EP1851351B1

EP1851351B1 - Austenitic stainless steel

Info

Publication number: EP1851351B1
Application number: EP06708260A
Authority: EP
Inventors: Mario Cusolito; Marco Valsecchi; Pedro M. Corcuera Amurzia
Original assignee: RODACCIAI SpA
Current assignee: RODACCIAI SpA
Priority date: 2005-02-14
Filing date: 2006-02-14
Publication date: 2012-08-15
Anticipated expiration: 2026-02-14
Also published as: ES2390678T3; AU2006212194B2; AU2006212194A1; WO2006084919A1; US20080206088A1; CA2597750A1; EP1690957A1; ZA200705778B; EP1851351A1

Abstract

A new austenitic stainless steel with the following composition by weight is described: 0.03 % < carbon < 0.07 %, 7.0 % < manganese< 8.5 %, 0.3 % < silicon 0.7 %, sulphur < 0.030 %, phosphorus < 0.045 %, 16.5 % < chromium < 18.0 %, 3.5 % < nickel < 4.5 %, 0.1 % < molybdenum < 0.5 %, 1.0 % < copper < 3.0 %, 0.1 % < nitrogen < 0.3 %, the difference consisting in iron and common process impurities. The steel thus obtained has an optimum combination of corrosion resistance, deformability and work- hardening properties, which make it suitable as a substitute for normal steel type 1.4301 in various specific applications.

Description

The present invention relates to a new austenitic stainless steel with a low nickel content which has special characteristics in terms of corrosion resistance in given environments, deformability and suitability for work-hardening. The steel according to the present invention is characterized by the following chemical composition: $0.03 % < carbon < 0.07 %$
$7.0 % < manganese < 8.5 %$
$0.3 % < silicon < 0.7 %$
$sulphur \leq 0.030 %$
$phosphorus \leq 0.045 %$
$16.5 % < chromium < 18.0 %$
$3.5 % < nickel < 4.5 %$
$0.1 % < molybdenum < 0.5 %$
$1.0 % < copper < 3.0 %$
$0.1 % < nitrogen < 0.3 %$

the difference consisting in iron and common process impurities.
A very important characteristic of the new steel is the small amount of nickel it contains: it is in fact well known that the price of this element is unstable, with a continuous tendency to increase, resulting in continuous variations in the costs of the articles produced with materials which contain this element.

Background art

Austenitic stainless steel is an iron and carbon alloy containing various other elements, the main ones of which are chromium and nickel. The combination of these elements gives the steel a basic property of corrosion resistance owing to the formation of a protective surface film which is due to the presence of a chromium content of at least 1.11% and whose qualities are improved by the presence of nickel and other elements. Other typical properties of austenitic stainless steels are the very low magnetic permeability (non-magnetic property), heat resistance, cold deformability and suitability for work-hardening. Owing to these properties, austenitic stainless steels are used in a very wide range of applications.

1.4301 steel

The most well known and widely used type of austenitic stainless steel contains about 18% chromium 10% nickel and has always been referred to as 18/10 steel. In the European standard EN 10088-3 1997 this steel has been called X5CrNi18-1 and has been attributed the steel number 1.4301. In the United States standard AISI this steel is called 304. The percentage by weight chemical composition envisaged for this steel by the European standard is as follows:

C = 0.07 max
Si = 1.00 max
Mn = 2.00 max
P = 0.045 max
S = 0.030 max
N = 0.11 max
Cr = between 17.00 and 19.50
Ni = between 8.00 and 10.50

In the case of products which are intended to be machined, the same standard envisages a variant whose sulphur content is controlled (or "micro-resulphurised") where

S = between 0.015 and 0.030

It should be noted that the maximum sulphur content coincides with that of basic steel, so that in fact this is not another steel, but only a variation of the same type 1.4301 obtained by dividing the analytical range permitted by sulphur. Sulphur has the capacity to weaken the metallic matrix and therefore improve the machinability during the swarf removal operations. At the same time, however, sulphur, even though present in limited amounts, modifies the corrosion resistance. This micro-resulphurised variant is cited here because below it will often be used for comparison with the type 1.4301 steel and with the steel of this invention.
1.4301 steel has extremely broad technological and corrosion properties such it has been become very widely established in the engineering sector as a structural material as well as in the environmental sector: it is in fact widely employed in the transportation, architecture and the domestic sectors, being used at high temperatures and in corrosive environments. The type 1.4301 is the most well known, widespread and researched in the sector of austenitic stainless steels and therefore is used as a reference type for comparing the characteristics of other austenitic stainless steels.

