Stainless steel for high-purity gases

Stainless steels for high-purity gases which are superior in non-dusting characteristics required at the time of welding, corrosion resistance and non-catalytic property and which can be widely utilized in the manufacturing process of semiconductors, liquid crystal displays or the like. The austenitic stainless steels of the present invention are characterized by having decreased Mn, Al, Si and O contents. The austenitic stainless steels meet the non-dusting characteristics which are required at the time of welding. In addition, corrosion resistance, abrasion resistance and machinability are improved. The ferritic and the duplex stainless steels of the present invention are characterized in that they can readily form thereon a Cr oxide layer when subjected to oxidation treatment. The ferritic and two-phase stainless steels are superior in corrosion resistance to corrosive gases, and contain non-catalytic property against chemically-unstable gases.

SPECIFICATION 
1. Technical Field 
The present invention relates to stainless steels for high-purity gases 
used in the manufacturing process of semiconductors or the like. 
2. Background Art 
In the field of the manufacturing of semiconductors or liquid crystal 
displays, the degree of the integration of devices has increased in recent 
years. 
In the manufacturing of a device called VLSI, a fine pattern of 1 micron or 
less is required. In such a manufacturing process, fine dust or an 
extremely small amount of gas impurities are deposited to or adsorbed by a 
wiring pattern to cause a circuit failure. It is therefore necessary that 
both a reaction gas and a carrier gas used have high purity; that is, only 
a few particles and gas impurities can be present in these gases. For this 
reason, a pipe or a member used for such gases that have high-purity is 
required that the inner surface thereof discharges as contaminants only 
minimum amounts of particles and gases. Besides inert gases such as 
nitrogen and argon, many gases called speciality gases are also used as 
gases for manufacturing semiconductors. Examples of the speciality gases 
include corrosive gases such as chlorine, hydrogen chloride and hydrogen 
bromide, and chemically-unstable gases such as silane. For the former 
gases is required corrosion resistance, and for the latter gases is 
required non-catalytic property (the property of preventing the 
decomposition of silane gas or the like to produce particles, which is 
caused due to the catalytic property of the inner surface of a pipe). 
Heretofore, in order to reduce the deposition or adsorption of dust or 
water, the inner surface of the pipe or the member for gases used for 
manufacturing semiconductors has been smoothed until the roughness thereof 
in R.sub.max becomes 1 micron or less. Cold drawing, mechanical polishing, 
chemical polishing, electropolishing, or the combination thereof can be 
mentioned as the method for smoothing the inner surface of the pipe or the 
member. However, a highly-smoothed material having an R.sub.max of 1 
micron or less is chiefly obtained by means of electropolishing. The pipe 
or the like whose inner surface is smoothed is then washed with 
high-purity water, and dried by a high-purity gas to obtain a final 
product. 
Welding is generally adopted when a pipe line is laid. This is because 
welding can ensure high strength and good airtightness to the pipe line. 
In the laying of a pipe line by welding, usually a high-purity inert gas, 
typically argon gas is allowed to run as a shielding gas through a pipe 
whose inner surface will come into contact with a high-purity gas, in 
order to avoid, as much as possible, contamination and oxidation of a part 
which is heated to high temperatures. Further, after the pipe line is 
laid, the pipes are purged with high-purity argon or nitrogen gas to 
remove those particles which are still remaining in the pipes. It takes 
several days to several weeks for this purging operation when the pipe 
line is long and complicated, such as a plant pipe line. Recently, 
decrease in the cost of the construction of a semiconductor-manufacturing 
plant and the early operation of the plant have been strongly demanded. To 
meet these demands, it is now required to shorten the purging time. 
Besides the aforementioned properties, the pipe and the member for high 
purity gases are required to have weldability; the joint area thereof to 
which mechanical sealing is applied is required to have abrasion 
resistance; and when parts such as joints are produced by machining, 
machinability is required. 
On the other hand, it has been known that corrosion resistance to and 
non-catalytic property against speciality gases, which are required for 
the pipe or the like for gases used for manufacturing semiconductors, can 
be improved by forming a Cr oxide layer on the surface of stainless steel 
by heating the steel under such an atmosphere in that the partial pressure 
of oxygen is controlled (see "Special Technique for Non-Corrosive, 
Non-Catalytic Cr.sub.2 O.sub.3 Stainless Steel Pipes", The 24th VLSI 
Ultra-Clean Technology Workshop held by Ultra Clean Society, pp. 55-67, 
Jun. 5, 1993). It is noted that the objective material for the pipes 
reported in this literature is assumed to be SUS 316L stainless steel. 
