FI93126C - Austenitic-ferritic stainless steel - Google Patents

Abstract

Austeno-ferritic stainless steel alloy having good corrosion resistance and a machinability index comprising a low content of molybdenum and a high content of copper dissolved by heat treatment of the alloy above 900 DEG C, the composition being the following: C < 0.06 % by weight Si < 1.2 Mn < 3 21 < Cr < 25 3 < Ni < 6 0.06 < N < 0.3 Mo < 1 1 < Cu < 3.5 the remainder being iron. The composition is balanced to obtain between 30 and 70% of ferrite to austenite.

Description

93126

Austenitic-ferritic stainless steel

The present invention relates to austenitic-ferritic stainless steel.

5 Austenitic-ferritic stainless steels are known which have good mechanical properties, good corrosion resistance and good weldability.

Such alloys contain not only iron, which forms a residue, chromium and molybdenum so as to improve corrosion resistance; nickel and nitrogen so as to improve the stability of the austenite phase; carbon in a low percentage as it impairs corrosion resistance, given its poor Liu hardness to ferrite; silicon; manganese.

EP patent 0 156 778 thus describes an austenitic-fer-20 sufficient stainless steel alloy, the austenitic phase of which remains stable, enabling cold working between 10 and 30%, good weldability and good corrosion resistance.

The composition of such a mixture is as follows: 25 C <0.06% by weight

Si <1.5% by weight

Mn <4.0% by weight 21% by weight <Cr <24.5% by weight 2% by weight <Ni <5.5% by weight 30 0.01% by weight <Mo <1.0% by weight ·· 0.05% by weight <N <0.3% by weight 0.01% by weight <Cu <1.0% by weight of the residue being iron (Fe), whereby the following conditions are also met when the above-mentioned 35 substances are met: 2 93126 percent ferrite a between 35 and 65 percent ferrite a <0.20 (Cr% / N%) + 23 (Cr% + Mn%) / N% <120.

22.4 x Cr% + 30 x Mn% + 22 x Mo% + 26 x Cu% + 5 110 x N%> 540.

Mo% + Cu%> 0.15, with at least 0.005% copper.

Such alloys have a stable austenitic phase which does not tend to change to martensitic, but are difficult to machine and have poor mechanical properties.

It is an object of the present invention to provide an austenitic-ferritic alloy which has better corrosion resistance than current alloys and has a high machining coefficient.

Such a mixture has a low molybdenum content but a high copper content and is dissolved by heat treatment above 900 ° C and then rapidly cooled to ambient temperature, the composition of this mixture being 20% by weight: C <0.06% by weight

Si <1.2 "

Mn <3 21 <Cr <25 25 3 <Ni <6 0.06 N <0.30 "

Mo <1 1 <Cu <3.5 "with 30 residues being iron (Fe). The composition is equilibrated to give 38-70% ferrite at 300 ° K.

Other advantages and features will become apparent upon reading the description followed by specific embodiments of the composition of the invention, with the sole accompanying Figure 35 showing the cure ranges of the composition as a graph of time and temperature.

3 93126

The two separate mixtures A and B are analyzed in comparison with mixtures of a generally known composition, in particular composition UNS 32304, which corresponds to the mixture described in EP patent 0 156 778.

5

C Si Mn Ni Cr Mo Cu N

A 0.02 0.06 1.9 4.1 23.5 0.13 1.60 0.1 10 B 0.02 0.5 2 3.9 24.3 0.14 2.8 0.09 AISI 304L 0.02 0.6 1.3 10 18.2 0.03 0.02 0.05 AISI 316 0.025 0.5 1.5 11.5 17.5 2.3 0.03 0.05 15 UNS 32304 0.02 0.5 1.8 4.2 23 0.13 0.127 0.123 UNS 31803 0.02 0.5 1.7 5.7 21.9 2.75 0.135 0.120 The table above summarizes the mixture ratios of iron additives according to the invention. mixtures A and B as well as in generally known mixtures.

The compositions of the invention are obtained by melting at at least 1600 ° C and reheating after solidification at about 1180 ° C. They are rolled into sheets. Sampling is carried out in order to determine the structural resistance in relation to the heat treatments and, more particularly, the hardening, mechanical and physical properties, corrosion resistance and workability.

