New duplex stainless steels

Abstract

The present invention relates to a duplex stainless steel comprising the following elements in weight percent: C max 0.030; Si max 0.30; Mn 0.20-2.50; P max 0.030; S max 0.030; Cr 28.5-30.5; Ni 6.0-8.0; Mo 0.70-3.00; W 2.00-4.40; Cu≦0.50; N 0.30-0.55; with the remainder being Fe and unavoidable impurities; the ferrite content is 40-60 volume percent; the duplex stainless steel meets the following requirements: a. [Cr]+4.0*[Mo]+2.0*[W]<42.5, where the values ​​of [Cr], [Mo], and [W] are in weight percent; and b. the coverage of quenched nitrides in the ferrite grain boundaries is less than 10% when in the solution treated state. The present invention also relates to an object made of the duplex stainless steel of the present invention having an Rp0.2 greater than 650 MPa according to ISO 6892-1, 2019.

Description

[Technical field]

[0001] The present invention relates to a duplex stainless steel having high Cr, W and N contents. [Background technology]

[0002] Today, well pressures are increasing and there are plans to develop wells at pressures of 20 kpsi. Offshore, long liners, also called umbilicals, usually connect the surface and subsea equipment. These umbilicals contain tubing for hydraulic and chemical injection into the sea as well as electrical and signal lines for carrying fiber optic signals and power. Umbilical tubing consists of long tubes that can be welded together.

[0003] Super duplex stainless steels, i.e., austenitic-ferritic iron-chromium-nickel alloys with molybdenum additions, with a PRE greater than 40, are used in conventional subsea wells because they provide sufficient pitting resistance and mechanical strength. However, the pressure increases significantly in deeper wells, which means that umbilicals need to withstand higher internal pressures. This means that the wall thickness of the umbilical tubes needs to be increased to withstand the high pressures. Therefore, if the weight of these tubes increases and the wall thickness is increased to meet the high pressure requirements, the possibility of failure will increase because today's duplex stainless steels do not have sufficient strength to support their own weight. Furthermore, in certain future applications, today's duplex stainless steels may not have sufficient corrosion resistance and problems with sigma phase formation during welding may also occur, especially in the heat affected zone. This will be very problematic in the manufacture of umbilicals, as these tubes may contain a large number of weld joints where cracks may form.

[0004] Therefore, there is a need for new duplex stainless steels that provide high strength and high corrosion resistance, both as stainless steels themselves and for welding applications. Summary of the Invention

[0005] The present disclosure is further illustrated by the following non-limiting figures: [Brief description of the drawings]

[0006] [Figure 1A] LOM image of Heat 7 in a representative area of ​​0.05mm2 [Figure 1B] The same LOM image as in Figure 1A, but also showing 10 randomly oriented test lines in a representative area of ​​0.05 mm2. [Figure 2A] LOM image of Heat 8 in a representative area of ​​0.05mm2 [Figure 2B] The same LOM image as in Figure 2A, but also showing 10 randomly oriented test lines in a representative area of ​​0.05 mm2. [Figure 3A] LOM image of Heat 13 in a representative area of ​​0.05mm2 [Figure 3B] The same LOM image as in Figure 3A, but also showing 10 randomly oriented test lines in a representative area of ​​0.05 mm2. [Figure 4A] LOM image of Heat 14 in a representative area of ​​0.05mm2 [Figure 4B] The same LOM image as in Figure 4A, but also showing 10 randomly oriented test lines in a representative area of ​​0.05 mm2. [Figure 5A] LOM image of Heat 9 in a representative area of ​​0.05mm2 [Figure 5B] The same LOM image as in Figure 5A, but also showing 10 randomly oriented test lines in a representative area of ​​0.05 mm2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] The present disclosure provides a duplex stainless steel that combines high strength with excellent corrosion resistance due to the low coverage of quenched nitrides in the ferrite grain boundaries in the final form and essentially no sigma phase in the steel itself and in the heat affected zone when subjected to welding. The low coverage of quenched nitrides in the ferrite grain boundaries improves corrosion resistance and impact resistance. In addition, the low content of sigma phase improves corrosion resistance. Both of these properties are important for the durability of the product during operation, such as when used in umbilical tubes. The term "final form" refers to the exposure to the metallurgical processes required to use the duplex stainless steel of the present invention in various applications. Thus, the duplex stainless steel of the present invention, as defined below, has a low or essentially no sigma phase content both in the duplex stainless steel itself and in the heat affected zone (HAZ) after welding, and further has a low coverage of quenched nitrides in the ferrite grain boundaries after being subjected to solution treatment and quenching.

