Pitting corrosion resistant austenite stainless steel

Abstract

A pitting corrosion resistant austenite stainless steel comprising not more than 0.08% by weight of carbon, not more than 4% by weight of silicon, not more than 5.0% by weight of manganese, not more than 0.04% by weight of phosphorus, not more than 0.03% by weight of sulfur, 10 to 18% by weight of nickel, 23 to 30% by weight of chromium and 0.30 to 0.45% by weight of nitrogen, the balance being iron and unavoidable impurities, and satisfying the condition of Ni % + 30 (C % + N %) ≧ 20%.

Description

The present invention relates to an austenite stainless steel having excellent resistance against pitting corrosion.

One object of the present invention is to provide a stainless steel having excellent resistance against pitting corrosion as well as good hot and cold workability.

Another object of the present invention is to provide a stainless steel having excellent resistance against pitting corrosion as well as good weldability.

Still another object of the present invention is to provide a stainless steel having excellent resistance against pitting corrosion as well as high strength and toughness.

Pitting corrosion is one of the three remarkable defects of a stainless steel; the other two defects are stress corrosion cracking and grain boundary attack. There are available appropriate counter-measures against stress corrosion cracking and grain boundary attack; for example, stress relieving annealing for the former and stabilization by Nb and Ti or extremely low carbon content for the latter. But there is no effective counter-measure against pitting corrosion.

Many accidents due to pitting corrosion have been experienced in chemical plants and equipments, and demand for pitting corrosion resistant stainless steel is increasing and will increase as the ocean development progresses.

As metal materials which show strong resistance against pitting corrosion in the sea water, we can mention titanium and Hostelloy C (Ni: 55%, Cr: 16%, Mo: 16% and W: 4%). However, these materials are very precious and prohibited to be used in chemical equipments and sea applicances from the point of economy, and are rarely used in practical application.

Pitting corrosion by of chromium and nitrogen. Although nitrogen shows some improvement of pitting corrosion resistance with 18% chromium content, the corrosion rate does not decrease below a certain value even when nitrogen is increased and no effective prevention against pitting corrosion can be assured. However, when the chromium content reaches 25%, the effects on the improvement of pitting corrosion resistance of nitrogen become very remarkable, and as shown in FIG. 4 substantially no pitting corrosion is observed even by a more severe testing method when the nitrogen content exceeds 0.30%. For this reason, the lower limit of nitrogen content is set as at nd∅30% in the present invention.

From the point of pitting corrosion resistance, an increased amount of nitrogen is desired, but since nitrogen is a gaseous component, the amount is limited by its solubility in the solid in order to prevent blow holes, which determines an upper limit of the nitrogen content. The amount of nitrogen in solid solution depends on the chromium content as shown in FIG. 5, and with a chromium content of 30%, the amount of solid soluble nitrogen is only 0.45%. Also the amount of solid soluble nitrogen can be increased by an increased content of manganese, but as understood from FIG. 5, 0.45% is the upper limit for nitrogen when the chromium content is 30% even when manganese is added in an amount of 5.0%. Chromium is limited up to 30% from the points of hot and cold workabilities and manganese is limited up to 5.0% from the aspects of pitting corrosion resistance and hot workability, so that the upper limit of the nitrogen content is 0.45% in the present invention.

Nickel is an essential element for assuring the austenite structure of the present inventive stainless steel, and is also effective for improving pitting corrosion resistance to some degree, and also is an essential element for attaining good hot workability in the present inventive stainless steel.

Effects on pitting corrosion resistance by nickel are shown in FIG. 6. With nickel contents more than 5%, the pitting corrosion potential becomes remarkably good, and no pitting corrosion whatever is observed with nickel contents in the range more than 10%. As mentioned above, nickel is essential for assuring the austenite base structure of the present inventive stainless steel, and the effects of nickel are considered to be explained by the following formula:

Equivalent Ni (%) = Ni + 0.5 Mn + 30 C + 30 N

Results of measurements of δ -ferrite in the steel products according to the above formula are shown in FIG. 7. When the equivalent nickel is more than 19% the δ-ferrite disappears and an austenite structure is obtained. But as understood from FIG. 7, even if the equivalent nickel is increased by addition of manganese, the austenite structure cannot be obtained. Thus addition of manganese is of no use for obtaining an austenite structure. On the other hand, carbon and nitrogen are stron strong austenizers. As hereinafter stated, the upper limit of carbon content is .[∅12% #x2205;08% because of its harmful effects on pitting corrosion resistance and resistance against intergranular attacks, and the upper limit of the nitrogen content is 0.45% from the point of its solid soluble limitation as mentioned before. Therefore, when carbon and nitrogen are added up to their upper limits, the equivalent nickel is 17% and the deficit of more than 2% may be supplemented by nickel addition. The lower limit of the nickel content has been explained from the points of pitting corrosion resistance and the structure, but in conclusion, 10% is the minimum amount of nickel for assuring the desired pitting corrosion resistance.

