KR20090078813A - Duplex stainless steel alloy and use of this alloy - Google Patents

Abstract

The present invention relates to a duplex stainless steel alloy containing in weight-%: C max 0.03%, Si < 0.30%, Mn 0-3.0%, P max 0,030%, S max 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%, balance Fe and normal occurring impurities, wherein the ferrite content is 30-70 volume-%, and wherein each weight-% of Mo above may optionally be replaced by two (2) weight-% W.

Description

Duplex stainless steel alloys and their uses {DUPLEX STAINLESS STEEL ALLOY AND USE OF THIS ALLOY}

The present invention relates to a duplex stainless steel alloy, which is a steel alloy comprising a ferrite-austenite matrix, which steel alloy has particularly high corrosion resistance with good structural stability and high temperature workability. The ferrite content is from 30 to 70% by volume and such steel alloys include well-balanced compositions that provide material corrosion properties that make them suitable for use in, for example, chloride containing environments such as the sea.

Recently, the environment in which corrosion-resistant metal materials are used has become more active, and the demand for corrosion and mechanical properties has increased. For example, a duplex steel alloy was used that had been formed as an alternative to other steel grades until high alloy austenitic steel, nickel-based alloys or other high steel alloys were part of the development. An established measurement method for corrosion resistance in chloride-containing environments is the so-called Pitting Resistance Equivalent (abbreviated PRE). This is defined as follows.

PRE =% Cr + 3.3% Mo + 16% N

Here, the percentage of each element refers to the weight percentage.

The larger the value, in particular, the better the corrosion resistance to pitting corrosion. Essential alloying elements affecting this property depend on Cr, Mo, and N of the above formula. Examples for such steel grades are apparent from EP0220141, which is incorporated herein by reference. The steel grades labeled SAF2507 (UNS S32750) were mainly alloyed with high contents of Cr, Mo, and N. As a result, among other things, it has good corrosion resistance in the chloride environment and there has been development in its properties.

Recently, elements of Cu and W have also been found to be effective alloying additives to further optimize the corrosion properties of steel in chloride environments. The element W has been used as a replacement for some of Mo, such as, for example, commercial alloy DP3W (UNS S39274) containing 2.0% W or Zeron100 containing 0.7% W. Zeron100 even contains 0.7% Cu to improve the corrosion resistance of the alloy in acid environment.

The addition of alloys of tungsten further developed the survey method for corrosion resistance and hence the PRE or PREW equation, whereby the relationship between the influence of Mo and W on alloy corrosion resistance is described, for example, in EP 0 545 753. Becomes clearer. :

PREW =% Cr + 3.3 (% Mo + 0.5% W) + 16% N

This publication is directed to duplex stainless alloys with improved overall corrosion properties.

The steel grade described above has a PRE / PREW value of 40 or more, regardless of the calculation method.

An alloy with good corrosion resistance in chloride environments is SAF 2906, the composition of which can be found in EP 0 708 845. This alloy is characterized by containing more Cr and N than SAF 2507, for example, and is particularly suitable for use in environments where intergranular corrosion and resistance to corrosion in ammonium carbamate are important. It has been found that it has high corrosion resistance even in chloride containing environment.

US-A-4 985 091 describes alloys aimed at use in hydrogen chloride and sulfuric acid environments where grain boundary corrosion occurs mainly. Primarily, this alloy is a replacement for the recently used austenitic steel.

US-A-6 048 413 describes a duplex stainless alloy as an alternative to austenitic stainless steels used in chloride containing environments.

EP 0 683 241 describes a duplex stainless steel alloy with a composition having improved properties against stress corrosion cracking and formula both resistance in chloride ion containing environments compared to most other known duplex stainless steel alloys. However, this alloy and the alloys described above are very susceptible to intermetallic precipitation, in particular sigma phase precipitation, which hardens or embrittles the material. Thus, the use of duplex stainless steel alloys according to EP 0 683 241 makes it very difficult to produce materials with good ductility.

It is an object of the present invention for a duplex stainless steel alloy of the kind described above and in particular in European Patent 0 683 241, having improved properties, in particular ductility and toughness, with respect to such alloys already known, at least to a degree similar to those alloys. To provide a duplex stainless steel alloy that maintains corrosion resistance. The alloy has good high temperature workability.

