Duplex stainless steel

Abstract

The invention relates a duplex ferritic austenitic stainless steel having high formability utilizing the TRIP effect and high corrosion resistance with the balanced pitting resistance equivalent. The duplex stainless steel contains less than 0.04 weight % carbon, less than 0.7 weight % silicon, less than 2.5 weight % manganese, 18.5-22.5 weight % chromium, 0.8-4.5 weight % nickel, 0.6-1.4 weight % molybdenum, less than 1 weight % copper, 0.10-0.24 weight % nitrogen, the rest being iron and inevitable impurities occurring in stainless steels.

Claims

1. A duplex ferritic austenitic stainless steel, heat treated at a temperature of 950° C. to 1150° C., the stainless steel having 35 volume %-44 volume % ferrite, the rest being austenite, wherein the stainless steel contains less than 0.04 weight % carbon, less than 0.7 weight % silicon, less than 2.0 weight % manganese, 18.5-22.5 weight % chromium, 1.5-3.5 weight % nickel, 0.99-1.4 weight % molybdenum, less than 1 weight % copper, 0.16-0.21 weight % nitrogen, greater than 0-0.5 weight % tungsten, the rest being iron and inevitable impurities, the stainless steel having: a measured M.sub.d30 temperature value of 10-70° C., a pitting resistance equivalent value (PRE) of 27-29.5, wherein PRE=[Cr]+3.3[Mo]+30 [N]−[Mn], wherein [Cr], [Mo], [N] and [Mn] represent the contents of Cr, Mo, N and Mn in weight %, a tensile strength R.sub.m in a longitudinal direction within a range of 766.7-805 MPa, and a critical pitting temperature CPT of 20-33° C.

2. The duplex ferritic austenitic stainless steel according to claim 1, wherein the chromium content is 19.0-22 weight %.

3. The duplex ferritic austenitic stainless steel according to claim 1, wherein the copper content is up to 0.7 weight %.

4. The duplex ferritic austenitic stainless steel according to claim 1, wherein the stainless steel contains one or more added elements: less than 0.04 weight % Al, less than 0.003 weight % B, less than 0.003 weight % Ca, less than 0.1 weight % Ce, up to 1 weight % Co, up to 0.1 weight % Nb, up to 0.1 weight % Ti, up to 0.2 weight % V.

5. The duplex ferritic austenitic stainless steel according to claim 1, wherein the stainless steel has a chemical composition window defined by weight % as: Si+Cr % being within a range of 18.5-21.9; Cu+Mo % being within a range of 1.36-2.394; C+N % being within a range of 0.175-0.215; and Mn+Ni % being within a range of 1.5-<2.9.

6. The duplex ferritic austenitic stainless steel according to claim 1, wherein the stainless steel has a chemical composition window defined by weight % as: Si+Cr % being within a range of 19.8-20.9; Cu+Mo % being within a range of 1.7-1.9; C+N % being within a range of 0.20-<0.25; and Mn+Ni % being within a range of 1.5-<2.9.

7. The duplex ferritic austenitic stainless steel according to the claim 1, wherein the stainless steel is produced as ingots, slabs, blooms, billets, plates, sheets, strips, coils, bars, rods, wires, profiles and shapes, seamless and welded tubes and/or pipes, metallic powder, formed shapes and profiles.

8. The duplex ferritic austenitic stainless steel according to claim 1, the stainless steel contains less than 0.010 weight % sulfur (S), less than 0.040 weight % phosphorous (P), and oxygen content below 100 ppm, wherein the sum (S+P) is less than 0.04 weight %.

9. The duplex ferritic austenitic stainless steel according to claim 1, the stainless steel having a yield strength R.sub.p0.2 in a longitudinal direction within a range of 479.7-562 MPa.

10. The duplex ferritic austenitic stainless steel according to claim 1, the stainless steel having a yield strength R.sub.p1.0 in a longitudinal direction within a range of 552-626 MPa.

11. The duplex ferritic austenitic stainless steel according to claim 1, the stainless steel having an elongation value A.sub.g in a longitudinal direction within a range of 34.6%-37.6%.

12. The duplex ferritic austenitic stainless steel according to claim 1, the stainless steel having a tensile strength R.sub.m in a transverse direction within a range of 757.5-858 MPa.

13. The duplex ferritic austenitic stainless steel according to claim 1, the stainless steel having a yield strength R.sub.p0.2 in a transverse direction within a range of 499-596 MPa.

14. The duplex ferritic austenitic stainless steel according to claim 1, the stainless steel having a yield strength R.sub.p1.0 in a transverse direction within a range of 556-648 MPa.

