DEVELOPMENT OF HIGH STRENGTH INTERSTITIAL FREE STEEL STHEET WITH 340-440 MPA UTS (ULTIMATE TENSILE STRENGTH) FOR AUTOMOTIVE APPLICATION

摘要

The invented product is a family of steel grades for automotive stamping applications. All the grades have ultra low carbon based steel compositions, strengthened by suitable additions of phosphorus and manganese, in order to achieve various strength levels. The processing schedules, including steel making, hot rolling, cold rolling and annealing, have been determined appropriately so as to achieve desired drawability and strength. The steel grades are also adequately weldable. Chemical composition %C %Mn %5 %P %Si %AI %Ti %Nb N B 0.0035 0.5-0.95 0.01 0.02-0.065 0.015 0.04-0.05 0.05-0.06 0.012-0.02 30ppm 4-11ppm Physical processing a) Hot charging of slabs to reheat furnace for grades IFHS-309 and IFHS-440. Hot charging is not absolutely necessary for IFHS-340. Hot charging desirable for high P content (more than 0.03%). b) Slab drop out @ around-1200°C c) Finish rolling @ higher than Ar3 temperature - 900 - 910°C d) Coiling @ 550 - 560°C e) Batch annealing @ hot spot temperature-720°C and cold spot temperature 690°C Test results (two results are given below for each grade of the IFHS family): SteelGrades Width(mm) Thickness(mm) YS(MPa) UTS (MPa) %EI r-bar IFHS- 1260 0.8 198 375 42 1.78 340 1245 1.2 185 360 41 1.72 Desirable properties 145-245 340 min 40 min 1.4 min IFHS- 1025 1.0 237 415 37 1.55 390 1025 0.9 232 411 37 1.6 Desirable properties 195-295 390 min 33 min 1.4 min IFHS- 1150 1.8 293 444 36 1.52 440 1150 1.4 310 446 37 1.42 Desirable Properties 235-345 440 min 33 min 1.3 min Secondary work embrittlement property: OK upto - 30°C (tested for all grades)