Materials Transactions, Vol. 60, No. 3 (2019) pp. 450 to 457 ©2019 The Japan Society for Technology of Plasticity

Evaluation of Work Hardening and Tensile Strength for Press Worked Products Plastic Strained from Multiaxial Direction

Tetsuro Ishimura1 and Souichiro Nishino2

1Power Generation Systems Div. Power Business Unit, Hitachi, Ltd., Hitachi 317-8511, Japan 2Institute of Quantum Beam Science, Graduate School of Science and Engineering, Ibaraki University, Hitachi 316-8511, Japan

Tensile strength and work hardening characteristics in the case that strain was applied from multi-axial directions by press processing and in the case that uniaxial strain was applied by tensile test were compared. Furthermore, in a unified manner with the results of the tensile tests, fatigue tests on plate material subjected to multiaxial strain were conducted, and the strength reliability of the pressed product was evaluated. Tensile strength of mild-steel plate subjected to uniaxial or multiaxial plastic strain was increased. If a tensile test is carried out again after a certain period of time has elapsed by applying strain, not only yield stress but also tensile strength are increased. This trend is due to strain aging inside the material. Degree of work hardening is higher when multiaxial strain is applied compared to when uniaxial strain is applied. For materials work hardened by applying strain, their hardness and the tensile strength are proportional regardless of the load status of strain. Accordingly, tensile strength of a product subjected to different strains can be evaluated uniformly by measuring the product’s hardness. When strain is applied from multiple axes in press working, as well as tensile strength, fatigue strength is improved by work hardening. According to the results the microscopic analysis by X-ray diffraction combined with the full-width-at-half-maximum method, dislocation density is increased when multiaxial strain is applied compared to when single-axis strain is applied, and the degree of work hardening is greater. The full width at half maximum is correlated with tensile strength regardless of the state of strain applied to the material. TEM observation of dislocations revealed that the third-step-pressed product subjected to multiaxial strain has a coaxial, fine-grain dislocation-cell structure. On the other hand, in the case of pre-strained (plastic strain of 20%) material subjected to a uniaxial tensile load, the cell structure was unclear, and even the cells that were produced had coarse grains stretched in the tensile direction. [doi:10.2320/matertrans.P-M2018854]

(Received May 21, 2018; Accepted December 12, 2018; Published February 25, 2019) Keywords: mild steel plate, uniaxial and multiaxial straining, work hardening, tensile strength, fatigue strength, X-ray diffraction, full width at half maximum intensity, TEM (Transmission Electron Microscope), dislocation cell

1. Introduction It can therefore be considered that when plastic strain is applied from multiple axial directions, as in the manner of In regard to the manufacture of automotive parts, press press forming, the change in mechanical properties of the (plastic) working is indispensable, and among the available strained material will be larger than that in the case of processing methods, press forming of steel sheets produces uniaxial tension. Car parts are molded from steel sheets by press-worked products with a large specific gravity. More- various press-working processes, and clarifying the influence over, various steel plates with different microstructures and of the plastic strain received in those processes on the static mechanical properties®ranging from mild steel plate to strength (yield stress, tensile strength, etc.) of the parts is a 1180-MPa-class high-strength tensile steel plate®are used key task in regard to correctly evaluating the strength of the for press-forming steel sheets. Many of these press-formed press-worked parts. Although many researches on tensile parts are given plastic strain by the press working, and the strength7­13) and fatigue strength14­20) of steel plates loaded hardness and microstructure of the processed material are with plastic strain have been reported, the majority of those changed by this plastic strain. However, in the case of the reports deal with strain loads in the form of simple tensile current automobile design process, the strength of a designed deformation.5­20) Regarding strain load from multiaxial part is set on the basis of the tensile strength of the steel sheet directions, although some examples of investigations on before pressing. the strength characteristics of steel plate after rolling have If a steel plate is subjected to a certain amount of plastic been reported,5,6) few examples of directly investigating deformation in an unloading and reloading manner, its yield the strength characteristics of pressed products have been stress is increased by work hardening (which is caused by reported. increased dislocation density in the steel sheet). In addition, it In consideration of the above background, in this study, is known that the hardness of a material is improved by strain focusing on the difference in strain-loading methods for press applied by plastic working, and an example in which the forming, we compared the influences on strength reliability hardness of a mild-steel plate is nearly doubled by plastic of (i) strain in one direction (“uniaxial” strain hereafter) under working the plate has been reported.1) As reported, hardness simple tension (plastic strain of 5 to 20%) and (ii) strain from corresponds to tensile strength,2­4) so tensile strength can also multiple axial directions (“multiaxial” strain hereafter). In be improved by plastic deformation. When a steel sheet is particular, car parts made of mild-steel plate and manufac- subjected to uniaxial tensile plastic strain, its yield stress is tured by 10-step press working were targeted. Tensile increased, but its tensile strength is assumed to stay constant. specimens were cut out from blank material, molded into On the contrary, it is known that rolled steel sheet has not pressed products by one of three different pressing processes, only increased yield stress and hardness but also increased and subjected to a tensile test. The effect of the plastic strain tensile strength and fatigue strength compared to those in the multiaxial direction received by the material in each properties of the sheet before rolling.5,6) process on the mechanical properties of the specimens was Evaluation of Work Hardening and Tensile Strength for Press Worked Products Plastic Strained from Multiaxial Direction 451

