High-strength hot-rolled steel sheet and method for manufacturing the same

Abstract

A high-strength hot-rolled steel sheet has a chemical composition containing, by mass %, C: 0.02% to 0.23%, Si: 0.10% to 3.00%, Mn: 0.5% to 3.5%, P: 0.100% or less, S: 0.02% or less, and Al: 1.5% or less, the balance being Fe and incidental impurities, in which the total area fraction of martensite and bainite is 80% to 100%, the maximum orientation density of grains is less than 2.5 in a region extending from a position of 5 μm to a position of 10 μm from a surface in the thickness direction, and the maximum orientation density of grains is 2.5 or more in a region extending from a position of 50 μm to a position of 100 μm from the surface in the thickness direction.

1 . A high-strength hot-rolled steel sheet having a chemical composition comprising, by mass %:
 C: 0.02% to 0.23%,   Si: 0.10% to 3.00%,   Mn: 0.5% to 3.5%,   P: 0.100% or less,   S: 0.02% or less, and   Al: 1.5% or less, the balance being Fe and incidental impurities,   wherein a total area fraction of martensite and bainite is 80% to 100%, a maximum orientation density of grains is less than 2.5 in a region extending from a position of 5 μm to a position of 10 μm from a surface in a thickness direction, and the maximum orientation density of grains is 2.5 or more in a region extending from a position of 50 μm to a position of 100 μm from the surface in the thickness direction.   
     
     
         2 . The high-strength hot-rolled steel sheet according to  claim 1 , wherein the chemical composition further contains one or more selected from, by mass %:
 Cr: 0.005% to 2.0%,   Mo: 0.05% to 2.0%,   V: 0.05% to 1.0%,   Cu: 0.05% to 4.0%,   Ni: 0.005% to 2.0%,   Ti: 0.005% to 0.20%,   Nb: 0.005% to 0.20%,   B: 0.0003% 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, comprising:
 heating a slab having the chemical composition according to  claim 1 ;   performing rough rolling;   then performing finish rolling under conditions in which a total rolling reduction in a temperature range of 1,000° C. or lower is 50% or more, a total number of passes in a temperature range of 1,000° C. or lower is 3 passes or more, a final pass rolling temperature is 750° C. to 900° C., and a total rolling reduction from the final pass rolling temperature to the final pass rolling temperature+50° C. is 35% or less;   performing cooling under conditions in which a natural cooling time after completion of the finish rolling is 2.0 seconds or less, and an average cooling rate to a temperature of 550° C. is 50° C./s or more;   performing cooling at an average cooling rate of 100° C./s or more in a temperature range of 300° C. to 400° C.; and   performing coiling at 300° C. or lower.   
     
     
         4 . A method for manufacturing a high-strength hot-rolled steel sheet, comprising:
 heating a slab having the chemical composition according to  claim 2 ;   performing rough rolling;   then performing finish rolling under conditions in which a total rolling reduction in a temperature range of 1,000° C. or lower is 50% or more, a total number of passes in a temperature range of 1,000° C. or lower is 3 passes or more, a final pass rolling temperature is 750° C. to 900° C., and a total rolling reduction from the final pass rolling temperature to the final pass rolling temperature+50° C. is 35% or less;   performing cooling under conditions in which a natural cooling time after completion of the finish rolling is 2.0 seconds or less, and an average cooling rate to a temperature of 550° C. is 50° C./s or more;   performing cooling at an average cooling rate of 100° C./s or more in a temperature range of 300° C. to 400° C.; and   performing coiling at 300° C. or lower.