Alloys or metallic compositions – Ferrous – Eight percent or more manganese containing
Patent
1997-10-29
1998-12-01
Yee, Deborah
Alloys or metallic compositions
Ferrous
Eight percent or more manganese containing
148325, C22C 3826, C22C 3828
Patent
active
058433708
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a ferritic stainless steel having excellent formability at an ordinary temperature and excellent strength at a high temperature for use in exhaust system equipment for cars.
BACKGROUND ART
In current production cars, the exhaust gas temperature has become higher and higher with higher output and higher performance of the engines, and a further improvement in high temperature strength has been required in the steel materials for the exhaust gas system equipment for the cars.
As technologies for improving the high temperature strength by adding Nb to a ferritic stainless steel so as to satisfy the requirements described above, Japanese Unexamined Patent Publication (Kokai) No. 3-294417, for example, discloses a technology which anneals a ferritic stainless steel having C and N contents of not greater than 0.03% and containing 0.1 to 1% of Nb added thereto at a temperature within the range of 1,100 to 1,250.degree. C., and Japanese Unexamined Patent Publication (Kokai) No. 5-331551 discloses a method which conducts finish annealing of a ferritic stainless steel having a C content of not greater than 0.02% and an N content of not greater than 0.03% and containing 0.4 to 1% of Nb added thereto at a temperature within the range of 1,100 to 1,200.degree. C. As disclosed in these prior art technologies, a greater quantity of Nb is required to obtain a high temperature strength in the relatively high C and N contents, so that the recrystallization temperature becomes excessively high and annealing must be carried out at a high temperature exceeding 1,100.degree. C.
On the other hand, a ferritic stainless steel for use in exhaust system equipment is described, for example, in Japanese Unexamined Patent Publication (Kokai) No. 6-248394. This reference describes a steel which contains Cr in a specific range, limits Nb and Ti as stabilization elements in specific ranges besides C and N, and supplementarily adds Si, Mo and Ni to improve a high temperature salt water corrosion resistance of a blank as a technology for improving an intergranular corrosion resistance of a welding heat affected zone of a front pipe, a center pipe, etc, of the car. However, because this reference adds large quantities of Si, Nb and Mo, it is not free from the problems that the recrystallization temperature of the steel becomes higher and the high temperature finish annealing requires in addition to a deterioration of formability at the ordinary temperature. In Japanese Unexamined Patent Publication (Kokai) No. 6-184705 developed for the same purpose as the reference described above and Japanese Unexamined Patent Publication (Kokai) No. 3-264652 describing an exhaust gas muffler material, on the other hand, the C+N value is limited to a low value by adding Nb and Ti. Because large quantities of Nb are added and the C+N value which still remains at a high level, however, the same problems as those of the prior art technologies described above occur.
U.S. Pat. No. 4,834,808 discloses a ferritic stainless steel for use in exhaust system equipment of cars. Though this patent uses Nb and Ti in combination, it cannot secure a low C+N value because the steel contains a high N content. In consequence, the problems that the solid solution Nb quantity decreases because of a small amount of Nb addition and the high temperature strength is deteriorated remain unsolved. Further, U.S. Pat. No. 4,964,926 shows no technical concept of increasing the solid solution Nb quantity by keeping a low C+N content, though the steel of this patent has a high Si content in order to secure the high temperature strength.
A YUS450-MS steel (Japanese Unexamined Patent Publication (Kokai) No. 5-821356) is commercially available on the market as an exhaust system material, for cars, having an improved high temperature strength. This material has a component composition containing 1% of Mo which is added in addition to 14% of Cr, 0.020% of C+N, 0.1% of Ti and 0.3% of Nb. This technology adds Nb in order to improve the high temperat
REFERENCES:
patent: 4964926 (1990-10-01), Hill
patent: 5685923 (1997-11-01), Takata et al.
Fujita Nobuhiro
Koyama Yuji
Maeda Shigeru
Shimada Tetsuya
Takahashi Akihiko
Nippon Steel Corporation
Yee Deborah
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