Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Patent
1993-11-08
1995-04-11
Yee, Deborah
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
164476, 164477, 428683, 428685, B22D 1100, B22B 1518, C21D 802
Patent
active
054054605
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to Fe-Cr-Al alloy steel sheets (including foil materials) free from surface flaws, and a process for producing the same.
BACKGROUND ART
Recently, Fe-Cr-Al alloys have become used more and more in heat resisting applications, such as an outer smoke pipe of an oil stove, an electric heater and a metallic carrier for an automobile catalyst, because of their excellent oxidation resistance.
It is known that oxidation resistance of this alloy is enhanced with increase of Cr and Al concentrations, and that a remarkable improvement in the adhesion of an oxide film serving to maintain the heat resistance can be attained by the addition of a small amount of at least one rare earth element, including Y, to the alloy. For this reason, in recent years, cold-rolled steel products or foil materials have been produced wherein the Cr and Al concentrations were maximized within such a range that the materials are still rollable and, at the same time, rare earth elements including Y were added.
These alloys, however, had a drawback that the producibility of the products from these alloys is poor and the yield decreases with an increase in the heat resistance. In particular, regarding alloys containing rare earth elements including Y, scabs are likely to occur on the surface of steel sheets in the step of hot rolling or cold rolling. The formation of the scabs incurs an increase in the cost of the process for removing the scabs and, in addition, causes a remarkable lowering in the yield.
An object of the present invention is to avoid scabs generation in the step of hot rolling or cold rolling of an Fe-Cr-Al alloy steel having high Cr and Al concentrations with rare earth elements including Y, thereby providing an Fe-Cr-Al alloy steel sheet capable of inhibiting the lowering in yield.
CONSTRUCTION OF INVENTION
In order to attain the object mentioned above, the present invention is characterized in that a cast slab comprising a surface layer and an inner layer with the concentration of at least one element selected from the group consisting of Cr, Al and REM (REM being at least one element selected from the group consisting of Y and rare earth elements (lantanoids) with atomic numbers of 57 to 71) in the surface layer being 70% or less of that of the inner layer is provided, hot-rolled and cold-rolled into an Fe-Cr-Al alloy steel sheet.
Specific examples of methods for producing the cast slab mentioned above include a double layer continuous casting method and a method wherein a REM wire covered with a metal provided so that the covering metal melts within a continuous casting mold at its position of a depth where the surface layer of the slab is solidified is continuously poured into the continuous casting mold at said position. These methods will now be described.
Among ferritic stainless steels, the Fe-Cr-Al alloy steel has inherently poorer hot workability and cold workability and is likely to cause flaws during rolling. Further, in Fe-Cr-Al alloy steels containing REM, these REM elements often segregate in the form of an oxide or a low-melting intermetallic compound or alloy in the vicinity of the surface of a cast slab. This phenomenon in combination with the poor hot workability and cold workability of the matrix gives rise to the formation of scabs starting from the segregates during rolling.
The present inventors have made various studies with a view to avoid these scabs generation and, as a result, have found that the scabs generation can be effectively avoided by one of two techniques, i.e., a technique where the Cr or Al concentration of the surface of the cast slab is lowered to improve the hot workability and cold workability of the surface layer and a technique where the average concentration of REM in the surface layer portion is lowered to eliminate the segregation of the REM elements in the surface layer portion of the cast slab. It is a matter of course that the resultant steel sheets or foil materials have concentration distributions of Cr, Al, REM or othe
REFERENCES:
patent: 4536455 (1985-08-01), Maeda et al.
patent: 5045404 (1991-09-01), Ohmura et al.
patent: 5228932 (1993-07-01), Shimizu et al.
Fukaya Masuhiro
Hisatomi Ryoichi
Imamura Akira
Ohmura Keiichi
Tanaka Hiroyuki
Nippon Steel Corporation
Yee Deborah
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