[0008] Thus, the present disclosure relates to duplex stainless steels having the element ranges disclosed below. For the duplex stainless steels of the present invention, the inventors have carefully tailored both the elements used and the ranges of these elements to obtain duplex stainless steels having the above or below described properties.

[0009] The duplex stainless steel of the present invention therefore contains the following elements: C max 0.030; Si max 0.30; Mn 0.20-2.50; P max 0.030; S max 0.030; Cr 28.5-30.5; Ni 6.0-8.0; Mo 0.70~3.00; W 2.00-4.40; Cu max 0.50; N 0.30-0.55; In weight percent, The remainder is Fe and unavoidable impurities; The ferrite content is 40-60% by volume; Duplex stainless steels meet the following requirements: a) [Cr] + 4.0 * [Mo] + 2.0 * [W] < 42.5 (wherein the values ​​of [Cr], [Mo], and [W] are in weight percent); and b) When in the solution-annealed condition, the coverage of quenched nitrides at the ferritic grain boundaries is less than 10%. Meet the following.

[0010] Thus, in other words, the duplex stainless steel of the present invention, when quenched after solution treatment, will have low coverage of quenched nitrides at the ferrite grain boundaries. The coverage of quenched nitrides at the ferrite grain boundaries is measured as described in the method of Example 4. Furthermore, the duplex stainless steel of the present invention, due to its composition, will be resistant to the formation of sigma phase in the heat affected zone (HAZ) and therefore will be essentially free of sigma phase.

[0011] In the present disclosure, the term "solution-treated condition" is intended to mean the condition obtained after the duplex stainless steel of the present invention has been subjected to a heat treatment for the purpose of solution treatment, followed by cooling from the solution treatment temperature to a temperature between about 900 and about 1000°C, and then quenching. Moreover, in the present disclosure, the term "solution-treated" is intended to include both the heat treatment and the cooling from the solution treatment temperature to a temperature between about 900 and about 1000°C.

[0012] In this disclosure, the term "quenched-in nitrides" refers to nitrides that form at the ferrite grain boundaries when the steel is cooled rapidly because the elements do not have time to migrate between the phases and therefore precipitate at the ferrite grain boundaries. The coverage of the quenched nitrides at the ferrite grain boundaries is measured according to the method described in the Examples.

[0013] Therefore, the inventors have found that, since sigma phase has a detrimental effect on strength, it is important to balance the elements that influence the formation of sigma phase during welding in order to obtain the properties of the duplex stainless steel of the present invention. Therefore, the inventors have found that the following conditions are met: [Cr]+4.0*[Mo]+2.0*[W]<42.5 (Formula 1) (where the values ​​must be expressed as weight percent) It has been found that when the above conditions are met, the sigma phase content of the duplex stainless steel of the present invention will be less than 0.2% by volume when cooled from the solution treatment temperature to room temperature at a cooling rate of 40° C. / min, as shown in Table 2 of the Examples. This means that the sigma phase content in the HAZ after welding will be low or essentially zero. This low sigma phase content therefore means that the duplex stainless steel will have high strength and impact toughness when used as a base metal (e.g., in the form of a tube or pipe) or as a welding material. Furthermore, the impact toughness and strength are increased because the brittleness is reduced due to the low sigma phase content in the heat affected zone.

[0014] According to the present disclosure, Equation 1 may also satisfy 40<[Cr]+4.0*[Mo]+2.0*[W]<42.