On the other hand, the upper limit of the nickel content is determined from the point of hot workability which is one of the important properties of the present stainless steel. FIG. 8 shows the effects on the hot workability by various chemical compositions. The hot workability is estimated by the maximum edge cracking rate calculated from the maximum value of edge cracking depth of steel billets after hot rolling.

In FIG. 8, the hot workabilities of the grades containing 10-12% nickel and the grades containing 13-18% nickel are considerably different even when the equivalent nickel is the same. As understood from the figure, the hot workability tends to improved improve as the equivalent nickel is increased. However, at the border line of 12-13% nickel content, the hot workability is deteriorated when the nickel content is increased beyond the border line. This deterioration of hot working is due to hot embrittlement peculiar to highly alloyed steels, and can be alleviated by limiting the amounts of substitution type solid solution elements (Ni, Mo, Nb, Cr). Thus it is necessary to increase the equivalent nickel by the addition of nitrogen to assure Ni + 30 (C + N) ≧ 20% and further to limit the additions of Mo and Nb, respectively, at less than 1%. Thus as for a desired range of nickel content, 12% is its upper limit within this desired range, but it is possible to improve the hot workability by increasing nitrogen addition and limiting the amount of Mo and Nb so far long as the nickel content is up to 18%. Thus the upper limit of the nickel content is set as at 18% in the present invention.

Both Mo and Nb are effective not only for improving the pitting corrosion resistance as well as Cr, N and Ni, but also Mo is effective for improving the resistance against sulfuric acid and Nb is effective for improving the resistance against intergranular attacks. For applications in chemical equipment and appliances, not only pitting corrosion resistance, but also ordinary corrosion resistance may be required. Thus Mo and Nb are sometimes added selectively for such purposes.

Mo contents in an amount more than 0.1% are required from the points of pitting corrosion resistance and sulfuric acid resistance as shown in FIGS. 9 and 10, and the lower limit of the Mo content is set as at nd∅1%. Regarding Nb, at least 0.05% of Nb is required for pitting corrosion resistance and resistance against intergranular attacks, and the lower limit of Nb addition is set as at nd∅05%.

A larger content of Mo is desirous from the points of pitting corrosion resistance and sulfuric acid resistance, but in order to maintain good hot workability as well as the austenite structure, the content of Mo is limited.

First, there is the complicated aspect that the effect of molybdenum on hot workability varies depending on the amount of nickel. When the nickel content is not more than 12%, good hot workability is obtained by adding Mo in a range not more than 4% with appropriate adjustment of the amounts of chromium and nitrogen. However, when the nickel content exceeds 12%, the hot embrittlement peculiar to highly alloyed steels appears and good hot workability is hard to maintain with an Mo addition more than 1%. Thus the upper limit of the molybdenum content must be limited in accordance with the following nickel contents:

(1) With nickel contents of 10% - 12% the upper limit of Mo is not more than 4%

(2) With nickel contents of 12% to 18% the upper limit of Mo is not more than 1%.

The upper limit of the Nb content is set as 2% in the present invention for the following reason: when the Nb content exceeds 2%, carbides and nitrides of niobium are produced in a large amount, thus deteriorating the pitting corrosion resistance, and in addition, fixation of carbon and nitrogen lowers the effective equivalent nickel so that it becomes more difficult to maintain the austenite structure.

Copper is not particularly effective for pitting corrosion resistance, but effective for ordinary corrosion resistance, such as, sulfuric acid resistance and hydrochloric acid resistance, and, similarly as Mo and Nb, it is desired to add copper according to the kinds and types of applications in which the steel is used. The reason for setting its lower limit at 0.2% is that at copper contents below this limit, no substantial improvement is obtained, while the reason for its upper limit of 5% is that copper contents of more than 5% remarkably deteriorate the hot workability because of an excess beyond the solid solution limit of copper.

Carbon not only deteriorates the pitting corrosion resistance, but also accelerates intergranular attacks remarkably and therefore it is limited to an upper limit of 0.08%.

Silicon is effective to improve the pitting corrosion resistance to some degree, but is less effective in this respect as compared with Cr, Mo and Nb. Since silicon is a strong ferrite former its content is limited from the point of maintenance of the austenite structure and its upper limit is set as at 4%.

Manganese is an element which lowers the pitting corrosion resistance, but it is useful in a limited amount for increasing the amount of solid soluble nitrogen and thus increasing the addition of nitrogen which is remarkably effective for improving the pitting corrosion resistance. With manganese contents beyond 6% 5%, the deterioration of the pitting corrosion resistance increases too much in spite of the above favourable effects, and thus its upper limit is set as at 6% 5%.