Other comparison steels

There exist other steels with a similar composition which differ owing to small analytical variations of a certain element which give them an improved property. Some of these steels are mentioned here because below they have been used for comparison with the steel according to the invention in order to highlight its characteristics. The type 1.4307 - X2CrNi18-9 (AISI 304L in the US standards) is a steel similar to the preceding one, but with a limited carbon content which improves the intergranular corrosion resistance. The chemical composition of type 1.4307 steel is as follows:

C = 0.03 max
Si = 1.00 max
Mn = 2.00 max
P = 0.045 max
S = 0.030 max
N = 0.11 max
Cr = between 17.50 and 19.50
Ni = between 8.00 and 10.00

The type 1.4306 - X2CrNi19-11 is a further low-carbon variant with a greater content of nickel which is added in order to improve the cold deformability and the corrosion resistance. The chemical composition of this type is as follows:

C = 0.03 max
Si = 1.00 max
Mn = 2.00 max
P = 0.045 max
S = 0.030 max
N = 0.11 max
Cr = between 18.00 and 20.00
Ni = between 10.00 and 12.00

The type 1.4567 - X3CrNiCul8-9-4 is a version with the addition of copper in large amounts for the purpose of improving the cold deformability: it is used for those particular cold-pressed products where the preceding types are unable to withstand the extreme deformation, such as, for example, hexagonal socket head screws. The chemical composition is as follows:

C = 0.04 max
Si = 1.00 max
Mn = 2.00 max
P = 0.045 max
S = 0.030 max
N = 0.11 max
Cr = between 17.00 and 19.00
Cu = between 3.00 and 4.00
Ni = between 10.00 and 12.00

The characteristics of austenitic stainless steels

The main characteristics of an austenitic stainless steel are its corrosion resistance, non-magnetic nature, cold-deformability and suitability for work-hardening. These characteristics are obtained by modifying various factors, including the chemical composition: in addition to chromium and nickel, the other secondary elements have an important effect. The effect of chromium, referred to as "alphagenic", tends to stabilize the ferritic phase of the materials (alpha phase): other elements, such as silicon and molybdenum, behave in the same manner as chromium, although to a lesser degree. The same applies to nickel, which is a "gammagenic" element, and therefore has a stabilizing effect on the austenitic phase (gamma phase): various elements such as carbon, nitrogen, copper and manganese behave in the same manner as nickel.

The nickel content of austenitic stainless steels

Most of the known austenitic stainless steels used on the market have nickel contents of about 8-10%, as in the case of the types mentioned hitherto. During the last few years, the worldwide economic situation has resulted in the price of nickel being very unstable, with a marked tendency to increase.
Manufacturers and retailers of stainless steels therefore have difficulty in operating within a fluctuating market, so much so that nowadays in Europe the price of these products is composed of a base price and an additional price, referred to as "alloy add-on", which is defined at the time of delivery: the "alloy add-on" varies with predefined mechanisms depending on the value of nickel on the world market. Steel product processing companies, for their part, have difficulty in establishing the prices of the parts produced since they cannot know the exact price of the raw material until the time of delivery.
For this reason, different austenitic stainless steels with low nickel contents have been researched: some of these, which are more widely used and have been known for some time, are included in various standards and used because of their specific characteristics. Others have been recently developed with the aim of obtaining some of the basic characteristics of austenitic stainless steel. In fact, by suitably increasing the content of the less costly "gammagenic" elements (nitrogen, copper and manganese), it is possible to obtain an austenitic stainless steel which is equally stable, but has a low percentage content of nickel (and therefore a price which is less dependent on the fluctuations of the cost of nickel) and with one or more technological properties the same as those of normal conventional austenitic steels with a higher nickel content. Austenitic steels with a low nickel content are for example described in EP593158 , EP694626 , EP896072 , EP969113 e WO 00/26428 .

