| 摘要 |
PROBLEM TO BE SOLVED: To provide a low yield ratio high tensile strength steel sheet having yield strength of 440 MPa or more, tensile strength of 590 MPa or more, and a yield ratio of 80% or less.SOLUTION: The low yield ratio high tensile strength steel sheet excellent in weld heat affected zone toughness contains, in mass%, 0.06-0.09% of C, 0.01-0.07% of Si, 1.0-1.6% of Mn, 0.009% or less of P, 0.0007% or less of S, 0.20-0.60% of Mo, and 0.005-0.060% of Al, and further contains 0.005-0.030% of Nb, 0.005-0.020% of Ti, and 0.0025-0.0055% of N so as to satisfy formula (1): 3.0×10≤(Ti+Nb/5)×N≤9.0×10and formula (2): 2.5≤(Ti+Nb/3)/N≤5.5, with the balance comprising Fe and unavoidable impurities, wherein B is 0.0003% or less, carbon equivalent Ceq is 0.40-0.47%, and a weld crack sensitivity index Pcm is 0.21% or less. |
