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Deformation-induced martensitic transformation behavior of type 304 stainless steel sheet in draw-bending process
Journal of Materials Processing Technology 223 (2015) 34–38
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Deformation-induced martensitic transformation behavior of type 304 stainless steel sheet in draw-bending process E. Ishimaru a,∗ , H. Hamasaki b , F. Yoshida b a b
Nippon Steel & Sumikin Stainless Steel Corporation, 3434, Shimata, Hikari 743-8550, Yamaguchi, Japan Department of Mechanical Science and Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima 739-8527, Hiroshima, Japan
a r t i c l e
i n f o
Article history: Received 26 October 2014 Received in revised form 25 March 2015 Accepted 27 March 2015 Available online 7 April 2015 Keywords: Stainless steel Draw-bending Martensitic transformation Workhardening Deformation microstructure
a b s t r a c t Deformation-induced martensitic transformation and workhardening behavior in draw bending process was investigated, on a Type 304 stainless steel sheet, in comparison with that in uniaxial tension experiments. The Vickers hardness of the draw-bent sheet at the surface is much larger than that at the mid-plane, and it becomes remarkably larger with increasing blank holder force. The significant increase of hardness in the deformed sheet is due to ␣ -martensitic transformation. The volume fraction of ␣ martensite in the draw-bent sheet is smaller than that in the uniaxially pulled sheet with the same plastic strain. In uniaxial tension the sheet is plastically deformed in one direction monotonically, but in contrast, in draw-bending tension-to-compression (i.e., bending-to-unbending) deformation takes place when the sheet is drawn over the die-corner. The difference in the evolution of the martensite between draw-bending and uniaxial tension is explained from such a difference in deformation mode. Under cyclic deformation, in the reverse deformation, the martensitic transformation stagnates in a certain extent of plastic strain because of the Bauschinger effect. Including such a case of stress reversal, the evolution of the martensitic transformation is given as a unique function of the effective stress, rather than the effective plastic strain. Thus the behavior of the martensitic transformation of the material during plastic deformation would be understood from the stress-induced phase transformation mechanism. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Austenitic stainless steels are widely used in many fields of industry because of their excellent mechanical and functional properties, such as high ductility and high strength, as well as excellent corrosion and heat resistances. Type 304 steel, one of the most popular austenitic stainless steel, has a very high formability, e.g., over 50% elongation is possible under uniaxial tension (Fukase et al., 1968), owing to the deformation-induced ␣ -martensitic transformation (Tamura et al., 1966). However, when using such a hard sheet metal for press forming operation, one may encounter some difficulties, such as large springback (Ohashi et al., 1977) and die galling (Hyashi, 1977). In addition, the risk of delayed cracking in deep-drawn cups becomes higher with increasing ␣ -martensite volume in the formed products (Sumitomo et al., 1976). Therefore, in order to predict the formability of type 304 stainless steel sheet, it is of vital importance to have a model describing the evolution of ␣ -martensitic phase. A model of the kinetics for
∗ Corresponding author. Tel.:+81 833 71 5118; fax: +81 833 71 5166. E-mail address: [email protected] (E. Ishimaru). http://dx.doi.org/10.1016/j.jmatprotec.2015.03.048 0924-0136/© 2015 Elsevier B.V. All rights reserved.
the deformation-induced martensitic transformation was first proposed by Olson and Cohen (1975), and later the strain rate effect was considered in the model by Stringfellow et al. (1992). In these models the volume fraction of the transformation-induced martensite is expressed as a function of accumulated plastic strain. However, there are some articles reporting that the evolution of the martensite is not only a unique function of plastic strain but it is also dependent on deformation mode. Hamasai et al. (2014), recently found that the martensitic phase transformation behavior under cyclic deformation is not the same as one under uniaxial tension. Accordingly, in the present study, the martensitic transformation behavior of type 304 stainless steel sheets during draw-bending process, where a sheet is subjected to bendingunbending when the sheet is drawn over the die corner, is investigated. The Vickers hardness distributions along the sheet thickness were determined for drawn sheets for several levels of blank holder force (BHF). The volume fractions of ␣ -martensite were measured both at the surface and the mid-plane of the drawn sheet for various BHFs, and they were compared to those obtained from uniaxially pulled specimens. From these, the effect of stress reversal on the martensitic transformation is discussed.
