DE2150731A1 - Corrosion-resistant, especially stainless steel - Google Patents

Description

Dipl.-Ing. H. Sauerland · Dr.-lng. R. König · Dipl.-Ing. K. Bergen Patent Attorneys · 4odo Düsseldorf ■ Cecilienallee 7e · Telephone 43373s

Our file: 26 988 October 11, 1971

International Nickel Limited, Thames House, Millbank,

London , SW 1, England

"Corrosion-resistant, especially stainless steel"

The invention relates to a corrosion-resistant, in particular stainless steel. Obgleüi not the invention is limited to this, it arises from the need to have one usable in sea water or a marine atmosphere Wire with sufficient resistance to crevice corrosion and pitting in standing sea water as well as with high Tensile strength of for example 246 to 280 cb with sufficient toughness to create the material suitable for deep-sea cables, ropes and cables.

Stainless steels have also been used in seawater, but the usual austenitic stainless steels have Steels do not have sufficient resistance to standing or low-speed flow Lake water. At moderate or high flow rates, the seawater contains enough oxygen for oxidation of chromium or for the construction or addition of a thin one consisting essentially of chromium oxides Protective film, which normally protects the surface of stainless steels against the ingress of corrosive media, is often destroyed, however, so that crevice corrosion occurs

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or pitting occurs. On the other hand, standing or flowing seawater does not contain enough water Oxygen to form new chromium oxide so that a surface film once destroyed is not built up again. In addition comes that chlorites cause to a large extent the destruction of passivating films and the natural ones, to restore affecting such films leading process. In addition to corrosion resistance, there are also difficulties to create a corrosion-resistant steel with high tensile strength and at the same time sufficient toughness. The usual austenitic stainless steels can only be hardened by cold deformation bring from about 210 cb. Some ferritic or martensitic Although stainless steels have good seawater resistance, they can often only be deformed with difficulty and have the desired strength for further processing up to the end product, for example for stranding wires, insufficient toughness more.

The invention is based on the surprising finding that high corrosion resistance, high strength and sufficient toughness when the corrosion " index, i.e. the chromium content and twice the molybdenum content are above a certain minimum, and the Ferrite index, the austenite index and the index for the austenite stability are carefully matched to one another. The aforementioned indices are defined as follows:

Ferrite index

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Austenite index

+ 0.65 (% Co) + (% Mn) + 1O (96x) + 20 (% y)

Austenite stability index

+ 0.15 (% Co) + (96Mh) + 1O (96x) + 2O (96y),

where χ the total content of carbon and nitrogen in solution up to 0.196 and y the total content Carbon and nitrogen in solution correspond to 0.1 to 0.396.

The invention resides in a stainless steel having 14 to 27.6596 chromium, 3 to 1296 molybdenum for a total of Chromium and twice the molybdenum content of over 28.596, 17.5% nickel and / or 25.5% cobalt, up to 0.3% carbon, 0 to 0.3% nitrogen with a total content of carbon and nitrogen up to 0.3%, 0 to 2% silicon, 0 to 5% Manganese, 0 to 2% niobium, 0 to 4% tantalum with a total content of niobium and half the tantalum content of 0 to 2%, 0 to 5% copper, the remainder including impurities caused by the melting process, iron, its ferrite index, austenite index and Austenite stability index within the ABCA field of Fig. or within the field JKLMJ of FIG.

Fig. 1 of the drawing shows the dependence of the ferrite indices from 22 to 30.65 on the austenite indices between 14.1 and 29.9, while FIG. 2 shows the dependence of the aforementioned ferrite indices on the austenite stability indices between 4.25 and 17.4 reproduces.

In the annealed state, the stainless steel of the present invention has Steel has a predominantly austenitic structure, the austenite of which

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is stable enough to allow a relatively strong work hardening, which is increased by an in At least a part of the austenite is transformed into martensite due to stress.

