High-strength hot-rolled steel sheet and method for manufacturing same

Abstract

A high-strength hot-rolled steel sheet has a tensile strength (TS) of 540 to 780 MPa, only small variations in strength, and excellent uniformity in strength using a general-purpose Ti-containing steel sheet, which is inexpensive. The high-strength hot-rolled steel sheet includes, on a mass percent basis, 0.05%-0.12% C, 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti, and the balance being Fe and incidental impurities. The microstructure have a volume fraction of polygonal ferrite of 70% or more, and the amount of Ti present in a precipitate having a size of less than 20 nm is 50% or more of the value of Ti* calculated using formula (1): Ti*=[Ti]−48/14×[N] (1) where [Ti] and [N] represent a Ti content (percent by mass) and a N content (percent by mass), respectively, of the steel sheet.

1 . A high-strength hot-rolled steel sheet comprising, on a mass percent basis, 0.05%-0.12% C, 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti, the balance being Fe and incidental impurities, and microstructures whose volume fraction of polygonal ferrite is 70% or more, wherein the amount of Ti present in a precipitate having a size of less than 20 nm is 50% or more of the value of Ti* calculated using formula (1):
   Ti*=[Ti]−48/14×[N]  (1)
   
       where [Ti] and [N] represent a Ti content (percent by mass) and a N content (percent by mass), respectively, of the steel sheet. 
     
     
         2 . A method for manufacturing a high-strength hot-rolled steel sheet comprising:
 heating a steel slab to 1150° C. to 1300° C., the steel slab containing, on a mass percent basis, 0.05%-0.12% C; 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti, and the balance being Fe and incidental impurities;   subjecting the slab to finish hot rolling at a finishing temperature of 800° C. to 950° C.;   starting cooling at a cooling rate of 20° C./s or more within 2 seconds after completion of the finish hot rolling;   stopping cooling at 650° C. to 750° C.;   performing natural cooling for 2 seconds to 15 seconds;   cooling the steel sheet at a cooling rate of less than 100° C./s; and   winding the steel sheet into a coil in a temperature range of 550° C. to 650° C.