Duplex stainless steel

Abstract

A duplex ferritic austenitic stainless steel having high formability utilizing the TRIP effect and high corrosion resistance with the balanced pitting resistance equivalent is formed with less than 0.04 weight % carbon, 0.2-0.8 weight % silicon, less than 2.0 weight % manganese, 16.5-19.5 weight % chromium, 3.0-4.7 weight % nickel, 1.5-4.0 weight % molybdenum, less than 3.5 weight % tungsten, less than 1 weight % copper, 0.13-0.26 weight % nitrogen, the rest being iron and inevitable impurities occurring in stainless steels.

Claims

1. A duplex ferritic austenitic TRIP stainless steel having a proportion of austenite phase in a microstructure of the duplex stainless steel of 55-70 volume %, the rest being ferrite, having undergone heat treatment at a temperature range of 900-1200° C., wherein the duplex ferritic austenitic TRIP stainless steel includes greater than 0 and less than 0.04 weight % carbon, 0.2-0.8 weight % silicon, greater than 0 and less than 2.0 weight % manganese, 16.5-19.5 weight % chromium, 3.0-4.7 weight % nickel, 1.5-4.0 weight % molybdenum, greater than 0 and less than 3.5 weight % tungsten, greater than 0 and less than 1 weight % copper, 0.13-0.26 weight % nitrogen, the rest being iron and inevitable impurities occurring in stainless steels; the duplex ferritic austenitic TRIP stainless steel has a pitting resistance equivalent value (PRE) of the duplex stainless steel is 30-36, the PRE calculated as follows: PRE=Cr+3.3(Mo+0.5 W)+30 N—Mn, wherein Cr, Mo, W, N and Mn are expressed in weight %; the duplex ferritic austenitic TRIP stainless steel has a critical pitting temperature CPT in the range of 34-45° C., where the CPT is measured in a 1 M sodium chloride (NaCl) solution according to ASTM G150 test; the duplex ferritic austenitic TRIP stainless steel has a yield strength value (R.sub.p0.2) of 430-471 MPa; and the duplex ferritic austenitic TRIP stainless steel has a predicted M.sub.d30 temperature (Moo Nohara) of the austenite phase of −9° C. to 42° C. calculated as follows: M.sub.d30 Nohara=551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)−185.Mo−68Nb.

2. A duplex ferritic austenitic TRIP stainless steel having a proportion of austenite phase in a microstructure of the duplex stainless steel of 55-70 volume %, the rest being ferrite, having undergone heat treatment at a temperature range of 900-1200° C., wherein: the duplex ferritic austenitic TRIP stainless steel includes greater than 0 and less than 0.04 weight % carbon, 0.2-0.8 weight % silicon, greater than 0 and less than 2.0 weight % manganese, 16.5-19.5 weight % chromium, 3.0-4.7 weight % nickel, 1.5-4.0 weight % molybdenum, greater than 0 and less than 3.5 weight % tungsten, greater than 0 and less than 1 weight % copper, 0.13-0.26 weight % nitrogen, the rest being iron and inevitable impurities occurring in stainless steels; the duplex ferritic austenitic TRIP stainless steel includes 16.9<(Si+Cr)<19.5, 2.0<(Cu+Mo+0.5W)<4.0, 0.16<(C+N)<0.29, 3.0<(Mn+Ni)<5.5, wherein Si, Cr, Cu, Mo, W, C, N, Mn and Ni are expressed in weight %; the duplex ferritic austenitic TRIP stainless steel has a measured M.sub.d30 temperature greater than 10° C. and less than 60° C.; the measured M.sub.d30 temperature defined as the temperature at which 0.3 true strain yields 50% transformation of the austentite to martensite, the fraction of the transformed martensite measured with Satmagan equipment; and the duplex ferritic austenitic TRIP stainless steel has a pitting resistance equivalent value (PRE) of the duplex stainless steel is 30-36, the PRE calculated as follows: PRE=Cr+3.3(Mo+0.5 W)+30 N—Mn, wherein Cr, Mo, W, N and Mn are expressed in weight %; the duplex ferritic austenitic TRIP stainless steel has a critical pitting temperature CPT in the range of 34-45° C., where the CPT is measured in a 1 M sodium chloride (NaCl) solution according to ASTM G150 test.

