JP2010508439A - Duplex stainless steel and use of this steel - Google Patents

Abstract

  The present invention is, by weight, up to 0.03% C, less than 0.30% Si, 0-3.0% Mn, up to 0.030% P, up to 0.050%. S, 25-29% Cr, 5-9% Ni, 4.5-8% Mo, 0-3% W, 0-2% Cu, 0-3% Co, 0-2% Ti, 0-0.05% Al, 0-0.01% B, 0-0.01% Ca, 0.35-0.60% N, the balance Fe and the impurities normally present Further, the present invention relates to a duplex stainless steel in which the ferrite content is 30 to 70% by volume and each weight% of the Mo can be arbitrarily replaced with 2% by weight of W.

Description

  The present invention relates to a duplex stainless steel which is a steel having a ferrite-austenite matrix and having a particularly high corrosion resistance combined with good structural stability and hot workability. Ferrite content is 30-70% by volume, and such steels have a well-balanced composition, thus giving material corrosion properties and suitable for use in chloride-containing environments such as in the sea.

Over the last few years, the environment in which corrosion resistant metal materials have been used has become more intense, increasing the requirements for corrosion properties and their mechanical properties. Duplex steels were established as an alternative to other steel grades previously used, such as high alloy austenitic steels, nickel-based alloys or other high alloy steels, but were also part of their development. The established standard for corrosion resistance in chloride-containing environments is the so-called pitting resistance equivalent (abbreviated as PRE),
PRE =% Cr + 3.3% Mo + 16% N
Where the percentage of each element is weight percent.

  Higher values indicate better corrosion resistance, especially to prevent pitting. The essential alloying elements that affect this property are Cr, Mo, and N according to the previous equation. Examples of such steel grades are evident from European patent EP0220141 which is hereby incorporated by reference. This steel grade with the name SAF2507 (UNS S32750) was mainly doped with high contents of Cr, MO and N as alloying elements. It is therefore developed against this property with good resistance to corrosion, especially in chloride environments.

  Recently, the elements Cu and W have also been shown to be effective alloying elements for further optimization of the corrosion properties of steel in chloride environments. Element W has been used to date as a replacement for part of Mo, for example, commercially available alloys DP3W (UNS S39274) or Zero 100, which contain 2.0% and 0.7% W, respectively. . The latter contains even 0.7% Cu for the purpose of enhancing the corrosion resistance of the steel in an acidic environment.

The alloying of tungsten leads to further development of corrosion resistance standards, which translates from the PRE equation to the PREW equation, further clarifying the relationship of the effects of Mo and W on steel corrosion resistance:
PREW =% Cr + 3.3 (% Mo + 0.5% W) + 16% N
For example, as described in European Patent EP0554553. This publication relates to a duplex stainless steel alloy with generally improved corrosion properties.

  The steel grades have a PRE / PREW number greater than 40, regardless of the calculation method.

  SAF 2906 is also mentioned from alloys with good corrosion resistance in the chloride environment, the composition of which comes from European patent EP 0708845. This alloy is characterized by a high Cr and N content, for example compared to SAF 2507, and is particularly suitable for use in environments where intergranular corrosion and resistance to corrosion in ammonium carbamate are important. However, it has high corrosion resistance even in a chloride-containing environment.

  U.S. Pat. No. 4,498,091 describes a steel for use in a hydrochloric acid and sulfuric acid environment where intergranular corrosion occurs primarily. This is mainly intended as an alternative to the recently used austenitic steel.

  U.S. Pat. No. 6,048,413 describes a duplex stainless steel alloy as an alternative to austenitic stainless steel for use in a chloride-containing environment.

  EP 0683241 discloses a duplex stainless steel having a composition that produces improved properties with respect to both stress corrosion cracking and pitting corrosion in a chloride ion containing environment over most of the other known duplex stainless steels. Yes. However, this steel as well as the above-mentioned steels are very susceptible to precipitation of intermetallic compounds, particularly sigma phase, so that the material becomes hard and brittle. Therefore, it is very difficult to produce a material with good ductility using the duplex stainless steel of EP 0683241.

