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Effect of increase in r-value on press formability for cold-rolled steel sheet
JSAE Review 23 (2002) 139–141
Technical Note
Effect of increase in r-value on press formability for cold-rolled steel sheet Hidetaka Kawabea, Saiji Matsuokaa, Tetsuo Shimizua, Osamu Furukimib, Kei Sakataa, Yoshinori Itob,c b
a Sheet Laboratory, Technical Research Laboratories, Kawasaki Steel Corporation, 1-Kawasaki-cho, Chuo-ku, Chiba 260-0835, Japan Stainless Steel Laboratory, Technical Research Laboratories, Kawasaki Steel Corporation, 1-Kawasaki-cho, Chuo-ku, Chiba 260-0835, Japan c Stamping Engineering Department No. 2, Stamping Production Engineering Division, Toyota Motor Corporation, 1, Motomachi, Toyota, Aichi 474-8573, Japan
Received 10 May 2001; received in revised form 5 July 2001
1. Introduction Much research has been done into press formability by relating to the Lankford-value [1,2]. However the combined contribution of tensile strength (TS) and r-value on press formability have not been adequately investigated. Recently, cold-rolled steel sheet with the highest r-value of 3.0 has been developed by controlling chemical composition and the optimization of hotrolling process [3]. In this paper, press formability of cold-rolled steel sheet with the above mentioned high r-value is compared with conventional steel sheet and the effect of increase in r-value on press formability for cold-rolled steel sheet is also discussed.
2. Experimental procedure 2.1. Materials tested Cold-rolled steel sheets with various r-value levels were prepared for investigating formability. The mechanical properties of the employed materials with a thickness of 0.7 mm are shown in Table 1. 2.2. Deep drawing test Deep drawability was estimated by measuring formable range, defined as the region between blank holder force with no fracture and no wrinkle. Diameter of punch, curvature of punch at the corner portion and diameter of blank sheet sample were 100, 15 and 90 mm, respectively. Forming height was 35 mm and blank
holder force was varied from 20 to 103 kN for estimating formable range. 2.3. Hat channel drawing test To investigate the formability of plane strain deformation, a hat channel drawing test was applied. Formability was evaluated by the limit blank holder force with no fracture. Height of channel, radius of punch at corner and radius of die were 80, 5 and 8 mm, respectively. Blank holder force was varied up to 590 kN. 2.4. Sliding test Frictional coefficient (m) was measured by a sliding test carried out at normal pressure of 10 MPa using a flat surface tool. Rust preventive oil of 1.5 103 kg/m2 was applied uniformly to both top and bottom surface of the sample. Frictional coefficient of the steel sheet obtained in this study is shown in Table 1. 2.5. Analysis of combined contribution of mechanical properties on formability The combined contribution of the mechanical properties such as TS, r-value and m on press formability at deep drawing and hat channel drawing were analyzed on the assumption that a dependent variable was the formable range and limit blank holder force in each case, and that independent variables were TS, r-value and m in both cases. In the case of deep drawing, formable range would be regressed as Eq. (1) in the case of deep drawing,
0389-4304/02/$ 22.00 r 2002 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. PII: S 0 3 8 9 - 4 3 0 4 ( 0 1 ) 0 0 1 6 9 - 2
JSAE20024023
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H. Kawabe et al. / JSAE Review 23 (2002) 139–141
Table 1 Mechanical properties and frictional coefficient of materials Steel YS (MPa) TS (MPa) El (%) r-Value n-Value Frictional coefficient A B C D E
176 174 157 155 153
330 317 299 284 287
47 49 51 54 53
1.80 2.16 2.37 2.46 2.68
0.24 0.24 0.23 0.24 0.24
0.148 0.152 0.145 0.148 0.145 Fig. 1. Formable blank holder force range of deep drawing test.
Formable range ¼ a TS þ br-valueþgm þ d;
ð1Þ
where a; b; g; m and d are numerical constants. If the frictional coefficient is the same in deep drawing, the equivalent relationship between TS and rvalue for the same formable range is obtained as DTS ¼ a=bDr;
ð2Þ
where DTS is the change in tensile strength and Dr the change in r-value.
