CR-MN-NI-CU AUSTENITIC STAINLESS STEEL

Abstract: An austenitic stainless steel alloy is disclosed that has the following broad weight percent composition: C 0.025-0.15; Mn 4-12; Si 1.0 max.; P 0.2 max.; S 0.1 max.; Cr 15.5-17.5; Ni 1-4; Mo 0.25-1.5; Cu 1.5-4; W 1.0 max.; Co 1.0 max.; N 0.05-0.30. The balance of the alloy is iron and the usual impurities. Within the foregoing ranges, the elements are balanced such that the combined amount of carbon and nitrogen is at least about 0.19 %, the combined amount of chromium and molybdenum is less than about 17.75 % and %Cr + 3.3(%Mo) + 13(%N) > 20.5. The alloy provides a unique combination of corrosion resistance, formability, and resistance to work hardening.

Description: CR-MN-NI-CU AUSTENITIC STAINLESS STEEL John H. Magee, Jr. FIELD OF THE INVENTION This invention relates to Cr-Mn austenitic stainless steels, and in particular to a Cr-Mn-Ni-Cu austenitic stainless steel having a unique combination of corrosion resistance in chloride-containing environments, hot workability, and resistance to work hardening BACKGROUND OF THE INVENTION A known austenitic stainless steel has the following we: composition C 0.15 max. Mn 6.4-8 Si 1.0 max Cr 16.5-17.5 Cu 2.0-3.0 N 0.2 max. C+N 0.30 max The balance of the alloy is iron and the usual impurities Data published for that alloy appears to show that the alloy has corrosive pitting resistance that is comparable to AISI Type 201L and AISI Type 430 stainless steels when determined by the anodic polarization test The published data also shows that the pitting resistance leaves much to be desired relative to AISI Type 304 stainless steel, one of the most commonly used stainless steels. Furthermore, it has been found that commercially produced products of the aforesaid austenitic stainless steel have significantly reduced chloπde crevice corrosion resistance and undergo an undesirable degree of rusting in the salt fog test. A need has aπsen for an austenitic stainless steel that has resistance to chloπde pitting corrosion comparable to that of Type 304 stainless steel, but which contains lower nickel than that alloy. It is generally known that molybdenum is beneficial to the chloπde pitting corrosion resistance of an austenitic stainless steel alloy. However, molybdenum is also known to adversely affect the hot workability of such alloys and tends to upset the phase balance of such alloys because it is a strong ferπte former. In view of the foregoing, it would be desirable to have an austenitic stainless steel that provides pitting and crevice corrosion resistance comparable to Type 304 stainless steel, but which has acceptable hot workability so that it can be readily formed into a vaπety of product shapes. SUMMARY OF THE INVENTION The present invention provides an austenitic stainless steel alloy having the broad, intermediate, and preferred compositional ranges set forth below in weight percent. Element Broad Intermediate Preferred C 0.025-0.15 0.05-0.12 0.08-0.11 Mn 4-12 6-10 7-9 Si 1.0 max. 0.75 max. 0.50 max. P 0.2 max. 0.1 max. 0.05 max. S 0.1 max. 0.05 max. 0.025 max. Cr 15.5-17.5 16.0-17.0 16.25-16.75 Ni 1-4 1.5-3.5 2.0-3.0 Mo 0.25-1.5 0.5-1.2 0.7-1.0 Cu 1.5-4 2.0-3.5 2.5-3.0 W 1.0 max. 1.0 max. 0.75 max. Co 1.0 max. 1.0 max. 0.75 max. N 0.05-0.30 0.10-0.25 0.12-0.20 Fe Balance Balance Balance Within the weight percent limits specified above, the elements are balanced to provide the unique combination of properties that are characteπstic of the alloy according to this invention More specifically, the combined amount of chromium and molybdenum is maintained below about 17.75 wt.%; the combined amount of carbon and nitrogen is at least about 0.19 wt.%; and the alloy composition is balanced such that it contains not more than about 9% by volume (vol %) ferπte in the as-cast condition. Further, chromium, molybdenum, and nitrogen are carefully balanced in accordance with the following relationship %Cr + 3.3(%Mo) + 13(%N) > 20.5 This relationship defines the pitting resistance equivalent number (PRE) for this alloy Preferably, the PRE for this alloy is greater than about 21.0. The foregoing tabulation is provided as a convenient summary and is not intended thereby to restπct the lower and upper values of the ranges of the individual elements for use in combination with each other, or to restπct the ranges of the elements for use solely in combination with each other Thus, one or more of the ranges can be used with one or more of the other ranges for the remaining elements. In addition, a minimum or maximum for an element of a broad, intermediate, or preferred composition can be used with the minimum or maximum for the same element in another preferred or