  The object of the present invention is a duplex stainless steel of the kind defined above and in particular in European Patent 0683241, which has improved properties, in particular ductility and toughness, as compared to known such steels. It is to provide a steel that maintains a level of corrosion resistance at least similar to that of a new steel. This steel naturally has good hot workability.

  According to the present invention, by weight%, C is a maximum of 0.03%, Si is less than 0.30%, Mn is 0 to 3.0%, P is a maximum of 0.030%, and S is a maximum of 0.050%. Cr: 25-29%, Ni: 5-9%, Mo: 4.5-8%, W: 0-3%, Cu: 0-2%, Co: 0-3%, Ti: 0-2 %, Al 0-0.05%, B 0-0.01%, Ca 0-0.01% and N 0.35-0.60%, the balance Fe and impurities that are normally present, This object is obtained by providing a duplex stainless steel alloy having a ferrite content of 30-70% by volume and in which each weight% of the Mo can be arbitrarily replaced by 2% by weight of W.

  It has been found that a duplex stainless steel having this composition has particularly improved ductility and toughness as compared to steel according to EP 0683241 and also has improved corrosion resistance. By reducing the Si content to less than 0.30% by weight, sigma phase precipitation is greatly reduced, which is the key to the improved ductility and toughness of the steel of the present invention. Therefore, when using a relatively high content of Mo, reducing the Si content is very effective in reducing the risk of precipitation of intermetallic compounds.

FIG. 1 shows the calculated phase content of a duplex stainless steel according to an embodiment of the invention as a function of temperature. FIG. 2 is a graph similar to FIG. 1 of a comparative steel according to EP 0683241. FIG. 3 is a photomicrograph of a continuously cooled sample of the steel of FIGS. 1 and 2 with three different cooling rates.

  According to one embodiment of the invention, the Si content is at most 0.25% by weight, which further increases the ductility and toughness of the material by making the steel more difficult to form a sigma phase. The same may be true if molybdenum is partially or wholly replaced with tungsten.

  According to another embodiment of the invention, the Si content is at most 0.23% by weight.

  According to another embodiment of the present invention, the Mo content is a wt%, the W content is b wt% and a + b / 2> 5.0. Such a high content of Mo and / or W produces excellent resistance to corrosion, in particular pitting and crevice corrosion resistance, but the intermetallic precipitation associated with such high contents of these elements. The increased fear is effectively counteracted by the combination and low content of Si. According to another embodiment of the invention, a> 5.0. When starting from the gap between the contents of Mo (4.5-8%) and W (0-3%), if the W content is at least 3%, the Mo content is, for example, 3%, It is pointed out that claim 1 is to be construed that each% of Mo can be replaced by 2% W or vice versa. According to a preferred embodiment, in order to keep the costs at a reasonable level, a + b / 2 <8, ie the total content of Mo and W does not exceed 8%. According to another preferred embodiment, b = 0 and the steel contains only Mo.

  According to still another embodiment of the present invention, the Co content is 0 to 0.010% by weight. Co is an expensive material, and it has been found that the effect of improving its structural stability and corrosion resistance is not an essential factor in the steel having the composition of the present invention.

  In another embodiment of the invention, the ferrite content is 40-60% by volume.

  According to another embodiment of the present invention, the average PRE value or PREW value of the two phases of steel is greater than 44, but PRE =% Cr + 3.3% Mo + 16% N and PREW =% Cr + 3.3 (% Mo + 0.5 % W) + 16% N, where% is% by weight. The PRE value or PREW value of both the ferrite phase and the austenite phase may be higher than 47, preferably higher than 48.5, and the average PRE value or PREW value may be higher than 48, preferably higher than 49. It has been found that the pitting and crevice corrosion resistance of the steel according to the invention is particularly improved by increasing the PRE value or PREW value of the phase having the lowest such value. It has been found that the steel of the present invention has good hot workability even with a PRE value or PREW value greater than 49.

