Title of Invention	AN AUSTENITIC STAINLESS STEEL HAVING A VERY LOW NICKEL CONTENT
Abstract	Austenitic stainless steel having a very low nickel content, of the following composition by weight: Carbon < 0.1% 0.1% < silicon < 1% 5% < manganese < 9% 0.1% < nickel < 2% 13% < chromium < 19% 1% < copper < 4% 0.1% < nitrogen < 0.40% 5xl0-4% < boron < 50x10-4% phosphorus < 0.05% sulphur < 0.01%.

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AUSTENITIC STAINLESS STEEL HAVING A VERY LOW NICKEL CONTENT The invention relates to an austenitic stainless steel having a very low nickel content. Stainless steels are classified into large families depending on their metallurgical structure• Austenitic steels are steels generally having a nickel content greater than 3% in their composition by weight. For example, an NF EN 10 08 8 standard No. 1,4301 austenitic steel (AISI 304) has more than 8% nickel in its composition. The high cost of the element nickel and the uncontrollable, variations in its price have led steelmakers , to develop austenitic steels whose composition does not contain nickel or else contains very little of it. The object of the invention is to produce an austenitic steel referred to as "having a very low nickel content", with, in particular, mechanical and welding properties which are , equivalent, and even superior, to those of austenitic steels having a high nickel content • International directives are aimed at reducing the release of nickel from materials, especially in the water and skin-contact fields . The subject of the invention is an austenitic steel having a very low nickel content, characterized by the following composition by weight: Carbon 0.1% 5% 0,1% 5x10-4% phosphorus The other characteristics of the invention are: the composition satisfies the relationship which defines a ferrite index FI1: FI1 = 0.03422 + 0.284X - 6.347 X = 6.903[-6.998 + Cr% - 0.972(Ni% + 21.31 N% + 20.04C% + 0.46Cu% + 0.08Mn%)]; the composition satisfies the following relationship, using a martensite stability index SI: SI = 0.0267x2 + 0.4332X - 3.1459 - the steel contains, in its composition, less than 1% nickel; - from 15 to 17% chromium; - Ifess than 0.08% carbon; - from 0,5% to 0.7% silicon; - less than 2% molybdenum; - less than 0.0020% sulphur; and the steel furthermore , contains in its composition less than 0.030% aluminium, preferably less than 50x10"^% aluminium and less than 20x10"^% calcium and preferably less than 5x10-4% calcium. The description which follows, together with the appended figure, all given by way of non-limiting example, will make the invention more clearly understood. The single figure shows the reduction-insect ion characteristics as a function of temperature for various steels. The austenitic steel according to the invention is smelted, with the nickel content of the composition being limited. The austenizing effect, usually attributed to the element nickel, must necessarily be compensated for by gammagenic elements, such as manganese, copper, nitrogen and carbon, and it is necessary to reduce as far as possible the contencs or alphagenic elements, such as chromium, molybdenum and silicon. The steel according to the invention undergoes ferritic-type solidification. The ferrite solidified reverts to austenite as the steel cools down after casting. At the casting stage, the steel being cooled, the residual ferrite content in per cent by volume is approximately given by the following experimentally established index: FI2 = 0.1106x2 + 0,0331x + 0.403 where X = 2.52 [-7,65 + Cr% + 0.03Mn% - 0.8 64 (Ni% + 16.10C% + 19.53N% + 0,35Cu%)]. At this stage, the ferrite content of the steels according to the invention is less than 5%, Next, the steel is reheated, in order to be hot rolled, at 1240°C for 30 min. It is observed that the ferrite content is then given by the equation: FI1 = 0.034x2 H- 0,284x - 0.347 where X = 6.903[-6.998 + Cr% - 0.972(Ni% + 21.31N% + 20.04C% + 0.46Cu% + 0.08Mn%)]. The steel according to the invention contains less than 20% ferrite after reheating for 30 min at 1240°C. After hot rolling and over hardening at 1100ºC for 30 min., the steel according to the invention has a ferrite content of less than 5%. After hot working, annealing, cold working and annealing a steel is obtained which has only a few traces of residual ferrite. The austenite/ferrite ratio was measured by saturation magnetization or by X-ray diffraction analysis. From the standpoint of the role of the elements contained in the composition, carbon is limited to a content of less than 