Subject of the invention

The subject of the present invention is a steel having a nickel content which is markedly lower than that of basic steel type 1.4301 (AISI 304) and which, with suitable balancing of the other elements, has many properties similar to the corresponding properties of basic steel type 1.4301 (AISI 304); it has the composition shown below: $0.03 % < carbon < 0.07 %$
$7.0 % < manganese < 8.5 %$
$0.3 % < silicon < 0.7 %$
$sulphur \leq 0.030 %$
$phosphorus \leq 0.045 %$
$16.5 % < chromium < 18.0 %$
$3.5 % < nickel < 4.5 %$
$0.1 % < molybdenum < 0.5 %$
$1.0 % < copper < 3.0 %$
$0.1 % < nitrogen < 0.3 %$

where the difference consists in iron and common process impurities.
The steel according to the present invention may be obtained by means of the conventional processes for the preparation of austenitic stainless steels, such as those for example described in "ASM Specialty Handbook - Stainless Steels" edited by "The Material Information Society" - USA. Preferably it has the composition indicated below: $0.04 % < carbon < 0.06 %$
$7.5 % \leq manganese < 8.0 %$
$0.4 % < silicon 0.6 %$
$0.002 % < sulphur 0.004 %$
$0.030 % < phosphorus < 0.035 %$
$17.0 % \leq chromium < 17.5 %$
$3.8 % < nickel < 4.2 %$
$0.1 % < molybdenum < 0.3 %$
$2.0 % \leq copper < 2.5 %$
$0.15 % < nitrogen < 0.2 %$
According to one of the possible embodiments of the invention, the sulphur is less than 0.005 %. According to another possible embodiment, which does not exclude the previous embodiment, the nickel is higher than 4.0 %. According to the best embodiment of the invention, the carbon is about 0.055 %, the manganese is about 7.50 %, the silicon is about 0.52 %, the sulphur is about 0.003 %, the phosphorus is about 0.032 %, the chromium is about 17.0 %, the nickel is about 4.0 %, the molybdenum is about 0.19 %, the copper is about 2.0 % and/or the nitrogen is about 0.17 %.
In order to define the characteristics of the product obtained with the newly invented steel, its main performance features have been studied and compared with those normally encountered in basic 1.4301 steel and similar steels: the results have proved to be very positive since, for the same functional characteristics, the cost of the steel is decidedly lower than that of basic steel type 1.4301 and in any case not so closely dependent on the nickel market.
The characteristics considered on the pages below have been obtained by means of varying castings of the new steel, all carried out with analyses similar to that of the best embodiment mentioned above.

Stress corrosion cracking

The steel according to the present invention presents a higher resistance to "stress corrosion cracking" (also called "delayed corrosion") than the steels commonly known in the art and, in particular, than those disclosed by WO 00/26428 , EP896072 or EP969113 . Such a higher resistance can be explained through the selected nickel range of between 3.5 and 4.5% by weight, as for instance subsequently demonstrated by J. Charles, Stainless Steel '05, Proceedings of the 5th European Congress Stainless Steel Science and Market, Seville, September 27-30, 2005 (pages 19-26).
This improved resistance to "stress corrosion cracking" makes the steel of the present invention particularly suitable for the manufacture of wires having a "deep drawing ratio" and which could be exposed to aggressive environments as for instance wires for agricultural use, electric household appliances, bicycle spokes; wires for laundry drying frames; wires for architecture, for meshwork and for hooks used on slate roofs.

Cold deformability by means of drawing

For the reduction in cross-section r the following relation is applicable: $r = \frac{A_{0} - A_{1}}{A_{0}} \cdot 100$

where:

A₀ = Initial cross-sectional area
A₁ = Final cross-sectional area

The drawing of the rolls is performed by means of successive passes through the tools (drawing dies) which deform the product, gradually decreasing its cross-section.
During deformation, a phenomenon called work-hardening, proportional to the reduction, occurs, said phenomenon resulting in an increase in the tensile properties of the material (R_m , R_p(0,2) ) and a decrease in the plastic properties (A, Z), up to the point where the material is no longer deformable. When work-hardening is such that the material no longer possesses plasticity, the wire breaks during further passes through the drawing dies and the product can no longer be drawn.
Under normal conditions with multiple-pass drawing machines operating at suitable industrial speeds, the reference stainless steel 1.4301 (AISI 304) is able to withstand drawing reductions of up to 88%. Beyond these values the work-hardening is such that the material breaks and is no longer capable of being deformed.
The stainless steel according to this invention, under identical conditions, is able to withstand drawing with reductions in the cross-section in the region of 92-94%.
This data is very important for detailed work where small diameters of the drawn wire are required, with the result that a certain amount of annealing during the reduction cycles may be dispensed with.

Table 1 shows the tensile strength and elongation at break values of the steel according to the invention for various degrees of reduction during drawing, compared with two reference steels: steel type 1.4307 with a low carbon content (about 0.02%) and steel type 1.4301 with a slightly higher carbon content (0.04%).

Table 1: Mechanical properties depending on work-hardening
% reduction	New steel		1.4307 low C		1.4301
	R	A	R	A	R	A
	MPa	%	Mpa	%	MPa	%
0	659	42	580	42	569	42
17.4			770	23	810
35.1	1045	12			952	12
56.4	1390	3.5	1140	4.0
67.9					1420	4.0
70	1583	2.5	1320	3.0
76					1610	1.5
84	1803	1.5	1490	2.5	1700	1.5
87.7					1750	1.5
90.3	1932	1.2
92	2000	1.0

Figure 1 shows in graph form the tensile strength values as a function of the drawing reduction for these steels, while Figure 2 shows the same type of comparative graph relating this time to the percentage elongation at break value.
The work-hardening is due to the partial and progressive transformation of part of the austenite into martensite, which is the hardest component of steel. A metallographic study was carried out on samples taken from materials in the annealed and work-hardened state, these revealing both the deformation of the grain, with elongation in the drawing direction, and the austenite-martensite transformation.
Figure 3 shows a longitudinal metallographic cross-section through the product in an ultra work-hardened state of the wire obtained with the new steel, in which the work-hardening lines due to the martensitic transformation are clearly visible.
Figure 4 shows the same type of cross-section carried out on a sample of the reference steel type 1.4301 (AISI 304).

Relative magnetic permeability µ _r

The relative magnetic permeability measures the ratio between the magnetic permeability of a material µ and that of a vacuum µ₀. $μ_{r} = \frac{μ}{μ_{0}}$
The magnetic permeability of a material µ (measured in Henry/metre [H/m]) is defined by the ratio between the magnetic induction value B and the value of the magnetizing force H.
The magnetic permeability of the vacuum is equal to µ₀ = 1.256 x 10^-6 H/m.
The magnetic permeability of a material basically measures the ferromagnetism, i.e. the property of a steel to react with a magnetic field of given value.
In the case of stainless steels, the martensitic structure is ferromagnetic (µ_r=700-1000), while austenite is practically non-magnetic (µ_r<1,2).
An austenitic steel in the solubilized state, and hence with a totally austenitic structure, is completely non-magnetic: when it is subjected to a magnetic field, for example that of a magnet, it does not react.
An austenitic steel in the work-hardened state, for example after undergoing drawing reductions, is increasingly more magnetic depending on the percentage of austenite transformed into martensite (basically dependent on the drawing reduction and the chemical composition).
For this reason, a steel type 1.4301 (AISI 304) which in the solubilized state is non-magnetic, after reductions with value of about 65%, has a structure which is partially ferromagnetic with a relative magnetic permeability of about µ_r = 1.50 (with a magnetic field of 4000 A/m); after reductions of 85%, its relative magnetic permeability rises to µ_r = 2.20 with the same magnetic field.
The steel according to the present invention remained perfectly non-magnetic also following numerous drawing operations: under the same test conditions, with reductions of 65%, we obtained a permeability µ_r = 1.10, while with reductions of 85% the permeability rose only to µ_r = 1.30.
The magnetic permeability in a stainless steel assumes particular importance both in the case of more complex applications (e.g. solenoid valve bodies, where the part must not be influenced by the magnetic field of excitation of the valve), but also for more straightforward applications, where recognition of the material is simply carried out by means of a magnet, as in the case of laundry drying frames sold at markets or in supermarkets: if the wire of the laundry drying rack is not attracted by the magnet, it is recognised as being austenitic stainless steel and is much more highly valued than the corresponding wire made of ferritic stainless steel or even galvanized iron, which are both highly ferromagnetic. The possibility of obtaining drawn wires with high work-hardening values (required by the product itself in order to withstand the load of wet laundry), without any significant variation in the magnetic permeability, results in the invention being particularly suitable for this type of use.

