High-strength steel sheet, hot-dipped steel sheet, and alloy hot-dipped steel sheet that have excellent fatigue, elongation, and collision characteristics, and manufacturing method for said steel sheets

Abstract

This high-strength steel sheet includes: in terms of percent by mass, 0.03 to 0.10% of C; 0.01 to 1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of S; 0.01 to 1.2% of Al; 0.06 to 0.15% of Ti; and 0.01% or less of N; and contains as the balance, iron and inevitable impurities, wherein a tensile strength is in a range of 590 MPa or more, and a ratio between the tensile strength and a yield strength is in a range of 0.80 or more, a microstructure includes bainite at an area ratio of 40% or more and the balance being either one or both of ferrite and martensite, a density of Ti(C,N) precipitates having sizes of 10 nm or smaller is in a range of 10 10 precipitates/mm 3 or more, and a ratio (Hvs/Hvc) of a hardness (Hvs) at a depth of 10 μm from a surface to a hardness (Hvc) at a center of a sheet thickness is in a range of 0.85 or more.

1 - 7 . (canceled) 
     
     
         8 . A method for producing a high-strength steel sheet having excellent fatigue properties, elongation and collision properties comprising:
 heating a slab comprising: in terms of percent by mass %,
 0.03 to 0.10% of C; 
 0.01 to 1.5% of Si; 
 1.0 to 2.5% of Mn; 
 0.1% or less of P; 
 0.02% or less of S; 
 0.01 to 1.2% of Al; 
 0.06 to 0.15% of Ti; and 
 0.01% or less of N; and 
   containing as the balance, iron and inevitable impurities, at a temperature in a range of 1,150 to 1,280° C. and performing hot rolling under conditions where a finish rolling is finished at a temperature in a range of not less than an Ar 3  point, thereby obtaining a hot-rolled material;   coiling the hot-rolled material in a temperature range of 600° C. or less, thereby obtaining a hot-rolled steel sheet;   subjecting the hot-rolled steel sheet to acid pickling;   subjecting the pickled hot-rolled steel sheet to first skin pass rolling at an elongation rate in a range of 0.1 to 5.0%;   annealing the hot-rolled steel sheet under conditions where a maximum heating temperature (Tmax° C.) is in a range of 600 to 750° C. and a holding time (t seconds) in a temperature range of 600° C. or higher fulfills expressions (1) and (2) as follows; and   subjecting the annealed hot-rolled steel sheet to second skin pass rolling,
   530−0.7 ×T max≦ t≦ 3,600−3.9 ×T max  (1)
 
     t> 0  (2).
 
   
     
     
         9 . The method for producing the high-strength steel sheet having excellent fatigue properties, elongation and collision properties according to  claim 8 ,
 wherein an elongation rate is set to be in a range of 0.2 to 2.0% in the second skin pass rolling.   
     
     
         10 . The method for producing the high-strength steel sheet having excellent fatigue properties, elongation and collision properties according to  claim 8 ,
 wherein ½ or more of the amount of Ti contained in the hot-rolled steel sheet after the coiling exists in a solid-solution state.   
     
     
         11 . A method for producing a hot-dipped steel sheet having excellent fatigue properties, elongation and collision properties comprising:
 heating a slab comprising: in terms of percent by mass %,
 0.03 to 0.10% of C; 
 0.01 to 1.5% of Si; 
 1.0 to 2.5% of Mn; 
 0.1% or less of P; 
 0.02% or less of S; 
 0.01 to 1.2% of Al; 
 0.06 to 0.15% of Ti; and 
 0.01% or less of N; and 
   containing as the balance, iron and inevitable impurities, at a temperature in a range of 1,150 to 1,280° C. and performing hot rolling under conditions where a finish rolling is finished at a temperature in a range of not less than an Ar 3  point, thereby obtaining a hot-rolled material;   coiling the hot-rolled material in a temperature range of 600° C. or less, thereby obtaining a hot-rolled steel sheet;   subjecting the hot-rolled steel sheet to acid pickling;   subjecting the pickled hot-rolled steel sheet to first skin pass rolling at an elongation rate in a range of 0.1 to 5.0%;   annealing the hot-rolled steel sheet under conditions where a maximum heating temperature (Tmax° C.) is in a range of 600 to 750° C. and a holding time (t seconds) in a temperature range of 600° C. or higher fulfills expressions (1) and (2) as follows, and performing hot dipping to form a hot-dipped layer on a surface of the hot-rolled steel sheet, thereby obtaining a hot-dipped steel sheet; and   subjecting the hot-dipped steel sheet to second skin pass rolling,
   530−0.7 ×T max≦ t≦ 3,600−3.9 ×T max  (1)
 
     t> 0  (2).
 
   
     
     
         12 . The method for producing the hot-dipped steel sheet having excellent fatigue properties, elongation and collision properties according to  claim 11 ,
 wherein an elongation rate is set to be in a range of 0.2 to 2.0% in the second skin pass rolling.   
     
     
         13 . A method for producing an alloyed hot-dipped steel sheet having excellent fatigue properties, elongation and collision properties comprising:
 heating a slab comprising: in terms of percent by mass %,
 0.03 to 0.10% of C; 
 0.01 to 1.5% of Si; 
 1.0 to 2.5% of Mn; 
 0.1% or less of P; 
 0.02% or less of S; 
 0.01 to 1.2% of Al; 
 0.06 to 0.15% of Ti; and 
 0.01% or less of N; and 
   containing as the balance, iron and inevitable impurities, at a temperature in a range of 1,150 to 1,280° C. and performing hot rolling under conditions where a finish rolling is finished at a temperature in a range of not less than an Ar 3  point, thereby obtaining a hot-rolled material;   coiling the hot-rolled material in a temperature range of 600° C. or less, thereby obtaining a hot-rolled steel sheet;   subjecting the hot-rolled steel sheet to acid pickling;   subjecting the pickled hot-rolled steel sheet to first skin pass rolling at an elongation rate in a range of 0.1 to 5.0%;   annealing the hot-rolled steel sheet under conditions where a maximum heating temperature (Tmax° C.) is in a range of 600 to 750° C. and a holding time (t seconds) in a temperature range of 600° C. or higher fulfills expressions (1) and (2) as follows, performing hot dipping to form a hot-dipped layer on a surface of the hot-rolled steel sheet so as to obtain a hot-dipped steel sheet, and subjecting the hot-dipped steel sheet to an alloying treatment to convert the hot-dipped layer into an alloyed hot-dipped layer; and   subjecting the hot-dipped steel sheet on which the alloying treatment is performed to second skin pass rolling,
   530−0.7 ×T max≦ t≦ 3,600−3.9 ×T max  (1)
 
     t> 0  (2).
 
   
     
     
         14 . The method for producing the alloyed hot-dipped steel sheet having excellent fatigue properties, elongation and collision properties according to  claim 13 ,
 wherein an elongation rate is set to be in a range of 0.2 to 2.0% in the second skin pass rolling.