| 摘要 |
Provided is a low-yield-ratio, high-strength hot-rolled steel sheet having excellent low-temperature toughness and being appropriate as a steel pipe raw material. The present invention has a composition including 0.03-0.10% C, 0.01-0.50% Si, 1.4-2.2% Mn, no more than 0.025% P, no more than 0.005% S, 0.005-0.10% Al, 0.02-0.10% Nb, 0.001-0.030% Ti, 0.01-0.50% Mo, 0.01-0.50% Cr, and 0.01-0.50% Ni, Moeq preferably satisfying a range of 1.4-2.2%, and the inner layers have a structure that includes bainitic ferrite as a main phase and massive martensite having a surface area ratio of 1.4-15% and an aspect ratio of less than 5.0 as a second phase, the lath spacing of the bainitic ferrite phase being 0.2-1.6 µm. Regarding the size of the massive martensite, the maximum is preferably no more than 5.0 µm and the average is preferably 0.5-3.0 µm. |
