High-strength hot-rolled steel sheet and method for manufacturing the same

Abstract

There are provided a high-strength hot-rolled steel sheet having high burring formability and a method for manufacturing the high-strength hot-rolled steel sheet. A high-strength hot-rolled steel sheet having high burring formability contains, on a mass percent basis, C: 0.03% or more and 0.1% or less, Si: less than 0.5%, Mn: more than 0.7% and less than 1.2%, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.1% or more and 0.25% or less such that C, S, N, and Ti satisfy Ti*≧0.1 and C×(48/12)−0.14<Ti*<C×(48/12)+0.08 (wherein Ti*=Ti−N×(48/14)−S×(48/32), and C, S, N, and Ti denote the amounts (% by mass) of the corresponding elements), the remainder being Fe and incidental impurities, wherein the high-strength hot-rolled steel sheet has a microstructure in which a ferrite phase fraction is more than 90%, a carbide containing Ti is precipitated, and 70% or more of the carbide has a grain size of less than 9 nm.

1 . A high-strength hot-rolled steel sheet comprising, on a mass percent basis:
 C: 0.03% or more and 0.1% or less, Si: less than 0.5%,   Mn: more than 0.7% and less than 1.2%, P: 0.05% or less,   S: 0.005% or less, N: 0.01% or less,   Al: 0.1% or less, and Ti: 0.1% or more and 0.25% or less   such that C, S, N, and Ti satisfy the following formulae (1) and (2), the remainder being Fe and incidental impurities, wherein the high-strength hot-rolled steel sheet has a microstructure in which a ferrite phase fraction is more than 90%, a carbide containing Ti is precipitated, and 70% or more of the carbide has a grain size of less than 9 nm.
   Ti*≧0.1  (1)
 
   C×(48/12)−0.14<Ti*<C×(48/12)+0.08  (2)
 
   wherein Ti*=Ti−N×(48/14)−S×(48/32), and C, S, N, and Ti denote the amounts (% by mass) of the corresponding elements.   
     
     
         2 . The high-strength hot-rolled steel sheet according to  claim 1 , wherein 50% by mass or more of Ti is precipitated as precipitates containing Ti having a grain size of less than 20 nm. 
     
     
         3 . The high-strength hot-rolled steel sheet according to  claim 1 , further comprising at least one of V: 0.002% or more and 0.1% or less and Nb: 0.002% or more and 0.1% or less on a mass percent basis. 
     
     
         4 . The high-strength hot-rolled steel sheet according to  claim 1 , further comprising, on a mass percent basis: at least one of Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.002% or more and 0.2% or less, and Mo: 0.002% or more and 0.2% or less. 
     
     
         5 . The high-strength hot-rolled steel sheet according  claim 1 , further comprising B: 0.0002% or more and 0.003% or less on a mass percent basis. 
     
     
         6 . The high-strength hot-rolled steel sheet according to  claim 1 , further comprising at least one of Ca: 0.0002% or more and 0.005% or less and REM: 0.0002% or more and 0.03% or less on a mass percent basis. 
     
     
         7 . A method for manufacturing a high-strength hot-rolled steel sheet, comprising: heating steel having a composition according to  claim 1  to 1100° C. or more, hot-rolling the steel at a finish-rolling temperature of (Ar 3 +20° C.) or more and at a total reduction ratio of 60% or less at last two finish rolling stands, cooling the hot-rolled steel sheet at an average cooling rate of 40° C./s or more, and coiling the hot-rolled steel sheet at a coiling temperature in the range of 560° C. to 720° C. 
     
     
         8 . A method for manufacturing a high-strength hot-rolled steel sheet, comprising: heating steel having a composition according to  claim 1  to 1100° C. or more, hot-rolling the steel at a finish-rolling temperature of (Ar 3 +20° C.) or more and at a total reduction ratio of 60% or less at last two finish rolling stands, cooling the hot-rolled steel sheet at an average cooling rate of 40° C./s or more, coiling the hot-rolled steel sheet at a coiling temperature in the range of 500° C. to 660° C., annealing the hot-rolled steel sheet at a soaking temperature of 750° C. or less after pickling, and plating the hot-rolled steel sheet by immersing the hot-rolled steel sheet in a molten zinc bath. 
     
     
         9 . The method for manufacturing a high-strength hot-rolled steel sheet according to  claim 8 , wherein the plating treatment is followed by alloying treatment.