30 It is necessary to study in advance the effect of the various additives. Carbon is reduced to low concentrations of less than 0.06% to reduce the risk of carbide formation during heat treatments, which would be detrimental to the resistance to certain forms of corrosion.

• * 4 93126

Silicon is reduced to low concentrations of less than 1.2% to reduce the risk of formation of metal compounds, which are brittle in the mixture.

Manganese is used to increase the solubilization of nitrogen in the mixture, but the nitrogen content must be limited to 3% so as not to impair the resistance to local and general corrosion in some cases.

The chromium is controlled so that the volume fractions of the ferritic and austenitic phases are close to each other. Too low a concentration does not make it possible to obtain a sufficient volume fraction of ferrite.

Too high a concentration may require significant additions of nickel and nitrogen, which, given the price of nickel, must be avoided. In addition, the alloy has an increased tendency to precipitate into brittle metal compound phases during heat treatments.

Thus, chromium contents of between 21 and 25%, in particular 23.5%, are used as usual. At that percentage, the alloy has different corrosion resistance.

Such a chromium content combined with a low nickel and molybdenum content prevents, even in the case of a few hours of heat treatment, the formation of phase a1, in contrast to phase a1, which a1 must be hardened and brittle. Phase a1 is formed during heat treatments between 300 and 500 ° C.

Nickel is an element that stabilizes the austenitic phase, optimizing the austenite / ferrite balance. Given the price of nickel, its addition is limited to 3 to 6%, more particularly to 4.2%.

Nitrogen is involved in maintaining the austenite / ferrite balance, and its addition also makes it possible to improve mechanical properties as well as resistance to pitting corrosion. Nitrogen addition is limited to 0.30 and 35 is often close to 0.13%.

93126 5

Molybdenum is limited to a maximum of 1% in order to reduce the manufacturing costs of the alloy and to reduce the formation of metal compound phases. Molybdenum improves the corrosion resistance of the alloy. 5 years

Copper, in contrast to generally known alloys, is present in relatively high percentages between 1 and 3.5%. This element is usually present in small amounts in generally known alloys, as its solubility in austenitic-ferritic alloys during cooling is limited.

Instead, dissolution according to the invention is possible by heat treatment at a high temperature above 950 ° C. This step must be followed by rapid cooling to room temperature so that the austenitic-ferritic structure does not precipitate and remains supersaturated with copper. Copper increases the resistance of the alloy to certain acid environments, especially sulfur environments, and improves workability.

The durability of the structure of mixture B has been studied with respect to time and temperature, as shown in the attached figure.

Between 300 and 600 ° C, considerable curing of the mixture occurs by precipitation of copper-enriched particles in the ferritic phase of the mixture.

This curing is proportional to the heat treatment given to the copper content.

Instead, a precipitation delay occurs when the temperature is maintained at 700 to 900 ° C due to the stability of the ferritic phase with respect to the metal compound phase, the latter being obtained with a very low molybdenum content.

The mechanical properties are summarized in the table below: »• t 93126 6

Tensile properties:

Hardness _

HV5 Re 0.2% Re 1% Rm λ Z

5 _ AISI 304 148 205 260 520 51 75

Mixture A 223 449 514 660 30.5 50.6

Mixture B 270 566 639 735 17.5 48.7

Seos B

10 cured 350,647,788,900 18.5 39

As for the cured mixture B, it is a mixture B which has been heat-treated for 5 hours at 400 eC.

The alloys according to the invention have better mechanical properties, in particular the values of the normal elastic limit (Re 0.2%) and the 1% elastic limit, while maintaining the value of the impact toughness of the V-groove test rod (KCV) and the elongation (Elongation A) sufficient.

As for the hardness, it clearly increases, especially after heat treatment.

The processing coefficient of the mixtures according to the invention is considerably improved compared to the generally known mixtures and in particular to the mixture of EP patent 0 156 778.