[0015] The ferrite content of the duplex stainless steel of the present invention is between 40-60% by volume, with the remainder essentially composed of austenite. If the ferrite content is too low, the mechanical strength will be too low, and if the ferrite content is too high, the corrosion properties will be reduced. For example, if the ferrite content is too high, the resistance to hydrogen induced cracking will be reduced. According to an embodiment, the ferrite content is at least 44% by volume, with the remainder essentially austenite. Phase balance is important in duplex stainless steels because it helps to obtain optimal mechanical properties and corrosion resistance. In addition, the ferrite content of the duplex stainless steel of the present invention is adapted so that the phase balance in the heat affected zone is still within an acceptable range. This is important because the ferrite content in the heat affected zone is often increased and will have a negative effect on the mechanical properties and corrosion resistance.

[0016] During solution treatment and subsequent quenching, the quench-in nitrides formed at the ferrite grain boundaries will adversely affect both the corrosion resistance and the mechanical properties, as previously described. It is therefore important to avoid the formation of these quench-in nitrides. The inventors have found that the duplex stainless steels of the present invention, having the specific element ranges defined above or below, have low coverage of quenched nitrides at the ferrite grain boundaries (less than 10%) in the solution treatment state. Thus, according to the present disclosure, low coverage of quenched nitrides at the ferrite grain boundaries means that the percentage of the ferrite grain boundaries covered by quench-in nitrides is less than 10%. For typical grain sizes expected in the finished duplex stainless steels of the present invention, a coverage of 10% of the ferrite grain boundaries by quench-in nitrides corresponds to a very low percentage of quench-in nitrides in the entire material. Thus, a coverage of quench-in nitrides is considered low when less than 10% of the ferrite grain boundaries are covered by quench-in nitrides.

[0017] The terms "weight %" and "wt%" are used interchangeably herein.

[0018] The terms "HAZ" or "heat affected zone" are used interchangeably and are the non-melted area of ​​metal that has undergone a change in material properties or microstructure as a result of exposure to heat generated by welding. Thus, the HAZ is the area between the weld and the unaffected base metal. In the duplex stainless steel of the present invention, the formation of sigma phase in the HAZ is significantly reduced by combining selected elemental ranges and certain elemental balancing conditions.

[0019] The advantages of the duplex stainless steel of the present invention and the selection of the ranges of elements of the duplex stainless steel that lead to the unexpected advantages of the duplex stainless steel of the present invention are described in more detail below. It should be noted that the inventors have tailored the composition with respect to both the selected elements and these ranges to obtain a duplex stainless steel with the desired properties discussed herein. However, the present disclosure is not limited to the exemplary embodiments discussed, and may vary within the scope of the appended claims. The upper and lower limits of the individual elements of the composition are freely combinable within the broadest ranges set in the claims, unless expressly disclosed otherwise. In addition, where ranges are disclosed in the present disclosure, such ranges include each end of the range, unless expressly disclosed otherwise. Similarly, where an open range is disclosed, the open range also includes a single end of the open range, unless expressly disclosed otherwise.

[0020] Chromium (Cr) Chromium is an essential element and this range has been selected to obtain good strength and corrosion resistance of the duplex stainless steel of the present invention. To obtain good strength properties and corrosion resistance, the chromium content must be at least 28.5 wt.%.

[0021] However, although chromium is a beneficial element, its content should not exceed 30.5 wt.%, since a high Cr content increases the risk of sigma phase formation both during cooling in the manufacturing process and in the HAZ, in addition, the formation of quench-in nitrides during solution treatment will be increased.

[0022] Furthermore, it has been found that in order to avoid the formation of sigma phase and obtain maximum strength, it is important to balance the respective chromium, molybdenum, and tungsten contents, more specifically, the chromium, molybdenum, and tungsten contents are balanced such that the condition of Equation 1 discussed above is satisfied.

[0023] Thus, the chromium content of the duplex stainless steel of the present invention is therefore 28.5-30.5 wt.%. According to the present disclosure, to further ensure obtaining the above properties, the chromium content may be 29.0-30.5 wt.%, for example 29.5-30.5 wt.%. According to the present disclosure, the chromium content may be from 29.1, 29.2, 29.3, 29.4 or 29.5 to 29.6, 29.7, 29.8, 29.9, 30.0, 30.1, 30.2, 30.3, 30.4 or 30.5 wt.%.