Cold deformability by means of pressing

Tests for the production of screws by means of cold deformation were carried out as follows:

Hexagonal-head screws (DIN 933 M5 x 25): for this product a steel type 304L with Cu content of about 0.9% is used.
Socket-head cap screws (DIN 912 M5 x 12): for this type of product normally a steel type 304Cu is used, with the addition of 3-4% Cu in order to improve the deformability.

The characteristics of the screws produced were determined by means of tensile tests carried out in accordance with the standard UNI EN ISO 3506 part 1 edition February 2000 and HV 500 microhardness tests.

The results of the tensile test are shown in Table 2

Table 2: Results of tensile tests carried out on cold-pressed screws
Type of screw	Dimensions	Material	Breaking load R_m Mpa	Upper yield point R_{p(0, 2)} Mpa	Elongation at break A %
DIN 933 Sheared hexagonal head screw	M 5 x 25	1.4306 (304L)	967	754	2.7
DIN 933 Sheared hexagonal head screw	M 5 x 25	New steel	1137	887	2.8
DIN 912 Socket head screw	M 5 x 12	1.4567 (304Cu)	865	675	2.3
DIN 912 Socket head screw	M 5 x 12	New steel	1160	905	2.2

Figure 6 shows the microhardness values determined at various points in the longitudinal section of the screws DIN 933 M5 x 25 produced.
In the same manner, Figure 7 shows the microhardness values detected at various points of the cross-section of screws DIN 912 M5 x 12.
Before commenting on these results, it should be noted that the reference standard for stainless steel screws (UNI EN ISO 3506-1 "Mechanical properties of corrosion-resistant stainless steel connecting elements - screws and stud screws") does not permit at the moment this type of austenitic steel. It may, however, be possible to apply for and obtain inclusion of the newly invented type in the future standard for screws, thus allowing its use.
The comparisons have been made, as always, with screws made of normal steel type 1.4301 (AISI 304). The screws made with the steel according to the present study had a higher tensile strength of about 70 MPa in the case of hexagonal head screws and 95 MPa in the case of socket head screws: this greater difference is due to the very poor work-hardening property of the 304Cu steel used for the comparison. Likewise, the hardness values are about 100 HV points higher in the case of the steel according to the invention. All the mechanical properties recorded are, however, within the limits stipulated by the standard for quality A4 screws (corresponding to the reference steel 1.4301) with strength class 70 or 80 (relating, therefore, to "work-hardened" or "ultra work-hardened" materials).
These results of pressability must be related to the technical possibility of producing screws by means of cold deformation using the new steel. Considering the corrosion-resistance properties of this steel (described in the following paragraphs), it seems possible to request, in due course, broadening of the range of steels accepted for the production of screws, at least as regards the strength class 80 (that of ultra work-hardened steels), which is sometimes difficult to achieve with normal austenitic steels.

Corrosion resistance of the semifinished starting product

Corrosion-resistance tests were carried out using samples obtained by means of machine-tool processing of solubilized wire rod.
The types of steel which underwent the tests were, in addition to the steel of the present invention, also two castings of austenitic steel type 1.4307 consisting of the micro-resulphurised variant (S=0.030 for machine-tool processing) and the variant with a very low sulphur content (0.003).

The tests carried out and the corresponding reference standard, where applicable, are listed in Table 3.

