Nickel-chromium stainless steel having improved corrosion resistances and machinability

A nickel-chromium stainless steel having improved corrosion resistances and machinability, comprising less than 0.08 wt. % of C, less than 1.0 wt. % of Si, less than 0.7 wt. % of Mn, less than 0.04 wt. % of P, less than 0.005 wt. % of S, 8.0 to 12.0 wt. % of Ni, 17.0 to 20.0 wt. % of Cr, 0.40 to 0.80 wt. % of Mo, less than 0.3 wt. % of Cu, 0.03 to 0.5 wt. % of Sn and the balance of Fe. In another embodiment of the present invention, 0.03 to 0.1 wt. % of Bi is added to the above-described composition to further improve the machinability of the stainless steel.

BACKGROUND OF THE INVENTION 
The present invention relates to a nickel-chromium stainless steel 
comprising nickel-chromium SUS 304 stainless steel as the base, having 
improved corrosion resistances and machinability and usable particularly 
as a preferred material for food machines. 
The chemical composition of the SUS 304 steel as stipulated by JIS is as 
shown in Table 1. 
TABLE 1 
______________________________________ 
(wt. %) 
C Si Mn P S Ni Cr 
______________________________________ 
&lt;0.08 &lt;1.0 &lt;2.0 &lt;0.04 &lt;0.03 8.0.about.10.5 
18.0.about.20.0 
______________________________________ 
Although the SUS 304 steel is widely used as a corrosion-resistant 
material, its machinability is quite poor. When a free cutting property is 
required, usually a sulfide inclusion such as MnS was formed purposely at 
the considerable sacrifice of the corrosion resistance in the prior art. 
However, it was reported that when the corrosion resistance was regarded 
as particularly important to make the SUS 304 steel resistant to a strong 
corrosion environment (for example, a chloride environment or acidic drink 
environment), it was effective to reduce the ratio of manganese to sulfur 
(Mn/S) which were the main constituents of MnS in the steel and to 
increase the amount of chromium dissolved in MnS [see "TETSU-TO-HAGANE 
(The Journal of the Iron & Steel Inst. of Japan)", 70 (1984), P. 741]. 
A solid solution treatment at high temperature has been also known to 
improve the corrosion resistance. For example, when the SUS 304 steel as 
shown in Table 1 is maintained at about 1300.degree. to 1400.degree. C. 
for about 1 to 60 minutes and then quenched, the resulting steel exhibits 
the improved corrosion resistance as compared with the SUS 304 steel 
treated by a conventional heat treatment at 1050.degree. C. [see 
Preconference Text of "33rd Conference concerning Corrosion and 
Anticorrosion" (held on Oct. 15-17, 1986 at Naha, Okinawa, Japan), P. 
119-122(1986)]. 
Though the machinability can be improved to some extent without reducing 
the corrosion resistance by suitably balancing the formation of MnS with 
the reduction of the Mn/S or by the solid solution treatment at high 
temperature, the improvement is yet insufficient. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a nickel-chromium 
stainless steel comprising SUS 304 stainless steel as the base and having 
further improved corrosion resistances and machinability. 
The first embodiment of the nickel-chromium stainless steel of the present 
invention having improved corrosion resistances and machinability has a 
partially modified chemical composition of SUS 304 stainless steel, and 
comprises less than 0.08 wt. % of C, less than 1.0 wt. % of Si, less than 
0.7 wt. % of Mn, less than 0.04 wt. % of P, less than 0.005 wt. % of S, 
8.0 to 12.0 wt. % of Ni, 17.0 to 20.0 wt. % of Cr, 0.40 to 0.80 wt. % of 
Mo, less than 0.3 wt. % of Cu, 0.03 to 0.5 wt. % of Sn and the balance of 
Fe. 
The second embodiment of the nickel-chromium stainless steel of the present 
invention having improved corrosion resistances and machinability 
comprises the stainless steel of the above-mentioned first embodiment plus 
bismuth which is an element capable of improving the machinability. The 
chemical composition of this steel comprises less than 0.08 wt. % of C, 
less than 1.0 wt. % of Si, less than 0.7 wt. % of Mn, less than 0.04 wt. % 
of P, less than 0.005 wt. % of S, 8.0 to 12.0 wt. % of Ni, 17.0 to 20.0 
wt. % of Cr, 0.40 to 0.80 wt. % of Mo, less than 0.3 wt. % of Cu, 0.03 to 
0.1 wt. % of Bi, 0.03 to 0.2 wt. % of Sn and the balance of Fe. 
