[Designation of Document] SPECIFICATION [Title of the Invention] METHOD, DEVICE, PROGRAM, AND RECORDING MEDIUM OF ANALYZING CAUSE OF SPRINGBACK [Technical Field] [0001] The present invention relates to a method, a device, a program, and a recording medium of analyzing a cause of springback in a formed product that is press-formed from a steel plate or other metal plates into parts of automobiles or household appliances. Although the invention will be described with reference to a steel plate, the invention may also be applied to other metal plates, plastic plates and linear materials. Priority is claimed on Japanese Patent Application No. 2008-329099, filed December 25,2008, the content of which are incorporated herein by reference. [Background Art] [0002] Many parts of automobiles, such as doors and bumpers, or household appliances, such as refrigerator panels, are press-formed from a steel plate or other metal plates. There is an increasing demand for lightweight press-formed products. Therefore, high-strength steel plates are used to provide thin and lightweight products. High-strength steel plates, however, have greater deformation resistance, which may increase the likelihood of occurrence of springback caused by residual stress during the process of press-forming. [0003] There is a recent trend that a forming process planning for forming products is started at the same time as a design stage of automobiles or the like, in order to reduce development man-hours and manufacturing costs. In order to follow the trend, a configuration of a press-formed product and its forming data are analyzed by a computer. The analysis provides calculation of a springback amount of the press-formed product estimated from the residual stress after the forming. The die configuration is corrected in accordance with the calculated springback amount. [0004] Patent Document 1 and Non Patent Document 1 disclose a method of determining a die configuration by estimating springback as described above. In particular, residual stress in a steel plate pressed in a die at the press bottom dead center is analyzed by a finite element method, and a die having a configuration of deformation (i.e., spring forward) caused by a residual stress toward the direction opposite to the aforementioned residual stress is numerically analyzed. In this manner, a die configuration which addresses the problem of springback can be obtained easily. [0005] However, it is very difficult to design a die through numerical analysis taking the problem of the springback into consideration in a complete manner, because it is a nonlinear problem. The methods in the foregoing documents are proposed only to obtain a simple die which is designed taking a consideration of springback by the finite element method. The documents therefore suggest no countermeasures against a product obtained by press-forming in a die that is outside of the tolerance for springback, which is a phenomenon that is difficult to analyze numerically. [0006] If a formed product which satisfies the tolerance value for the springback cannot be obtained using a die designed considering the springback problem, countermeasures to be undertaken must be determined by experienced technical personnel. Accordingly, there is a need to produce an actual die and to repeatedly modify the die configuration while pressing steel plates in the die. [0007] Another approach to reduce the springback is to modify configurations of steel plates or formed products, not the configuration of the die, to remove residual stress. An exemplary modification method is to provide an opening or a slit in the formed product at an area where springback is occurring. [0008] This approach can reduce residual stress which may otherwise cause springback by undertaking a countermeasure against areas where springback is occurring. However, since cutting or punching may decrease rigidity of the product itself, only slight residual stress tends to cause great springback. For this reason, this approach fails to completely eliminate the springback problem. In addition, such an approach needs tests with an actual test die and a steel plate, which increases man-hours and costs at the design stage. [0009] Patent Documents 2-5 also disclose simulations by the finite element method. The methods disclosed in Patent Documents 2-4 employ partial stress release and modification. In Patent Document 2, however, evaluation is only made with respect to an amount of angle variations, i.e., torsion, before and after springback occurs in parts and thus factors that cause deformation other than torsion are outside of the discussion. In Patent Document 2, all the stress components at release positions during stress release are set to 0. If deformation is large, linear approximation performed with respect to stress gradients produces larger inconsistency between the linear approximation and actual nonlinear transition. [Related Art Documents] [Patent Documents] [0010] [Patent Document 1] Japanese Unexamined Patent Application, First Application No.2003-33828 [Patent Document 2] Japanese Unexamined Patent Application, First Application No. 2007-229724 [Patent Document 3] Japanese Unexamined Patent Application, First Application No. 2008-49389 [Patent Document 4] Japanese Unexamined Patent Application, First Application No. 2008-55476 [Patent Document 5] Japanese Unexamined Patent Application, First Application No. 2004-148381 [Non Patent Documents] [0011] [Non Patent Document 1] Mitsubishi Motors Corporation technical review (No. 18,2006, pages 126 to 131) [Disclosure of the Invention] [Problem that the Invention is to solve] [0012] As described above, although the press-forming process and the press-formed product have been analyzed through numerical analysis, it is difficult to accurately specify the cause of springback in a press-formed product at the design stage before conducting actual forming tests. [0013] It is therefore an object of the invention to provide analysis of the cause of springback with which an area of a press-formed product in