E. Ishimaru et al. / Journal of Materials Processing Technology 223 (2015) 34–38
35
Table 1 Chemical compositions (wt%). C
Si
Mn
Ni
Cr
Cu
N
0.05
0.5
1.1
8.0
18.0
0.3
0.04
Table 2 Tensile properties (test specimen: JIS 13B). 0.2% Proof stress (MPa)
Tensile stress (MPa)
Elongation (%)
n Value (10–30%)
313
750
56
0.45
2. Experimental procedure
Fig. 2. Locations for micro-Vickers hardness measurement (a) in the longitudinal direction; (b) in the thickness direction.
2.1. Specimen Type 304 stainless steel sheets of 1 mm thick (as annealed) was used for the experiment. Tables 1 and 2 show the chemical compositions (wt%) of the sheet and its tensile properties (test specimen: JIS 13B) in the rolling direction, respectively. 2.2. Draw-bending The experimental set-up for draw-bending and the specimen is schematically illustrated in Fig. 1. The draw-bending experiments were conducted for three levels of BHFs, (10, 30 and 50 kN). The specimen was drawn over the die-corner (die-corner radius was 3 mm) at a punch speed of 0.167 mm/s up to 40 mm punch travel, where a lubricant with high viscosity (Johnson wax no. 122) was applied on the die and blank holder surfaces.
electron microscope (JSM-7000F FE-SEM, JEOL Co. Ltd.). The EBSD measurement was conducted with an acceleration voltage of 25 kV at several locations along the sheet thickness direction starting from near the sheet surface (approximately 100 m away from the sheet surface) to the mid-plane of the sheet with an interval of 0.5 m. Furthermore, hardness distributions in the sheet, both in the longitudinal and thickness directions, were determined by the micro-Vickers hardness measurement (HV0.1), as schematically illustrated in Fig. 2. In the longitudinal direction, the hardness at the mid-point of the sheet was measured throughout the side-wall of the draw-bent sheet (from the flange to the punch-corner) with an interval of 1 mm. In the sheet thickness direction, the measurement was conducted at a position of 10 mm away from the end of die-corner with an interval of 100 m in the thickness direction from the surface to the mid-plane of the sheet.
2.3. Evaluation of materials completed testing 3. Results and discussion The volume fraction of the deformation-induced martensite at the surface of the specimen was measured using a ferrite meter (MP-30, Fischer Co. Ltd.). For an accurate determination of the volume fraction of the martensite phase, thus measured volume fraction of the martensite had been calibrated using the X-ray analysis. For calibration, cold-rolled samples with different ␣ martensite volume fractions were employed. In the X-ray analysis, the volume fraction was calculated from integrated strength of ␥ (2 0 0) and ␣ (2 1 1) using the target Cr-K␣ line with a micro point X-ray stress measurement device (PSPC-MSF, Rigaku Co. Ltd.). In the analysis of ␣ -martensitic transformation behavior in the thickness direction, separation of the ferritic phase from the austenitic phase was performed by the Electron beam Backscatter Diffraction (EBSD) method using a field emission-type scanning
Fig. 1. Experimental set-up and specimen for draw-bending.
3.1. Workhardening of draw-bent sheet The specimens after draw-bending experiments are shown in Fig. 3, where one can see that the side-wall curl decreases significantly with increasing BHF since it gives a tensile load to the sheet, thus increasing tension over thickness and shifting the neutral line toward the die. The hardness distributions of draw-bent sheets are summarized in Fig 4(a) and (b), where Fig. 4(a) shows the hardness at the midplane, and Fig. 4(b) the result in the thickness direction, for three
Fig. 3. Outlook of specimens after draw-bending experiments.
36
E. Ishimaru et al. / Journal of Materials Processing Technology 223 (2015) 34–38
(a)
BHF 350
Hardness HV(0.97N)
different BHFs, 10, 30 and 50 kN. In draw-bending, the sheet is subjected to bending strain together with stretching strain. From these figures, it is clear that the overall workhardening increases with increasing BHF because of a larger stretching strain. Under a low BHF (see Fig. 4(b) for BHF = 10 kN), the bending strain is dominant compared to the stretching strain, thus the hardness values at the sheet surfaces are much larger than that at the mid plane. In contrast, under a large BHF (see Fig. 4(b) for BHF = 50 kN), the difference in hardness between at the surface and at the mid-plane is not so large, since a large stretching strain is induced by the BHF.