15. The duplex ferritic austenitic stainless steel according to claim 1, the stainless steel having an elongation value A.sub.g in a transverse direction within a range of 30.8%— 39.8%.

Description

(1) The present invention is described in more details referring to the drawings where FIG. 1 illustrates the dependence of the minimum and maximum M.sub.d30 temperature and PRE values between the element contents Si+Cr and Cu+Mo in the tested alloys of the invention, FIG. 2 illustrates an example with constant values of C+N and Mn+Ni for the dependence of the minimum and maximum M.sub.d30 temperature and PRE values between the element contents Si+Cr and Cu+Mo in the tested alloys of the invention according to FIG. 1, FIG. 3 illustrates the dependence of the minimum and maximum M.sub.d30 temperature and PRE values between the element contents C+N and Mn+Ni in the tested alloys of the invention, and FIG. 4 illustrates an example with constant values of Si+Cr and Cu+Mo for the dependence of the minimum and maximum M.sub.d30 temperature and PRE values between the element contents C+N and Mn+Ni in the tested alloys of the invention according to FIG. 3.

(2) Based on the effects of the elements the duplex ferritic austenitic stainless steel according to the invention is presented with the chemical compositions A to G as named in the table 1. The table 1 contains also the chemical composition for the reference duplex stainless steel of the FI patent application 20100178 named as H, all the contents of the table 1 in weight %.

(3) TABLE-US-00001 TABLE 1 Alloy C % Si % Mn % Cr % Ni % Cu % N % Mo % A 0.03 0.30 0.50 20.7 4.0 0.42 0.165 1.27 B 0.023 0.29 1.4 20.4 3.5 0.41 0.162 0.99 C 0.024 0.28 1.36 20.6 2.7 0.42 0.18 1.14 D 0.02 0.37 1.82 19.6 1.7 0.42 0.198 1.17 E 0.021 0.31 0.76 20.1 2.9 0.42 0.194 1.19 F 0.017 0.33 0.83 19.8 3.1 0.41 0.19 1.2 G 0.026 0.46 0.99 20.08 3.03 0.36 0.178 1.19 H 0.04 0.40 3.0 20.2 1.2 0.40 0.22 0.40

(4) The alloys A-F were manufactured in a vacuum induction furnace in 60 kg laboratory scale to small slabs that were hot rolled and cold rolled down to 1.5 mm thickness. The alloy G was produced in 100 ton production scale followed by hot rolling and cold rolling to coil form with varying final dimensions.

(5) When comparing the values in the Table 1 the contents of carbon, nitrogen, manganese, nickel and molybdenum in the duplex stainless steels of the invention are significantly different from the reference stainless steel H.

(6) The properties, the values for the M.sub.d30 temperature, the critical pitting temperature (CPT) and the PRE were determined for the chemical compositions of the table 1 and the results are presented in the following table 2.

(7) The predicted M.sub.d30 temperature (M.sub.d30 Nohara) of the austenite phase in the table 2 was calculated using the Nohara expression (1) established for austenitic stainless steels
M.sub.d30=551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)−18.5Mo−68Nb  (1)
when annealed at the temperature of 1050° C.

(8) The actual measured M.sub.d30 temperatures (M.sub.d30 measured) of the table 2 were established by straining the tensile samples to 0.30 true strain at different temperatures and by measuring the fraction of the transformed martensite with Satmagan equipment. Satmagan is a magnetic balance in which the fraction of ferromagnetic phase is determined by placing a sample in a saturating magnetic field and by comparing the magnetic and gravitational forces induced by the sample.

(9) The calculated M.sub.d30 temperatures (M.sub.d30 calc) in the table 2 were achieved in accordance with a mathematical constraint of optimization from which calculation the expressions (3) and (4) have also been derived.

(10) The critical pitting temperature (CPT) is measured in a 1 M sodium chloride (NaCl) solution according to the ASTM G150 test, and below this critical pitting temperature (CPT) pitting is not possible and only passive behaviour is seen.

(11) The pitting resistance equivalent (PRE) is calculated using the formula (2):
PRE=% Cr+3.3*% Mo+30*% N−% Mn  (2).

(12) The sums of the element contents for C+N, Cr+Si, Cu+Mo and Mn+Ni in weight % are also calculated for the alloys of the table 1 in the table 2. The sums C+N and Mn+Ni represent austenite stabilizers, while the sum Si+Cr represents ferrite stabilizers and the sum Cu+Mo elements having resistance to martensite formation.