The object is achieved by providing a duplex stainless steel alloy, which has a C max of 0.03%, Si <0.30%, Mn 0 to 3.0%, P max of 0.030%, S max of 0.050%, Cr 25 to 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 is an impurity that usually occurs with Fe, the content of ferrite is 30 to 70% by volume, and each weight% of the Mo is selectively selected by 2% by weight W Can be replaced by

Duplex stainless steel alloys comprising the composition have particularly improved ductility and toughness for alloys according to EP 0 683 241 and also have improved corrosion resistance. By reducing the Si content to less than 0.30% by weight, the sigma phase precipitation is significantly reduced, which is an important factor for the improved ductility and toughness of the steel alloy according to the invention. Therefore, when using a relatively high content of Mo, it is very effective to reduce the Si content to reduce the risk of intermetallic precipitation.

According to embodiments of the present invention, the content of Si is at most 0.25% by weight, so that the steel alloy is less affected by sigma formation, thereby improving the ductility and toughness of the material. This would be effective even if molybdenum could be partially or completely replaced by tungsten.

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

According to another embodiment of the invention, when the content of Mo is a weight% and the content of W is b weight%, a + b / 2> 5.0. Such high contents of Mo and / or W provide excellent resistance to corrosion, in particular formula and crack corrosion. However, such high content of these elements increases the risk of intermetallic precipitation, which is effectively reduced by mixtures with low content of Si. According to another embodiment of the present invention, a> 5.0. Claim 1 claims that, starting from the content intervals of Mo (4.5-8%) and W (0-3%), each% of Mo can be replaced by 2% W, or vice versa, whereby When the content is at least 3%, it is interpreted that the content of Mo may be for example 3%. According to a preferred embodiment, a + b / 2 ≦ 8, ie the total content of Mo and W does not exceed 8% in order to keep costs at an ideal level. According to another preferred embodiment, b = 0, ie the alloy comprises only Mo.

According to another embodiment of the present invention, the content of Co is 0 to 0.010% by weight. Co is an expensive material, and it has been found that the influence on the structural performance and corrosion resistance improvement of this material is not an essential element of the steel alloy having the composition according to the present invention.

According to another embodiment of the present invention, the content of ferrite is 40 to 60% by volume.

According to another embodiment of the invention, the average PRE or PREW value of the two phases of the alloy is greater than 44, and PRE =% Cr + 3.3% Mo + 16% N and PREW =% Cr + 3.3 (% Mo + 0.5% W) + 16% N, where% is weight%. The PRE or PREW value for the ferrite and austenite phase is greater than 47, preferably greater than 48.5, and the average PRE or PREW value may be greater than 48, preferably greater than 49. It has been found that in the steel alloy according to the invention the formula and crack corrosion resistance are improved in particular by increasing the PRE or PREW values of the phase with the smallest value. It has been found that the steel alloy according to the invention still has good high temperature workability even with a PRE or PREW value greater than 49.

According to another embodiment of the invention, the ratio between the PRE (W) value for the austenite phase and the PRE (W) value for the ferrite phase is in the range 0.90 to 1.15, preferably in the range 0.95 to 1.05.

The alloy according to the invention is in the form of products such as tubes, plates, strips, wires, welding wires, such as construction parts such as pumps, valves, flanges and couplings, in chloride-containing environments, such as bars, welded tubes and seamless tubes. Used.

1 shows the estimated phase content of a duplex stainless steel alloy according to an embodiment of the invention as a function of temperature.

FIG. 2 is a graph similar to FIG. 1 for a comparative steel alloy according to EP 0 683 241. FIG.

3 is a micrograph of a continuous cooling sample of the alloy according to FIGS. 1 and 2 with three different cooling rates.

The combination of elements in the duplex stainless steel alloy according to the invention results in high ductility and toughness as well as good corrosion resistance. This steel alloy also has good workability, which makes it possible to extrude, for example, to seamless tubes. Alloys according to the present invention include the following (% by weight).

C max. 0.03%

Si <0.30%

Mn 0-3.0%

P max. 0.030%

S max 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 ~ 0.60%

The remainder is Fe and commonly occurring impurities, the ferrite content is 30 to 70% by volume, and the weight% of each Mo may be optionally replaced by 2% by weight W.

Carbon (C)

Carbon has limited solubility in both ferrite and austenite. Limited solubility means the risk of precipitation of chromium carbides, whereby the content should not exceed 0.03% by weight, preferably not exceeding 0.02% by weight.

Silicon (Si)

Silicon is used as a reducing agent in steel products and improves fluidity during production and welding. However, an excess of Si results in undesirable precipitation of the intermetallic phase, whereby the content is limited to less than 0.30% by weight, preferably up to 0.25% by weight, more preferably up to 0.23% by weight.