400
10kN,
30kN,
50kN
Die R
300
Punch R
250
3.2. Relationship between volume fraction of martensite and the hardness
200
150 -10
0
10
20
30
40
50
60
Distance from end of die corner (mm)
(b)
400
BHF
10kN,
30kN,
50kN
Hardness HV(0.97N)
350
300
250
200
150 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Distance from surface of Punch side (mm) Fig. 4. Hardness distributions in the draw-bent sheets: (a) in the longitudinal direction (measured at the mid-plane); (b) in the thickness direction (measured at 10 mm away from the end of die corner).
The volume fractions of ␣ -martensite were measured with either the ferrite meter or the EBSD analysis. EBSD images for the draw-bent sheets at the surfaces on punch-side, die-side and the mid-plane, tested under BHF = 10, 30 and 50 kN, are shown in Fig. 5. The result for the uniaxially pulled specimen, at 22% tensile strain, is also shown in the figure. From these images, one can clearly see that ␣ -martensite volume fraction increases with increasing BHF. The difference in appearance of the martensite phase among both sides of the sheet and mid-plane is not so obvious for BHF = 50 kN, compared to the case of BHF = 10 kN, which is corresponding to the afore-mentioned result of hardness distribution. The relationship between volume fraction of the martensite and the Vickers hardness in the draw-bent sheets, together with the result in the uniaxially pulled specimen, is shown in Fig. 6, where the hardness is found to be expressed as a unique function of the volume fraction of martensite. From this, it would be concluded that the major mechanism of workhardening of type 304 stainless steel is the evolution of martensite during plastic deformation. 3.3. Effect of bending-unbending on the martensitic transformation Based on the kinetics for the deformation-induced martensitic transformation Olson and Cohen (1975) preformed the uniaxial
Fig. 5. The EBSD analysis on the deformation-induced transformation of the samples after draw-bending and tensile test.
E. Ishimaru et al. / Journal of Materials Processing Technology 223 (2015) 34–38
37
Hardness HV(0.97N)
350
300
250 Draw-bending surface of Punch side mid-plane surface of Die side Tensile test mid-plane
200
150 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Volume fraction of martensite (%) Fig. 6. Relationship between the volume fraction of ␣ -martensite and the Vickers hardness.
tension test for an austenitic stainless steel, and presumed that the volume fraction of ␣ -martensite is given as a unique function of accumulated plastic strain (i.e., effective plastic strain). To verify the effective plastic strain of the draw-bent sheet was calculated with a dynamic explicit FE code PAM-STAMP 2G (ESI Co. Ltd.). Fig. 7 shows the relationship between the volume fraction of the martensite obtained from EBSD analysis and the effective plastic strain in the draw-bending experiment, together with the result of uniaxial tension experiment. One can see that the result of a material element at the mid-plane of the draw-bent sheet is almost identical with one of uniaxial tension. Contrary to this, it should be noted that the evolution of the martensite of a material element at the surface of the draw-bent sheet, with increasing effective plastic strain, is significantly slower than that in uniaxial tension. In draw-bending, the sheet is subjected to stretch bending-unbending when it is drawn over the die-corner. Therefore, a material element at the sheet surface is subjected to severe tension-to-compression (or compression-totension) plastic deformation, but in contrast, a material element at the mid-plane of the sheet is mostly subjected to monotonically increasing tensile strain. Such a difference in deformation mode, between at the surface and the mid-plate, would be the reason why the evolution of martensite is so slow at the surface of the sheet.
Volume fraction of martensite (%)
0.30 0.25 0.20
Draw-bending surface of Punch side mid-plane surface of Die side Tensile test mid-plane
0.15 0.10 0.05 0.00 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Plastic strain Fig. 7. Evolution of volume fraction of martensite with increasing effective plastic strain under draw-bending and uniaxial tension.
Fig. 8. Behavior of cyclic tension–compression tests; (a) stress–strain curves: (b) martensite volume fraction vs. plastic strain amount.