[0024] Nickel (Ni) Ni is used as an austenite stabilizing element. To obtain a ferrite content between 40-60% by volume, the nickel content must be at least 6.0% by weight. In addition, too high a Ni content will increase the austenite content, leading to a decrease in strength. However, since nickel is an expensive element, it is desirable to limit its content. According to the present disclosure, the maximum nickel content is 8.0% by weight.

[0025] According to the present disclosure, the Ni content may be less than 7.9, 7.8, 7.7, or 7.6 wt%. According to the present disclosure, the minimum amount of Ni may be 6.0 wt%. According to the present disclosure, the range may be between 6.0 and 7.6 wt%.

[0026] Molybdenum (Mo) Mo is an active element that improves strength and corrosion resistance in chloride environments and reducing acids.

[0027] However, the combination of an excessive Mo content and a high Cr content will increase the risk of sigma phase formation. Molybdenum can sometimes be replaced by tungsten. It is generally accepted in the field that the replacement of molybdenum by tungsten can be carried out in a ratio such as 1 wt% Mo replaced by 2 wt% W. However, if the amount of Mo contained in the duplex stainless steel of the present invention is too low, excessive quench-in nitrides will be formed at the ferrite grain boundaries during the solution treatment and subsequent quenching.

[0028] Therefore, it is important to optimize the chromium, molybdenum, and tungsten contents in duplex stainless steels to avoid the formation of sigma phase and obtain maximum strength. Therefore, the Mo content should be in the range of at least 0.70 wt% and not more than 3.00 wt%. According to the present disclosure, the Mo content can be at least 0.70-2.00 wt%, such as at least 0.70-1.70 wt%, such as at least 0.90-1.70 wt%.

[0029] Tungsten(W) W will improve corrosion resistance in chloride environments and resistance to pitting and crevice corrosion. However, too high W content in combination with high Cr content increases the risk of sigma phase precipitation. Therefore, it is very important to balance the total amount of chromium, molybdenum, and tungsten to avoid the formation of sigma phase. More specifically, the contents of chromium, molybdenum, and tungsten are balanced so that the condition of formula 1 discussed above is satisfied.

[0030] Therefore, the W content is 2.00 to 4.40% by weight. According to the present disclosure, the W content can be 2.50 to 4.40% by weight, such as 3.00 to 4.00% by weight.

[0031] Nitrogen (N) Nitrogen is a highly reactive element that enhances the corrosion resistance and strength of duplex stainless steels. To obtain a good effect, at least 0.30 wt% of N must be added.

[0032] However, since the added N needs to be dissolved in the duplex stainless steel to provide increased strength and corrosion resistance, the range of N needs to be carefully selected (too high a N content increases the risk of chromium nitride precipitation, especially if the chromium content is also high). The N content should therefore be limited to a maximum of 0.55 wt.%. The nitrogen content is therefore 0.30-0.55 wt.%.

[0033] According to the present disclosure, the N content may be 0.35-0.55 wt %; for example, 0.38-0.55 wt %, such as 0.38-0.50 wt %.

[0034] Manganese (Mn) Mn is added to increase the solubility of N in steel. However, Mn may also form manganese sulfides that act as the starting point of pitting corrosion. Therefore, the Mn content is 0.20 wt% or more and 2.50 wt% or less. According to the present disclosure, the Mn content may be 0.30-2.00 wt%.

[0035] Silicon (Si) Si is frequently used as a deoxidizer in steel production. However, it is known that high silicon content stabilizes the sigma phase. Therefore, the silicon content should be limited to a maximum of 0.30 wt%. According to the present disclosure, the silicon content can be 0.10-0.30 wt%.

[0036] Carbon (C) Carbon is a very difficult element to completely avoid in duplex stainless steels. Trying to keep the carbon content very low would unduly increase production costs. Therefore, at least for cost reasons, carbon may be present in an amount of at least 0.005% by weight. C strengthens stainless steels, but also promotes the formation of chromium carbides, which are detrimental to corrosion. Carbon has limited solubility in both ferrite and austenite. Therefore, the carbon content must be limited to a maximum of 0.030% by weight, e.g., a maximum of 0.025% by weight.

