| 摘要 |
<p num="1"><br/><br/><br/>A bake-hardening hot-rolled steel sheet which contains 0.01-0.2 mass% carbon, <br/>0.01-2 mass% silicon, 0.1-2 mass% manganese, up to 0.1 mass% phosphorus, up to <br/>0.03 mass% sulfur, 0.001-0.1 mass% aluminum, up to 0.01 mass% nitrogen, and <br/>0.005-0.05 mass% niobium, the remainder comprising iron and unavoidable <br/>impurities. The microstructure of the sheet is a polygonal ferrite having an <br/>average particle diameter of 2-8 ~m and/or a continuous cooling transformation <br/>texture. The proportion of a solid solution of carbon and/or a solid solution <br/>of nitrogen present at the grain boundaries in the sheet is 0.28 or lower. <br/>Also provided is a process for producing the hot-rolled steel sheet which <br/>comprises heating a steel billet comprising those ingredients to at least a <br/>temperature satisfying the equation SRT (~C) = 6670/{2.26-log(%Nb)(%C)}-273 <br/>(A) rough-rolling the steel billet heated, finish-rolling the sheet under such <br/>conditions that the final temperature is in the range of from the Ar3 <br/>transformation point to (Ar3 transformation point + 100~C), cooling it at a <br/>cooling rate of 80~C/sec or higher for cooling from cooling initiation to <br/>500~C or lower, and winding it up.<br/>
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