3. Result and discussion 3.1. Deep drawability Fig. 1 shows the formable range defined as the region between blank holder force with no fracture and no wrinkle obtained by deep drawing test. Formable range tends to be significantly wider with increasing r-value. Careful comparison with Steel D and Steel E, which shows that, with almost the same mechanical properties, such as YS, TS, El and n-value, except for the r-value, the formable range of Steel E becomes dramatically greater than Steel D by increasing the r-value to 0.22, resulting in significant effect of r-value on deep drawability. From these experimental results, the combined contribution of mechanical properties on deep drawability was analyzed as follows. In the case of cylindrical cup drawing, it is well known that a deformation at the flange portion is governed by rvalue, and both high TS and high r-value properties are effective for plane strain deformation at the wall portion. Lowering frictional coefficient also leads to good formability. Therefore, numerical analysis was done to estimate the combined contributions of TS, r-value and frictional coefficient (m) for obtaining formable range in the same way as Eq. (1). If frictional coefficient is the same, Eq. (3) is obtained for the equivalent formable range in the same way as Eq. (2) using the above experimental results: DTS ¼ 212:6Dr:
ð3Þ
Fig. 2. Contribution of r-value and tensile strength (TS) on drawability.
This equation shows that increase in TS of 21 MPa is equivalent to increase in r-value of 0.1 for obtaining the same formable range in the case of deep drawing. Fig. 2 indicates the combined contribution of TS and r-value obtained by Eq. (3) on deep drawability. The steel with higher TS and r-value shows higher formability, though contribution of TS on drawability is very small. The steel sheets on the same line in this figure would show the same formable range when deep drawing is applied, and in fact, Steel C and Steel D showed the same formable range, as shown in Fig. 1, resulting in good agreement between numerical analysis and experimental results. Increase in r-value can decrease the deformation resistance at flange portion and increase the main stress at fracture part on plane strain deformation, with the result that the steel with higher r-value shows good formability. 3.2. Plane strain deformation Since many fractured parts on plane strain deformation were observed in a stamped panel on the actual press line, a hat channel drawing test was applied to estimate formability of plane strain deformation.
H. Kawabe et al. / JSAE Review 23 (2002) 139–141
141
Fig. 3. Formable blank holder force range of plane strain deformation.
The limit blank holder force, which is the maximum blank holder force with no fracture, is shown in Fig. 3. Compared with Steel C and Steel D, limit blank holder force of Steel C with higher TS is higher than that of Steel D with higher El. Furthermore, compared with Steel D and Steel E, limit blank holder force of Steel E with higher r-value is higher than that of Steel D with almost the same TS and El and is the same as Steel C. These experimental results show that the limit blank holder force does not have a correlation to only one mechanical property, and that increase in TS or r-value are both effective to prevent fracture on plane strain deformation. Numerical analysis was done to estimate the combined contributions of TS, r-value and frictional coefficient for limit blank holder force in plane strain deformation similar to deep drawing. If frictional coefficient is the same, Eq. (4) is also obtained using experimental results: DTS ¼ 36:5Dr:
ð4Þ
Increase in r-value of 0.5 is equivalent to increase in TS of 18 MPa for limit blank holder force in the case of hat channel drawing. Fig. 4 shows the combined contribution of TS and r-value on limit blank holder force. Steel with higher TS or r-value is effective for preventing fracture on plane strain deformation. The steel sheet on the same line in this figure would show the same limit blank holder force, and steel C and Steel E showed almost the same limit blank holder force as that in Fig. 3, resulting in good agreement between numerical analysis and experimental results. Since the r-value of Steel E is the highest at 2.68 and El is higher at 53%, it could be possible to increase the main stress on plane strain deformation corresponding to Steel C with higher TS and it is suggested that Steel E is suitable for
Fig. 4. Contribution of r-value and tensile strength (TS) on plane strain deformation.
applying stamped panels exposed to fracture part on both plane strain deformation and stretch forming.
4. Summary Research into the effect of r-value on the press formability for cold-rolled steel sheet has observed the following points. (1) Equation DTS ¼ 212:6 Dr-value is obtained for deep drawing deformation and this shows that r-value is a more effective mechanical property than TS on deep drawing by estimating the combined contribution of mechanical properties. (2) On plane strain deformation, equation DTS= 36.5 Dr-value is obtained and the effect of r-value on plane strain deformation is relatively small compared with that on deep drawing deformation. (3) The steel with the highest r-value of 2.68 shows superior formability, especially in the case of a pressed panel required for decreased in deformation resistance at the flange portion and increase in main stress at the fracture part on plane strain deformation
References [1] Yoshida, K., Handbook of Ease and Difficulty in Press Forming, National Center for Manufacturing Science, Inc., Ann Arbor, Michgan (1993). [2] Iizuka, E., et al., Effect of coating on deep drawability for steel sheet with high r-value, CAMP-ISIJ, Vol. 13, p. 630 (2000). [3] Nishimura, K., et al., Cold rolled steel sheets with ultra high Lankford value and excellent press formability, Kawasaki Steel Technical Report, No. 42, pp. 8–11 (2000).