The corrosion index must be above 28.5%, however
Corrosion resistance is greatly improved when the index is at least 3096. The total content of chromium and molybdenum can be up to 30.65% * but preferably does not exceed 27.5 or 28%, primarily in order to avoid the occurrence of delta ferrite as far as possible, since this structural component causes difficulties in hot forming and, for example, to ferrite and a brittle intermetallic compound breaks down. In addition, the delta ferrite allows less work hardening, since less austenite is then available for the transformation into martensite. This can lead to a lower strength. Nevertheless, 5 or 6 % by volume of delta ferrite can still be tolerated, although advantageously the proportion of delta ferrite does not exceed 2 or 3% by volume.

Alloys with ferrite and austenite indices on the left and above the line DE do not contain any delta ferrite after cooling from the annealing heat, for example from 1150 to 1205 ° C. Lower indices to the right of the line DE result in delta ferrite proportions up to 5 or
6% by volume, in particular at annealing temperatures of 1175 ° C. upwards, the proportion of delta ferrite increasing with the distance from the line DE in the direction of the line AB. That
Nickel has a very beneficial effect on the suppression of delta ferrite. For this and other reasons, the steel according to the invention contains at least V / o, preferably 2 to 10% nickel. Alloys are included in the DEFD field

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preferable, especially with a total chromium and molybdenum content of at least 24%, as they are not only free of delta ferrite but also allow work hardening at room temperature. Accordingly, it is a low temperature treatment below room temperature to -73 ° C not required, as they are between alloys lines FH and CB of Fig. 1 with the temperature falling towards line CB. The ones in the Alloys covered by DEHG not only have excellent technological properties but are also relatively inexpensive.

Although the steel according to the invention has an essentially austenitic structure after cooling from the annealing temperature should have, the austenxt stability must not be too great in order to still achieve sufficient strength. In this context cobalt is of particular importance because, although it is austenitizing promotes during annealing, the M temperature is only slightly reduced, so that there are fewer difficulties than they would in the case of nickel or chrome and thus too great a structural stability for work hardening with the result that the structural transformation into martensite caused by the cold deformation is not without the circumstances can be achieved, which result in a low temperature treatment. The stainless steel according to the invention Steel therefore preferably contains at least 15% cobalt. The correct structural stability is achieved by setting the ferrite and austenite stability index according to the diagram of Fig. 2 are placed on top of each other. The alloys below and to the left of line JK in the diagram of FIG. 2 are not commercially interesting because their work hardening takes place too quickly, while the structure of the alloys

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above the line ML is too stable. The work hardening in the area between the lines RS and ML must however take place below room temperature. Steels with increasing strength, on the other hand, are produced at a distance from the line RS towards the JK line. Alloys that fall into the PQRS field acquire excellent strength and, as a fully formed wire, meet the toughness requirements the winding and the kink test. To the extent that the field adjoining the JK line forms the line When PQ is reached, the work hardening also increases and difficulties can arise in view of the greater reduction in cross-section occur during cold forming.

Although the total content of carbon and nitrogen can be up to 0.3%, there are considerable advantages when the total content of these elements does not exceed 0.2% and the carbon content is 0.01 to 0.08%. The nitrogen content should preferably not exceed 0.1% and is advantageously up to 0.05%. The Elements Carbon and nitrogen are major contributors to the strength of drawn wire and others work-hardened products, but reduce the M -Tem-

temperature is quite considerable, especially if their total content exceeds 0.1 to 0.2%. On the other hand, this can lead to a Lead steel that is more stable than required in the annealed state.

The silicon content should preferably not exceed 1% and the manganese content should preferably not exceed 2%. Silicon stabilizes the ferrite, so that with increasing silicon content either higher contents of nickel and / or cobalt and thus higher costs or lower contents of chromium and / or molybdenum and consequently an impairment of the corrosion resistance are necessary . Manganese can lead to the formation of a

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detrimental sigma phase.

Low but effective niobium content, e.g. 0.25 to 2%, improve the corrosion resistance in particular together with carbon contents not above 0.06 to 0.08%. Tantalum can take the place of niobium, with two parts tantalum replacing one part niobium. Niobium and tantalum are in of the formula for the ferrite index used without a factor because they are quite strong ferrite formers. The inventive Alloy can also contain up to 5% copper.

