PROCESS FOR MANUFACTURING HOT-ROLLED PLATE, STRIP OR COIL MADE OF DUPLEX STAINLESS STEEL

Abstract

The invention relates to a duplex stainless steel composition, the composition of which consists of, in % by weight: C0.05% 21%Cr25% 1%Ni2.95% 0.16%N0.28% Mn2.0% Mo+W/20.50% Mo0.45% W0.15% Si1.4% Al0.05% 0.11%Cu0.50% S0.010% P0.040% Co0.5% REM0.1% V0.5% Ti0.1% Nb0.3% Mg0.1% the balance being iron and impurities resulting from the smelting, and the microstructure consisting of austenite and 35 to 65% ferrite by volume, the composition furthermore satisfying the following relationships:

40I.sub.F70

where

I.sub.F=6(% Cr+1.32% Mo+1.27% Si)10(% Ni+24% C+16.15% N+0.5% Cu+0.4% Mn)6.17

and

I.sub.LCR30.5

where

I.sub.LCR=% Cr+3.3% Mo+16% N+2.6% Ni0.7% Mn, and also to a process for manufacturing plate, strip, coil, bar, rod, wire, sections, forgings and castings made of this steel.

Claims

1. A process for manufacturing a hot-rolled plate, strip or coil made of steel comprising: providing an ingot or slab of a steel of composition comprising in % by weight: C0.05% 21%Cr25% 1%Ni2.95% 0.16%N0.28% Mn2.0% Mo+W/20.50% Mo0.45% W0.15% Si1.4% Al0.05% 0.11%Cu0.50% S0.010% P0.040% Co0.5% REM0.1% V0.5% Ti0.1% Nb0.3% Mg0.1% the balance being iron and impurities resulting from the smelting, and the microstructure consisting of austenite and 35 to 65% ferrite by volume, the composition furthermore satisfying the following relationships:
40I.sub.F70 where
I.sub.F=6(% Cr+1.32% Mo+1.27% Si)10(% Ni+24% C+16.15% N+0.5% Cu+0.4% Mn)6.17 and
I.sub.LCR30.5 where
I.sub.LCR=% Cr+3.3% Mo+16% N+2.6% Ni0.7% Mn; and hot rolling the ingot or slab at a temperature between 1150 and 1280 C. in order to obtain plate, strip or coil.

2. The process for manufacturing hot-rolled plate made of steel according to claim 1, in which: hot-rolling the ingot or slab at a temperature between 1150 and 1280 C. to obtain a quarto plate; performing a heat treatment at a temperature between 900 and 1100 C.; and cooling the plate by air quench.

3. The process for manufacturing hot-rolled bar or rod made of steel comprising the steps of: providing a continuously cast ingot or bloom of steel of composition comprising in % by weight: C0.05% 21%Cr25% 1%Ni2.95% 0.16%N0.28% Mn2.0% Mo+W/20.50% Mo0.45% W0.15% Si1.4% Al0.05% 0.11%Cu0.50% S0.010% P0.040% Co0.5% REM0.1% V0.5% Ti0.1% Nb0.3% Mg0.1% a balance being iron and impurities resulting from the smelting; hot-rolling the ingot or bloom from a temperature between 1150 and 1280 C. in order to obtain a bar, which is air-cooled, or a coil of wire stock which is water-cooled; and then, optionally: performing a heat treatment at a temperature between 900 and 1100 C.; and quench cooling the bar or coil of wire stock.

4. The process for manufacturing according to claim 3, further comprising the step of: performing a cold-drawing operation carried out on the bar or a die-drawing operation on the rod, after being cooled.

5. The process for manufacturing a steel section according to claim 3, further comprising the step of: performing a cold-forming operation on the hot-rolled bar.

6. The process for manufacturing a steel forging comprising the steps of: providing a hot-rolled bar of claim 3; cutting the hot-rolled bar into slugs; and performing a forging operation on the slugs between 1100 C. and 1280 C.

7. The process for manufacturing according to claim 1, wherein 30.5I.sub.LCR38.6.

8. The process for manufacturing according to claim 3, wherein 40I.sub.F70where
I.sub.F=6(% Cr+1.32% Mo+1.27% Si)10(% Ni+24% C+16.15% N+0.5% Cu+0.4% Mn)6.17 and where
I.sub.LCR=% Cr+3.3% Mo+16% N+2.6% Ni0.7% Mn.