The above demand of corrosion resistance and non-catalytic property is made 
not only for a pipe line for gases. The same demand is also made for 
stainless steels which are used for various types of apparatus for 
manufacturing semiconductors, in which a wafer is finely processed. 
Austenitic stainless steels, in particular, type SUS 316L is mainly used 
as a material for the pipes and the members of such apparatus. 
Japanese Laid-Open Patent Publication No. 161145/1988 discloses 
non-standard high-cleanness austenitic stainless steels which are used for 
steel pipes arranged in a clean room. Non-metallic inclusions are reduced 
by limiting Mn, Si, Al and O (oxygen) contents so as to decrease the 
production of the previously-mentioned particles from the inner surface of 
the pipes. 
Further, Japanese Laid-Open Patent Publication No. 198463/1989 discloses 
stainless steel members for an apparatus used for manufacturing 
semiconductors. These members are produced in such a manner in that 
stainless steel after subjected to electropolishing is heated in an 
oxidizing gas which is under the specific conditions to form thereon an 
oxide layer having a thickness of 100 to 500 angstrom, in which the 
proportion of the number of Ni atoms in the outer part of the layer and 
that of the numbers of Cr atoms in the inner part of the layer are in 
respective predetermined ranges. 
Furthermore, Japanese Laid-Open Patent Publication No. 59524/1993 discloses 
stainless steel members for an ultra-high vacuum apparatus, which are 
obtained by forming a Cr.sub.2 O.sub.3 layer having a thickness of 20 to 
150 angstrom on the surface layer of stainless steel in which Cr and Mo 
contents are in a specific relation. It is described that this layer can 
be obtained, for example, by heating the stainless steel at 250.degree. to 
550.degree. C. under such an atmosphere in that the partial pressure of 
oxygen is 5 Pa (50 ppm) or less. 
It can be expected that non-dusting characteristics under steady state 
conditions, which are indispensable for a stainless steel pipe for 
high-purity gases, are obtained by smoothing the inner surface of the 
pipe, and by reducing non-metallic inclusions as described in Japanese 
Laid-Open Patent Publication No. 161145/1988. However, when pipes or 
members are laid by welding, the welds thereof produce a large amount of 
dust. This is an essential problem for a pipe line for high-purity gases, 
for which the characteristics of producing no dust or only a few dust 
particles are important. 
Regarding the dust which is produced when the pipes or members are welded, 
the particles remaining therein are removed by means of purging after they 
are laid as described previously. However, to purge a complicated pipe 
line in a whole plant creates two problems from the viewpoints of 
decreasing the cost of plant construction and of the necessitating the 
early operation of the plant. These problems cannot be successfully solved 
by the conventionally adopted methods, such as the smoothing of the 
surface of stainless steel, and the simple reduction of non-metallic 
inclusions contained in steel. 
Further, the previously-described corrosion resistance and non-catalytic 
property against speciality gases can be improved by forming a Cr oxide 
layer on the surface of stainless steel. When the method for producing a 
pipe or a member for gases used for manufacturing semiconductors is taken 
into consideration, the treatment for forming a Cr oxide layer should be 
carried out after the surface of the stainless steel which will come into 
contact with a gas is smoothed by means of electropolishing. However, 
since the diffusion of Cr is slow in conventional austenitic stainless 
steel, it is not easy to form on the steel a Cr oxide layer which can 
sufficiently show the above properties even when the steel is subjected to 
the oxidation treatment after it is smoothed by electropolishing. This 
problem cannot be solved even by reducing the amount of non-metallic 
inclusions. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to provide austenitic stainless 
steels used for a pipe line for high-purity gases, which meet the 
non-dusting characteristics required when a pipe line is laid by welding, 
as well as corrosion resistance, abrasion resistance, machinability and 
weldability. Another object of the invention is to provide high Cr 
stainless steels (ferritic stainless steels and duplex stainless steels) 
used for a pipe line for high-purity gases, which can readily form thereon 
a Cr oxide layer having excellent corrosion resistance and non-catalytic 
property after they are smoothed by means of electropolishing. 
The above objects can be attained by the following stainless steels (1) to 
(3) for high-purity gases. 
