MXPA99006215A - Corrosion-resistant low-nickel austenitic stainless steel - Google Patents

Abstract

Corrosion-resistant low-nickel austenitic stainless steel having the following compostion in percentages by weight:0.01%.

Description

AUSTENIC STAINLESS STEEL THAT COMPRISES A LOW NICKEL CONTENT AND IS RESISTANT TO, CORROSION FIELD OF THE INVENTION The invention concerns an austenitic stainless steel comprising a low nickel content and is resistant to corrosion, especially to generalized corrosion, pitting corrosion, and cavernous corrosion.

BACKGROUND OF THE INVENTION Patents are known which have steels whose composition contains, in proportion, the base elements such as chromium, nickel, manganese, copper, silicon, giving a structure of the austenitic type. For example, in the French Patent Application No. 70 27 948 an austenitic steel is presented whose composition is as follows: carbon: 0.05% -0.15%; Silicon: 0.3% -1.0%; manganese: 4% -12%; nickel: 0.5% -3%; Chrome: 13% -16%; nitrogen: 0.05% -0.2%. This patent application discloses compositions of stainless steels Ref. 30724 austenitic with a low nickel content and a relatively high content of manganese, which have corrosion resistance equivalent to or higher than that of the commercial types with a high content of nickel, such as AISI 304, 301, 201, 202, according to immersion tests in a chlorinated medium and test in S02. The influence of copper, molybdenum and nickel has been well defined, nickel should be in a small content, but the influence of elements such as calcium, boron and sulfur has not been addressed. In another example, Japanese Patent JP 54038217 concerns an austenitic manganese steel having the following composition: carbon: less than 0.04%; silicon: less than 1%; manganese: 6% -13%; nickel: 1.0% -3.5%; Chrome: 13% -19%; niobium: less than 0.3%; copper: 1.0% -3.5%, rare earths: 0.005% -0.3%. The steel described has a corrosion resistance that is at least equivalent to that of stainless steel of the AISI 304 type, and a high resistance to intergranular corrosion. It is not about the elements sulfur, calcium and boron nor their influence on the different types of corrosion. In another example, Japanese Patent JP 52024914 presents an austenitic steel whose composition is as follows: carbon: 0.11% -0.15%; silicon: less than 1%; Manganese: 8.0% -ll%; nickel: 1.0% -3.5%; Chrome: 16% -18%; nitrogen: 0.05% -0.15%; copper: 0.5% -3.5%; molybdenum: less than 0.5%. It is taught that the decrease in nickel content does not harm corrosion. The influence of elements such as sulfur and boron is not present.

DESCRIPTION OF THE INVENTION The purpose of the present invention is the production of an austenitic steel with a very low nickel content, which has a corrosion resistance close to that of AISI 304 steel, in particular in the field of resistance to pitting, cavernous corrosion and generalized. The invention relates to an austenitic stainless steel comprising a low nickel content, it is resistant to corrosion and has the following composition, in percentages by weight: 0.01% < carbon < 0.08%, 0.1% < silicon < 1%, 5% < manganese < 11%, 15% < chrome < 17.5%, 1% < nickel < 4%, 1% < copper < 4%, 1 - 10 ~ 4% < sulfur < 20 - 10"4%, 1-10 ~ 4% <calcium <50-10 ~ 4%, 0% <aluminum <0.03%, 0.005% < phosphorus < 0.1%, boron < 5- 10 ~ 4%, oxygen <0.01%, the rest being iron and impurities resulting from the preparation, preferably the composition is as follows: 0. 01% < carbon < 0.05%, 0.1% ~ < silicon < 1%, 5% < manganese < 11%, 15% < chrome < 17%, 1% < nickel < 2%, 2% < copper < 4%, 1-10 ~% < sulfur < 1-10"4%, 1-10" 4% < calcium 1-10"%, 0% <aluminum <0.01% 0.005% <phosphorus <0.1%, oxygen <0.01%, the remainder being iron and impurities resulting from processing. , from 0.01% to 2% molybdenum The description that follows and the appended figures, given the totality as a non-limiting example, will make the invention well understood. Figures 1 and 2 present the comparative values of the pitting potential, respectively in NaCl, 0.02 M, pH 6.6 and 23 ° C and in NaCl, 0.5 M, pH 6.6 and 23 ° C, for different types of steel taken as reference and for three compositions according to the invention, marked with an asterisk. Figure 3 shows the evolution of the pitting potentials, in NaCl, 0.02 M, pH 6.6 and 23 ° C, depending on the sulfur content, for two reference steels and two steels according to the invention, one of which carries in its composition a small chromium content. Figure 4 shows the characteristics of resistance to cavernous corrosion in a chlorinated medium for three steels taken as a reference and three steels according to the invention and which comprise in their composition different nickel contents. Figures 5 and 6 show the comparative values of the pitting potential respectively in NaCl, 0.02 M, pH 6.6 and 23 ° C and in NaCl, 0.5 M, pH 6.6 and 23 ° C, for different types of steels, which show the influence of boron. The steel according to the invention has been developed in order to meet the criteria of and especially the criteria of pitting, generalized and cavernous corrosion. To this end, the effect of the following alloying elements has been analyzed: chromium in a range between 15.5% and 17.5%, nickel in a range between 0.5% and 2.7%, carbon in a range comprised between 0.05% and 0.11%, nitrogen in a range between 0.12% and 0. 26%, sulfur in a range between 0.001% and 0.007-%, copper in a range between 2% and 3%, boron in concentration levels between 0.0025% and less than 0.0005%, calcium in levels of concentration between 0.0025% and less than 0.0005%. The chemical compositions of the steels tested are presented in Table 1, the first column giving the references of castings of steels tested, being marked with an asterisk steels according to the invention. Table 2 presents the chemical compositions of the known and tested reference steels, by way of comparison. The different forms of corrosion studied are: - pitting corrosion in a medium of 0.02 M NaCl and 0.5 M at 23 ° C, with a pH of 6.6, - cavernous corrosion in a chlorinated medium at 23 ° C by a tracing of polarization curves in a NaCl medium, 2 M at different acidic pHs, and then a measurement of the activity currents, generalized corrosion in a 2 M concentrated sulfuric medium at 23 ° C by plotting polarization curves and by measurement of the activity current, - the intergranular corrosion by means of the STRAUSS test with a steel sensitized by thermal treatment and with a steel welded by TIG. Tables 3 and 4 group the results of corrosion tests that justify the choice of the composition according to the invention. For pitting corrosion, the potential is given which corresponds to the probability of 1 sting per cm2. For cavernous corrosion, the values of the critical current densities i, measured in different solutions of NaCl, 2 M of variable pH, are given. For generalized corrosion, the values of the critical current densities i are given in a H2S04, 2 M acid solution. The results of the intergranular corrosion are given in Table 4 in the form of mass losses? M and of * the maximum depth of fissures in μm.