which springback occurs can be analyzed more accurately than ever before through numerical analysis and thus the time and cost needed to determine a process for a forming product can be reduced. [Means to Solve the Problems] [0014] The invention has the following aspects in order to solve the foregoing problems. (1) A first aspect of the invention is a method of analyzing a cause of springback, which includes: performing a forming analysis through a numerical simulation based on a forming condition of a plastically formed product so as to calculate forming data of the formed product; decomposing, over an entire of the formed product, stress data included in the forming data of the formed product into an in-plane stress component and a bending moment component with respect to at least one directional component of directional components of the stress; generating, from the forming data of the formed product, an individual decomposition data including at least one of a first individual decomposition data and a second individual decomposition data as a before-calculation individual decomposition data, the first individual decomposition data having only an in-plane stress component regarding the stress of the decomposed directional component, the second individual decomposition data having only a bending moment component regarding a stress of the decomposed directional component; performing a calculation for at least one directional component of stress in the before-calculation individual decomposition forming data regarding each of areas divided from the formed product, so as to generate an after-calculation individual decomposition forming data; analyzing a first springback configuration obtained through a numerical simulation with respect to the before-calculation individual decomposition forming data and a second springback configuration obtained through a numerical simulation with respect to the after-calculation individual decomposition forming data; obtaining a degree of influence of a stress in each of the areas with respect to a springback deformation, calculated based on a before-springback configuration of the formed product included in the forming data, the first springback configuration, and the second springback configuration; and displaying the degree of influence with respect to the springback deformation calculated for each area. (2) In the method of (1), the performing of the forming analysis may be executed through numerical simulation by a finite element method using multiple elements; an average stress in a plate thickness direction of each directional component for each element in the forming data of the formed product may be used as the in-plane stress component of the directional component; and a value obtained by subtracting the average of the in-plane stress from each of the directional components of the stress value for all of the integration points which arises for each element may be used as the bending moment component of the directional component. (3) In the method of (1), the calculation may be executed by multiplying at least one of directional components of a stress of the before-calculation individual decomposition forming data by a coefficient k in a range of-2 < k < 2. (4) In the method of (3), the range of the coefficient k may be 0 < k < 1. (5) In the method of (4), the range of the coefficient k may be 0.5 < k < 0.95. (6) In the method of (1), the formed product may be a press-formed product. (7) A second aspect of the invention is a springback cause analysis device, which includes: a forming analysis section that performs a forming analysis through a numerical simulation based on a forming condition of a plastically formed product so as to calculate forming data of the formed product; a decomposing section that decomposes, over an entire of the formed product, stress data included in the forming data of the formed product into an in-plane stress component and a bending moment component with respect to at least one directional component of directional components of the stress; a before-calculation individual decomposition forming data generating section that generates, from the forming data of the formed product, an individual decomposition data including at least one of a first individual decomposition data and a second individual decomposition data as a before-calculation individual decomposition data, the first individual decomposition data having only an in-plane stress component regarding the stress of the decomposed directional component, the second individual decomposition data having only a bending moment component regarding a stress of the decomposed directional component; a calculation section that performs a calculation for at least one directional component of stress in the before-calculation individual decomposition forming data regarding each of areas divided from the formed product, so as to generate an after-calculation individual decomposition forming data; a springback analyzing section that analyzes a first springback configuration obtained through a numerical simulation with respect to the before-calculation individual decomposition forming data and a second springback configuration obtained through a numerical simulation with respect to the after-calculation individual decomposition forming data; an influence obtaining section that obtains a degree of influence of a stress in each of the areas with respect to a springback deformation, calculated based on a before-springback configuration of the formed product included in the forming data, the first springback configuration, and the second springback configuration; and a display section that displays the degree of influence with respect to the springback deformation calculated for each area. (8) In the device of (7), the display section may display the degree of influence with respect to the springback deformation calculated for each of the areas as a contour display. (9) A third aspect of the invention is a program for analyzing a cause of springback, which includes: performing a forming