Amend pages 5 and 6 
(1) Austenitic stainless steel for high-purity gases, characterized by 
comprising 10 to 40% by weight of Ni, 15 to 30% by weight of Cr, 0 to 7% 
by weight of Mo, 0 to 3% by weight of Cu, 0 to 3% by weight of W, 0.005 to 
0.30% by weight of N, 0 to 0.02% by weight of B, 0 to 0.01% by weight of 
Se, and Fe and unavoidable impurities as the remaining part, provided that 
the impurities contain 0.03% by weight or less of C, 0.50% by weight or 
less of Si, 0.20% by weight or less of Mn, 0.01% by weight or less of Al, 
0.02% by weight or less of P, 0.003% by weight or less of S and 0.01% by 
weight or less of 0, and that the Ni-bal. value obtained from the 
following equation &lt;1&gt; is 0 or more and less than 2: 
EQU Ni-bal.=Ni eq.-1.1.times.Cr eq.+8.2 &lt;1&gt; 
where 
Ni eq.(%)=%Ni+%Cu+0.5%Mn+30 (%C+%N) 
Cr eq.(%)=%Cr+1.5%Si+%Mo+%W 
It is desirable that the B and Se contents of this stainless steel be in 
the following respective ranges: 
B: 0.001 to 0.02%; and 
Se: 0.0005 to 0.01%. 
(2) Ferritic stainless steel for high-purity gases, characterized by 
comprising 20 to 30% by weight of Cr, 0.1 to 5% by weight of Mo, 0 to 3% 
by weight of Ni, 0 to 1% by weight of Ti, 0 to 1% by weight of Nb, 0.03% 
by weight or less of N, 0 to 0.5% by weight of Cu, 0.1 to 0.5% by weight 
of W, and Fe and unavoidable impurities as the remaining part, provided 
that the impurities contain 0.03% by weight or less of C, 0.5% by weight 
or less of Si, 0.2% by weight or less of Mn, 0.05% by weight or less of 
Al, 0.02% by weight or less of P, 0.003% by weight or less of S and 0.01% 
by weight or less of 0. 
It is desirable that the Ti, Nb and Cu contents of this stainless steel be 
in the following respective ranges: 
Ti:0.1 to 1%; 
Nb:0.1 to 1%; and 
Cu:0.1 to 0.5% 
(3) Duplex stainless steel for high-purity gases, characterized by 
comprising 4 to 8% by weight of Ni, 20 to 30% by weight of Cr, 0.1 to 5% 
by weight of Mo, 0.1 to 0.3% by weight of N, 0 to 0.5% by weight of Cu, 0 
to 0.5% by weight of W, and Fe and unavoidable impurities as the remaining 
part, provided that the impurities contain 0.03% by weight or less of C, 
0.5% by weight or less of Si, 0.2% by weight or less of Mn, 0.05% by 
weight or less of Al, 0.02% by weight or less of P, 0.003% by weight or 
less of S and 0.01% by weight or less of 0. 
It is desirable that the Cu and W contents of this stainless steel be in 
the following respective ranges: 
Cu, W: both are 0.1 to 0.5%

BEST MODE FOR CARRYING OUT THE INVENTION 
In order to develop pipes for high-purity gases having superior non-dusting 
characteristics by clarifying dusting behavior at the time of welding, the 
inventors of the present invention welded SUS 316L stainless steel pipes 
whose inner surface had been smoothed by means of electropolishing, 
counted the number of particles produced during the welding, and analyzed 
the particles to determine the chemical composition thereof. As a result, 
it became clear that the main component of the particles produced was Mn, 
which is an alloying element of the stainless steel. The reason of this 
fact will be explained by referring to FIG. 1. 
FIG. 1 is a graph showing the relationship between vapor pressure and 
temperature in terms of the main alloying elements of stainless steel (see 
"Handbook of Chemistry", pp. 702-705, Maruzen Co., Ltd., 1975). As shown 
in the graph, the vapor pressure of Mn is remarkably higher than those of 
the other elements in the range of 1400.degree. to 1600.degree. C. in 
which the melting point of SUS stainless steel falls. This graph shows the 
above relationship in terms of the metals which are pure. However, it is 
understood that this tendency can be applied as it is to stainless steel 
when the vapor pressure of the gas phase at the upper part of molten 
stainless steel at the time of welding is considered. It is therefore 
considered that Mn is preferentially evaporated from the molten steel when 
welding is conducted, and cooled and solidified in a shielding gas to 
become a particle. 
Further, the effect of the chemical composition of stainless steel, and 
particularly that of the content of Mn, by which almost all of the 
particles are made, on the amount of dust produced; that is, the number of 
the particles produced were examined. As a result, it was found that when 
Mn content is 0.20% by weight or less, the amount of dust which by welding 
is drastically reduced. In addition, the relationship between weldability 
or machinability and chemical composition was examined. As a result, it 
was found that Se content has an influence on weldability and that N and B 
contents have an influence on machinability. 
Furthermore, in order to develop stainless steels which can readily form 
thereon a Cr oxide layer having high corrosion resistance and excellent 
non-catalytic property, the inventors of the present invention smoothed, 
by means of electropolishing, the inner surface of pipes made of stainless 
steels having various chemical compositions, and subjected the pipes to 
oxidation treatment. The properties, corrosion resistance and 
non-catalytic property of the oxide layers thus obtained were then 
examined. 