Table 1: Chemical analysis of austenitic steels with low amount of Ni that have been studied.

* Steels according to the invention 8? 5 1 » J5 e e? • a co to .2 F W 8 3 cr 52 J2 c < cc 2 cs Table 3: Results of pitting, cavernous and generalized corrosion tests Table 4: Results of intergranular corrosion tests COMMENTS ON THE EFFECTS OF THE DIFFERENT ALLOY ELEMENTS INTRODUCED IN THE COMPOSITION ACCORDING TO THE INVENTION.

The effect of sulfur.

Sulfur has no effect on the generalized corrosion resistance. In the field of cavernous corrosion, it slightly degrades the resistance to finishing and the propagation of corrosion, with a higher critical current and a pH greater than or equal to 2.0 when the sulfur content increases. In the field of pitting corrosion, its effect is on the contrary much more important. By decreasing the sulfur content to levels of the order of 10-10 ~ 4% in the composition of the steels that contain little nickel in their composition, the resistance to the initiation of the bite is greatly improved. From the point of view of pitting corrosion, the steel according to the invention has the same properties as a reference steel AISI 304 or an AISI 430 Ti steel comprising on the order of 30 • 10 ~ 4% sulfur, while steel with low nickel content, with a sulfur content of 30'1Q "%, behaves like a reference steel AISI 430 Nb.

The effect of the sulfur observed in the compositions according to the invention is unexpected. In austenitic steels of reference or in ferritic steels of type 430 Nb, the effect is much less important and more regular, as shown in Figure 3.

The effect of nickel.

- It is shown that nickel is very beneficial in the field of generalized corrosion and cavernous corrosion. In the generalized corrosion sector, a nickel content of 1.6% makes it possible to obtain a steel that behaves the same as an AISI 304 steel, while showing that a nickel content of 0.6% remains insufficient. In the field of cavernous corrosion, a minimum nickel content of 1% is necessary to obtain a level of strength that is acceptable and clearly superior to that of a steel of the AISI 430 Ti type. However, a nickel content of less than 2% is preferable to obtain a good resistance to pitting corrosion. Figure 4 shows, in the form of curves giving the values of the activity currents as a function of the pH of a chlorided solution, the behavior against cavernous corrosion of different reference steels and of steels according to the invention. The currents of activity are proportional to the corrosion rate. The closer to the abscissa the curve is located, the lower the corrosion rates and therefore the better the resistance to corrosion.

The effect of copper.

