[0001] This application claims priority of Taiwanese patent Application No. 091124567, filed
on October 23, 2002.
[0002] This invention relates to an austenitic stainless steel, more particularly to a low
nickel containing chromium-nickel-manganese-copper austenitic stainless steel.
[0003] U.S. Patent No. 5,286,310 discloses a low nickel containing chromium-nickel-manganese-copper
austenitic stainless steel that has a reduced nickel content and acceptable metallographic
structure, mechanical strength, corrosion resistance and workability. The aforesaid
austenitic stainless steel contains at least 16.5% by weight of chromium so as to
provide acceptable corrosion resistance. However, the chromium content should not
exceed 17.5% by weight so as to prevent undesired formation of delta ferrite (δ-ferrite)
during hot working and impairment to hot workability. The aforesaid austenitic stainless
steel further contains at least 2.5% by weight of nickel so as to improve cold workability
and so as to inhibit transformation of austenite into martensite. However, nickel
content should not exceed 5% by weight due to the relatively high price thereof.
[0004] Although the aforesaid austenitic stainless steel is capable of providing acceptable
corrosion resistance and cold or hot workability, the chromium content thereof is
still high (previous investigation has shown that at least 17% by weight of chromium
is necessary to provide minimum levels of corrosion resistance), which can impair
stability of the austenitic stainless steel and which can cause cracking during hot
rolling.
[0005] The disclosure of U.S. Patent No. 5,286, 310 is incorporated herein by reference.
[0006] Therefore, it is an object of the present invention to provide a low nickel containing
chromium-nickel-manganese-copper austenitic stainless steel that is capable of overcoming
the aforesaid drawbacks of the prior art.
[0007] According to this invention, there is provided an austenitic stainless steel that
comprises: (a) 0.03wt% to 0.12wt% of C; (b) 0.2wt% to 1.0wt% of Si; (c) 7.5wt% to
10.5wt% of Mn; (d) 14.0wt% to 16.0wt% of Cr; (e) 1.0wt% to 5.0wt% of Ni; (f) 0.04wt%
to 0.25wt% of N; (g) 1.0wt% to 3.5wt% of Cu; (h) trace amount of Mo; and the balance
being Fe and incidental impurities. The austenitic stainless steel has a δ - ferrite
content that is less than 8.5 and that satisfies the following formula
[0008] In drawing which illustrates an embodiment of the invention,
Fig. 1 is a diagram illustrating the relationship between δ-ferrite content of the
preferred embodiment of the austenitic stainless steel of this invention and hot working
temperature.
[0009] The preferred embodiment of the low nickel containing chromium-nickel-manganese-copper
austenitic stainless steel of the present invention comprises: (a) 0.03wt% to 0.12wt%
of C; (b) 0.2wt% to 1.0wt% of Si; (c) 7.5wt% to 10.5wt% of Mn; (d) 14.0wt% to 16.0wt%
of Cr; (e) 1.0wt% to 5.0wt% of Ni; (f) 0.04wt% to 0.25wt% of N; (g) 1.0wt% to 3.5wt%
of Cu; (h) trace amount of Mo; and the balance being Fe and incidental impurities.
The austenitic stainless steel has a δ-ferrite content that is less than 8.5 and that
satisfies the following formula
wherein (a), (b), (c), (d), (e), (f), (g), (h) in the formula mean the content of
the respective elements (wt%).
[0010] The austenitic stainless steel can further comprise 5 to 30 ppm of B so as to improve
hot workability. The contents of harmful impurities, such as S (sulfur) and P (phosphorous),
are as small as possible. However, due to cost concerns associated with removal of
these impurities, the S content is limited to 150 ppm, and the P content is limited
to 0.06wt%.
[0011] Fig. 1 illustrates the relationship between the δ-ferrite content of the preferred
embodiment of the austenitic stainless steel of this invention and temperature. The
results show that when temperature is raised to above 1250°C during hot rolling, the
δ-ferrite content rises sharply, which results in the risk of edge cracking of a rolled
plate of the austenitic stainless steel. In addition, a minimum temperature of 1050°C
during hot rolling is required so as to obtain the requisite mechanical strength.
Examples and Comparative Examples
[0012] The following Examples and Comparative Examples illustrate the unexpectedly better
results of this invention over the prior art.