The above-mentioned and other objects of the present invention will be 
obvious from the following description made with reference to the 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Tin added to the stainless steel in the first embodiment of the present 
invention serves to improve not only the machinability but also the 
overall corrosion resistance and crevice corrosion resistance. 
Particularly in a dilute aqueous sulfuric acid solution, tin deposits on 
the surface of the steel to increase the hydrogen overvoltage and to 
improve the resistance to sulfuric acid (overall corrosion resistance). 
When the amount of tin is less than 0.03 wt. %, the above-mentioned 
effects of improving the corrosion resistance cannot be exhibited. On the 
other hand, when it exceeds 0.5 wt. %, the forging property of the steel 
is reduced and the effect of improving the corrosion resistance is not so 
much increased. 
Although molybdenum and copper are effective in improving both the overall 
and crevice corrosion resistances, the resistance to corrosion by organic 
acids might be reduced when the amount of copper is excessive. However, 
the corrosion resistance for organic acid can be increased by controlling 
the amount of copper to less than 0.3 wt. % as in the present invention. 
The optimum amount of molybdenum is 0.40 to 0.80 wt. %, since when the 
amount is less than 0.40 wt. %, no corrosion resistance can be exhibited 
and, on the contrary, even when it exceeds 0.80 wt. %, the effect of 
improving the corrosion resistance is no more significantly increased and 
the cost is increased. 
Although the corrosion resistance can be improved by reducing the amounts 
of sulfur and manganese as described above, the machinability is reduced. 
In the present invention, the corrosion resistance can be improved by 
limiting the amounts of sulfur and manganese to less than 0.005 wt. % and 
less than 0.7 wt. %, respectively, while the reduced machinability can be 
made up by the addition of tin. 
Nickel is an essential element of an austenitic (.gamma.) stainless steel 
to make the .gamma.-phase stable. In an aspect of the strength, nickel 
contributes to an improvement of the toughness. When the nickel content is 
insufficient, the .gamma.-phase becomes unstable and martensite is formed 
by the processing to harden the steel and reduce the toughness thereof. 
Since nickel is electrochemically nobler than iron and chromium, it 
inhibits the corrosion in an active region. Further nickel imparts to the 
steel a remarkable resistance to the corrosion by a solution of a neutral 
chloride or a non-oxidizing acid and it also intensifies the passivity. In 
the present invention, nickel is used in a content larger than the 
standard nickel content of the SUS 304 steel to stabilize the 
.gamma.-phase, since tin which is a ferrite-forming element is contained 
therein. 
Chromium is an essential component of the stainless steel. It contributes 
to the passivation of the stainless steel under an oxidizing atmosphere. 
Namely, the corrosion resistance of the stainless steel is maintained by 
the passive film. Chromium is thus an indispensable element of the 
stainless steel. 
In the second embodiment of the present invention, the machinability is 
improved by further adding bismuth to the steel composition of the first 
embodiment. The amount of bismuth used in the preesent invention is in the 
range of 0.03 to 0.1 wt. %, since any effect of improving the 
machinability cannot be obtained with less than 0.03 wt. % and, on the 
contrary, the forging property is reduced with more than 0.1 wt. % thereof 
to cause the pitting corrosion and also to affect the corrosion 
resistances. The stainless steel of the second embodiment of the present 
invention containing both bismuth and tin has more improved overall 
corrosion resistance and crevice corrosion resistance than those of the 
stainless steel containing only bismuth. In the stainless steel of the 
second embodiment, the amount of tin is 0.03 to 0.2 wt. %, since it 
contains also bismuth. 
Other alloying elements in the present invention, i.e. carbon, silicon and 
phosphorus, can be used in amounts as stipulated by JIS (SUS 304). 
EXAMPLE 1 
Steel samples 2 to 6 (tin-containing steel samples) of the first embodiment 
of the present invention and comparative steel sample 1 (tin-free steel 
sample) having the chemical compositions shown in Table 2 were prepared. 
The samples 2 to 6 had various tin contents. The copper content of only 
the sample 4 was less than 0.3 wt. % (0.28 wt. %) and those of other 
samples 2, 3, 5 and 6 were less than 0.02 wt. %. The components other than 
tin and copper were contained substantially in the same amounts in all of 
the samples. The tin-free comparative sample 1 includes less than 0.02 wt. 
