Hot-rolled steel sheet and method of manufacturing same

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, in which a microstructure contains, by area %, bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in the microstructure: 10.0% or less, a total density of a length L7 of a grain boundary having a crystal orientation difference of 7° and a length L68 of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 μm/μm2, and a tensile strength is 780 MPa or more.

1 . A hot-rolled steel sheet comprising, as a chemical composition, by mass %:
 C: 0.030% to 0.200%;   Si: 0.05% to 2.50%;   Mn: 1.00% to 4.00%;   sol. Al: 0.001% to 2.000%;   Ti: 0.030% to 0.200%,   P: 0.020% or less;   S: 0.020% or less;   N: 0.010% or less;   Nb: 0% to 0.200%;   B: 0% to 0.010%;   V: 0% to 1.00%;   Mo: 0% to 1.00%;   Cu: 0% to 1.00%;   W: 0% to 1.00%;   Cr: 0% to 1.00%;   Ni: 0% to 1.00%;   Co: 0% to 1.00%;   Ca: 0% to 0.010%;   Mg: 0% to 0.010%;   REM: 0% to 0.010%;   Zr: 0% to 0.010%; and   a remainder consisting of iron and impurities,   wherein a microstructure contains, by area %,
 bainite: 80.0% or more, 
 ferrite: 10.0% or less, and 
 a remainder in the microstructure: 10.0% or less, 
   a total density of a length L 7  of a grain boundary having a crystal orientation difference of 7° and a length L 68  of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 μm/μm 2 , and a tensile strength is 780 MPa or more.   
     
     
         2 . The hot-rolled steel sheet according to  claim 1 ,
 wherein the hot-rolled steel sheet includes, as a chemical composition, by mass %, one or more selected from the group of:   Nb: 0.005% to 0.200%;   B: 0.001% to 0.010%;   V: 0.005% to 1.00%;   Mo: 0.005% to 1.00%;   Cu: 0.005% to 1.00%;   W: 0.005% to 1.00%;   Cr: 0.005% to 1.00%;   Ni: 0.005% to 1.00%;   Co: 0.005% to 1.00%;   Ca: 0.0005% to 0.010%;   Mg: 0.0005% to 0.010%;   REM: 0.0005% to 0.010%; and   Zr: 0.0005% to 0.010%.   
     
     
         3 . The hot-rolled steel sheet according to  claim 1 ,
 wherein in the microstructure, an average grain size of prior austenite grains is 10 to 30 and   a ratio I d /S d  between a long axis I d  and a short axis S d  of the prior austenite grains is 2.0 or less.   
     
     
         4 . A method of manufacturing the hot-rolled steel sheet according to  claim 1 , comprising:
 a heating step of retaining a slab having the chemical composition according to  claim 1 , at a heating temperature of 1200° C. or higher for 1.0 hour or longer;   a hot rolling step of performing rough rolling so that a rough rolling completion temperature is 1000° C. or higher and a total rolling reduction is more than 65%, and performing finish rolling so that a finish rolling completion temperature is 860° C. to 980° C.; and   a cooling step of performing cooling to a temperature range of 570° C. to 620° C. at an average cooling rate of 20° C./s or higher and performing winding, then, performing retaining at a temperature range of 500° C. to 580° C. for 2.0 to 12.0 hours, and then performing cooling to a room temperature.   
     
     
         5 . The method of manufacturing the hot-rolled steel sheet according to  claim 4 ,
 wherein in the hot rolling step, the total rolling reduction in the rough rolling is set to 70% or more, and the finish rolling is performed so that all rolling reductions of final three stages of the finish rolling are less than 25%.   
     
     
         6 . The hot-rolled steel sheet according to  claim 2 ,
 wherein in the microstructure, an average grain size of prior austenite grains is 10 to 30 μm, and   a ratio I d /S d  between a long axis I d  and a short axis S d  of the prior austenite grains is 2.0 or less.   
     
     
         7 . A hot-rolled steel sheet comprising, as a chemical composition, by mass %:
 C: 0.030% to 0.200%;   Si: 0.05% to 2.50%;   Mn: 1.00% to 4.00%;   sol. Al: 0.001% to 2.000%;   Ti: 0.030% to 0.200%,   P: 0.020% or less;   S: 0.020% or less;   N: 0.010% or less;   Nb: 0% to 0.200%;   B: 0% to 0.010%;   V: 0% to 1.00%;   Mo: 0% to 1.00%;   Cu: 0% to 1.00%;   W: 0% to 1.00%;   Cr: 0% to 1.00%;   Ni: 0% to 1.00%;   Co: 0% to 1.00%;   Ca: 0% to 0.010%;   Mg: 0% to 0.010%;   REM: 0% to 0.010%;   Zr: 0% to 0.010%; and   a remainder comprising iron and impurities,   wherein a microstructure contains, by area %,
 bainite: 80.0% or more, 
 ferrite: 10.0% or less, and 
 a remainder in the microstructure: 10.0% or less, 
   a total density of a length L 7  of a grain boundary having a crystal orientation difference of 7° and a length L 68  of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 μm/μm 2 , and   a tensile strength is 780 MPa or more.