analysis through a numerical simulation based on a forming condition of a plastically formed product so as to calculate forming data of the formed product; decomposing, over an entire of the formed product, stress data included in the forming data of the formed product into an in-plane stress component and a bending moment component with respect to at least one directional component of directional components of the stress; generating, from the forming data of the formed product, an individual decomposition data including at least one of a first individual decomposition data and a second individual decomposition data as a before-calculation individual decomposition data, the first individual decomposition data having only an in-plane stress component regarding the stress of the decomposed directional component, the second individual decomposition data having only a bending moment component regarding a stress of the decomposed directional component; performing a calculation for at least one directional component of stress in the before-calculation individual decomposition forming data regarding each of areas divided from the formed product, so as to generate an after-calculation individual decomposition forming data; analyzing a first springback configuration obtained through a numerical simulation with respect to the before-calculation individual decomposition forming data and a second springback configuration obtained through a numerical simulation with respect to the after-calculation individual decomposition forming data; obtaining a degree of influence of a stress in each of the areas with respect to a springback deformation, calculated based on a before-springback configuration of the formed product included in the forming data, the first springback configuration, and the second springback configuration; and displaying the degree of influence with respect to the springback deformation calculated for each area. (10) In the program of (9), the performing of the analysis may be executed through numerical simulation by a finite element method using multiple elements; an average stress in a plate thickness direction of each directional component for each element in the forming data of the formed product may be used as an in-plane stress component of the directional component; and a value obtained by subtracting average of the in-plane stress from each of the directional components of the stress value for all of the integration points for each element is used as a bending moment component of the directional component. (11) A fourth aspect of the invention is a computer-readable recording medium in which the program for analyzing the cause of springback according to (9) is recorded. (12) A fifth aspect of the invention is a method of analyzing a cause of springback, which includes: performing a forming analysis through a numerical simulation based on a forming condition of a plastically formed product so as to calculate forming data of the formed product; decomposing, over an entire of the formed product, stress data included in the forming data of the formed product into an in-plane stress component and a bending moment component with respect to at least one directional component of directional components of the stress; generating, from the forming data of the formed product, an individual decomposition data including at least one of a first individual decomposition data and a second individual decomposition data as a before-calculation individual decomposition data, the first individual decomposition data having only an in-plane stress component regarding the stress of the decomposed directional component, the second individual decomposition data having only a bending moment component regarding a stress of the decomposed directional component; performing a calculation for at least one directional component of stress in the before-calculation individual decomposition forming data regarding each of areas divided from the formed product, so as to generate an after-calculation individual decomposition forming data; analyzing a springback configuration obtained through a numerical simulation with respect to the after-calculation individual decomposition forming data; obtaining a degree of influence of a stress in each of the areas with respect to a springback deformation calculated based on a before-springback configuration of the formed product included in the forming data and the springback configuration; and displaying the degree of influence with respect to the springback deformation calculated for each area. [Effects of the Invention] [0015] According to the present invention, the cause of springback can be accurately analyzed and the time needed to determine a forming process for a formed product can be reduced. [0016] Further, the present invention provides a springback cause analysis which cannot be performed with actual products. Countermeasures against the springback can be taken by decomposing the problem into small components. [0017] The present invention includes dividing a press-formed product into areas and multiplying, by a coefficient k, at least one directional component of stress in individual decomposition forming data in an area of interest for each of the areas. The coefficient k is preferably in a range of -2 < k < +2 (including 0). If the coefficient k is 0, calculation is simplified and influence of stress for each area with respect to springback deformation can be evaluated clearly, based on the calculated degree of influence. If the coefficient k is a value close to +1, the degree of influence can be calculated and evaluated with higher accuracy. Evaluation accuracy is improved with the value of the coefficient k close to 1 as compared to the coefficient k close to 0 because a relationship between stress and displacement is practically nonlinear. If deformation is small, there is almost no difference in stress gradients before and after editing with respect to displacement between linear approximation simulation and an actual nonlinear process. In this case, even if calculation