intermediate composition. Here and throughout this application, the term "percent" or the symbol "%" means percent by weight, unless otherwise indicated DETAILED DESCRIPTION In this alloy, at least about 15.5%, better yet at least about 16.0%, and preferably at least about 16.25% chromium is present to benefit the general corrosion resistance of the alloy Chromium also contπbutes to the chloπde corrosion resistance of this alloy. The amount of chromium in the alloy is restπcted to not more than about 17.5%. better yet to not more than about 17.0% and preferably to not more than about 16.75% because too much chromium adversely affects the hot workability of the alloy. At least about 0.25%, better yet at least about 0.5%, and preferably at least about 0.7% molybdenum is present in this alloy because it too benefits the chloπde corrosion resistance of the alloy. However, too much molybdenum adversely affects the hot workability of the alloy. Therefore, the amount of molybdenum in this alloy is restπcted to not more than about 1.5%, better yet to not more than about 1.2%, and preferably to not more than about 1.0%. Chromium and molybdenum are both ferπte forming elements in this alloy Too much ferπte adversely effects the hot-workability of this alloy Therefore, chromium and molybdenum are restπcted to avoid the formation of a deleteπous amount of ferπte. As noted above, the combined amount of chromium and molybdenum (%Cr + %Mo) in this alloy is not greater than about 17.75%, and preferably, not more than about 17 5%. Nitrogen also contπbutes to the chloπde corrosion resistance of this alloy and benefits the austenitic phase balance of the alloy. For those reasons, the alloy contains at least about 0.05%, better yet at least about 0.10%, and preferably at least about 0.12% nitrogen The amount of nitrogen is restπcted in this alloy because too much nitrogen adversely affects the cold formabihty of the alloy by increasing the work hardening rate of the alloy Therefore, the alloy contains not more than about 0 30%, better yet not more than about 0.25%, and preferably not more than about 0.20% nitrogen. Carbon also benefits the austenitic phase balance of this alloy Accordingly, at least about 0 025%, better yet at least about 0.05%, and preferably at least about 0.08% carbon is present in this alloy. Too much carbon leads to carbide precipitation which adversely affects the corrosion resistance and cold formabihty of the alloy Therefore, the amount of carbon is restπcted to not more than about 0.15%, better yet to not more than about 0.12%, and preferably to not more than about 0.11% in this alloy. Because of the amounts of chromium and molybdenum that are present in this alloy, as descπbed above, the alloy contains a combined amount of carbon and nitrogen (%C + %N) of at least about 0.19%, and preferably, at least about 0.25%, to avoid the formation of a deleterious amount of ferrite. At least about 1%, better yet at least about 1.5%, and preferably at least about 2.0% nickel is present in this alloy to benefit the austenitic phase balance of the alloy. Nickel also contributes to the low work hardening rate of the alloy, and thus benefits the good cold formability of the alloy. Nickel is restricted to not more than about 4%, better yet to not more than about 3.5%, and preferably to not more than about 3.0% in this alloy. Copper is present in this alloy because it lowers the work hardening rate of the alloy to an even greater extent than nickel. Thus, it is highly beneficial to the good cold formability of this alloy. Copper also benefits the cold formability of this alloy because it lowers the annealed strength provided by this alloy. To achieve those desired properties the alloy contains at least about 1.5%, e.g., at least about 1.6 or 1.7% copper. Better yet, the alloy contains at least about 2.0%, and preferably at least about 2.5% copper. The amount of copper is restricted because too much copper causes hot shortness, especially when the amount of ferrite in the as-cast alloy is about 9 vol.% or more. Hot shortness results in severe cracking and/or tearing of the alloy during hot working. Therefore, the alloy contains not more than about 4%, better yet not more than about 3.5%, and preferably not more than about 3.0% copper. I have found that alloys containing as little as 9 vol. % ferrite in the as- cast condition could not be hot worked and were subject to severe cracking or tearing. To minimize hot shortness in this alloy, the composition is balanced so that the alloy contains less than about 9 vol.% ferrite in the as-cast condition. The alloy is preferably balanced such that it contains not more than about 8.5 vol.