High-strength hot-rolled steel sheet and a method of manufacturing the same which is excellent in the hole expansion workability

摘要

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet excellent in bore expanding workability with tensile strength of 980 MPa or more.;SOLUTION: A steel raw material containing, by mass%, C:over 0.1% to 0.2%, Si:1.0% or less, Mn:1.5 to 2.5%, Al:0.10% or less, N:0.007% or less, Ti:0.07 to 0.2%, V:over 0.1% to 0.3%, is heated at 1200°C or more, applied to rough rolling and finish rolling at completion temperature of 850 to 950°C, first step cooling for cooling to a cooling stop temperature of 500 to 600°C at an average cooling rate of 20 to 80°C/s, then second step cooling for cooling to a cooling stop temperature of 330 to 470°C at the average cooling rate of 90°C/s or more, and winding up at winding temperature of 330 to 470°C. This leads a high strength hot rolled steel sheet having a structure of a bainite phase of 90% or more by area ratio as a main phase, dispersed cementite of 0.8 mass% or less and an average particle diameter of 150 nm or less, and excellent bore expanding workability with tensile strength of 980 MPa or more.;COPYRIGHT: (C)2015,JPO&INPIT