DESCRIPTION TITLE OF INVENTION: PRESS-FORMED PRODUCT AUTOMOBILE STRUCTURAL MEMBER INCLIIDING THE SAME, PRODUCTION METHOD AND PRODUCTION DEVICE FOR THE PRESS-FORMED PRODUCT TECHNICAL FIELD [0001] The present invention relates to a press-formed product of a steel plate, and particularly relates to a press-formed product suitable as a reinforcing member for a structural member constituting an automobile body. Further, the present invention relates to an automobile structural member including the press-formed product, and a production method and a production device for the press-formed product. BACKGROLIND ART [0002] Al automobile body includes various structural members (for example, a rocker (also referred to as a side sill), a cross member, a side member, a center pillar, and the like). For the structural members, press-formed products of steel plates are frequently used. The press-formed products for use in the structural members have char¡rel-shaped or hat-shaped sectional shapes, for example. A press-formed product having a channel-shaped section includes a top plate part and a pair of flange parts connecting to the top plate part. A press-formed product having a hatshaped section includes a top plate part, apair of vertical wall parts connecting to the top plate part, and a pair of flange parts connecting to the respective vertical wall parts. In general, mutual edge portions of a plurality of press-formed products are laid on one another, and are joined by resistance spot welding. In this way, a hollow structural member is assembled. [0003] ln an automobile structural member as above, reinforcing members are 3 1l joined by resistance spot welding to important parts such as those where the formed products are joined to each other, and those where high loads are applied. For the reinforcing members, press-formed products of steel plates are frequently used. In the case of a rocker, for example, reinforcing members that partition an inside of the rocker are disposed at one spot or a plurality of spots in a lengthwise direction of the rocker. This is for ensuring three-point bending crush performance of the rocker with respect to the load of a lateral collision. The reinforcing member is referred to as a bulkhead or a stiffener. [0004] FIG. 1 is a perspective view showing an example of a rocker including a conventional bulkhead inside. FIG. I shows a state in which a bulkhead 110 inside is seen through. In the following explanation, the case where the structural member is a rocker 1 is taken as an example. [000s] As shown in FIG. 1, the rocker 1 is a long hollow cylindrical body. A sectional shape of the rocker 1 illustrated in FIG. 1 is substantially rectangular. The bulkhead 110 includes a top plate part 111 and flange parts 114. An outline shape of the top plate part 11 1 substantially corresponds to an inner sectional shape of the rocker 1. The flange parts I 14 are formed at respective four sides 112 of the top plate part I11. The bulkhead 110 is disposed in a predetermined position in a lengthwise direction in the inside of the rocker 1 . The respective flange parts 1 14 of the bulkhead 110 are overlaid on inner surfaces of the rocker 1, and are joined to the rocker 1 by resistance spot welding or the like. From the viewpoint of reducing the weight of an automobile body, press-formed products formed from a high+ensile strength steel plate are used for the rocker I and the butkhead 110. Tensile strength of the high-tensile strength steel plate in this case is usually set aT 440 MPa or more, and is sometimes set to be more than 440 MPa in accordance with required performance or the like. [0006] Conventionally, the flange parts 114 have been formed by individually folding respective edge parts of the four sides 112 of the top plate part 111. Hereinafter, the individual flange parts 114 which extend frorn the respective sides 1 12 of the top plate part 1 I I will also be referred to as "side flange parts". In this case, the flange parts 114 are separated to be discontinuous at comer parts 113 that connectthe adjacent sides 112 of the topplatepart 111. In some cases, cutouts 117 are provided at both side ends of each of the side flange parts 1 14, as shown in FIG. 1. [0007] Speaking from a performance aspect of the bulkhead 1 10 (the reinforcing member) in the rocker 1 (the automobile structural member), it is more desirable that the flange parts are configured as follows, than the flange parts 114 are discontinued in the corner parts 113 of the top plate part 111 as in the conventional bulkhead 110 shown in FIG. 1, from an idealistic viewpoint. The flange parts also extend from therespectivecornerparts 113 ofthetopplatepart 111. Hereinafter,theindividual flange parts which extend from the respective corner parts 113 of the top plate part 111 will also be referred to as "corner flange parts". The adjacent side flange parts are connected via the comer flange part. In this case, the flange parts continue from the certain side 1 12 of the top plate part 1 11 throughout the corner parts I 13 and the adjacent sides 112. Hereinafter, the flange part that continues without being separated in the corner part I I 3 of the top plate part 1 I I will also be referred to a "continuous flange part". [0008] However, it is very diffrcult to obtain the product