Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2002-04-30
2004-09-21
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S325000, C420S042000
Reexamination Certificate
active
06793746
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field in the Industry
The present invention concerns parts made of free cutting stainless steel, from which release of so-called “sulfide gas”, i.e., corrosive gas consisting mainly of H
2
S, is suppressed to such a level that no practical problem is caused. The invention also concerns a method of producing the parts. The stainless steel here encompasses both ferritic and martensitic stainless steel. The parts according to the invention can be finished by precision machining, and therefore, useful as the parts for electronic apparatus such as rotation axes of hard disk drives to which high hardness and corrosion resistance are required, and the parts installed near the printed circuit substrates.
2. State of the Art
As is well known, Ag, Cu and Al are usually used as the material for contacting points and circuits of electronic apparatus. These metals are easily sulfurized. If the atmosphere contains H
2
S, the formation of sulfides by S in the atmosphere occurs and the sulfides may cause troubles. Therefore, material such as steel used in an electronic apparatus must be such one that it may not react with moisture in the environment and release substantially no sulfide gas such as H
2
S. To meet this requirement it is necessary to lower the S-content in the material.
Because the electronic apparatus are generally precision apparatus, very high accuracy is required to dimensions of the parts thereof, and therefore, high machinability is essential to the material. Further, hardness and corrosion resistance are also required.
To date, there has been used, for production of these parts, ferritic and martensitic stainless steel. However, machinability of the conventional steel of this kind is so low that sufficient accuracy may not be realized on the product parts, and that productivity of processing is low. Steels such as SUS 430F (ferritic), SUS416 and SUS420F (martensitic) exhibit improved machinability by addition of S. These free cutting steels do not fit for use due to release of the sulfide gas. There has been, therefore, strong demand for a stainless steel which releases no sulfide gas and has high machinability.
A proposal to meet this demand is to lower the Mn/S ratio in the steel so that the sulfides formed may be less soluble (for example, Japanese Patent Disclosure Hei.10-237603). In this steel the S-content is controlled to a suitable low level. The machinability decreased by lowering the S-content is compensated by addition of other free cutting elements such as Se and the like. The drawback of this countermeasure to the problem is, instead of the suppressed release of the sulfide gas given by the decreased Mn/S ratio, processability of the alloy is low and the production cost is high.
SUMMARY OF THE INVENTION
The object of the present invention is to improve the parts made of ferritic or martensitic stainless steel, particularly, the parts of electronic apparatus, in which troubles caused by the sulfide gas should be avoided, and to provide such parts with suppressed release of sulfide gas to such extent that no practical problem is caused. The parts have high hardness and corrosion resistance, as well as high finishing accuracy.
DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS
As noted above, the stainless steel parts of the present invention with suppressed release of sulfide gas include the parts made of both ferritic and martensitic stainless steel.
The ferritic stainless steel consists essentially of, as the basic alloy composition, at percent by weight, C: up to 0.12%, Si: 0.05-1.00%, Mn: 0.5-2.5%, S: 0.02-0.25%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Cr: 15.0-25.0%, N: up to 0.05% and O: 0.005-0.040%, and the balance of Fe, provided that P: 0.04% or less and the ratio Mn/S is 5.0 or higher. The steel is processed to blanks having the shape of the parts, and the blanks are treated with a solution of oxidative acid to dissolve out sulfides existing on the surfaces of the blanks.
The martensitic stainless steel consists essentially of, as the basic alloy composition, also by weight percent, C: 0.05-0.65%, Si: 0.01-1.00%, Mn: 0.5-2.5%, S: more than 0.010%-0.25%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Cr: 10.0-16.0%, N: 0.01-0.15% and O: 0.004-0.030%, and the balance of Fe, provided that P: 0.04% or less and the ratio Mn/S is 8.5 or higher. The steel is also processed to blanks having the shape of the parts, and the blanks are treated with a solution of oxidative acid to dissolve out sulfides existing on the surfaces of the blanks.
The stainless steel used as the material for the parts with suppressed release of sulfide gas may contain, in both ferritic and martensitic, in addition to the above described basic alloy components, one or more of the elements belonging to the following groups I to IV.
To the ferritic stainless steel,
Group I: one or two of Mo: up to 3.0% and W: up to 3.0%;
Group II: one or more of Se: up to 0.50%, Pb: up to 0.30%, Bi: up to 0.20% and Te: up to 0.10%;
Group III: one or more of B, Ca, Mg and REM: 0.0005-0.010%; and
Group IV: one or more of Nb, V, Ti, Zr, Hf and Ta: 0.03-0.50%.
To the martensitic stainless steel,
Group I: one or two of Mo: up to 2.0% and W: up to 2.0%;
Group II: one or more of Se: up to 0.50%, Pb: up to 0.30%, Bi: up to 0.20% and Te: up to 0.10%;
Group III: one or more of B, Ca, Mg and REM: up to 0.0005%; and
Group IV: one or more of Nb, V, Ti, Zr, Hf and Ta: 0.03-0.50%.
The method of producing the stainless steel parts of the present invention comprises:
processing a ferritic or a martensitic stainless steel of any alloy composition defined above by forging and/or machining to the shape of the part, followed by quenching and tempering; or quenching and tempering, followed by machining to the shape of the parts; and applying a solution of oxidative acid to the surfaces of the parts so as to dissolve and remove sulfides existing on the surfaces of the parts.
The reason why the alloy compositions of the ferritic and martensitic stainless steel are decided as noted above will be explained below. Alloying elements in which the effects are common in the two kinds of stainless steels and the ranges of addition amounts are the same or similar are explained in the lumps.
C: up to 0.12% for ferritic stainless steel, and 0.05-0.65% for martensitic stainless steel
In the ferritic stainless steel carbon combines with chromium to precipitate carbides, which lowers corrosion resistance. Carbon also increases hardness of the matrix as a solution-strengthening element to impair machinability of the steel. Therefore, it is preferable to decrease the carbon content as low as possible. In view of the technological limit in steel making the above upper limit, 0.12%, was decided as the limit at which there is no significant increase in costs and little influence on the steel properties. Preferably, carbon content is less than the lower limit of 0.03%.
For the martensitic stainless steel, carbon is essential to ensure the hardness. However, carbon is undesirable in regard to the corrosion resistance. It is advisable to add, with expectation of hardness on nitrogen by adding such a large amount of nitrogen that is possible in ordinary steel making technology, suitable amount of carbon which may give a desired hardness. From this point of view, the lower limit, 0.05%, is the necessary amount for ensuring the hardness. The upper limit, 0.65%, is decided in view of the facts that increase in hardness saturates at a higher addition amount and that, if the carbon content exceeds this limit, too large primary carbide particles occur and damage processability such as machinability after annealing.
Si: for the ferritic stainless steel 0.05-1.00% for the martensitic stainless steel 0.01-1.00%
Si is used as a deoxidizing agent in steel making, and for this purpose, it is necessary to add Si of the amount of the lower limit, 0.01%, or higher, preferably, 0.05% or higher. However, Si enhances formation of intermetallic compounds and increases strength of the matrix to decrease machinability of the stainless steel. Therefore, large amou
Okabe Michio
Shimizu Tetsuya
Daido Steel Co. Ltd.
King Roy
Varndell & Varndell PLLC
Wilkins, III Harry D
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