Hot rolled high tensile strength steel sheet and method for manufacturing same

Abstract

The steel sheet has a chemical composition containing, by mass %, C: 0.04-0.08%, Si: 0.50% or less, Mn: 0.8-2.2%, P: 0.02% or less, S: 0.006% or less, Al: 0.1% or less, N: 0.008% or less, and Cr: 0.05-0.8%, and further Nb: 0.01-0.08%, V: 0.001-0.12%, and Ti: 0.005-0.04% in adjusted amounts, with the balance including Fe and incidental impurities. The steel sheet has a surface layer having a microstructure containing bainite as a main phase, martensite as a second phase in a volume fraction of 0.5-4%, and at lease one of ferrite phase, pearlite, and cementite as a third phase in a total volume fraction of 10% or less.

1 .- 9 . (canceled) 
     
     
         10 . A hot rolled high tensile strength steel sheet comprising a chemical composition containing, by mass %,
 C: 0.04% to 0.08%,   Si: 0.50% or less,   Mn: 0.8% to 2.2%,   P: 0.02% or less,   S: 0.006% or less,   Al: 0.1% or less,   N: 0.008% or less,   Cr: 0.05% to 0.8%,   Nb: 0.01% to 0.08%,   V: 0.001% to 0.12%, and   Ti: 0.005% to 0.04%,   
       the contents of Nb, V, and Ti being adjusted to satisfy Formula (1), the chemical composition further containing the balance including Fe and incidental impurities, 
       wherein the steel sheet has a surface layer having a microstructure containing bainite as a main phase, martensite as a second phase in a volume fraction of 0.5% to 4%, and at least one of ferrite phase, pearlite, cementite as a third phase in a total volume fraction of 10% or less:
   0.05≦Nb+V+Ti≦0.20  (1)
 
 
       where Nb, V, and Ti each represent the content (mass %) of niobium, vanadium, and titanium in steel, respectively. 
     
     
         11 . The hot rolled high tensile strength steel sheet according to  claim 10 , wherein the steel sheet has a middle portion in the sheet thickness direction, the middle portion having a microstructure containing bainite as a main phase, martensite as a second phase in a volume fraction of 0.5% to 4%, and at least one of ferrite phase, pearlite, and cementite as a third phase in a total volume fraction of 20% or less, the middle portion exhibiting a uniform elongation of 10% or more. 
     
     
         12 . The hot rolled high tensile strength steel sheet according to  claim 10 , wherein the chemical composition further comprises at least one group selected from (A) to (C), wherein,
 (A) by mass %, at least one of Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, and B: 0.001% or less,   (B) by mass %, at least one of Zr: 0.04% or less and Ta: 0.07% or less,   (C) by mass %, at least one of Ca: 0.005% or less and REM: 0.005% or less.   
     
     
         13 . The hot rolled high tensile strength steel sheet according to  claim 11 , wherein the chemical composition further comprises at least one group selected from (A) to (C), wherein,
 (A) by mass %, at least one of Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, and B: 0.001% or less,   (B) by mass %, at least one of Zr: 0.04% or less and Ta: 0.07% or less,   (C) by mass %, at least one of Ca: 0.005% or less and REM: 0.005% or less.   
     
     
         14 . A method of manufacturing a hot rolled high tensile strength steel sheet comprising:
 heating a steel material;   subjecting the steel material to hot rolling to obtain a hot rolled sheet;   subjecting, immediately after the hot rolling, the hot rolled sheet to accelerated cooling; and   coiling the sheet at a coiling temperature,   wherein the steel material comprises a chemical composition containing, by mass %,   C: 0.04% to 0.08%,   Si: 0.50% or less,   Mn: 0.8% to 2.2%,   P: 0.02% or less,   S: 0.006% or less,   Al: 0.1% or less,   N: 0.008% or less,   Cr: 0.05% to 0.8%,   Nb: 0.01% to 0.08%,   V: 0.001% to 0.12%, and   Ti: 0.005% to 0.04%,   
       the contents of Nb, V, and Ti being adjusted to satisfy Formula (1), the chemical composition further containing the balance including Fe and incidental impurities,
 wherein heating of the steel material is performed to heat the steel material to temperatures of 1100° C. to 1250° C., 
 wherein a cumulative rolling reduction ratio in a temperature range of 930° C. or lower is 50% or more and a finisher delivery temperature is 760° C. or higher during finish rolling in the hot rolling, 
 wherein the accelerated cooling is adapted to start cooling, immediately after completion of the finish rolling, at an average cooling rate CR of 7° C./s to 50° C./s and to stop the cooling at a cooling stop temperature of 550° C. or higher to a temperature SCT+30° C., the SCT being defined by Formula (2), 
 wherein the sheet is allowed to cool or gradually cooled during a period of time after the accelerated cooling is stopped and before the coiling is started so that the sheet is retained at temperatures in a temperature range of (SCT−20° C.) to (SCT+30° C.) for 10 seconds to 60 seconds, and 
 wherein the coiling temperature is 430° C. or higher to (SCT−50° C.)
   0.05≦Nb+V+Ti≦0.20  (1)
 
 
 
       where Nb, V, and Ti each represent the content (mass %) of niobium, vanadium, and titanium in steel, respectively,
   SCT(° C.)=750−270C−90Mn+4Si(25−CR)−80Mo−30(Cu+Ni)  (2)
 
 
       where C, Mn, Si, Mo, Cu, and Ni each represent the content (mass %) of carbon, manganese, silicon, molybdenum, copper, and nickel in steel, and CR is an average cooling rate (° C./s) during the accelerated cooling. 
     
     
         15 . The according to  claim 14 , wherein the chemical composition further comprises at least one group selected from (A) to (C), wherein,
 (A) by mass %, at least one of Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, and B: 0.001% or less,   (B) by mass %, at least one of Zr: 0.04% or less and Ta: 0.07% or less,   (C) by mass %, at least one of Ca: 0.005% or less and REM: 0.005% or less.