Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Patent
1996-08-21
1998-02-03
Yee, Deborah
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
148325, C22C 3820, C22C 3822
Patent
active
057141142
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND ART
The present invention relates to a high hardness martensitic stainless steel with good pitting corrosion resistance suitable for use as materials of products which require both good corrosion resistance, particularly pitting corrosion resistance, and high hardness, such as nails, bolts, screws edged tools, springs and so on which are used in the open air and may be possibly exposed to tap water, rainwater, condensed dew or the like, including molds for plastic molding, parts of plastic injection molding machines, etc.
Heretofore, carbon steel containing a relatively large content of carbon or low-alloy steel have seen widespread use as materials of nails, bolts, screws edged tools, springs and so on which require high hardness. But because the content of alloy elements which contribute to corrosion resistance, such as Cr, are small in those types of steel, the steel tends to easily corrode even when exposed to tap water, rainwater, condensed dew or the like that are relatively less corrosive. The problems of marring an appearance and deteriorating the strength have been thus encountered.
On the other hand, stainless steel is employed for applications in which corrosion resistance is required. Austenitic stainless steel represented by SUS304 or SUS316, for example, has good corrosion resistance, but shows large work hardening and poor cold workability and also exhibits hardness of about 43 HRC at maximum even when subjected to considerably heavy cold working. Therefore, austenitic stainless steel is not suitable for applications in which high hardness is required. Further, ferritic stainless steel represented by SUS430, for example, has small work hardening and is relatively easy to perform cold working, but exhibits very low hardness. Accordingly, ferritic stainless steel is also not suitable for applications in which high hardness is required.
Meanwhile, martensitic stainless steel is known as stainless steel having high hardness. However, even SUS410 which is a typical material having achieved extensive use in the fields of automobiles and other industries is not satisfactory in points of corrosion resistance and hardness because the corrosion resistance does not meet a sufficient level and the hardness is about 42 HRC at most. There is SUS440C as martensitic stainless steel having very high hardness. This steel has a C content as high as about 1% and hence shows high hardness not less than 58 HRC, but its corrosion resistance is not satisfactory as stainless steel. Further, stainless steel is relatively highly resistant against corrosion in general, but may locally corrode in the form of small pits, i.e., cause so-called pitting corrosion, in spite of less corrosion as a whole. This has raised the problem that the steel is apt to cause fracture in high hardness materials starting from the corroded pits.
In addition, Japanese Patent Laid-Open No. 57-70265 proposes a high hardness martensitic stainless steel, and Japanese Patent Laid-Open No. 6-264194 proposes a martensitic stainless steel with good corrosion resistance and a drilling tapping screw.
The martensitic stainless steel disclosed in Japanese Patent Laid-Open No. 57-70265 contains 1.0 to 3.0% Cu and not more than 0.2% Ni with 0.5 to 3.0% Mo added, if necessary. However, this steel has had the problem of not surely meeting a satisfactory degree of hot workability because the Cu content is large although the amount of Ni added is small. Depending on combinations of the composition, delta ferrite is apt to be formed and, in this case, there is caused the problem of deteriorating pitting corrosion resistance.
Further, the martensitic stainless steel proposed in Japanese Patent Laid-Open No. 6-264194 does not contain Cu, but a relatively large content of Ni and Mo. But this steel has the problem that hardness after annealing is not sufficiently lowered in a case of a single time of annealing treatment because of the high content of Ni. Therefore, the annealing treatment is required to be repeated several times, which makes the process
REFERENCES:
patent: 4450006 (1984-05-01), Uyehara et al.
Hitachi Metals Ltd.
Yee Deborah
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