Both phosphorus and sulfur are elements which deteriorate pitting corrosion resistance and thus it is desirous to minimize their contents. But these elements are unavoidable impurities which are present during a steel making process. The reason for the upper limit of 0.040% for phosphorus is that phosphorus contents beyond this limit cause remarkable damage to weldability which is one of the most important properties of the present inventive steel. The reason for the upper limit of 0.030% for sulfur is that its contents beyond this limit deteriorates the hot workability as well as the weldability.

All of boron, cerium and titanium improve the hot workability of the present inventive steel, and it is desired to add these elements particularly when severe hot workings are conducted. Although these elements are effective in a very small addition, excessive addition of those these elements on the contrary damages the cleanness of the steel and deteriorates the hot workability. Thus the upper limits of B, Ce and Ti are set as at nd∅01, 0.05 and 0.5% respectively.

Explanations have been made for the reasons of limitations of each of the constituents of the present inventive steel composition. Further it is necessary to limit the steel composition for balancing the composition from the points of the structure, hot workability and weldability as under: ##EQU1##

Examples of the present inventive steel are set forth in Table 1 with conventional standard grades of stainless steel for comparison, and their mechanical properties and corrosion resistance are set forth in Table 2.

Table 1
__________________________________________________________________________
Examples of Present Steel
Chemical Compositions
Sample      C   Si  Mn  Ni  Cr  N    Mo  Cu  Others
__________________________________________________________________________
A    0.06
0.65
1.28
10.1
23.5
0.35 --  --
B    0.05
0.70
1.43
15.2
25.6
0.36 --  --
C    0.06
0.62
1.18
10.9
24.7
0.35 0.8 --
Inventive
D    0.07
0.66
1.52
11.7
23.4
0.31 2.4 --
Steels E    0.04
0.73
1.04
16.2
26.3
0.38 0.5 0.4
F    0.06
0.65
1.22
11.4
23.2
0.32 1.4 --  Nb 0.8
G    0.05
0.61
1.64
14.8
25.3
0.34 0.7 1.2 Nb 0.5
H    0.05
0.64
4.87
10.6
15.8
0.43 --  --
I    0.06
3.04
1.10
13.8
23.5
0.37 --  --
SUS 27
0.05
0.64
1.11
9.1
18.4
0.024
--  --
Conventional
SUS 32
0.06
0.70
1.34
13.2
17.1
0.022
2.4 --
Steels SUS 42
0.06
0.62
1.33
19.7
25.2
0.025
--  --
SUS 64
0.05
0.68
1.42
14.2
18.7
0.023
3.3 --
__________________________________________________________________________

Table 2
__________________________________________________________________________
Mechanical Properties and
Corrosion Resistance
Bending
Pitting Corrosion
Corrosion
Tensile Properties
Property
Resistance      Resistance
Pitting
Corrosion
Corrosion
Potential
Rate    5%
VSCE    g/m2.h
H2 SO4
Yield
Tensile
Elon-     (at 1mA/cm2)
50g/l   boil ×
Strength
Strength
gation
r: 0.5t
3%NaCl + 5%
FeCl2 +1/20N
6 hrs.
Sample      kg/mm2
kg/mm2
%   180°
H2 SO4 : 335° C
HCl: 50° C
g/m2.h
__________________________________________________________________________
A    48.3 87.1 48.9
good  1.01    2.1     140
B    42.9 80.5 46.8
"     0.99    0       59
C    40.2 80.2 51.7
"     1.02    0       2.3
Inventive
D    47.8 83.6 48.8
"     1.02    0       0.8
Steels E    40.7 80.8 51.0
"     1.02    0       1.6
F    55.4 90.9 40.0
"     1.02    0       1.8
G    51.6 85.4 44.5
"     1.02    0       1.2
H    55.8 91.3 39.8
"     0.96    3.2     135
I    43.5 81.6 45.4
"     1.02    0       62
SUS 27
26.5 59.5 63.6
good  0.36    17      206
Conventional
SUS 32
25.7 54.9 61.7
"     0.81    12      40
Steels SUS 42
25.3 56.6 64.8
"     0.54    6.9     27
SUS 64
28.0 60.1 54.0
"     0.93    5.5     3.7
__________________________________________________________________________

Claims

1. A pitting corrosion resistant austenite stainless steel consisting essentially of

not more than nd∅08% by weight of carbon,

silicon in an amount not exceeding 4.0% by weight,

manganese in an amount not exceeding 5.0% by weight,

phosphorus in an amount not exceeding 0.04% by weight,

sulfur in an amount not exceeding 0.03% by weight,

10 to 18% by weight of nickel,

23 to 30% by weight of chromium,

0.30 to 0.45% by weight of nitrogen, and

0.1 to 4.0% by weight of molybdenum,

the balance being iron and unavoidable impurities, said steel satisfying the condition of Ni % + 30 (C % + N %) ≧ 20%.