[0037] Copper (Cu) Cu can be added to improve resistance to certain corrosive environments, such as acidic environments, and also reduces susceptibility to stress corrosion cracking. Additionally, Cu increases strength and reduces the formation of sigma phase.

[0038] However, Cu will have a negative effect on nitrogen solubility, since less nitrogen will dissolve in the stainless steel and the formation of chromium nitrides will increase. Therefore, if copper is added, it is very important to carefully adjust its content. Therefore, the content of Cu is limited to a maximum of 0.50 wt%, for example less than 0.50 wt%. According to one embodiment, Cu is added and the content of Cu is between 0.15 and ≦0.50 wt%, according to an embodiment, the Cu content can be 0.25 to 0.45 wt%.

[0039] Sulfur (S) Sulfur is an impurity element usually found in duplex stainless steels. The sulfur content should not exceed 0.030 wt. % since above this range it would affect the hot workability.

[0040] Rin(P) Phosphorus is also an impurity element usually included in duplex stainless steels. The phosphorus content should not exceed 0.030 wt. % since above this range it will affect the hot workability.

[0041] The balance of the duplex stainless steel of the present invention is iron (Fe) and unavoidable impurities. Unavoidable impurities are elements that are not intentionally added and may be present in the steel of the present invention due to scrap and / or manufacturing processes used to provide the duplex stainless steel. Examples of such elements include, but are not limited to, Co, V, Ti, Nb, Pb, Sn, and Ce. The total content of these elements is less than 1.0 wt.%.

[0042] Furthermore, the duplex stainless steel of the present invention may optionally include one or more elements that may be added to improve the manufacturing process. Examples of such process-improving elements include, but are not limited to, aluminum (Al), magnesium (Mg), calcium (Ca), and boron (B). The total content of these improving elements is less than 0.50 wt%, for example less than 0.30 wt%, provided that the content of Al does not exceed 0.050 wt% and the content of B does not exceed 0.050 wt%.

[0043] In addition, the duplex stainless steels of the present invention can comprise or consist of the elements described herein within any of the ranges of specific elements described herein and meeting the requirements described herein.

[0044] According to an embodiment, the present disclosure relates to an object comprising or consisting of a duplex stainless steel as defined above or below. According to an embodiment, the object is made of a duplex stainless steel as defined above or below. According to an embodiment, the object is in a solution-treated state.

[0045] Accordingly, the present disclosure provides an article comprising a duplex stainless steel, the article comprising the following elements: C max 0.030; Si max 0.30; Mn 0.20-2.50; P max 0.030; S max 0.030; Cr 28.5-30.5; Ni 6.0-8.0; Mo 0.70~3.00; W 2.00-4.40; Cu max 0.50; N 0.30-0.55; In weight percent, The remainder is Fe and unavoidable impurities; The ferrite content is 40-60% by volume; Duplex stainless steels meet the following requirements: a) [Cr] + 4.0 * [Mo] + 2.0 * [W] < 42.5 (wherein the values ​​of [Cr], [Mo], and [W] are in weight percent); and b) When in the solution-annealed condition, the coverage of quenched nitrides at the ferritic grain boundaries is less than 10%. The present invention relates to an object comprising a duplex stainless steel, the object satisfying the above.

[0046] The objects of the invention may have an Rp0.2 greater than 650 MPa per ISO 6892-1, 2019, measured at room temperature.

