Method for producing high-strength duplex stainless steel

Abstract

The invention relates to a method for producing a high-strength ferritic austenitic duplex stainless steel with the TRIP (Transformation induced plasticity) effect with deformation. After the heat treatment on the temperature range of 950-1150 C. in order to have high tensile strength level of at least 1000 MPa with retained formability the ferritic austenitic duplex stainless steel is deformed with a reduction degree of at least 10%, preferably at least 20% so that with a reduction degree of 20% the elongation (A.sub.50) is at least 15%.

Claims

1. A method for producing a high-strength ferritic austenitic duplex stainless steel with the TRIP (Transformation induced plasticity) effect with deformation, comprising: heat treating the ferritic austenitic duplex stainless steel in a temperature range of 950-1150 C. in order to have high tensile strength level of at least 1000 MPa with retained formability; deforming the ferritic austenitic duplex stainless steel with a reduction degree of at least 20%; heating the ferritic austenitic duplex stainless steel from room temperature 25 C. to 250 C. so the yield strength of the stainless steel reaches a maximum increase by approximately 10% and the elongation remains above 15%; wherein the ratio R.sub.d50 %(TR %)/R.sub.d50 %(0%) is more than 1.2; wherein the deforming of the ferritic austenitic duplex stainless steel comprises temper rolling, and wherein R.sub.d50 %(TR %) is the fatigue limit of the ferritic austenitic duplex stainless steel after the temper rolling, and R.sub.d50 %(0%) is the fatigue limit of the ferritic austenitic duplex stainless steel before the temper rolling.

2. The method according to the claim 1, wherein at a reduction degree of 40%, a tensile strength level of at least 1300 MPa is achieved.

3. The method according to claim 1, wherein a mean volumetric wear rate for erosion resistance after deforming is below 6.0 mm.sup.3/kg.

4. The method according to claim 1, wherein the deforming of the ferritic austenitic duplex stainless steel comprises tension levelling.

5. The method according to claim 1, wherein the deforming of the ferritic austenitic duplex stainless steel comprises roller levelling.

6. The method according to claim 1, wherein the deforming of the ferritic austenitic duplex stainless steel comprises drawing.

7. The method according to claim 1, wherein the ferritic austenitic duplex stainless steel contains in weight % greater than 0% and less than 0.05% carbon (C), 0.2-0.7% silicon (Si), 2-5% manganese (Mn), 19-20.5% chromium (Cr), 0.8-1.5% nickel (Ni), greater than 0% and less than 0.6% molybdenum (Mo), greater than 0% and less than 1% copper (Cu), 0.16-0.26% nitrogen (N), the sum C+N being 0.2-0.29%, greater than O weight % and less than 0.010 weight % S, greater than 0 weight % and less than 0.040 weight % P so that the sum (S+P) is less than 0.04 weight %, and the total oxygen (O) above O ppm and below 100 ppm, optionally contains one or more added elements: 0-0.5% tungsten (W), 0-0.2% niobium (Nb), 0-0.1% titanium (Ti), 0-0.2% vanadium (V), 0-0.5% cobalt (Co), 0-50 ppm boron (B), and 0-0.04% aluminium (Al); the balance being iron (Fe) and inevitable impurities.

8. The method according to claim 1, wherein the ferritic austenitic duplex stainless steel contains in weight % greater than 0% and less than 0.05% carbon (C), 0.2-0.7% silicon (Si), 2-5% manganese (Mn), 19-20.5% chromium (Cr), 0.8-1.5% nickel (Ni), greater than 0% and less than 0.6% molybdenum (Mo), greater than 0% and less than 1% copper (Cu), 0.16-0.26% nitrogen (N), optionally contains one or more added elements: 0-0.5% tungsten (W), 0-0.2% niobium (Nb), 0-0.1% titanium (Ti), 0-0.2% vanadium (V), 0-0.5% cobalt (Co), 0-50 ppm boron (B), and 0-0.04% aluminium (Al); the balance being iron (Fe) and inevitable impurities.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention is described in more details referring to the following drawings wherein

(2) FIG. 1 illustrates the tensile strength (R.sub.m) of the steels versus elongation (A.sub.50) of the steels,

(3) FIG. 2 illustrates the tensile strength (R.sub.m) and the elongation (A.sub.50) of the steels versus the cold rolling reduction by temper rolling of the steels,

(4) FIG. 3 illustrates the erosion resistance of the steels, and

(5) FIG. 4 illustrates the influence of a 10 minute heat treatment at different temperatures on the yield strength (R.sub.p0,2) and elongation (A.sub.50).

