High strength hot rolled steel sheet

Abstract

There is provided a high strength hot rolled steel sheet including a predetermined chemical composition. A structure includes, by area ratio, 80% or more of polygonal ferrite, a total of 5% or less of martensite and austenite, and a total of 5% or less of pearlite and cementite, and a remainder is at least one selected from bainitic ferrite and bainite. When a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 μm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller. A grain of the polygonal ferrite contains 5×10 7 pieces/mm 2 or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, the aspect ratio which is a ratio of a length of a long side to a length of a short side is less than 3. The tensile strength is 540 MPa or higher.

1 - 4 . (canceled) 
     
     
         5 . A high strength hot rolled steel sheet comprising, as a chemical composition, by mass %,
 C: 0.010% to 0.200%,   Si: 0.001% to 2.50%,   Mn: 0.001% to 1.50%,   P: 0.050% or less,   S: 0.010% or less,   N: 0.0070% or less,   Al: 0.001% to 0.50%,   Ti: 0.050% to 0.30%,   V: 0% to 0.50%,   Nb: 0% to 0.090%,   Cr: 0% to 0.50%,   Ni: 0% to 0.50%,   Cu: 0% to 0.50%,   Mo: 0% to 0.50%,   B: 0% to 0.0050%,   Ca: 0% to 0.01%,   Mg: 0% to 0.01%,   Bi: 0% to 0.01%, and   a remainder of Fe and impurities,   wherein a structure includes, by area ratio, 80% or more of a polygonal ferrite, a total of 5% or less of a martensite and an austenite, and a total of 5% or less of a pearlite and a cementite, and the remainder is at least one selected from a bainitic ferrite and a bainite,   wherein when a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 μm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller,   wherein a grain of the polygonal ferrite contains 5×10 7  pieces/mm 2  or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, an aspect ratio which is a ratio of a length of a long side to a length of a short side is less than 3, and   wherein a tensile strength is 540 MPa or higher.   
     
     
         6 . The high strength hot rolled steel sheet according to  claim 5 ,
 wherein the chemical composition includes, by mass %, at least one selected from the group consisting of V: 0.010% to 0.50%, Nb: 0.001% to 0.090%, Cr: 0.001% to 0.50%, Ni: 0.001% to 0.50%, Cu: 0.001% to 0.50%, Mo: 0.001% to 0.50%, and B: 0.0001% to 0.0050%.   
     
     
         7 . The high strength hot rolled steel sheet according to  claim 5 ,
 wherein the chemical composition includes, by mass %, at least one selected from the group consisting of Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, and Bi: 0.0001% to 0.01%.   
     
     
         8 . The high strength hot rolled steel sheet according to  claim 6 ,
 wherein the chemical composition includes, by mass %, at least one selected from the group consisting of Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, and Bi: 0.0001% to 0.01%.   
     
     
         9 . The high strength hot rolled steel sheet according to  claim 5 , further comprising:
 a hot-dip galvanized layer on a surface.   
     
     
         10 . The high strength hot rolled steel sheet according to  claim 6 , further comprising:
 a hot-dip galvanized layer on a surface.   
     
     
         11 . The high strength hot rolled steel sheet according to  claim 7 , further comprising:
 a hot-dip galvanized layer on a surface.   
     
     
         12 . The high strength hot rolled steel sheet according to  claim 8 , further comprising:
 a hot-dip galvanized layer on a surface.