Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
1998-07-29
2001-03-27
Jones, Deborah (Department: 1775)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C420S036000, C420S037000, C420S038000, C420S062000, C420S063000
Reexamination Certificate
active
06207103
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to Fe—Cr—Si steel sheet having excellent corrosion resistance and high toughness, and to a method for manufacturing the same.
BACKGROUND OF THE INVENTION
Fe—Cr alloy sheets have been known for excellent corrosion resistance. To secure even more corrosion resistance and better heat resistance properties under far more severe conditions, various elements have been added to the alloys used in the sheets. Representative examples are Mo, Co and Al. As a result, quite excellent corrosion resistance has been achieved. Pitting corrosion potential is used as a representative index for corrosion resistance (as measured in a 3.5 vol % aqueous solution of NaCl at 30° C. at a current density of 10 &mgr;A/cm
2
). With the added elements the pitting corrosion potential of the sheets can reach 500 mV or even higher. However, all of those elements are expensive. Accordingly, in the working industry, the added amount in the sheet is limited at a sacrifice of corrosion resistance and heat resistance.
Si is less expensive than Mo, Co or Al and, in addition, improves corrosion resistance or heat resistance. Accordingly, use of Fe—Cr—Si alloys in industry is expected. As an example Japanese Laid-Open Patent Publication Sho-57/134,542 discloses ferritic stainless steel containing 0.01-5.00 wt % of Si, 0.01-5.00 wt % of Mn and 0.20-1.00 wt % of Nb and having an excellent corrosion resistance.
Unfortunately, Si has the disadvantage that, when its content is about 3.5 wt % or more, toughness of the iron alloy is radically reduced. This limits its use as a material. Moreover, processing steps such as rolling and press forming become difficult. Further, it has been said that the effect of Si for improving corrosion resistance is inferior to that of Mo, Co, Al, etc. However, when the Si content is unduly restricted, its usefulness as an anticorrosive material for an Fe—Cr—Si alloy cannot be maintained.
It has been known that, in Fe—Cr alloy systems, reduction of impurities can sometimes improve toughness and processing ability without changing the main component system. A representative example is Japanese Laid-Open Patent Publication Hei-06/033,197 in which it is mentioned that, in some products, even when Si is present, processing ability can be improved by decreasing impurities. However, when a large amount of Si is present, there is a far more significant deterioration of toughness than is common in Fe—Cr alloys and there is concern that this deterioration cannot be compensated for by any degree of improvement of toughness of a common Fe—Cr alloy as disclosed in the patent. Further, it has not yet been investigated whether corrosion resistance can be kept as high as 500 mV, expressed as pitting corrosion potential.
In Japanese Laid-Open Patent Publication Hei-03/053,025, it is disclosed that when rapid cooling is conducted after hot rolling under high stress, the toughness of an Fe—Cr—Si alloy containing 0.01-0.50 wt % of rare earth metal elements (REM) can be improved. However, such a rolling process is not common and adds cost and delay. In addition, when the properties of the conventional Fe—Cr—Si alloy are taken into consideration, it is only to be expected that the resulting corrosion resistance will have to be less than 500 mV of pitting corrosion potential.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome these barriers, and to create an Fe—Cr—Si alloy having excellent corrosion resistance and high toughness, and also to conduct cold rolling or hot rolling taking advantage of such high toughness.
We have found that, even in the case of a high content of Si, we can avoid reducing the amounts of C and N and Cr, as usually presumed to improve toughness, but, on the contrary, Cr in more than a certain amount is actually present, and that this achieves surprisingly high toughness. We have also found that, with regard to corrosion resistance, Cr and Si in more than certain amounts can be present while the contents of C and N are reduced, and that this achieves corrosion resistance at such a high level that it surpassed anything possible up to now.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to Fe—Cr—Si steel sheet having excellent corrosion resistance and high toughness. The steel sheet comprises about 10-30 wt % of Cr and about 3.5-10 wt % of Si; the total amount of C and N in the sheet is not more than about 100 ppm, while the remainder comprises Fe and incidental impurities. When the total amount of C and N is about 40 ppm or less, very significant corrosion resistance and toughness is achieved.
Moreover, when not more than about 5 wt % of one or more metals selected from Mo, Co and Al is added to such a steel sheet, the corrosion resistance and toughness are further improved.
We have further found that, when the final thickness formed by hot rolling is less than about 3 mm, very high toughness or workability can be achieved, even if the amount of Si is as high as about 3.5-10 wt %. It has been also found that the greater the Cr, the more the advantageous effect.
The effect is significant when a cast ingot containing about 10-30 wt % of Cr and about 3.5-10 wt % of Si, where the total amount of C and N is not more than about 100 ppm and the remainder is mostly composed of iron and incidental impurities, is subjected to hot rolling to a thickness of not more than about 3 mm. The effect is promoted when not more than about 10 wt % of Ni is further added.
The sheet which is hot rolled to a thickness of not more than about 3 mm can surprisingly be subjected to cold rolling or warm rolling without annealing.
Experimental results whereby the present invention has been achieved will now be illustrated. They are not included to define or to limit the scope of the invention, which is defined in the appended claims.
Fe, Cr and Si each having a purity of at least 99.99% were used as materials. Each sample comprising 10 kg of highly pure Fe—Cr (0-30wt %)—Si (5wt %) alloy (wherein the weight percentage of Cr of each was either 0, 2, 10, 18 or 30%) was prepared by melting in a small melting furnace. For deoxidation, 0.01 wt % of Al was added. Amounts of the impurities in the resulting alloy were 1-4 ppm of C, 3-7 ppm of P, 3-5 ppm of S, 6-15 ppm of N, 5-11 ppm of 0 and 8-17 ppm of C and N.
Cast blocks were cut out at a thickness of 60 mm, heated at 1,100° C. and rolled into a sheet having a thickness of 3.5 mm. Charpy impact test specimens having a sheet thickness of 2.5 mm, a width of 10 mm, a length of 55 mm and a V notch of 2 mm were taken from each steel sheet in parallel to the rolling direction. Each was subjected to measurement of impact values at various temperatures. The temperature at which the percent brittle fracture became 50% (i.e. the ductile-brittle transition temperature) was determined and served as an index of toughness.
The transition temperature for each composition (0, 2, 10, 18 or 30 wt % of Cr and 5 wt % of Si) was as follows.
Cr (wt%)
Transition Temperature (° C.)
0
+180
2
+160
10
−20
18
−40
30
−30
This unexpectedly shows that, when the amount of Cr is about 10 wt % or more, a very low transition temperature or, in other words, a very high toughness is achieved, ven if 5 wt % of Si is present.
Then, the composition Cr(18 wt %)—Si(5 wt %) was subjected to the same treatment as above except that iron nitride and mother alloy containing 5 wt % of C were used for the adjustment of C and N. The resulting alloy samples having various amounts of C and N, were subjected to a Charpy test in the same manner as above. The results were:
C + N (ppm)
Transition Temperature (° C.)
11
−40
22
−10
43
+70
86
+90
117
+180
This shows that, when the amount of C plus N is about 100 ppm or less, toughness is markedly improved and that, when C plus N is about 40 ppm or less, toughness is drastically improved.
Those hot rolled sheets were made into thin sheets having a thickness of 0.35 mm by warm rolling, annealed at 850° C. in
Kondo Osamu
Matsuzaki Akihiro
Takajo Shigeaki
Yamashita Takako
Jones Deborah
Kawasaki Steel Corporation
Miller Austin R.
Miranda Lymarie
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