Table 3: Corrosion tests carried out on samples obtained from solubilized wire rod
Test in 20% sulphuric acid	--	1 cycle of 96 hours at +20°C
Test in 65% nitric acid	ASTM A262 test C	3 cycles at 48 hours at boiling temperature - change of solution with each new cycle
Test in 6% ferric chloride	ASTM G-48	1 cycle of 72 hours at 22°C +/-2

The results for the test with 20% sulphuric acid are shown in the graphs of Figure 9. Similarly, Figure 10 shows the results of the test in 65% nitric acid carried out on the same steels. In Figure 11, the corrosion test was carried out in 6% ferric chloride.
From the results it can be easily understood that in this type of test, the progression of the corrosion is greatly influenced by the sulphur content of the steel, while the decidedly lower nickel content did not result in a substantial deterioration.
The new steel in fact has a performance perfectly in keeping with that of the reference types and only in the nitric acid test is the corrosion value slightly higher than that of the type 1.4307 micro-resulphurised steel.
Before reaching conclusions in connection with these tests it is necessary to point out again that both the steels used for comparison had an extremely low carbon content (type 1.4307 corresponds to the type AISI 304L, Low Carbon): the new steel is therefore not affected, all other conditions being equal, by the C content which is higher than in the basic comparison steels.
By way of conclusion, these tests show that the sulphur content in a steel type 1.4307 (with a corrosion-resistance considerably higher than the basic type 1.4301) has a decisive influence on the corrosion resistance. Both the compared types (1.4307 steel with low sulphur content and micro-resulphurised steel) are able to form part of a perfectly compliant supply of "normal" 1.4301 steel since this type envisages only a maximum limit for the elements C (0.07 max) and S (0.030 max).
The steel according to the invention, in the solubilized state and on test pieces obtained by means of machining, has corrosion-resistance properties which are practically the same as those of the reference steels.

Corrosion resistance of drawn and solution annealed steels

Corrosion tests were carried out, in different work-hardening conditions, on some samples of drawn wire and drawn + solution annealed wire made from the new steel and, by way of comparison, various other qualities of stainless steel.
Most of the tests were carried out in accordance with international standards which describe the methods to be applied, but do not describe the threshold values (exposure time or the like) which must be surpassed: these threshold values are established contractually in each case during placing of the order. In the present test program only comparative tests were carried out between the new steel and some reference steels, subjecting all the parts together to variable exposure times, until oxidation appeared in some of the parts or for time periods which were sufficiently long to guarantee the applicability thereof.

Table 4 lists the types of materials which underwent this type of test, their diameters and the associated working conditions.

Table 4: Wire samples subjected to corrosion tests
Quality		Diameter	State	Reference number
European standard	AISI standard	mm	State	Reference number
1.4301	304	2.30	Partially work-hardened	1
1.4301	304	2.00	Solution annealed	2
1.4301	304	1.30	Work-hardened	3
New steel		1.40	Solution annealed	4
New steel		1.40	Work-hardened	5
New steel		2.25	Solution annealed	6
New steel		2.00	Partially work-hardened	8

Table 5 instead lists the tests which these samples underwent and the reference standards.

Table 5: Corrosion tests on wire
Test	Reference standard	Duration
Neutral saline mist	UNI ISO 9227 NSS	168 / 400 hours
Copper acetic acid mist	UNI ISO 9227 CASS	120 hours
Kesternich cycles (corrosion in an industrial atmosphere)	DIN 50018 21	4 cycles of 24 hours consisting of 8 hours exposure to SO₂ and 16 hours exposure to the laboratory air
Immersion test in a solution of NaCl 2M with pH 6.6	--	168 hours
Intercrystalline corrosion test	ASTM A262 test E	24 hours in copper/copper sulphate/sulphuric acid solution

Outcome of tests:

• Neutral saline mist test

After exposure for 150 hours, no sample showed signs of corrosion.
Only after 200 hours were some spots of rust detected on the surface of samples 5 and 6 and some more extensive areas found on ferritic sample 7.
After 400 hours these rust spots were extensive, so much so that the ferritic steel was widely oxidised, while some rust areas affected the new steel (the extent of these areas is proportional to the degree of work-hardening); at the same time only small sporadic spots appeared on the type 1.4301 steel in the work-hardened state.