Technical Note
Effect of increase in r-value on press formability for cold-rolled steel sheet Hidetaka Kawabea, Saiji Matsuokaa, Tetsuo Shimizua, Osamu Furukimib, Kei Sakataa, Yoshinori Itob,c b
a Sheet Laboratory, Technical Research Laboratories, Kawasaki Steel Corporation, 1-Kawasaki-cho, Chuo-ku, Chiba 260-0835, Japan Stainless Steel Laboratory, Technical Research Laboratories, Kawasaki Steel Corporation, 1-Kawasaki-cho, Chuo-ku, Chiba 260-0835, Japan c Stamping Engineering Department No. 2, Stamping Production Engineering Division, Toyota Motor Corporation, 1, Motomachi, Toyota, Aichi 474-8573, Japan
Received 10 May 2001; received in revised form 5 July 2001
1. Introduction Much research has been done into press formability by relating to the Lankford-value [1,2]. However the combined contribution of tensile strength (TS) and r-value on press formability have not been adequately investigated. Recently, cold-rolled steel sheet with the highest r-value of 3.0 has been developed by controlling chemical composition and the optimization of hotrolling process [3]. In this paper, press formability of cold-rolled steel sheet with the above mentioned high r-value is compared with conventional steel sheet and the effect of increase in r-value on press formability for cold-rolled steel sheet is also discussed.
2. Experimental procedure 2.1. Materials tested Cold-rolled steel sheets with various r-value levels were prepared for investigating formability. The mechanical properties of the employed materials with a thickness of 0.7 mm are shown in Table 1. 2.2. Deep drawing test Deep drawability was estimated by measuring formable range, defined as the region between blank holder force with no fracture and no wrinkle. Diameter of punch, curvature of punch at the corner portion and diameter of blank sheet sample were 100, 15 and 90 mm, respectively. Forming height was 35 mm and blank
holder force was varied from 20 to 103 kN for estimating formable range. 2.3. Hat channel drawing test To investigate the formability of plane strain deformation, a hat channel drawing test was applied. Formability was evaluated by the limit blank holder force with no fracture. Height of channel, radius of punch at corner and radius of die were 80, 5 and 8 mm, respectively. Blank holder force was varied up to 590 kN. 2.4. Sliding test Frictional coefficient (m) was measured by a sliding test carried out at normal pressure of 10 MPa using a flat surface tool. Rust preventive oil of 1.5 103 kg/m2 was applied uniformly to both top and bottom surface of the sample. Frictional coefficient of the steel sheet obtained in this study is shown in Table 1. 2.5. Analysis of combined contribution of mechanical properties on formability The combined contribution of the mechanical properties such as TS, r-value and m on press formability at deep drawing and hat channel drawing were analyzed on the assumption that a dependent variable was the formable range and limit blank holder force in each case, and that independent variables were TS, r-value and m in both cases. In the case of deep drawing, formable range would be regressed as Eq. (1) in the case of deep drawing,
0389-4304/02/$ 22.00 r 2002 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. PII: S 0 3 8 9 - 4 3 0 4 ( 0 1 ) 0 0 1 6 9 - 2
JSAE20024023
140
H. Kawabe et al. / JSAE Review 23 (2002) 139–141
Table 1 Mechanical properties and frictional coefficient of materials Steel YS (MPa) TS (MPa) El (%) r-Value n-Value Frictional coefficient A B C D E
176 174 157 155 153
330 317 299 284 287
47 49 51 54 53
1.80 2.16 2.37 2.46 2.68
0.24 0.24 0.23 0.24 0.24
0.148 0.152 0.145 0.148 0.145 Fig. 1. Formable blank holder force range of deep drawing test.
Formable range ¼ a TS þ br-valueþgm þ d;
ð1Þ
where a; b; g; m and d are numerical constants. If the frictional coefficient is the same in deep drawing, the equivalent relationship between TS and rvalue for the same formable range is obtained as DTS ¼ a=bDr;
ð2Þ
where DTS is the change in tensile strength and Dr the change in r-value.
