新しい自動車用高強度鋼板の開発 - kobelco.co.jp

1 FEATURE : Progress of Technology in 100-year History of Kobe Steel .. Development of New High-strength Steel Sheets for Automobiles . Youichi Mukai Improved fuel economy through car weight reduction and higher impact resistant passenger areas are both becoming increasingly popular in the automotive industry. Kobe Steel has developed and produced various kinds of high strength steels for automotive use including 590 MPa grade dual-phase hot dip galvannealed steel, and hot and cold-rolled TRIP-aided steel. In this report a few new high strength steels are reported on including a very ductile 980 MPa grade hot dip galvannealed steel, and an ultra high strength cold-rolled steel which has excellent delayed fracture resistance.. CO2 . 2 .. 590 980 MPa . 590 980 MPa . GA 5 7 1 .. 1970 .. 1 4 . 980 MPa . 590 MPa .. 980 MPa .. 100 .. 1 .. 590 MPa . 980 MPa.

2 1 . Fig. 1 Application of high strength steel sheet for body in white .. 30 KOBE STEEL ENGINEERING REPORTS/Vol. 55 No. 2 Sep. 2005 . 80 . 70 . Developed 60 980 MPa . steel ( ) 50 . 40 . 30 Conventional 1 . 1 980 MPa . steel 1 . 1 1 . 20.. 10. 5 10 15 20 25 590 MPa 780 MPa GA . El. ( ).. 2 980 MPa El.. Fig. 2 Elongation and value combination of 980 MPa grade cold rolled steel .. 980 MPa . 2 .. Mn .. Cr Mo . C .. C 1 .. C 780 980 MPa GA . 8 1 . 1 2 .. TRIP: 2 . TRansformation Induced Plasticity 3 .. 4 . 50mm R punch radius rp 10mm . GA780 MPa .. 1 980 MPa .. Table 1 Chemical compositions of 980 MPa grade hot dip galvannealed 9 11 steel (mass ). Other Steel TS grade C Si Mn P. element Developed steel 980 MPa Cr, Mo . Conventional steel 780 MPa Cr, Mo .. 2 . Table 2 Mechanical properties of steels 12 YP TS El. Steel TS grade (MPa) (MPa) ( ). Developed steel GA 980 MPa (GA980) 660 1 030 15.

3 Cold-rolled 980 MPa (CR980) 621 1 051 17. Conventional steel GA 780 MPa (GA780) 445 832 20. Thickness , JIS specimen /Vol. 55 No. 2 Sep. 2005 31. 1 1 3 . 6 . 3 . 5 .. JIS .. 40.. 30. GA780 1 . 1 4 . 20. GA980 . y ( ). 10. CR980 7. 0 50mm . 0 10 20 30 40 50 60. 10 300mm 3 . 20 1 000mm 4 . 30 . x ( ).. 3 GA980 MPa 50mm . Fig. 3 Forming limit diagram of GA 980 MPa grade steel 8 . 45 GA980 Formimg load . GA980 Rupture load Forming, rupture load (kN). 40 . CR980 Forming load 35 CR980 Rupture load GA780 Forming load 1 . 2 . 30. GA780 Rupture load . 25. 980 MPa . 20. 15 14. Cross tension strength (kN). Expulsion 10 12. 5 10. Drawing ratio 8. 4 GA980 MPa 6. Fig. 4 Relation between drawing ratio and forming, rupture load of 4 GA980-Fracture in nugget GA980 MPa steel Expulsion GA980-Button fracture 2 CR980-Fracture in nugget 3 CR980-Button fracture 0.

