| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet suitable for automobiles or the like, and having a wide proper current range, and to provide a spot-welded member. SOLUTION: The hot rolled steel sheet is made of a carbon steel or a low alloy steel containing ferrite as the main phase, and having a C content of≤0.15 mass%, and in which the average crystal grain diameter D (μm) of the ferrite in the depth of the 1/4 of the sheet thickness from the surface of the steel sheet satisfies the following inequalities (1) and (2), further, the increasing speed X (μm/min) at 700°C in the average crystal grain diameter of the ferrite in the depth position of the 1/4 of the sheet thickness from the surface of the steel sheet, and the average crystal grain diameter D (μm) satisfy the following inequality (3), and also, tensile strength TS (MPa) satisfies the following inequality (4): inequality (1) 1.2≤D≤7; inequality (2) D≤2.7+5000/(5+350*C+40*Mn)<SP>2</SP>; inequality (3) D*X≤0.1; and inequality (4) TS(TS/170-1)/1000≥Si+Al+Mn. COPYRIGHT: (C)2008,JPO&INPIT
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