High strength hot rolled thick steel sheet excellent in strength and toughness after heat treatment and method for manufacturing the same

Abstract

A high strength hot rolled thick steel sheet has a tensile strength of 440 to 640 MPa, preferably 490 to 590 MPa, and an elongation of 20% or more; is excellent in uniformity in a sheet thickness direction and in strength and toughness after heat treatment; and is suitably used for structural components of automobiles, construction machines, and the like, and a method for manufacturing the hot rolled thick steel sheet. A steel material having a composition including C: 0.10 to 0.20%, Ti: 0.01 to 0.15%, B: 0.0010 to 0.0050%, and proper amounts of Si, Mn, Al, P, S, and N is hot-rolled at a finisher delivery temperature of 820 to 880° C. in finish rolling, is cooled at a cooling rate of 15 to 50° C./s until a temperature reaches a cooling stop temperature of 500 to 600° C., and is coiled.

1 . A high strength hot rolled thick steel sheet with a sheet thickness of 6 mm or more and 12 mm or less that is excellent in strength and toughness after heat treatment, the hot rolled thick steel sheet comprising a composition including C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, and B: 0.0010 to 0.0050% by mass with the balance Fe and incidental impurities; and a bainitic ferrite phase having an area ratio of 95% or more, wherein a deviation of hardness along thickness is within 10% from an average; and having a tensile strength of 440 to 640 MN and an elongation of 20% or more (gauge length GL: 50 mm). 
     
     
         2 . A method for manufacturing a high strength hot rolled thick steel sheet that is excellent in strength and toughness after heat treatment, the method comprising:
 hot-rolling a steel material at a finisher delivery temperature of 820 to 880° C. in finish rolling to obtain a hot rolled steel sheet having a sheet thickness of 6 mm or more and 12 mm or less, the steel material having a composition including C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, and B: 0.0010 to 0.0050% by mass with the balance Fe and incidental impurities;   cooling the hot rolled steel sheet at a cooling rate of 15 to 50° C./s on a surface temperature basis until a surface temperature reaches a temperature range of 550 to 650° C.; and   coiling the hot rolled steel sheet in the temperature range, such that the steel sheet has a deviation of hardness along thickness within 10% from an average, a tensile strength of 440 to 640 MPa and an elongation of 20% or more (gauge length GL: 50 mm).