4 Table 3 Spot welding conditions 5 10 15 20. Welding current (kA). Electrode tip Dome type Cu-Cr, Tip diameter 6mm 6 . Electrode force 4 410N. Fig. 6 Effect of welding current on cross tension strength Welding time 16cycle (16Hz). Welding current 5-20kA 70. Cooling water 3l/min (Upper , Lower ). R5 60. 35 R5. Button fracture Tensile shear strength (kN). GA980. 30. CR980. 25 Unit mm 130. 20. 7 . Button fracture 15 Fig. 7 Cross section of crash test specimen 10. 4 . JIS Z 3140 A-class Table 4 Crash test condition 5. min. 0 Axial crash Crash bending 2 4 6 8 10 12 Weight 2 843N 1 667N. Button diameter (mm). Impact speed 50km/h 50km/h 5 Displacement Fig. 5 Relation between tensile shear strength and button diameter (span 900mm). 32 KOBE STEEL ENGINEERING REPORTS/Vol. 55 No. 2 Sep. 2005 . 5. Absorbed energy (kN). Axial crash 4 Crash bending 3. 2. 1.

5 0. GA980 CR980 GA780. 8 . Fig. 8 Absorbed energy on dynamic crash test 250nm 13 . 1 1 470 MPa TBF TEM . Photo 1 TEM micrograph of 1 470 MPa TBF steel . 5 . Table 5 Mechanical properties of developed steel . YP TS El. Steel 14 (MPa) (MPa) ( ). Developed steel TBF1470 1 059 1 495 11. Conventional steel DP1470 1 313 1 527 7. 1 180 MPa Thickness , JIS specimen . Punch .. TBF Trip-aided Bainitic R Specimen Ferrite 15 17 Die . 1 2 1 . 980 MPa . 9 V . Fig. 9 V-bending test method . 6 V .. Table 6 Effect of bending angle and radius on V-bend cracking test . Bending angle Bending radius (R). Steel 5mm 4mm 3mm 2mm 1mm 60 . TBF1470. 90 . 60 . DP1470. 90 . No fracture, Fracture .. TBF . 1 . 2 2 . 5 . DP 1 470 MPa . DP . 9 V . V . TRIP 60 90 1 5mm . 6 V DP . 1 TBF . 1 m DP .. TBF . /Vol. 55 No. 2 Sep. 2005 33. U-bending Fastenning with bolt HCl 5 . Specimen Bolt R.

6 Roller-die Strain gage R Residual stress . Fig. 10 Experimental procedure of delayed fracture resistance test 7 . Table 7 Effect of bending radius, residual stress and time on delayed fracture performance Bending Residual Time in 5 -HCL. Steel radius(mm) stress(MPa 1h 4h 8h 24h 48h 1 000 . 10 1 500 . 2 000 . TBF1470. 1 000 . 15 1 500 . 2 000 . 1 000 . 10 1 500 . 2 000 . DP1470. 1 000 . 15 1 500 . 2 000 . No fracture, Fracture USS . PRO-TEC COATING . 1 2 3 GA 590 . 980 MPa .. 18 780 MPa TRIP GA .. 2 . 2 . U .. 10 440 . 10mm 15mm U 590 MPa . 1 470 MPa . 1 000 2 000 MPa 20 . 5 48 2 . 3 . 7 . DP R . Si Mn .. 19 .. 21 .. 3 .. 2 .. 2 1 .. 34 KOBE STEEL ENGINEERING REPORTS/Vol. 55 No. 2 Sep. 2005 . 4 R&D , 2002 , 5 R&D , 2000 6 R&D , 2001 , . 7 R&D , 2002 , 8 2862186 . 9 K. Sugimoto et al ISIJ-Int, , 1995 , 10 CAMP-ISIJ, 2000 , . 11 CAMP-ISIJ, 2001 , 12 R&D , 2002 , 13 , 1994 , 14 228 15 K.)

7 Sugimoto et al ISIJ-Int, , 2000 , . 16 51. 2002 , 17 CAMP-ISIJ, 2004 , 18 1989 . 19 CAMP-ISIJ, 2005 , . 20 2002 241895. 21 CAMP-ISIJ, 2004 , . 1 R&D , 1985 , 2 H. Shirasawa et al. Trans. ISIJ , 1986 . , 3 R&D , 2000 , /Vol. 55 No. 2 Sep. 2005 35.