Cold-rolled steel sheet

Patent metadata

Field	Value
Publication number	US-9771638-B2
Application number	US-201615047990-A
Country	US
Kind code	B2
Filing date	Feb 19, 2016
Priority date	Mar 5, 2007
Publication date	Sep 26, 2017
Grant date	Sep 26, 2017

Abstract

A high strength cold-rolled steel sheet has excellent surface appearance and a composition consisting essentially of, in mass %, C: 0.0005-0.025%, Si: at most 0.2%, Mn: 0.3-2.5%, P: at most 0.15%, S: at most 0.02%, N: at most 0.006%, sol. Al: less than 0.005%, Ti: 0.005-0.05%, and Nb: 0.020-0.200% with the mass ratio (Nb/Ti) of the contents of Nb and Ti being at least 2, either boron at most 0.0020% or one or more of Cr, Mo, V, W, Cu, and Ni, each at most 1%, and a remainder of Fe and impurities. The sheet has excellent press formability and tensile strength by having an r value in a direction at 45° with respect to the rolling direction (r 45 ) of at least 1.80 and/or a mean r value (r m ) of at least 1.60, and a tensile strength of at least 340 MPa.

First claim

The invention claimed is:

1. A cold-rolled steel sheet characterized by having a chemical composition consisting of, in mass percent, C: 0.0005-0.025%, Si: at most 0.2%, Mn: 0.3-2.5%, P: at most 0.15%, S: at most 0.02%, N: at most 0.006%, sol. Al: 0.0001 to less than 0.005%, Ti: 0.005-0.05%, Nb: 0.020-0.200%, and optionally one or more elements selected from the group consisting of B: at most 0.0020%, Cr: at most 1%, Mo: at most 1%, V: at most 1%, W: at most 1%, Cu: at most 1%, and Ni: at most 1%, with the mass ratio (Nb/Ti) of the contents of Nb and Ti being at least 2, sol. Ti: 0.004% or more, and a remainder of Fe and impurities, and by having an r value in a direction at 45° with respect to the rolling direction (r 45 ) of at least 1.80 and/or a mean r value (r m ) of at least 1.60, along with a tensile strength of at least 340 MPa, wherein a dissolved oxygen concentration ranges between 0.003 and 0.018% in a molten steel used to make the steel sheet, the dissolved oxygen concentration measured prior to addition of Ti to the molten steel.

Assignees

• Nippon Steel & Sumitomo Metal Corp

Inventors

• Furuhashi Seiji
• Haga Jun
• Nishi Takayuki

Classifications

• C22C38/04

  containing manganese · CPC title

• C22C38/12

  containing tungsten, tantalum, molybdenum, vanadium, or niobium · CPC title

• C23C2/28

  Thermal after-treatment, e.g. treatment in oil bath · CPC title

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title

• C22C38/02

  containing silicon · CPC title