• Copper acetic acid mist test

After 120 hours exposure, the behaviour of the various wires was sufficiently varied and it was possible to detect that the ferritic steel 1.4016 had the most area covered by corrosion products (about 40%).
The behaviour of the new steel and the 1.4301 steel is instead greatly influenced by the degree of work-hardening: as known from the literature, the best corrosion resistance is obtained with the material in the solution annealed state, while it is worsened by work-hardening. It was noted, however, that the behaviour of the steel considered in this study is midway between the type 1.4301 and the type 14016.

• Corrosion tests in an industrial atmosphere using Kesternich cycles

After 4 cycles the behaviour of the new steel was entirely similar to all the other types of austenitic steel, there being no appreciable corrosion (the surface remained substantially unchanged).

• Tests with immersion in a solution of NaCl 2M with pH 6. 6:

In this case the best behaviour was that of the type 1.4301, followed very closely by the new type, while the type 1.4016 had various rust spots.

• Intercrystalline corrosion tests

After attack, all the test pieces were able to be bent through 180° without any signs of cracking or flaking on the surface subject to tensile stress.
The corrosion tests carried out were particularly numerous and covered all the possible ranges of applications such that it was possible to determine the characteristics of the new material with a wide series of tests.
The tests were carried out on products in the wire state, in various finishing conditions, and confirmed, as is well known in the literature, that materials in the work-hardened state behave in general less well when subjected to aggressive agents: the explanation of this phenomenon is due mainly to the tensioning of the grains and the grain edges which make the individual points more unstable and therefore more prone to attack and also the partial martensitic transformation, since this structure has a corrosion resistance which is less than that of austenite.
Overall the corrosion behaviour of the new steel was scarcely inferior to that of the reference type 1.4301, for the same work-hardening conditions.
Particularly positive was the behaviour in relation to atmospheric corrosion and intergranular corrosion, where no differences were noted compared to the reference type.
Additional tests in acid environment (H₂SO₄ 0.2 M + NaCl 1g/l) evidenced that the steel of the present invention also presents better anodic polarization curves than similar steels having a lower nickel content.

Hot tensile strength

One of the main characteristics of stainless steels is the possibility of use at high temperatures. Rapid hot tensile tests were carried out in order to verify the mechanical properties at temperatures higher than room temperature. The samples of the new steel which underwent this test, in the form of 3 mm diameter solubilized wires, were compared with identical samples of 1.4307, 1.4310 and 1.4301 steel.

The tests were carried out at 900°C in accordance with the standard EN 10002 part 5, giving the results listed in Table 6.

Table 6: Mechanical properties during high temperature tests
Material		Test piece diameter mm	Cross sectional area mm²	Test temperature °C	R_p0,2	R_m
European standard	AISI Standard	Test piece diameter mm	Cross sectional area mm²	Test temperature °C	MPa	Mpa
1.4301	304	3	7.1	900	91	141
1.4307	304L	3	7.1	900	94	155
1.4310	302	3	7.1	900	103	169
New steel		3	7.1	900	90	155

The rapid hot tensile tests were carried out at a decidedly high temperature (900°C) compared to the operating temperatures normally permitted. The results show that the new steel has a behaviour very similar to that of the normal reference steel, type 1.4301, while only the type with a higher carbon content (1.4310) has a slightly higher hot strength, even though it as of the same order of magnitude.

High temperature stay test

The basic stainless steel 1.4301 (AISI 304) is resistant for fairly long periods in a high temperature oxidising environment: in particular the most common uses for this material are those which envisage stays in air up to about 500°C. The new steel was also tested for its resistance to temperatures higher than room temperature by means of air heating tests inside a muffle furnace. The results can be seen in Figure 10.
The resistance was evaluated by measuring the depth of surface oxidation, i.e. the loss of diameter as a result of oxidation. It is possible to note that the new steel behaves in a manner perfectly similar to that of the of the various types with a high nickel content up to a temperature of higher than 800°C. As mentioned, the temperatures commonly used for normal austenitic steels (belonging to the family of 1.4301 steel) are about 500°C, while for higher temperatures refractory alloys (with high nickel contents) or superalloys (nickel based alloys, not belonging to the family of steels) are used. The new steel is therefore perfectly utilisable at the same temperatures at which the basic type is used since there is no variation in its characteristics.