3.4. Evolution of martensitic transformation under cyclic tension–compression From the above discussion, it was found that the evolution of the martensite is strongly affected by the deformation mode. To examine the martensitic transformation behavior under cyclic deformation, cyclic tension–compression experiment on the material was conducted. The stress–strain response under cyclic tension–compression is shown in Fig. 8(a), where the Bauschinger effect is clearly seen in the reverse deformation. In Fig. 8(b) the evolution of the martensite during the cyclic deformation is depicted. From this figure, it should be noted that the evolution of the martensite stagnates in a certain extent of reverse deformation even though the plastic strain progresses. This phenomenon is consistent with the martensitic transformation behavior in draw-bending. The flow stress in a reverse deformation is remarkably lower than that in the preceding forward deformation, in a certain extent of the reversed strain, because of the Bauschinger effect. If it would be assumed that the stress is the major driving force of the deformation-induced martensitic transformation, such behavior of the stagnation of martensitic transformation in a strain reversal would be easily understood. To validate this, the relationship between the volume fraction of the martensite and the maximum effective stress which a material element has ever been subjected to is shown in Fig. 9. From this figure, it would be concluded that the volume fraction of martensite is given as a unique function of the effective stress, in other words, the major mechanism of this phenomenon would be the stress-induced phase transformation.
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E. Ishimaru et al. / Journal of Materials Processing Technology 223 (2015) 34–38
volume fraction of martensite (%)
0.30 Cyclic tension-compression test Draw-bending
0.25 0.20 0.15 0.10 0.05 0.00
0
200
400
600
800
1000
1200
Maximum equivalent stress (MPa) Fig. 9. Relationship between martensite volume fraction and maximum equivalent stress in cyclic tension–compression.
4. Conclusions Deformation-induced martensitic transformation behavior during draw-bending was investigated on type 304 stainless steel. The present findings are summarized as follows: (1) For the hardness distribution in the sheet thickness direction, the hardness at the surface is higher than that at the mid-plate because of the bending effect. The hardness is given as a unique function of the volume fraction of ␣ -martensite. From this fact, it is concluded that the major mechanism of workhardening of the material is the evolution of the martensitic phase. (2) The evolution of the martensite of a material element at the surface of the draw-bent sheet, with increasing effective plastic strain, is significantly slower than that in uniaxial tension, although at the mid-plane it is almost identical with the
result in uniaxial tension. This shows that bending–unbendig process strongly affects the evolution of martensite. This phenomenon cannot be described by the Olsen–Stringfellow model which assumes that the volume fraction of the martensite is given as a unique function of accumulated plastic strain. (3) From cyclic tension–compression experiments, it was found that the evolution of the martensite stagnates in a certain extent of reverse deformation even though the plastic strain progresses. This phenomenon is consistent with the martensitic transformation behavior in draw-bending. The volume fraction of martensite was found to be given as a unique function of the maximum effective stress which a material element has ever been subjected to. Such behavior of the martencitic transformation would be understood from the stress-induced phase transformation mechanism. References Fukase, Y., Ebato, K., Okubo, N., Murao, S., 1968. On the anomalus behavior of mechanical properties in metastable Cr–Ni austenitic stainless steels at ambient temperatures. J. Jpn. Inst. 32 (1), 38–44. Hamasai, H., Ishimaru, E., Yoshida, F., 2014. Cyclic stress–strain response and martensitic transformation behavior for type 304 stainless steel. Appl. Mech. Mater. 510, 114–117. Hyashi, H., 1977. Sheet forming of stainless steels. Curr. Adv. Mater. Proc 10, 1185–1188. Ohashi, N., Ono, Y., Nohara, K., 1977. Press formability of stainless steel sheets. TetsuTo-Hagane 63 (5), 812–823. Olson, G.B., Cohen, M., 1975. Kinetics of strain-induced martensitic nucleation. Metall. Trans. A 6A, 791–795. Stringfellow, R.G., Parks, D.M., Olson, G.B., 1992. A constitutive model for transformation plasticity accompanying strain-induced martensitic transformations in metastable austenitic steels. Acta Metall. Mater. 40 (7), 1703–1716. Sumitomo, H., Arakawa, M., Sawatani, T., Ohoka, T., 1976. Delayed cracking of deepdrawn cup of marastable austenitic stainless steel. J. Jpn. Soc. Technol. Plast. Sosei-To-Kako 17 (11), 891–898. Tamura, I., Maki, M., Hato, H., Aburai, K., 1966. On the plasticity induced by martensitic transformation in Fe–Ni alloys and Fe–Cr–Nu alloys. J. Jpn. Inst. Met. 33 (12), 1383–1389.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Deformation-induced martensitic transformation behavior of type 304 stainless steel sheet in draw-bending process E. Ishimaru a,∗ , H. Hamasaki b , F. Yoshida b a b