  According to another embodiment of the invention, the ratio between the PRE (W) value of the austenite phase and the PRE (W) value of the ferrite phase is between 0.09 and 1.15, preferably 0.95. Between 1.05.

  The steel of the present invention is used in a chloride-containing environment in product forms such as rods, pipes such as welded pipes and seamless pipes, plate materials, steel bars, wire rods, welded wire rods, structural members such as pumps, valves, flanges and couplings. Suitable for use.

Good corrosion resistance and high ductility and toughness are obtained by the combination of elements in the duplex stainless steel of the present invention. This steel has good workability and can be extruded into, for example, a seamless tube. The steel of the present invention (in weight%)
C 0.03% at maximum
Si <0.30%
Mn 0-3.0%
P 0.030% at maximum
S up to 0.050%
Cr 25-29%
Ni 5-9%
Mo 4.5-8%
W 0-3%
Cu 0-2%
Co 0-3%
Ti 0-2%
Al 0-0.05%
B 0-0.01%
Ca 0-0.01%
N 0.35 to 0.60%
Including the remaining Fe and impurities that are normally present, the ferrite content is 30-70% by volume, and each weight% of the Mo can be optionally replaced with 2% by weight of W.

  Carbon (C) has limited solubility in both ferrite and austenite. Limited solubility indicates the risk of chromium carbide precipitation, so its content should be 0.03% by weight or less, preferably 0.02% by weight or less.

  Silicon (Si) is used as a deoxidizer in steel making and increases the fluidity during manufacturing and welding. However, if the Si content is too high, undesirable intermetallic phase precipitation occurs, so the content is less than 0.30% by weight, preferably up to 0.25% by weight, more preferably up to 0%. Limited to 23 wt%.

  Manganese (Mn) is added to increase the solubility of N in the material. However, it has been shown that Mn alone has a limited effect on N solubility in the type of steel in question. Instead, other elements have been found that have a large effect on solubility. Moreover, Mn combined with a high content of sulfur can cause the formation of manganese sulfide, which acts as a starting point for pitting corrosion. Therefore, the Mn content should be limited to 0-3.0 wt%, preferably 0.5-1.2 wt%.

  Phosphorus (P) is a normal impurity element. If present in an amount exceeding about 0.05%, it may adversely affect, for example, hot ductility, weldability and corrosion resistance. Therefore, the amount of P in the steel should not exceed 0.05%.

  Sulfur (S) adversely affects corrosion resistance due to the formation of soluble sulfides. Furthermore, since the hot workability is reduced, the sulfur content is limited to a maximum of 0.030% by weight, preferably less than 0.010% by weight.

  Chromium (Cr) is a much more active element because it increases resistance to the majority of corrosion types. Furthermore, a high content of chromium means obtaining very good N solubility in the material. Therefore, it is desirable to keep the Cr content as high as possible in order to increase the corrosion resistance. For a very good amount of corrosion resistance, the chromium content must be at least 25% by weight. However, a high content of Cr increases the risk of precipitation of intermetallic compounds, so the chromium content must be limited to a maximum of 29% by weight, preferably 25.5 to 28% by weight.

  Nickel (Ni) is used as an austenite stabilizing element and is added at a suitable content to obtain the desired content of ferrite. In order to obtain the desired relationship between the austenite and ferrite phases and 30-70% by volume of ferrite, 5-9% by weight of nickel is required, preferably 6-8% by weight.

  Molybdenum (Mo) is an active element that improves the corrosion resistance in chloride environments and preferably in reducing acids. Too high Mo content combined with high Cr content means increased risk of intermetallic compound precipitation. The Mo content of the present invention should be in the range of 4.5-8 wt%, preferably should exceed about 5.0 wt%, each wt% of Mo being arbitrarily 2 It can be replaced by W% by weight.