As a result, it was found that stainless steels in which Cr level is higher 
and Ni level is lower than those in SUS 316L stainless steel; that is. 
ferritic stainless steel and duplex stainless steel, readily form thereon 
a Cr oxide layer when they are subjected to oxidization treatment after 
smoothed by means of electropolishing, and that the Cr oxide layer offers 
high superiority in both corrosion resistance and non-catalytic property. 
The present invention has been accomplished on the basis of the above 
findings. The reasons why the chemical compositions of the stainless 
steels defined in the present invention, and the Ni-bal. value of the 
austenitic stainless steels of the invention are restricted to the 
previously-mentioned ranges will now be explained. Hereinafter, "%" means 
"% by weight". 
Ni: 
10 to 40% in the austenitic stainless steels; 
0 to 3% in the ferritic stainless steels; and 
4 to 8% in the two-phase stainless steels. 
Ni is an important element for the corrosion resistance and structure 
control of the austenitic stainless steels. In order to maintain and 
stabilize the structure of austenite, and to keep the corrosion resistance 
of the steels, the range of Ni content was restricted to 10 to 40%. When 
Ni content is less than 10%, the structure of austenite cannot be 
stabilized. On the other hand, when Ni content is in excess of 40%, the 
effects of Ni are saturated, and the production cost is also increased; 
such a high Ni content is uneconomical. 
An addition of a small amount of Ni to the ferritic stainless steels is 
effective for improving toughness, so that it is desirable to incorporate 
Ni into the steels, when necessary. In the case where Ni is intentionally 
added to the ferritic stainless steels to obtain this effect, it is 
desirable to make the lowest limit of the amount of Ni added to 0.1%. The 
more preferable amount of Ni is 0.2% or more. On the other hand, when more 
than 3% of Ni is added to the ferritic stainless steels, an extremely 
small amount of austenite is produced therein, and toughness and corrosion 
resistance are thus impaired. 
In order to maintain the corrosion resistance and toughness of the duplex 
stainless steels, it is necessary to control the proportion of austenite 
contained in the whole structure to 40 to 60%. When Ni content is less 
than 4%, the proportion of austenite is insufficient. On the contrary, the 
proportion of austenite becomes excessively high when Ni content exceeds 
8%. Thus,corrosion resistance and toughness are impaired in either cases. 
The preferable range of Ni content is from 5 to 7%. 
Cr: 15 to 30% in the austenitic stainless steels; and 20 to 30% in the 
ferritic stainless steels and in the duplex stainless steels. 
Cr is also, like Ni, an important element for the corrosion resistance and 
structure control of the austenitic stainless steels. The range of the Cr 
content of the austenitic stainless steels was restricted to 15 to 30%. 
When Cr content is less than 15%, even minimum corrosion resistance 
required for stainless steels cannot be obtained. On the other hand, when 
Cr content is in excess of 30%, intermetallic compounds tend to separate 
out, so that hot-workability and mechanical properties are impaired. 
Cr is an important element in high Cr stainless steels. This is because Cr 
improves the corrosion resistance of the steels themselves, and, at the 
same time, makes the steels easily form thereon a Cr oxide layer. For this 
reason, with respect to the ferritic stainless steels and the duplex 
stainless steels, the range of Cr content was fixed to 20 to 30%. When Cr 
content is less than 20%, a Cr oxide layer cannot be satisfactorily 
formed. On the other hand, when Cr content is more than 30%, intermetallic 
compounds tend to separate out, and toughness is thus impaired. The 
preferable range of Cr content is from 24 to 30%. 
Mo: 0 to 7% in the austenitic stainless steels; and 0.1 to 5% in the 
ferritic stainless steels and in the duplex stainless steels. 
Reduction of the amount of dust which is produced when welding is conducted 
is the main purpose of the austenitic stainless steels of the present 
invention. However, corrosion resistance is also one of the important 
properties for the austenitic stainless steels as mentioned previously. 
Therefore, Mo, which has the effect of improving corrosion resistance, may 
be added to the steels within such a range that the other properties such 
as hot-workability and weldability are not marred. In the case where Mo is 
intentionally added to the steels, one or more elements selected from Mo, 
and Cu and W, which will be described later, are added. In order to obtain 
the above effect, it is desirable to make the lowest limit of Mo content 
to 0.1%. When Mo content is in excess of 7%, the effect of improving 
corrosion resistance is saturated. 
