KR20190099232A - Uses of Duplex Stainless Steel Articles - Google Patents

Abstract

The present invention relates to the use of a solution melted annealed article comprising duplex stainless steel having the following composition in weight percent (wt%): C 0.03 or less; Si 0.5 or less; Mn 1.0 or less; Ni 5.0-7.0; Cr 22.0-26.0; Mo 2.5-4.5; N 0.1-0.2; P 0.03 or less; S 0.03 or less; Cu 0.3 or less; Al 0.10 or less; The balance is Fe and unavoidable impurities, and the duplex stainless steel satisfies the formula Cr + 50N ≦ 35; The duplex stainless steel also has a ferrite phase content of 40% by volume to 60% by volume and an austenite phase content of 40% to 60% by volume in seawater applications.

Description

Uses of Duplex Stainless Steel Articles

The present invention relates to the use of articles made of duplex (ferrite-austenite) stainless steel in seawater applications, which articles have surprisingly good resistance to hydrogen organic stress corrosion (HISC).

Cathodic protection (CP) offshore for the prevention of formulas of duplex and superduplex stainless steels used in subsea components in high temperature wells has been used for more than 20 years. Cathodic protection is defined as electrochemical protection by reducing the corrosion potential to a level where the corrosion rate of the metal is significantly reduced. Thus, this is a technique for reducing the corrosion of metal surfaces by making the metal surfaces from the cathode of the electrochemical cell. Thus, duplex stainless steel is the cathode and the other metal is the anode (usually Zn).

Although duplex stainless steel has been a very good choice of materials for use with cathode protection, several breakdowns have occurred in the last few years with respect to hydrogen organic stress corrosion cracking, also known as HISC. HISC is a non-ductile failure mode that originates in the bond between stress, the use of cathode protection systems and the use of materials with sensitive microstructures, and is also caused by atomic hydrogen diffusion. This breakdown affects the strength and ductility of duplex stainless steels, as these materials due to HISC are more susceptible to brittle cracking, especially during high load applications.

Thus, in seawater applications, in particular, in addition to duplex (ferrite-austenite) stainless steels used to produce articles which should be used in applications where duplex stainless steel is used for cathode protection (duplex stainless steel acts as cathode). There is still a need to improve.

Accordingly, an aspect of the present invention is to provide an article made of duplex (ferrite-austenite) stainless steel, which article should be used for seawater applications. Such duplex stainless steel articles have an elemental composition that, together with the manufacturing method, provides good resistance to hydrogen organic stress corrosion (HISC). Accordingly, the present invention relates to the use of a solution-annealed annealed article made of duplex (ferrite-austenite) stainless steel having the following composition by weight (wt%):

C 0.03 or less;

Si 0.5 or less;

Mn 1.0 or less;

Ni 5.0 to 7.0;

Cr 22.0 to 26.0;

Mo 2.5 to 4.5;

N 0.1 to 0.2;

P 0.03 or less;

S 0.03 or less;

Cu 0.3 or less;

Al 0.10 or less;

The balance is Fe and inevitable impurities,

The duplex stainless steel satisfies the condition of Cr + 50N ≦ 35; In addition, the duplex stainless steel has a ferrite content of 40% by volume to 60% by volume and austenite content of 40% by volume to 60% by volume in seawater applications.

According to one embodiment, the use of the article comprises cathodic protection, ie the use of a duplex stainless steel alloy as described above or below, such as a cathode.

By optimizing the element compositions of the duplex stainless steel and processes for making the article of the present invention, articles comprising duplex stainless steel will have high corrosion resistance and good structural stability. Thus, the duplex stainless steel of the present invention has been found to combine several good properties due to this composite optimization as described in the description below.

