The present invention relates to a press forming method for a workpiece material which is made of steel, and a tool for press fol-ming which is used in the press forming method. Priority is claimed on Japanese Patent Application No. 2014-103735, filed on May 19,2014, the content of \vIiich is incorporated herein by reference. [Related Art] [0002] As a method for fom~inga final product such as a bottoriied cylindrical member having a vertical wall portion and a bottom \vall portion which is continuous with the vertical wall portion from a plate-shaped material, a cup-shaped intermediate material, or the like, a drawing method is widely used. For example, Non-Patent Document 1 discloses a method \vIlich fornis a cylindrical container having a constant inner diameter frotii a bottorn portion to an opening portion, or a stepped cylindrical product having a step portion in \vhich an inner diameter changes on the way from the bottom portion to the opening portion. That is, in general, a method is widely used, in which an intermediate material ~vhichis fornled into a cup shape fro111 a disk-shaped material in a first process is drawn in a second process again, and tlie cup-shaped ititelmediate material is further dtawn by the re-dta~vingm ethod. [0003] In this re-drawing method, tlie cup-shaped intermediate material formed in the first process is nipped between a die in ~vhichth e intermediate matet ial is accotnmodated, and a blank holder \\4iicli is a cplindrical tool inserted into the inner portion of the intermediate material. hi addition, a punch coaxially passing through the inner portion of the blank holder is pushed to be inserted into a colur~mars pace \\~hichi s fornled on the - 1 - bottom of the die, and a cylindrical protrusion is for~ned on the bottom \\call portio~io f the cup-shaped intermediate material. Ho\vever, in this fortiiing method, the material configuring the bottom \\(all portion of tlie cop-shaped intermediate material may not be sutficiently fed into the columnar space by the punch. In this case, there are problems that tlie bottom wall portion of the intermediate material nlay be broken by the tip angle portion of the punch, and a forming failure due to insufficient supply of a material into the colu~iinasrp ace may occur. [0004] With respect to the above-described problems, in Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2, a method for preventing a forming failure using a tool divided into multiple portions is disclosed. That is, as \vitli the re-drawing method of the related art, the upper edge portion of the intertnediate material is pressed by the second punch \vhile the first punch is poslied into the bottom wall portion of tlie cup-shaped intermediate material so as to fonn a cylindrical protnision. According to this nietl~ods, upply of a material into the periphery of the tip angle portion of the first punch is promoted doe to a pressing force by the second punch, and as a result, it is possible to prevent a forming failure due to a material breakage or the like. In addition, Patent Document 2 discloses a method in which fortning is not perfonned on a cup-shaped intermediate material, and a f i ~ aplr oduct is obtained from a plate-shaped material by a single process. [OOOS] In these forming methods, in order to perform forming in a state where a fortning failure does not occur, it is impo~tantto maintain the movement speed of each tool divided into multiple portions (for example, first punch and second punch) at an appropriate value. In this case, in consideration of variation in material dimensions before fortning, or variation in lubrication states between the tool and the material during the fortning, it is preferable to proceed fortiling while the movement speed of each portion of the tool is suitably corrected to an appropriate value according to the fornring progress situation such as filling of the niatcrial into tlie tool. - 2 - Patent Documents 3 to 5 disclosc a method and a device for tlleasuring a load distribution or a strain amount in a tool during press fonning. Ho~vever, in a fonning method which is used in general, forming is only perfonned wl~ilee ach tool dividcd into nlultiple portions llloves at a co~lstatlst peed which is set in advance before shaping starts. Accordinglj: the move~lle~slpte ed is not corrected according to the material dimensions 01. the progress sittiation of press fornling during for~ning. [Prior Art Documents] [Patent Docoment] [0006] [Patent Document I] Japanese Unexamined Patent Application, First Publication No. 2004-322104 [Patent Documeut 21 Japanese Unexa~ninedP atent Applicatioa, First Publication No. 2010-214381 [Patent Document 31 Japanese Unexamit~edP atent Application, First Pr~blicationN o. 2008-149349 [Patent Docume~l4t 1 Japanese Unexamiued Patent Application, First Publicatio~Nl o. 2008-173686 [Patent Document 51 Japanese Uuexanlined Patent Application, First Publication No. 2010-1 15702 won-Patent Document] [0007] [Nan-Patent Document 11 Takashi SUZUMURA, JOURNAL OF THE JAPAN SOCIETY FOR TECHNOLOGY OF PLASTTCITY, P. 9, vol. 5 I, No. 594 (2010) won-Patent Document 21 Michiharo YOKOI, JOURNAL 01: THE JAPAN SOCIETY FOR TECHNOLOGY