High-strength thin steel sheet and manufacturing method therefor

Abstract

A high-strength thin steel sheet having a high yield ratio and excellent formability and a manufacturing method therefor are provided. The steel sheet contains C: 0.04 to 0.25%, Si: 0.5 to 2.0%, Mn: 0.1 to 2.0%, P: 0.05% or less, S: 0.030% or less, Al: 0.10% or less, and N: 0.010% or less, with the balance being iron and incidental impurities; and has a microstructure containing, in area fraction, 90% or more of ferrite and bainite in total, where ferrite and bainite each have an aspect ratio of 3.0 or higher.

1 . A high-strength thin steel sheet having
 a component composition containing, in mass %,   C: 0.04 to 0.25%,   Si: 0.5 to 2.0%,   Mn: 0.1 to 2.0%,   P: 0.05% or less,   S: 0.030% or less,   Al: 0.10% or less, and   N: 0.010% or less,   with the balance being iron and incidental impurities; and having   a microstructure containing, in area fraction, 90% or more of ferrite and bainite in total, wherein   ferrite and bainite each have an aspect ratio of 3.0 or higher.   
     
     
         2 . The high-strength thin steel sheet according to  claim 1 , wherein the component composition further contains, one or more element selected from Groups A, B, C, D, E and F in mass %,
 Group A: 0.01 to 1.0% of each of one or two or more of Ti, Nb, and V   Group B: 0.005 to 1.0% of each of one or two or more of Mo, Ta, and W   Group C: 0.01 to 1.0% of each of one or two or more of Cr, Ni, and Cu   Group D: 0.0002 to 0.0050% of B   Group E: 0.0005 to 0.01% of each of one or two of Ca and REM   Group F: 0.005 to 0.050% of Sb.   
     
     
         3 . The high-strength thin steel sheet according to  claim 1 , having a coated layer on a steel sheet surface. 
     
     
         4 . The high-strength thin steel sheet according to  claim 2 , having a coated layer on a steel sheet surface. 
     
     
         5 . A method of manufacturing a high-strength thin steel sheet, comprising:
 hot-rolling a steel slab having the component composition according to  claim 1  at a finishing temperature of 800° C. or higher and at a coiling temperature of 400° C. or higher;   pickling; then   cold-rolling at a cold reduction of 30 to 80%; and subsequently   annealing at a soaking temperature of 650° C. to 800° C. for a soaking time of 600 s or less.   
     
     
         6 . A method of manufacturing a high-strength thin steel sheet, comprising:
 hot-rolling a steel slab having the component composition according to  claim 2  at a finishing temperature of 800° C. or higher and at a coiling temperature of 400° C. or higher;   pickling; then   cold-rolling at a cold reduction of 30 to 80%; and subsequently   annealing at a soaking temperature of 650° C. to 800° C. for a soaking time of 600 s or less.   
     
     
         7 . The method of manufacturing a high-strength thin steel sheet according to  claim 5 , further comprising a hot-dip coating process after the annealing. 
     
     
         8 . The method of manufacturing a high-strength thin steel sheet according to  claim 6 , further comprising a hot-dip coating process after the annealing. 
     
     
         9 . The method of manufacturing a high-strength thin steel sheet according to  claim 7 , wherein the hot-dip coating process is a hot-dip galvanizing process, the method further comprising a galvannealing process at a galvannealing temperature of 460° C. to 600° C. for a holding time of 1 s or more after the hot-dip galvanizing process. 
     
     
         10 . The method of manufacturing a high-strength thin steel sheet according to  claim 8 , wherein the hot-dip coating process is a hot-dip galvanizing process, the method further comprising a galvannealing process at a galvannealing temperature of 460° C. to 600° C. for a holding time of 1 s or more after the hot-dip galvanizing process. 
     
     
         11 . The method of manufacturing a high-strength thin steel sheet according to  claim 9 , further comprising an electroplating process after the annealing. 
     
     
         12 . The method of manufacturing a high-strength thin steel sheet according to  claim 16 , further comprising an electroplating process after the annealing. 
     
     
         13 . The method of manufacturing a high-strength thin steel sheet according to  claim 5 , comprising processing at a thickness reduction of 0.1 to 3.0% after any of the coiling, the annealing, the hot-dip coating process, the galvannealing process, and the electroplating process. 
     
     
         14 . The method of manufacturing a high-strength thin steel sheet according to  claim 6 , comprising processing at a thickness reduction of 0.1 to 3.0% after any of the coiling, the annealing, the hot-dip coating process, the galvannealing process, and the electroplating process. 
     
     
         15 . A method of manufacturing a high-strength thin steel sheet comprising a coating process for the high-strength thin steel sheet according to  claim 1 . 
     
     
         16 . A method of manufacturing a high-strength thin steel sheet comprising a coating process for the high-strength thin steel sheet according to  claim 2 .