  Tungsten (W) enhances pitting corrosion resistance and crevice corrosion resistance. However, if the tungsten content is too high in addition to the high Cr content and the Mo content, it means that the risk of precipitation of intermetallic compounds increases. The W content in the present invention should be in the range of 0 to 3.0% by weight.

  Copper (Cu) may be added to enhance the general corrosion resistance in an acidic environment such as sulfuric acid. At the same time, Cu affects the structural stability. However, a high content of Cu means that the solid solubility will be exceeded. Therefore, the Cu content should be limited to a maximum of 2.0 wt%, preferably 0 to 1.5 wt%, more preferably 0.1 to 0.5 wt%.

  Cobalt (Co) has intermediate properties between iron and nickel. Therefore, even if these elements are replaced with a small amount of Co, or a Co-containing raw material (Ni scrap metal usually contains some Co, sometimes in excess of 10%), there is no significant change in properties. There will be no. Co can be used to replace part of Ni as an austenite stabilizing element. Since Co is a relatively expensive element, the addition of Co is limited to a range of 0 to 3% by weight.

  Titanium (Ti) has a high affinity with N. Thus, titanium can be used, for example, to increase the solubility of N in molten steel and avoid the formation of nitrogen bubbles during casting. However, excessive amounts of Ti in the material can cause precipitation of nitrides during casting and interfere with the casting process, and the formed nitrides can act as defects that reduce corrosion resistance, toughness and ductility. . Therefore, the addition of Ti is limited to 2% by weight.

  Aluminum (Al) and calcium (Ca) are used as deoxidizers in steelmaking. The Al content should be limited to a maximum of 0.05% by weight, preferably a maximum of 0.03%, in order to limit the formation of nitrides. Ca has an advantageous effect on hot workability. However, to avoid an undesirable amount of slag, the Ca content must be limited to a maximum of 0.010% by weight.

  Boron (B) may be added to increase the hot workability of the material. If the boron content is too high, weldability and corrosion resistance may be reduced. Therefore, the boron content must be limited to a maximum of 0.01% by weight.

  Nitrogen (N) is a very active element that increases corrosion resistance, structural stability, and material strength. Furthermore, a high N content enhances the recovery of austenite after welding and gives good properties in the weld joint. In order to obtain a positive effect of N, at least 0.35% by weight of N must be added. High N content increases the risk of chromium nitride precipitation, especially when the chromium content is simultaneously high. Furthermore, a high N content means an increased risk of porosity due to the extreme solubility of N in the molten steel. For these reasons, the N content must be limited to a maximum of 0.60% by weight, preferably less than 0.35 to 0.45% by weight of N is added.

  The ferrite content is important to obtain good mechanical and corrosion properties as well as good weldability. From the viewpoint of corrosion and weldability, a ferrite content of 30-70% is desirable in order to obtain good properties. Furthermore, a high content of ferrite means that the impact strength at low temperatures and the resistance to the risk of hydrogen embrittlement are reduced. Therefore, the ferrite content is 30 to 70% by volume, preferably 40 to 60% by volume.

  Two experimental steels were produced mainly to test the effect of different concentrations of Si. Table 1 below shows the contents of two types of steel, No. 1 and No. 2, where No. 1 is a duplex stainless steel according to an embodiment of the present invention and No. 2 steel is a steel according to EP 0683241.

