Metal treatment – Stock – Ferrous
Patent
1995-03-28
1996-07-02
Yee, Deborah
Metal treatment
Stock
Ferrous
148603, C21D 804, C22C 3814
Patent
active
055318398
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a continuously annealed cold-rolled steel sheet excellent in balance between deep drawability and resistance to secondary-work embrittlement, using ultra-low-carbon steel as a material, and a method for manufacturing same. The continuously annealed cold-rolled steel sheet of the present invention is suitable for the application of a surface treatment such as a dip-plating. A continuous annealing line for manufacturing the continuously annealed cold-rolled steal sheet of the present invention may include a dip-plating treatment equipment and an alloying treatment equipment of a dip-plating layer.
BACKGROUND OF THE INVENTION
The recent progress of a degassing technology in the steel making industry has made it possible to manufacture ultra-low-carbon steel in which a carbon (C) content is reduced to up to 30 ppm at a relatively low cost in a large quantity. Steel known as IF steel (abbreviation of interstitial atoms free steel) comprising the above-mentioned ultra-low-carbon steel added with at least one of niobium (Nb), titanium (Ti), boron (B) and zirconium (Zr), is popularly used as a preferred material for manufacturing, through a continuous annealing treatment, a cold-rolled steel sheet for ultra-deep drawing of the EDDQ (abbreviation of excellent deep drawing quality)-class required to have deep drawability and non-aging property.
IF steel commonly used as a material for a continuously annealed cold-rolled steel sheet is ultra-low-carbon steel added with any one or both of titanium and niobium. Titanium is a strong element forming carbide and nitride in steel, and furthermore, titanium has a function of fixing sulfur in steel by forming sulfide through combination with sulfur in steel. IF steel added with titanium (hereinafter referred to as "Ti-IF steel") therefore provides an advantage of permitting stable availability of very excellent deep drawability and ductility within a wide range of a chemical composition of steel.
However, since titanium is an element easily oxidized, titanium oxide produced in molten Ti-IF steel during the continuous casting thereof, adheres and accumulates onto the surface of a bore of a pouring nozzle of a tundish, thus causing reduction or clogging of the bore, or surface defects of a slab are caused by titanium oxide. Addition of titanium in an amount sufficient to completely fix carbon in steel in the form of titanium carbide (TiC) to steel, causes a degradation in grain boundary strength of the annealed cold-rolled steel sheet, and upon subjecting the annealed cold-rolled steel sheet to the deep drawing, a problem of secondary-work embrittlement is caused in the annealed cold-rolled steel sheet. For the solution of secondary-work embrittlement, addition of boron in a slight amount to steel is known to be effective. Addition of boron to steel however results in deterioration of deep drawability of the cold-rolled steel sheet.
There is known another IF steel added with niobium (hereinafter referred to as "Nb-IF steel") as steel solving the above-mentioned problems. In Nb-IF steel, in which carbon in steel is fixed in steel in the form of niobium carbide (NbC), a cold-rolled steel sheet excellent in deep drawability is available, as in Ti-IF steel. A problem in Nb-IF steel is however that a range of an appropriate niobium content is tight. Since surface defects of a slab are hardly caused by oxide inclusions in Nb-IF steel, on the other hand, it is not necessary to scarf the surface of a continuously cast Nb-IF steel slab. This provides an advantage that it is possible to manufacture a hot-rolled steel strip from a high-temperature continuously cast slab of Nb-IF steel by the application of a method known as the hot-direct rolling method comprising directly hot-rolling a slab without reheating same. When using IF steel as a material for an alloying-treated zinc dip-plated cold-rolled steel sheet, it is known that adhesiveness of an alloying-treated zinc dip-plating layer to a cold-rolled steel sheet is improved m
Hosoya Yoshihiro
Morita Masaya
Tsuyama Seishi
NKK Corporation
Yee Deborah
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