The steel according to the invention can be melted from commercially pure or highly pure starting materials; In addition to the residual iron content, it can also contain the usual impurities caused by the melting process, so-wät these do not affect the material properties. These contaminants also include deoxidation residues and refining agents such as zirconium, boron, calcium, magnesium and titanium. Impurities such as sulfur, phosphorus However, hydrogen and oxygen should be kept as low as is economically feasible.

With regard to its corrosion resistance, strength and toughness, a steel according to the invention has proven itself with 16 to 22% Chromium, 4 to 8.5% molybdenum with a total content of chromium and twice the molybdenum content of at least 30 2.5 to 6.5% nickel, 15 to 25% cobalt, 0.01 to 0.15% carbon, 0 to 0.1% nitrogen for a total of carbon and nitrogen of at most 0.2%, with 0 to 0.75% silicon and 0 to 2% manganese, particularly proven. One A steel according to the invention with 17 to 20% chromium also has an excellent combination of technological properties, 5.5 to 7.5% molybdenum with a total content of chromium and the

twice the molybdenum content of at least 30.5, with 3 to 5% nickel, 17 to 23% cobalt, 0.01 to 0.08% carbon, 0 to 0.04% nitrogen, 0 to 0.75% silicon and 0 to 2% manganese. In the case of the two steels mentioned above, the Alloy residue each made of iron including impurities caused by the melting process and the alloy components are coordinated in such a way that the ferrite, the austenite and the austenite stability index within of the ABCA or JKLMJ field, preferably within the DEFD or PQRS field.

The steel according to the invention can be produced in the usual way, although the temperature during the equalization annealing preferably does not exceed 1230 to 1260 ° C to limit the formation of delta ferrite. The annealing is advantageously carried out at 1150 to 1205 ° C. For applications that require a large reduction in cross section a single or multiple intermediate annealing is carried out in order to achieve maximum strength and toughness. aside from that If the heat treatment is chosen appropriately, the manufacturing costs can be reduced. The temperature of the Intermediate annealing should be 260 to 510, preferably 370 to 480 ° C, if not the structure again first is austenitized. In this case considerable cold working should be carried out to avoid any loss to keep corrosion resistance as low as possible. The intermediate glow can be very short and, for example, 1 minute last; its effectiveness depends on the martensite content of the work-hardened Good. With a structure with less than 10% martensite there is no noticeable increase of strength. Since a completely martensitic wire tends to become brittle, it should be ensured that during the intermediate annealing the martensite content is at least 10% but not more than 60%.

209873/0 587

The invention is summarized below with reference to those in Table I. Alloys 1 to 14 according to the invention explained in more detail. These alloys were excepted of alloy 6, which was melted under argon and cast in air, all melted in a vacuum induction furnace. Also in Table I are two comparative alloys A and B with compositions outside the scope of the invention listed.

Table I.

Legie : Mon tion : (90 1 : 7.0 2 : 8.3 3: : 6.2 4: : 9.2 VJl : 7.8 6: : 6.9 7: : 5.6 8th : : 4.0: 9: : 6.2: 10: . 4.0. 11: . 6.0: 12: . 6.1: 13: : 7.0: 14: : 6.1 i A: . 4.0: B: : 4.0 i : Cr : Ni ί (#) : (90 : 18.3 : 5.2 : 16.4 20.1 ■ 2.2 : 15.6 : 2.1 i . 17.8 : 2.2! 18.3 : 4.5. : 17.8 : 3.5 i 21.9 . 2.2: 18.1 2.5: 20.7: : 3.7: 18.2 ' 7.4 i 18.1 ■ 5.1 i 18.1 5.3: 17.8 3.9: 18.2: 6.5: 16.2 -; : Co : (90 : 20.2 : 23.4 i : 25.2: . 25.2: • 25.1: , 20.2: : 21.0: 25.3: 21.5: 21.4: 13.9: 17.5: 20.2: 19.4: 13.5: 19.2: : C ι (%) ι 0.021 ; 0.086 ; 0.053 ί . 0.061: : 0.050: . 0.038 , 0.030: : 0.052: 0.079: 0.048 j 0.094 j 0.065: 0.005: 0.060: 0.036: 0.037: : Si ί (90 : 0.10 : 0.17 . 0.13 y : 0.13 0.12: : 0.16: . 0.13 . 0.13: 0.30: . 0.13! . Oo30: Oo30 j 0.16: 0.30: 0.14: 0.22: : Mn • (QJL \ : 0.18 : 0.10 : 0.14: : 0.12: . 0.15: . 0.20. : Ο _Ο 14: 0.16: OZlO: 0.15: 0.17: 0.19: 0.20: 0.16: 0.13: 0ο18: : Weight ratio ί lust (Mg : 3.6 ί 0.2 4.6 . 4ο8 5.2 : 5.2 • 6.9 7.4 9.0 : 9.5 11.0 13.0 16.5 19.5 . 635 454

