High strength hot rolled steel sheet having excellent blanking workability and method for manufacturing the same

Abstract

A high strength hot rolled steel sheet having excellent blanking workability is provided. The composition contains C: 0.050 to 0.15%, Si: 0.1 to 1.5%, Mn: 1.0 to 2.0%, P: 0.03% or less, S: 0.0030% or less, Al: 0.01 to 0.08%, Ti: 0.05 to 0.15%, N: 0.005% or less, and the balance being Fe and unavoidable impurities. More than 95% of the microstructure is formed of a bainite phase in terms of area fraction. Average grain diameters of the bainite phase in a region having a thickness equal to ¼ of the sheet thickness from the surface in the sheet thickness direction is 5 μm or less in an L-direction cross section and 4 μm or less in a C-direction cross section. The number of crystal grains extended in the rolling direction and having an aspect ratio of 5 or more is 7 or less in a sheet thickness center portion.

1 . A high strength hot rolled steel sheet having excellent blanking workability and a tensile strength TS of 780 MPa or more, wherein the high strength hot rolled steel sheet has a composition that includes, in terms of mass %,
 C: 0.050 to 0.15%, Si: 0.1 to 1.5%,   Mn: 1.0 to 2.0%, P: 0.03% or less,   S: 0.0030% or less, Al: 0.01 to 0.08%,   Ti: 0.05 to 0.15%, N: 0.005% or less, and   the balance being Fe and unavoidable impurities,   and a microstructure, more than 95% of which is formed of a bainite phase in terms of area fraction throughout the entire region in a thickness direction, wherein average grain diameters of the bainite phase in a region having a thickness equal to ¼ of the sheet thickness from the surface in the sheet thickness direction is 5 μm or less in a sheet thickness cross section taken in a direction parallel to a rolling direction and 4 μm or less in a sheet thickness cross section taken in a direction perpendicular to the rolling direction, and wherein the number of crystal grains extended in the rolling direction and having an aspect ratio of 5 or more is 7 or less in a region having a thickness equal to 1/10 of the sheet thickness with its center at the center of the sheet in the thickness direction.   
     
     
         2 . The high strength hot rolled steel sheet according to  claim 1 , wherein, in addition to the composition, one or both of Nb: 0.005 to 0.1% and V: 0.005 to 0.2% are contained in terms of mass %. 
     
     
         3 . The high strength hot rolled steel sheet according to  claim 1 ,  wherein, in addition to the composition, at least one selected from Cu: 0.005 to 0.3%, Ni: 0.005 to 0.3%, Cr: 0.005 to 0.3%, and Mo: 0.005 to 0.3% is contained in terms of mass %. 
     
     
         4 . The high strength hot rolled steel sheet according to  claim 1 , wherein, in addition to the composition, one or both of Ca: 0.0005 to 0.03% and REM: 0.0005 to 0.03% are contained in terms of mass %. 
     
     
         5 . A method for manufacturing a high strength hot rolled steel sheet having excellent blanking workability, the method comprising heating a steel having a composition that includes, in terms of mass %,
 C: 0.050 to 0.15%, Si: 0.1 to 1.5%,   Mn: 1.0 to 2.0%, P: 0.03% or less,   S: 0.0030% or less, Al: 0.01 to 0.08%,   Ti: 0.05 to 0.15%, N: 0.005% or less,   and the balance being Fe and unavoidable impurities, to 1200 to 1350° C. and subjecting the heated steel to hot rolling that includes rough rolling and finish rolling, wherein a finish rolling delivery temperature of the finish rolling is set to a temperature within a range of “Ar 3  transformation point +30° C. or more” and “Ar 3  transformation point +150° C. or less”, cooling is immediately started after completion of the finish rolling, the cooling is conducted in two stages including first-stage cooling in which the finish rolled sheet is cooled at an average cooling rate of 35° C./sec. or more from the finish rolling delivery temperature to a first stage cooling end temperature of 520 to 580° C. and a second-stage cooling in which the finish rolled sheet is cooled at an average cooling rate of 80° C./sec. or more from the first stage cooling end temperature to a coiling temperature, and coiling is performed at a coiling temperature of 300 to 500° C.   
     
     
         6 . The method for manufacturing a high strength hot rolled steel sheet according to  claim 5 , wherein the steel contains, in addition to the composition, one or both of Nb: 0.005 to 0.1% and V: 0.005 to 0.2% in terms of mass % of the steel. 
     
     
         7 . The method for manufacturing a high strength hot rolled steel sheet according to  claim 5 , wherein the steel contains, in addition to the composition, at least one selected from of Cu: 0.005 to 0.3%, Ni: 0.005 to 0.3%, Cr: 0.005 to 0.3%, and Mo: 0.005 to 0.3% in terms of mass %. 
     
     
         8 . The method for manufacturing a high strength hot rolled steel sheet according to  claim 5 , wherein the steel contains, in addition to the composition, one or both of Ca: 0.0005 to 0.03% and REM: 0.0005 to 0.03% in terms of mass %.