Frishauf, Holtz, Goodman & Chick, P.C.
High strength steel sheet and method for manufacturing the same
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 0, 0.005% or less S, by weight, having 10 &mgr;m or less of average ferritic grain size, and 20 mm/mm2 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 mm2 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 Ar3 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&deg; C. at cooling speeds of from 100 to 2,000&deg; C./sec, followed by coiling the cooled steel sheet at temperatures of from 450 to 650&deg; 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.
NKK Corporation
C C C C C C C C C
C23C C22C C21D C21D C22C C22C C22C C22C C21D
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Sadanori Takayuki Hiroyasu Tadashi
Imada Odake Kikuchi Inoue