Hot-rolled steel sheet

Abstract

This hot-rolled steel sheet has a desired chemical composition, and contains, in a microstructure, by area %, bainite: 10% or more, tempered martensite: 10% or more, a total of bainite and tempered martensite: 70% to 96%, fresh martensite: 20% or less, residual austenite: 4% to 12%, ferrite: 5% or less, and pearlite: 5% or less, in which an average grain size of prior austenite grains is 20.0 μm or less, an average aspect ratio of the prior austenite grains is 3.00 or less, and an area ratio of aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more is 5% or less.

1 . A hot-rolled steel sheet comprising, as a chemical composition, by mass %:
 C: 0.13% to 0.23%;   Si: 0.70% to 1.79%;   Mn: 1.79% to 3.00%;   P: 0.060% or less;   S: 0.005% or less;   N: 0.0070% or less;   O: 0.010% or less;   Al: 0.010% to 0.430%;   Ti: 0.006% to 0.055%;   Nb: 0.005% to 0.040%;   B: 0.0001% to 0.0030%;   Cr: 0% to 0.660%;   Mo: 0% to 0.300%;   Cu: 0% to 1.000%;   Ni: 0% to 1.000%;   Sn: 0% to 0.100%;   Ca: 0% to 0.0200%;   As: 0% to 0.100%;   Bi: 0% to 0.020%;   Mg: 0% to 0.0200%;   Zr: 0% to 0.400%;   V: 0% to 0.200%;   REM: 0% to 0.1000%;   Co: 0% to 0.2000%;   W: 0% to 0.2000%;   Zn: 0% to 0.2000%;   and a remainder including Fe and impurities,   in a microstructure,   by area %,
 bainite: 10% or more, 
 tempered martensite: 10% or more, 
 a total of bainite and tempered martensite: 70% to 96%, 
 fresh martensite: 20% or less, 
 residual austenite: 4% to 12%, 
 ferrite: 5% or less, and 
 pearlite: 5% or less, 
   an average grain size of prior austenite grains: 20.0 μm or less,   an average aspect ratio of the prior austenite grains: 3.00 or less, and   an area ratio of aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more: 5% or less.   
     
     
         2 . The hot-rolled steel sheet according to  claim 1 ,
 wherein pearlite is less than 3% in the microstructure.   
     
     
         3 . The hot-rolled steel sheet according to  claim 1 ,
 wherein a region of bainite in which an average value of nanoindentation hardnesses at a load of 5,000 μN and a loading rate of 500 μN/s is 4.7 GPa or less is 30% or less.   
     
     
         4 . The hot-rolled steel sheet according to  claim 1 ,
 wherein the chemical composition comprises, by mass %, one or more of
 Cr: 0.020% to 0.660%, 
 Mo: 0.001% to 0.300%, 
 Cu: 0.001% to 1.000%, 
 Ni: 0.001% to 1.000%, 
 Sn: 0.001% to 0.100%, 
 Ca: 0.0005% to 0.0200%, 
 As: 0.001% to 0.100%, 
 Bi: 0.001% to 0.020%, 
 Mg: 0.0005% to 0.0200%, 
 Zr: 0.001% to 0.400%, 
 V: 0.001% to 0.200%, 
 REM: 0.0005% to 0.1000%, 
 Co: 0.0005% to 0.2000%, 
 W: 0.0005% to 0.2000%, and 
 Zn: 0.0005% to 0.2000%. 
   
     
     
         5 . The hot-rolled steel sheet according to  claim 2 ,
 wherein the chemical composition comprises, by mass %, one or more of
 Cr: 0.020% to 0.660%, 
 Mo: 0.001% to 0.300%, 
 Cu: 0.001% to 1.000%, 
 Ni: 0.001% to 1.000%, 
 Sn: 0.001% to 0.100%, 
 Ca: 0.0005% to 0.0200%, 
 As: 0.001% to 0.100%, 
 Bi: 0.001% to 0.020%, 
 Mg: 0.0005% to 0.0200%, 
 Zr: 0.001% to 0.400%, 
 V: 0.001% to 0.200%, 
 REM: 0.0005% to 0.1000%, 
 Co: 0.0005% to 0.2000%, 
 W: 0.0005% to 0.2000%, and 
 Zn: 0.0005% to 0.2000%. 
   
     
     
         6 . The hot-rolled steel sheet according to  claim 2 ,
 wherein a region of bainite in which an average value of nanoindentation hardnesses at a load of 5,000 μN and a loading rate of 500 μN/s is 4.7 GPa or less is 30% or less.   
     
     
         7 . The hot-rolled steel sheet according to  claim 3 ,
 wherein the chemical composition comprises, by mass %, one or more of
 Cr: 0.020% to 0.660%, 
 Mo: 0.001% to 0.300%, 
 Cu: 0.001% to 1.000%, 
 Ni: 0.001% to 1.000%, 
 Sn: 0.001% to 0.100%, 
 Ca: 0.0005% to 0.0200%, 
 As: 0.001% to 0.100%, 
 Bi: 0.001% to 0.020%, 
 Mg: 0.0005% to 0.0200%, 
 Zr: 0.001% to 0.400%, 
 V: 0.001% to 0.200%, 
 REM: 0.0005% to 0.1000%, 
 Co: 0.0005% to 0.2000%, 
 W: 0.0005% to 0.2000%, and 
 Zn: 0.0005% to 0.2000%. 
   
     
     
         8 . The hot-rolled steel sheet according to  claim 6 ,
 wherein the chemical composition comprises, by mass %, one or more of
 Cr: 0.020% to 0.660%, 
 Mo: 0.001% to 0.300%, 
 Cu: 0.001% to 1.000%, 
 Ni: 0.001% to 1.000%, 
 Sn: 0.001% to 0.100%, 
 Ca: 0.0005% to 0.0200%, 
 As: 0.001% to 0.100%, 
 Bi: 0.001% to 0.020%, 
 Mg: 0.0005% to 0.0200%, 
 Zr: 0.001% to 0.400%, 
 V: 0.001% to 0.200%, 
 REM: 0.0005% to 0.1000%, 
 Co: 0.0005% to 0.2000%, 
 W: 0.0005% to 0.2000%, and 
 Zn: 0.0005% to 0.2000%.