High strength steel sheet and method for manufacturing the same

Abstract

The present invention relates to a high strength steel sheet consisting essentially of 0.04 to 0.1% C, 0.5% or less Si, 0.5 to 2% Mn, 0.05% or less P, 0.005% or less O, 0.005% or less S, by weight, having 10 μm or less of average ferritic grain size, and 20 mm/mm 2 or less of generation frequency A, which generation frequency A is defined as the total length of a banded secondary phase structure observed per 1 mm 2 of steel sheet cross section along the rolling direction thereof. The steel sheet is manufactured by, for example, a method comprising the steps of: hot-rolling a continuously cast slab having the composition described above at temperatures of Ar 3 transformation point or above directly or after reheating thereof; and cooling the hot-rolled steel sheet within 2 seconds down to the temperatures of from 600 to 750° C. at cooling speeds of from 100 to 2,000° C./sec, followed by coiling the cooled steel sheet at temperatures of from 450 to 650° C. The present invention provides a high strength steel sheet having strengths of 340 MPa or more and having excellent stretch flanging performance, ductility, and shock resistance, providing a sufficient coil shape with good surface properties, even when hot dip zinc-coating is applied.

1 . A method for manufacturing high strength hot dip zinc-coated steel sheet comprising the steps of: hot-rolling a steel slab consisting essentially of 0.01 to 0.3% C, 0.7% or less Si, 1 to 3% Mn, 0.08% or less P, 0.01% or less S, 0.08% or less sol.Al, and 0.007% or less N, by weight, at temperatures of Ar 3  transformation point or above; cooling the hot-rolled steel sheet within 2.5 seconds down to the temperatures of from above 500° C. to 700° C. at average cooling speeds of 100° C./sec or more, followed by coiling the cooled steel sheet; and pickling the coiled steel sheet, or pickling the coiled steel sheet and cold-rolling the pickled steel sheet, then annealing thereto in a continuous hot dip zinc-coating line at temperatures of 720° C. or above to perform zinc coating.  
   
   
       2 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 1 , wherein the steel slab further contains at least one element selected from the group consisting of 0.005 to 0.5% Nb, 0.005 to 0.5% Ti, and 0.0002 to 0.005% B.  
   
   
       3 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 1 , wherein the steel slab further contains at least one element selected from the group consisting of 0.01 to 1% V, 0.01 to 1% Cr, and 0.01 to 1% Mo.  
   
   
       4 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 2 , wherein the steel slab further contains at least one element selected from the group consisting of 0.01 to 1% V, 0.01 to 1% Cr, and 0.001 to 1% Mo.  
   
   
       5 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 1 , wherein the steel sheet after completed the hot-rolling is cooled in a period of from more than 0.5 second to 2.5 seconds at average cooling speeds of 100° C./sec or more.  
   
   
       6 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 2 , wherein the steel sheet after completed the hot-rolling is cooled in a period of from more than 0.5 second to 2.5 seconds at average cooling speeds of 100° C./sec or more.  
   
   
       7 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 3 , wherein the steel sheet after completed the hot-rolling is cooled in a period of from more than 0.5 second to 2.5 seconds at average cooling speeds of 100° C./sec or more.  
   
   
       8 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 4 , wherein the steel sheet after completed the hot-rolling is cooled in a period of from more than 0.5 second to 2.5 seconds at average cooling speeds of 100° C./sec or more.