OF PLASTICITY, P 13, vol. 51, No. 594 (2010) [Disclosure of the Invention] [Problems to be Solved by the Invention] [0008] In the above-described press fornliog method, if a movement speed ratio between - 3 - a first p~tncal~nd a second punch which move indepet~dentlyfr om eacll other during forming is not appropriate, a load of any one of the turo punches becotnes excessive, the load may exceed a fortning load lirnit of the drawing device, and there is a concern that further fornling may be inlpossible. On the contrary, although both loads of the first punch and the second punch are within the forming load limit of the drawing device, an [tnfilled portion where the tool is not filled with a material remains, and as a result, there is a concern that a forming failure of a product may occur. [0009] The present invention is made in consideratiot~o fthe above-described circutnstances, and an object thereof is to provide a press fanning method and a tool for press fortning in \\~hich it is not inlpossible to perform fortning if a forming load exceeds a load limit of a press forn~ingd evice when each portions of a tool divided into multiple portions are operated independently from each other, atid a product in which a for~ning failure due to unfilling the tool with a material does not occur can be stably fonned . [Means for Solving the Problem] [OO lo] In order to solve the problem and achieve the object, the inventors investigated a method for ascertaining a material inflo~va t a predetermined position inside a tool in a non-contact manner. In addition, as an example of the method, the inventor adopted a nlethod which provides a sensor in tlie tool for measuring deformation of the tool, measures a defomlation amount generated in the tool by the sensor, and detects an overload situation of the tool during forn~ing. According to this method, it is possible to prevent the load applied to the tool from excessively exceeding the load limit of the press fortning device so as not to be itnpossible to perforn forming, and it is possible to prevent a fortning failore of a product associated \\rith the unfilling the tool ~vitha material. [OOll] That is, the primary points of the present invention are as follo~\~s. (1) According to an aspect of the present invention, there is provided a press - 4 - forming metllod, including: a first process of obtaining a pressing force applied to each portion of a tool by an workpiece nlaterial during press forn~ing\v hile independently driving the respective each portion of the tool divided into nll~ltiplep ortions and press fortning the \\arkpiece material; and a second process of adjusting at least one of an applied driving force, an applied driving speed, and an applied driving tinling for each portion of the tool to cause a processing portion of the workpiece material in which the state approacl~inga 1 overload state is detected based on the pressing force to flow to other processing portions of the workpiece material. [OO 121 (2) In the aspect according to (I), in the first process, the pressing force may be obtained based on a deformation amount of the tool generated according to the flow of the workpiece material during press forn~ing. [OO 131 (3) In the aspect according to (1) or (2), in the second process, whether or not the state has approached the overload state may be determined by whether or not the pressing force exceeds a predetermined threshold valae. [0014] (4) In the aspect according to any one of (I) to (3), press fonning may be drawing for forming the workpiece material into a cylindrical member having an axis line, and the pressing force rnay be obtained at multiple locations along a circmnferential direction of which the center is the axis line. [OO 151 (5) 111 the aspect according to any one of (1) to (3), press forming may be drawing for fonning the workpiece rnaterial into a cylindrical member having a1 axis line, and the pressing force may be obtained at multiple locations along an extension direction of the axis line. [OO 161 (6) In the case of (S), the pressing force may be further obtained at multiple locations along a circumferential direction of \vhich the center is the axis line. - 6 . 100 I 71 (7) In the aspect according to any one of (I) to (6), the tool nlay include a die and a punch, and tile pressing force may be obtained by a strain sensor which is provided on at least one of the die-and the poncl~. 100 1 81 (8) In the aspect according to any one of (1) to (7), a preliminary process may be perfot-riled before the first process, and the preli~ninatyp rocessing rnay include: a calculation process of obtaining a prediction correspondence relationship between at least one of the driving force, the driving speed, and the driving timing, and the pressing force in which the overload state is not generated, in numerical calculations; a measurement process of measuring the pressing force applied to each portion of the tool by the workpiece material during forming while indepelldently driving the respective each portions of the tool and press forming the workpiece material according to the prediction correspondence relationship obtained by the calculation process, and obtaining a tneasuretnent correspondence relationship between the measured pressing force and at least one of the driving force, the driving speed, and the driving timing; and a correction process of obtaining a difference between the prediction correspondence relationship obtained by the calcolation process and the lneasilrement correspondence relationship obtained by the measurement process, and correcting the prediction correspondence relationship, in which the first process may be performed according to the corrected prediction correspondence relationship obtained by the preliniinary process. 