Accordingly, the present invention provides an article of duplex stainless steel, which article will have high corrosion resistance, high strength and toughness. In addition, the articles of the present invention are easy to manufacture and have good processability, for example, to allow extrusion into a seamless tube. Due to its composition and its manufacturing process, the article will essentially contain no sigma phase (essentially no gamma phase). This means that the problems with corrosion, brittle fracture and nitride formation during welding are reduced and / or eliminated, which is very advantageous.

As described above or below, the process for making the article of the present invention should include the step of solving annealing. Solvent annealing means that the article is heat treated at a temperature above the recrystallization temperature of the duplex stainless steel as described above or below.

Alloying elements of the duplex stainless steel and their composition range according to the present invention will now be described further.

Carbon (C) is an impurity contained in duplex stainless steel. If the content of C exceeds 0.03 wt%, the corrosion resistance is lowered due to precipitation of chromium carbide at the grain boundaries. Thus, the content of C is 0.03 wt% or less, such as 0.02 wt% or less.

Silicon (Si) is an element that may be added for deoxidization. However, too much Si will promote precipitation of intermetallic phases, such as sigma phases, so the content of Si is below 0.5 wt%.

Manganese (Mn) is used in most duplex stainless steels at levels up to about 1.0 wt%. One important reason is that Mn has the ability to bind sulfur, an impurity, to MnS, which is advantageous for high temperature ductility. Therefore, in order to have such an effect, the content of Mn is 1.0 wt% or less.

Nickel (Ni) is an austenite stabilizing element and also needs to be present to achieve the desired phase equilibrium between the ferrite phase and the austenite phase. Therefore, the content of N is 5.0 to 7.0 wt%, such as 6.0 to 7.0 wt%.

Chromium (Cr) is a very important element in duplex stainless steel because Cr is essential to form a passive oxide film that protects the duplex stainless steel from corrosion. In addition, the addition of Cr will increase the solubility of nitrogen (N). If the Cr content is too low, pitting resistance is reduced. If the Cr content is too high, the resistance to HISC is reduced. As shown in FIG. 1, a linear relationship between HISC resistance and the formula Cr + 50N was found, which indicates that resistance to HISC in the duplex stainless steel is related to the content of both Cr and N, as described above or below. Means that. As can be seen from FIG. 1, if Cr and N are too high, resistance to HISC will be reduced. Therefore, the content of Cr is 22.0 to 26.0 wt%, such as 23.0 to 24.0 wt%.

Molybdenum (Mo) is an element effective in stabilizing a passivation film formed on the surface of duplex stainless steel, and is also effective in improving stress corrosion cracking and pitting resistance. When the content of Mo is less than 2.5 wt%, the stress corrosion cracking and pitting resistance are not high enough. If the Mo content is too high, there will be a risk of forming an intermetallic phase that causes the material to break easily. Thus, the content of Mo is 2.5 to 4.5 wt%, such as 2.8 to 4.0 wt%.

Nitrogen (N) is an effective element for increasing the strength of duplex stainless steel by solvent curing. If the N content is too low, mechanical properties and pitting resistance will be reduced. If the N content is too high, the resistance to HISC will be reduced. As shown in FIG. 1, a linear relationship between HISC resistance and the formula Cr + 50N was found. Therefore, the content of N is 0.10 to 0.20 wt%, such as 0.12 to 0.20 wt%.

Phosphorus (P) is an impurity contained in duplex stainless steel and it is well known that P adversely affects hot workability. Therefore, the content of P is set at 0.03 wt% or less, for example 0.02 wt% or less.

Sulfur (S) is an impurity contained in duplex stainless steel, which will also worsen hot workability at low temperatures. Thus, the acceptable content of S is 0.03 wt% or less, such as 0.02 wt% or less.

Copper (Cu) is an optional element that may or may not be included in the duplex stainless steel of the present invention as scrap is used as a starting material for producing the melt. Such Cu may stabilize the passive film formed on the surface of the duplex stainless steel, and may improve pitting resistance and corrosion resistance at low concentrations. Thus, the acceptable content of Cu is 0.3 wt% or less, such as 0.2 wt% or less.