Table 1 Mechanical properties. piston,1) which was processed from blank material with a diameter of 195 mm in 10 steps, were used. Photographs of the processed (press-worked) items after the first to third steps and their cross-sectional shapes are shown in Fig. 1. The first step is optimal cylindrical drawing with minimum thickness reduction. Specifically, minimum reduction in plate thickness is achieved by setting optimum punch radius Rp, die radius Rd (as shown in Fig. 2) and minimum blank- holding pressure. The second step is reverse-press forming 1st process 2nd process 3rd process in which the first-step-processed item is “reverse pressed” in order to greatly improve re-draw ability. Specifically, as shown in the schematic diagram of the forming process shown in of Fig. 2, the front and back surfaces of the steel sheet are inverted and processed by utilizing the rigidity of 5cm 5cm 5cm the pressed product itself. By this reversed forming of the front and back surfaces, stress in the direction opposite to that acted in the first step can be applied to the work-piece. 5cm 5cm 5cm As a result of this reverse press forming, which utilizes the so-called Bauschinger effect, formability is remarkably improved compared with that in the case of redrawing that is usually used. The third step involves intrusion forming to increase the Fig. 1 Press worked products and cross section. wall thickness of the second-step-molded item to more than the material wall thickness. Specifically, as shown in the investigated. In addition to the mechanical properties schematic diagram of the forming process in Fig. 2, the determined by the tensile tests, strength reliability of the diameter of the vertical wall portion is reduced by pushing pressed products was comprehensively evaluated by cutting the second-step-molded product into the mold, and the wall out test specimens from the pressed products and investigat- thickness is increased according to the constant-volume law. ing their fatigue characteristics. The difference between the Measured wall thicknesses of the first- to third-step- multiaxial strain load and the uniaxial strain load was pressed products are shown in Fig. 3. Tensile test specimens investigated by observing the respective microscopic were prepared by cutting them out from vertical wall portions dislocation structures by TEM. of these products by wire cutter. And these specimens were flattened in a vice and treated by machine before tensile test. 2. Specimens and Test Methods To confirm the influence of the anisotropy of the steel sheet, as shown in Fig. 4, the test specimens was cut in three 2.1 Test materials and test-specimen preparation meth- directions, namely, 0° (rolling direction), 45°, and 90° (right- od angle), to the rolling direction of the steel sheet. The cutout The mechanical properties of the test material (JSH 270 D) positions and dimensions of the tensile-test specimens are are listed in Table 1. For the pressed parts to be investigated, shown in Fig. 5 and Fig. 6, respectively. These cutting the first- to third-step pressed products of CVT pulley positions were determined vertical wall portion due to small

㻰㼞㼍㼣㼕㼚㼓 㻾㼑㼢㼑㼞㼟㼑 㻰㼞㼍㼣㼕㼚㼓 㻾㼑㻙㼐㼞㼍㼣㼕㼚㼓

㻿㼠㼑㼑㼘 㼟㼔㼑㼑㼠 㻼㼡㼚㼏㼔 㼙㼛㼠㼕㼛㼚 㻝㼟㼠 㼟㼠㼑㼜 㻞㼚㼐 㼟㼠㼑㼜 㻟㼞㼐 㼟㼠㼑㼜

㻰㼕㼑

㻼㼡㼚㼏㼔 㻰㼕㼑 㼓㼑㼛㼙㼑㼠㼞㼥

Fig. 2 Schematic illustration of 2nd and 3rd press working process. 452 T. Ishimura and S. Nishino

4.8

4.6

4.4 10 15 Blank thickness

() 4.2 4 10 3.8 1st process Thickness mm 2nd process 3.6 3rd process 50 3.4 0 20 40 60 80 100 120 Distance from a top () mm Fig. 6 Geometry of specimen on tensile test.