9. The process for manufacturing according to claim 8, wherein 30.5I.sub.LCR38.6.

10. A hot-rolled steel quarto plate, obtained by the process according to claim 2 and having a thickness between 5 and 100 mm.

11. Use of hot-rolled coil obtained by the process according to claim 1, for the manufacture of structural components for material production or energy production installations.

12. Use according to claim 11, in which said material and energy production installations operate between 100 and 300 C., preferably between 50 and 300 C.

13. A cold-rolled steel strip that can be obtained by cold-rolling a hot-rolled coil obtained by the process according to claim 1.

14. A hot-rolled bar obtained by the process according to claim 3 and having a diameter of 18 mm to 250 mm or a hot-rolled rod that can be obtained by the process according to claim 3 and having a diameter of 4 to 30 mm.

15. A cold-drawn bar that can be obtained by the process according to claim 4, having a diameter of 4 mm to 60 mm, or die-drawn rod or wire that can be obtained by the process according to claim 3, having a diameter of 0.010 mm to 20 mm.

16. A mechanical part such as pumps, valve shafts, motor and engine shafts and couplings operating in corrosive media comprising: a bar according to claim 14.

17. Use of a rod or wire according to claim 14 for the manufacture of cold-formed assemblies, for the agri-foodstuff industry, for oil and ore extraction, or for the manufacture of woven or knitted metal fabrics for the filtration of chemicals, ores or foodstuffs.

18. A steel section obtained by the process according to claim 5.

19. A steel forging obtained by the process according to claim 6.

20. Brackets or couplings comprising: a steel forging according to claim 19.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0101] To illustrate the invention, trials were made and these will be described in particular with reference to FIGS. 1 to 5 which show:

[0102] FIG. 1: A correlation between % ferrite after treatment at 1100 C. and I.sub.F for as-processed products;

[0103] FIG. 2: A relative diametral change as a function of the deformation temperature;

[0104] FIG. 3: Pitting potentials E1 and E2, determined on forged bars, as a function of the index I.sub.LCR;

[0105] FIG. 4: The uniform corrosion rate V, determined on forged bars, as a function of the index I.sub.LCR; and

[0106] FIG. 5: Critical temperatures T.sub.CC and T.sub.CP, determined on forged bars, as a function of the index I.sub.LCR.

DETAILED DESCRIPTION

Examples

[0107] 25 kg laboratory ingots were produced by vacuum induction melting pure ferro-alloy raw materials, followed by introducing nitrogen by addition of ferro-alloys nitrided under a nitrogen partial pressure and cast into a metal mould under an external nitrogen pressure of 0.8 bar. Among these, only trials 14441 and 14604 were according to the invention.

[0108] An industrial heat according to the invention of 150 tonnes referenced 8768 was produced. This grade was smelted by melting in an electric furnace then vacuum-refined with decarburization in order to achieve the intended carbon level. It was then continuously cast into slabs measuring 2201700 mm in cross section before being hot-rolled, after reheating to 1200 C., into quarto plates with thicknesses of 7, 12 and 20 mm. The plates thus obtained then underwent a heat treatment at around 1000 so as to put the various precipitates present at this stage back into solution. After the heat treatment, the plates were water-cooled, then levelled, cut and pickled.

[0109] The compositions in percentages by weight of the various grades smelted on a laboratory scale or an industrial scale are given in Table 1, together with the compositions of the various industrial products or semi-finished products smelted in an electric furnace, followed by AOD refining and cast into ingots or continuously cast, these being mentioned for comparison.

