thus being gradually bent and, then, being formed into two layers. With such operations, the projecting portions <NUM> are formed each of which includes an overlapping portion, and projects obliquely downward. The press formed product <NUM> which includes the projecting portions <NUM> can be acquired in this manner.

In the case of performing the second step by hot stamping, the heated preformed product <NUM> is cooled at the time of performing press forming so that press forming and quenching are performed.

Each projecting portion <NUM> of the press formed product <NUM> projects obliquely downward. Accordingly, it is preferable to move the press formed product <NUM> upward before the slide dies <NUM> are returned to an original position. Specifically, first, as shown in <FIG>, after the step (ii) is performed, constraint of the vertical wall portions <NUM> performed by the lower die <NUM> and the slide dies <NUM> is released, and the upper die <NUM> and the pad 12b are moved upward, thus moving the press formed product <NUM> upward (step (iii)). At this point of operation, the press formed product <NUM> is moved upward such that the lower edges of the projecting portions <NUM> (distal end portions 115t) are positioned above the upper edges of the slide dies <NUM>. Constraint of the vertical wall portions <NUM> can be released by slightly separating the slide dies <NUM> from the lower die <NUM>.

Next, as shown in <FIG>, after the step (iii) is performed, the slide dies <NUM> are separated from the lower die <NUM> (step (iv)). For example, as shown in <FIG>, the slide dies <NUM> are caused to slide such that the slide dies <NUM> are positioned further outward than the distal end portions 115t of the projecting portions <NUM>. Thereafter, the press formed product <NUM> is taken out from the pressing device.

In the case of forming a press formed product which includes flange portions, it is sufficient to further pressing the press formed product <NUM> which is acquired through the above-mentioned steps, thus forming the flange portions (third step). One example of a method for forming a flange portion is shown in <FIG>.

A pressing device shown in <FIG> includes an upper die <NUM>, a lower die <NUM>, and a posture holding die <NUM>. The upper die <NUM> includes a projecting portion 21a, a recessed portion 21b, a pad 21c, and an extension and contraction mechanism 21d. The pad 21c is connected to the recessed portion 21b by the extension and contraction mechanism 21d which can be extended and contracted.

First, as shown in <FIG>, the press formed product <NUM> is set on the pressing device. The posture holding die <NUM> is provided for preventing the press formed product <NUM> from being inclined so that the posture holding die <NUM> does not constrain the press formed product <NUM>. Next, the upper die <NUM> is moved downward. With the downward movement of the upper die <NUM>, first, a portion of the vertical wall portion <NUM> is fixed by the pad 21c and the lower die <NUM>. When the upper die <NUM> is further moved downward, the extension and contraction mechanism 21d is constrained, and a third portion 301d which is contiguous from the vertical wall portion <NUM> is bent. One flange portion <NUM> is formed as described above. The other flange portion is formed in the same manner so that the press formed product (P) which includes two flange portions can be acquired.

In the case of producing the structural member (S) of this embodiment, it is sufficient to fix another member (M) to the press formed product (P) acquired through the above-mentioned steps by any desired method.

The present invention is described in more detail with reference to examples.

In the example, a simulation of a three-point bending test was performed on a structural member (S) with the press formed product (P) of this embodiment. In the simulation, a general-purpose FEM (finite element method) software (made by LIVERMORE SOFTWARE TECHNOLOGY, trade name: LS-DYNA) was used.

<FIG> is a cross-sectional view schematically showing a sample <NUM> used in the simulation as a Comparative Example. The sample <NUM> shown in <FIG> is formed of two U-shaped members <NUM> and <NUM>. Each of the U-shaped members <NUM> and <NUM> includes two vertical wall portions and a top plate portion which connects the two vertical wall portions. As shown in <FIG>, it is assumed that the U-shaped member <NUM> and the U-shaped member <NUM> are joined by spot welding at fixing portions <NUM> of the vertical wall portions. Sizes of respective portions of the sample <NUM> are shown in <FIG>. The length of the sample <NUM> in the longitudinal direction is set to <NUM>.

<FIG> is a cross-sectional view schematically showing a sample <NUM> used in the simulation as a reference example <NUM>. The sample <NUM> shown in <FIG> includes a press formed product 100a and a member <NUM> having a U shape in cross section. In the press formed product 100a shown in <FIG>, the angle X is set to <NUM>°. Accordingly, the press formed product 100a shown in <FIG> is different from the press formed product (P) of this embodiment. The press formed product 100a includes projecting portions <NUM>. It is assumed that the press formed product 100a and the member <NUM> are joined by spot welding at fixing portions <NUM> of vertical wall portions <NUM>. The shape of the sample <NUM> is as follows.

Samples of this embodiment used in the simulation are obtained by changing the angle X of the projecting portion <NUM> of the sample <NUM> of the reference example <NUM> shown in <FIG> to <NUM>°, <NUM>°, <NUM>°, and <NUM>°. <FIG>, <FIG> are cross-sectional views respectively showing projecting portions of the samples having the angle X of <NUM>°, <NUM>°, <NUM>°, and <NUM>°, and areas around the projecting portions.