DETAILED DESCRIPTION OF THE INVENTION

(6) The duplex stainless steels according to the embodiments (A) and (B) of the invention after a heat treatment, solution annealing on the temperature range of 950-1150 C. were temper rolled in accordance with the invention with the reduction degree of at least 10%, preferably at least 20%. The yield strength R.sub.p0,2 and the tensile strength R.sub.m values were determined for both duplex stainless steels (A) and (B) and the results are in the table 1. As the reference alloys the table 1 also contains the respective values for the ferritic austenitic duplex stainless steels LDX 2101, 2205 and 2507 as well as for the standard austenitic stainless steels 1.4307 (304L) and 1.4404 (316L).

(7) TABLE-US-00001 TABLE 1 Thickness Reduction R.sub.p0.2 R.sub.m A.sub.50 Alloy mm % MPa MPa % A 3.36 0 599 788 46 1.45 0 611 845 42.4 0.4 0 521 774 43 0.69 20 894 1068 18.3 2.72 20 973 1107 15.2 0.59 30 999 1278 8.3 0.25 40 1096 1400 7.2 0.51 40 1113 1426 6.3 1.1 40 1165 1418 4.5 1.72 50 1271 1544 2.6 0.41 50 1284 1642 3.5 1.45 60 1439 1697 1.7 0.16 60 1305 1750 3 B 0.46 0 519 808 42.1 2.06 0 580 797 40.5 0.8 0 611 836 38.6 1.65 10 918 1057 22.6 0.88 10 826 937 26.5 1.32 10 883 1035 23.4 1.65 20 936 1082 19.2 0.68 30 998 1171 10.6 0.59 40 1056 1346 8 1.2 40 1162 1403 7.2 1 50 1298 1551 3.7 0.47 50 1251 1560 2.9 0.8 60 1468 1687 1.6 LDX 2101 1 0 592 803 28 0.8 20 976 1184 5 0.6 40 1100 1400 3 0.4 60 1216 1559 3 2205 0.7 0 698 894 22 0.56 20 1080 1232 5 0.42 40 1235 1400 3 0.28 60 1331 1612 2 0.203 71 1367 1692 2 2507 1 0 834 920 26 0.8 20 1099 1273 6 0.6 40 1362 1623 3 0.4 60 1423 1736 2 0.2 80 1548 1894 2 304L 0 270 600 55 14 648 800 30 17 719 839 24 17 710 837 27 22 780 925 17 23 779 911 16 23 775 899 20 23 780 900 22 24 788 912 18 29 838 979 14 31 863 1005 10 35 910 1063 9 36 908 1057 12 37 1050 1100 9 48 1059 1208 8 48 1150 1200 7 50 1040 1211 7 58 1250 1300 5 72 1350 1400 3 316L 0 260 580 55 29 820 925 14 45 1000 1100 6 60 1050 1200 4 73 1150 1300 3 80 1250 1400 2

(8) The results of the table 1 for the tensile strength R.sub.m versus the retained ductility (elongation A.sub.50) are illustrated in FIG. 1 for the ferritic austenitic duplex stainless steels A and B of the invention and as the reference materials for the standard ferritic austenitic duplex steel (LDX 2101 and 2507) as well as for the standard austenitic stainless steel (304L).

(9) The dashed line in FIG. 1 shows the trend for both standard duplex stainless steel and austenitic stainless steel grades, whereas the solid line is for the alloys A and B.

(10) The results in FIG. 1 show that for a given tensile strength R.sub.m the retained ductility is substantially greater for the alloys A and B than for the standard duplex stainless steel and standard austenitic stainless steel grade 304L. Alternatively, for a given elongation A.sub.50 the alloys A and B have up to 150 MPa greater tensile strength R.sub.m than the tensile strength R.sub.m for the standard duplex stainless steel and austenitic stainless steel grade 304L.

(11) FIG. 2 shows clearly the difference in retained ductility (elongation A.sub.50) with respect to the cold rolling reduction when comparing the alloys A and B with the standard duplex stainless steel and austenitic stainless steel grade 304L. For instance, for a 20% cold rolling reduction of the standard duplex stainless steels only 5% of elongation A.sub.50 is remaining, whereas the alloys A and B have 15-20% of elongation A.sub.50 still remaining with the similar tensile strength R.sub.m. Furthermore, the alloys A and B require a smaller cold rolling reduction degree than the standard austenitic stainless steel 304L to achieve the same target tensile strength R.sub.m. Consequently, the retained ductility (elongation A.sub.50) is greater in the alloys A and B than in the standard austenitic stainless steel 304L at the same tensile strength R.sub.m.

