| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a thick hot-rolled steel sheet which is suitable as a square steel pipe material for a building structural member. <P>SOLUTION: A steel material having a composition including, by mass%, C: 0.07-0.18%, Mn: 0.3-1.5%, Al: 0.01-0.06%, N: 0.006% or less is heated to 1,100-1,300°C, and then subjected to a rough rolling with rough rolling end temperature of 950-1,150°C and a finish-rolling with rolling start temperature of 1,100-850°C and rolling end temperature of 900-750°C. The steel material is successively subjected to three stages of cooling: a primary cooling with a cooling stop temperature of 550°C or higher in terms of surface temperature, a secondary cooling of air-cooling for 3-15 s, and a tertiary cooling of cooling at a cooling rate such that an average cooling rate is 4-15°C/s in terms of plate thickness center temperature, in which the time until the temperature reaches 650°C in terms of plate thickness center temperature after the cooling start is within 35 s. Then, the steel material is wound at 500-650°C. <P>COPYRIGHT: (C)2012,JPO&INPIT |