[0013] Table 1 illustrates an edge crack effect test for different test specimens of the
austenitic stainless steel of Examples 1 to 9 and comparative Examples 1 to 5, which
differ in composition (only elements Ni, C, Si, Mn, Cr, and Cu are shown). The test
was conducted by hot rolling at a temperature ranging from 1050°C to 1250°C. The test
results show that each Example of the austenitic stainless steel of this invention
has a δ-ferrite content less than 8.5, and that no edge crack was observed for the
test specimens of Examples 1 to 9. Each of the test specimens of the Comparative Examples
1 to 5 has a δ-ferrite content greater than 8.5. Edge cracks were found in each of
the test specimens of the Comparative Examples 1 to 5.
Table 1
Examples |
Ni |
C |
Si |
Mn |
Cr |
Cu |
δ-ferrite |
Edge crack |
1 |
4.31 |
0.053 |
0.50 |
7.60 |
16.30 |
1.60 |
8.49 |
None |
2 |
4.05 |
0.032 |
0.53 |
7.85 |
15.36 |
1.71 |
6.636 |
None |
3 |
4.07 |
0.032 |
0.54 |
8.00 |
15.33 |
1.66 |
6.259 |
Noen |
4 |
4.55 |
0.032 |
0.58 |
7.54 |
15.23 |
1.59 |
4.984 |
None |
5 |
4.15 |
0.059 |
0.62 |
7.44 |
15.26 |
1.65 |
3.859 |
None |
6 |
4.24 |
0.046 |
0.42 |
7.86 |
15.68 |
1.66 |
3.278 |
None |
7 |
4.21 |
0.051 |
0.49 |
7.63 |
15.16 |
1.62 |
1.684 |
None |
8 |
4.09 |
0.060 |
0.50 |
8.08 |
15.14 |
1.70 |
0.109 |
None |
9 |
4.19 |
0.066 |
0.54 |
7.76 |
14.99 |
1.65 |
-1.989 |
None |
Comparative Examples |
Ni |
C |
Si |
Mn |
Cr |
Cu |
δ-ferrite |
Edge crack |
1 |
4.31 |
0.039 |
0.47 |
7.07 |
19.04 |
2.15 |
28.58 |
Cracking |
2 |
4.36 |
0.05 |
0.45 |
7.58 |
17.53 |
2.03 |
15.82 |
Cracking |
3 |
4.37 |
0.046 |
0.47 |
7.96 |
18.33 |
1.71 |
22.60 |
Cracking |
4 |
4.77 |
0.052 |
0.51 |
7.54 |
18.13 |
1.73 |
19.85 |
Cracking |
5 |
4.45 |
0.051 |
0.53 |
7.5 |
16.20 |
1.5 |
9.1 |
cracking |
[0014] Table 2 illustrates a corrosion resistance test (ASTM B117) using salt fog for different
test specimens of the austenitic stainless steel of Examples 10 to 12 and comparative
Example 6 (type 304 stainless steel), which differ in composition (only elements Ni,
C, Si, Mn, Cr, Cu, and B are shown). The test results show that each Example of the
austenitic stainless steel of this invention has a corrosion rate that is as low as
that of the type 304 stainless steel (no more than 0.1%) of the prior art.
Table 2
Examples |
Ni |
C |
Si |
Mn |
Cr |
Cu |
B |
Corrosion rate |
10 |
4.40 |
0.058 |
0.48 |
7.56 |
15.26 |
1.79 |
0.0001 |
≦ 0.1wt% |
11 |
4.11 |
0.051 |
0.54 |
7.86 |
15.35 |
1.69 |
0.0032 |
≦ 0.1wt% |
12 |
3.40 |
0.059 |
0.77 |
7.84 |
14.94 |
1.78 |
0.0001 |
≦ 0.1wt% |
Comparative Example |
Ni |
C |
Si |
Mn |
Cr |
Cu |
B |
Corrosion rate |
6 |
8.02 |
0.045 |
0.53 |
1.25 |
18.19 |
0.23 |
0.0008 |
≦ 0.1wt% |
[0015] It is noted that the chromium content in each of the Examples 1 to 12 of the austenitic
stainless steel of this invention is less than 17wt%, which is a minimum requirement
of the prior art for providing minimum levels of corrosion resistance.
