High strength steel sheet and manufacturing method therefor

Abstract

One aspect of the present invention is a high strength steel sheet having a specific component composition, wherein a metal structure of the steel sheet comprises polygonal ferrite, bainite, tempered martensite, and retained austenite; when the metal structure is observed with a scanning electron microscope, the metal structure satisfies polygonal ferrite: 10 to 50 area %, bainite: 10 to 50 area %, and tempered martensite: 10 to 80 area % with respect to the metal structure overall; and when the metal structure is measured by X-ray diffractometry, the metal structure satisfies retained austenite: 5.0 volume % or more, retained austenite with a carbon concentration of 1.0 mass % or less: 3.5 volume % or more, and retained austenite with a carbon concentration of 0.8 mass % or less: 2.4 volume % or less with respect to the metal structure overall.

1 . A high strength steel sheet satisfying comprising, in mass %:
 C: 0.10% to 0.5%,   Si: 1.0% to 3%,   Mn: 1.5% to 3%,   P: more than 0% and 0.1% or less,   S: more than 0% and 0.05% or less,   Al: 0.005% to 1%,   N: more than 0% and 0.01% or less, and   iron   wherein   (1) a metal structure of the steel sheet comprises polygonal ferrite, bainite, tempered martensite, and retained austenite;   (2) when the metal structure is observed with a scanning electron microscope, the metal structure satisfies:   polygonal ferrite: 10 to 50 area %,   bainite: 10 to 50 area %, and   tempered martensite: 10 to 80 area %,   with respect to the metal structure overall; and   (3) when the metal structure is measured by X-ray diffractometry, the metal structure satisfies:   retained austenite: 5.0 volume % or more,   retained austenite with a carbon concentration of 1.0 mass % or less: 3.5 volume % or more, and   retained austenite with a carbon concentration of 0.8 mass % or less: 2.4 volume % or less,   with respect to the metal structure overall.   
     
     
         2 . The high strength steel sheet according to  claim 1 , further comprising one or more selected from the group consisting of:
 Cr: more than 0% and 1% or less,   Mo: more than 0% and 1% or less,   Ti: more than 0% and 0.15% or less,   Nb: more than 0% and 0.15% or less,   V: more than 0% and 0.15% or less,   Cu: more than 0% and 1% or less,   Ni: more than 0% and 1% or less,   B: more than 0% and 0.005% or less,   Ca: more than 0% and 0.01% or less,   Mg: more than 0% and 0.01% or less, and   a rare earth element: more than 0% and 0.01% or less, in mass %.   
     
     
         3 . The high strength steel sheet according to  claim 1 , having an electrogalvanized layer, a hot-dip galvanized layer, or a hot-dip galvannealed layer on a surface of the steel sheet. 
     
     
         4 . A method for manufacturing the high strength steel sheet according to  claim 1 , the method comprising:
 a steel sheet satisfying the component composition of  claim 1  to a T1 temperature region, which is 800° C. or higher and an Ac 3  point or lower, and holding the steel sheet in the T1 temperature region for 40 seconds or more for soaking;   a first cooling after the soaking, wherein   in cooling the steel sheet down to an arbitrary cooling stop temperature T2 satisfying 350° C. or lower and 100° C. or higher when an Ms point represented by the following formula (I) is 350° C. or higher, or else,   in cooling the steel sheet down to an arbitrary cooling stop temperature T2 satisfying the Ms point or lower and 100° C. or higher when the Ms point represented by the following formula (1) is lower than 350° C.,   the steel sheet is cooled at an average cooling rate of 5° C./sec or more from 700° C. down to a temperature which is the higher one of 300° C. and the cooling stop temperature T2;   reheating the steel sheet to a T3 temperature region exceeding 350° C. and being 540° C. or lower and holding the steel sheet for 50 seconds or more in the T3 temperature region; and   a second cooling after the holding, wherein the steel sheet is cooled at an average cooling rate of 10° C./sec or more from the T3 temperature region down to 300° C., and further the steel sheet is cooled at an average cooling rate exceeding 0° C./sec and being less than 10° C./sec from 300° C. down to 150° C.,   the Ms point satisfying:
   Ms point (° C.)=561−474×[C]/(1−Vf/100)−33×[Mn]−17×[Ni]−17×[Cr]−21×[Mo]  (I)
 
   where, in the formula (I), Vf represents a polygonal ferrite fraction (area %) in a sample that is separately obtained by performing the soaking under the same conditions as in the manufacturing conditions for the high strength steel sheet and thereafter cooling down to room temperature at the same average cooling rate as in the manufacturing conditions for the high strength steel sheet in the first cooling, and   in the formula (I), brackets [ ] represents a content (mass %) of each element, where calculation is made assuming that the content of elements not contained in the steel sheet is 0 mass %.   
     
     
         5 . The method for manufacturing a high strength steel sheet according to  claim 4 , further comprising electrogalvanization is carried out after the second cooling. 
     
     
         6 . The method for manufacturing a high strength steel sheet according to  claim 4 , further comprising hot-dip galvanization or hot-dip galvannealing is carried out in the reheating.