Fig. 3 Thickness distribution of press products. Blank

rolling 50 direction 60 70 195 150 φ

rolling direction 60

Fig. 7 Specimen for uniaxial tensile pre-straining.

Fig. 4 Angle for rolling direction. 1st process 2nd process 3rd process 1st process 2nd process 3rd process

Fig. 8 Cutting location of specimen on fatigue test. Fig. 5 Cutting location of specimen on tensile test.

0.6 influence for the shape by press working. And the distance of vertical wall portions from the center of products on the first- 5 to third-step-pressed products were difference. Therefore only 12 maximum distance of 1st process were determined as shown in Fig. 5. Tensile-test specimens, as shown in Fig. 7 were cut out 50 from blank steel plate and subjected to three tensile strains (pre-strains) of 5%,10%,and20% as loads for uniaxial pre- Fig. 9 Geometry of specimen on fatigue test. strain. After tensile loading, a tensile specimen with the same shape as that made from the pressed product (Fig. 6) was cut out from the parallel part of the test specimen by wire cutting, 2.2 Test methods and the tensile test was performed again. For the uniaxial tensile pre-straining and tensile tests, an For fatigue tests under multiaxial strain, test specimens INSTRON universal testing machine (maximum load: were cut out from the press-molded products and blank 100 kN) was used. materials by wire cutting in the same manner as the tensile Conditions for the tensile test were as follows. Tensile specimens described above. Regarding the direction at an speed was 2 mm/min, Elongation was total elongation of angle to the rolling direction, it was taken as the 45° direction the specimen. Number of specimen n = 1. Also, a Vickers only. The cutting positions of the fatigue-test specimens and hardness tester (measuring load: 200 gf ) was used for the specimen dimensions are shown in Fig. 8 and Fig. 9, measuring hardness of the test material. Furthermore, respectively. These test specimens were used for a tensile- X-ray-diffraction peak profiles of the central part of the test tensile fatigue tests. specimens subjected to multiaxial and uniaxial strain were Evaluation of Work Hardening and Tensile Strength for Press Worked Products Plastic Strained from Multiaxial Direction 453 obtained by X-ray diffractometer, and the dislocation density 700 1st process (a) (i.e., state of work hardening) in each specimen was 2nd process 600 investigated. Conditions for the X-ray-diffraction were as 3rd process Blank follows: Tube globe was used Cr material, Tube voltage 500 was 40 kV, Tube current was 30 mA, Diameter of the 400 collimater was 1 mm and 211 face was investigated. For the TEM, pick samples in the central part of specimen, and 300 observed with 200 kV voltage. 200 For the fatigue test, a hydraulic fatigue tester (load control/ Tensile stressTensile (MPa) dynamic maximum load: 80 kN) was used for fatigue tests 100 with stress ratio R = 0.1. 0 02 468 10 12 14 16 3. Test Results and Considerations Elongation (mm)