Nippon Steel & Sumikin Stainless Steel Corporation, 3434, Shimata, Hikari 743-8550, Yamaguchi, Japan Department of Mechanical Science and Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima 739-8527, Hiroshima, Japan
a r t i c l e
i n f o
Article history: Received 26 October 2014 Received in revised form 25 March 2015 Accepted 27 March 2015 Available online 7 April 2015 Keywords: Stainless steel Draw-bending Martensitic transformation Workhardening Deformation microstructure
a b s t r a c t Deformation-induced martensitic transformation and workhardening behavior in draw bending process was investigated, on a Type 304 stainless steel sheet, in comparison with that in uniaxial tension experiments. The Vickers hardness of the draw-bent sheet at the surface is much larger than that at the mid-plane, and it becomes remarkably larger with increasing blank holder force. The significant increase of hardness in the deformed sheet is due to ␣ -martensitic transformation. The volume fraction of ␣ martensite in the draw-bent sheet is smaller than that in the uniaxially pulled sheet with the same plastic strain. In uniaxial tension the sheet is plastically deformed in one direction monotonically, but in contrast, in draw-bending tension-to-compression (i.e., bending-to-unbending) deformation takes place when the sheet is drawn over the die-corner. The difference in the evolution of the martensite between draw-bending and uniaxial tension is explained from such a difference in deformation mode. Under cyclic deformation, in the reverse deformation, the martensitic transformation stagnates in a certain extent of plastic strain because of the Bauschinger effect. Including such a case of stress reversal, the evolution of the martensitic transformation is given as a unique function of the effective stress, rather than the effective plastic strain. Thus the behavior of the martensitic transformation of the material during plastic deformation would be understood from the stress-induced phase transformation mechanism. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Austenitic stainless steels are widely used in many fields of industry because of their excellent mechanical and functional properties, such as high ductility and high strength, as well as excellent corrosion and heat resistances. Type 304 steel, one of the most popular austenitic stainless steel, has a very high formability, e.g., over 50% elongation is possible under uniaxial tension (Fukase et al., 1968), owing to the deformation-induced ␣ -martensitic transformation (Tamura et al., 1966). However, when using such a hard sheet metal for press forming operation, one may encounter some difficulties, such as large springback (Ohashi et al., 1977) and die galling (Hyashi, 1977). In addition, the risk of delayed cracking in deep-drawn cups becomes higher with increasing ␣ -martensite volume in the formed products (Sumitomo et al., 1976). Therefore, in order to predict the formability of type 304 stainless steel sheet, it is of vital importance to have a model describing the evolution of ␣ -martensitic phase. A model of the kinetics for
∗ Corresponding author. Tel.:+81 833 71 5118; fax: +81 833 71 5166. E-mail address: [email protected] (E. Ishimaru). http://dx.doi.org/10.1016/j.jmatprotec.2015.03.048 0924-0136/© 2015 Elsevier B.V. All rights reserved.
the deformation-induced martensitic transformation was first proposed by Olson and Cohen (1975), and later the strain rate effect was considered in the model by Stringfellow et al. (1992). In these models the volume fraction of the transformation-induced martensite is expressed as a function of accumulated plastic strain. However, there are some articles reporting that the evolution of the martensite is not only a unique function of plastic strain but it is also dependent on deformation mode. Hamasai et al. (2014), recently found that the martensitic phase transformation behavior under cyclic deformation is not the same as one under uniaxial tension. Accordingly, in the present study, the martensitic transformation behavior of type 304 stainless steel sheets during draw-bending process, where a sheet is subjected to bendingunbending when the sheet is drawn over the die corner, is investigated. The Vickers hardness distributions along the sheet thickness were determined for drawn sheets for several levels of blank holder force (BHF). The volume fractions of ␣ -martensite were measured both at the surface and the mid-plane of the drawn sheet for various BHFs, and they were compared to those obtained from uniaxially pulled specimens. From these, the effect of stress reversal on the martensitic transformation is discussed.
