EP1431408B1 - Low nickel containing chromium-nickel-manganese-copper austenitic stainless steel - Google Patents

Abstract

An austenitic stainless steel includes (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 delta - ferrite content less than 8.5 and equal to 6.77Ä(d)+(h)+1.5(b)Ü-4.85Ä(e)+30(a)+30(f)+0.5(c)+0.3(g)Ü-52.75. <IMAGE>

Description

• This application claims priority of Taiwanese patent Application No. 091124567, filed on October 23, 2002.
• This invention relates to an austenitic stainless steel, more particularly to a low nickel containing chromium-nickel-manganese-copper austenitic stainless steel.
• 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.
• 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.
• The disclosure of U.S. Patent No. 5,286, 310 is incorporated herein by reference.
• ES 2142756 discloses austenitic stainless steels developed as potential replacements for AISI 304 and AISI 316.
• 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.
• 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 $$\mathit{\delta }-\mathrm{ferrite}=6.77\left[\left(\mathrm{d}\right)+\left(\mathrm{h}\right)\right]-1.5\left(\mathrm{b}\right)]-4.85\left[\right(\mathrm{e})+30\left(\mathrm{a}\right)+30\left(\mathrm{f}\right)+0.5\left(\mathrm{c}\right)+0.3\left(\mathrm{g}\right)]-\mathrm{52.75.}$$

  
• 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.
• 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 $$\mathit{\delta }\mathrm{-}\mathrm{ferrite}\mathrm{=}\mathrm{6.77}\mathrm{\left[}\mathrm{\left(}\mathrm{d}\mathrm{\right)}\mathrm{+}\mathrm{\left(}\mathrm{h}\mathrm{\right)}\mathrm{+}\mathrm{1.5}\left(\mathrm{b}\right)\mathrm{\right]}\mathrm{-}\mathrm{4.85}\left[\mathrm{\left(}\mathrm{e}\mathrm{\right)}\mathrm{+}\mathrm{30}\left(\mathrm{a}\right)\mathrm{+}\mathrm{30}\left(\mathrm{f}\right)\mathrm{+}\mathrm{0.5}\left(\mathrm{c}\right)\mathrm{+}\mathrm{0.3}\left(\mathrm{g}\right)\right]\mathrm{-}\mathrm{52.75}\mathrm{,}$$

  

  wherein (a), (b), (c), (d), (e), (f), (g), (h) in the formula mean the content of the respective elements (wt%).
• 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%.
• 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
• The following Examples and Comparative Examples illustrate the unexpectedly better results of this invention over the prior art.
• 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

  *steels out of the scope of the invention

• 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%

• 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.
• 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

• 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.

Claims (3)

1. An austenitic stainless steel having

   (a) 0.03wt% to 0.064wt% 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; and optionally one or more of the following 5 to 30 ppm of B; no more than 150 ppm of S, and/or no more than 0,06 wt% of P.
   wherein said austenitic stainless steel has a δ-ferrite content that is less than 8.5 and satisfies the following formula $$\mathrm{\delta }\mathrm{-}\mathrm{ferrite}\mathrm{=}\mathrm{6.77}\mathrm{\left[}\mathrm{\left(}\mathrm{d}\mathrm{\right)}\mathrm{+}\mathrm{\left(}\mathrm{h}\mathrm{\right)}\mathrm{+}\mathrm{1.5}\left(\mathrm{b}\right)\mathrm{\right]}\mathrm{-}\mathrm{4.85}\left[\mathrm{\left(}\mathrm{e}\mathrm{\right)}\mathrm{+}\mathrm{30}\left(\mathrm{a}\right)\mathrm{+}\mathrm{30}\left(\mathrm{f}\right)\mathrm{+}\mathrm{0.5}\left(\mathrm{c}\right)\mathrm{+}\mathrm{0.3}\left(\mathrm{g}\right)\right]\mathrm{-}\mathrm{52.75}\mathrm{,}$$

   

   wherein the elongation is between 53.1 and 55.3 % and the hardness is between 82.3 and 84.7 HRBO.
2. The austenitic stainless steel according claim 1 characterized in that
   the yield strength is between 287.6 and 319.7 MPa.
3. The austenitic stainless steel according to claim 1 or 2 characterized in that the tensile strength of said austenitic stainless steel is between 618.4 and 642.6 MPa.

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