High-strength steel sheet and method for manufacturing the same

Abstract

A high strength steel sheet has a tensile strength of 900 MPa or higher that can achieve both high strength and good formability and a composition including, on a mass basis, C: 0.1% or more and 0.3% or less; Si: 2.0% or less; Mn: 0.5% or more and 3.0% or less; P: 0.1% or less; S: 0.07% or less; Al: 1.0% or less; and N: 0.008% or less, with the balance Fe and incidental impurities. The steel sheet microstructure includes, on an area ratio basis, 5% or more and 80% or less of ferrite, 15% or more of autotempered martensite, 10% or less of bainite, 5% or less of retained austenite, and 40% or less of as-quenched martensite; the mean hardness of the autotempered martensite is HV≦700; and the mean number of precipitated iron-based carbide grains each having a size of 5 nm or more and 0.5 μm or less and included in the autotempered martensite is 5×10 4 or more per 1 mm 2 .

1 . A high strength steel sheet having a tensile strength of 900 MPa or higher, comprising a composition including, on a mass basis:
 C: 0.1% or more and 0.3% or less;   Si: 2.0% or less;   Mn: 0.5% or more and 3.0% or less;   P: 0.1% or less;   S: 0.07% or less;   Al: 1.0% or less; and   N: 0.008% or less, with the balance Fe and incidental impurities,   with a steel microstructure including, on an area ratio basis, 5% or more and 80% or less of ferrite, 15% or more of autotempered martensite, 10% or less of bainite, 5% or less of retained austenite, and 40% or less of as-quenched martensite; a mean hardness of the autotempered martensite is HV≦700; and the mean number of precipitated iron-based carbide grains each having a size of 5 nm or more and 0.5 μm or less and included in the autotempered martensite is 5×10 4  or more per 1 mm 2 .   
     
     
         2 . The high strength steel sheet according to  claim 1 , further comprising, on a mass basis, at least one element selected from:
 Cr: 0.05% or more and 5.0% or less;   V: 0.005% or more and 1.0% or less; and   Mo: 0.005% or more and 0.5% or less.   
     
     
         3 . The high strength steel sheet according to  claim 1 , further comprising, on a mass basis, at least one element selected from:
 Ti: 0.01% or more and 0.1% or less;   Nb: 0.01% or more and 0.1% or less;   B: 0.0003% or more and 0.0050% or less;   Ni: 0.05% or more and 2.0% or less; and   Cu: 0.05% or more and 2.0% or less.   
     
     
         4 . The high strength steel sheet according to  claim 1 , further comprising, on a mass basis, at least one element selected from:
 Ca: 0.001% or more and 0.005% or less; and   REM: 0.001% or more and 0.005% or less.   
     
     
         5 . The high strength steel sheet according to  claim 1 , wherein the area ratio of autotempered martensite in which the number of precipitated iron-based carbide grains each having a size of 0.1 μm or more and 0.5 μm or less is 5×10 2  or less per 1 mm 2  to the entire autotempered martensite is 3% or more. 
     
     
         6 . The high strength steel sheet according to  claim 1 , wherein a galvanized layer is disposed on a surface of the steel sheet. 
     
     
         7 . The high strength steel sheet according to  claim 1 , wherein a galvannealed layer is disposed on a surface of the steel sheet. 
     
     
         8 . A method for manufacturing a high strength steel sheet, comprising:
 hot-rolling and then cold-rolling a slab to be formed into a steel sheet having the composition according to  claim 1  to form a cold-rolled steel sheet;   annealing the cold-rolled steel sheet in a first temperature range of 700° C. or higher and 950° C. or lower for 15 seconds or longer and 600 seconds or shorter;   in a second temperature range, which is a temperature range from the first temperature range to 420° C., cooling the steel sheet from the first temperature range to 550° C. at an average cooling rate of 3° C./s or higher and cooling the steel sheet from 550° C. to 420° C. within 600 seconds; and   cooling the steel sheet at a cooling rate of 50° C./s or lower in a third temperature range of 250° C. or higher and 420° C. or lower to perform, in the third temperature range, autotempering treatment in which martensite transformation is caused while at the same time the transformed martensite is tempered.   
     