3. The duplex ferritic austenitic TRIP stainless steel according to claim 1, wherein the chromium content is 16.5-18.8 weight %.

4. The duplex ferritic austenitic TRIP stainless steel according to claim 1, wherein the nickel content is 3.0-4.5 weight %.

5. The duplex ferritic austenitic TRIP stainless steel according to claim 1, wherein the manganese content is greater than 0 and less than 1.0 weight %.

6. The duplex ferritic austenitic TRIP stainless steel according to claim 1, wherein the copper content is greater than 0 and less than 0.7 weight %.

7. The duplex ferritic austenitic TRIP stainless steel according to claim 1, wherein the tungsten content is 1—less than 3.5 weight %.

8. The duplex ferritic austenitic TRIP stainless steel according to claim 1, wherein the sum of the molybdenum (Mo) and tungsten (W) contents according to the formula (Mo+0.5W) is less than 4.0 weight %.

9. The duplex ferritic austenitic TRIP stainless steel according to claim 1, wherein the nitrogen content is 0.16-0.25 weight %.

10. The duplex ferritic austenitic TRIP stainless steel according to claim 1, characterized in that the steel is produced in a form selected from a group consisting of 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.

11. The duplex ferritic austenitic TRIP stainless steel according to claim 1, wherein: the duplex ferritic austenitic TRIP stainless steel has a tensile strength (Rm) of 721-765 MPa; and the duplex ferritic austenitic TRIP stainless steel has a yield strength values (R.sub.p1.0) of 512-538 MPa.

12. The duplex ferritic austenitic TRIP stainless steel according to claim 2, wherein the duplex ferritic austenitic TRIP stainless steel has a yield strength value (R.sub.p0.2) of 430-471 MPa.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The present invention is described in more details referring to the drawings where

(2) 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+0.5W in the tested alloys of the invention,

(3) 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+0.5W in the tested alloys of the invention according to FIG. 1,

(4) 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

(5) FIG. 4 illustrates an example with constant values of Si+Cr and Cu+Mo+0.5W 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.

DETAILED DESCRIPTION OF THE INVENTION

(6) 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 P as named in the table 1. The table 1 contains also the chemical composition for the reference duplex stainless steels of the WO patent application 2011/135170 named as R and the WO patent application 2013/034804 named as Q, all the contents of the table 1 in weight %.

(7) TABLE-US-00001 TABLE 1 Mn Alloy C % Si % % Cr % Ni % Cu % N % Mo % W % A 0.021 0.54 0.62 17.61 4.25 0.41 0.181 1.59 3.08 B 0.023 0.48 0.65 17.85 4.31 0.43 0.189 1.65 1.5 C 0.024 0.51 0.72 18.16 4.04 0.42 0.201 2.26 D 0.029 0.48 0.75 18.24 3.32 0.42 0.225 2.27 E 0.027 0.53 1.6 18.75 3.42 0.39 0.191 2.56 F 0.029 0.5 0.73 18.34 3.4 0.43 0.215 2.57 G 0.027 0.53 1.62 18.67 3.42 0.39 0.171 2.68 H 0.023 0.54 0.61 16.99 4.38 0.44 0.176 2.73 1.92 I 0.027 0.52 0.68 17.98 3.6 0.31 0.23 2.96 J 0.026 0.55 1.54 18.19 3.27 0.48 0.168 2.97 K 0.022 0.57 1.31 18.58 3.28 0.48 0.178 3.11 L 0.022 0.46 0.69 18.14 4.38 0.44 0.185 3.33 M 0.031 0.58 1.54 18.19 3.78 0.42 0.174 3.72 N 0.024 0.57 1.52 18.29 3.81 0.42 0.193 3.72 O 0.028 0.53 0.71 16.98 3.45 0.43 0.208 3.76 P 0.027 0.47 0.76 17.31 3.44 0.43 0.187 3.77 Q 0.04 0.40 3.0 20.2 1.2 0.40 0.22 0.40 R 0.026 0.46 0.99 20.08 3.03 0.36 0.178 1.19

(8) The alloys A-P were manufactured in a vacuum induction furnace in 1 kg laboratory scale to small slabs that were forged and cold rolled down to 1.5 mm thickness.