E. Ishimaru et al. / Journal of Materials Processing Technology 223 (2015) 34–38
35
Table 1 Chemical compositions (wt%). C
Si
Mn
Ni
Cr
Cu
N
0.05
0.5
1.1
8.0
18.0
0.3
0.04
Table 2 Tensile properties (test specimen: JIS 13B). 0.2% Proof stress (MPa)
Tensile stress (MPa)
Elongation (%)
n Value (10–30%)
313
750
56
0.45
2. Experimental procedure
Fig. 2. Locations for micro-Vickers hardness measurement (a) in the longitudinal direction; (b) in the thickness direction.
2.1. Specimen Type 304 stainless steel sheets of 1 mm thick (as annealed) was used for the experiment. Tables 1 and 2 show the chemical compositions (wt%) of the sheet and its tensile properties (test specimen: JIS 13B) in the rolling direction, respectively. 2.2. Draw-bending The experimental set-up for draw-bending and the specimen is schematically illustrated in Fig. 1. The draw-bending experiments were conducted for three levels of BHFs, (10, 30 and 50 kN). The specimen was drawn over the die-corner (die-corner radius was 3 mm) at a punch speed of 0.167 mm/s up to 40 mm punch travel, where a lubricant with high viscosity (Johnson wax no. 122) was applied on the die and blank holder surfaces.
electron microscope (JSM-7000F FE-SEM, JEOL Co. Ltd.). The EBSD measurement was conducted with an acceleration voltage of 25 kV at several locations along the sheet thickness direction starting from near the sheet surface (approximately 100 m away from the sheet surface) to the mid-plane of the sheet with an interval of 0.5 m. Furthermore, hardness distributions in the sheet, both in the longitudinal and thickness directions, were determined by the micro-Vickers hardness measurement (HV0.1), as schematically illustrated in Fig. 2. In the longitudinal direction, the hardness at the mid-point of the sheet was measured throughout the side-wall of the draw-bent sheet (from the flange to the punch-corner) with an interval of 1 mm. In the sheet thickness direction, the measurement was conducted at a position of 10 mm away from the end of die-corner with an interval of 100 m in the thickness direction from the surface to the mid-plane of the sheet.
2.3. Evaluation of materials completed testing 3. Results and discussion The volume fraction of the deformation-induced martensite at the surface of the specimen was measured using a ferrite meter (MP-30, Fischer Co. Ltd.). For an accurate determination of the volume fraction of the martensite phase, thus measured volume fraction of the martensite had been calibrated using the X-ray analysis. For calibration, cold-rolled samples with different ␣ martensite volume fractions were employed. In the X-ray analysis, the volume fraction was calculated from integrated strength of ␥ (2 0 0) and ␣ (2 1 1) using the target Cr-K␣ line with a micro point X-ray stress measurement device (PSPC-MSF, Rigaku Co. Ltd.). In the analysis of ␣ -martensitic transformation behavior in the thickness direction, separation of the ferritic phase from the austenitic phase was performed by the Electron beam Backscatter Diffraction (EBSD) method using a field emission-type scanning
Fig. 1. Experimental set-up and specimen for draw-bending.
3.1. Workhardening of draw-bent sheet The specimens after draw-bending experiments are shown in Fig. 3, where one can see that the side-wall curl decreases significantly with increasing BHF since it gives a tensile load to the sheet, thus increasing tension over thickness and shifting the neutral line toward the die. The hardness distributions of draw-bent sheets are summarized in Fig 4(a) and (b), where Fig. 4(a) shows the hardness at the midplane, and Fig. 4(b) the result in the thickness direction, for three
Fig. 3. Outlook of specimens after draw-bending experiments.
36
E. Ishimaru et al. / Journal of Materials Processing Technology 223 (2015) 34–38
(a)
BHF 350
Hardness HV(0.97N)
different BHFs, 10, 30 and 50 kN. In draw-bending, the sheet is subjected to bending strain together with stretching strain. From these figures, it is clear that the overall workhardening increases with increasing BHF because of a larger stretching strain. Under a low BHF (see Fig. 4(b) for BHF = 10 kN), the bending strain is dominant compared to the stretching strain, thus the hardness values at the sheet surfaces are much larger than that at the mid plane. In contrast, under a large BHF (see Fig. 4(b) for BHF = 50 kN), the difference in hardness between at the surface and at the mid-plane is not so large, since a large stretching strain is induced by the BHF.