<FIG> is a cross-sectional view schematically showing a sample <NUM> used in the simulation as a reference example <NUM>. In the sample <NUM> shown in <FIG>, a steel sheet is folded over into three layers at an upper edge portion of each vertical wall portion <NUM> so that portions which correspond to the projecting portions <NUM> are brought into close contact with the vertical wall portions <NUM>. That is, portions which correspond to the projecting portions <NUM> do not substantially project from boundary portions each of which connects the vertical wall portion <NUM> and the top plate portion <NUM> with each other. The shape of the sample <NUM> other than the above-mentioned portions is equal to the shape of the sample <NUM>.

It is assumed that each of all samples is formed of a steel sheet having a thickness of <NUM>, and tensile strength of <NUM> MPa. It is assumed that the press formed product and another member are fixed by spot welding at pitches of <NUM>.

A method of the three-point bending test used in the simulation is schematically shown in <FIG>. The three-point bending test was performed such that a sample is placed on two fulcrums <NUM>, and the sample is pressed from above by an impactor <NUM>. A distance S between the two fulcrums <NUM> was set to <NUM> or <NUM>. The radius of curvature of the fulcrum <NUM> was set to <NUM>. The radius of curvature of the impactor <NUM> was set to <NUM>. The collision speed of the impactor <NUM> was set to <NUM>/h. The simulation was performed by taking into account spot welding and breaking off of material.

In the three-point bending test, the impactor <NUM> was caused to collide with each sample from above (from the top plate portion side). The collision direction of the impactor <NUM> is indicated by an arrow in <FIG>.

The simulation results of the three-point bending test are shown in <FIG>. <FIG> show the simulation results of the sample of the Comparative Example (sample <NUM>) and the sample of an Inventive Example of the present invention (angle X = <NUM>°). <FIG> shows the results of the case where the distance S is set to <NUM>. <FIG> shows the results of the case where the distance S is set to <NUM>. The abscissa in <FIG> shows the amount of movement (amount of displacement) of the impactor <NUM> after the impactor <NUM> collides with the sample. The ordinate in <FIG> shows a load generated in the impactor <NUM>.

<FIG> show the simulation results of the Comparative Example (sample <NUM>), the reference example <NUM> (sample <NUM>), the Inventive Examples of the present invention (angle X = <NUM>°, <NUM>°, <NUM>°, and <NUM>°), and the reference example <NUM> (sample <NUM>). The ordinate in <FIG> shows the amount of energy absorption until the displacement amount reaches <NUM>. <FIG> shows a maximum load generated in the impactor <NUM>. Both of <FIG> show the results of the case where the distance S is set to <NUM>.

As shown in <FIG>, in the case where the distance S is set to <NUM>, the sample of the Inventive Example of the present invention has higher characteristics than the Comparative Example (sample <NUM>), the reference example <NUM> (sample <NUM>), and the reference example <NUM> (sample <NUM>).

Particularly, the Comparative Example (sample <NUM>) and the reference example <NUM> (sample <NUM>) have remarkably low characteristics. When the distance S was set to <NUM>, and the displacement amount was set to <NUM>, the vertical wall portions of the sample <NUM> (Comparative Example) were inclined outward. In the same manner, the vertical wall portions of the sample <NUM> (reference example <NUM>) were also inclined outward. On the other hand, when the distance S was set to <NUM>, and a displacement amount was set to <NUM>, the vertical wall portions of the sample (angle X = <NUM>°) of the Inventive Example of the present invention were inclined inward. Although it is not clear at present, there is a possibility that high characteristics of the sample of the Inventive Example of the present invention are caused due to the vertical wall portions being inclined inward.

As shown in the above-mentioned examples, according to this embodiment, it is possible to acquire a structural member having high characteristics in the three-point bending test. With the use of the structural member of this embodiment, it is possible to improve collision safety of an automobile, and reduce the weight of an automobile.

Claim 1:
An automobile structural member (<NUM>) comprising a press formed product (<NUM>) formed from a single steel sheet (<NUM>), the press formed product (<NUM>) comprising:
two vertical wall portions (<NUM>);
a top plate portion (<NUM>) which connects the two vertical wall portions (<NUM>) with each other;
at least one projecting portion (<NUM>) which projects outward from at least one boundary portion of two boundary portions each of which connects the vertical wall portion (<NUM>) and the top plate portion (<NUM>) with each other, and
a portion which is bent toward the projecting portion (<NUM>) from an upper edge side of the vertical wall portion (<NUM>) is a corner portion (<NUM>), wherein
in the projecting portion (<NUM>), the steel sheet (<NUM>) extending from the vertical wall portion (<NUM>) and the steel sheet (<NUM>) extending from the top plate portion (<NUM>) project from the boundary portion so as to overlap at the overlapping portion (115d) located at least at a distal end of the projecting portion (<NUM>),
the projecting portion (<NUM>) is present at least at a portion of the press formed product (<NUM>) in a longitudinal direction, and
an angle (X) formed between the top plate portion (<NUM>) and the overlapping portion (115d) is larger than <NUM>° and <NUM>° or less, and
wherein the automobile structural member is a side sill, a pillar, a front pillar, a front pillar lower, a center pillar, a roof rail, a roof arch, a bumper beam, a belt line reinforcement, or a door impact beam.