Aluminum (Al) is a deoxidation element and may optionally be contained in the duplex stainless steel of the present invention. If the Al content exceeds 0.10 wt%, the formation of an intermetallic phase such as a sigma phase will be promoted. In addition, if Al is added at a level above 0.10 wt%, AlN or NiAl may be formed, which will affect the mechanical properties. Therefore, in order to obtain duplex stainless steel having the above-mentioned or as described below, the Al content is 0.10 wt% or less.

Surprisingly, solution-annealed articles made of duplex stainless steel as described above or below and satisfying the formula of Cr + 50N (see Fig. 1) of 35 or less have better resistance to HISC, in which Cr and It was found that the amount of N was in weight percent. This means that the content of Cr is related to the content of N, which means that the contents of Cr and N are found to be low in the duplex stainless steels described above or below (as compared with other known duplex stainless steels). Since sensitivity to HISC is initially due to the microstructure of the duplex stainless steel rather than the chemical composition of the duplex stainless steel, it should be recognized that, according to common general knowledge, the relationship between Cr and N cannot be predicted. It can further be appreciated that the duplex grades generally used for such applications have a content of 25 wt% Cr and more than 0.25 wt% N. According to one embodiment, Cr + 50N is 34 or less, such as 33 or less.

In accordance with the present invention, the process for making articles comprising duplex stainless steel as described above or below should include the step of annealing before being used in seawater applications. Solvent annealing means that the article is heat treated, and this step will improve the microstructure of the duplex stainless steel, thereby increasing the ductility and toughness. Solvent annealing should be performed at a temperature above the recrystallization temperature of the duplex stainless steel. According to one embodiment, the solution annealing temperature is between 1030 and 1150 ° C. According to one embodiment, it is rapidly cooled in air or water after solution dissolution. Solvent annealing is carried out after cold processing steps such as cold deformation, such as pressing, bending, shearing, pilgering or drawing.

The microstructure of duplex stainless steel is a biphasic structure comprising austenite islands embedded in a ferrite matrix. The more closely packed austenite phase (FCC) has larger pores in the tissue than the ferrite BCC tissue. These tissues affect hydrogen diffusion and hydrogen solubility. The diffusion rate of hydrogen is much faster in the ferrite phase than in the austenitic phase, but the solubility of hydrogen is higher in the austenite phase than in the ferrite phase. It has been found that the cracks due to HISC often start on the ferrite phase and in most cases the austenite phase acts as a crack inhibitor. Thus, in the present invention, the distribution of the two phases is balanced in the article to provide approximately the same amount of ferrite phase and austenite phase in the solution annealing conditions. Thus, the ferrite phase content of the article is 40 to 60% by volume, such as 45 to 55% by volume, balanced by the austenite phase.

According to one embodiment, the other elements are selective to the duplex stainless steel as described above or below during the manufacturing process to improve processability, for example hot workability, machinability and the like. It may be added as. Examples of such elements are titanium (Ti), calcium (Ca), cerium (Ce) and boron (B). When added, these elements are added in amounts up to 0.5 wt% in total. According to one embodiment, the stainless steel according to the invention consists of all of the elements as described above or below in the range described above or below.

The balance of duplex stainless steel is iron (Fe) and unavoidable impurities. Examples of unavoidable impurities are elements and compounds that have not been intentionally added but cannot be completely avoided, for example, as they generally occur as impurities in materials used to make duplex stainless steel.

Microstructure features such as austenite spacing (average distance of ferrite between austenite regions) and grain size are affected by the manufacturing method. Austenitic spacing can be reduced by a greater degree of hot working and / or cold working prior to the solution annealing heat treatment. Duplex stainless steel with smaller austenite spacing has better HISC resistance. According to one embodiment, the austenitic spacing of the duplex stainless steel as described above or below in solution annealed conditions may be less than 35 μm, such as 5 to 35 μm, such as 5 to 20 μm, such as 5 to 15 μm. .