100 191 (9) According to another aspect of the present invention, there is provided a tool for press fortning including a tool divided into tnultiple portions in which each portion individually receives a driving force and press fortns an workpiece material; in which a sensor which acquires a pressing force \\~11ichi s applied to a forming surface of the tool from the workpiece ~nateriald uring press forming. [0020] (10) Jn tllc aspect according to (9), a configuration may be adopted, in which - G - the tool for prcss forn~ingis i~sedf or drawing so that the \vorkpiece material is fortncd into a cylindrical member having an axis line, and the sensor is provided at multiple locations along a circumferential direction of which the center is the axis line. [0021] (11) In the aspect according to (9), a configuration nlay be adopted, in ~vhich the tool for press fornting is usetl for drawing so that the \vorkpiece material is formed into a cylindrical n~embehr aviag i u ~as is line, and the sensor is provided at multiple locations along an extension direction of the axis line. [0022] (12) In the case of (1 l), the sensors be further provided at multiple locations along a circumferential direction of which the center is the axis line. [0023] (13) hl the aspect according to any one of (9) to (12), a configi~rationtn ay be adopted, in which the tool for press forming inclodes a die and a puocl~a, nd the sensor is a strain sensor which is provided on at least one of the die and the punch. [0024] (14) In the case of (13), a detection unit of the strain sensor may be provided at a position at a depth of 5 mrn to 50 tntn frotn the fortning surface of at least one of the die and the punch on \vIvhich the strain sensor is provided. [Effects of the Invention] [0025] According to the aspect described in (I) of the present invention, after the flow state in the material of the workpiece material in the tool is ascertained based on the pressing force acquired by the first process, it is possible to control the operation of each portion of the tool in the second process. Accordingly, it is not itnpossible to perfonn fortning if a fonning load exceeds a load limit of a press forming device when each portions of a tool are operated independently from each other, and it is possible to perform press fonning a ptoduct in whic11 a forming failure due to unfilling the tool \\it11 a inaterial does not occur. . 7 . [0026] 111 the case of (2), since the flo\v in the material of the workpiece tnaterial can be ascertained \writ11 favorable responsiveness, even wllell press forming is performed in a short time, it is possible to secure a tilne required for controlling the driving of each portion of the tool, and it is possible to acarrately perfor111 press fonning of the \vorkpiece material. I11 tlie case of (3), it is possible to control the operation of each portion of the tool, hen tlie flow state of the Iwrorkpiece material during press fortning is instantaneously determined. [0027] In the case (4), since pressing forces are obtained at multiple locations along the circumferential direction of wl~ichth e center is the axis line, it is possible to reliably prevent failed operations doe to variation in the flow states of the workpiece material in the circu~nferer~tidailr ection. In the case (5), since pressing forces are obtained at multiple locations along the extension direction of the axis line, it is possible to ascertain the fortning process of the \vorkpiece material with higher sensitivity. In addition, an application can be performed, in whicll data of the pressing forces obtained along the axis line direction is input to a numerical calculation model so that press forn~ingis sit~~ulatetod increase calculation accuracy. In the case of (6), since the pressing forces are obtained both along the extension direction of the axis line and the circunlferential direction thereof, it is possible to three-dimensionally ascertain the fortning process of the workpiece ~naterial. [0028] In the case of (7), since the flow of the \\forkpiece material can be ascertained with appropriate sensitivity and responsiveness by the strain sensol; it is possible to more accurately perfonn press fonning of the workpiece material. [0029] In the case of (S), since the first process and the second process can be perfortned, - 8 - \vl~ena t least one of the driving force, the driving speed, and the driving timing is optitnized by the preliminaty process, it is possible to tnore accurately perfortn press forming. [0030] According to the aspect described in (9) of the present invention, it