is performed with the coefficient k set to 0, the value of the degree of influence of the stress with respect to the springback for each area can be sufficiently accurate for analysis and evaluation. If deformation is large, on the contrary, difference in the stress gradients before and after editing with respect to the displacement becomes large between the linear approximation simulation and an actual nonlinear process. Therefore, the linear approximation may include errors. If the calculation is performed so that a value of stress after editing is close to a value of stress before editing (i.e., if the coefficient k is close to 1), the calculation is performed with the stress gradients before and after the editing with respect to the deformation are close to that of an actual nonlinear process. Accuracy in evaluating values of the degree of influence of the stress with respect to the springback of each area is therefore improved as compared to a case where the coefficient k is 0 (see Fig. 10). It is especially advantageous to set the coefficient k to be a value close to +1. [BRIEF DESCRIPTION OF THE DRAWINGS] [0018] [Fig. 1] Fig. 1 illustrates a configuration of a device which analyzes a cause of springback according to an embodiment of the present invention. [Fig. 2] Fig. 2 schematically illustrates a method of analyzing the cause of springback according to an embodiment of the present invention. [Fig. 3] Fig. 3 illustrates an exemplary hardware configuration of a device which analyzes a cause of springback. [Fig. 4] Fig. 4 is a perspective view illustrating a configuration of a press-formed product in Example 1. [Fig. 5] Fig. 5 illustrates divided areas of the press-formed product in Example 1. [Fig. 6] Fig. 6 illustrates a result of springback analysis based on original data acquired from press-forming analysis. [Fig. 7A] Fig. 7A illustrates a springback amount in each area for which calculation is performed with respect to bending moment component (i.e., deviator stress) decomposition data. [Fig. 7B] Fig. 7B illustrates a springback amount in each area for which calculation is performed with respect to in-plane stress component (i.e., mean stress) decomposition data. [Fig. 8 A] Fig. 8 A illustrates a configuration of a press-formed product in Example 2. [Fig. 8B] Fig. 8B illustrates divided areas and fixed points of the press-formed product in Example 2. [Fig. 9A] Fig. 9A illustrates a configuration of a press-formed product in Example 3. [Fig. 9B] Fig. 9B illustrates divided areas and fixed points of the press-formed product in Example 3. [Fig. 10] Fig. 10 is a graph which illustrates a relationship between stress and displacement. [Fig. 11 A] Fig. 11A illustrates a configuration of a press-formed product in Example 5. [Fig. 12B] Fig. 11B illustrates divided areas and fixed points of the press-formed product in Example 5. [Fig. 11C] Fig. 11C illustrates a torsional angle about X-axis relating to the press-formed product in Example 5. [Fig. 12A] Fig. 12A illustrates a configuration of a press-formed product in Example 6. [Fig. 12B] Fig. 12B illustrates divided areas and fixed points of the press-formed product in Example 6. [Fig. 13 A] Fig. 13 A illustrates a configuration of a press-formed product in Example 7. [Fig. 13B] Fig. 13B illustrates divided areas and fixed points of the press-formed product in Example 7. [Fig. 14A] Fig. 14A illustrates a configuration of a press-formed product in Example 8. [Fig. 14B] Fig. 14B illustrates divided areas and fixed points of the press-formed product in Example 8. [Fig. 14C] Fig. 14C illustrates relative displacement (i.e., torsion) of four nodes relating to the press-formed product in Example 5. [Fig. 15 A] Fig. 15 A illustrates a configuration of a press-formed product in Example 9. [Fig. 15B] Fig. 15B illustrates divided areas and fixed points of the press-formed product in Example 9. [Fig. 16A] Fig. 16A illustrates a global coordinate system of the press-formed product in Example 9. [Fig. 16B] Fig. 16B is a cross-sectional view of Fig. 16A taken along line F-F. [Fig. 17A] Fig. 17A illustrates a local coordinate system of the press-formed product in Example 9. [Fig. 17B] Fig. 17B is a cross-sectional view of Fig. 17A taken along line G-G [Embodiments of the Invention] [0019] Hereinafter, preferred embodiments of the present invention will be described referring to analysis of a cause of springback in a product press-formed from a thin plate material. Application of the present invention, however, is not limited to the same, and may include roll-formed products and formed linear materials. [0020] Fig. 1 illustrates a functional configuration of a springback cause analysis device 1 according to an embodiment of the present invention. The springback cause analysis device 1 includes a forming condition input section 2, a press-forming analysis section 3, a decomposition forming data generating section 4, an area division and calculation section 5, a springback analysis section 6, a degree of influence calculating section 19, a degree of influence output screen 20 which is a display section, and a file storage section S. [0021] The forming condition input section 2 is for inputting forming conditions, which includes configuration data (including plate thickness, length, width, curvature and distortion), nature (including quality of material, such as strength and extension), a die configuration (including configurations of a die and a punch, curvature, diameter, clearance and lubrication condition), press conditions (including load for pressing wrinkles, pad load, bead tension, pressing pressure and temperature) regarding a steel plate to be analyzed in the press-forming analysis section 3 and the springback analysis section 6. Data areas used for forming analysis, data areas used in the decomposition forming data generating section 4, data areas used in the area division and calculation section 5 and divided areas used for displaying an analysis result on an output screen can be set up separately and can be input. [0022] The press-forming analysis