TABLE-US-00001 TABLE 1 Heat No. 14441 14604 8768 14382 14383 14439 14426 14422 14425 14424 14660 Product 25 kg 25 kg 150 t 25 kg 25 kg 25 kg 25 kg 25 kg 25 kg 25 kg 25 kg Al 0.014 0.012 0.0042 0.010 0.015 0.014 <0.002 <0.002 0.024 C 0.016 0.028 0.020 0.020 0.020 0.017 0.021 0.022 0.019 0.020 0.024 Cr 23.07 22.80 22.83 23.03 23.01 23.05 26.67 26.56 26.68 26.61 22.79 Cu 0.301 0.300 0.15 0.304 0.297 0.299 0.279 0.280 0.280 0.208 0.284 Mn 1.282 1.284 1.25 1.288 1.277 1.309 0.724 0.706 0.723 0.705 4.780 Mo 0.249 0.249 0.35 0.251 0.250 0.251 1.322 1.337 1.327 1.328 0.296 N 0.212 0.239 0.21 0.110 0.110 0.290 0.119 0.117 0.300 0.237 0.199 Ni 2.539 1.692 2.50 4.249 1.552 1.485 4.532 1.419 1.541 2.549 2.470 O 0.0049 0.0038 0.0042 0.0031 0.0039 0.0052 0.0316 0.0284 0.0205 0.0221 0.0033 P 0.023 0.023 0.024 0.024 0.024 0.022 0.025 0.022 0.025 0.022 0.025 S 0.0009 0.0010 0.0005 0.0008 0.0008 0.0009 0.0209 0.0203 0.0210 0.0203 0.0014 Si 0.430 0.358 0.44 0.399 0.455 0.403 0.424 0.391 0.407 0.408 0.494 V 0.121 0.061 0.064 0.123 0.122 0.120 0.106 0.102 0.109 0.107 0.013 W <0.010 <0.010 0.019 <0.010 <0.010 <0.010 <0.010 <0.010 <0.010 <0.010 <0.010 Ti 0.0048 0.0017 0.007 0.0027 0.0039 0.0027 0.0041 0.0059 0.0047 0.0050 0.0011 Zr 0.0048 0.0052 0.0042 0.0049 0.0055 0.0064 0.0055 0.0060 0.0058 0.0072 0.0083 Co <0.002 <0.002 0.041 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 Ca <0.0005 <0.0005 0.0003 <0.0005 <0.0005 <0.005 <0.0005 <0.005 <0.0005 <0.0005 <0.0002 Nb <0.002 <0.002 0.0009 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 Se <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 As <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 Ce + La <0.0002 <0.0002 <0.0002 <0.0002 <0.0002 <0.0002 <0.0002 <0.0002 <0.0002 <0.0002 <0.0002 Mg <0.0005 <0.0005 0.0004 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 B <0.0005 <0.0005 0.0024 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005 Heat No. 304 304L 316L UNS32101 UNS32304 140301 436002 517077 533054 150091 Product * * * * * * * * * * Al 0.026 0.006 C 0.020 0.018 0.022 0.018 0.015 0.021 0.026 0.029 0.011 Cr 18.23 18.4 16.5 21.6 22.9 23.01 22.30 22.14 22.32 23.02 Cu 0.15 0.11 0.31 0.24 0.163 0.303 0.260 0.284 0.083 Mn 0.79 1.20 1.66 5.2 1.26 1.563 1.097 1.082 1.054 1.584 Mo 0.37 0.16 2.08 0.3 0.24 2.802 0.277 0.285 0.275 3.118 N 0.044 0.074 0.067 0.224 0.12 0.164 0.143 0.119 0.106 0.150 Ni 8.96 10.2 10.24 1.5 4.20 5.500 4.022 3.995 4.364 8.672 O 0.0037 P 0.023 0.020 0.019 0.027 0.028 0.022 0.022 0.023 0.019 S 0.0013 0.0011 0.0004 0.0008 0.0006 0.0004 0.0004 0.0006 0.0009 Si 0.37 0.50 0.71 0.40 0.206 0.414 0.464 0.400 0.390 V 0.103 0.114 0.058 0.126 W 0.028 0.017 0.013 0.022 Ti 0.0065 0.0040 0.0030 0.0033 Co 0.063 0.129 0.056 0.035 Zr Ca 0.0007 0.0026 0.0028 0.0007 Nb 0.0046 0.009 0.012 0.0063 Se <0.0020 <0.0020 <0.0020 <0.0020 <0.0020 Ce + La Mg 0.0014 <0.0005 <0.0005 <0.0005 <0.0005 B 0.0008 <0.0005 <0.0005 0.0022 <0.0005 *: Rolled plate or billet or bar.

[0110] 1. Ferrite Contents

[0111] 1.1 Ferrite Contents on As-Processed Products

[0112] Specimens ranging from 1 to 8 cm.sup.3 in volume were cut from these laboratory heats in the as-cast state or from industrial products in the as-cast state, and heat treatments for 30 minutes at various temperatures were carried out on these specimens, in a salt bath, followed by an end-of-treatment water quench, in order to determine the proportion of ferrite at high temperature. Since ferrite is magnetic, unlike austenite, carbides and nitrides possibly present, an assaying method was used in which the saturation magnetization was measured. The ferrite contents thus determined are given in Table 2 and plotted in FIG. 1.