Manganese (Mn)

Manganese is added to the material to improve N-solubility. However, only Mn was shown to have a limited effect on N-solubility in the type of alloys in question. Instead, it was found that other elements exist that have a greater impact on solubility. In addition, Mn combined with high content of sulfur can result in the formation of manganese sulfide, which serves as a starting point for pitting corrosion. Therefore, the content of Mn should be limited to 0 to 3.0% by weight, preferably 0.5 to 1.2% by weight.

Phosphorus (P)

Phosphorus is a common impurity element. If phosphorus is present at about 0.05% or more, it may adversely affect, for example, high temperature ductility, weldability, and corrosion resistance. Therefore, the amount of P in the alloy should not exceed 0.05%.

Sulfur (S)

Sulfur negatively affects corrosion resistance by forming soluble sulfur. In addition, the sulfur content is limited to a maximum of 0.030% by weight, preferably 0.010% by weight or less in view of deterioration of high temperature workability.

Chromium (Cr)

Chromium is a very active element, which improves resistance to most corrosion types. In addition, the high content of chromium means that very good N-solubility can be obtained in the material. Therefore, it is desirable to keep the Cr-content as high as possible to improve the corrosion resistance. For good amounts on corrosion resistance, the content of chromium should be at least 25% by weight. However, since the high content of Cr may increase the risk of intermetallic precipitation, the content of chromium should be limited up to 29% by weight, preferably 25.5 to 28% by weight.

Nickel (N)

Nickel is used as an austenite stabilizing element and an appropriate amount is added to obtain a desired content of ferrite. In order to obtain the desired relationship between the ferrite phase and the austenite phase with 30 to 70 volume% ferrite, the addition of 5 to 9 weight% nickel is necessary, preferably 6 to 8 weight%.

Molybdenum (Mo)

Molybdenum is an active element that improves corrosion resistance in chloride environments and preferably when reducing acids. Excess Mo combined with a high Cr content represents a risk of increasing intermetallic precipitation. The content of Mo of the present invention should be 4.5-8% by weight, preferably 5.0% by weight or more, and optionally each weight% of Mo can be replaced by 2% by weight W.

Tungsten (W)

Tungsten improves the resistance to formula and crack corrosion. However, addition of excess tungsten combined with high Cr content and high Mo content means there is a risk of increasing intermetallic precipitation. W content in the present invention should be 0 to 3.0% by weight.

Copper (Cu)

Copper may be added to enhance general corrosion resistance in acid environments such as sulfuric acid. At the same time Cu affects the structural stability. However, the high content of Cu means that it will surpass solid solution. Therefore, the Cu content should be limited to a maximum of 2.0% by weight, preferably 0 to 1.5% by weight, more preferably 0.1 to 0.5% by weight.

Cobalt (Co)

Cobalt has the property of being an intermediate between iron and nickel. Therefore, small-scale substitution of the element with Co or the use of a Co-containing raw material (in some cases, in which the scrap metal of Ni contains an amount of Co of more than 10%) does not cause major property changes. Co can be used to substitute a degree of Ni as an austenite stabilizing element. Since Co is a relatively expensive element, the addition of Co is limited to 0 to 3% by weight.

Titanium (Ti)

Titanium has a high affinity for N. Therefore, titanium can be used, for example, to improve the dissolution of N in the melt during casting and to prevent the formation of nitrogen bubbles. However, an excess of Ti in the material can cause precipitation of nitrides that can interfere with the casting process during casting, and the resulting nitrides can act as defects that can cause a decrease in corrosion resistance, toughness and ductility. Therefore, the addition of Ti is limited to 2% by weight.

Aluminum (Al) and Calcium (Ca)

Aluminum and calcium are used as reducing agents in steel production. The content of Al should be limited to at most 0.05% by weight, preferably at most 0.03%, to limit the formation of nitrides. Ca has a favorable effect on high temperature ductility. However, the Ca content should be limited to a maximum of 0.010% by weight so as to avoid undesirable amounts of slag.

Boron (B)

Boron may be added to improve the high temperature workability of the material. In excess of boron, weldability and corrosion resistance may be impaired. Therefore, the content of boron should be limited to at most 0.01% by weight.

Nitrogen (N)

Nitrogen is a very active element that improves corrosion resistance, structural stability, and strength of the material. In addition, the high content of N improves the recovery of austenite after welding, thereby providing good properties in the weld joint. In order to obtain the good effect of N, at least 0.35% by weight of N must be added. At high contents of N, at the same time the risk for precipitation of chromium nitride increases, especially when the chromium content is high. In addition, high N content means that the risk of porosity is increased due to excessive solubility of N in the melt. For this reason, the N content should be limited to a maximum of 0.60 weight%, preferably more than 0.35 to 0.45 weight% N is added.