% ferrite in the as-cast condition, better yet not more than about 7 vol.% ferrite, and for best results, not more than about 6 vol.% ferrite. At least about 4%, better yet at least about 6%, and preferably at least about 7% manganese is present in this alloy because it contributes to austenite stability, i.e., it helps to stabilize the austenite against transformation to martensite during mechanical deformation. Manganese also contributes to the retention of nitrogen in the alloy by increasing the solubility limit of nitrogen in the solid solution. The alloy contains not more than about 12%, better yet not more than about 10%, and preferably not more than about 9% manganese because too much manganese leads to the formation of ferrite which is undesirable in this alloy. A small quantity of silicon may be present in this alloy as a retained amount from deoxidizing additions during melting. The amount of silicon present in the alloy is restricted to not more than about 1.0%, better yet to not more than about 0.75%, and preferably to not more than about 0.50% because silicon is a ferrite forming element in this steel. Optionally, up to about 1.0% tungsten can be present in this alloy to benefit the chloride coπosion resistance of the alloy. However, the amount of tungsten in the alloy is preferably restricted to not more than about 0.75 % because tungsten is also a ferrite former in this alloy and too much tungsten adversely affects the hot workability of the alloy. A small amount of cobalt up to about 1.0% or 0.75% may also be present in this alloy. When present, cobalt provides benefits similar to those provided by nickel and copper. A small amount of phosphorus, up to about 0.2% or 0.1%, and/or a small amount of sulfur, up to about 0.1% or 0.05%, may be present in this alloy when desired to benefit the machinability of the alloy. However, the amounts of phosphorus and sulfur are preferably restricted because they adversely affect the corrosion resistance and formability of the alloy. Accordingly, phosphorus is preferably restricted to not more than about 0.05% and sulfur is preferably restricted to not more than about 0.025% to obtain good formability. For optimum formability, sulfur is restricted to not more than about 0.01%, and preferably to not more than about 0.005% in this alloy. The balance of the alloy is essentially iron except for the usual impurities found in commercial grades of steels intended for similar service. No special techniques are required in melting, casting, or working the alloy of this invention. Arc melting followed by argon-oxygen decarburization is the preferred method of melting and refining the alloy, but other practices such as vacuum melting can be employed. In addition, this alloy can be made using powder metallurgy techniques, such as powder injection molding and metal injection molding techniques. This alloy can also be prepared using continuous casting methods. The alloy of the present invention can be formed into a variety of shapes for a wide variety of uses. Preferred product forms include billets, bars, rod, wire, strip, plate, and sheet. The formability properties of this alloy are such that it readily lends itself to cold formed articles such as cold-drawn wire or rod. Furthermore, useful articles may be easily formed by cold bending or flattening the wire or rod that has been prepared from this alloy. Examples To demonstrate the unique combination of properties provided by the alloy of this invention, Examples 1 and 2 having the compositions in weight percent shown in Table 1 were prepared and tested. For comparison purposes, comparative Heats A-L with compositions outside the range of the present invention were also prepared and tested. Their weight percent compositions are also shown in Table 1. CN f CN SO — CN OO CO WO CN sO CN CN so ^ °. !__> ^ O D3 CN SO f- "* so CN CN S oo o so 2S =* so —i o _ ca wo o —i O so CN CN r _- — ^ . 0 CN o CN CN CO t- CN O CN oo SO CN ^N 0O O SO r- o CO o CN SO O SO sO S3 —β C 9 r-: o o 2 i CN ^ 03 CN sd — * c- .» o < Ϊ f • - ϋ = r°ι ≥ Z Lb Z+ + as u Heats A, F, and G are representative of the alloy descπbed in U.S. Patent No. 5,268,510. Heats B-E and H-K are similar to Heats A, F, and G, but contain different levels of carbon, copper, nickel, molybdenum, and/or nitrogen. Heat L is representative of a commercial grade of Type 304 stainless steel. Example 1 and Heats B to K were induction melted under a cover of argon gas and cast into 2% inch square ingots each weighing 17 lbs. The ingots were forged at 2200 °F to 1 inch square bars. A 4" long piece was taken from each of the bars and was milled to 7/8" x 1 ". All of the pieces were hot rolled at 2200°F to approximately 0.200" thick