High-strength cold-rolled steel sheet and method of producing the same

Abstract

A steel sheet contains C: 0.15% to 0.22%, Si: 1.0% to 2.0%, Mn: 1.7% to 2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01% to 0.05%, N: 0.005% or less, O: 0.01% or less, and the balance being iron and unavoidable impurities while satisfying [Si]/[Mn]≧0.5 ([Si] and [Mn] represent Si and Mn contents, wherein the steel sheet has a structure including, in terms of area fraction, 60% to less than 100% of tempered martensite, 5% or less including 0% of untransformed austenite, and the balance being ferrite, and the ferrite has an average crystal grain size of less than 3.5 μm, wherein less than 10 particles/100 μm 2 of Si—Mn compound oxide particles having a circle equivalent diameter of 5 μm or less are present on a surface of the steel sheet, and the surface is covered with Si-based oxide at a coverage of 1% or less.

1 - 9  (canceled) 
     
     
         10 . A high-strength cold-rolled steel sheet having a composition comprising, in terms of % by mass, C: 0.15% or more and 0.22% or less, Si: 1.0% or more and 2.0% or less, Mn: 1.7% or more and 2.5% or less, P: 0.05% or less, S: 0.02% or less, Al: 0.01% or more and 0.05% or less, N: 0.005% or less, and the balance being iron and unavoidable impurities, the composition satisfying formula (1), wherein the steel sheet has a structure including, in terms of area fraction, 60% or more and less than 100% of tempered martensite, 5% or less of untransformed austenite, and the balance being ferrite, and the ferrite has an average crystal grain size of less than 3.5 μm, wherein less than 10 particles/100 μm 2  of Si—Mn compound oxide particles having a circle equivalent diameter of 5 μm or less are present on a surface of the steel sheet, and the surface of the steel sheet is covered with Si-based oxide at a coverage of 1% or less, wherein the steel sheet has a tensile strength of 1,300 MPa or higher:
   [Si]/[Mn]≧0.5   (1)
 
 
       in the formula, [Si] represents a Si content (% by mass), and [Mn] represents a Mn content (% by mass). 
     
     
         11 . The high-strength cold-rolled steel sheet according to  claim 10 , wherein the composition further comprises at least one element selected from at least one group consisting of, in terms of % by mass,
 group I: Ti: 0.010% or more and 0.020% or less,   group II: Nb: 0.02% or more and 0.10% or less,   group III: B: 0.0002% or more and 0.0020% or less,   group IV: at least one selected from V: 0.01% or more and 0.30% or less, Mo: 0.01% or more and 0.30% or less, and Cr: 0.01% or more and 0.30% or less,   group V: at least one selected from Cu: 0.01% or more and 0.30% or less and Ni: 0.01% or more and 0.30% or less, and   group VI: at least one selected from Sn: 0.001% or more and 0.100% or less, Sb: 0.001% or more and 0.100% or less, Ca: 0.0002% or more and 0.0100% or less, W: 0.01% or more and 0.10% or less, Co: 0.01% or more and 0.10% or less, and REM: 0.0002% or more and 0.0050% or less.   
     
     
         12 . A method of producing a high-strength cold-rolled steel sheet, comprising:
 heating a steel having the composition according to  claim 10  to a temperature of 1,200° C. or higher, performing hot rolling at a finish rolling delivery temperature equal to or higher than 800° C., performing coiling at a temperature of 450° C. or higher and 700° C. or lower, and performing cold rolling;   then performing an annealing treatment comprising performing heating to an annealing temperature of Ac 1  point or higher and Ac 3  point or lower where a holding time in a temperature range from Ac 1  point to Ac 3  point is 30 seconds or longer and 1,200 seconds or shorter, performing primary cooling from the annealing temperature to a primary cooling finishing temperature equal to or higher than 600° C. at an average cooling rate below 100° C./s, and performing secondary cooling to a secondary cooling finishing temperature equal to or lower than 100° C. at an average cooling rate of 100° C./s or higher and 1,000° C./s or lower;   then performing a tempering treatment comprising performing heating to a temperature of 100° C. or higher and 300° C. or lower where a holding time at a temperature of 100° C. to 300° C. is 120 seconds or longer and 1,800 seconds or shorter; and   performing pickling and re-pickling.   
     
     
         13 . A method of producing a high-strength cold-rolled steel sheet, comprising:
 heating a steel having the composition according to  claim 11  to a temperature of 1,200° C. or higher, performing hot rolling at a finish rolling delivery temperature equal to or higher than 800° C., performing coiling at a temperature of 450° C. or higher and 700° C. or lower, and performing cold rolling;   then performing an annealing treatment comprising performing heating to an annealing temperature of Ac 1  point or higher and Ac 3  point or lower where a holding time in a temperature range from Ac 1  point to Ac 3  point is 30 seconds or longer and 1,200 seconds or shorter, performing primary cooling from the annealing temperature to a primary cooling finishing temperature equal to or higher than 600° C. at an average cooling rate below 100° C./s, and performing secondary cooling to a secondary cooling finishing temperature equal to or lower than 100° C. at an average cooling rate of 100° C./s or higher and 1,000° C./s or lower;   then performing a tempering treatment comprising performing heating to a temperature of 100° C. or higher and 300° C. or lower where a holding time at a temperature of 100° C. to 300° C. is 120 seconds or longer and 1,800 seconds or shorter; and   performing pickling and re-pickling.   
     
     
         14 . The method according to  claim 12 , wherein the re-pickling uses, as a pickling solution, a non-oxidizing acid, which is different from a pickling solution used in the pickling. 
     
     
         15 . The method according to  claim 13 , wherein the re-pickling uses, as a pickling solution, a non-oxidizing acid, which is different from a pickling solution used in the pickling.