Ferrite content is important for obtaining good mechanical properties, corrosion properties and good weldability. From the standpoint of weldability and corrosiveness, the ferrite content is preferably 30 to 70% in order to obtain good properties. In addition, the high content of ferrite affects the strength at low temperatures, which means that the resistance to hydrogen-induced brittleness risks deteriorates. Therefore, the content of ferrite is 30 to 70% by volume, preferably 40 to 60% by volume.

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

The alloy was also modeled using the Thermo-Calc software of the database CCTSS (eg a slightly modified version of the commercial database TCFE3 with an improved model for duplex alloy compositions). 1 and 2 show the estimated phase contents of alloy No. 1 and alloy No. 2 as a function of temperature, respectively. In the figures, numerical values are as follows. :

1: ferrite content. In the alloy according to the invention (FIG. 1), a heat treatment of 1, 100 to 1, 300 ° C. is required to obtain the desired ferrite content.

2: austenite content. Heat treatment is performed to obtain only ferrite phase and austenite phase.

3: N content.

4: liquid metal.

5: sigma award. The formation of this sigma phase can be prevented by rapid cooling.

6: content of Cr ₂ N which causes a decrease in brittleness and corrosion resistance.

7: Carbide content that must be kept in small amounts without affecting welding. If carbide tends to be precipitated, there is a risk of reduced corrosion resistance near welds. Therefore, the equilibrium amount of carbides should be kept low.

8: intermetallic phase. The sum of this intermetallic phase and the sigma phase should be kept as low as possible.

Table 2 shows the total PRE of the two alloys, the expected PRE, the ratio between the austenitic and ferrite PREs for each phase upon quench from 1100 ° C. Also shown is the expected ferrite content, final expected dissolution temperature for Cr ₂ N and sigma (σ) phases after quench from 1100 ° C., and the expected precipitate present at 1100 ° C. FIG. Since the precipitation of Cr ₂ N is faster than that of the phase σ, two T _max, Cr ₂ N appear, one for slow cooling (with “σ”) when precipitation of the equilibrium amount of σ is allowed, and the other One is the case of fast cooling when σ does not precipitate (without “s”). It is evident that both alloys meet the requirements for ferrite content, total PRE and PRE balance and minimum PRE in each phase as described in WO 03020994.

Sample manufacturing

The alloy was produced by melting, ingot casting, and final press forging. Table 3 shows the results of forging. Forging was stopped when serious surface defects began to form, and a reduction in the overall cross-sectional area during the forging process can be used as an estimate of the forging of the two alloys.

The forged bar was annealed at 1100 ° C. and then quenched in water before another process started. The preliminary material used as the sample was divided into smaller pieces at 1100 ° C. for 1 hour, then annealed once more, and then quenched in water. After this treatment, other samples were processed.

exam

Impact test

Impact tests were performed on four different material conditions on a 10 × 10 mm Charpy v − notched sample (55 mm in length). Ie annealed (eg, quenched at 1100 ° C./water), and the impact sample was further annealed at low temperature. Table 4 shows the different material conditions and the resulting impact toughness values. Two samples were tested for each composition and annealing conditions.

Alloy 1 with high Mo and low Si and Co has good impact toughness when sufficiently high annealing temperatures are used. The table shows the drawbacks of alloy 2 according to EP 0 683 241, ie the content of Si higher than 0.5% with high Mo content potentially feeds brittle materials. Only by reducing the Si content is provided a greatly improved toughness (such as alloy 1 according to the invention).

Continuous cooling

Nine samples from each heat were annealed at 1100 ° C. and then reheated from each heat to three different temperatures of 1050, 1100 and 1150 ° C., respectively. Samples were cooled at three different constant cooling rates from different holding temperatures, ie 20, 60 and 140 ° C./min. That means that nine different annealing cycles were used for each hit. There was no nitride in any of the samples. Table 5 summarizes the optical microscopy observations. The following relative grade indices are used for the σ phase inclusions of the other samples.

0: σ phase not detected

1 to 2 σ phase particles detected on average in a field of view of 500 x magnification

2: small amount of σ phase detected at 500 × magnification (but not less than 2 particles per field of view)

3: relatively large amount of σ (but less than 5% ferrite deteriorated)

4: 5% or more ferrite deteriorated to σ

5: more than 25% ferrite is changed to σ

6: more than 50% ferrite is changed to σ

It is shown that Alloy 1 suffers less σ precipitation than Alloy 2. A "note" of 2, preferably 1, is necessary to properly prepare the material.