Amend page 10 
In the case of the high Cr stainless steels of the present invention, Mo is 
added in order to improve corrosion resistance to corrosive gases. When Mo 
content is less than 0.1%, this effect cannot be obtained. On the other 
hand, when Mo content is in excess of 5%, intermetallic compounds separate 
out, and toughness is impaired. The preferable range of Mo content is from 
1 to 4%. 
Cu, W: both Cu and W are 0 to 3% in the austenitic stainless steel; Cu is 0 
to 0.5% and W is 0.1 to 0.5% in the ferritic stainless steels and both of 
them are 0 to 0.5% in the duplex stainless steels. 
As mentioned above, corrosion resistance is also one of the important 
properties for the austenitic stainless steels which require non-dusting 
characteristics. Cu and W are elements which have, like Mo, the effect of 
improving corrosion resistance. Therefore, they may be added to the 
austenitic stainless steels within such a range that the other properties 
such as hot-workability and weldability are not marred. In the case where 
Cu or W is intentionally added, one or more elements selected from Mo, Cu 
and W are incorporated into the steels. In this case, it is desirable to 
make both the lowest limit of Cu content and that of W content to 0.1% in 
order to obtain the above effect. When both Cu and W contents are in 
excess of 3%, the effect of improving corrosion resistance is saturated. 
In the ferritic stainless steel according to the present invention, it is 
preferred to use W as the essential ingredient for ensuring corrosion 
resistance and use Cu as necessary. When the W content is less than 0.1%, 
the effect of improving corrosion resistance can not be obtained and the 
effect is saturated when it exceeds 0.5%, so that the content is defined 
as 0. 1 to 0.5%. If Cu is added intentionally, the content is preferably 
from 0.1 to 0.5%. 
In the duplex stainless steel, since Cu and W improve corrosion resistance, 
one or both of them may be used preferably as necessary. In a case of 
intentional addition for obtaining the effect, the lower limit for the 
content is preferably 0.1% for each of them. On the other hand, if each of 
them exceeds 0.5%, the effect described above is saturated. 
C: 0.03% or less. 
C makes Cr carbide separate out at a weld to impair corrosion resistance, 
so that it is necessary to reduce C content. C content was therefore 
restricted to 0.03% or less in consideration of the use of the steels of 
the present invention for strongly-corrosive gases. The preferable range 
of C content is 0.02% or less. 
Si: 0.50% or less. 
Although Si has the action of deoxidizing steels to purify the steels, it 
also produces, at the same time, oxide inclusions. When Si content is in 
excess of 0.50%, the inclusions become large, and non-dusting 
characteristics under steady state conditions are particularly impaired. 
It is therefore necessary to reduce Si content. For this reason, Si 
content was restricted to 0.50% or less. The desirable range of Si content 
is 0.1% or less in the case of the austenitic stainless steels which are 
required to have non-dusting characteristics, and 0.2% or less in the case 
of the high Cr stainless steels. 
Mn: 0.20% or less. 
Mn has, like Si, the action of deoxidizing steels to purify the steels. 
However, it is the most harmful element for non-dusting characteristics 
required when welding is conducted. When Mn content is in excess of 0.2%, 
the amount of dust which is produced by welding is drastically increased. 
For this reason, Mn content was restricted to 0.2% or less. The desirable 
range of Mn content is 0.1% or less. 
Al: 0.01% or less in the austenitic stainless steels; and 0.05% or less in 
the ferritic stainless steels and in the duplex stainless steels. 
Al also has, like Si, the action of deoxidizing steels to purify the 
steels. However, Al produces oxide inclusions, and cause these oxide 
inclusions to become enlarged. Further, Al is oxidized much more easily 
than the other alloying elements, so that Al on the molten metal surface 
of pipes is reacted, when the pipes are welded, with an extremely small 
amount of oxygen present in the atmosphere in the pipes, whereby Al oxide 
is produced. Dust is produced due to either of these reasons. It is 
therefore necessary to reduce Al content in the case of the austenitic 
stainless steels. For this reason, the Al content of the austenitic 
stainless steels was restricted to 0.01% or less, and that of the high Cr 
stainless steels was restricted to 0.05% or less. The preferable range of 
Al content is 0.01% or less. 
P: 0.02% or less. 
P is harmful for hot-workability, so that it is necessary to reduce P 
content. However, it is difficult to reduce P content to extremely low 
level from the viewpoint of steel making. Further, a material in which P 
level is low and which is needed to produce stainless steel whose P 
content is extremely low is expensive. Therefore, it is not economical to 
reduce P content to excessively low level. For this reason, it is 
desirable to make P content to such a level that does not adversely affect 
the properties of the steels. The range of P content was thus restricted 
to 0.02% or less. 
S: 0.003% or less. 