0.1% in order to avoid sensitizing the steel to intergranular corrosion after treatment at temperatures between 550°C and B00°C. Preferably, the carbon content is less than 0.08% for the same reason. Nitrogen and carbon have a similar ettect on the mode of solidification, the equilibrium of the ferrite and austenite phases and the stability of the austenite with respect to martensite formation, although nitrogen has a slightly more austenizing character than carbon. Manganese increases the solubility of nitrogen. A minimum content of 5% of this element is necessary in order to dissolve enough nitrogen and to guarantee that the steel has an austenitic structure. A 9% upper limit of the manganese content in the composition of the steel of the invention is related to the use, in the smelting of the steel according to the invention, of carburized ferro-manganese, preferably refined ferro-manganese. The effect of manganese on the amount of ferrite is constant for contents of between 5% and 9%. Furthermore, the manganese content must also be limited in order to prevent deterioration of the hot ductility. Silicon is intentionally limited to less than 1%, and preferably to less than 0.7%, in order to prevent the formation of ferrite and to have satisfactory behaviour of the steel during pickling. The 0.1% minimum content is necessary in smelting and 0.5% minimum content is preferable in order to prevent the formation of olivine-type oxide, This is because, during conversion of the steel by hot rolling, low-melting-point oxides of the olivine (FeO/SiO2/MnO) type form on a steel according to the invention and containing only a low silicon content, for example less than 0.5%. If the silicon content is less than 0.5%, a hybrid zone having a metal matrix containing these oxides in the liquid state is formed during the hot-rolling operation. This results in a poor surface finish of the steel strip, especially after pickling. In order to prevent the formation of these low-melting-point oxides, it has proved necessary to enrich the composition of the steel with silicon to a level above 0.5%, ' Oxides with a high melting point are then formed, which no longer cause a surface-finish problem during hot rolling. Silicon is limited to a content of less than 2%, and preferably less than 1%, as, taking into account the other elements of the composition, it does not contribute to the formation of an austenitic structure when its content is higher. Nickel is an essential element in austenitic steels in general and the posed problem of the invention is, in particular, to obtain an austenitic steel containing little nickel, an element which is expensive, the price of which is highly variable and uncontrollable, and which, because of the price fluctuations, disturbs the proper operation of the enterprise responsible for producing the steel. Nickel also has the drawback of increasing the sensitivity to stress corrosion of austenitic steels, We have also found that limiting the nickel content has allowed us to produce a new generation of steels having improved properties, as will be described below. A chromium content greater than 13%, and preferably greater than 15%, is necessary in order to guarantee corrosion resistance of the stainless steel. The 19%, and preferably 17%, limit of the chromium content is related to the fact that the steel according to the invention must remain with a ferrite content of less than 5% after the overhardening treatment. Chromium contents greater than 19% result in excessively high ferrite contents which do not guarantee a sufficient tensile elongation. A minimum of 1% copper is necessary to guarantee an austenitic-type structure because of the reduction in the nickel content. Above a 4% copper content, the forgeability of the steel deteriorates significantly and hot conversion of the said steel becomes difficult. Copper has approximately 40% of the austenizing effect of nickel. Also to guarantee an austenitic-type structure in the steel according to the invention, a nitrogen content of at least 0.1% is required. Above a 0.4% nitrogen content, bubbles of this gas, called "blowholes", form within the steel during solidification. The necessary nitrogen content may be high when molybdenum with contents of less than 2% is introduced into the composition of the steel in order to improve the corrosion resistance. Molybdenum contents greater than 2% require the addition of more than 0.4% of nitrogen in order to avoid the presence of ferrite, which is not realizable when smelting the steel at normal