Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1995-09-22
1997-05-06
Yee, Deborah
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
148610, 148654, C21D 802
Patent
active
056266944
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a process for the production of stainless steel, and more particularly to a process for the production of stainless steel sheets having an excellent corrosion resistance.
BACKGROUND ART
Stainless steel sheets are excellent in the corrosion resistance under various corrosive environments and are widely used as building materials, materials for automobiles, materials for chemical plants and so on. Recently, there are observed many examples of service environments which are becoming more severe and the stainless steel sheet is demanded to have a more excellent corrosion resistance. On the other hand, stainless steels which take too much labor in their production, even though the corrosion resistance is excellent, are unfavorable to stainless steel manufacturers, so that it is desired that the stainless steel is excellent in the productivity, particularly hot workability.
Under the above circumstances, it has recently become possible to reduce impurities in steel with the advance of steel-making techniques, so as to improve the above corrosion resistance and hot workability by decreasing C, S and O in the stainless steel. For example, JP-B-60-57501 discloses a method of improving anti-corrosion in sea water and hot workability by decreasing C, S and O, and JP-B-2-46662 and JP-B-2-14419 disclose a method of likely improving the hot workability.
According to the above conventional improving techniques, however, there may be created remarkable chapping in a surface of stainless steel sheet after hot rolling-annealing-pickling. Such a chapping falls down during cold rolling to remain as a scab-like defect after the cold rolling, which undesirably deteriorates the corrosion resistance in hot rolled steel sheet and cold rolled steel sheet.
Of course, it has been attempted to trim the chapped surface of the steel sheet by means of a grinder or the like, which brings about the decrease of productivity and the rise of cost and is not an advantageous countermeasure. For this end, it is strongly desired to establish a technique of not creating the above chapping on the surface of the stainless steel sheet after annealing-pickling.
DISCLOSURE OF INVENTION
It is, therefore, a main object of the invention to solve the aforementioned problems in the production of the present stainless steel sheets, particularly stainless steel sheets having extreme-low amounts of C, S and O and to provide a process for the production of stainless steel sheets having more improved corrosion resistance as compared with the conventional ones without trimming the surface of the steel sheet after annealing-pickling.
In order to achieve the above object, there have been made various studies with respect to causes of creating the chapping on the surface of the conventional stainless steel sheet after annealing-pickling and also means for the prevention thereof has been examined. As a result, the following facts have been confirmed. That is,
1) The chapping of the steel sheet surface is caused due to the fact that Cr-removed layer formed in the annealing is eroded with an acid to form unevenness on the surface of the steel sheet.
2) The Cr-removed layer grows as an amount of scale (Fe.sub.3 O.sub.4) in the hot rolled sheet becomes large.
3) The Cr-removed layer grows as an adhesion property of scale (Fe.sub.3 O.sub.4) in the hot rolled sheet to iron matrix becomes strong.
4) The scale Fe.sub.3 O.sub.4 in the hot rolled sheet is formed at a relatively low temperature below 830.degree. C.
From the above facts, the inventors have noticed the following:
5) In order to prevent the chapping of the steel sheet surface, it is effective to decrease the amount of scale Fe.sub.3 O.sub.4 and to lower the adhesion property to iron matrix.
6) In order to decrease the amount of scale Fe.sub.3 O.sub.4 and lower the adhesion property to iron matrix, it is effective to control a finish temperature of hot rolling, and a cooling rate and a coiling temperature followed thereto.
Although a mechanism of forming the Cr-remov
REFERENCES:
patent: 5314549 (1994-05-01), Misao et al.
Fujisawa Mitsuyuki
Fukuda Kunio
Kawabata Yoshikazu
Satoh Susumu
Kawasaki Steel Corporation
Miller Austin R.
Yee Deborah
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