* The results are summarized in the following table: 30 HB V 0,500 Number of cavities m / min per 500 mm

Relationship A 223 26 72 AISI 304L 148 8 33 35 UNS 31803 241 16 56 UNS 32304 234 11 33 93126 7

The three parameters studied are Brinell hardness (HB), the machining factor at a cutting speed of 0.5 m / mn, and the drilling test as the number of holes corresponding to a cumulative length of 500 mm (0.5 m).

The generally known alloys have hardness values located on both sides of the hardness value of the specimen A of the mixture according to the invention, and the two workability tests together show the clearly better performance values of the mixture A.

10 Corrosion tests show that the benefits obtained have not been obtained at the expense of corrosion resistance.

The dimensions summarized in the table below have been obtained in acidic environments (H 2 SO 4 at 50 ° C).

15 _ E corrosion Ia lp E fracture mV / ecs μΑ / αη2 μΑ / cm2 mV / ecs UNS 32304 -430 1250 14 250 20 Alloy A -460 1270 3 480

Alloy B -460 2000 3.8 400 To obtain the polarization curves leading to these results, the output potential is -600 mV with respect to one supersaturated calomel electrode (ees) and a scan rate of 0.25 mV / sec. The backflow is implemented for a current of 100 μΑ up to -1 100 mV / ecs.

The passivation current lp is reduced while the mur-30 topotential is increased, which makes it possible to extend the field of application of the mixture according to the invention to the range of redox potential.

This is also due to copper, which is confirmed by the toughness of mixture B after heat treatment in an acidic environment in the presence of abrasive particles 9ό ί 26 8 with a diameter of 0.5, 1.9 and 2.38 mm (see table below):

Weight loss result (mg) 8 h H2SO4 (2N) 5_____ UNS 32304 Mixture B AISI 304 hardened

Static testing 25 4 28 10 Dynamic testing without particles 808

Dynamic testing of particles 0.5 mm 34 35 58

Dynamic testing ^ particles 1.19 mm 97 73 110

Dynamic testing particles 2.38 mm 130 99 136 20 The mixture according to the invention solves the problem posed by improving the mechanical properties and machinability without these improvements being detrimental to the corrosion resistance properties.

Improvements in the properties of this alloy are achieved by increasing the percentage of copper and by dissolving or partially precipitating the latter.

These results are significant considering that generally known mixtures, especially UNS 32304, require percentages of 30 Cu + Mo * 1% as the most preferred embodiment.

However, the copper content of the mixture according to the invention must be limited to 3.5% in order to avoid high risks of tearing of the products during implementation.

In this range of 1 to 3.5%, the skilled artisan will apply the percentage according to the utilization of the mixture.

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Likewise, well-known complementary additives such as sulfur and bismuth make it possible to improve workability.

Claims (6)

93126 1. Austenitic-ferritic stainless steel alloy, characterized in that it has very good corrosion resistance and a good working coefficient, a low molybdenum content and a high copper content, which is dissolved by heat treatment at a temperature above 900 ° C and then cooled rapidly at ambient temperature, the composition of the steel alloy being as follows: 10 C <0.06% by weight Si <1.2 "Mn <3 21 <Cr <25 3 <Ni <6 15 0.06 <N <0.30" Mo <1 1 <Cu <3.5 " the rest being iron. Stainless steel alloy according to Claim 1, characterized in that it has the following composition: C = 0.02% by weight Si = 0.6 "Mn * 1.9" Ni = 4.1 "Cr = 23.5" Mo = 0.13 "N = 0.1" Cu = 1.6 ". Stainless steel alloy according to Claim 1, characterized in that it has the following composition: C = 0.02% by weight Si = 0.5 " 35 Mn = 2 93126 Ni = 3.9 "Cr = 24.3" Mo = 0.14 "N = 0.09" Stainless steel alloy according to one of Claims 1 to 3, characterized in that the copper is dissolved by a heat treatment at at least 1600 ° C, which heat treatment is followed by a reprocessing at 1180 ° C after solidification. Steel alloy according to Claim 4, characterized in that the alloy is subjected to a heat treatment at a temperature of between 300 and 500 ° C by alloying the partially dissolved copper. 5 Cu = 2.8 ". Stainless steel alloy according to Claim 5, characterized in that the duration of the heat treatment is 5 hours at 400 ° C. 951 26