Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-09-05
2002-12-31
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S610000
Reexamination Certificate
active
06500280
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ferritic Cr-containing steel sheet suitable for use for building facing materials, kitchen utensils, chemical plants, water tanks, automobile heat resistant members, etc. Particularly, the present invention relates to a ferritic Cr-containing steel sheet having excellent ductility, formability, and anti-ridging property, and a method of producing the same. In the present invention, the steel sheet includes a steel plate, and a steel strip.
2. Description of the Related Art
Stainless steel sheets have beautiful surfaces and excellent corrosion resistance, and are thus widely used for building facing materials, kitchen utensils, chemical plants, water tanks, etc. Particularly, austenitic stainless steel sheets have excellent ductility and excellent press-formability, and thus cause no ridging as a result of pressing, and are widely used for the above applications.
On the other hand, ferritic Cr-containing steel sheets such as ferritic stainless steel sheets need to be improved in formability. This is done by purifying the steel. The use for the above applications, instead of austenitic stainless steel, sheets of SUS 304, SUS 315, etc. have recently been studied. This is because the properties of the ferritic stainless steel are widely known, for example, low thermal expansion coefficient, low sensitivity to stress corrosion cracking, and low cost due to the absence of the expensive Ni ingredient.
However, for application to formed products, the ferritic stainless steel sheets have lower ductility than the austenitic stainless steel sheets, and this causes problems in that “ridging” occurs in the surfaces of the formed products. Ridging is an unevenness that spoils or downgrades the beauty of the formed products, significantly increasing the polishing load. Therefore, in order to further extend the application of ferritic stainless steel sheets, improvement in all of ductility, formability and anti-ridging properties are required.
For these requirements, ferritic stainless steel having excellent formability comprises 0.03 to 0.08 wt % of C, 0.01 wt % or less of Ni, and 2×N wt % to 0.2 wt % of Al and is proposed in, for example, Japanese Unexamined Patent Publication No. 52-24913. In the technique disclosed in Japanese Unexamined Patent Publication No. 52-24913, the C and N contents are decreased, and the Al content is twice or more as much as the N content decreasing the amount of solute nitrogen and making the crystal grains fine, thereby improving ductility, anti-ridging properties, and secondary formability.
However, in the technique disclosed in the aforesaid Publication No. 52-24913, the formability is greatly improved, but the anti-ridging properties are not significantly improved. Therefore, when working such as press forming or the like is performed, polishing is required for improving the beauty of the metal surface, increasing cost due to increased polishing load.
On the other hand, for example, Japanese Unexamined Patent Publication No. 51-123720 discloses a method for reducing ridging, in which after hot rolling, rolling is performed with a rolling reduction of 15% or more in a temperature region of 450 to 700° C., followed by annealing, cold rolling and final annealing.
Although the technique disclosed in the aforesaid Publication No. 51-123720 improves the anti-ridging properties, the technique does not significantly improve ductility or formability. Therefore, the various further attempts have been made to improve all of ductility, formability and anti-ridging properties concurrently.
Japanese Unexamined Patent Publication No. 2-170923 discloses a method of producing a chromium stainless steel sheet having excellent anti-ridging properties and press-formability, in which a hot-rolled sheet obtained by hot-rolling a chromium stainless steel containing 13.0 to 20.0 wt % of chromium is subjected to pre-cold rolling with a rolling reduction of 2 to 30%, followed by continuous annealing, descaling, cold rolling, and finish annealing. Strain is achieved by cold rolling before annealing to promote recrystallization in annealing, permitting continuous annealing for improving formability and anti-ridging properties.
The occurrence of ridging is a fundamental problem and is inherent in a ferritic stainless steel sheet. It needs to be fully resolved. On the other hand Japanese Unexamined Patent Publications Nos. 9-263900 and 10-330887 disclose a technique for improving anti-ridging properties by controlling a colony of similarly oriented crystal grains.
Although the technique disclosed in Japanese Unexamined Patent Publication No. 2-170923 improves the so-called “r” value (Lankford value) and the anti-ridging properties, the technique has a problem in that there is still considerable room for further improvement of both those properties, and that it fails to improve significantly the anti-ridging property and r value of the stainless steel.
Although the techniques disclosed in Japanese Unexamined Patent Publications Nos. 9-263900 and 10-330887 can prevent the occurrence of a colony of similarly oriented grains, they both face the problem that the occurrence of ridging cannot be completely suppressed, with the products exhibiting poor surface qualities after forming.
Furthermore, deeply drawing a ferritic stainless steel by press forming or the like encounters the problem of planar anisotropy of the “r” value and elongation of the steel sheet. Even when the steel sheet has a high mean “r” value and a mean elongation value in each direction, with a low minimum “r” value and a minimum elongation value, deep drawing cannot be sufficiently performed. In the steel sheets produced by the above-described conventional techniques, the mean “r” value and mean elongation are improved, while the minimum “r” value and minimum elongation value are low enough to cause a problem of high planar anisotropy of the “r” value and the elongation value.
The above-described conventional techniques cannot produce a ferritic stainless steel sheet satisfying the need for better ductility, formability, and anti-ridging properties at low cost. Namely, in the conventional techniques, formability is greatly improved, while the effect of improving the anti-ridging property is insufficient. Therefore, in an application using working such as press forming or the like an increased polishing load is necessary for improving the surface beauty of the formed product. In addition, although the mean “r” value and mean elongation value are improved, the problem remains that sufficient formability cannot be obtained in actual press forming (or the like) because of the high planar anisotropy of the “r” value and elongation, thereby causing difficulties in producing steel having sufficient levels of ductility, formability and anti-ridging properties at low cost.
SUMMARY OF THE INVENTION
The present invention has been achieved for solving the problems associated with the above-described conventional techniques.
An object of the present invention is to provide a ferritic Cr-containing steel sheet having good ductility and formability, while also having excellent anti-ridging properties, particularly an anti-ridging property equivalent to that of stainless steel SUS304, and excellent surface qualities after forming, and a method of producing the same. Another object of the present invention is to provide a ferritic Cr-containing steel sheet having good ductility and formability, excellent anti-ridging properties, and low planar anisotropy of the “r” value, along with excellent elongation characteristics.
This invention also relates to a method of producing such a ferritic Cr-containing steel sheet.
We have discovered the importance of specific chemical components and proportions in the steel, and the steps of pre-rolling performed by warm or cold rolling with a relatively low rolling reduction between hot-rolling and hot-rolled sheet annealing to improve ductility, formability and the anti-ridging property. We have further found
Kato Yasushi
Ota Hiroki
Satoh Susumu
Ujiro Takumi
Kawasaki Steel Corporation
Schnader Harrington Segal & Lewis LLP
Yee Deborah
LandOfFree
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