section 3 obtains, through numerical analysis, a configuration, stress, distortion, plate thickness of a formed product to be press-formed on the basis of the data input from the forming condition input section 2. The numerical analysis may be performed by an elastic-plastic finite element method, a rigid-plastic finite element method, a one-step finite element method and a boundary element method. The press-forming analysis section 3 outputs results of the numerical analysis in a form of variables, such as plate thickness of a workpiece, component values of stress and component values of strain or a distribution of these variables. Output data (i.e., original data) is output to the decomposition forming data generating section 4, the area division and calculation section 5, the springback analysis section 6 and the degree of influence calculating section 19 as, for example, a file "P org.k." and is stored in the file storage section S. [0023] The numerical analysis in the press-forming analysis section 3 may include setting up the forming conditions, such as configuration data, nature, die configuration and press conditions, using the finite element method and performing forming analysis to numerically obtain distribution of stress and strain after forming. Examples of software used for the numerical analysis in the finite element method include commercially-available software, such as PAM-STANP, LS-DYNA, AUTOFORM, OPTRIS, ITAS-3D, ASU/P-FORM, ABAQUS, MARC, HYSTAMP, HYPERFORM, SIMEX, FASTFORM-3D and QUICKSTAMP. [0024] The decomposition forming data generating section 4 decomposes, over the entire press-formed product, the forming data regarding the press-formed product acquired in the press-forming analysis section 3 into an in-plane stress component and a bending moment component with respect to at least one of directions of directional components of the stress for each element. Regarding the stress of the directional components obtained by the decomposition of the forming data of the press-formed product acquired in the press-forming analysis section 3, individual decomposition data having only an in-plane stress component and individual decomposition data having only a bending moment component are generated. The in-plane stress component herein is a mean stress component of the distribution in a plate thickness direction of the in-plane direction stress of the formed product. The bending moment component is a deviator stress of the distribution in the plate thickness direction in the in-plane direction stress of the formed product, that is, a stress component having a distribution in the plate thickness direction obtained by subtracting the mean stress component from the distribution in the plate thickness direction, in the in-plane direction stress. [0025] The mean stress of the distribution in the plate thickness direction for every element of the result of the forming analysis is assigned to all the integration points in the plate thickness direction for every element to generate in-plane stress component decomposition data. Bending moment component decomposition data is also generated by subtracting the mean stress extracted from the original forming analysis result from the stress values of all the integration points in plate thickness direction generated for every element. That is, the mean stress in the forming data is used as the in-plane stress component and a value obtained by subtracting the in-plane mean stress from the stress values of all the integration points in plate thickness direction generated for every element is used as the bending moment component. [0026] Decomposition to the directions of the stress herein may be performed on the basis of a global coordinate system or a local coordinate system. The local coordinates system is based on a coordinate system of nodes constituting each of the elements. The local coordinate system may be set to each element on the basis of the global coordinate system in an initial state in the press-forming analysis of each element, i.e., in a state of an initial blank of the pressing, and the stress may be decomposed on the basis of a coordinate system after the press-forming obtained by moving and rotating the local coordinate system set for each element following deformation of each element in the press-forming. [0027] In this manner, "P rem.hei.k" and "P rem.hen.k" are acquired. "P rem.hei.k" is individual decomposition data obtained by decomposing data of the forming analysis result obtained by numerically analyzing the forming conditions of the press-formed product into data of the in-plane stress component with respect to at least one of directions of the directional components of the stress over the entire press-formed product. "P rem.hen.k" is individual decomposition data obtained by decomposing data of the forming analysis result obtained by numerically analyzing the forming conditions of the press-formed product into data of the bending moment component with respect to at least one of directions of the directional components of the stress over the entire press-formed product. These individual decomposition data are output to the area division and calculation section 5 and the springback analysis section 6, and are stored in file storage section S. [0028] The area division and calculation section 5 inputs data files "P rem.hei.k" and "P rem.hen.k" output from the decomposition forming data generating section 4, performs area division on the basis of configuration data of the press-formed product, performs calculation for each area, outputs "P rem2.hei.k" and "P rem2.hen.k" for each area as a calculation result to the springback analysis section 6 and stores the data in the file storage section S. The calculation is performed with respect to at least one of directional components of the stress in an area of interest for each of the divided areas regarding "P rem.hei.k" and "P rem.hen.k." The calculation is a multiplication using a coefficient k, which is preferably -2 < k < +2, more preferably 0 < k < 1 and even more preferably 0.5