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

Abstract

A high-strength hot-rolled steel sheet according to the present invention has a specific chemical composition and a steel microstructure including, as main phases, 80% to 100% of martensite and bainite in terms of total area fraction. An entire area fraction of the martensite dispersed in the bainite is 2% to 20%. Among the martensite dispersed in the bainite, an area fraction of martensite each having an orientation difference of 15° or more between a crystal orientation of the martensite and a crystal orientation of at least one of bainite adjacent to the martensite is more than 50% relative to the whole martensite dispersed in the bainite. When regions surrounded by boundaries between adjacent crystals having an orientation difference of 15° or more are defined as crystal grains, an average aspect ratio of the crystal grains present in a region extending from a surface of the steel sheet to a depth of 5 μm is 2.0 or less.

1 . A high-strength hot-rolled steel sheet comprising:
 a chemical composition containing, by mass %,   C: 0.04% to 0.18%,   Si: 0.1% to 3.0%,   Mn: 0.5% to 3.5%,   P: more than 0% and 0.100% or less,   S: more than 0% and 0.020% or less, and   Al: more than 0% and 1.5% or less, and   further containing one or two or more selected from Cr: 0.005% to 2.0%, Ti: 0.005% to 0.20%, Nb: 0.005% to 0.20%, Mo: 0.005% to 2.0%, and V: 0.005% to 1.0%,   with the balance being Fe and incidental impurities; and   a steel microstructure including, as main phases, 80% to 100% of martensite and bainite in terms of total area fraction,   wherein an entire area fraction of the martensite dispersed in the bainite is 2% to 20%,   among the martensite dispersed in the bainite, an area fraction of martensite each having an orientation difference of 15° or more between a crystal orientation of the martensite and a crystal orientation of at least one of bainite adjacent to the martensite is more than 50% relative to the whole martensite dispersed in the bainite, and   when regions surrounded by boundaries between adjacent crystals having an orientation difference of 15° or more are defined as crystal grains, an average aspect ratio of the crystal grains present in a region extending from a surface of a steel sheet to a depth of 5 μm is 2.0 or less.   
     
     
         2 . The high-strength hot-rolled steel sheet according to  claim 1 , comprising:
 in addition to the chemical composition, by mass %,   one or two or more selected from:   Cu: 0.05% to 4.0%,   Ni: 0.005% to 2.0%,   B: 0.0002% to 0.0050%,   Ca: 0.0001% to 0.0050%,   REM: 0.0001% to 0.0050%,   Sb: 0.0010% to 0.10%, and   Sn: 0.0010% to 0.50%.   
     
     
         3 . A method for manufacturing a high-strength hot-rolled steel sheet according to  claim 1 or 2 , the method comprising:
 heating a slab having the chemical composition; and   subsequently subjecting the slab to hot rolling,   wherein the hot rolling includes performing rough rolling, performing finish rolling under conditions in which a total number of passes at 1,000° C. or higher is 3 times or more, a total rolling reduction at 1,000° C. or lower is less than 50%, and a total rolling reduction from a final pass rolling temperature to the final pass rolling temperature+50° C. is 35% or less, subsequently starting cooling in less than 1.0 s, performing cooling under a condition in which an average cooling rate from a cooling start temperature to 550° C. is 50° C./s or more, and subsequently performing coiling at a coiling temperature of (Ms temperature−50)° C. to 550° C.   
     
     
         4 . A method for manufacturing a high-strength hot-rolled steel sheet according to  claim 2 , the method comprising:
 heating a slab having the chemical composition; and   subsequently subjecting the slab to hot rolling,   wherein the hot rolling includes performing rough rolling, performing finish rolling under conditions in which a total number of passes at 1,000° C. or higher is 3 times or more, a total rolling reduction at 1,000° C. or lower is less than 50%, and a total rolling reduction from a final pass rolling temperature to the final pass rolling temperature+50° C. is 35% or less, subsequently starting cooling in less than 1.0 s, performing cooling under a condition in which an average cooling rate from a cooling start temperature to 550° C. is 50° C./s or more, and subsequently performing coiling at a coiling temperature of (Ms temperature−50)° C. to 550° C.