400
10kN,
30kN,
50kN
Die R
300
Punch R
250
3.2. Relationship between volume fraction of martensite and the hardness
200
150 -10
0
10
20
30
40
50
60
Distance from end of die corner (mm)
(b)
400
BHF
10kN,
30kN,
50kN
Hardness HV(0.97N)
350
300
250
200
150 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Distance from surface of Punch side (mm) Fig. 4. Hardness distributions in the draw-bent sheets: (a) in the longitudinal direction (measured at the mid-plane); (b) in the thickness direction (measured at 10 mm away from the end of die corner).
The volume fractions of ␣ -martensite were measured with either the ferrite meter or the EBSD analysis. EBSD images for the draw-bent sheets at the surfaces on punch-side, die-side and the mid-plane, tested under BHF = 10, 30 and 50 kN, are shown in Fig. 5. The result for the uniaxially pulled specimen, at 22% tensile strain, is also shown in the figure. From these images, one can clearly see that ␣ -martensite volume fraction increases with increasing BHF. The difference in appearance of the martensite phase among both sides of the sheet and mid-plane is not so obvious for BHF = 50 kN, compared to the case of BHF = 10 kN, which is corresponding to the afore-mentioned result of hardness distribution. The relationship between volume fraction of the martensite and the Vickers hardness in the draw-bent sheets, together with the result in the uniaxially pulled specimen, is shown in Fig. 6, where the hardness is found to be expressed as a unique function of the volume fraction of martensite. From this, it would be concluded that the major mechanism of workhardening of type 304 stainless steel is the evolution of martensite during plastic deformation. 3.3. Effect of bending-unbending on the martensitic transformation Based on the kinetics for the deformation-induced martensitic transformation Olson and Cohen (1975) preformed the uniaxial
Fig. 5. The EBSD analysis on the deformation-induced transformation of the samples after draw-bending and tensile test.
E. Ishimaru et al. / Journal of Materials Processing Technology 223 (2015) 34–38
37
Hardness HV(0.97N)
350
300
250 Draw-bending surface of Punch side mid-plane surface of Die side Tensile test mid-plane
200
150 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Volume fraction of martensite (%) Fig. 6. Relationship between the volume fraction of ␣ -martensite and the Vickers hardness.
tension test for an austenitic stainless steel, and presumed that the volume fraction of ␣ -martensite is given as a unique function of accumulated plastic strain (i.e., effective plastic strain). To verify the effective plastic strain of the draw-bent sheet was calculated with a dynamic explicit FE code PAM-STAMP 2G (ESI Co. Ltd.). Fig. 7 shows the relationship between the volume fraction of the martensite obtained from EBSD analysis and the effective plastic strain in the draw-bending experiment, together with the result of uniaxial tension experiment. One can see that the result of a material element at the mid-plane of the draw-bent sheet is almost identical with one of uniaxial tension. Contrary to this, it should be noted that the evolution of the martensite of a material element at the surface of the draw-bent sheet, with increasing effective plastic strain, is significantly slower than that in uniaxial tension. In draw-bending, the sheet is subjected to stretch bending-unbending when it is drawn over the die-corner. Therefore, a material element at the sheet surface is subjected to severe tension-to-compression (or compression-totension) plastic deformation, but in contrast, a material element at the mid-plane of the sheet is mostly subjected to monotonically increasing tensile strain. Such a difference in deformation mode, between at the surface and the mid-plate, would be the reason why the evolution of martensite is so slow at the surface of the sheet.
Volume fraction of martensite (%)
0.30 0.25 0.20
Draw-bending surface of Punch side mid-plane surface of Die side Tensile test mid-plane
0.15 0.10 0.05 0.00 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Plastic strain Fig. 7. Evolution of volume fraction of martensite with increasing effective plastic strain under draw-bending and uniaxial tension.
Fig. 8. Behavior of cyclic tension–compression tests; (a) stress–strain curves: (b) martensite volume fraction vs. plastic strain amount.
