| 简介 |
PROBLEM TO BE SOLVED : To provide a hot rolled steel sheet having excellent press formability, secondary working brittleness resistance and fatigue resistance, and suitable as the stock for an automobile under carriage components, or the like.
SOLUTION : The hot rolled steel sheet has a chemical composition containing <0.0030% C, ≤0.1% Si, ≤1.0% Mn, ≤0.02% P, ≤0.015% S, 0.015 to 0.10% Ti, ≤0.10% Al, ≤0.005% N and 0.0010 to 0.0050% B, and the balance Fe with impurities, and has a structure composed of bainitic ferrite and polygonal ferrite each having a mean crystal grain size of 5 to 30 μm, and in which the area ratio of the bainitic ferrite is 1 to 80%, and the hardness of the polygonal ferrite is ≤120 by Vickers hardness. One or more kinds of metals selected from at least one group of the following (1) to (3) may be incorporated therein : (1) 0.002 to 0.04% Nb; (2) 0.005 to 1.0% Ni and 0.005 to 1.0% Cr; and (3) 0.0005 to 0.050% Ca and 0.0005 to 0.050% rare earth metals.
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