Conclusions

The new stainless steel according to the present invention with a low nickel content possesses technical characteristics similar or comparable to those of steel type 1.4301.
The main advantage of this new steel from the commercial point of view is its lesser dependency on the nickel market and therefore its greater stability from a price point of view. From the technical point of view, the main advantage is the extremely high suitability for drawing which allows a large reduction during drawing and a small number of intermediate annealing operations.
The new material is particularly suitable as a substitute for traditional types of steel in certain specific applications

agricultural wire, owing to its optimum atmospheric corrosion resistance and the excellent mechanical properties which can be obtained;
glossy wire for domestic use, electric household appliances, gratings, luggage racks, bicycle spokes, owing to the optimum combination of corrosion resistance and mechanical strength in the work-hardened state;
wire for laundry drying frames, owing to the good resistance to saline mist (traces of chlorides may remain on the washed laundry) and also good mechanical strength and non-magnetic property;
special wires and screws for electronic components, owing to its non-magnetic property in the deformed state and good cold deformability;
wires for architecture, for meshwork and for hooks used on slate roofs, owing to the mechanical strength and resistance to environmental corrosion;
wire and tie-rods for industrial furnaces operating at a medium to low temperature (up to 550°C, for treatment of copper, aluminium and other alloys), owing to the excellent resistance to temperatures up to 800°C.

Claims

Use of an austenitic stainless steel having the following composition by weight: $0.03 % < carbon < 0.07 %$
$7.0 % < manganese < 8.5 %$
$0.3 % < silicon < 0.7 %$
$sulphur \leq 0.030 %$
$phosphorus \leq 0.045 %$
$16.5 % < chromium < 18.0 %$
$3.5 % < nickel < 4.5 %$
$0.1 % < molybdenum < 0.5 %$
$1.0 % < copper < 3.5 %$
$0.1 % < nitrogen < 0.3 %$

the difference consisting in iron and impurities, for the preparation of articles selected from: wires for agricultural use, wires for domestic use, electric household appliances, gratings, luggage racks, bicycle spokes; wires for laundry drying frames; wires and screws for electronic components; wires for architecture, for meshwork and for hooks used on slate roofs; wires and tie-rods for industrial furnaces.
Use of an austenitic stainless steel according to Claim 1, characterized in that: 0.04 % < carbon < 0.06 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 7.5 % ≤ manganese < 8.0 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 0.4 % < silicon 0.6 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: sulphur < 0.005 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 0.002 % < sulphur < 0.004 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 0.030 % < phosphorus < 0.035 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 17.0 % ≤ chromium < 17.5 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 3.8 % < nickel < 4.2 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 0.1 % < molybdenum < 0.3 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 2.0 % ≤ copper < 2.5 %.
Use of an austenitic stainless steel according to any one of the preceding claims, characterized in that: 0.15 % < nitrogen < 0.2 %.
Use of an austenitic stainless steel according to claim 1 having the following composition by weight: $0.04 % < carbon < 0.06 %$
$7.5 % \leq manganese < 8.0 %$
$0.4 % < silicon < 0.6 %$
$0.002 % < sulphur < 0.004 %$
$0.030 % < phosphorus < 0.035 %$
$17.0 % \leq chromium < 17.5 %$
$3.8 % < nickel < 4.2 %$
$0.1 % < molybdenum < 0.3 %$
$2.0 % \leq copper < 2.5 %$
$0.15 % < nitrogen < 0.2 %$

the difference consisting in iron and impurities.
Use of an austenitic stainless steel according to claim 1 having the following composition by weight:
carbon 0.055 %

manganese 7.50 %

silicon 0.52 %

sulphur 0.003 %

phosphorus 0.032 %

chromium 17.0 %

nickel 4.0 %

molybdenum 0.19 %

copper 2.0 %

nitrogen 0.17 %
the difference consisting in iron and impurities.
Articles containing or consisting of austenitic stainless steel according to any one of the preceding claims, characterized in that they are selected from among: wires for agricultural use, wires for domestic use, electric household appliances, gratings, luggage racks, bicycle spokes; wires for laundry drying frames; wires and screws for electronic components; wires for architecture, for meshwork and for hooks used on slate roofs; wires and tie-rods for industrial furnaces.