[0113] FIG. 1 shows that there is a good correlation between the index I.sub.F and the ferrite contents measured on the base metal after treatments at 1100 C.

[0114] It has also been shown that heat 14441 according to the invention has, below 1300 C., a ferrite content appropriate to hot transformation to a duplex structure. Furthermore, after heat treatment in the 950 C. to 1100 C. range, it has a ferrite content appropriate for stress corrosion resistance.

TABLE-US-00002 TABLE 2 Heat 14382 14383 14441 14426 14422 14425 14424 140301 436002 517077 533054 150091 Product Ingot Ingot Ingot Ingot Ingot Ingot Ingot CCB CCB CCB CCB CCB As- 55.6 50.5 52.6 50.3 25.4 processed state +900 C. 45.6 89.5 54.4 71.2 98.7 100 91.9 45 51.0 47.2 20.5 +950 C. 48.7 87.1 51.7 71.1 98.8 99.6 94.6 42.8 48.9 46.1 25.4 +1000 C. 50.9 90.0 54.5 71.8 99.4 99.4 93.4 50.8 42.1 50.7 46.0 28.8 +1050 C. 55.7 81.0 53.0 77.8 98.6 99.1 78.8 54 44.2 54.6 48.3 33.7 +1100 C. 60.8 84.6 55.5 82.0 99.0 87.4 75.4 58.6 47.6 59.4 51.3 36.1 +1150 C. 65.2 88.6 59.0 88.1 98.9 75.6 78.1 64.6 52.7 66.7 57.9 41.1 +1200 C. 76.6 94.2 64.0 95.4 98.8 78.4 71.6 59.3 75.5 64.8 46.7 +1250 C. 92.3 98.1 67.7 100 99.2 81.0 86.2 81.5 67.4 86.0 73.2 55.1 +1300 C. 95.2 97.7 72.6 99.4 98.7 85.9 93.5 100 78.3 99.0 85.0 66.4 CCB = continuously cast bloom.

[0115] 1.2. Ferrite Contents On End-Products

[0116] The ferrite content was also measured by the grid method (according to ASTM E 562 standard) on forged bars after heat treatment at 1030 C. and in heat-affected zones of weld beads deposited by a coated electrode with a constant energy resulting in cooling rates at 700 C. of 20 C./s. The results (ferrite contents of the base metal and of the heat-affected zone) are given in Table 3. This shows that heats 14441 and 14604 according to the invention have a ferrite content in the base metal and in the heat-affected zone that is favourable to localized corrosion resistance and to stress corrosion resistance, and also favourable to toughness (cf. Table 5).

TABLE-US-00003 TABLE 3 Ferrite contents .sub.BM .sub.HAZ Reference Product (%) (%) 14441* forged rod 48 70 14604* forged rod 54 65 14382 forged rod 49 80 14383 forged rod 79 88 14660 forged rod 48 72 UNS S32101 HR plate 45 67 UNS S32304 HR plate 47 75 *according to the invention; HR: hot-rolled; .sub.BM (%): ferrite content measured on the base metal; .sub.HAZ: ferrite content measured in the heat-affected zone.

[0117] 2. Castability

[0118] Ingot 14439 exhibited blisters and was unusable. To avoid this phenomenon during casting in air at atmospheric pressure, it proved necessary to limit the nitrogen content of the heats according to the invention to less than 0.28% by weight.

[0119] 3. Hot-Transformation Capability

[0120] The hot-deformability was evaluated using hot tensile tests carried out on test specimens, the calibrated part of which, having a diameter of 8 mm and a length of 5 mm, was heated by Joule heating for 80 seconds at 1280 C. and then cooled at 2 C. per second down to the test temperature, which varied between 900 and 1280 C. When this temperature was reached, the rapid tensile test was immediately started, at the rate of 73 mm/s; after fracture, the necking diameter at the break was measured.

[0121] The relative diametral change (Table 4), as defined below, reflects the hot-deformability:

[0122] =100(1(final diameter/initial diameter)).