including the continuous flange part by press forming. It has been, of course, conventionally possible to obtain a product such as a tid of a can by press-forming a soft plate material of soft steel, aluminum or the like. However, it is difficult to produce a product including the continuous flange part by especially press-forming a high-tensile strength steel plate, in order to obtain a reinforcing member in the automobile structural member described above. This is for the following reason. [000e] When producing the product including a continuous flange part from a steel plate by press forming, forming of the corner flange part is shrink-flanging. Consequently, wrinkles easily occur in the corner flange parts. The wrinkles occur more remarkably as strength of the steel plate is higher. [0010] When wrinkles occur in the corner flange part, the following problem arises. Referring to the example shown in FIG. 1, when the bulkhead 110 is disposed inside the rocker 1 and mutual overlapping portions are joined by resistance spot welding, gaps due to wrinkles are generated in the overlapping portions. Thereby, a trouble such as poor welding easily occurs. Further, position of the bulkhead I l0 in the rocker I easily becomes unstable. Accordingly, if wrinkles occur in the corner fTange parts of the bulkhead 1 10, the bulkhead I I 0 cannot be used as an actual product. [0011] Prior arts that suppress occurrence of wrinkles in the regions to be shrinkflanged at the time of press-forming steel plates are as follows. [0012] For example, Japanese Patent No.2554768 (Patent Literature 1) and Japanese Patent Application Publication No. 07-1 12219 (Patent Literature 2) each disclose a technique of forming a roof panel having an opening for a sunroof. Each of the techniques of Patent Literatures I and2 indicates that in order to absorb an excess line length of the region to be shrink-flanged at a time of forming, an excess thickness part is given in advance. Japanese Patent No. 2560416 (Patent Literature 3) discloses a technique of square cylinder draw forming. In the technique of Patent Literature 3, a specif'rc shape is given to a region to be shrink-flanged. Japanese Patent Application Publication No. 04-l 18118 (Patent Literature 4) discloses a technique of press forming using a cam structure. It is indicated that in the technique of Patent Literature 4, forming is performed while presser bar pressure is given to a region to be shrink-flanged. CITAIION LIST PATENT LITERATURE [0013] Patent Literature 1: Japanese Patent No. 2554768 Patent Literature 2: Japanese Fatent Application Publication No. 07-l I22I9 Patent Literature 3: Japanese Patent No. 256041 6 Patent Literature 4: Japanese Patent Application Publication No. 04-118118 SUMMARY OF INVENTION TECHNICAL PROBLEM [0014] In development of automobiles, higher functionality and higher value addition of structural members are always studied, and study on enhancement of three-point bending crush performance of the structural members at a time of crushing is not an exception. [0015] Therefore, an objective ofthe present invention is to provide a pressformed product suitable as a reinforcing member (a bulkhead) that contributes to enhancement of three-point bending crush performance of a structural member (example: a rocker or the like) at a time of a collision, for example, in particular, a press-formed product including a continuous flange part in which wrinkles are suppressed in corner flange parts. Further, another objective of the present invention is to provide an automobile structural member that includes the pressformed product as a reinforcing membe¡ and is excellent in three-point bending crush performance. [00r6] In each of the aforementioned techniques of Patent Literatures 1 to 3, the excess line length to be the cause of wrinkles and excessive thickness at the time of press formíng is absorbed by the excess thickness part which is given in advance. The excess thickness part remains in the product after press forming. Consequently, not only the excess thickness part becomes a hindrance to resistance spot welding in a domain of the excess thickness part, but also the excess thickness part sometimes becomes a hindrance to resistance spot welding in other domains. [0017] In the aforementioned technique of Patent Literature 4, a shrinkage degree and a reaction force received by a cam structure are small when a product having a region with a large radius of curvature (2100 rnm, for example) is formed. consequently, wrinkles can be suppressed in the region with a large radius of .