HIGH-STRENGTH, COLD-ROLLED, THIN STEEL SHEET AND METHOD FOR MANUFACTURING SAME

Title:

HIGH-STRENGTH, COLD-ROLLED, THIN STEEL SHEET AND METHOD FOR MANUFACTURING SAME

Document Type and Number:

WIPO Patent Application WO/2016/132680

Kind Code:

A1

Abstract:

Provided are a high-strength, cold-rolled, thin steel sheet having low in-plane anisotropy, and a method for manufacturing the same. The method comprises subjecting a steel material having a composition that contains, in mass%, more than 0.20% to 0.45% C, 0.50-2.50% Si, and 2.00% to less than 3.50% Mn, and further contains one or two elements selected from 0.005-0.100% Ti and 0.005-0.100% Nb, to hot rolling, and to cold rolling at a rolling reduction of 30% or more, followed by a further step of heating to a temperature range of 800-950°C, and cooling at a cooling rate of 5°C/s or greater to a cooling stop temperature in a temperature range of 350-500°C to obtain a steel sheet having a structure in which the total of a martensite phase and a bainite phase make up 80 vol% or more, which is then heated and held in a temperature range of 700-840°C, cooled at a cooling rate of 5-50°C/s to a cooling stop temperature in a temperature range of 350-500°C, and held for 10-1,800 s in this temperature range. This produces a structure that includes, in vol%, 15%-70% of a ferrite phase, more than 15% to 40% of a retained austenite phase, and 30% or less of a martensite phase, the retained austenite phase containing fine needle-shaped grains having an average crystal grain size of 2.0 μm or smaller and an aspect ratio of 2.0 or above. This produces a high-strength, cold-rolled, thin steel sheet having excellent production stability, a TS value of 980 MPa or greater, high ductility, and low in-plane anisotropy.

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Inventors:

OBATA YOSHIE (JP)
KAWASAKI YOSHIYASU (JP)
UEDA KEIJI (JP)
KANEKO SHINJIRO (JP)
YOKOTA TAKESHI (JP)
SETO KAZUHIRO (JP)

Application Number:

PCT/JP2016/000339

Publication Date:

August 25, 2016

Filing Date:

January 25, 2016

Export Citation:

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Assignee:

JFE STEEL CORP (JP)

International Classes:

C21D9/46; C22C38/00; C22C38/14; C22C38/60

Domestic Patent References:

WO2013047808A1	2013-04-04
WO2015151419A1	2015-10-08

Foreign References:

JP2012153957A	2012-08-16
JP2013185196A	2013-09-19
JP2011190474A	2011-09-29
JP2007154283A	2007-06-21

Other References: