Ferritic-austenitic stainless steel

Abstract

The invention relates to a duplex stainless steel having austenitic-ferritic microstructure of 35-65% by volume, preferably 40-60% by volume of ferrite and having good weldability, good corrosion resistance and good hot workability. The steel contains 0.005-0.04% by weight carbon, 0.2-0.7% by weight silicon, 2.5-5% by weight manganese, 23-27% by weight chromium, 2.5-5% by weight nickel, 0.5-2.5% by weight molybdenum, 0.2-0.35% by weight nitrogen, 0.1-1.0% by weight copper, optionally less than 1% by weight tungsten, less than 0.0030% by weight one or more elements of the group containing boron and calcium, less than 0.1% by weight cerium, less than 0.04% by weight aluminium, less than 0.010% by weight sulphur and the rest iron with incidental impurities.

Claims

1. Duple stainlesss steel having austenitic-ferritic microstructure of 35-65% by volume of ferrite and having good weldability, good corrosion resistance and good hot workability, wherein the steel contains less than 0.03 % by weight carbon, less than 0.7% by weight silicon, 2.8-4.0% by weight manganese, 23-25% by weight chromium, 3.0-4.5% by weight nickel, 1.5-2.0% by weight molybdenum, 0.23-0.30% by weight nitrogen,0.1-0.8% by weight copper, less than 1% by weight tungsten, less than 0.003% by weight one or more elements of the group containing boron and calcium, less than 0.1% by weight cerium, less than 0.04% by weight aluminum, less than 0.003% by weight sulphur and less than 0.035% phosphorus, and the rest iron with incidental impurities, wherein,the steel has an area contraction (Ψ) at the temperature range 1000-1200° C. of between 90.0 and 97.1%.

2. Duplex stainless steel according to claim 1, wherein the yield strength of the steel is at least 500 MPa.

3. Duplex stainless steel according to claim 1, wherein the fracture strength of the steel is more than 700 MPa.

4. Duplex stainless steel according to claim 1, wherein the critical pitting temperature, CPT, of the steel is more than 40 ° C.

5. Duplex stainless steel according to claim 1, wherein the pitting resistance equivalent, PRE, of the steel is between 30 and 36.

6. Duplex stainless steel according to claim 5, wherein the pitting resistance equivalent, PRE, of the steel is between 32 and 36.

7. Duplex stainless steel according to claim 6, wherein the pitting resistance equivalent, PRE, of the steel is between 33 and 35.

8. The duplex stainless steel according to claim 1, wherein the steel contains 0.025% by weight carbon, less than 0.36% by weight silicon, 3.0% by weight manganese, 23.92% by weight chromium, 3.66% by weight nickel, 1.61% by weight molybdenum, 0.279% by weight nitrogen, and 0.39% by weight copper.

Description

(1) The duplex stainless steel of the invention is further described in the test results, which are compared with two reference duplex stainless steels in tables and in one drawing wherein

(2) FIG. 1 shows coil edges made of the duplex stainless steel of the invention, and

(3) FIG. 2 shows coil edges made of the full-scale reference grade.

(4) For the property tests of the duplex stainless steel of the invention a series of 30 kg laboratory heat alloys A to F as well as Ref1 and Ref2 were produced in a vacuum induction furnace with compositions as listed in Table 3. Alloys Ref1 and Ref2 are typical compositions of two commercial grades AL2003 (similar to the grade described in the U.S. Pat. No. 6,551,420) and 2205 (EN 1.4462) respectively. The 100 mm square ingots were conditioned, re-heated and forged to approximately 50 mm thickness and then hot rolled down to 12 mm thick strips. The strips were re-heated and further hot rolled to 3 mm thickness. The hot rolled material was solution annealed at 1050° C. and pickled for various tests. Welding trials were performed with gas tungsten arc (GTA) welding on 3 mm material using 22-9-3 LN welding filler material. The heat input was 0.4-0.5 kJ/mm.

(5) TABLE-US-00003 TABLE 3 Chemical compositions of tested heats Alloy C Si Mn P S Cr Ni Mo Cu N W A 0.031 0.48 3.87 0.013 0.004 24.7 2.65 1.53 0.17 0.251 0.01 B 0.015 0.47 1.59 0.013 0.001 24.43 4.06 1.56 0.18 0.25 0.01 C 0.018 0.29 3.85 0.012 0.003 24.06 3.95 1.72 0.12 0.283 0.01 D 0.011 0.31 2.72 0.015 0.007 23.81 4.13 1.71 0.13 0.307 0.01 E 0.019 0.32 4.08 0.024 0.002 23.71 4.12 1.71 0.96 0.245 0.01 F 0.018 0.31 4.09 0.016 0.004 23.64 4.08 1.72 0.16 0.253 0.9 G 0.025 0.36 3.00 0.022 0.001 23.92 3.66 1.61 0.39 0.279 0.01 Ref1 0.02 0.54 0.67 0.013 0.002 21.66 3.56 1.78 0.23 0.166 0.01 Ref2 0.018 0.41 1.43 0.021 0.001 22.07 5.67 3.18 0.2 0.171 0.01 Ref3 0.013 0.38 1.50 0.021 0.001 22.22 5.76 3.18 0.25 0.185 0.04

(6) The alloy G and Ref3 are the full-scale heats and these alloys G and Ref3 were tested separately from the laboratory heats. The Ref3 is a full-scale heat of the Ref2.

(7) The laboratory heat alloys A to F as well as Ref1 and Ref2 were evaluated regarding mechanical properties in solution-annealed condition. Tensile tests were performed on 3 mm sheet material. For the full-scale material the test was carried out on 6 mm annealed material. The results are listed in Table 4. All tested alloys according to present invention have yield strength Rp.sub.0.2 above 500 MPa, valid for the thickness range and the tested coil process route, and higher than the reference materials of the commercial steels. The fracture strength Rm of heat alloys according to the invention is well above 700 MPa, preferably above 750 MPa, and fracture A50 elongation is greater than 25%, preferably more than 30%.

(8) TABLE-US-00004 TABLE 4 Mechanical properties of tested heats Rp0.2 Rp1.0 Rm A50 Alloy [MPa] [MPa] [MPa] [%] A 567 617 749 31 B 528 594 741 34 C 539 603 769 38 D 518 596 775 36 E 523 593 748 29 F 549 606 763 34 G 561 632 802 34 Ref1 498 542 690 35 Ref2 502 563 715 36

(9) Evaluations of the microstructures in the laboratory heat alloys A to F as well as Ref1 and Ref2 were made using light optical microscopy. The ferrite contents were measured in 3 mm thick material after solution annealing at 1050° C. using quantitative metallography. The results are listed in Table 5. An important feature of a duplex stainless steel of the invention is to show a good microstructure in both as solution annealed in the parent metal (PM) and as welded condition (WM). Steel A shows high ferrite levels in both conditions, which can be explained by a too low Ni content in the steel. Steel B shows acceptable ferrite contents but the nitride level in the welded condition is high, which can be explained by the low manganese content in the steel. With the steel according to the invention a good phase balance has been achieved in both solution annealed and as welded conditions. Further, the amount of nitride precipitates in the heat-affected zone (HAZ) is clearly lower in the steel of this invention.

(10) TABLE-US-00005 TABLE 5 Metallographic investigations Nitride Ferrite % in Alloy PM HAZ WM HAZ A 66 84.3 80.5 high B 57 75.2 73.3 high C 47 69.3 69.6 low D 49 63.3 59.1 low E 51 77 74.1 low F 53 76.9 72.4 low G 49 71 68.7 low Ref1 56 83.6 79.5 high Ref2 51 81.1 75.5 med

(11) In order to evaluate the resistance to pitting corrosion of different laboratory heat alloys A to F as well as Ref1 and Ref2 the critical pitting corrosion temperature, CPT was measured for the heat alloys A to F as well as Ref1 and Ref2. The CPT is defined as the lowest temperature at which pitting occurs in a specific environment. CPT of the different laboratory heat alloys A to F as well as Ref1 and Ref2 was measured on 3 mm material of solution annealed condition and in a 1M NaCl solution using ASTM G150 standard procedure. The results are listed in Table 6. The steels of the invention have CPT in excess of 40° C. The table 6 also contains the PRE value calculated using the formula (1) for the laboratory heat alloys A to F and for the reference materials Ref1 and Ref2.

(12) TABLE-US-00006 TABLE 6 Critical Pitting Temperatures obtained according to ASTM G150 with PRE values Alloy PRE CPT [° C.] A 34 36 B 34 45 C 33 44 D 33 47 E 33 43 F 35 47 G 34 43 Ref1 30 39 Ref2 35 60

(13) This level of critical pitting resistance also compares favourably with that of several, more costly, commercial steels as listed in Table 7.

(14) TABLE-US-00007 TABLE 7 Critical Pitting Temperatures (ASTM G150) of some steel grades Material PRE CPT [° C.] This invention 33-35 ≧40 EN 1.4362 26 25 EN 1.4462 34 50 EN 1.4438 28 35 EN 1.4401 26 10

(15) The test results described for the full-scale alloy G in the tables 4, 5 and 6 are based on the tests, which were carried out on the material having a thickness of 6 mm and received from the full-scale production. The annealing of this alloy G was done in the laboratory circumstances.

(16) An important property of duplex stainless steels is the ease of the manufacture of these steels. For various reasons it is difficult to evaluate such effects on laboratory heats, as the steel refining is not optimal in small scale. Therefore, in addition to the laboratory heat alloys A to F for the duplex stainless steel of the invention above, the full-scale heats (90 ton) were produced (Alloy G and Ref3 in the table 3). These heats were produced using conventional electric arc furnace melting, AOD processing, ladle furnace refining and continuous casting into slabs with a section of 140×1660 mm.

(17) For the manufacture of the duplex stainless steel the hot workability was evaluated of full-scale alloy G of the invention and of Ref3 using hot tensile testing of cylindrical specimens cut from the continuously cast slab and heat treated for 30 minutes at 1200° C. and water quenched. The results are shown in Table 8 where the workability (evaluated as area contraction (ψ [%]) and flow stress (σ [MPa]) for alloy G are compared with a full-scale reference of Ref3, where the specimens for both the alloy G of the invention and the Ref3 were prepared in the same way. The area contraction, ψ, was determined by measuring the sample diameter before and after the tensile test. The flow stress, σ, is the necessary sample stress to attain a deformation rate of 1s.sup.−1. Table 8 also contains the calculated ferrite contents at three temperatures using the thermodynamical database ThermoCalc TCFE6.

(18) TABLE-US-00008 TABLE 8 Results of hot tensile testing Alloy G Ref3 Temperature σ Ferrite Ferrite [° C.] ψ [%] [MPa] [%] ψ [%] σ [MPa] [%] 950 92.5 133 73.3 146 1000 90.0 110 71.6 116 1050 90.9 95 39 75.5 91 38 1100 93.5 81 82.0 77 1150 96.0 65 51 89.4 55 51 1200 97.1 55 66 98.0 46 68

(19) The alloy G, according to the invention, shows a surprisingly good hot ductility in the entire hot working temperature range as compared to the reference material (Ref3) that exhibits a loss in ductility (ψ) towards lower temperatures. Because the phase balance between austenite and ferrite is similar in the compared Alloy G and Ref3, the different compositions of these two steels are the main cause of the different hot workability. This is a crucial property for the duplex stainless steels that will be hot rolled to coils. In order to test the edge cracking in a hot rolled coil, a 20-ton coil of the alloy G was hot rolled in a Steckel mill from 140 to 6 mm thickness resulting in very smooth coil edges as illustrated in FIGS. 1 and 2, where a comparison with a similar coil of Ref3 is shown. FIG. 1 shows coil edges for the alloy G and FIG. 2 coil edges for the Ref3.

(20) The duplex stainless steel according to present invention shows a superior strength level to other duplex stainless steels and exhibits comparable corrosion performance to other duplex stainless steels and austenitic stainless steel alloys with higher raw material costs. It is evident that steel of the invention also possesses a balanced microstructure that makes it respond to welding cycles very favourably.

(21) This description illustrates some important aspects of the invention. However, variations and modifications will be evident to those of ordinary skill in the art without departing from the scope and spirit of the present invention and appended claims.