Continuously cast slab of extremely low carbon steel with less s

Details Continuously cast slab of extremely low carbon steel with less s Continuously cast slab of extremely low carbon steel with less s

1995-07-21

1996-11-26

Yee, Deborah

Metal treatment

Stock

Carburized or nitrided

148541, 148565, 148903, 428682, 164498, 164473, B22D 2702, B32B 1518, C21D 104

Patent

active

055781438

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a continuously cast slab of extremely low carbon steel with less surface defects in a steel sheet-producing step; an extremely low carbon sheet steel; and a process for producing the same, and more particularly to an extremely low carbon sheet steel with a good shapability now in much use. Thus, the present invention relates not only to iron-making technology for producing the steel sheets, but also to a broad range of industrial field including automobiles, household electric appliances, housing materials, etc. as their consumers.


BACKGROUND ART

(Prior Art)
Extremely low carbon steel produce by reducing the carbon content of steel refined in the steel-making process to a few 10 ppm in a vacuum degassing apparatus such as RH, etc., and further by adding carbide-nitride-forming elements such as Ti, Nb, etc. thereto, thereby fixing the remaining carbon, that is, the so called IF steel (interstitial free steel) has a very excellent formability such as deep drawing, etc. and is now much used in automobile application, etc. IF-based, high strength sheet steel, intensified by adding solid solution-strengthening elements such as P, Mn, etc. to the IF steel has also now established a dominant position as the main steel species of high strength sheet steels. However, the IF steel has several process and product drawbacks.
First process drawback is a surface defect. IF steel is vacuum degassed in the steel-making process, and consequently the oxygen content is increased owing to the CO equilibrium, and thus deoxidation is carried out. It is hard to completely remove the deoxidation products, which are liable to remain as inclusions.
Furthermore, solidification in the continuous casting process substantially eliminates carbon, and thus there are no coexisting solid-liquid zones substantially at all. Unstability such as a temperature fluctuation, etc. is directly connected to unstability of steel slab quality, resulting in deterioration of surface properties. Furthermore, the IF steel can be deemed to be nearly pure iron in the composition and has a higher Ar.sub.3 transformation point. Thus, the hot rolling finishing temperature must be made higher, resulting in inevitable generation of surface defects. The serious situation can be understood by a forum "Technique to prevent surface defects in the hot rolling and heavy plate rolling" held in No. 126 Autumn Lecture Conference of Japan Iron and Steel Institute, where the IF steel was taken as the main subject (see, for example, Lecture summary, CAMP-ISIJ, vol. 6, pp 1328-1331 and pp 1332-1335). In spite of the forum, measures to be taken were of on-the-spot type and allopathic and no fundamental solution was made at all yet.
Other drawbacks include several types, depending on differences in species of sheet steels and surface treatment, such as cold rolled steel sheets, electroplated steel sheets, and hot zinc-dipped steel sheets. The IF steel is generally sensitive to the nature of surface treatment. Particularly the iron-zinc alloy coated steel (galvannealed steel) is strongly influenced by steel components. Thus, in case of cold rolled steel sheets and galvannealed steel sheets, it is an ordinary expedient to classify the IF steel, depending on their purposes. For example, Nb-contained IF steel having a better hot dip galvanizing property is preferably used for the galvannealed products, whereas Ti-containing IF steel having a high Lankford value (hereinafter referred to as "a r value") as an indicator of material quality, particularly deep drawability is preferably used for the cold rolled steel sheet, as classified depending on the purposes. This is true also of IF-based high strength steel sheets. However, the minute classification of steel species was against the mass production as the basis of iron and steel industries and largely deteriorated the economy.
Still furthermore, satisfactory metallographic structure cannot be obtained at the heat-influenced parts of welded joints, because the IF steel is extremel

REFERENCES:
Suminaga et al., "Prevention Of Rolled-In-Scale Problems In Ultra Low Carbon Steels", CAMP-ISIJ vol. 6 (1993)-1328.
Tsuyama et al., "Hot Ductility Of Ultra-Low Carbon Steels", CAMP-ISIJ vol. 6 (1993)-1332.

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