| 摘要 |
PURPOSE:To obtain a low yield ratio high strength steel for an automotive door reinforcing material or the like excellent in toughness by normalizing or low temp. tempering as well, by forming the componental compsn. of the steel in which components, particularly, the bainitic index is specified and sufficient strength and toughness can be obtd. CONSTITUTION:This low yield ratio high strength steel is constituted of, by weight, 0.15 to 0.40% C, 0.1 to 0.7% Si, 1.0 to 2.7% Mn, 1.0 to 3.5% Cr, 0.01 to 0.05% sol.Al, <=0.025% P and <=0.015% S, and the balance Fe. Then, its bainitic index (%) shown by -209C+43Si-48Mn-58Cr-0.416R+317 is regulated to 0 to 50%; where R denotes its cooling rate ( deg.C/min) at the time of hardening for obtaining a martensitic structure. This steel is provided with >=120kgf/mm<2> tensile strength, <=75% yield ratio and -40 deg.C toughness value vTrs. Moreover, the same steel may be incorporated with prescribed amounts of Mo and Ni and further V, Ti, Nb and B.
|