(12) The results in FIG. 2 also show that for instance in order to achieve a tensile strength R.sub.m of 1100-1200 MPa it is required a 20% temper rolling reduction degree for the standard duplex stainless steels and for the alloys A and B whereas a 50% temper rolling reduction degree is required for the austenitic stainless steel 304L in order to achieve the same tensile strength R.sub.m of 1100-1200 MPa. At the same time the alloys A and B have a greater retained ductility (A.sub.50 15-20%) compared to the standard duplex stainless steels (A.sub.50 about 5%) and standard austenitic grade 304L (A.sub.50 7-8%).

(13) For many applications where duplex stainless steels are used, the fatigue strength is important. Table 2 demonstrates the fatigue limit R.sub.d50 % of the steels before (R.sub.d50 %(0%)) and after temper rolling (R.sub.d50 %(TR %)) as well as the ratio R.sub.d50 %(TR %)/R.sub.d50 %(0%), i.e. the ratio of the fatigue limit between the temper rolled and the non-temper rolled material. The fatigue limit R.sub.d50 % describes 50% probability of failure after 2 million cycles, determined at stress maximum and R=0.1, where R is the ratio between maximum and minimum stress in the fatigue cycle.

(14) TABLE-US-00002 TABLE 2 Reduction R.sub.p0.2 R.sub.m R.sub.d(50%) R.sub.d50%(TR %)/ Alloy % MPa MPa MPa R.sub.d50%(0%) A 0 594 799 596 A 30 1032 1235 719 1.21 B 0 580 797 594 B 10 918 1057 748 1.26

(15) Table 2 demonstrates the fatigue limit itself and the value for the ratio R.sub.d50 %(TR %)/R.sub.d50 %(0%), the ratio being more than 1.2 for the temper rolled alloys A and B. The temper rolling according to the invention thus also improves the fatigue limit more than 20% for the alloys A and B.

(16) Table 3 shows results for the erosion resistance of a range of stainless grades where for the mean volumetric wear rate was tested with the standardized test configuration GOST 23.208-79.

(17) TABLE-US-00003 TABLE 3 Alloy Mean volumetric wear rate mm3/kg 316L 10.3 304L 10.5 2507 9.3 2205 10.3 LDX 2101 9.8 Alloy B 6.9 Alloy A 7.1 Alloy A(TR) 5.7

(18) The results for the mean volumetric wear rate in Table 3 and in FIG. 3 demonstrate the high erosion resistance for the alloys A and B when comparing with the reference alloys of the austenitic stainless steel grades 316L and 304L as well as the duplex stainless steels 2507, 2205 and LDX 2101. The temper rolling according to the invention further improves the erosion resistance, as shown for the alloy A(TR), the alloy A after temper rolling in accordance with the invention. The mean volumetric wear rate after temper rolling is below 6.0 mm.sup.3/kg.

(19) The table 4 shows the favorable effect of the heat treatment to the yield strength (R.sub.p0,2) and the elongation (A.sub.50). The heat treatment is carried out after cold deformation.

(20) TABLE-US-00004 TABLE 4 Heat temperature ( C.) R.sub.p0.2 (MPa) R.sub.m (MPa) A.sub.50 (%) 25 883 1035 23.4 100 897 1026 23.2 150 906 1022 23.6 200 947 1032 21.7 250 961 1059 21.2 275 955 1062 21.0 300 950 1076 20.4 360 949 1075 18.2 420 951 1067 18.0

(21) The material tested in table 4 is the alloy B with a 10% rolling reduction from the table 1 and with the heat treatment period of 10 minutes. The original material corresponds to the room temperature (25 C.) sample in the table 4. The results in the table 4 and in FIG. 4 demonstrate that heating for 10 minutes gives an increase in the strength. In particular, the yield strength (R.sub.p0,2) is improved reaching a maximum increase by approximately 10% at the temperature 250 C. The elongation (A.sub.50) is fairly stable up until the temperature 250 C. at 20%. Above this temperature 250 C. the elongation decreases but still remains above 15%. Therefore, short heat treatments within the temperature range 175 C. to 420 C. are shown to improve the yield strength (R.sub.p0,2) and whilst maintaining good ductility.

(22) The duplex stainless steels temper rolled in accordance with the invention can be used for replacing the temper rolled standard austenitic stainless steels 1.4307 (304L) and 1.4404 (316L) in applications where a need for better general corrosion resistance, erosion and fatigue problems exist as well as in applications where these austenitic stainless steels are not able to reach a desired strength/ductility ratio. Possible applications of use can be for instance machinery components, building elements, conveyor belts, electronic components, energy absorption components, equipment casings and housings, flexible lines (carcass and armouring wire), furniture, lightweight car and truck components, safety midsole, structural train components, tool parts and wear parts.