High-strength hot-rolled steel sheet and method for producing same

Abstract

The present invention provides a high-strength hot-rolled steel sheet having both excellent strength and excellent workability (particularly, bending workability), and a method of producing the same. The steel sheet of the present invention has a certain composition as well as microstructures such that an area ratio of ferrite phase is 95% or more, an average grain size of the ferrite phase is 8 μm or less, and carbides in grains of the ferrite phase have an average particle size of less than 10 nm. The steel sheet of the present invention also has a tensile strength of 980 MPa or more.

1 - 14 . (canceled) 
     
     
         15 . A high-strength hot-rolled steel sheet comprising a chemical composition containing, in mass %,
 C: 0.06% or more and 0.1% or less,   Si: 0.09% or less,   Mn: 0.7% or more and 1.3% or less,   P: 0.03% or less,   S: 0.01% or less,   Al: 0.1% or less,   N: 0.01% or less,   Ti: 0.14% or more and 0.20% or less,   V: 0.07% or more and 0.14% or less, and   the balance being Fe and incidental impurities,   wherein the steel sheet has microstructures such that an area ratio of ferrite phase is 95% or more, an average grain size of the ferrite phase is 8 μm or less, and carbides in grains of the ferrite phase have an average particle size of less than 10 nm, and   wherein the steel sheet has a tensile strength of 980 MPa or more.   
     
     
         16 . The high-strength hot-rolled steel sheet according to  claim 15 , wherein the chemical composition further contains at least one group selected from (A) to (C), wherein
 (A) in mass %, Nb: 0.01% or more and 0.05% or less   (B) in mass %, at least one element selected from   Mo: 0.05% or less,   W: 0.05% or less,   Zr: 0.05% or less, and   Hf: 0.05% or less,   (C) in mass %, at least one of O (oxygen), Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be, Sr, REM, B, Ni, Cr, Sb, Cu, Sn, Mg, and Ca, in a total amount of 0.2% or less.   
     
     
         17 . The high-strength hot-rolled steel sheet according to  claim 15 , further comprising a plating layer on a surface of the steel sheet. 
     
     
         18 . The high-strength hot-rolled steel sheet according to  claim 16 , further comprising a plating layer on a surface of the steel sheet. 
     
     
         19 . The high-strength hot-rolled steel sheet according to  claim 17 , wherein the plating layer is a galvanized layer. 
     
     
         20 . The high-strength hot-rolled steel sheet according to  claim 17 , wherein the plating layer is a galvannealed layer. 
     
     
         21 . The high-strength hot-rolled steel sheet according to  claim 18 , wherein the plating layer is a galvanized layer. 
     
     
         22 . The high-strength hot-rolled steel sheet according to  claim 18 , wherein the plating layer is a galvannealed layer. 
     
     
         23 . A method of producing a high-strength hot-rolled steel sheet, comprising:
 heating a steel material;   subjecting the steel material to hot rolling including rough rolling and finish rolling; and   after completion of the finish rolling, cooling and coiling thus rolled steel material to gain a hot-rolled steel sheet,   wherein the steel material has a chemical composition containing, in mass %,
 C: 0.06% or more and 0.1% or less, 
 Si: 0.09% or less, 
 Mn: 0.7% or more and 1.3% or less, 
 P: 0.03% or less, 
 S: 0.01% or less, 
 Al: 0.1% or less, 
 N: 0.01% or less, 
 Ti: 0.14% or more and 0.20% or less, 
 V: 0.07% or more and 0.14% or less, and 
 the balance being Fe and incidental impurities, and 
   wherein the steel material is heated at a temperature of 1100° C. or higher and 1350° C. or lower, the finish rolling is operated at a finish rolling temperature of 850° C. or higher, the cooling is initiated within 3 seconds after completion of the finish rolling, the cooling is operated at an average cooling rate of 20° C./s or higher, and the coiling is operated at a coiling temperature of 550° C. or higher and 700° C. or lower.   
     
     
         24 . The method of producing a high-strength hot-rolled steel sheet according to  claim 23 , wherein the chemical composition further contains at least one group selected from (A) to (C), wherein
 (A) in mass %, Nb: 0.01% or more and 0.05% or less   (B) in mass %, at least one element selected from   Mo: 0.05% or less,   W: 0.05% or less,   Zr: 0.05% or less, and   Hf: 0.05% or less,   (C) in mass %, at least one of 0 (oxygen), Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be, Sr, REM, B, Ni, Cr, Sb, Cu, Sn, Mg, and Ca, in a total amount of 0.2% or less.   
     
     
         25 . The method of producing a high-strength hot-rolled steel sheet according to  claim 23 , comprising forming a plating layer on a surface of the hot-rolled steel sheet. 
     
     
         26 . The method of producing a high-strength hot-rolled steel sheet according to  claim 24 , comprising forming a plating layer on a surface of the hot-rolled steel sheet. 
     
     
         27 . The method of producing a high-strength hot-rolled steel sheet according to  claim 25 , wherein the plating layer is a galvanized layer. 
     
     
         28 . The method of producing a high-strength hot-rolled steel sheet according to  claim 25 , wherein the plating layer is a galvannealed layer. 
     
     
         29 . The method of producing a high-strength hot-rolled steel sheet according to  claim 26 , wherein the plating layer is a galvanized layer. 
     
     
         30 . The method of producing a high-strength hot-rolled steel sheet according to  claim 26 , wherein the plating layer is a galvannealed layer.