The corrosion resistance and crevice corrosion resistance of stainless steels are mainly determined by the wt% content of Cr, Mo and N. The index used to compare this resistance is PRE (Pitting Resistance Equivalent), which is expressed as Cr + 3.3Mo + 16N. For duplex stainless steels, pitting resistance depends on the PRE value in both the ferrite and austenite phases. This means that the phase with the lowest PRE value sets a limit on the local corrosion resistance of duplex stainless steel. Thus, according to one embodiment, the PRE of the duplex stainless steel according to the present invention may be at least 31, such as at least 34.

Stable strength is the load at which a material can deform without changing its dimensions. The stable strength (R _p0.2 ) of the duplex stainless steels according to the invention in solution _{melted conditions} is between 450 and 700 MPa, such as between 475 and 650 MPa.

High elongation means high ductility, which is considered in the molding manufacturing process. Thus, according to one embodiment of the invention, the elongation (A) of the duplex stainless steel according to the invention in solution annealed conditions is 15 to 45%, such as 20 to 45%, such as 25 to 45%.

Duplex stainless steel articles may be prepared according to conventional methods, ie casting or forging, followed by hot working and / or cold working, quenching annealing and any further heat treatment, or for example powder by hot isostatic process (HIP) It may also be manufactured as a product. An important step in the manufacturing process is the solution annealing step, since this will establish the final microstructure.

According to one embodiment, an article made of duplex stainless steel as described above or below is produced by a process comprising the following steps:

a. Melting step;

b. Casting step;

c. Hot processing step;

d. Cold processing step;

e. Solvation Releasing Step.

Duplex stainless steel articles include bars; tube; It may also be in the form of seamless or welded tubes, for example construction parts such as flanges and couplings, plates, sheets or strips, or wires.

The invention is further illustrated by the following non-limiting examples.

FIG. 1 discloses HISC testing at a constant load at 3 wt% sodium chloride (NaCl) at 4 ° C., thus simulating the environment in which duplex stainless steels are exposed to seawater.

Example

Five different hits with different compositions were melted as 270 kg of hits in a high frequency induction furnace and also cast into ingots using a 9 "mold. Table 1 shows the compositions of the duplex stainless steels used. Both Examples and Comparative Examples are described below: In Table 1, Points E1 and E2 represent Examples 1 and 2 of the present invention, while Points C1 through C3 represent Comparative Examples 1 through 3. Indicates.

Table 1. Chemical Composition of Various Heats

After casting, the mold was removed and the ingot was held at 1050 ° C. for 2 hours and then quenched in water. 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.

The resulting ingots were forged with billets 130 x 60 to 70 mm with a hammer. Prior to forging, the ingots were heated to 1250-1280 ° C. with a holding time of 2 hours. Forged billets were machined into 120 x 50 mm billets hot rolled from 10 to 12 mm on a Robertson mill. Prior to hot rolling, the billets were heated to 1150 ° C.-1212 ° C. with a holding time of 1.5-2 hours. After hot rolling, the billets were held at 1100 ° C.-1120 ° C. for 10 minutes and then cooled to 900 ° C.-950 ° C. in air, where the billets were quenched in oil. Duplex stainless steel billets were cold rolled to a thickness of 7-8 mm, then heat treated by solution anneal at 1000-1150 ° C. and then cooled in air.

After the final heat treatment step, the HISC test was performed at constant load with dead weight testers in a solution of 3 wt% NaCl at 4 ° C. and also received cathode protection at about 1050 mV _SCE . The test time was up to 500 hours or failure and the load was related to the stable strength. Prior to the experiment, the samples were charged galvanostatically with hydrogen at a current density of 0.02 A / cm ² .

