High strength steel sheet and method for manufacturing same

Abstract

The high strength steel sheet consists essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, and balance of Fe and inevitable impurities; and contains 60% or more polygonal ferrite by volume, and 5 to 30% martensite by volume. The steel sheet is manufactured by the steps of: casting a slab having the specified composition; hot-rolling the slab at Ar3 point or more temperature; beginning cooling the hot-rolled steel sheet within 2 seconds after completing the hot-rolling to a temperature of from 750° C. to 600° C. at a cooling rate of 150° C./s or more; holding the cooled steel sheet at a temperature between 750° C. and 600° C. for 2 to 15 seconds; cooling the steel sheet at a cooling rate of 20° C./s or more; and coiling the cooled steel sheet at a temperature of 400° C. or less.

1 . A high strength steel sheet consisting essentially of: 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, by mass, and the balance being Fe and inevitable impurities; and 
 60% or more polygonal ferrite by volume, and 5 to 30% martensite by volume.    
     
     
         2 . The high strength steel sheet according to  claim 1 , wherein the polygonal ferrite is 60 to 95% by volume.  
     
     
         3 . The high strength steel sheet according to  claim 1 , wherein the martensite is 10 to 20% by volume.  
     
     
         4 . The high strength steel sheet according to  claim 1 , wherein the polygonal ferrite has a mean grain size of 5 to 10 μm.  
     
     
         5 . The high strength steel sheet according to  claim 1 , further containing at least one element selected from the group consisting of 0.01 to 0.3% Mo, 0.001 to 0.05% Nb, 0.001 to 0.1% Ti, 0.0003 to 0.002% B, and 0.05 to 0.49% Cr, by mass.  
     
     
         6 . The high strength steel sheet according to  claim 1 , wherein the Si content is 0.01 to 0.5% by mass.  
     
     
         7 . The high strength steel sheet according to  claim 1 , wherein the Si content is 0.01 to 0.25% by mass.  
     
     
         8 . The high strength steel sheet according to  claim 1 , wherein the P content is 0.020 to 0.06% by mass.  
     
     
         9 . The high strength steel sheet according to  claim 1 , having a yield ratio of 0.6 or less.  
     
     
         10 . A method for manufacturing high strength steel sheet comprising the steps of: 
 casting a slab consisting essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, by mass, and the balance being Fe and inevitable impurities;    hot-rolling the cast slab, directly or heating thereof, at a temperature of Ar3 point or more to form a hot-rolled steel sheet;    a primary cooling step of cooling the hot-rolled steel sheet at a cooling rate of 150° C./s or more to a temperature of from 750° C. to 600° C., beginning cooling thereof within 2 seconds after completing the hot-rolling;    holding the cooled steel sheet at a temperature between 750° C. and 600° C. for 2 to 15 seconds;    a secondary cooling step of cooling the temperature-held steel sheet at a cooling rate of 20° C./s or more; and    coiling the cooled steel sheet at a coiling temperature of 400° C. or less.    
     
     
         11 . The method according to  claim 10 , wherein the slab further contains at least one element selected from the group consisting of 0.01 to 0.3% Mo, 0.001 to 0.05% Nb, 0.001 to 0.1% Ti, 0.0003 to 0.002% B, and 0.05 to 0.49% Cr, by mass.  
     
     
         12 . The method according to  claim 10 , wherein the cooling rate in the primary cooling step is from 150 to 1000° C./s.  
     
     
         13 . The method according to  claim 12 , wherein the cooling rate in the primary cooling step is from 200 to 700° C./s.  
     
     
         14 . The method according to  claim 10 , wherein the cooling rate in the secondary cooling step is from 20 to 1000° C./s.  
     
     
         15 . The method according to  claim 10 , wherein the coiling temperature is from 0° C. to 400° C.