(13) TABLE-US-00002 TABLE 2 M.sub.d30 M.sub.d30 M.sub.d30 calc Nohara measured CPT Alloy C + N % Si + Cr % Mn + Ni % Cu + Mo % ° C. ° C. ° C. ° C. PRE % A 0.195 21 4.5 1.7 7.7 −18.4 12.5 29.2 29.3 B 0.185 20.7 4.9 1.4 19.9 6.5 22 22.5 27.1 C 0.204 20.9 4.1 1.6 17.2 −5.5 15.5 25.2 28.4 D 0.218 19.97 3.52 1.59 44.7 21.8 32.5 — 27.6 E 0.215 20.41 3.66 1.61 27.7 6.3 30.0 25.3 29.1 F 0.207 20.13 3.93 1.61 36.9 −81 56.0 22.8 28.6 G 0.204 20.54 4.02 1.55 29.6 5 19 30.0 28.4 (1.5 mm) G 0.204 20.54 4.02 1.55 29.6 5 21 30.6 28.4 (2.5 mm) H 0.26 20.7 4.3 1.0 24.9 23 27 <10 25

(14) When comparing the values in the Table 2 the PRE value having the range of 27-29.5 is much higher than the PRE value in the reference duplex stainless steel H which means that the corrosion resistance of the alloys A-G is higher. The critical pitting temperature CPT is in the range of 21-32° C., which is much higher than the CPT for austenitic stainless steels, such as EN 1.4401 and similar grades.

(15) The predicted M.sub.d30 temperatures using the Nohara expression (1) are essentially different from the measured M.sub.d30 temperatures for the alloys on the table 2. Further, from the table 2 it is noticed that the calculated M.sub.d30 temperatures agree well with the measured M.sub.d30 temperatures, and the mathematical constraint of optimization used for the calculation is thus very suitable for the duplex stainless steels of the invention.

(16) The sums of the element contents for C+N, Si+Cr, Mn+Ni and Cu+Mo in weight % for the duplex stainless steel of the present invention were used in the mathematical constraint of optimization to establish the dependence in one hand between C+N and Mn+Ni, and in another hand between Si+Cr and Cu+Mo. In accordance with this mathematical constraint of optimization the sums of Cu+Mo and Si+Cr, respectively the sums Mn+Ni and C+N, form the x and y axis of a coordination in the FIGS. 1-4 where the linear dependence for the minimum and maximum PRE values (27<PRE<29.5) and for the minimum and maximum M.sub.d30 temperature (10<M.sub.d30<70) values are defined.

(17) In accordance with FIG. 1 a chemical composition window for Si+Cr and Cu+Mo is established with the preferred ranges of 0.175-0.215 for C+N and 3.2-5.5 for Mn+Ni when the duplex stainless steel of the invention was annealed at the temperature of 1050° C. It is also noticed in FIG. 1 a limitation of Cu+Mo<2.4 because of the maximum ranges for copper and molybdenum.

(18) The chemical composition window, which lies within the frame of the area a′, b′, c′, d′ and e′ in FIG. 1, is defined with the following labelled positions of the coordination in the table 3.

(19) TABLE-US-00003 TABLE 3 Si + Cr % Cu + Mo % C + N % Mn + Ni % a′ 22.0 0.45 0.175 3.2 b′ 21.4 1.9 0.175 3.2 c′ 19.75 2.4 0.21 3.3 d′ 18.5 2.4 0.215 5.5 e′ 18.9 1.34 0.215 5.5

(20) FIG. 2 illustrates one chemical composition example window of FIG. 1 when constant values of 0.195 for C+N and 4.1 for Mn+Ni are used at all points instead of the ranges for C+N and Mn+Ni in FIG. 1. The chemical composition window, which lies within the frame of the area a, b, c and d in FIG. 2, is defined with the following labelled positions of the coordination in the table 4.

(21) TABLE-US-00004 TABLE 4 Si + Cr % Cu + Mo % C + N % Mn + Ni % a 21.40 0.80 0.195 4.1 b 20.10 1.60 0.195 4.1 c 19.15 2.25 0.195 4.1 d 19.50 1.40 0.195 4.1

(22) FIG. 3 illustrates a chemical composition window for C+N and Mn+Ni with the preferred composition ranges 19.7-21.45 for Cr+Si and 1.3-1.9 for Cu+Mo, when the duplex stainless steel was annealed at the temperature of 1050° C. Further, in accordance with invention the sum C+N is limited to 0.1<C+N<0.28 and the sum Mn+Ni is limited to 0.8<Mn+Ni<7.0. The chemical composition window, which lies within the frame of the area p′, q′ r′, s′, t′ and u′ in FIG. 3, is defined with the following labelled positions of the coordination in the table 5.