TABLE-US-00004 TABLE 4 Relative diametral changes (hot tensile test) Test temperature (in %) ( C.) Heat 14382 Heat 14383 Heat 14441* 1280 85.0 100.0 96.7 1250 98.3 86.7 1200 75.0 98.3 76.7 1150 70.0 95.0 61.7 1100 63.3 93.3 56.7 1050 51.7 75.0 44.2 1010 45.0 1000 65.0 40.0 980 36.7 960 58.3 950 35.8 900 35.0 51.7 36.7 *according to the invention.

[0123] On examining Table 4 and FIG. 2, which represents the data in the form of curves, it may be seen that heat 14441 according to the invention has a hot-deformability comparable to that of comparative reference heat 14382.

[0124] 4. Mechanical Properties

[0125] The tensile properties R.sub.e0.2 and R.sub.m were determined according to the NFEN 10002-1 standard. The toughness K.sub.v was determined at various temperatures according to the NF EN 10045 standard.

TABLE-US-00005 TABLE 5 Mechanical properties K.sub.V K.sub.V R.sub.e0.2 R.sub.m 20 C. 50 C. Reference Product (MPa) (MPa) (J) (J) 14441* forged bar 477 716 334 51 14604* forged bar 477 691 288 18 14382 forged bar 436 664 >339 339 14383 forged bar 458 604 79 9 14660 forged bar 493 701 293 31 304L HR plate 218 523 312 301 316L HR plate 232 537 307 298 UNS S32101 HR plate 466 720 101 60 UNS S32304 HR plate 438 663 268 153 8768* HR plate 519 743 *according to the invention; HR: hot-rolled; R.sub.e0.2: at yield strength 0.2% strain; R.sub.m: tensile strength.

[0126] The results of the laboratory heats 14441 and 14604 and of the industrial heat 8768, all three according to the invention, show that a yield strength of greater than 450 MPa, i.e. twice that obtained for austenitic steels of AISI 304L type, may be obtained.

[0127] The toughness values determined at 20 C. for the laboratory heats 14441 and 14604 and for the industrial heat 8768, all three according to the invention, are all greater than 200 J, this being satisfactory taking into account the yield strength level of these grades. For heat 14383 not according to the invention, having a low nitrogen content and a high ferrite content in the annealed state, the toughness values at 20 C. are below 100 J. This confirms the need for a sufficient addition of nitrogen in order to obtain a satisfactory toughness level.

[0128] 5. Corrosion Resistance

[0129] Corrosion resistance tests were carried out both on the forged bars from laboratory heats and on coupons removed from hot-rolled plates coming from the industrial heats.

[0130] 5.1 Localized Corrosion Resistance

[0131] The pitting corrosion resistance was evaluated by plotting the current-potential curves and determining the pitting potential for i=100 A/cm.sup.2. This parameter was measured in a neutral medium (pH=6.4) with a high chloride concentration ([Cl.sup.]=30 g/l) at 50 C. (E.sub.1), representative of the brine encountered in seawater desalination plants, and in a slightly acid (pH=5.5) medium with a low chloride concentration ([Cl.sup.]=250 ppm) at room temperature (E.sub.2) representative of drinking water. The critical pitting temperature in a ferric chloride medium (6% FeCl.sub.3) was also determined according to the ASTM G48-00 standard, method C.

[0132] In another series of trials, the pitting corrosion resistance was determined in a deaerated neutral medium containing 0.86 mol/litre of NaCI, corresponding to 5% NaCl by weight, at 35 C. A floating potential measurement for 900 seconds was carried out. Next, a potentiodynamic curve was plotted at a rate of 100 mV/min of the floating potential up to the pitting potential. The pitting potential (E.sub.3) was determined for i=100 A/cm.sup.2. Under these conditions, specimens according to the invention and reference specimens of 304L grade and austenitic-ferritic duplex grades of 1.4362 type and others were tested.

[0133] The crevice corrosion resistance was studied by measuring the critical crevice temperature in the neutral medium (pH=6.4) with a high chloride concentration ([Cl.sup.]=30 g/l). The arrangement favouring floating crevice corrosion was in accordance with the recommendations given in the ASTM G78-99 standard. The critical crevice temperature is the minimum temperature for which crevices with a depth of greater than 25 m are observed.

[0134] The values obtained are given in Table 6. Comparison between the results obtained on the plate made of UNS S32304 and the bar obtained from heat 14382, these two being of similar chemical composition, indicates that the corrosion resistance of a bar is lower than that of a hot-rolled plate of the same composition.

