High-strength steel sheet excellent in workability and manufacturing method thereof

Abstract

Provided are: a high-strength steel sheet which is improved in both elongation and local formability and thus exhibits excellent workability; and a manufacturing method thereof. The high-strength steel sheet contains C, Si, Mn, Al, P and S with the remainder including iron and unavoidable impurities, and has a metal structure which includes polygonal ferrite, bainite, tempered martensite, and retained austenite. In the metal structure, (1) the bainite has a composite microstructure including both a high-temperature-formed bainite having an average distance between adjacent regions of retained austenite and/or carbide of 1 μm or more and a low-temperature-formed bainite having an average distance between adjacent regions of retained austenite and/or carbide of less than 1 μm each identified upon observation with a scanning electron microscope; and (2) the retained austenite is present in a volume percentage of 5% or more of the entire metal structure as determined by a saturation magnetization measurement.

1 - 20 . (canceled) 
     
     
         21 . A high-strength steel sheet comprising, by mass percent based on a total mass of the steel sheet, iron and:
 from 0.10% to 0.3% of C;   from 1.0% to 3% of Si;   from 1.0% to 2.5% of Mn;   from 0.005% to 3% of Al;   0.1% or less of P; and   0.05% or less of S,   wherein:   the steel sheet has a metal structure comprising polygonal ferrite, bainite, tempered martensite, and retained austenite;   (1) when the metal structure is observed with a scanning electron microscope,   (1a) the polygonal ferrite is present in an area percentage “a” of greater than 50% of the entire metal structure,   (1b) the bainite has a composite microstructure comprising:
 a high-temperature-formed bainite having an average distance between adjacent regions of retained austenite and/or carbide of 1 μm or more; and 
 a low-temperature-formed bainite having an average distance between adjacent regions of retained austenite and/or carbide of less than 1 μm, 
 the high-temperature-formed bainite is present in an area percentage “b” of from 5% to 40% of the entire metal structure; and 
 the low-temperature-formed bainite and the tempered martensite are present in a total area percentage “c” of from 5% to 40% of the entire metal structure; and 
   (2) the retained austenite is present in a volume percentage of 5% or more of the entire metal structure as determined by a saturation magnetization measurement.   
     
     
         22 . The high-strength steel sheet of  claim 21 , wherein, when martensite-austenite constituents comprising both as-quenched martensite and retained austenite are observed at a cross-section of the metal structure with an optical microscope, martensite-austenite constituents each having an equivalent circle diameter “d” of greater than 7 μm are present in a number percentage of from 0% to less than 15% of a total number of entire martensite-austenite constituents at the observed cross-section. 
     
     
         23 . The high-strength steel sheet of  claim 21 , wherein grains of the polygonal ferrite have an average equivalent circle diameter D of from greater than 0 μm to 10 μm. 
     
     
         24 . The high-strength steel sheet of  claim 21 , further comprising, by mass percent based on a total mass of the steel sheet:
 from greater than 0% to 1% of Cr; and/or   from greater than 0% to 1% of Mo.   
     
     
         25 . The high-strength steel sheet of  claim 21 , further comprising, by mass percent based on a total mass of the steel sheet, at least one element selected from the group consisting of:
 from greater than 0% to 0.15% of Ti;   from greater than 0% to 0.15% of Nb; and   from greater than 0% to 0.15% of V.   
     
     
         26 . The high-strength steel sheet of  claim 21 , further comprising, by mass percent based on a total mass of the steel sheet:
 from greater than 0% to 1% of Cu; and/or   from greater than 0% to 1% of Ni.   
     
     
         27 . The high-strength steel sheet of  claim 21 , further comprising, by mass percent based on a total mass of the steel sheet, from greater than 0% to 0.005% of B. 
     
     
         28 . The high-strength steel sheet of  claim 21 , further comprising, by mass percent based on a total mass of the steel sheet, at least one element selected from the group consisting of:
 from greater than 0% to 0.01% of Ca;   from greater than 0% to 0.01% of Mg; and   from greater than 0% to 0.01% of a rare-earth element.   
     
     
         29 . The high-strength steel sheet of  claim 29 , comprising a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface thereof. 
     
     
         30 . A method of manufacturing the high-strength steel sheet of  claim 21 , the method comprising, in the following order:
 heating a steel sheet to a temperature range of from [Ac 1  point+20° C.] to [Ac 3  point+20° C.];   holding the steel sheet in the temperature range for 50 seconds or longer;   cooling the steel sheet down to an arbitrary temperature T at an average cooling rate of from 2° C. to 50° C. per second, the temperature T falling within a range specified by Expression (1);   holding the steel sheet in the temperature range specified by Expression (1) for 10 to 100 seconds; and   holding the steel sheet in a temperature range specified by Expression (2) for 200 seconds or longer,   wherein Expressions (1) and (2) are as follows:
   400° C.≦ T 1(° C.)≦540° C.  (1)
 
   200° C.≦ T 2(° C.)<400° C.  (2).