     
         9 . The method according to  claim 8 , wherein, when the steel sheet is cooled at a cooling rate of 50° C./s or lower in the third temperature range of 250° C. or higher and 420° C. or lower, the steel sheet is cooled at a cooling rate of 1.0° C./s or higher and 50° C./s or lower in a temperature range of at least (Ms temperature−50)° C. or lower to perform, in the third temperature range, autotempering treatment in which martensite transformation is caused while at the same time the transformed martensite is tempered. 
     
     
         10 . The method according to  claim 8 , wherein martensite start temperature Ms of the slab is approximated by M represented by Formula (1) below, and the M is 300° C. or higher:
   M(° C.)=540−361×{[C %]/(1−[α%]/100)}−6×[Si %]−40×[Mn %]+30×[Al %]−20×[Cr %]−35×[V %]−10×[Mo %]−17×[Ni %]−10×[Cu %]  (1)
 
 
       where [X %] is mass % of a constituent element X of the slab and [α%] is an area ratio (%) of polygonal ferrite. 
     
     
         11 . The method according to  claim 9 , wherein martensite start temperature Ms of the slab is approximated by M represented by Formula (1) below, and the M is 300° C. or higher:
   M(° C.)=540−361×{[C %]/(1−[α%]/100)}−6×[Si %]−40×[Mn %]+30×[Al %]−20×[Cr %]−35×[V %]−10×[Mo %]−17×[Ni %]−10×[Cu %]  (1)
 
 
       where [X %] is mass % of a constituent element X of the slab and [α%] is an area ratio (%) of polygonal ferrite. 
     
     
         12 . The high strength steel sheet according to  claim 2 , further comprising, on a mass basis, at least one element selected from:
 Ti: 0.01% or more and 0.1% or less;   Nb: 0.01% or more and 0.1% or less;   B: 0.0003% or more and 0.0050% or less;   Ni: 0.05% or more and 2.0% or less; and   Cu: 0.05% or more and 2.0% or less.   
     
     
         13 . The high strength steel sheet according to  claim 2 , further comprising, on a mass basis, at least one element selected from:
 Ca: 0.001% or more and 0.005% or less; and   REM: 0.001% or more and 0.005% or less.   
     
     
         14 . The high strength steel sheet according to  claim 3 , further comprising, on a mass basis, at least one element selected from:
 Ca: 0.001% or more and 0.005% or less; and   REM: 0.001% or more and 0.005% or less.   
     
     
         15 . The high strength steel sheet according to  claim 2 , wherein the area ratio of autotempered martensite in which the number of precipitated iron-based carbide grains each having a size of 0.1 μm or more and 0.5 μm or less is 5×10 2  or less per 1 mm 2  to the entire autotempered martensite is 3% or more. 
     
     
         16 . The high strength steel sheet according to  claim 3 , wherein the area ratio of autotempered martensite in which the number of precipitated iron-based carbide grains each having a size of 0.1 μm or more and 0.5 μm or less is 5×10 2  or less per 1 mm 2  to the entire autotempered martensite is 3% or more. 
     
     
         17 . The high strength steel sheet according to  claim 4 , wherein the area ratio of autotempered martensite in which the number of precipitated iron-based carbide grains each having a size of 0.1 μm or more and 0.5 μm or less is 5×10 2  or less per 1 mm 2  to the entire autotempered martensite is 3% or more. 
     
     
         18 . The high strength steel sheet according to  claim 2 , wherein a galvanized layer is disposed on a surface of the steel sheet. 
     
     
         19 . The high strength steel sheet according to  claim 3 , wherein a galvanized layer is disposed on a surface of the steel sheet. 
     
     
         20 . The high strength steel sheet according to  claim 4 , wherein a galvanized layer is disposed on a surface of the steel sheet.