S produces sulfide inclusions even when the amount thereof is extremely 
small, and therefore impairing corrosion resistance. It is necessary to 
reduce S content. The range of S content was restricted to 0.003% or less 
so as not to impair corrosion resistance and economical efficiency. The 
desirable range of S content is 0.002% or less. 
O (oxygen): 0.01% or less. 
O is an element which produces oxide inclusions in steels, so that it is 
necessary to reduce O as much as possible. The oxide inclusions are 
agglomerated and become large at a weld when welding is conducted. In 
order to reduce the amount of dust particles during the weld, the range of 
O content in the steel was restricted to 0.01% or less so as not to 
adversely affect non-dusting characteristics. The preferable range of O 
content is 0.005% or less. 
N alone or N and B in combination is incorporated into the austenitic 
stainless steels of the present invention. Further, N content is 
suppressed as much as possible in the ferritic stainless steels, whereas N 
is incorporated into the duplex stainless steels. 
N: 0.005 to 0.30% in the austenitic stainless steel, 0.03% or less in the 
ferritic stainless steel and 0.1 to 0.3% the duplex stainless steel. 
In the austenitic stainless steels, N is an element contained inevitably in 
the steel. However, N acts as an alloying element having an effect of 
enhancing strength, hardness and corrosion resistance. In the austenitic 
stainless steel according to the present invention, since C, Si, Mn, P, S 
and O are elements having the dust enhancing effect are reduced as 
described above, hardness is lowered as compared with general stainless 
steels. Decrease in hardness is not a great problem for stainless steel 
pipes for high-purity gases. However, in the case of the pipeline parts 
having a slidable portion on a gas sealing surface such as various types 
of valves, it is necessary to increase hardness in order to improve the 
abrasion resistance of the slidable portion. For such a purpose, it is 
effective to increase hardness by addition of N. 
When the N content of the austenitic stainless steels is less than 0.005%, 
the above-described effect of increasing hardness can not be obtained. On 
the other hand, when it is more than 0.30%, it separates out as nitride 
and corrosion resistance is impaired. Therefore, the range of N content is 
0.005 to 0.30%. The desirable range is 0.1 to 0.25%. 
In the case of ferritic stainless steels, even if an extremely small amount 
of N is added to the steels, Cr nitride is produced, and toughness is 
impaired. In order to prevent the decrease in toughness, it is necessary 
to control N content to 0.03% or less. The preferable range of N content 
is 0.01% or less. 
In the case of the duplex stainless steels, N and the austenite phase form 
a solid solution to improve corrosion resistance. When N content is less 
than 0.1%, this effect cannot be obtained. On the other hand, when N 
content is in excess of 0.3%, Cr nitride is produced, and toughness is 
thus impaired. The preferable range of N content is from 0.15 to 0.3%. 
B: 0 to 0.02% in the austenitic stainless steels. 
B is an element which produces nitride. When B (in addition to the 
above-described N) is added to the austenitic stainless steels, not only 
hardness but also machinability is improved. This is because extremely 
fine nitride, BN, separates out to improve the crushability of shavings. 
In order to obtain this effect, it is necessary that N content be in the 
range of 0.01 to 0.30% and that B content be 0.001% or more. On the other 
hand, when B content is in excess of 0.02%, nitride separates out 
excessively so that corrosion resistance is impaired. For this reason, the 
range of B content was restricted to 0.001 to 0.02%. The desirable range 
of B content is from 0.005 to 0.01%. 
It is possible to further incorporate Se into the austenitic stainless 
steels of the present invention. 
Se: 0 to 0.01% in the austenitic stainless steels. 
Since Se has the effect of improving arc stability required in arc welding 
which is ordinarily conducted, and the effect of suppressing the change in 
shape of molten metals, Se is added to the austenitic stainless steels, 
when necessary. In the case where Se is intentionally added to the steels, 
the above effects cannot be obtained when Se content is less than 0.0005%. 
On the other hand, when Se content is in excess of 0.01%, non-metallic 
inclusions are formed, and corrosion resistance is thus impaired. For this 
reason, the range of Se content was restricted to 0.0005 to 0.01%. The 
desirable range of Se content is from 0.001 to 0.005%. 
One or both of Ti and Nb can be further incorporated into the ferritic 
stainless steels of the present invention, when necessary. 
Ti, Nb: both are 0 to 1% in the ferritic stainless steels. 
In order to stabilize C and N which produce Cr precipitates, it is 
effective to add Ti and/or Nb, which produces stable carbon nitride, to 
the ferritic stainless steels. It is therefore desirable to add Ti and/or 
Nb, when necessary. When they are intentionally added to the steels to 
obtain the above effect, it is desirable to make both the lowest limit of 
Ti content and that of Nb content to 0.1%. On the other hand, when both Ti 
and Nb contents are in excess of 1%, the above effect is saturated. The 
more preferable range of Ti content and that of Nb content are from 0.2 to 
0.5%. 