pressure. The composition of the steel according to the invention contains boron in an amount of between 5x10" % and 50x10-4%, The addition of boron to the composition consequently improves the hot ductility, especially between 900 ºC and 1150ºC, as is shown by the hot tensile reduction-in-section characteristics as a function of temperature. Above 50x10'*% of boron, too great a reduction in the burning point occurs, that is to say that there is a risk of areas of liquid metal forming during the reheat before rolling. Sulphur is introduced .into the steel in an amount of less than 0.01% in order to ensure that the steel has a satisfactory pitting corrosion behaviour. Preferably, the sulphur content is less than 20x10" %, which appreciably improves the hot ductility at lOOOºC and above. The low sulphur content may be obtained by the controlled use of calcium and aluminium, generating final aluminium contents of less than 0,03% and preferably less than 50x10' % or less than 30x10-4 % and calcium contents of 10x10-4 and preferably less than 5x10' %, the oxygen content which results therefrom generally ranging from 20x10-4 to 60x10-4%. The phosphorus content is limited to 0.05%, as in most austenitic stainless steels, in order to limit segregation during the solidification of welds and hot tearing phenomena which may consequently occur while the welds are cooling. The steel according to the invention is compared in the description with an AISI 304 type steel called "reference" steel. The composition of the steel according to the invention is given in Tables 1 and 2 of Annexes 1 and 2 below, pages 14 and 15. In the description, the compositions of the steel according to the invention are indicated by an aste risk, Table 3 below gives the calculated values of the indices FI1, FI2 and SI for various steels. Table 3 I Table 4 gives the measured values of FI2, FI1 and the measured SI value for martensite formed after a tensile strain of 30%. - Hot properties of the steel according to the invention The hot ductility was measured in hot tensile tests. The measurements were carried out on an as-solidified steel and on a worked-and-annealed steel. The worked steel is obtained by forging at a start temperature of 1250ºC. The steel is then annealed at a temperature of 1100ºC for 30 min. The thermal cycle of the tensile test consists of a temperature rise to 1240°C at a rate of 20ºC/5, a temperature hold at 124 0ºC for one minute and a fall at a rate of 2ºC/8 down to the deformation temperature- The diametral reduction in section is measured, this corresponding to the ratio, expressed in %, of the difference between the initial diameter and the final diameter to the initial diameter. The single figure shows the reduction-in-section behaviour as a function of the deformation temperature for steels 769- (B) and 771-(C) according to the invention compared with low-sulphur steel 774-(D) , boron-free . steel 768-(A) 'and steel 671 called the "reference" steel (AISI 304). Steel 768-(A), containing 30x10-4% sulphur and no boron, has a markedly lower hot ductility than the reference steel. The same applies to steel 774-(D) containing 9x10"*% sulphur and no boron. The addition of boron improves the ductility between 900°C and 1050ºC, as shown in the figure. Furthermore, it should be pointed out that, when boron is present, steel 771-(C) having a sulphur content of less than 20x10-4% has a superior hot ductility characteristic over the entire temperature range between 900°C and 1250°C and approaches the ductility of the reference steel 671, Mechanical properties of the steel according to the invention, at ambient temperature The mechanical properties were measured on an annealed worked steel, The steel is worked by forging starting at 1250°C. The steel is then annealed at a temperature of 1100°C for 3 0 min, in a salt bath. The test pieces used for the tensile test have a gauge part 5 0 mm in length with a circular cross-sect ion 5 mm in diameter. They are pulled at a rate of 20 mm/minute. The steels according to the invention have an elongation , of between 55% and 67%. By way of comparison, Table 5 below gives the measured properties of the steel according to the invention, of low-nickel-content steels outside the invention and of a reference steel of the AISI 304 type. I The amount of martensite after a true tensile strain of 30% was measured (Table 4) . In the case of the steel according to the invention, it is less than 20%. No trace of e-martensite was observed in the test pieces of the steel according to the