[0016] Table 3 illustrates compositions of test specimens of the austenitic stainless steel
of Examples 13 to 22 and comparative Examples 7 to 10 (only elements Ni, C, Si, Mn,
Cr, and Cu are shown). Table 4 illustrates a mechanical strength test for the test
specimens of the austenitic stainless steel of the Examples 13 to 22 and the comparative
Examples 7 to 10. The test results show that the austenitic stainless steel of this
invention has an elongation better than those of type 304 stainless steel of the prior
art. Other mechanical properties, such as tensile strength, yield strength, and hardness,
of the austenitic stainless steel of this invention are comparable to those of type
304 stainless steel of the prior art.
Table 3
Examples |
Ni |
C |
Si |
Mn |
Cr |
Cu |
13 |
4.26 |
0.036 |
0.56 |
7.7 |
15.12 |
1.67 |
14 |
4.21 |
0.039 |
0.47 |
7.97 |
15.32 |
1.66 |
15 |
4.21 |
0.056 |
0.54 |
7.69 |
15.26 |
1.79 |
16 |
4.15 |
0.049 |
0.48 |
7.7 |
15.26 |
1.66 |
17 |
4.20 |
0.040 |
0.49 |
7.93 |
15.35 |
1.67 |
18 |
4.21 |
0.039 |
0.48 |
7.96 |
15.29 |
1.66 |
19 |
4.22 |
0.044 |
0.46 |
7.93 |
15.01 |
1.70 |
20 |
4.17 |
0.064 |
0.5 |
7.71 |
15.16 |
1.65 |
21 |
4.20 |
0.055 |
0.52 |
7.70 |
15.32 |
1.68 |
22 |
4.41 |
0.058 |
0.48 |
7.56 |
15.27 |
1.80 |
Comparative Example |
Ni |
C |
Si |
Mn |
Cr |
Cu |
7 |
8.06 |
0.039 |
0.53 |
1.17 |
18.14 |
0.23 |
8 |
8.04 |
0.041 |
0.50 |
1.15 |
18.15 |
0.21 |
9 |
8.08 |
0.039 |
0.49 |
1.18 |
18.17 |
0.24 |
10 |
8.03 |
0.040 |
0.52 |
1.11 |
18.09 |
0.22 |
Table 4
Examples |
Tensile strength,
(MPa) |
Yield strength,
(MPa) |
Hardness,
(HRBO) |
Elongation,
(%) |
13 |
621.7 |
313.3 |
83.5 |
55.2 |
14 |
630.2 |
289.5 |
82.5 |
55.3 |
15 |
628.5 |
287.6 |
82.3 |
55.0 |
16 |
642.3 |
291.3 |
82.8 |
53.1 |
17 |
618.4 |
312.0 |
84.3 |
53.7 |
18 |
634.6 |
296.4 |
82.8 |
53.8 |
19 |
639.0 |
317.2 |
83.9 |
54.1 |
20 |
642.6 |
319.7 |
84.7 |
54.3 |
21 |
621.7 |
313.3 |
83.5 |
55.2 |
22 |
641.9 |
301.6 |
83.4 |
53.4 |
Comparative Examples |
Tensile strength,
(MPa) |
Yield strength,
(MPa) |
Hardness,
(HRBO) |
Elongation,
(%) |
7 |
660.0 |
324.6 |
83.2 |
49.1 |
8 |
660.6 |
325.0 |
82.6 |
46.8 |
9 |
663.8 |
328.9 |
82.4 |
48.8 |
10 |
657.8 |
322.8 |
81.8 |
48.5 |
[0017] The aforesaid tests show that the austenitic stainless steel of this invention is
capable of exhibiting excellent mechanical strength, corrosion resistance, and phase
stability during hot or cold working with a relatively low nickel content and a low
chromium content as compared to those of the prior art.
1. An austenitic stainless steel
characterized by:
(a) 0.03wt% to 0.12wt% of C;
(b) 0.2wt% to 1.0wt% of Si;
(c) 7.5wt% to 10.5wt% of Mn;
(d) 14.0wt% to 16.0wt% of Cr;
(e) 1.0wt% to 5.0wt% of Ni;
(f) 0.04wt% to 0.25wt% of N;
(g) 1.0wt% to 3.5wt% of Cu;
(h) trace amount of Mo; and
the balance being Fe and incidental impurities; wherein said austenitic stainless
steel has a δ-ferrite content that is less than 8.5 and that satisfies the following
formula
2. The austenitic stainless steel of Claim 1, further characterized by 5 to 30 ppm of B.
3. The austenitic stainless steel of Claim 1, further characterized by no more than 150 ppm of S.
4. The austenitic stainless steel of Claim 1, further characterized by no more than 0.06wt% of P.