stπps. The stπps were annealed at 1950°F for one-half hour, water-quenched, and then cleaned. Test specimens for crevice corrosion testing were prepared from the annealed mateπal. Heat A was prepared as a continuously-cast production size heat which was cast as 5" square billet. An 8" long section was cut from the billet and press forged at 2200°F to 2 1 /." square bar, reheated, and then forged to 1-1/16" square bar. A 4" long piece was cut from the 1-1/16" bar, milled to 7/8" x 1-1/16", and then hot rolled to 0.250" thick strip The stπp mateπal was annealed at 1950°F for 1 hour, water quenched, cleaned, and then cold rolled to 0.140" thick stπp. That mateπal was annealed at 1950°F for 15 minutes and water quenched. Test specimens for crevice corrosion testing were prepared from the annealed mateπal. Example 2 was induction melted under a cover of argon gas and cast as a 7'/ 2 inch square mgot. The ingot was press forged at 2200 °F to 5 inch square billet A slice was cut from one end of the billet and hot rolled to 0.200" thick stπp. The stπp mateπal was annealed at 1950 °F for 45 minutes, water quenched, and then pickled. After pickling, the annealed stπp mateπal was cold rolled to 0.140" thick and annealed again at 1950°F, this time for 15 minutes, and then water quenched. Heat L was obtained as a 4" long by 1 " round piece taken from a production heat of Type 304 stainless steel and processed into annealed stπp in a manner similar to Example 2. Specimens for crevice corrosion testing were prepared from the 0.140" annealed stπp mateπal. Duplicate specimens of Examples 1 and 2 and Comparative Heats A-C, E-I, and L were tested in a solution of 5% FeCl 3 and 1% NaNO 3 for 24 hours at 10 °C. If no pitting attack was observed after the first 24 hour period, then the specimen was ground and tested again at a temperature 5C° higher than the previous test temperature. Testing continued in this manner until pitting attack was observed on the specimen. The critical corrosion temperature is the highest temperature at which the specimen did not sustain any pitting. Set forth in Table 2 below are the results of critical coπosion temperature testing of each of duplicate specimens of the examples and comparative heats in Table 1. The results are presented as the critical corrosion temperature (CCT) in °C for each specimen. Also shown in Table 2 are the Cr, Mo, and N content of each example and comparative heat and the pitting resistance equivalence number (PRE) for each example and comparative heat. TABLE 2 ExJHt. No. % Cr %Mo % N PRE CCT 1 16.19 0.70 0.20 21.30 15,15 2 16.70 0.75 0.16 21.26 15,20 A 16.06 0.27 0.17 19.16 * * B 16.02 0.25 0.21 19.57 * * C 15.95 0.25 0.20 19.38 * * E 15.97 0.25 0.12 18.36 * * F 17.02 <0.01 0.14 18.84 * * G 17.08 0.01 0.11 18.55 * * H 15.96 0.26 0.11 18.22 * * I 16.06 0.74 0.10 19.85 * ,10 L 18.37 0.68 0.08 21.70 20,20 * Pitting occurred at or below the initial test temperature (10°C). The data presented in Table 2 show that Examples 1 and 2 of the alloy according to this invention provide better resistance to chloride induced corrosion than any of the comparative heats (Heats A-C and E-I). Type 304 stainless steel (Heat L) has slightly better chloride corrosion resistance than Examples 1 and 2. Salt spray testing was performed on a limited number of wire specimens of Example 2 and Heat A. The 0.264" diameter specimens were obtained from production material that was hot rolled to 0.306" round, annealed, cleaned, and then cold drawn to 0.264" round. The wire specimens were exposed to a 5%NaCl solution at 95 °F for 466 hours. At the end of the test period, the specimens of Heat A had up to 20% of the surface area rusted, whereas the specimens of Example 1 had essentially no rust. The salt spray data is additional evidence of the good chloride corrosion resistance provided by the alloy according to the present invention. Set forth in Table 3 below are the results of a work-hardening study performed on annealed strip for several of the examples from Table 1 above including the percent cold reduction per pass (% CR.) and the Rockwell hardness readings obtained after each reduction. In each case, the readings in parentheses are given as Rockwell B Scale (HRB) and the remaining readings are given as Rockwell C Scale (HRC). HRB is used up to a hardness value of 100, which is equivalent to HRC 22. The amount of Cu, Mo, and N in each Example/Heat is also shown for ease of comparison. TABLE 3 Ex.1 Ht. Ht. K Ht. B Ht. C Ht.E Ht. L (2% Cu, (02% Cu, (l%Cu, (2% Cu, (3% Cu, (3% Cu, (Type % CR. 70% Mo, 25% Mo, 25% Mo, 25% Mo, 25% Mo, 25% Mo, 304) 2%N) 2%N) 2%N) 2%N) 2%N) 1%N) 0 (915) (95) (91) (88) (87) (80) — 5 (93) (98) (97) (93) (93) (90) (91) 