High strength cold rolled steel sheet having excellent stretch flangeability and method for manufacturing the same

Abstract

A steel is subjected to a hot rolling step, a cold rolling step, and an annealing step having a maximum achieving temperature: 800° C. to 900° C., two-step heating, and two-step cooling. The two-step heating includes first-step heating to (maximum achieving temperature—(10° C. to 50° C.)) at an average temperature raising rate: 0.5° C. to 5.0° C./s and second-step heating in which the temperature raising time from that temperature range to the maximum achieving temperature is specified to be 30 to 150 s. Two-step cooling includes first-step cooling to cool from the maximum achieving temperature at a cooling rate of 10° C. to 40° C./s and second-step cooling to cool to a temperature range of 400° C. to 500° C. at a cooling rate of 0.2 to 0.8 times the first-step cooling rate for a cooling time of 0.2 to 0.8 times the total cooling time.

1 . A high strength cold rolled steel sheet having excellent stretch flangeability, comprising:
 a composition containing   
       C: 0.050% to 0.090%, Si: 0.05% or less, 
       Mn: 1.5% to 2.0%, P: 0.030% or less, 
       S: 0.0050% or less, Al: 0.005% to 0.1%, 
       N: 0.01% or less, Ti: 0.005% to 0.050%, 
       Nb: 0.020% to 0.080%, and the balance being Fe and incidental impurities, on a percent by mass basis, and
 a microstructure including 50% to 77% of ferrite phase, 20% to 50% of bainite phase, 2% to 10% of martensitic phase, and 1% to 5% of retained austenite phase. 
 
     
     
         2 . The high strength cold rolled steel sheet according to  claim 1 , further comprising Ca: 0.0001% to 0.0050% on a percent by mass basis in the composition. 
     
     
         3 . A method for manufacturing a high strength cold rolled steel sheet having excellent stretch flangeability, the method comprising: subjecting a steel to a hot rolling step, a cold rolling step, and an annealing step sequentially, so as to produce a cold rolled steel sheet, wherein
 the steel is specified to be a steel having a composition containing   
       C: 0.050% to 0.090%, Si: 0.05% or less, 
       Mn: 1.5% to 2.0%, P: 0.030% or less, 
       S: 0.0050% or less, Al: 0.005% to 0.1%, 
       N: 0.01% or less, Ti: 0.005% to 0.050%, 
       Nb: 0.020% to 0.080%, and the balance being Fe and incidental impurities, on a percent by mass basis,
 the annealing step is specified to be a step having a maximum achieving temperature: 800° C. to 900° C., two-step heating, and two-step cooling, 
 the two-step heating includes first-step heating to heat from 50° C. to a first-step heating achieving temperature in a temperature range of (maximum achieving temperature—50° C.) to (maximum achieving temperature—10° C.) at an average temperature raising rate: 0.5° C. to 5.0° C./s and second-step heating, in which the temperature raising time from that temperature range to the maximum achieving temperature is specified to be 30 to 150 s, 
 the two-step cooling includes first-step cooling to cool from the maximum achieving temperature at a first-step cooling rate of an average cooling rate: 10° C. to 40° C./s, and the following second-step cooling to cool to a cooling stop temperature in a temperature range of 400° C. to 500° C. at a cooling rate of an average cooling rate: (0.2 to 0.8)×first-step cooling rate for a cooling time of 0.2 to 0.8 times the total cooling time of the first-step cooling and the second-step cooling, and 
 after the second-step cooling is finished, staying is performed at a temperature range of 400° C. to 500° C. for 100 to 1,000 s. 
 
     
     
         4 . The method for manufacturing a high strength cold rolled steel sheet according to  claim 3 , wherein the steel further contains Ca: 0.0001% to 0.0050% on a percent by mass basis in the composition.