In addition, the steel was modeled using thermocalc software using the database CCTSS (eg, a slightly modified version of the commercial database TCFE3 containing an improved model of dual phase steel composition). Figures 1 and 2 show the calculated phase content of No. 1 and No. 2 steels as a function of temperature, respectively. In these figures, 1: Ferrite content. In the steel of the present invention (FIG. 1), it can be seen that heat treatment in the region of 1100-1300 ° C. is necessary to obtain the desired ferrite content.
2: Austenite content. Heat treatment is performed so that only a ferrite phase and an austenite phase are obtained.
3: N content 4: Liquid metal 5: Sigma phase. Its formation can be avoided by rapid cooling.
6: Content of Cr ₂ N, causing embrittlement and deterioration of corrosion resistance.
7: Carbide content and must be kept low so as not to affect the weld. If the tendency of carbide precipitation is high, it may lead to a decrease in corrosion resistance in the vicinity of the weld. Therefore, the equilibrium amount of carbide must be kept low.
8: Intermetallic phase. The sum of this and the sigma phase should be kept as low as possible.

Table 2 above shows the total PRE for the two steels and the predicted PRE for each phase when quenched from 1100 ° C and the ratio between the austenite and ferrite PRE. Also shown is the predicted ferrite content after quenching from 1100 ° C. and the predicted solid solution temperature of the Cr ₂ N and sigma (σ) phases and the predicted presence of precipitates at 1100 ° C. Since the precipitation of Cr ₂ N is faster than the precipitation of the σ phase, two T _{max and} Cr ₂ N are shown, one of which is the case of slow cooling where an equilibrium amount of σ precipitates (“with σ” ), And the other is a case of rapid cooling in which σ does not precipitate (“no s”). It is clear that both steels meet the requirements for ferrite content, total PRE and PRE balance and minimum PRE for each phase as described in our WO03020994.

Sample production Steel was produced by melting, ingot casting and finally press forging. Table 3 shows the results of forging. Since forging was interrupted when serious surface defects began to form, the total reduction in cross-sectional area during the forging process can be used as an evaluation of the forgeability of the two types of steel.

  The forged bar was annealed at 1100 ° C. and water-cooled before further processing. The raw material used for the sample was cut into small pieces, annealed once again at 1100 ° C. for 1 hour, and then water-cooled. After this treatment, various samples were made by machining.

Test Impact Test An impact test was performed on a 10 × 10 mm Shanopy V-notch specimen (length 55 mm) in four different material states: as-annealed (ie 1100 ° C./quenching in water) and at lower temperatures for impact samples. Performed additional annealing. Table 4 shows the different material states as well as the impact toughness values obtained. Two samples were tested at each composition and annealing condition.

  No. 1 steel with high Mo content and low Si and Co content has good impact toughness if a sufficiently high annealing temperature is utilized. This table represents the weakness of No. 2 steel according to EP 0683241, ie shows that Si and high Mo contents above 0.5% give a potentially brittle material. Only by reducing the Si content (like the No. 1 steel of the present invention) the toughness is greatly improved.

Continuous Cooling Nine samples from each heat were annealed at 1100 ° C. and then reheated to 1050, 1100 and 1150 ° C. from three different temperatures, respectively from each heat. Samples were cooled from three different holding temperatures at three different constant cooling rates, 20, 60 and 140 ° C./min. This means that 9 different annealing cycles were utilized for each heat. Nitride was not found in any sample. Table 5 summarizes the observations made with an optical microscope. Use relative rank index for σ phase content of various samples,
0: No sigma phase is detected 1: One or two sigma phase particles are detected on average in the field at 500 times magnification 2: A small amount of sigma phase is detected at 500 times magnification (but per field of view) More than 2 particles)
3: A relatively large amount of σ, but less than 5% of the ferrite is transformed 4: More than 5% of the ferrite is transformed into σ 5: More than 25% of the ferrite is transformed into σ 6: Ferrite More than 50% of the

  It is shown that steel 1 is slightly less likely to precipitate σ than steel 2. It is pointed out that a “description” of 2, preferably 1, is necessary in order to allow the proper production of the material in question.

  FIG. 3 is a photomicrograph of a continuously cooled sample heated to 1100 ° C. The light color is austenite, brown is ferrite, and the blackish color is σ phase. It is shown that the formation of the σ phase (blackish) is significantly weaker in the No. 1 steel of the present invention than the No. 2 steel according to EP 0683241, which is clearly due to the lower Si content.

