High-strength cold rolled sheet having excellent formability and crashworthiness and method for manufacturing the same

Abstract

A high-strength cold rolled steel sheet has excellent formability and crashworthiness and includes, on a mass % basis, C: 0.05 to 0.3%, Si: 0.3 to 2.5%. Mn: 0.5 to 3.5%, P: 0.003 to 0.100%, 5: 0.02% or less, Al: 0.010 to 0.5%, the balance being iron and unavoidable impurities, the high-strength cold rolled steel sheet having a microstructure including 20% or more of ferrite on an area fraction basis, 10 to 60% of tempered martensite on an area fraction basis, 0 to 10% of martensite on an area fraction basis, and 3 to 15% of retained austenite on a volume fraction basis.

1 . A high-strength cold rolled steel sheet having excellent formability and crashworthiness comprising, on a mass % basis, C: 0.05 to 0.3%, Si: 0.3 to 2.5%. Mn: 0.5 to 3.5%, P: 0.003 to 0.100%, S: 0.02% or less. Al: 0.010 to 0.5%, and balance being iron and unavoidable impurities, the high-strength cold rolled steel sheet having a microstructure including 20% or more of ferrite on an area fraction basis, 10 to 60% of tempered martensite on an area fraction basis, 0 to 10% of martensite on an area fraction, basis, and 3 to 15% of retained austenite on a volume fraction basis. 
     
     
         2 . The high-strength cold rolled steel sheet according to  claim 1 , wherein a low-temperature transformation-forming phase constituted of the martensite, the tempered martensite, and the retained austenite has an average crystal grain diameter of 3 μm or less. 
     
     
         3 . The high-strength cold rolled steel sheet according to  claim 1 , further comprising, on a mass % basis, at least one element selected from the group consisting of Cr: 0.005 to 2.00%, Mo: 0.005 to 2.00%. V: 0.005 to 2.00%, Ni: 0.005 to 2.00%, and Cu: 0.005 to 2.00%. 
     
     
         4 . The high-strength cold roiled steel sheet according to  claim 1 , further comprising, on a mass % basis, one or both of Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20%. 
     
     
         5 . The high-strength cold roiled steel sheet according to  claim 1 , further comprising, on a mass % basis, B: 0.0002 to 0.005%. 
     
     
         6 . The high-strength cold rolled steel sheet according to  claim 1 , further comprising, on a mass % basis, one or both of Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%. 
     
     
         7 . A method of manufacturing a high-strength cold rolled steel sheet having excellent formability and crashworthiness, comprising:
 hot-roiling and cold-rolling a slab having a composition described in  claim 1  to manufacture a cold rolled steel sheet; and   continuously annealing the cold rolled sheet, wherein, during the continuous annealing, the steel sheet is held at a temperature of 750° C. or more for 10 seconds or more, cooled from 750° C. to a temperature range of 150 to 350° C. at a cooling rate of 10° C./s or more on average, heated to a temperature of 350 to 600° C., held thereat for 10 to 600 seconds, and cooled to room temperature.   
     
     
         8 . The method according to  claim 7 , wherein the average heating rate at 500° C. to Ac 1  transformation point is 10° C./s or more. 
     
     
         9 . The high-strength cold rolled steel sheet according to  claim 2 , further comprising, on a mass % basis, at least one element selected from the group consisting of Cr: 0.005 to 2.00%. Mo: 0.005 to 2.00%. V: 0.005 to 2.00%, Ni: 0.005 to 2.00%, and Cu: 0.005 to 2.00%. 
     
     
         10 . The high-strength cold roiled steel sheet according to  claim 2 , further comprising, on a mass % basis, one or both of Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20%. 
     
     
         11 . The high-strength cold rolled steel sheet according to  claim 3 , further comprising, on a mass % basis, one or both of Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20%. 
     
     
         12 . The high-strength cold rolled steel sheet according to  claim 2 , further comprising, on a mass % basis, B: 0.0002 to 0.005%. 
     
     
         13 . The high-strength cold rolled steel sheet according to  claim 3 , further comprising, on a mass % basis, B: 0.0002 to 0.005%. 
     
     
         14 . The high-strength cold rolled steel sheet according to  claim 4 , further comprising, on a mass % basis, B: 0.0002 to 0.005%. 
     
     
         15 . The high-strength cold rolled steel sheet according to  claim 2 , further comprising, on a mass % basis, one or both of Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%. 
     
     
         16 . The high-strength cold rolled steel sheet according to  claim 3 , further comprising, on a mass % basis, one or both of Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%. 
     
     
         17 . The high-strength cold rolled steel sheet according to  claim 4 , further comprising, on a mass % basis, one or both of Ca; 0.001 to 0.005% and REM: 0.001 to 0.005%. 
     
     
         18 . The high-strength cold rolled steel sheet according to  claim 5 , further comprising, on a mass % basis, one or both of Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%.