3.1 Tensile-test results 700 1st process (b) The results of the tensile tests on the specimens cut at three 2nd process 600 angles to the rolling direction are plotted in Fig. 10(a) to (c), 3rd process Blank respectively. According to these results, tensile strength is 500 (MPa) improved as the pressing process progresses regardless of the 400 specimen-cutting angle to the rolling direction. The results of the tensile tests on the test specimens (each direction) during 300 each of the three process steps are plotted in Fig. 11(a) to (c), 200 respectively. According to these results, no anisotropy is observed in each process, and the cut direction (i.e., sheet stress Tensile 100 rolling direction) of the specimens does not significantly 0 improve tensile strength. 02 468 10 12 14 16 fi The above tensile-test results con rm that the tensile Elongation (mm) strength of the pressed product, namely, a mild-steel plate, was improved to approximately the same level as that of 700 1st process (c) 590-MPa-class high-tensile-strength steel sheet after the 600 2nd process third-step pressing was completed. In other words, this study 3rd process Blank verified that tensile strength is improved by work hardening 500 in pressed products subjected to multiaxial strain. 400 The results of the tensile tests in the case that uniaxial pre- strain was applied are shown in Fig. 12. Clearly, as the 300 amount pre-strain increases, both yield stress and tensile strength are increased. This result reveals that the stress-strain 200 diagram obtained by the tensile test repeated after the pre- stressTensile (MPa) straining (“re-tensile strength” hereafter) does not coincide 100 with the stress-strain diagram for the virgin mild-steel-sheet 0 material considered up until now (namely, yield stress was 02 468 10 12 14 16 improved, but tensile strength stays the same). The relation- Elongation (mm) ship between pre-strain amount and tensile strength is shown in Fig. 13. The broken line and dotted line show the tensile Fig. 10 Results of tensile test on each direction. (a) 0° direction, (b) 45° strength after each pressing step (as shown in Fig. 10(a)). As direction, (c) 90° direction. a result, although tensile strength is improved, even when the specimen is subjected to uniaxial strain (as shown in Fig. 12), passage of time is due to “strain aging.” Strain aging is a it can be seen that the rate of improvement in tensile strength phenomenon that occurs due to progress of sticking of is lower than that in the case of applying multiaxial strain dislocations with time as a result of stress fields generated in (as shown in Fig. 12 and Fig. 13). the material lattice by plastic strain. The results of a tensile test immediately after loading the Conventionally, it is considered that in a tensile test on a test specimens with uniaxial pre-strain with elongation uniaxial pre-strained material, yield stress improves but amount of 5 mm and those of a tensile test after a pre-strain tensile strength does not change if the tensile test is repeated load was applied for one month are shown in Fig. 14(a) and after strain loading.21) However, it was revealed by this study, (b), respectively. This result indicated that the tensile test not only yield stress but also tensile strength are improved by performed immediately after pre-strain loading produces strain aging. The improvement of tensile strength due to equivalent results to a stress-strain diagram obtained by a strain aging is also valid in the case of pressed products that continuous tensile test. On the contrary, when the tensile test undergo plastic strain from multiaxial directions. In the case is carried out after a lapse of time (one month), tensile of actual products, the time from press processing to product strength is improved. This improvement in strength with the shipment varies, and in some cases, the strain aging must be 454 T. Ishimura and S. Nishino

700 600 Blank (a) 0㼻direction Pre-strain 5% Pre-strain 10% 600 45㼻direction 500 Pre-strain 20% 90㼻direction 500 400

400 300

300 200

200 stressTensile (MPa) 100 Tensile stressTensile (MPa) 100 0 02 468 10 12 14 16 0 02 468 10 12 14 16 Elongation (mm)

Elongation (mm) Fig. 12 Tensile test results of uniaxial pre-straining specimen.

700 (b) 0㼻direction 600 45㼻direction 650 Blank 90㼻direction 500 600 Pre-strain 5% 䠅 Pre-strain 10% 400 Pre-strain 20% 3rd process of press

MPa 550

䠄 worked 300 500

200 450 2nd process of press Tensile stressTensile (MPa) 100 400 worked

0 strength Tensile 350 02 46 10 12 14 16 8 300 Elongation (mm) Blank 5 10 20 Pre- strain 䠄 䠂䠅 700 0㼻direction (c) Fig. 13 Comparison of tensile strength between press worked products and 600 45㼻direction uniaxial pre-straining specimen. 90㼻direction 500