10 (985) 28 235 (985) (98) (965) (99) 20 285 345 315 29 275 255 265 30 345 39 37 35 33 31 315 40 375 43 41 375 355 34 35 50 40 455 425 40 38 37 37 The data in Table 3 show that the alloy according to the present invention has very good resistance to work hardening duπng cold deformation The data also show the beneficial contπbution of copper in providing a low work hardening rate in a Cr-Mn-Ni austenitic stainless steel containing molybdenum Heats C and E have work hardening rates that are very similar to that of Type 304 (Ht. L) which contains 8% nickel However, a compaπson of the results for Heats C and E shows that even with 3% copper, increasing nitrogen has some adverse effect on the work hardening rate of a Cr-Mn-Ni austenitic stainless containing copper and nitrogen The terms and expressions which have been employed herein are used as terms of descπption, not of limitation There is no intention in the use of such terms and expressions of excluding any equivalents of the elements or features shown and descπbed or portions thereof However, it is recognized that vaπous modifications are possible within the scope of the invention claimed

Claims: What is claimed is: 1. An austenitic stainless steel alloy having a unique combination of corrosion resistance, formability, and resistance to work hardening, said alloy consisting essentially of, in weight percent, about wt. % C 0.025-0.15 Mn 4-12 Si 1.0 max. P 0.2 max. s 0.1 max. Cr 15.5-17.5 Ni 1-4 Mo 0.25-1.5 Cu 1.5-4 W 1.0 max. Co 1.0 max. N 0.05-0.30 and the balance is essentially iron and the usual impurities; wherein the combined amount of carbon and nitrogen is at least about 0.19%; the combined amount of chromium and molybdenum is less than about 17.75%; and %Cr + 3.3(%Mo) + 13(%N) > 20.5. 2. An alloy as set forth in Claim 1 which contains at least about 6% manganese. 3. An alloy as set forth m Claim 1 which contains at least about 1.5% nickel 4. An alloy as set forth in Claim 1 which contains at least about 0.5% molybdenum. 5. An alloy as set forth m Claim 1 which contains at least about 16.0% chromium 6 An alloy as set forth in Claim 1 which contains less than 9 vol % ferπte in the as-cast condition. 7. An alloy as set forth in Claim 1 which contains not more than about 8.5 vol. % ferπte in the as-cast condition 8. An alloy as set forth in Claim 1 wherein %Cr + 3.3(%Mo) + 13(%N) > 21.0. 9 An alloy as set forth in Claim 1 wherein the combined amount of chromium and molybdenum is not more than about 17.5%. 10. An austenitic stainless steel alloy having a unique combination of corrosion resistance, formability, and resistance to work hardening, said alloy consisting essentially of, in weight percent, about wt. % C 0.05-0.12 Mn 6-10 Si 0.75 max. P 0.1 max. s 0.05 max. Cr 16.0-17.0 Ni 1.5-3.5 Mo 0.5-1.2 Cu 2.0-3.5 W 1.0 max. Co 1.0 max. N 0.10-0.25 and the balance is essentially iron and the usual impurities; wherein the combined amount of carbon and nitrogen is at least about 0.19%; the combined amount of chromium and molybdenum is less than about 17.75%; and %Cr + 3.3(%Mo) + 13(%N) > 20.5. 11. An alloy as set forth in Claim 10 which contains at least about 7% 12. An alloy as set forth in Claim 10 which contains at least about 2.0% nickel. 13. An alloy as set forth in Claim 10 which contains at least about 0.7% molybdenum. 14. An alloy as set forth in Claim 10 which contains at least about 16.25% chromium. 15. An alloy as set forth in Claim 10 which contains less than 9 vol. % ferrite in the as-cast condition. 16. An alloy as set forth in Claim 10 which contains not more than about 8.5 vol. % ferrite in the as-cast condition. 17. An alloy as set forth in Claim 1 wherein %Cr + 3.3(%Mo) + 13(%N) > 21.0. 18. An alloy as set forth in Claim 10 wherein the combined amount of chromium and molybdenum is not more than about 17.5%. 19. An austenitic stainless steel alloy having a unique combination of corrosion resistance, formability, and resistance to work hardening, said alloy consisting essentially of, in weight percent, about wt. % C 0.08-0.11 Mn 7-9 Si 0.50 max. P 0.05 max. S 0.025 max. Cr 16.25-16.75 Ni 2.0-3.0 Mo 0.7-1.0 Cu 2.5-3.0 W 0.75 max. Co 0.75 max. N 0.12-0.20 and the balance is essentially iron and the usual impurities; wherein the combined amount of chromium and molybdenum is not more than about 17.5%; and %Cr + 3.3(%Mo) + 13(%N) > 21.0. 20. An alloy as set forth in Claim 19 wherein the combined amount of carbon and nitrogen is at least about 0.25%.

Inventors: MAGEE JOHN H JR, MAGEE, JOHN, H., JR.
Applicants: CRS HOLDINGS INC, MAGEE JOHN H JR, CRS HOLDINGS, INC., MAGEE, JOHN, H., JR.
Application year: 1999
Main classification: C22C38
IPC codes: C21D8, C22C38