[0047] Thus, the present disclosure also provides the following elements: TIFF2025513023000001.tif78170 by weight percent and the following conditions: a) [Cr] + 4.0 * [Mo] + 2.0 * [W] < 42 (wherein the values ​​of [Cr], [Mo], and [W] are in weight percent; the ferrite content is 40-60 volume percent); b) the ferrite grain boundaries in the solution-annealed condition have less than 10% coverage of quench-in nitrides; and c) Rp0.2 is greater than 650MPa at room temperature according to ISO 6892-1, 2019 The present invention also relates to an object comprising or consisting of a duplex stainless steel that satisfies the following:

[0048] The present disclosure also provides an article comprising a duplex stainless steel, the article comprising the following elements: Contains TIFF2025513023000002.tif86170 by weight percent, A ferrite content of 40-60 volume percent; and The following conditions: a) [Cr] + 4.0 * [Mo] + 2.0 * [W] < 42.5 (wherein the values ​​of [Cr], [Mo], and [W] are in weight percent); b) the ferritic grain boundaries in the solution-annealed condition have less than 10% coverage of quench-in nitrides; and c) the object has an Rp0.2, measured at room temperature, of greater than 650 MPa, per ISO 6892-1, 2019 The present invention also relates to an object comprising a duplex stainless steel, the object satisfying the following:

[0049] A Rp0.2 of greater than 650 MPa per ISO 6892-1, 2019 at room temperature also means that the duplex stainless steel articles of the present invention could be used in applications where the material is exposed to high internal pressures, such as those found in new developments where exploration has recently commenced at wellbore pressures as high as 20 ksi, and still have a strength high enough to support its own weight.

[0050] According to embodiments, the object is a tube or pipe, for example a seamless or coiled tube, or one or more tubes welded into one long tube. According to other embodiments, the object is a longitudinally welded tube or a welded pipe. According to embodiments, the object is a hollow body or a billet. According to other embodiments, the object may be a strip or a wire.

[0051] The present disclosure also relates to a process for manufacturing an object, such as a tube, of a duplex stainless steel as defined above or below, said process comprising the steps of: - Melting the raw materials; The melting can be carried out in a high frequency induction furnace or an electric arc furnace. - Casting; - hot processing; The hot working may be performed by forging and / or rolling and / or extrusion. - cold working; The cold working can be accomplished by rolling or pilgering. - solution treatment; the solution treatment is carried out in the temperature range of about 1000 to about 1200°C, for example about 1050 to about 1150°C; - cooling; Cooling can be performed at room temperature or in air to a temperature from the solution treatment temperature to about 900 to about 1000°C. - quench; The quench can be carried out in water and should be carried out in such a way as to avoid the formation of sigma phases. Optionally, other steps can be carried out, such as straightening, machining, etc.

[0052] The term "about" is intended to mean a 10% deviation from a numerical value.

[0053] List of embodiments;

[0054] 1. Duplex stainless steel containing the following elements: C max 0.030; Si max 0.30; Mn 0.20-2.50; P max 0.030; S max 0.030; Cr 28.5-30.5; Ni 6.0-8.0; Mo 0.70~3.00; W 2.00-4.40; Cu max 0.50; N 0.30-0.55; In weight percent, The remainder is Fe and unavoidable impurities; The ferrite content is 40-60% by volume; Duplex stainless steel meets the following requirements: a) [Cr] + 4.0 * [Mo] + 2.0 * [W] < 42.5, where the values ​​of [Cr], [Mo], and [W] are in weight percent; and b) When in the solution-annealed condition, the coverage of quenched nitrides at the ferritic grain boundaries is less than 10%. Meets the requirements of duplex stainless steel.

[0055] 2. The duplex stainless steel according to item 1, wherein the Cr content is 29.0 to 30.5% by weight.

[0056] 3. The duplex stainless steel according to item 1 or 2, wherein the Cr content is 29.5 to 30.5 wt.%.

[0057] 4. The duplex stainless steel according to any one of items 1 to 3, wherein the Ni content is 6.0 to 7.6% by weight.

[0058] 5. A duplex stainless steel according to any one of items 1 to 4, wherein the Mo content is at least 0.70 to 2.00 weight percent.

[0059] 6. A duplex stainless steel according to any one of items 1 to 5, having a W content of 2.50 to 4.40 weight percent.

[0060] 7. The duplex stainless steel according to any one of items 1 to 6, wherein the W content is 3.00 to 4.00 weight percent.

[0061] 8. The duplex stainless steel according to any one of items 1 to 7, wherein the Mn content is 0.30 to 2.00% by weight.

[0062] 9. The duplex stainless steel according to any one of items 1 to 8, wherein the Si content is 0.10 to 0.30% by weight.