Copper has a beneficial effect in the generalized corrosion sector. To have a behavior equivalent to that of a steel type AISI 304, it is shown by the behavior of steel 804 that a copper content of 2% can be considered as insufficient, while a copper content of 3% is more convenient, such as shown by the behavior of steel 801. The values of measured activity currents are presented in Table 3. It should be noted that in the case of steel 804, a second peak of activity is observed in the vicinity of a potential of -390 mV / ECS. This peak must also be taken into consideration to evaluate the corrosion rate in H2S04 acid. However, copper has a harmful effect on the resistance to pitting corrosion, as shown in Figures 1 and 2 or Table 3. Steel 801, whose copper content is 3%, has pitting potentials. less than those of steel 804, whose copper content is 2%. Also, the copper content is, according to the invention, limited in preference to 4%.

The effect of boron.

Boron has no effect on generalized corrosion. In the field of pitting corrosion, as shown in Figures 5 and 6, this appears to be slightly beneficial in steels containing a little calcium such as 841 steel, but it is harmful "in steels such as 881 and 801 that do not contain it For a steel that contains boron but not calcium, it would be necessary to perform a hypertemple at 1100 ° C followed by a quench with water to find a content of pitting corrosion that is close to that of a boronless steel. Finally, in the case of intergranular corrosion, as shown in Table 4, it is slightly harmful in certain cases, preferably the composition according to the invention does not contain the boron element or, if it contains it, in contents always lower than 5-10 ~ 4%.

The effect of calcium.

- It has been shown that calcium is harmful in the field of pitting corrosion, especially in a moderately chlorinated medium, that is to say with normal NaCl 0.02 M. This behavior is presented in Table 3. The steels 836 and 840 containing respectively 23-10 ~ 4% and 20-10"4% calcium, have pitting potentials lower than those of steels 881 (air-hardened) and 805, which do not have calcium. to pitting corrosion as close as possible to the reference AISI 304 and AISI 430 Ti steel, the calcium content must be very small, ie less than 20 * 10"4% and preferably less than 10-10 ~ 4% The effect of chromium.

Chromium is beneficial in the sector of generalized corrosion, pitting corrosion and cavernous corrosion, as shown in Table 3 comparing the values obtained in steels 584 and 723, 801 and 806. A minimum content of 15 % is necessary to ensure a good corrosion behavior, but a content equal to 16.5% is preferable to obtain a corrosion resistance that responds to a resistance to corrosion comparable to that of a reference steel of the type ftISI 304 or AISI 430 You. With a content greater than 17% chromium, as for steel 806, corrosion is further improved but it is difficult to obtain a steel having an entirely austenitic structure.

Effect of carbon and nitrogen.

Carbon has a preponderant behavior in the case of steels in the intergranular corrosion sector. Steels with varying carbon and nitrogen contents have been tested according to the STRAUSS test after welding or sensitization by thermal treatment. The results of this test are grouped in Table 4.

It is shown that a maximum carbon content of 0.07% is desirable and that a preferred content of 0.05% allows a corrosion behavior analogous to that of a reference steel AISI 304. A nitrogen content between 0.1% and 0.3% is acceptable. The steel according to the invention has a resistance to corrosion comparable to that of a reference steel AISI 304 but contains a small amount of nickel in its composition. Furthermore, the steel according to the invention has a behavior considerably superior to that of steels of the AISI 430 Ti type in the generalized and cavernous corrosion sector. It is noted that in relation to this date, the best method known to the applicant, to implement said invention is that which is clear from the manufacture of the objects to which it refers. Having described the invention as above, the content of the following is claimed as property.

Claims (3)

1. - Austenitic stainless steel comprising a low content of nickel and is resistant to corrosion and is characterized Doraue Dresenta the following composition in Dorcentaies by weight: 0. 01% < carbon < 0.08%, 0.1% < silicon < 1%, 5% < manganese < 11%, 15% < -cro? ao < 17.5%, 1% < nickel < 4%, 1% < copper < 4%, 1-10 ~ 4 < sulfur < 20-10"%, 1-10" 4% < calcium < 50-10 ~ 4%, I heard < aluminum < < 0.03%, 0.005% < phosphorus < 0.1%, boron < 5-10"oxygen <0.01%, the remainder being iron and impurities resulting from processing.

2. Steel according to claim 1, characterized in that, preferably, the composition is as follows: 0.01% < carbon < 0.05%, 0.1% < silicon < 1%, 5% < manganese < 11%, 15% < chrome < 17%, 1% < nickel < 2%, 2% < copper < 4%, 1-10"4% < sulfur < 10-l < 1-10 < 4 > calcium < 10-10"4%, 0% <aluminum <0.01%, 0.005% <phosphorus <0.1%, oxygen <0.01%, the remainder being iron and impurities resulting from processing.

3. Steel according to one of claims 1 and 2, characterized in that it also comprises 0.01% to 2% molybdenum.