The austenitic stainless steels of the present invention is further defined 
by the Ni-bal. value which is obtained from the previously-given equation 
&lt;1&gt;. 
Ni-bal. value: 0 or more and less than 2. 
When the Ni-bal. value is less than 0, the structure of austenite cannot be 
stably obtained, and only such a structure that contains a ferrite phase 
is obtained. Mechanical properties and corrosion resistance are thus 
impaired. On the other hand, when this value is 2 or more, hot-workability 
is impaired. When steel ingots are produced on a small laboratory scale, 
trouble will not occur even if hot-workability is poor. However, when the 
steel ingots are mass-produced on a commercial scale, these ingots tend to 
crack during forging and rolling processes. For this reason, the Ni-bal. 
value which is calculated from the contents of the alloying elements of 
the steels of the present invention was restricted to 0 or more and less 
than 2. 
The effects of the stainless steels for high-purity gases of the present 
invention will now be explained by referring to the following examples, 
that is, Test 1 to Test 3. 
Test 1 
The inner surface of seamless pipes having an outer diameter of 6.4 mm, a 
thickness of 1 mm and a length of 4 m, made of SUS 316L stainless steels 
having a chemical composition shown in FIG. 2 was smoothed by means of 
electropolishing until the R.sub.max of the surface became 0.7 micron or 
less. Thereafter, the inner surface of the pipes was washed with 
high-purity water, and dried by allowing 99.999% Ar gas to run through the 
pipes at 120.degree. C. The pipes made of a steel of the same type were 
welded by an automatic welder without conducting edge preparation under 
the conditions shown in FIG. 3 so that the weld, that is, the weld bead 
would come on the inner surface of the pipe. Ar shielding gas which was 
allowed to run through the pipe during this welding was introduced to a 
particle counter at the downstream side of the weld to count the number of 
particles produced. The amount of dust produced was evaluated in such a 
manner. 
Further, the above Ar shielding gas was directly blown into 1 mol/l 
hydrochloric acid. The concentrations of the metals in the hydrochloric 
acid were then measured, thereby determining the composition of the 
particles. 
The number of particles produced, the results of the composition analysis, 
and the hardnesses of the pipes made of the steels of the present 
invention at the central part thereof (the part not affected by the 
welding) are shown in FIG. 4. 
The results shown in FIG. 4 demonstrate that the austenitic stainless 
steels having a chemical composition defined in the present invention 
produce a minute amount of dust when the steels are welded. This effect is 
obtained due to the reduced Mn and Al contents of the steels. Further, 
those steels of the present invention which contain N have hardness 17-56% 
higher than those of the other steels. 
Test 2 
Stainless steels having a chemical composition shown in FIGS. 5 and 6 were 
produced in a vacuum induction heating furnace, and processed into pipes 
and plates by means of hot processing and cold processing. Thereafter, the 
pipes and the plates were treated at 1000.degree. C. under H.sub.2 gas 
atmosphere so as to form solid solutions. 
The steel pipes obtained were subjected to electropolishing, and then tests 
for evaluating the corrosion resistance and abrasion resistance thereof 
were carried out. Further, after the polished pipes were welded, the 
number of particles produced from the inner surface of the pipes were 
counted; the particles were subjected to composition analysis; a 
weldability test was carried out; and machinability was tested by using 
the plates obtained. 
The conditions of the electropolishing and those of the welding, the method 
for counting the number of the particles produced and that of the 
composition analysis of the particles, and the conditions such as the 
dimension of the steel pipes used are the same as those in Test 1. 
A corrosion resistance test was carried out as follows: The pipe after 
being subjected to electropolishing was cut lengthwise in half, and a 
filter paper impregnated with an aqueous ferric chloride solution was 
stuck to the inner surface of the pipe. This was preserved at 25.degree. 
C. for 6 hours, and the inner surface of the pipe was then observed as to 
whether corrosion occurred or not. The test was carried out by changing 
the concentration of the aqueous ferric chloride solution, and corrosion 
resistance was evaluated by the critical concentration of the solution for 
pitting. Abrasion resistance was evaluated by the Vickers hardness of the 
cross-section of the pipe which had been subjected to electropolishing. 
Weldability was evaluated in the following manner: The pipes after being 
subjected to electropolishing were welded at the circumference thereof 
under the same conditions as in Test 1. The weld was cut lengthwise in 
half, and the width of the bead on the inner surface of the pipe was 
measured. Weldability was evaluated by the variation of the width in the 
circumferential direction. 