[0063] 10. The duplex stainless steel according to any one of items 1 to 9, wherein the Cu content is 0.15 to 0.50% by weight.

[0064] 11. A duplex stainless steel according to any one of items 1 to 10, having an N content of 0.35 to 0.55% by weight.

[0065] 12. The duplex stainless steel according to any one of items 1 to 11, having an N content of 0.38 to 0.55% by weight.

[0066] 13. A duplex stainless steel according to any one of items 1 to 12, having an N content of 0.38 to 0.50% by weight.

[0067] 14. An object comprising or consisting of the duplex stainless steel according to any one of items 1 to 13.

[0068] 15. The object according to item 14, having an Rp0.2 of greater than 650 MPa according to ISO 6892-1, 2019 at room temperature.

[0069] According to an embodiment, the present disclosure also relates to an object in a solution-treated state comprising or consisting of any of features 1-15 or a combination thereof.

[0070] According to an embodiment, the present disclosure also relates to a treated duplex stainless steel comprising or consisting of any of features 1-13 or a combination thereof. The duplex stainless steel has been treated as described above or below.

[0071] The present disclosure is further illustrated by the following non-limiting examples. EXAMPLES

[0072] Example 1 – Sample preparation Heats having the chemical compositions shown in Table 1 were melted in a high frequency induction furnace as 50 kg heats or 270 kg heats, and cast into ingots using molds.

[0073] After casting, the molds were removed and the ingots were heat treated by solution treatment and then quenched. Samples were taken from each ingot for chemical analysis. Chemical analysis was performed using X-ray fluorescence spectroscopy, spark atomic emission spectroscopy, and combustion techniques, as the content of different elements is measured in different ways.

[0074] The resulting ingots were forged or hot rolled into billets, which were then machined as necessary to form smaller billets.

[0075] After hot working (rolling or forging), the billets were cold rolled to 7-10 mm specimens, then solution treated at a temperature of 1050-1150°C, then cooled at room temperature to a temperature between 950-1000°C, and then quenched in water.

[0076] Example 2 – Sigma Phase Measurement The results of the sigma phase measurements are shown in Table 2. The samples were taken after solution treatment and quenching, and then heated to about 1100°C to simulate the environment of the heat affected zone. The cooling of the samples was controlled, and the cooling rate from about 1100°C to room temperature was 40°C / min. The proportion of sigma phase formed during cooling was determined for a total area of ​​0.4 mm 2 The measurements were made using SEM and image analysis evaluation of 10 image fields covering 2. This test simulates the reality of welding operations, so if there is no sigma phase or if there is only a small amount of sigma phase, the product can be welded with low sigma phase formation in the HAZ. As mentioned before, if there is no sigma phase or if there is only a small amount of sigma phase, it will have good corrosion resistance and good impact toughness.

[0077] Example 3 - Rp0.2 measurement Rp0.2 was measured at room temperature according to ISO 6892-1, 2019.

[0078] The results can be seen from Table 2. All of the inventive stainless steel heats have an Rp0.2 greater than 650 MPa, such as greater than 660 MPa. TIFF2025513023000003.tif124170

[0079] Example 4 – Measurement of the coverage of quench-in nitrides at ferrite grain boundaries The coverage of quenched nitrides at the ferrite grain boundaries was measured in the solution-annealed condition. Samples were taken from Example 1. Standard optical microscope (LOM) photomicrographs taken at 500x magnification and 0.05mm 2 An intercept method using 10 randomly oriented test lines at a representative site of the specimen was used to measure the percentage of ferrite grain boundaries covered with quench-in nitride. Specimens for LOM were prepared by surface polishing followed by electrolytic etching with HNO3 and NaOH. The percentage of ferrite grain boundaries covered with quench-in nitride was calculated by dividing the number of intercepts between the test line and the ferrite grain boundaries covered with quench-in nitride by the total number of intercepts between the test line and the ferrite grain boundaries. An example of this is shown in Figures 1A-5B and the results of these calculations are shown in Table 3. TIFF2025513023000004.tif123170