Mechanical properties Table 6 shows the tensile strength results. The No. 2 steel is clearly less ductile than the No. 1 steel of the present invention.

Corrosion Test Critical crevice corrosion temperature (CCT) with MTI-2 and critical pitting corrosion in “Green Death” solution (1% FeCl ₃ + 1% CuCl ₂ + 111% H ₂ SO ₄ + 1.2% HCl) Table 7 shows the temperature (CPT). There is almost no difference in crevice corrosion resistance between different steels. The assumption that the resistance to pitting and crevice corrosion in a dual phase steel is mainly determined by the PRE of the phase with the lowest PRE is consistent with the fact that the No. 1 steel has the highest CCT. Furthermore, the improved No. 1 steel behavior over No. 2 steel appears in the form of lower weight loss due to corrosion and higher maximum temperature.

Outline Steel (No. 2) corresponding to EP 0683241 is very likely to cause sigma phase precipitation, so that it is very difficult to produce a material having good ductility. This problem is solved by a reduced Si content and a good balance between the PRE values of the two phases. Furthermore, No. 2 steel has low forgeability. By reducing the Si content of the type of steel defined in EP 0683241, ie by using the composition of No. 1 steel, not only increases ductility and toughness, but also improves corrosion resistance, but in fact it is very unexpected. It was an effect that did not.

Claims (15)

Duplex stainless steel with a weight percentage of C up to 0.03%
Si <0.30%
Mn 0-3.0%
P 0.030% at maximum
S up to 0.050%
Cr 25-29%
Ni 5-9%
Mo 4.5-8%
W 0-3%
Cu 0-2%
Co 0-3%
Ti 0-2%
Al 0-0.05%
B 0-0.01%
Ca 0-0.01%
N 0.35 to 0.60%
Two-phase, characterized in that it contains the balance Fe and impurities that are normally present, the ferrite content is 30-70% by volume, and each wt% of the Mo can be optionally replaced with 2 wt% W Stainless steel.   Steel according to claim 1, characterized in that the Si content is at most 0.25% by weight.   The steel according to claim 1, characterized in that the Si content is at most 0.23% by weight.   Steel according to any of the preceding claims, characterized in that the Mo content is a wt%, the W content is b wt% and a + b / 2> 5.0.   The steel according to claim 4, wherein a> 5.0.   The steel according to claim 4, wherein a + b / 2 ≦ 8.   The steel according to any one of the preceding claims, wherein the Co content is 0 to 0.010 wt%.   The steel according to any one of the preceding claims, wherein the Cr content is 25.5 to 28 wt%.   Steel according to any of the preceding claims, characterized in that the Ni content is 6-8 wt%.   The steel according to claim 1, wherein the N content is 0.35 to 0.45 wt%.   The steel according to any one of the preceding claims, wherein the ferrite content is 40 to 60% by volume.   The average PRE value or PREW value of the two phases of steel exceeds 44, PRE =% Cr + 3.3% Mo + 16% N and PREW =% Cr + 3.3 (% Mo + 0.5% W) + 16% N, where% is Steel according to any of the preceding claims, characterized in that it is% by weight.   The PRE value or PREW value of both the ferrite phase and the austenite phase is higher than 47, preferably higher than 48.5, and the average PRE value or PREW value is higher than 48, preferably higher than 49, Item 13. The steel according to Item 12.   The ratio between the PRE (W) value of the austenite phase and the PRE (W) value of the ferrite phase is from 0.90 to 1.15, preferably from 0.95 to 1.05, Item 14. The steel according to Item 12 or 13.   Any of the preceding claims in a chloride-containing environment in the form of products such as rods, pipes such as welded pipes and seamless pipes, plates, strips, wires, welded wires, eg structural members such as pumps, valves, flanges and couplings Use of the steel described in Crab.

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