(9) The referred alloys Q and R were produced in 100 ton production scale followed by hot rolling and cold rolling to coil form with varying final dimensions.

(10) When comparing the values in the Table 1 the contents of chromium, nickel, molybdenum and tungsten in the duplex stainless steels of the invention are significantly different from the reference stainless steels Q and R.

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

(12) 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)

(13) when annealed at the temperature of 1050° C.

(14) 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.

(15) The calculated M.sub.d30 temperatures (M.sub.d30 calc) in the table 2 were achieved in accordance with a mathematical constraint of optimization.

(16) The critical pitting temperature (CPT) is measured in a 1M 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.

(17) The pitting resistance equivalent (PRE) is calculated using the formula (2):
PRE=% Cr+3.3*(% Mo+0.5% W)+30*% N−% Mn  (2).

(18) The sums of the element contents for C+N, Cr+Si, Cu+Mo+0.5W 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+0.5W elements having resistance to martensite formation.

(19) TABLE-US-00002 TABLE 2 M.sub.d30 M.sub.d30 M.sub.d30 Cu + Mo + calc Nohara measured CPT Alloy C + N % Si + Cr % Mn + Ni % 0.5W % ° C. ° C. ° C. ° C. PRE A 0.202 18.15 4.87 3.54 22.8 41.9 — 39.2 32.7 B 0.212 18.33 4.96 2.83 33.7 30.8 — 38.2 30.8 C 0.225 18.67 4.76 2.68 30.7 16.6 18 36.6 30.9 D 0.254 18.72 4.07 2.69 40.5 22.8 54.9 — 31.7 E 0.218 19.28 5.02 2.95 1.0 17.7 2 35.5 31.3 F 0.244 18.84 4.13 3 28.4 17.9 32.7 — 32.5 G 0.198 19.2 5.04 3.07 1.6 25.7 — — 31.0 H 0.199 17.53 4.99 4.13 22.8 26.1 — 37.2 33.8 I 0.257 18.5 4.28 3.27 26.7 7.5 34 34.6 34.0 J 0.194 18.74 4.81 3.45 10.0 30.9 0 31.5 K 0.2 19.15 4.59 3.59 −1.6 21.6 — 39.8 32.9 L 0.207 18.6 5.07 3.77 −1.1 −4.4 — — 34.0 M 0.205 18.77 5.32 4.14 −21.0 −1.3 −29 — 34.1 N 0.217 18.86 5.33 4.14 −25.0 −8.9 — 45.1 34.8 O 0.236 17.51 4.16 4.19 35.4 16.6 41.6 — 34.9 P 0.214 17.78 4.2 4.2 28.8 22.5 34 34.8 34.6 Q 0.26 20.7 4.3 1.0 24.9 23 27 <10 25 R 0.204 20.54 4.02 1.55 29.6 5 19 30.0 28.4

(20) When comparing the values in the Table 2 the PRE value having the range of 30-36 is much higher than the PRE value in the referred duplex stainless steels Q and R which means that the corrosion resistance of the alloys A-P is higher. The critical pitting temperature CPT is in the range of 34-45° C., which is much higher than the CPT for the referred duplex stainless steels Q and R and further for instance for austenitic stainless steels, such as EN 1.4401 and similar grades.

(21) 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.

(22) The sums of the element contents for C+N, Si+Cr, Mn+Ni and Cu+Mo+0.5W 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+0.5W. In accordance with this mathematical constraint of optimization the sums of Cu+Mo+0.5W 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 (30<PRE<36) and for the minimum and maximum M.sub.d30 temperature (10<M.sub.d30<60) values are defined.

(23) In accordance with FIG. 1 a chemical composition window for Si+Cr and Cu+Mo+0.5W is established with the preferred ranges of 0.16-0.29 for C+N and 3.0-5.5 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 that the sum Si+Cr is limited to 16.5<Si+Cr<20.2 in accordance with the stainless steel of the invention.

