Hot-rolled steel sheet and method for manufacturing same

Abstract

This hot-rolled steel sheet has a predetermined chemical composition and has a metallographic structure containing ferrite with an area ratio of 53.0% or more and 76.0% or less, martensite with an area ratio of 3.0% or more and 10.0% or less, bainite with an area ratio of 14.0% or more and 39.0% or less, and pearlite with an area ratio of 2.6% or less, and with an average diameter of martensite of 0.26 μm or more and 0.70 μm or less, wherein, among all interfaces of the martensite, the total length of the interfaces between the martensite and the bainite is 75.0% or more with respect to the total length of all interfaces of the martensite.

1 . A hot-rolled steel sheet having a chemical composition containing, in mass %,
 C: 0.035% or more and 0.085% or less,   Si: 0.001% or more and 0.15% or less,   Mn: 0.70% or more and 1.80% or less,   P: 0.020% or less,   S: 0.0050% or less,   Ti: 0.075% or more and 0.170% or less,   Nb: 0.003% or more and 0.050% or less,   Al: 0.10% or more and 0.40% or less,   N: 0.0080% or less,   Cr: 0% or more and 0.27% or less,   B: 0% or more and 0.0050% or less,   Ca: 0% or more and 0.0050% or less,   Mo: 0% or more and 0.40% or less,   Ni: 0% or more and 0.50% or less,   Cu: 0% or more and 0.50% or less, and   REM: 0% or more and 0.0300% or less,   with the remainder consisting of Fe and impurities, and   having a metallographic structure containing,
 ferrite with an area ratio of 53.0% or more and 76.0% or less, 
 martensite with an area ratio of 3.0% or more and 10.0% or less, 
 bainite with an area ratio of 14.0% or more and 39.0% or less, and 
 pearlite with an area ratio of 2.6% or less, 
   with an average diameter of martensite of 0.26 μm or more and 0.70 μm or less,   wherein, among all interfaces of the martensite, a total length of the interfaces between the martensite and the bainite is 75.0% or more with respect to a total length of all interfaces of the martensite.   
     
     
         2 . The hot-rolled steel sheet according to  claim 1 ,
 wherein the chemical composition contains, in mass,   one, or two or more selected from the group consisting of
 Cr: 0.06% or more and 0.27% or less, 
 B: 0.0003% or more and 0.0050% or less, 
 Ca: 0.0003% or more and 0.0050% or less, 
 Mo: 0.01% or more and 0.40% or less, 
 Ni: 0.01% or more and 0.50% or less, 
 Cu: 0.01% or more and 0.50% or less, and 
 REM: 0.0003% or more and 0.0300% or less. 
   
     
     
         3 . A method for manufacturing a hot-rolled steel sheet that is performed on a slab having the chemical composition according to  claim 1 , the method comprising:
 a hot rolling process in which rolling is performed under conditions in which a final finishing temperature is 880° C. or higher and 950° C. or lower;   a primary cooling process in which cooling is performed to a primary cooling stop temperature of 680° C. or higher and 760° C. or lower at an average cooling rate of 60° C./sec or faster, after the hot rolling process;   a secondary cooling process in which cooling is performed at an average cooling rate of 20° C./sec or slower for 1.6 seconds or longer and 6.3 seconds or shorter, after the primary cooling process;   a tertiary cooling process in which cooling is performed to a tertiary cooling stop temperature of 195° C. or higher and 440° C. or lower at an average cooling rate of 60° C./sec or faster and 130° C./sec or slower, after the secondary cooling process;   a quaternary cooling process in which water cooling is performed at a water density of 2.0 m 3 /min/mm 2  or more and 7.2 m 3 /min/mm 2  or less for 0.33 seconds or longer and 1.50 seconds or shorter, after the tertiary cooling process;   a quinary cooling process in which air cooling is performed for 3.0 seconds or longer and 5.0 seconds or shorter, after the quaternary cooling process; and   a winding process in which winding is performed at lower than 180° C., after the quinary cooling process.   
     
     
         4 . A method for manufacturing a hot-rolled steel sheet that is performed on a slab having the chemical composition according to  claim 2 , the method comprising:
 a hot rolling process in which rolling is performed under conditions in which a final finishing temperature is 880° C. or higher and 950° C. or lower;   a primary cooling process in which cooling is performed to a primary cooling stop temperature of 680° C. or higher and 760° C. or lower at an average cooling rate of 60° C./sec or faster, after the hot rolling process;   a secondary cooling process in which cooling is performed at an average cooling rate of 20° C./sec or slower for 1.6 seconds or longer and 6.3 seconds or shorter, after the primary cooling process;   a tertiary cooling process in which cooling is performed to a tertiary cooling stop temperature of 195° C. or higher and 440° C. or lower at an average cooling rate of 60° C./sec or faster and 130° C./sec or slower, after the secondary cooling process;   a quaternary cooling process in which water cooling is performed at a water density of 2.0 m 3 /min/mm 2  or more and 7.2 m 3 /min/mm 2  or less for 0.33 seconds or longer and 1.50 seconds or shorter, after the tertiary cooling process;   a quinary cooling process in which air cooling is performed for 3.0 seconds or longer and 5.0 seconds or shorter, after the quaternary cooling process; and   a winding process in which winding is performed at lower than 180° C., after the quinary cooling process.