3 shows a micrograph of a continuous cooling sample heated to 1100 ° C. FIG. Light color is austenite, brown is ferrite, and black is σ phase. The formation of the σ phase (black) is much weaker for alloy No. 1 according to the invention than for alloy No. 2 according to EP 0 683 241, which is apparently due to the low content of Si.

Mechanical properties

Table 6 shows the results of the tensile test. Alloy No. 2 is an alloy No. 2 according to the present invention. Ductility is certainly smaller than 1

2 samples from each hit as a result of the tensile test

Corrosive Test

Critical cracking temperature (CCT) according to MTI-2 and critical formula temperature (CPT) in "Green Death" solution (1% FeCl ₃ + 1% CuCl ₂ + 11% H ₂ SO ₄ + 1.2% HCl) ) Is shown in Table 7. There is a slight difference in crack corrosion resistance between different alloys. The assumption that the formula and crack corrosion resistance in a duplex alloy is mainly determined by the PRE of the phase with the lowest PRE is consistent with the fact that Alloy 1 has the highest CCT. In addition, the improved behavior of Alloy 1 over Alloy 2 is manifested in the form of less weight loss due to corrosion and higher maximum temperatures.

Crack corrosion according to MTI-2, the result in the formula in the Green Death solution and the formula in ferric chloride. Two samples / alloys are used in each test.

summary

The alloy (No. 2) corresponding to EP 0 683 241 is very susceptible to sigma phase precipitation, which makes the production of materials with good ductility very difficult. This problem is solved by lowering the Si content and a good balance between the PRE values of the two phases. In addition, alloy No. 2 has low forging property. By reducing the Si content of the alloy of the type specified in EP 0 683 241, ie by using the composition of alloy No. 1, not only the ductility and toughness is increased, but also the corrosion resistance, which in practice was not expected at all. Effect.

Claims (15)

As a duplex stainless steel alloy containing the following by weight%, C max. 0.03% Si <0.30% Mn 0-3.0% P max. 0.030% S max 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 ~ 0.60% The remainder is Fe and commonly occurring impurities, the ferrite content is 30 to 70% by volume, the weight percentage of each of the Mo can be replaced by 2% by weight W optionally. The duplex stainless steel alloy of claim 1 wherein the content of Si is at most 0.25% by weight. The duplex stainless steel alloy of claim 1 wherein the content of Si is at most 0.23 weight percent. The duplex stainless steel alloy according to any one of claims 1 to 3, wherein the content of Mo is a weight% and the content of W is a + b / 2> 5.0 when b weight%. The duplex stainless steel alloy of claim 4 wherein a> 5.0. The duplex stainless steel alloy of claim 4 wherein a + b / 2 ≦ 8. The duplex stainless steel alloy according to any one of claims 1 to 6, wherein the content of Co is 0 to 0.010% by weight. The duplex stainless steel alloy according to any one of claims 1 to 7, wherein the content of Cr is 25.5 to 28% by weight. The duplex stainless steel alloy according to any one of claims 1 to 8, wherein the content of Ni is 6 to 8% by weight. 10. The duplex stainless steel alloy according to any one of claims 1 to 9, wherein the content of N is 0.35 to 0.45% by weight. The duplex stainless steel alloy according to any one of claims 1 to 10, wherein the content of ferrite is 40 to 60% by volume. 12. The alloy according to any of the preceding claims, wherein the average PRE or PREW value of the two phases of the alloy is greater than 44, and PRE =% Cr + 3.3% Mo + 16% N and PREW =% Cr + 3.3 (% Mo). + 0.5% W) + 16% N, where% is weight%, duplex stainless steel alloy. 13. The duplex stainless steel according to claim 12, wherein the PRE or PREW value for the ferrite and austenite phase is greater than 47, preferably greater than 48.5, and the average PRE or PREW value is greater than 48, preferably greater than 49. alloy. The duplex stainless steel according to claim 12 or 13, wherein the ratio of the PRE (W) value for the austenite phase and the PRE (W) value for the ferrite phase is 0.90 to 1.15, preferably 0.95 to 1.05. alloy. Claims 1 to 3 used in the form of products such as tubes, plates, strips, wires, welding wires such as pumps, valves, flanges and couplings, such as bars, welded tubes and seamless tubes, in chloride containing environments Use of an alloy according to claim 14.

Images