$) '': ;' cwvattue. However, when a product having a region with a small radius of curvature (5 mrr, for example) is formed, a shrinkage degree and a reaction force received by a cam structure are large. Consequently, wrinkles cannot be suppressed in the region with a small radius of curvature. Especially in press-forming with use of a high-tensile strength steel plate as a blank, yield stress of the blank is high, and therefore, out-of-plane deformation of the blank under elastic deformation becomes large. consequently, excessively large wrinkles occur. In addition, the reaction force received by the cam structure becomes excessively large, and therefore, handling is difficult with the cam structure. [0018] As above, it is difficutt to obtain a product including a continuous flange part by press forming in the prior technique. In particular, it is nearly impossible in the prior technique to press-form a high-tensile strength steel plate of 440 MPa-class or more, and produce a product with wrinkles suppressed, in order to obtain a pressformed product suitable for a reinforcing member for an automobile structural mernber. [001e] Therefore, still another objective of the present invention is to provide a production method and a production device capable of producing a press-formed product which is a press-formed product including a continuous flange, and in which wrilkles are suppressed in corner flange parts. SOLUTION TO PROBLEM [0020] A press-formed product according to one embodiment of the present invention is formed from a steel plate. The press-formed product includes a top plate part and flange parts. The flange parts each extend from some or all of sides constituting an outline of the top plate part. A comer flange part extends from a corner part connecting, among the sides of the top plate part, at least a pair of the sides which are adjacent to each other and from which the flange parts extend. The flange parts each extending from the pair of the sides are connected to each other via the corner flange part. In the comer flange part, a part having a minimum value of plate thickness is present in a central portion in a direction along a circumferential direction of the corner part of the top plate part, parts having a maximum value of the plate thickness are present on both sides of the central portion, and aratio of the maximum value and the minimum value is in a range of more than 1.0 to 1.6 or less. [0021] In the above described press-formed product, a radius of curvature of the corner flange part is preferably 30 mm or less. 100221 . h the above described press-formed product, a width of at least a part of the flange parts is preferably 10 mm or more. [0023] In the above described press-formed product, tensile strength of the pressformed product is preferably 440 MPa or more. L0o24l An automobile structural member according to one embodiment of the present invention is a hollow structural member constituting an automobile body. The automobile structural member includes therein the press-formed product descrjbed above as a reinforcing member, wherein the flange parts are joined to the automobile structural member. [0025] A production rnethod for a press-formed product according to one embodiment of the present invention is for producing the press-formed product from a blank ofa steel plate. The press-formed product includes a top plate part, flange parts and a corner flange part. The flange parts each extend from sorne or all of sides constituting an outline of the top plate part. The corner flange part extends from a corner part connecting, among the sides of the top plate part, at least a pair of the sides which are adjacent to each other and from which the flange parts extend. The corner flange part connects the flange parts each extending from the pair of the sides, to each other. The production method uses a punch, a die and a convex-shaped tool that is disposed in a position corresponding to the corner flange part, with both of the die and the convex-shaped tool being paired with the punch, folds the blank by relative movement of the die to the punch, and thereby forms the flange parts, and folds the blank prior to the folding of the blank by the die, by relative movement of the convex-shaped tool to the punch, and thereby forms the corner flange part. [0026] In the above described production method, the convex-shaped tool is preferably constructed integrally with or separately from the die. 10a27) In the above described production method, the production method preferably fuither uses a blank holder that is paired with the die, and performs forming of the corner flange part and the flange parts in a state where the blank is sandwiched by the die and the blank holder. [0028] A production device for a press-formed product according to one embodiment of the present invention is for producing a press-formed product from a blank ofa steel plate. The press-formed product includes a top plate part, flange parts, and a corner flange part. The flange parts each extend from some or all of sides constituting an outline of the top plate part. The comer flange part extends frorn a corner part connecting, among the sides of the top plate part, at least a pair of the sides which are adjacent to each other and from which the flange parts extend. The corner flange part connects the flange parts each extending from the pair of the sides, to each other. The production device includes a punch, a die and a convex-shaped tool that is disposed in a position corresponding to the corner flange part, with both of the die and the convex-shaped tool being paired with the punch. The die folds the blank by relative movement to the punch, and thereby forms the flange parts. The convex-shaped tool folds the blank prior to the folding of the blank by the die, by relative movement to the punch, and thereby forms the corner flange part. l0tzel In the production device, a radius Rt [mm] of curvature of a front end of the convex-shaped tool preferably satisfies conditions of Formula (a) as follows. Rt