(23) TABLE-US-00005 TABLE 5 Si + Cr % Cu + Mo % C + N % Mn + Ni % p′ 20.4 1.8 0.28 4.3 q′ 19.8 1.3 0.28 7.0 r′ 20.2 1.7 0.17 7.0 s′ 20.1 1.7 0.10 5.2 t′ 20.9 1.9 0.10 1.5 u′ 20.6 1.9 0.16 0.8

(24) The effect of the limitations for C+N and Mn+Ni with the preferred ranges for the element contents of the invention is that the chemical composition window of FIG. 3 is partly limited by the PRE maximum and minimum values and partly limited by the limitations for C+N and Mn+Ni.

(25) FIG. 4 illustrates one chemical composition example window of FIG. 3 with the constant values of 20.5 for Cr+Si and 1.6 for Cu+Mo and further, with the limitation of 0.1<C+N. The chemical composition window, which lies within the frame of the area p, q, r, s, t and u in FIG. 4, is defined with the following labelled positions of the coordination in the table 6.

(26) TABLE-US-00006 TABLE 6 Si + Cr % Cu + Mo % C + N % Mn + Ni % p 20.5 1.6 0.24 5.1 q 20.5 1.6 0.19 6.0 r 20.5 1.6 0.10 3.2 s 20.5 1.6 0.10 2.4 t 20.5 1.6 0.13 1.8

(27) Using the values of the table 2 and the values of the FIGS. 1-4 the following expressions for the minimum and maximum M.sub.d30 temperature values are established
19.14−0.39(Cu+Mo)<(Si+Cr)<22.45−0.39(Cu+Mo)  (3)
0.1<(C+N)<0.78−0.06(Mn+Ni)  (4)
when the duplex stainless steel of the invention is annealed at the temperature range of 950-1150° C.

(28) The alloys of the present invention as well as the reference material H above were further tested by determining the yield strengths R.sub.p0.2 and R.sub.p1.0 and the tensile strength R.sub.m as well as the elongation values for A.sub.50, A.sub.5 and A.sub.g both in the longitudinal (long) direction (alloys A-C, G-H) and in the transverse (trans) direction (all alloys A-H). The table 7 contains the results of the tests for the alloys A-G of the invention as well as the respective values for the reference H duplex stainless steel.

(29) TABLE-US-00007 TABLE 7 R.sub.p0.2 R.sub.p1.0 R.sub.m A.sub.50 A.sub.5 A.sub.g Alloy (MPa) (MPa) (MPa) (%) (%) (%) A trans 549.0 594.0 777.0 37.9 41.4 33.4 A long 527.8 586.0 797.3 40.0 44.0 34.6 B long 479.7 552.0 766.7 40.8 44.5 36.9 C trans 550.3 594.0 757.5 38.3 42.1 31.0 C long 503.8 583.0 772.3 42.5 46.7 34.6 D trans 1050° C. 526 577 811 41.6 45.7 37.4 D trans 1120° C. 507 561 786 44.0 48.3 39.8 E trans 1050° C. 540 588 810 44.0 48.2 38.8 E trans 1120° C. 517 572 789 43.6 47.8 38.5 F trans 1050° C. 535 577 858 37.2 40.8 34.7 F trans 1120° C. 499 556 840 39.8 43.7 35.9 G 1.5 mm trans 596 648 784 37.1 40.8 30.8 G 1.5 mm long 562 626 801 40.4 44.3 35.5 G 2.5 mm trans 572 641 793 40.7 43.3 34.9 G 2.5 mm long 557 622 805 43.3 45.9 37.6 H trans 493.7 543.7 757.3 44.6 48.6 40 H long 498.0 544.0 787.0 45.2 49.0 40

(30) The results in the table 7 show that the yield strength values R.sub.p0.2 and R.sub.p1.0 for the alloys A-G are much higher than the respective values for the reference duplex stainless steel H, and the tensile strength value R.sub.m is similar to the reference duplex stainless steel H. The elongation values A.sub.50, A.sub.5 and A.sub.g of the alloys A to G are lower than the respective values for the reference stainless steel.

(31) The duplex ferritic austenitic stainless steel of the invention can be produced as ingots, slabs, blooms, billets and flat products such as plates, sheets, strips, coils, and long products such as bars, rods, wires, profiles and shapes, seamless and welded tubes and/or pipes. Further, additional products such as metallic powder, formed shapes and profiles can be produced.