Machinability was evaluated as follows: The plate material having a 
thickness of 9 mm was bored by using a drill under the conditions shown in 
FIG. 7. Machinability was evaluated by the number of bores which were 
obtainable by using one drill. The results of the above tests are shown in 
FIGS. 8 and 9. 
The results shown in FIGS. 8 and 9 clearly demonstrate that the austenitic 
stainless steels having a chemical composition defined in the present 
invention produce only a minute amount of dust when they are welded. This 
effect is obtained due to the reduced Mn, Al, Si and O contents of the 
steels. It is clear that the austenitic stainless steels of the present 
invention are also superior in corrosion resistance, abrasion resistance 
and machinability. 
Test 3 
Stainless steels having a chemical composition shown in Table 10 were 
produced. They were subjected to hot extrusion, and then processed into 
seamless steel pipes having an outer diameter of 6.4 mm, a thickness of 1 
mm, and a length of 1 m by cold rolling and cold drawing. 
The inner surface of the pipes obtained was smoothed by means of 
electropolishing to make the R.sub.max of the surface to 0.7 micron or 
less, washed with high-purity water, and then dried by allowing 99.999% Ar 
gas to run through the pipe at 120.degree. C. The steel pipes finally 
obtained were subjected to oxidation treatment under the following 
conditions to form an oxide layer thereon. 
Conditions of oxidation treatment: Preserved at 550.degree. C. for 3 hours 
in the stream of Ar gas containing 10% of hydrogen and 100 ppm of water 
vapor. 
After the oxidation treatment was carried out, the thickness of the oxide 
layer and the Cr concentration in the oxide layer were measured, and the 
water-discharging property, corrosion resistance and catalytic property of 
the inner surface of the pipes were examined to totally evaluate the 
pipes. 
The Cr oxide layer was evaluated in the following manner: The pipe was cut 
lengthwise in half, and the distribution of elements in the direction of 
the depth of the inner surface of the pipe was determined by a secondary 
ion mass spectrometer. The maximum Cr concentration in all metal elements 
contained in the oxide layer, and the thickness of a Cr rich portion of 
the oxide layer were obtained. 
Water-discharging property was evaluated in the following manner: The pipe 
after being subjected to the oxidation treatment was allowed to stand for 
24 hours in a laboratory where the humidity was 50%. While high-purity Ar 
gas containing less than 1 ppb of water was being allowed to run through 
the pipe at a rate of 1 liter/min, the concentration of vapor in the gas 
was measured at the output end of the pipe by an atmospheric pressure 
ionization mass spectrometer. Water-discharging property was evaluated by 
the time required for the vapor concentration to become 1 ppb from the 
beginning of the measurement. 
Corrosion resistance was evaluated in the following manner: 5 atoms of 
hydrogen bromide gas was charged in the pipe which had been subjected to 
the oxidation treatment, and the pipe was sealed. This pipe was preserved 
at 80.degree. C. for 100 hours. Thereafter, the inner surface of the pipe 
was observed by a scanning electron microscope as to whether the surface 
underwent any change. 
Catalytic property was evaluated as follows: Ar gas containing 100 ppm of 
monosilane (SiH.sub.4) was allowed to run through the pipe which had been 
subjected to the oxidation treatment, by changing the temperature of the 
pipe. The concentration of H.sub.2 generated by the decomposition of the 
monosilane was measured at the output end of the pipe by gas 
chromatography. Catalytic property was evaluated by the minimum 
decomposition temperature of the monosilane. The results of the above 
tests are shown in FIG. 11. 
The results shown in FIG. 11 clearly demonstrate that the oxide layers 
formed by subjecting the ferritic or duplex stainless steels of the 
present invention to oxidation treatment have a high Cr concentration and 
are thick and that such oxide layers are useful for improving the 
water-discharging property and the non-catalytic property, as well as the 
corrosion resistance. 
Industrial Applicability 
The austenitic stainless steels of the present invention are steels which 
have decreased Mn, Al, Si and O contents and which meet the non-dusting 
characteristics required at the time of welding. In addition, corrosion 
resistance, abrasion resistance and machinability are more improved. The 
ferritic and duplex stainless steels of the present invention are steels 
which can readily form thereon a Cr oxide layer having superior corrosion 
resistance and non-catalytic property when they are subjected to oxidation 
treatment. Therefore, all of the steels of the present invention are 
suitable as stainless steels for high-purity gases used for apparatus for 
manufacturing semiconductors or liquid crystals, and can thus be utilized 
in the field of the manufacturing of semiconductors or liquid crystals.