Thin steel sheet and process for producing the same

Abstract

A thin steel sheet having sheet thickness ≦1.6 mm, but tensile strength ≧780 MPa and Young's modulus ≧240 GPa in transverse direction is provided, where the steel sheet has composition including, in mass %, C: 0.06-0.12%, Si: 0.5-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, Ti: 0.02-0.20%, and the balance being Fe and incidental impurities, where the composition satisfies relations of Formula (1) and (2), and microstructure such that ferrite phase has area ratio ≧60% and martensite phase has area ratio of 15-35%, ferrite and martensite phases are 95% or more in total, average grain size of ferrite is ≦4.0 μm and that of martensite is ≦1.5 μm, 0.05≦[ % C]−( 12/47.9 )×[% Ti*]≦ 0.10   (1), where Ti*=[% Ti]−( 47.9/14 )×[% N]−( 47.9/32.1 )×[% S]  (2).

1 - 6 . (canceled) 
     
     
         7 . A thin steel sheet having a composition including, in mass %, C: 0.06% to 0.12%, Si: 0.5% to 1.5%, Mn: 1.0% to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, Ti: 0.02% to 0.20%, and the balance being Fe and incidental impurities, where the composition satisfies Formulae (1) and (2) below,
 wherein the steel sheet has a microstructure comprising a ferrite phase at an area ratio of 60% or more and a martensite phase at an area ratio of 15% to 35%, where a total of the ferrite phase and the martensite phase is 95% or more, an average grain size of ferrite is 4.0 μm or less and an average grain size of martensite is 1.5 μm or less,   wherein the steel sheet has a tensile strength (TS) of 780 MPa or higher in a transverse direction perpendicular to the rolling direction, a Young's modulus of 240 GPa or higher in the transverse direction, and a strength-elongation balance (TS×El) of 16500 MPa·% or more in the transverse direction, the strength-elongation balance being expressed by a product of the tensile strength (TS) and total elongation (El),
   0.05≦[% C]−(12/47.9)×[% Ti*]≦0.10   (1),
 
   where 
   Ti*=[% Ti]−(47.9/14)×[% N]−(47.9/32.1)×[% S]  (2), and
 
   
       [% M] indicates the content (mass %) of M element. 
     
     
         8 . The thin steel sheet according to  claim 7 , wherein the composition of the steel sheet further comprises, in mass %, Nb: 0.02% to 0.10%, and satisfies Formula (3) below in place of the Formula (1):
   0.05 [% C]−(12/92.9)×[% Nb]−(12/47.9)×[% Ti*] 23  0.10   (3).
   
     
     
         9 . The thin steel sheet according to  claim 7 , wherein the composition of the steel sheet further comprises, in mass %, one or more elements selected from Cr: 0.1% to 1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 2.0% and B: 0.0005% to 0.0030%. 
     
     
         10 . A method of manufacturing a thin steel sheet comprising:
 in a hot rolling process, subjecting a steel material to finish rolling and completing the finish rolling at 850° C. to 950° C. to obtain a hot-rolled steel sheet, the steel material having a composition including, in mass %, C: 0.06% to 0.12%, Si: 0.5% to 1.5%, Mn: 1.0% to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, Ti: 0.02% to 0.20%, and the balance being Fe and incidental impurities, where the contents of C, N, S and Ti satisfy Formulae (1) and (2):
 then coiling the steel sheet at 650° C. or lower; 
 subjecting the steel sheet to pickling; and 
 then subjecting the steel sheet to cold rolling at a rolling reduction ratio of 60% or more; 
   in a subsequent annealing process, heating the steel sheet to a soaking temperature of 780 to 880° C. at an average heating rate from (Ac 1 -100° C.) to Ac 1  of 15° C./s or higher;   holding the steel sheet at the soaking temperature for 150 seconds or less; and   cooling the steel sheet to 350° C. or lower at an average cooling rate until at least 350° C. of 5° C./s to 50° C./s,   
     
     
         11 . The method according to  claim 10 , wherein the composition of the steel material further comprises, in mass %, Nb: 0.02% to 0.10%, and satisfies Formula (3) below in place of the Formula (1):
   0.05 [% C]−(12/92.9)×[% Nb]−(12/47.9)×[% Ti*] 23  0.10   (3).
   
     
     
         12 . The method according to  claim 10 , wherein the composition of the steel material further comprises, in mass %, one or more elements selected from Cr: 0.1% to 1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 2.0% and B: 0.0005% to 0.0030%. 
     
     
         13 . The thin steel sheet according to  claim 8 , wherein the composition of the steel sheet further comprises, in mass %, one or more elements selected from Cr: 0.1% to 1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 2.0% and B: 0.0005% to 0.0030%. 
     
     
         14 . The method according to  claim 11 , wherein the composition of the steel material further comprises, in mass %, one or more elements selected from Cr: 0.1% to 1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 2.0% and B: 0.0005% to 0.0030%.