3.4. Evolution of martensitic transformation under cyclic tension–compression From the above discussion, it was found that the evolution of the martensite is strongly affected by the deformation mode. To examine the martensitic transformation behavior under cyclic deformation, cyclic tension–compression experiment on the material was conducted. The stress–strain response under cyclic tension–compression is shown in Fig. 8(a), where the Bauschinger effect is clearly seen in the reverse deformation. In Fig. 8(b) the evolution of the martensite during the cyclic deformation is depicted. From this figure, it should be noted that the evolution of the martensite stagnates in a certain extent of reverse deformation even though the plastic strain progresses. This phenomenon is consistent with the martensitic transformation behavior in draw-bending. The flow stress in a reverse deformation is remarkably lower than that in the preceding forward deformation, in a certain extent of the reversed strain, because of the Bauschinger effect. If it would be assumed that the stress is the major driving force of the deformation-induced martensitic transformation, such behavior of the stagnation of martensitic transformation in a strain reversal would be easily understood. To validate this, the relationship between the volume fraction of the martensite and the maximum effective stress which a material element has ever been subjected to is shown in Fig. 9. From this figure, it would be concluded that the volume fraction of martensite is given as a unique function of the effective stress, in other words, the major mechanism of this phenomenon would be the stress-induced phase transformation.
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volume fraction of martensite (%)
0.30 Cyclic tension-compression test Draw-bending
0.25 0.20 0.15 0.10 0.05 0.00
0
200
400
600
800
1000
1200
Maximum equivalent stress (MPa) Fig. 9. Relationship between martensite volume fraction and maximum equivalent stress in cyclic tension–compression.
4. Conclusions Deformation-induced martensitic transformation behavior during draw-bending was investigated on type 304 stainless steel. The present findings are summarized as follows: (1) For the hardness distribution in the sheet thickness direction, the hardness at the surface is higher than that at the mid-plate because of the bending effect. The hardness is given as a unique function of the volume fraction of ␣ -martensite. From this fact, it is concluded that the major mechanism of workhardening of the material is the evolution of the martensitic phase. (2) The evolution of the martensite of a material element at the surface of the draw-bent sheet, with increasing effective plastic strain, is significantly slower than that in uniaxial tension, although at the mid-plane it is almost identical with the
result in uniaxial tension. This shows that bending–unbendig process strongly affects the evolution of martensite. This phenomenon cannot be described by the Olsen–Stringfellow model which assumes that the volume fraction of the martensite is given as a unique function of accumulated plastic strain. (3) From cyclic tension–compression experiments, it was found that the evolution of the martensite stagnates in a certain extent of reverse deformation even though the plastic strain progresses. This phenomenon is consistent with the martensitic transformation behavior in draw-bending. The volume fraction of martensite was found to be given as a unique function of the maximum effective stress which a material element has ever been subjected to. Such behavior of the martencitic transformation would be understood from the stress-induced phase transformation mechanism. References Fukase, Y., Ebato, K., Okubo, N., Murao, S., 1968. On the anomalus behavior of mechanical properties in metastable Cr–Ni austenitic stainless steels at ambient temperatures. J. Jpn. Inst. 32 (1), 38–44. Hamasai, H., Ishimaru, E., Yoshida, F., 2014. Cyclic stress–strain response and martensitic transformation behavior for type 304 stainless steel. Appl. Mech. Mater. 510, 114–117. Hyashi, H., 1977. Sheet forming of stainless steels. Curr. Adv. Mater. Proc 10, 1185–1188. Ohashi, N., Ono, Y., Nohara, K., 1977. Press formability of stainless steel sheets. TetsuTo-Hagane 63 (5), 812–823. Olson, G.B., Cohen, M., 1975. Kinetics of strain-induced martensitic nucleation. Metall. Trans. A 6A, 791–795. Stringfellow, R.G., Parks, D.M., Olson, G.B., 1992. A constitutive model for transformation plasticity accompanying strain-induced martensitic transformations in metastable austenitic steels. Acta Metall. Mater. 40 (7), 1703–1716. Sumitomo, H., Arakawa, M., Sawatani, T., Ohoka, T., 1976. Delayed cracking of deepdrawn cup of marastable austenitic stainless steel. J. Jpn. Soc. Technol. Plast. Sosei-To-Kako 17 (11), 891–898. Tamura, I., Maki, M., Hato, H., Aburai, K., 1966. On the plasticity induced by martensitic transformation in Fe–Ni alloys and Fe–Cr–Nu alloys. J. Jpn. Inst. Met. 33 (12), 1383–1389.