Each of the test alloys contained the balance including iron impurities caused by the melting process.

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Π 50731

- ίο -

The test alloys were made from very pure starting materials, ie from electrolyte iron, nickel and cobalt as well as molybdenum pellets under protective gas, melted chromium, low-carbon ferromanganese and spectographically pure carbon. The initial or partial deoxidation was carried out by adding carbon, ferromanganese and chromium were added at the same time. The final deoxidation was carried out with aluminum, zirconium and calcium-silicon. The test alloys were cast at around 1510 C to form 13.6 kg square blocks with an edge length of 89 mm, which were then subjected to equalizing annealing at 1205 ° C for two hours and then forged down to a square cross-section with an edge length of 70 mm. The samples were then reheated to 1205 ⁰ C, forged mm on a square cross-section with an edge length of 51, halved, heated again to 1205 ° C and then rolled to 15 mm RCS billets and thereafter to 6.4 mm thick plates.

The samples of the plates were ^{annealed for one hour at 1205 °} C. and quenched in water and ground down to a thickness of 5.7 mm and cold-rolled down to 1.3 mm. Square test pieces with an edge length of 15.4 were cut out of the cold-rolled sheet metal, deburred and ground to a surface roughness of 240.

In order to determine the corrosion resistance in standing sea water, the samples were the in "Journal of the Electrochemical Society, Vol. 103, No. 7, pp. 375-390. The samples were six times with wrapped in a rubber band to form artificial gaps, and then placed in a strong one at room temperature for 72 hours corrosive solution of 10% iron (III) chloride immersed. the

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Samples were then rinsed with water and removed from the corrosion products freed as well as dried and scalloped.

Two test pieces each were subjected to the aforementioned test, the ones listed in Table I above showed mean weight losses. The weight loss is an indication of the corrosion resistance of the material. A weight loss of up to 0.5 mg is an indication of a practically corrosion-resistant steel without pitting and crevice corrosion. With slight crevice corrosion there is a weight loss of 0.5 to 5 or 10 mg without pitting, but with little discoloration in at least one groove of at least one rubber turn. Moderate to strong Crevice corrosion is due to a weight loss of 10 to 100 mg without pitting, but with stronger discoloration and Pitting associated. The steels according to the invention should show a weight loss after the above-mentioned experiment do not have more than 20 mg. However, the weight loss is advantageously only 10 mg.

The corrosion test described above confirms that all fourteen alloys covered by the invention are one have adequate corrosion resistance. In addition, it should be noted that even with 18-month-olds Tests in seawater flowing at low speeds showed excellent corrosion resistance.

The tensile strength and toughness of three tests each result from the following Tables II and III. The test alloys were melted in the manner described above and heat-treated or deformed. Specifically, pieces of wire were examined which were similar to that given in Table II Cross-section decrease had been drawn. In addition, samples subjected to intermediate annealing were carried out with the

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Belle III examined the results given. In Tables II and IH, "P" means corrosion-resistant, "M" means less Attack, "F" not corrosion resistant.