[0135] The present inventors have found that the localized corrosion resistance index, that is to say resistance to the formation of pits or crevices, abbreviated to I.sub.LCR and defined by:

I.sub.LCR=Cr+3.3Mo+16N+2.6Ni0.7Mn

[0136] (contents in Cr, Mo, N, Ni and Mn in % by weight) [0137] accounts for the classification of all the compositions containing less than 6% nickel as regards localized corrosion resistance (see FIGS. 3, 4 and 5).

[0138] Heats 14383 and 14660 not according to the invention, having I.sub.LCR indices equal to 28.7 and 29.8, exhibit an inferior corrosion behaviour than a steel of AISI 304L type. Heats 14604 and 14441 according to the invention, having an I.sub.LCR of 30.9 and 33, behave at least as well as 304L type steel. To obtain a corrosion resistance at least equal to that of AISI 304L grade, it has been found that the steels according to the invention must preferably have an I.sub.LCR of greater than 30.5 and preferably greater than 32.

[0139] 5.2 Uniform Corrosion Resistance

[0140] Uniform corrosion was characterized by evaluating the corrosion rate by loss of weight after 72 hours' immersion in a 2% dilute sulphuric acid solution heated to 40 C.

[0141] Comparing the corrosion rates for the experimental heats containing 2.5% Ni and 0.2% N (14441 according to the invention and 14660 not according to the invention) clearly shows the negative effect of a high Mn content on the uniform corrosion resistance in a sulphuric medium.

TABLE-US-00006 TABLE 6 Localized and uniform corrosion resistance data E.sub.1 E.sub.2 E.sub.3 T.sub.CP T.sub.CC V Ref. Product I.sub.LCR (V/ECS) (V/ECS) (V/ECS) ( C.) ( C.) (mm/y) 14441* forged rod 33.0 0.165 1.058 0.320 7.5 50 0.73 14604* forged bar 30.9 0.159 0.802 5 45 1.8 14382 forged bar 35.8 0.302 1.323 0.420 15 60 0.24 14383 forged bar 28.7 0.049 0.595 0.050 0 35 4.95 14660 forged bar 29.8 0.094 0.707 7.5 45 1.11 304L HR plate NA 0.188 0.834 0.210 5 65 316L HR plate NA 0.266 0.865 7.5 75 UNS S32101 HR plate 26.4 0.163 0.855 12.5 UNS S32304 HR plate 35.7 0.413 1.330.sup.1 17.5 95 517077 rolled bar 34.6 0.415 140301 rolled bar 47.1 1.200.sup.1 8768* HR plate 33.1 0.227 1.273.sup.1 *according to the invention; .sup.1odixation potential of the solvent, no pitting observed; HR: hot-rolled; NA: not applicable; E.sub.1: pitting potential in neutral medium (pH = 6.4) having a high chloride concentration (30 g/l of Cl.sup.) at 50 C.; E.sub.2: pitting potential in slightly acid environment (pH = 5.5) having a low chloride concentration (250 ppm of Cl.sup.) at 25 C.; E.sub.3: pitting potential in neutral chloride medium (5% NaCl) at 35 C.; T.sub.CP: critical pitting temperature in a ferric chloride medium; T.sub.CC: critical crevice temperature in neutral medium (pH = 6.4) with a high chloride concentration (30 g/l of Cl.sup.) V: uniform corrosion rate in 2% sulphuric acid medium at 40 C.

[0142] 5.3 Repassivation Potential

[0143] The steel specimens were polished under water using SiC paper up to 1200 and then aged for 24 hours in air.

[0144] The cyclic polarization test in a chloride medium was carried out by starting with measurement of the floating potential for 15 min, followed by cyclic dynamic polarization at 100 mV/min starting from the floating potential up to the potential for which the current reached an intensity of 300 A/cm.sup.2, followed by return to the potential for which the current is zero.

[0145] Thus, the pitting potential (P.sub.pit) and the repassivation potentials (P.sub.repassivation) of the previously formed pits were determined. The results obtained are given in Table 7.

TABLE-US-00007 TABLE 7 Repassivation as a function of the nickel content Heat % Ni V.sub.pit-V.sub.repassivation (mV/ECS) 14382 4.5 460 14441 2.5 361 14383 1.5 227

[0146] From the repassivation potential tests in NaCl medium, the higher the nickel content the greater the difference between the pitting potential and the repassivation potential. This shows that nickel is not beneficial to the repassivation of a grade according to the invention that has previously undergone pitting corrosion.