3. Result and discussion 3.1. Deep drawability Fig. 1 shows the formable range defined as the region between blank holder force with no fracture and no wrinkle obtained by deep drawing test. Formable range tends to be significantly wider with increasing r-value. Careful comparison with Steel D and Steel E, which shows that, with almost the same mechanical properties, such as YS, TS, El and n-value, except for the r-value, the formable range of Steel E becomes dramatically greater than Steel D by increasing the r-value to 0.22, resulting in significant effect of r-value on deep drawability. From these experimental results, the combined contribution of mechanical properties on deep drawability was analyzed as follows. In the case of cylindrical cup drawing, it is well known that a deformation at the flange portion is governed by rvalue, and both high TS and high r-value properties are effective for plane strain deformation at the wall portion. Lowering frictional coefficient also leads to good formability. Therefore, numerical analysis was done to estimate the combined contributions of TS, r-value and frictional coefficient (m) for obtaining formable range in the same way as Eq. (1). If frictional coefficient is the same, Eq. (3) is obtained for the equivalent formable range in the same way as Eq. (2) using the above experimental results: DTS ¼ 212:6Dr:
ð3Þ
Fig. 2. Contribution of r-value and tensile strength (TS) on drawability.
This equation shows that increase in TS of 21 MPa is equivalent to increase in r-value of 0.1 for obtaining the same formable range in the case of deep drawing. Fig. 2 indicates the combined contribution of TS and r-value obtained by Eq. (3) on deep drawability. The steel with higher TS and r-value shows higher formability, though contribution of TS on drawability is very small. The steel sheets on the same line in this figure would show the same formable range when deep drawing is applied, and in fact, Steel C and Steel D showed the same formable range, as shown in Fig. 1, resulting in good agreement between numerical analysis and experimental results. Increase in r-value can decrease the deformation resistance at flange portion and increase the main stress at fracture part on plane strain deformation, with the result that the steel with higher r-value shows good formability. 3.2. Plane strain deformation Since many fractured parts on plane strain deformation were observed in a stamped panel on the actual press line, a hat channel drawing test was applied to estimate formability of plane strain deformation.
H. Kawabe et al. / JSAE Review 23 (2002) 139–141
141
Fig. 3. Formable blank holder force range of plane strain deformation.
The limit blank holder force, which is the maximum blank holder force with no fracture, is shown in Fig. 3. Compared with Steel C and Steel D, limit blank holder force of Steel C with higher TS is higher than that of Steel D with higher El. Furthermore, compared with Steel D and Steel E, limit blank holder force of Steel E with higher r-value is higher than that of Steel D with almost the same TS and El and is the same as Steel C. These experimental results show that the limit blank holder force does not have a correlation to only one mechanical property, and that increase in TS or r-value are both effective to prevent fracture on plane strain deformation. Numerical analysis was done to estimate the combined contributions of TS, r-value and frictional coefficient for limit blank holder force in plane strain deformation similar to deep drawing. If frictional coefficient is the same, Eq. (4) is also obtained using experimental results: DTS ¼ 36:5Dr:
ð4Þ
Increase in r-value of 0.5 is equivalent to increase in TS of 18 MPa for limit blank holder force in the case of hat channel drawing. Fig. 4 shows the combined contribution of TS and r-value on limit blank holder force. Steel with higher TS or r-value is effective for preventing fracture on plane strain deformation. The steel sheet on the same line in this figure would show the same limit blank holder force, and steel C and Steel E showed almost the same limit blank holder force as that in Fig. 3, resulting in good agreement between numerical analysis and experimental results. Since the r-value of Steel E is the highest at 2.68 and El is higher at 53%, it could be possible to increase the main stress on plane strain deformation corresponding to Steel C with higher TS and it is suggested that Steel E is suitable for
Fig. 4. Contribution of r-value and tensile strength (TS) on plane strain deformation.
applying stamped panels exposed to fracture part on both plane strain deformation and stretch forming.
4. Summary Research into the effect of r-value on the press formability for cold-rolled steel sheet has observed the following points. (1) Equation DTS ¼ 212:6 Dr-value is obtained for deep drawing deformation and this shows that r-value is a more effective mechanical property than TS on deep drawing by estimating the combined contribution of mechanical properties. (2) On plane strain deformation, equation DTS= 36.5 Dr-value is obtained and the effect of r-value on plane strain deformation is relatively small compared with that on deep drawing deformation. (3) The steel with the highest r-value of 2.68 shows superior formability, especially in the case of a pressed panel required for decreased in deformation resistance at the flange portion and increase in main stress at the fracture part on plane strain deformation
References [1] Yoshida, K., Handbook of Ease and Difficulty in Press Forming, National Center for Manufacturing Science, Inc., Ann Arbor, Michgan (1993). [2] Iizuka, E., et al., Effect of coating on deep drawability for steel sheet with high r-value, CAMP-ISIJ, Vol. 13, p. 630 (2000). [3] Nishimura, K., et al., Cold rolled steel sheets with ultra high Lankford value and excellent press formability, Kawasaki Steel Technical Report, No. 42, pp. 8–11 (2000).