400 600 (a) 300 500 Temporary 200 400 unloading Tensile stressTensile (MPa) 100 300 tress tress ( MPa) 200 0 loading instantly 02 468 10 12 14 16 100 Tensile s Tensile 0 Elongation (mm) 0 2 4 6 8 10121416 Fig. 11 Results of tensile on each process. (a) 1st process, (b) 2nd process, Elongation (mm) (c) 3rd process. 600 (b) considered. In particular, it is possible that a difference in 500 strengths of products may arise due to the difference between 400 trial manufacturing (in which a strength test is carried out 300 immediately after forming) and mass production (when there is a certain time from forming to market shipment), and care 200 loading after one month must be taken in evaluating strength reliability. 100 Tensile stress stress (MPa) Tensile Temporary unloading 0 3.2 Relation between work hardening and tensile 0246810121416 strength Elongation (mm) The results of measuring cross-sectional hardness of the Fig. 14 Results of uniaxial tensile test with and without strain aging. press-worked products in each of the three processes are (a) without strain aging, (b) with strain aging. Evaluation of Work Hardening and Tensile Strength for Press Worked Products Plastic Strained from Multiaxial Direction 455

600 240 1st process 2nd process 220 550 3rd process Pre-strain 5%

Hv) 200 ( 500 Pre-strain 10% 180 Pre-strain 20% 450 Blank 160

140 400 1st process 120 2nd process 350

3rd process (MPa) strength Tensile Vickers hardness Vickers 100 0 10 20 30 40 50 60 70 80 90 100 300 120 130 140 150 160 170 180 190 200 210 Distance from a top (mm) Average vickers hardness (Hv) Fig. 15 Hardness distribution of press worked product cross section. Fig. 17 Relationship between tensile strength and average hardness.

210 1st process 200 2nd process the manner described above can be uniformly evaluated by 190 3rd process Blank hardness measurement. 180 In press forming, the work-piece is subjected to reverse 170 drawing, by which not only tensile stress but also 160 complicated combined multiaxial strain (including compres- 150 sive stress) is applied to the product. As a result, work hardening is greater and tensile strength is increased more 140 Critical hardness than that obtained with uniaxial strain. Specifically, in the 130 on tensile test Vickers hardnessVickers (Hv) second step of the press-forming process, namely, reverse- 120 rd press forming, work hardening is performed by applying a 㻮㼘㼍㼚㼗 1st 2nd 3 process process process stress in the direction opposite to that of the first step. In the third step, compressive stress is applied by intrusion for Fig. 16 Work hardening on tri-axial and uniaxial straining. restoring the thickness of the vertical wall. It is conceivable that unlike applying unidirectional tension, imparting multi- plotted in Fig. 15. The horizontal axis represents the axial distortion can introduce work hardening. circumferential length to the end of the product with the It can therefore be concluded that by effectively utilizing central apex of the pressed product as the origin. The results the work hardening of a material during the forming process, of examining the increase in hardness accompanying press it is possible to considerably increase the tensile strength of working by using the hardness distribution in Fig. 15 are that material by forming and work hardening complicated shown in Fig. 16. In the figure, the measured hardness at the product shapes by press forming using a mild-steel plate time of tensioning until just before the blank material breaks (which as excellent formability) instead of using a high- is shown by the dotted line. It is clear that hardness is strength steel sheet (which faces with many problems, such increased by each press-working process, and in case of the as formability, shape accuracy, processing load, weld ability, third-step-pressed product, it is slightly less than twice that and paint ability). It is expected that effectively utilizing the of the blank material. Also, the hardness of the third-step- improvement in tensile strength accompanying the work pressed product exceeds the critical hardness under simple hardening will provide a weight-saving method enabling tension. stable production with high cost effectiveness. The relationship between tensile strength of the pressed product and pre-strained material and average hardness of 3.3 Fatigue strength of pressed products each test specimen is shown in Fig. 17. It is clear from the The results of the fatigue tests on pressed products figure that the hardness attained when multiaxial strain is subjected to multiaxial strain are shown in Fig. 18. When applied to the specimens by press working is higher than that the fatigue strength of the blank material and the third-step- attained under uniaxial strain; in other words, the amount of processed product are focused on, it is clear that as well as work hardening is larger. In addition, regardless of the tensile strength, fatigue strength is improved by press histories of uniaxial and multiaxial strains, the hardness working. Although the results of conventional research have obtained by applying processing strain is roughly propor- shown that hardness and fatigue strength have a proportional tional to tensile strength. It is generally known that material relationship,5,6) it was revealed by the present study that the hardness and tensile strength have a proportional relationship, change in hardness due to work hardening correlates with but it became clear from the present study that the same fatigue strength in the same manner as tensile strength. proportionality holds true for work-hardened material. According to this result, if the hardness of a pressed part is 3.4 Measurement of dislocation density by X-ray known, its tensile strength can be derived. In other words, diffraction tensile strength of products subjected to different strains in Differences in work hardening under applied uniaxial 456 T. Ishimura and S. Nishino