invention deformed to failure. The steels according to the invention, the SI indey of which is less than 20 and the FI1 index of which is less than 20, have a tensile elongation of greater than 55% after the conversion as defined above, Such an elongation is necessary in order to obtain a suitable cold ductility. - Corrosion resistance In the field of intergranular corrosion, a test according to the ASTM 2 62 E standard was carried out on steels having variable carbon and nitrogen contents. The steels on which the test is carried out are steels in the form of a 3 mm thick hot-rolled strip annealed at 1100ºC (overhardening) . Next, the steels are subjected to one of the following two sensitizing treatments: a) A 30-minute anneal at 700ºC followed by a water quench or b) a 10-minute anneal at 6 50ºC followed by a water quench. The results of the test are given in Table 6 below. Table 6 The steels outside the invention, containing more than 0.1% carbon, such as steels 594 and 596, do not have acceptable properties. The steels according to the invention, which contain less than 0.1% carbon in their composition, such as steels 567, 592 and 584, are comparable to the AISI 304 steel in terms of intergranular corrosion in the case of Test b. Only the steels according to the invention containing less than 0.080% carbon in their composition are comparable to the AISI 3 04 steel in the case of Test a. The carbon content according to the invention is therefore limited to less than 0.1% and preferably limited to less than 0.08%. Steels according to the compositions in Annex 3, having variable aluminium, calcium, oxygen and sulphur contents, were produced in an electric furnace and with AOD, these contents' having been measured using particularly accurate methods such as atomic absorption spectroscopy in the case of calcium and glow-discharge spectroscopy in the case of aluminium; using worked products, pitting corrosion tests were carried out in 0.02M NaCl at 23ºC at a pH of 6 .6, the results of which are given in Table 7, The potential El corresponds to the probability of 1 pit per cm2. It may be seen that the pitting potential is appreciably higher in steel's whose composition has an aluminium content not exceeding 50x10" % and which furthermore contain less than 10x10-4% calcium, less than 60x10-4% oxygen and less than 20x10"*% sulphur. It has also been able to be observed, using scanning electron microscopy, that steels A and B, having 110x10-4% aluminium and 115x10-4% [lacuna] in their composition, contain inclusions of the aluminate of lime type and of the alumina-magnesia type, these inclusions being surrounded by calcium sulphides, the sizes of which may be as much as several micrometres. No calcium sulphide was found in steels C and D containing less than 30x10-4% aluminium and less than 10x10"'*% calcium. 1. Austenitic stainless steel having a very low nickel content, characterized by the following composition by weight: 2. Austenitic steel according to Claim 1, characterized in that the composition satisfies the following relationship, which uses a ferrite index FI1: FI1 = 0,034x2 + 0.284X - 0.347 3. Austenitic steel according to Claim 1, characterized in that the composition satisfies the following relationship, which uses a martensite stability index SI: SI = 0.0267x2 + 0.4332X - 3.1459 4. Austenitic steel according to Claims 1 to 3, characterized in that it contains less than 1% nickel in its composition, 5. Austenitic steel according to Claims 1 to 3, characterized in that it contains from 15% to 17% chromium in its composition. 6. Austenitic steel according to Claims 1 to 3, characterized in that it contains less than 0.08% carbon in its composition. 1, Austenitic steel according to Claims 1 to 3, characterized in that it contains from 0.5% to 0.7% silicon in its composition. 8. Austenitic steel according to Claims 1 to 3, characterized in that it furthermore contains less than 2% molybdenum in its composition. 9. Austenitic steel according to Claims 1 to 3, characterized in that it furthermore contains less than 0.0020% sulphur in its composition, 10. Austenitic steel according to Claims 1 to 3, characterized in that it furthermore contains less than 0.030% aluminium, preferably less than 50x10-4% aluminium, and less than 20xl0-4 calcium, and preferably less than 5x10-4% calcium, in its composition- 10. Austenitic stainless steel having a very low nickel content substantially as herein described with reference to the accompanying drawing.

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