% of copper and substantially the same amounts of the same components as 
in the samples 2 to 6. 
TABLE 2 
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(wt. %) 
C Si Mn P S Ni Cr Mo Cu Sn 
__________________________________________________________________________ 
Sample 1 
0.009 
0.48 
0.28 
0.001 
0.002 
11.02 
19.04 
0.64 
0.01 
0.00 
Sample 2 
0.003 
0.47 
0.27 
0.001 
0.002 
11.08 
18.98 
0.65 
0.01 
0.04 
Sample 3 
0.006 
0.48 
0.27 
0.000 
0.002 
11.12 
18.84 
0.65 
0.00 
0.08 
Sample 4 
0.004 
0.48 
0.31 
0.001 
0.002 
11.13 
18.79 
0.69 
0.28 
0.10 
Sample 5 
0.003 
0.48 
0.27 
0.001 
0.002 
11.09 
18.66 
0.70 
0.00 
0.20 
Sample 6 
0.003 
0.48 
0.26 
0.001 
0.002 
11.14 
18.20 
0.70 
0.00 
0.50 
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(1) Improvement of overall corrosion resistance 
FIG. 1 shows the corrosion rates of the samples 1 to 6 in dilute 
hydrochloric acid (boiling 0.5% and 0.8% hydrochloric acid solution). 
Generally the tin-containing steel samples had a lower corrosion rate and 
more excellent corrosion resistance than those of the tin-free steel 
sample. Particularly, the samples 3 and 4 were not corroded in 0.5% 
hydrochloric acid. 
FIG. 2 shows the corrosion rates of the samples 1 to 6 in dilute sulfuric 
acid (boiling 5% sulfuric acid) and in a solution of lactic acid plus 
common salt (boiling solution containing 50% of lactic acid and 1% of 
common salt). In the dilute sulfuric acid, the tin-containing steel 
samples had a far lower corrosion rate and far more excellent corrosion 
resistance than those of the tin-free steel sample. Also in the solution 
of lactic acid plus common salt, the tin-containing steel samples had a 
more excellent corrosion resistance, though the difference between it and 
that of the tin-free steel sample was not so great unlike in the dilute 
sulfuric acid. 
FIG. 3 shows the anodic polarization curves of the samples 1 and 6 in a 
solution of dilute sulfuric acid plus common salt (a solution containing 
5% of sulfuric acid and 1% of common salt at 30.degree. C.). The critical 
current density for passivation (i.sub.crit) as shown by .circle.1 and 
.circle.2 in FIG. 3 was lower and an active region was narrower in the 
sample 6 (tin containing steel sample). This fact suggests that the 
corrosion resistance in the solution of dilute sulfuric acid plus common 
salt is improved by the addition of tin. 
(2) Evaluation based on the eluting amounts of iron and chromium 
FIG. 4 shows the results of the determination of the amounts of iron and 
chromium eluted out when immersing the samples 1 to 6 in a solution of 
lactic acid plus common salt (10% lactic acid and 0.3% common salt) at 
40.degree. C. for 55 days. The amount of iron eluted from the sample 1 was 
50 ppm, while that eluted from the tin-containing steel samples was as 
small as less than a half of that of the sample 1. As for the eluting 
amount of chromium, the similar inclination as that of the iron was 
observed, though the eluting amounts of chromium were various. It is thus 
apparent also from the eluting amounts of iron and chromium that the 
tin-containing steel samples had improved corrosion resistances. 
(b 3) Improvement in crevice corrosion resistance 
FIG. 5 shows the results of measurement of the repassivation potentials 
(E.sub.R) of the samples 1, 2, 4 and 6 in a solution of common salt (a 3% 
solution at 30.degree. C.). Usually the higher the value of E.sub.R, the 
easier the repassivation of the sample after the crevice corrosion. 
Namely, the higher the value of E.sub.R, the easier the termination of the 
crevice corrision. In FIG. 5, the E.sub.R of the tin-containing steel 
samples was higher than that of the sample 1 to indicate that tin serves 
to improve the crevice corrosion resistance. 
(4) Improvement in machinability 
FIG. 6 shows the life of a high-speed steel tool SKH-51 (4 mm.phi.) used 
for drilling the steel samples of the present invention. The life of the 
tool used for drilling the tin-containing steel samples was at least twice 
as long as that used for drilling the tin-free steel sample. The larger 
the tin content, the longer the tool life. It is apparent, therefore, that 
the machinability is improved by the addition of tin. 