is possible to ascertain the flow state of the material of the \\corkpiece material in the tool based on the pressing force acquired by the sensor. Accordingly, it is not inlpossible to perfor111 forming if a fonning load exceeds a load limit of a press fonning device when each portions of a tool are operated independently frotn each other, and it is possible to control so as to be stably drawn a product in which a fornlitig failure due to i~nfillingth e tool with a material does not occur. [003 11 In the case of (lo), since the pressing forces can be obtained at n~ultiplelo cations along the circumferential direction of which the center is the axis line, it is possible to reliably prevent failed operations due to variation in the flow states of the material of the workpiece material in the circumferential direction. In the case of (1 l), since the pressing forces are obtained at nn~ltiplelo cations along the extension direction of the axis line, it is possible to ascertain the forn~ingp rocess of the workpiece material with higher sensitivity. In addition, at1 application can be performed, in which data of the pressing forces obtained along the axis line direction is input to a numerical calculation model so that press forming is simulated to increase calculation accuracy. In the case of (12), since the pressing forces are obtained both along the extension dil-ection of the axis line and the circumferential direction thereof, it is possible to three-dimensionally ascertain the fortning process of the \\forkpiece material. [0032] In the case of (13), since the flow in the material ofthe workpiece material cat) be ascertained with favorable respot~sivenessb y the strain sensor, even when press forming is performed in a short time, it is possible to secure a time requit-ed for controlling the - 9 - driving of each portion of the tool, and it is possible to accurately perform press forming of the workpiece material. 111 the case of (14), measurement can be accurately perfomled \vitIlin a sensitivity range of the strain sensor. [Brief Description of the Drawings] [0033] Fig. 1A is a view showing a first enlbodinlent of a press forming method of the present invention, and is a longitudit~als ectional view when viewed from a cross section i~lcludinga n axis line of a tool. Fig. 1B is a view showing subsequence of the press fonllirlg method, and is a longit~ldinals ectional view when viewed from the same cross section as that of Fig. IA. Fig. 1C is a vie111 showing further subsequence of the press fornling method, and is a longitudinal sectional view w11en viewed fro111 the sarlle cross section as that of Fig. 1A. Fig. 2 is a functional block diagram of a press forming device used in the embodiment. Fig. 3 is a view sho\ving cracks of a tip angle portion of a punch \\fhich beconles a problem in drawing, and is a sectional view \\&en viewed from the cross section including the axis line of the tool. Fig. 4A is a vie\\( showiug an example of a filling process of a nlaterial inside the tool in the press fortning method, and is a longitudinal sectional view when viewed from the cross section including the axis line of the tool. Fig. 4B is a view showing subsequence of the press forming rnethod, and is a longitudinal sectional view when vielved from the same cross section as that of Fig. 4A. Fig. 4C is a view showing subsequence of the press forniing method, and is a longitudinal sectional view when viewed fro111 the sanle cross section as that of Fig. 4A. Fig. 5 is a flo\rcIlart of a calcr~lationp rogram used to control the press forming device. Fig. 6A is a view showing disposition of a sensor of the tool for press for~ning - 10- used in the en~bodimcnat nd tile press forming method using the sensor, and is a longitt~dinasl ectional view when vie\ved fro111t he cross section i~lcludi~th~eg a xis line of the tool. Fig. 6B is a view shoxving subsequence of the press forming method, and is a longitudit~asl ectio~lavl iew ~vhenv ie~vedf rom the sane cross section as that of Fig. 6A. Fig. 7A is a view sho~\~inthge press forming tnethod of the embodin~ent,a nd is a longitudinal sectional view \vheo viewed from the cross section i~lcludingth e axis line of the tool. Fig. 7B is a view sho\ving the subsequence of the press forming method, and is a longitudinal sectional view when viewed from the same cross section as that of Fig. 7A. Fig. 8A is a view showing a modification example of the first embodiment, and is a plan sectional view when viewed from an A-A cross section of Fig. 1A. Fig. 8B is a view showing the n~odificatione xample, and is a plan sectional view mhen viewed fiom line B-B of Fig. 1A. Fig. 9 is a view showing the modification example of the first embodiment, and is a partial sectio~~vaile w corresponding to a C portion of Fig. 1C. Fig. 10A is a view showing a second embodiment of the press forming tnethod of the present invention, and is a longitudinal sectional view when viewed from the cross section including the axis line of the tool. Fig. 10B is a view showing subsequence of the press forming method, and is a lo~lgitudinals ectio~~vaile w when viewed from the same cross section as that of Fig. 10A. Fig. 11A is a vie\\