600 1st process 䠅 䠅 550 2nd process 3rd process MPa

MPa Pre-strain 5% 䠄 䠄

Open marks : 䠄 500 Pre-strain 10% not broken Pre-strain 20% 450 Blank 400

350 Stress amplitude amplitude Stress Tensile Strength Strength Tensile 300 1.4 1.5 1.6 1.7 1.8 1.9 2

Number of cycles (Cycles) Full width of half maximum intensity ( 㼻)

Fig. 18 Fatigue tests of blank and press worked products. Fig. 20 Relationship between tensile strength and full width at maximum intensity.

12000 (a) Blank The relationship between full width at half maximum of 10000 3rd process the peak-intensity profile and tensile strength of the three-step 8000 pressed product and the uniaxial-pre-strained material is

6000 shown in Fig. 20. It is clear from this result that in the same 1.444㼻 manner as the case of hardness explained in the previous 4000 1.977㼻 section, not only strain history but also full-width-at-half- Intensity (cpm) Intensity 2000 maximum intensity and tensile strength are correlated.

0 Dislocation density is higher when multiaxial strain is 150 152 154 156 158 160 162 applied by press working than that produced when uniaxial Angle 2θ ( 㼻) distortion is imposed. This result demonstrates that it is 12000 possible to understand the difference in work hardening from Blank (b) Pre-strain 20% the microscopic point of view. 10000

8000 3.5 Observation of dislocations by TEM 6000 In the previous section, dislocation density was inves- 1.444㼻 tigated by analyzing the peak intensity profile obtained by 4000 1.855㼻 X-ray diffraction. In this section, the dislocation structures of Intensity (cpm) 2000 the third-step-pressed product subjected to multiaxial strain 0 and the uniaxial tension/pre-strained material (plastic strain 150 152 154 156 158 160 162 of 20%) were observed by TEM. As shown in Fig. 21, Angle 2θ 䠄 㼻䠅 in both cases of plastic deformation, a “dislocation-cell” ® Fig. 19 Peak profile on X-ray diffraction. (a) pre-straining (press worked texture due to the entanglement of dislocations proliferated products), (b) pre-straining. by plastic deformation®is observed. However, differences in that condition can be recognized. That is, in the case of the pressed product, fine grains and equal-axial dislocation cells strain or multiaxial strain were investigated from the are observed; however, in the case of uniaxial-tension-loaded microscopic viewpoint of dislocation density by using the specimen, in some regions, the cells are obscure, and the half-width method with X-ray diffraction. X-ray-diffraction observed cells are coarse and extend in the tensile direction. peak profiles of the third-step-pressed product and the blank The shape and size of such cells are closely related to the material subjected to uniaxial strain of 20% are compared in degree of work hardening, and they are therefore judged to Fig. 19(a) and (b), respectively. The ordinate represents be correlated with the increase in tensile strength that was diffraction intensity, the abscissa represents diffraction angle. revealed in this study. The full width at half maximum of the peak profile (i.e., width of the diffraction angle at half of peak diffraction 4. Conclusion intensity) is also shown in the figure. Before the strain was applied, the profile shows a sharp rise (labelled “blank”), but In this study, which focused on parts for cars made of mild- when multiaxial and uniaxial strain was imposed, the profile steel plate, test specimens were cut out from blank material shows a lower diffraction intensity and a wider angular and pressed products formed in three steps and subjected to distribution. It is generally known that this tendency is tensile tests. The effects of plastic strain applied from observed when dislocation density increases. Moreover, multiple-axial directions during each of the pressing process- when the full widths at half maximum in the cases of es (three steps) on mechanical properties of the material were multi-axis and single-axis strain are compared, it is clear that investigated. In addition, tensile strength and work-hardening the full width at half maximum in the case of receiving characteristics in the case that strain was applied from multi- multiaxial strain is large, so dislocation density must be axial directions by press processing and in the case that large too. uniaxial strain was applied by tensile test were compared. Evaluation of Work Hardening and Tensile Strength for Press Worked Products Plastic Strained from Multiaxial Direction 457

Blank Press worked product Uniaxial tensile loaded specimen 䠄3rd process䠅 (Pre-strain 20%)

Fig. 21 TEM observation of dislocation cell.