Table II

Legie 6.1 » 6.0 ι
« ; _ P. tion 6.0: : P 15th ■
ι : P • : - 5.7: : P 16 : P : - 6.2: . P. 17th P. - 6.1: P. 18: ρ - . Co .Ni. C .Si .Mn .Ab-ZugSßiid P. 19: & ■ wrap P. : (%) Ά%) 1%) *. (%): (%) Yisbssi do-ysp ^: vers. 21.5: 2.5: 0.079: 0.30 <D.10: 90: 243: - + - : 17.5: 5.1: 0.065: 0.30: 0.19: 90: 221: - ::::: 98: 290: F: F 20: :::::: 98: 264: P ::::: 99: 297: F: P :::::: 99: 271: P 24.3: n.a: 0.099: 0.30: 0.18: 90: 243: -: - : 19.4: 3.9: 0.06: 0.30: 0.16: 90: 237: - ::::: 95: 295: F: F 21: :::::: 98: 284: M ::::: 97: 287: F: :::::: 99: 290: M : 15.6: 6.2: 0.085: 0.30: 0.17: 90: 204: - :::::: 98: 243: P: • / θ / ι
ν / 0 J ::::: 99: 243: P: : 18.1 13.9: 7.4: 0.094: 0.30: 0.17: 90: 269: - s • ::::: 98: 221: P ι ::::: 99: 226: P: : 17.8 11.3: 8.7: 0.050: 0.30: 0.17: 90: 185: -: ::::: 98: 226: P: ::::: 99: 225: P: : 18.0 18.2: • 17.1: « β
• 18.1: 17.6: . Mon ; (ji) : 6.1 * >

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Alloy cross section Zugf. Decrease cb v% J

Zugf.

"A"

cb

Table III Heat treatment

Zugf. "B" cb

Zugf. "C" cb

toughness

j ^ β "C"

Knickv.Wickelv.Knickv.Wickelv.Kni ^ ckv.Wickel

16

17th

18th

19th

20th

21

90

98 99 90 98 99 90 98 99 90

98 99

90 98

99 90 98 99 90 95 97

: 221: 257: : 264 y . 288: : 271 : 299: : 237 : 284: : 284 : 318: ; 290 : 327: : 198 : 230: : 221 : 266: : 226 : 255: : 198 : 212: : 221 : 252: : 226 : 241: : 185 : 207: : 226 : 236: : 225 : 236: : 243 : 318: : 290 : 288: : 297 : 333: : 243 : 297: : 295 : 325: : 287 y _ j

265 305 315 brittle

222 246 248 217

219 223

206 ·

229 229

brittle

H U U

225 290 295 265 316 320 220

255 264 211

231 234

203

229 234

284 326 *

307 339 **

F.
F.
F.
F.

P P

F P

P P

P P

P P

F F F F

M F

P P

P P

P P

P P

P P

P P

F F

P P

P P

P P

F F F

P P

P F

-W

P P

P P

MP F

* ■ Cross-sectional decrease 96% ** Cross-sectional decrease 93% 209823/0587

In the tests of Table II, rods 12.7 mm thick were machined and used on a draw bench a copper pad with soap as a lubricant drawn down to a diameter of 2.5 mm or 1.5 mm. Afterward the copper plates were removed and the samples annealed, if necessary. After sandblasting and pickling The test wires were annealed to their final diameter with a diamond drawing die and oil as a lubricant pulled down. Three samples of each composition were drawn with cross-sectional reductions of 90%, 98% and 99%. In the kink attempt, the wires were tied in a loop and this was pulled so tight that no through the box More light shimmered through. During the winding attempt, the wire was wrapped around itself several times.

Alloys 15 to 17 show that the exceptionally high tensile strength is accompanied by sufficient toughness is. Alloys 18 and 19 are marginal in terms of their strength; however, this can be the same as the data in the table III show, can be noticeably improved by a heat treatment. Alloys 20 and 21 survived the Both corrosion tests are not and are therefore not suitable as a material for submarine cables and cables, although they are is used for fasteners, especially for screws with cold applied threads, and other cold formed items such as sheet metal and reinforcing steel.

Heat treatment A mentioned in Table III consisted of 15 minutes. Annealing a wire drawn to 1.3 mm in diameter at 427 ^° C.; it was followed by a drawing to the final diameter. In the heat treatment B, the annealing temperature was 538 ⁰ C, while the temperature of the heat treatment C at 65O ⁰ C was. The heat treatment B brought about hardening and the heat treatment C brought about austenitizing, in which the during drawing to a diameter of 1.3 mm

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The martensite that has been left is completely converted back to austenite became. However, this austenite was quite unstable and easily settled in the final drawing to the final diameter convert to martensite.