In analyzing the results from the HISC test, it was found that solution-annealed duplex stainless steels with surprisingly lower Cr and N contents have better resistance to HISC. As can be seen in FIG. 1, a linear relationship between the linear relationship for the formula Cr + 50N and the maximum load without breakage in the HISC test associated with the stable strength (Rp _0.2 ) at 4 ° C. was observed. In FIG. 1, points E1 and E2 represent Examples 1 and 2 of the present invention, while points C1-C3 represent Comparative Examples 1-3. Therefore, articles made of duplex stainless steel must satisfy an equation with a Cr + 50xN of 35 or less to have improved HISC resistance.

In addition, annealed duplex stainless steel articles were analyzed. Tensile test (R _p0.2 and R _m ) was performed at room temperature to determine yield strength. Elongation (A) was measured according to ISO 6892-1. Ferrite content was measured according to ASTM E562. Austenitic spacing was measured according to DNV-RP-F112. These experimental results are listed in Table 2.

Table 2. Experimental Results

In Table 2, points E1 and E2 represent Example 1 and Example 2 of the present invention, while points C1 to C3 represent Comparative Examples 1 to 3.

As can be seen from the results in Table 3, the solution annealed articles made of the duplex stainless steel of the present invention have an advantageous microstructure with very good mechanical properties as well as corrosion properties. This means that articles made of the duplex stainless steel can withstand hydrogen intrusion and the load / stress of hydrogen formed on the steel surface due to cathode protection in seawater applications. Thus, duplex stainless steel articles have an increased lifetime since the risk of equipment damage or any serious accident due to hydrogen organic stress corrosion is minimized or completely possible.

Claims (12)

As a use of a solution-annealed article comprising duplex stainless steel having the following composition by weight (wt%):
C 0.03 or less;
Si 0.5 or less;
Mn 1.0 or less;
Ni 5.0-7.0;
Cr 22.0-26.0;
Mo 2.5-4.5;
N 0.1-0.2;
P 0.03 or less;
S 0.03 or less;
Cu 0.3 or less;
Al 0.10 or less;
The balance is Fe and inevitable impurities,
The duplex stainless steel satisfies the formula Cr + 50N ≦ 35; Also
Wherein said duplex stainless steel has a ferrite phase content of 40% by volume to 60% by volume and an austenite phase content of 40% by volume to 60% by volume in seawater applications. The method of claim 1,
Use of an analyzed annealed article comprising duplex stainless steel having a Cr content of 23.0 to 24.0 wt%. The method according to claim 1 or 2,
The use of a solution melted annealed article comprising duplex stainless steel with a Ni content of 6.0-7.0 wt%. The method according to any one of claims 1 to 3,
Use of an analyzed annealed article comprising duplex stainless steel having an N content of 0.12 to 0.20 wt%. The method according to any one of claims 1 to 4,
Use of an analyzed annealed article comprising duplex stainless steel having a Mo content of 2.8 to 4.0 wt%. The method according to any one of claims 1 to 5,
Use of a solution melted annealed article comprising duplex stainless steel having a Cu content of 0.2 wt% or less. The method according to any one of claims 1 to 6,
Wherein said duplex stainless steel satisfies the formula Cr + 50N ≦ 34. The method according to any one of claims 1 to 7,
Wherein said duplex stainless steel satisfies the formula Cr + 50N ≦ 33. The method according to any one of claims 1 to 8,
The article of claim 1, wherein the article is a duplex stainless steel in the form of a bar, tube, seamless or welded tube, constructive part, plate, sheet, strip or wire. The method according to any one of claims 1 to 9,
The article is
a. Melting step;
b. Casting step;
c. Hot processing step;
d. Cold processing step;
e. Solvent Release Step
Manufactured by a process comprising,
And wherein said solution solution annealing step is performed at a temperature above the recrystallization temperature of said duplex stainless steel. The method of claim 10,
And wherein said solution solution annealing step is performed at a temperature of 1030 to 1150 ° C. The method according to any one of claims 1 to 11,
Use of a solution-annealed annealed article comprising duplex stainless steel, used as cathode in seawater applications.

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