Furthermore, in a unified manner with the results of the cell structure was unclear, and even the cells that were tensile tests, fatigue tests on plate material subjected to produced had coarse grains stretched in the tensile multiaxial strain were conducted, and the strength reliability direction. of the pressed product was evaluated. The results of this study are summarized as follows. REFERENCES (1) Tensile strength of mild-steel plate subjected to uniaxial ­ or multiaxial plastic strain was increased. If a tensile test 1) K. Ohya, T. Yagasaki and I. Saitho: JSAE Review 50(12) (1996) 31 37. is carried out again after a certain period of time has 2) SAE International: J417 Conversion Table of Hardness, (Society of elapsed by applying strain, not only yield stress but also Automotive Engineers. Inc., 1983 rev.) p. 584. tensile strength are increased. This trend is due to strain 3) N. Suzuki: Materials Data Book, rev. 4, (Japan Institute of Metals, aging inside the material. Degree of work hardening is 2004). higher when multiaxial strain is applied compared to 4) G. Oppel: Exp. Mech. 4 (1964) 135. 5) JSMS, JP.: Data book on Fatigue Strength of Metallic Materials, (The when uniaxial strain is applied. Society of Materials Science, Japan, 1996). (2) For materials work hardened by applying strain, their 6) SAE International: SAE Fatigue Design Handbook, Third Edition AE- hardness and the tensile strength are proportional 22, (Society of Automotive Engineers. Inc., 2002). regardless of the load status of strain. Accordingly, 7) E. Ishimaru, H. Hamasaki, T. Ohno and F. Yoshida: J. JSTP 55 (2014) ­ tensile strength of a product subjected to different 615 619. 8) G.B. Olson and M. Cohen: Mater. Trans. 6A (1975) 791­795. strains can be evaluated uniformly by measuring the 9) R.G. Stringfellow, D.M. Parks and G.B. Olson: Acta Metall. Mater. 40 product’s hardness. (1992) 1703­1716. (3) When strain is applied from multiple axes in press 10) Y. Tomita and T. Iwamoto: Int. J. Mater. Sci. 37 (1995) 1295­1305. working, as well as tensile strength, fatigue strength is 11) T. Iwamoto, T. Tsuta and T. Tomota: Int. J. Mater. Sci. 40 (1998) 173­ improved by work hardening. 182. 12) G.W. Powell, E.R. Mashal and W.A. Backofen: Trans. ASM 50 (1958) (4) According to the results the microscopic analysis by 478. X-ray diffraction combined with the full-width-at-half- 13) J.R. Patel and M. Cohen: Acta Metall. Mater. 1 (1953) 531­538. maximum method, dislocation density is increased 14) H. Sigwart: Proc. Intern. Conf. ASME & IME, (1954). when multiaxial strain is applied compared to when 15) J. Morrow: T. and A. H. Report No. 288, (Univ. of Illinois, 1966). single-axis strain is applied, and the degree of work 16) K. Tokaji, Z. Ando, N. Nakano and K. Takegoshi: Int. J. Mater. Sci. 27(294) (1978) 285­290. hardening is greater. The full width at half maximum is 17) M. Jono, M. Hanai and M. Kikukawa: Int. J. Mater. Sci. 32(361) (1982) correlated with tensile strength regardless of the state of 1137­1143. strain applied to the material. 18) L.F. Coffin, Jr.: Trans. ASME 76 (1954) 931. (5) TEM observation of dislocations revealed that the third- 19) K. Hayashi and S. Doi: Int. J. Mater. Sci. 19(207) (1970) 1075­1080. 20) K. Sugai, M. Yamashita, T. Hattori and N. Nishimura: Collected step-pressed product subjected to multiaxial strain has ­ fi Abstracts of Annual meeting of Japan, Inst. JSME, (2008) pp. 245 a coaxial, ne-grain dislocation-cell structure. On the 246. other hand, in the case of pre-strained (plastic strain of 21) A.S. Khan and S. Huang: Continuum Theory of Plasticity, (John Wiley 20%) material subjected to a uniaxial tensile load, the & Sons, Inc., Hoboken, 1995) pp. 2­4.