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

(25) TABLE-US-00003 TABLE 3 Si + Cr % Cu + Mo + 0.5W % C + N % Mn + Ni % a′ 20.2 1.4 0.29 4.5 b′ 20.2 3.4 0.16 3.0 c′ 19.9 3.7 0.16 3.0 d′ 16.5 4.75 0.16 4.0 e′ 16.5 3.15 0.29 5.5 f′ 17.3 2.27 0.29 5.5

(26) FIG. 2 illustrates one chemical composition example window of FIG. 1 when constant values of 0.257 for C+N and 4.28 for Mn+Ni are used at all points instead of the ranges for C+N and Mn+Ni in FIG. 1. The same limitations are given to the sum of Si+Cr in FIG. 2 as in FIG. 1. The chemical composition window, which lies within the frame of the area a, b, c, d, e, f and g in FIG. 2, is defined with the following labelled positions of the coordination in the table 4.

(27) TABLE-US-00004 TABLE 4 Si + Cr % Cu + Mo + 0.5W % C + N % Mn + Ni % a 20.2 2.0 0.257 4.28 b 18.7 3.7 0.257 4.28 c 16.5 4.35 0.257 4.28 d 16.5 4.2 0.257 4.28 e 18.7 1.85 0.257 4.28 f 20.2 1.4 0.257 4.28

(28) FIG. 3 illustrates a chemical composition window for C+N and Mn+Ni with the preferred composition ranges 16.9-19.5 for Cr+Si and 2.0-4.0 for Cu+Mo+0.5W, 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 0.13<C+N<0.30 and the sum Mn+Ni is limited to 3.0<Mn+Ni<6.7. The chemical composition window, which lies within the frame of the area p′, q′ r′ and s′ in FIG. 3, is defined with the following labelled positions of the coordination in the table 5.

(29) TABLE-US-00005 TABLE 5 Si + Cr % Cu + Mo + 0.5W % C + N % Mn + Ni % p′ 17.2 2.5 0.3 6.7 q′ 16.9 4.0 0.13 6.7 r′ 18.71 4.0 0.13 3.0 s′ 19.5 2.0 0.3 3.0

(30) 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 solely by the limitations for the minimum and maximum sums of C+N and Mn+Ni.

(31) FIG. 4 illustrates one chemical composition example window of FIG. 3 with the constant values of 18.5 for Cr+Si and 3.27 for Cu+Mo+0.5W and further, with the limitations of 0.13<C+N<0.30 and 3.0<Mn+Ni. The chemical composition window, which lies within the frame of the area p, q, r, s, t, u and v in FIG. 4, is defined with the following labelled positions of the coordination in the table 6.

(32) TABLE-US-00006 TABLE 6 Si + Cr % Cu + Mo + 0.5W % C + N % Mn + Ni % p 18.5 3.27 0.30 4.4 q 18.5 3.27 0.30 4.9 r 18.5 3.27 0.14 5.6 s 18.5 3.27 0.13 5.2 t 18.5 3.27 0.13 3.3 u 18.5 3.27 0.19 3.0 v 18.5 3.27 0.26 3.0

(33) The alloys of the present invention A-P as well as the reference materials Q and R 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 in the longitudinal direction. The table 7 contains the results of the tests for the alloys A-P of the invention as well as the respective values for the reference duplex stainless steels Q and R.

(34) 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 454 534 755 43.0 46.0 33.4 B 439 518 743 42.5 45.0 32.8 C 491 577 760 43.3 40.7 32.8 D 430 498 862 39.3 41.8 34.7 E — — — — — — F 432 512 823 41   43.5 36.6 G 476 538 725 36.7 40.0 25.4 H 440 525 742 47.9 51.2 41.0 I 471 536 853 42.7 45.3 37.7 J — — — — — — K 471 557 721 30.7 32.9 19.8 L 427 535 743 45.1 48.1 38.6 M — — — — — — N 453 537 732 36.8 39.6 24.4 O 474 565 765 45.7 49.5 32.0 P 452 534 800 46.1 49.6 39.4 Q   498.0   544.0   787.0 45.2 49.0 40   R 562 626 801 40.4 44.3 35.5

(35) 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-P are lower than the respective values for the reference duplex stainless steels Q and R, and the tensile strength value R.sub.m is similar to the reference duplex stainless steels Q and R. The elongation values A.sub.50, A.sub.5 and A.sub.g of the alloys A-P are lower than the respective values for the reference stainless steels Q and R. Because the alloys A-P according to the invention are manufactured in the laboratory scale and the reference duplex stainless steels Q and R are produced in the production scale, the strength values of the table 7 are not directly comparable with each other.

(36) 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.