[0080] Example 5 – Ferrite measurements The amount of ferrite was measured using LOM and manual point counts in accordance with ASTM E562. The remainder is austenitic in nature. Ferrite was measured on cold rolled, solution treated, and quenched specimens. TIFF2025513023000005.tif123170TIFF2025513023000006.tif243170TIFF2025513023000007.tif242170

Claims

1. Duplex stainless steel containing the following elements: C maximum 0.030; Si maximum 0.30; Mn 0.20-2.50; P maximum 0.030; S maximum 0.030; Cr 28.5-30.5; Ni 6.0-8.0; Mo 0.70-3.00; W 2.00-4.40; Cu ≤ 0.50; N 0.30-0.55; in weight percent, The remainder is Fe and unavoidable impurities; A ferrite content of 40 to 60 volume percent; Duplex stainless steel meets the following requirements: a. [Cr] + 4.0 * [Mo] + 2.0 * [W] < 42.5, where the values ​​of [Cr], [Mo], and [W] are in weight percent; and b. When in the solution-annealed condition, the coverage of quenched nitrides at the ferrite grain boundaries is less than 10%. Meets the requirements of duplex stainless steel.

2. The duplex stainless steel according to claim 1, wherein the Cr content is 29.0 to 30.5 wt%.

3. The duplex stainless steel according to claim 1, wherein the Ni content is 6.0 to 7.6 wt%.

4. 2. The duplex stainless steel according to claim 1, wherein the Mo content is at least 0.70 to 2.00 wt.%.

5. The duplex stainless steel according to claim 1, wherein the W content is 2.50 to 4.40 wt.% or 3.00 to 4.00 wt.%.

6. 2. The duplex stainless steel according to claim 1, wherein the Mn content is 0.30 to 2.00 wt.%.

7. The duplex stainless steel according to claim 1, wherein the Si content is 0.10 to 0.30 wt%.

8. The duplex stainless steel according to claim 1, wherein the Cu content is 0.15 to 0.50 wt%.

9. The duplex stainless steel according to claim 1, wherein the N content is 0.35 to 0.55 wt%, or 0.38 to 0.55 wt%, or 0.38 to 0.50 wt%.

10. 1. An object comprising a duplex stainless steel, the duplex stainless steel comprising the following elements: C maximum 0.030; Si maximum 0.30; Mn 0.20-2.50; P maximum 0.030; S maximum 0.030; Cr 28.5-30.5; Ni 6.0-8.0; Mo 0.70-3.00; W 2.00-4.40; Cu ≤ 0.50; N 0.30-0.55; in weight percent, The remainder is Fe and unavoidable impurities; A ferrite content of 40 to 60 volume percent; Duplex stainless steel meets the following requirements: a) [Cr] + 4.0 * [Mo] + 2.0 * [W] < 42.5, where the values ​​of [Cr], [Mo], and [W] are in weight percent; and b) When in the solution-annealed condition, the coverage of the quenched nitrides at the ferrite grain boundaries is less than 10%. An object that satisfies.

11. 11. The object of claim 10, wherein the object has an Rp0.2 per ISO 6892-1, 2019 greater than 650 MPa.

12. 11. The body of claim 10, wherein the body, when in a solution-treated state, will contain less than 0.2 vol.% sigma phase after cooling from 1,100°C to room temperature at 40°C / min.

13. 11. The object of claim 10, wherein the object comprises an element in the range of any one of claims 2 to 9.

14. The object of claim 10 , wherein the object is a tube, a sheet, a hollow body, a billet, a strip, or a wire.

15. The object of claim 10 , wherein the object is in a solid solution treated state.

16. 11. Use of the article of claim 10 in applications requiring resistance to high pressures, such as applications where the internal pressure exceeds about 20 kpsi.

17. 11. A method for manufacturing an object according to claim 10, comprising the steps of: - melting; - Casting; - hot working; - cold working; - solution treatment carried out in a temperature range of about 1000 to about 1200°C, such as about 1050 to about 1150°C; - cooling from the solution treatment temperature to a temperature of about 900 to about 1000°C - Quench A method comprising:

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