EXAMPLE 2 
A steel sample 8 (steel sample containing both bismuth and tin) of the 
second embodiment of the present invention and a comparative steel sample 
7 (steel sample containing only bismuth) having the chemical compositions 
shown in Table 3 were prepared. 
TABLE 3 
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(wt. %) 
C Si Mn P S Ni Cr Mo Cu Bi Sn 
__________________________________________________________________________ 
Sample 7 
0.03 
0.50 
0.30 
0.003 
0.001 
10.97 
18.85 
0.70 
0.01 
0.08 
0.00 
Sample 8 
0.03 
0.46 
0.30 
0.003 
0.001 
10.99 
18.92 
0.70 
0.01 
0.08 
0.10 
__________________________________________________________________________ 
(1) Improvement of overall corrosion resistance 
FIG. 7 shows the corrosion rates of the samples 7 and 8 in dilute 
hydrochloric acid (boiling 0.8% hydrochloric acid). The corrosion rate of 
the steel sample containing both bismuth and tin was less than 1/3 of that 
of the steel sample containing only bismuth to suggest that the former had 
an improved corrosion resistance. 
FIG. 8 shows the corrosion rates of the samples 7 and 8 in dilute sulfuric 
acid (boiling 5% sulfuric acid ) and in a solution of lactic acid plus 
common salt (boiling solution containing 50% of lactic acid and 1% of 
common salt). In both solutions, the corrosion rate of the steel sample 
containing both bismuth and tin was lower than that of the sample 
containing only bismuth to suggest that the former had an improved 
corrosion resistance. 
(2) Evaluation based on the eluting amounts of iron and chromium 
FIG. 9 shows the results of the detemrination of the amounts of iron and 
chromium eluted when immersing the samples 7 and 8 in a solution of lactic 
acid plus common salt (10% lactic acid and 0.3% common salt) at 40.degree. 
C. for 55 days. The amount of iron eluted from the steel sample 7 was more 
than 150 ppm, while that eluted from the steel sample 8 was less than 1/10 
of that of the sample 7. Also the amount of chromium eluted from the 
sample 8 was about 1/10 of that eluted from the sample 7. Thus, it is 
apparent from the eluting amounts of iron and chromium that the steel 
containing both bismuth and tin had a remarkably improved corrosion 
resistance. 
(3) Improvement in pitting corrosion resistance 
FIG. 10 shows the results of the determination of the pitting potential 
(V'c100) of the samples 7 and 8 in a solution of common salt (a 3% 
solution at 30.degree. C.). Usually the higher the value of V'c100, the 
more difficult the occurrence of the pitting corrosion. The V'c100 of the 
steel sample containing both bismuth and tin was nobler than that of the 
sample containing only bismuth by 50 mV on average. It is apparent, 
therefore, that the addition of both bismuth and tin improves the pitting 
corrosion resistance. 
(4) Improvement in crevice corrosion resistance 
FIG. 11 shows the results of the measurement of the repassivation potential 
(E.sub.R) of the samples 7 and 8 in a solution of common salt (a 3% 
solution at 30.degree. C.). Usually the higher the value of E.sub.R, the 
easier the termination of the crevice corrosion. It is apparent from FIG. 
11 that the E.sub.R of the steel sample containing both bismuth and tin 
was higher than that of the steel sample containing only bismuth and that 
the addition of both bismuth and tin served to improve the crevice 
corrosion resistance. 
(5) Improvement in machinability 
FIG. 12 shows the life of a high-speed steel tool SKH-51 (4 mm.phi.) used 
for drilling the samples 7 and 8. The life of the tool used for drilling 
the sample containing both bismuth and tin was longer than that used for 
drilling the sample containing only bismuth. It is apparent, therefore, 
that the improvement in the machinbility realized by the addition of both 
bismuth and tin was more remarkable than that realized by the addition of 
only bismuth. 
As described above, the stainless steel of the present invention is 
remarkably superior in corrosion resistances and machinability to SUS 304 
stainless steel. This stainless steel can be particularly preferably used 
as a material for food machines which requires excellent corrosion 
resistances. 
While the invention has been described with respect to preferred 
embodients, it should be apparent to those skilled in the art that 
numerous modifications may be made thereto without departing from the 
scope of the invention.