The heat treatments A and C show that a substantial increase in strength can be achieved with them. It also shows that hardening according to heat treatment B should be avoided because of the associated tendency to become brittle. When using intermediate annealing, the temperature should not exceed 480 ° C. or, if possible, 510 ^° C. unless the structure of the steel has first been converted into austenite according to heat treatment C.

The stainless steel according to the invention is not only suitable as a material for wire, in particular for submarine cables and lines in standing water but also for objects such as fastening elements, e.g. screws, structural parts and reinforcing steel, springs, nets and screens, cold forged parts, precision forgings, prostheses, pipes and containers.

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Claims (15)

Claims; 1. Stainless steel, consisting of 14 to 27.65% chromium, 3 to 12% molybdenum for a total of chromium and twice that Molybdenum content of a maximum of 28.5 to 17.5% nickel and / or up to 25.5% cobalt, up to 0.3% carbon, 0 to 0.3% nitrogen with a total content of carbon and nitrogen of no more than 0.3%, from 0 to 2% silicon, 0 to 5% manganese, 0 to 2% niobium, 0 to 4% tantalum with a total content of niobium and half the tantalum content of 0 to 2% and 0 to 5% Copper, the remainder iron including impurities caused by the melting process, their ferrite, austenite and austenite stability index are in field ABCA of FIG. 1 or JKLMJ of FIG. 2. 2. The stainless steel of claim 1 but containing at least 1% nickel and 15% cobalt. 3. Stainless steel according to claim 1 or 2, the total content of chromium and twice the molybdenum content, however, at least 30%. 4. Stainless steel according to one or more of claims 1 to 3, but containing 0.01 to 0.08% carbon and a total content contains not more than 0.02% of carbon and nitrogen? 209823/0587 5. Stainless steel according to one or more of claims 1 "to 4, the ferrite and austenite index within the Field DEFD of FIG. 6. Stainless steel according to claim 5, but whose ferrite and austenite index are within the DEHG field of FIG lie. 7. Stainless steel according to one or more of claims 1 to 6, its ferrite and austenite stability index lie within the field PQRS of FIG. 8. Stainless steel according to one or more of claims 1 to 7, which, however, 16 to 22% chromium, 4 to 8.5% molybdenum a total content of chromium and twice the molybdenum content of at least 30%, as well as 2.5 to 6.5% nickel, 15 to 25% cobalt, 0.01 to 0.15% carbon, 0 to 0.1% nitrogen for a total of carbon and nitrogen Contains no more than 0.2% and 0 to 0.75% silicon and 0 to 2% manganese. 9. Stainless steel according to one or more of claims 1 to 7, but 17 to 20% chromium, 5.5 to 7.5% molybdenum with a total chromium content and twice the molybdenum content of at least 30.5% and 3 to 5% nickel, 17 to 23% Cobalt, 0.01 to 0.08% carbon, 0 to 0.04% nitrogen Contains 0 to 0.75% silicon and 0 to 2% manganese. 10. Rust mill steel according to one or more of claims 1 up to 9, but containing up to 2% niobium. 11. A method for heat treatment of a steel according to the claims 1 to 10 after work hardening to a structure containing at least 10% martensite, g e - 209873/0587 characterized by annealing at 260 to 510 ⁰ C and subsequent cooling. 12. The method according to claim 11, characterized by annealing at 370 to 480 ⁰ C. 13. A method for heat treating a steel according to claims 1 to 10 after a work hardening on a A structure containing at least 10% martensite, characterized by intermediate austenitizing before further work hardening, 14. Use of a steel according to claims 1 to 13 as a material for objects such as wire and submarine cables or In the cold-hardened state, cables have a high corrosion resistance in standing sea water as well as a high one Must have strength and toughness. 15. Use of a steel according to claims 1 to 13, as a material for objects such as screws, structural parts, Reinforcing steel, springs, nets, sieves, cold and precision forgings, prostheses, pipes and containers in the work-hardened state, in addition to good corrosion resistance, high strength and toughness must own. 209823/0587