KR101668534B1 - Super duplex stainless steel and manufacturing method thereof - Google Patents

Abstract

The super duplex stainless steel according to the present invention and the method for producing the super duplex stainless steel according to the present invention are improved in corrosion resistance and impact toughness. A first deoxidation process in which Si of 1 to 2 wt% of the total molten steel amount is injected and a second deoxidation process of injecting Al of 0.2 to 1 wt% of the total molten steel amount and Si of 1 wt% or less (excluding 0) A decarburization step of removing carbon contained in the molten steel; A desulfurization step of adding Ca to the molten steel to form CaO, and controlling the amount of Ca to be added so that the ratio (At%) of CaO to Al ₂ O ₃ produced during the second deoxidation is 1: 0.8 to 1.2; A continuous casting step of making the molten steel as a slab; And a hot rolling step of making the slab as a thick plate; .

Description

TECHNICAL FIELD [0001] The present invention relates to a super duplex stainless steel and a method of manufacturing the same.

The present invention relates to a super duplex stainless steel and a manufacturing method thereof, and more particularly to a super duplex stainless steel improved in corrosion resistance and impact toughness and a method of manufacturing the same.

Conventional crude oil mining methods, which are classified as 'easy oil', are bound to meet the growing demand for energy. Therefore, the energy mining method is becoming more diversified, and it is seeking the answer in the deep sea and the polar coast. The demand for mining equipment, which requires corrosion resistance even under extreme conditions such as deeper and colder, is further intensified. The selection of materials for coastal industrial facilities is based on various factors such as safety, light weight, durability, cost and procurement ability, and corrosion resistance, but safety and durability are also the most important requirements.

Recently, the use of stainless steels excellent in corrosion resistance has been increasing as a material for coastal mining equipment and desalination facilities, and the use of super duplex stainless steels, which have higher strength and higher corrosion resistance than expensive super austenitic stainless steels, is being used.

When producing oil and gas, it is classified into sweet and sweet sour, depending on the amount of hydrogen sulfide present in the oil well. The sour environment has been attracting attention in the oil and gas industry because material breakage may occur due to the action of hydrogen sulfide gas (H ₂ S).

In a sour environment, the metal is brittle by hydrogen sulfide, and stress corrosion caused by the sulfide generated thereby may occur. When sulfides (S ₂ , HS, H ₂ S, etc.) are adsorbed on the metal surface, they accelerate the adsorption of hydrogen atoms on the metal. As a result, hydrogen organic rupture is induced by the adsorbed hydrogen. This causes SSCC (Sulphide Stress Corrosion Cracking ) Is corrosion. If the adsorption of hydrogen caused by hydrogen sulfide and the stress corrosion caused by chlorine ion are combined, the risk of fracture becomes higher.

On the other hand, such an offshore structure facility is also important in terms of shock characteristics at low temperatures because it uses cold water of the deep sea to cool the facility or is exposed to the polar environment of below minus 20 degrees on average.

Super duplex stainless steels containing a large amount of chromium and nickel are used in members exposed to harsh environments compared to lean or standard duplex stainless steels. Therefore, factors that impair the impact characteristics and corrosion resistance at low temperatures need to be identified and improved .

In the present invention, a correlation between purity, impact resistance and corrosion resistance of a super duplex steel, particularly corrosion resistance in a sour environment, is examined and a manufacturing method for improving the same is proposed.

SUMMARY OF THE INVENTION The present invention has been made to solve such problems, and an object of the present invention is to provide a super duplex stainless steel capable of improving corrosion resistance and low-temperature impact properties in a sour environment and a method of manufacturing the same.

According to an aspect of the present invention, there is provided a method of manufacturing a super duplex stainless steel having improved corrosion resistance and impact toughness, the method comprising: preparing a molten steel; A first deoxidation process in which Si of 1 to 2 wt% of the total molten steel amount is injected and a second deoxidation process of injecting Al of 0.2 to 1 wt% of the total molten steel amount and Si of 1 wt% or less (excluding 0) A decarburization step of removing carbon contained in the molten steel; A desulfurization step of adding Ca to the molten steel to form CaO, and controlling the amount of Ca to be added so that the ratio (At%) of CaO to Al ₂ O ₃ produced during the second deoxidation is 1: 0.8 to 1.2; A continuous casting step of making the molten steel as a slab; And a hot rolling step of making the slab as a thick plate; .

Wherein the molten steel preparation step comprises the steps of: C: 0.03% or less (excluding 0), Si: 0.8% or less (excluding 0), Mn: 1.2% or less (excluding 0), Cr: 24 to 26% To 8%, Mo: 3 to 5%, N: 0.24 to 0.32%, Cu: 0.3 to 1.2%, W: 0.3 to 1.2%, the balance being Fe and unavoidable impurities .

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In order to achieve the above object, the super duplex stainless steel according to an embodiment of the present invention is characterized in that it contains W: 0.03% or less (excluding 0), Cr: 24-26%, Ni: 6-8% (Excluding 0), Mn: not more than 1.2% (excluding 0), Cu: 0.3 to 1.2%, W: 0.3 to 1.2%, Ca: 0.002 to 3% 0.004%, the balance Fe, and inevitable impurities are contained in the base material at a ratio (At%) of 1: 0.8 to 1.2 in the CaO and Al ₂ O ₃ inclusions.

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And an impact toughness of 150 J or more at -46 캜.

The super duplex stainless steel according to the present invention and the manufacturing method thereof have the following effects.

First, it is possible to improve the corrosion resistance and impact characteristics in a low temperature and extreme environment.

Second, by reducing the melting point of the inclusions, internal defects during rolling can be reduced.

Third, a small process change can achieve a great effect.

FIG. 1 is a photograph of a microscope photographing a high temperature inclusion broken at the time of rolling,
FIG. 2 is a micrograph of a low temperature inclusion drawn upon rolling,
3 is a graph showing the melting point of each sample of the high melting point inclusion,
4 is a graph showing the melting points of low melting point inclusions for each sample,
5 is a graph comparing the impact characteristics of the high melting point inclusions and the low melting point inclusions according to the temperature,
6 is a photograph and a table showing the results of the HIC test for the high melting point inclusions and the low melting point inclusions,
7 is a view showing specimen specifications for impact characteristics test,
8 is a view showing a specimen specification for HIC test.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms as used herein include plural forms as long as the phrases do not expressly express the opposite meaning thereto. Means that a particular feature, region, integer, step, operation, element and / or component is specified, and that other specific features, regions, integers, steps, operations, elements, components, and / And the like.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

Hereinafter, a super duplex stainless steel according to a preferred embodiment of the present invention and a method of manufacturing the same will be described with reference to the accompanying drawings.

In order to improve the corrosion resistance and impact toughness, molten steel is prepared through a molten steel preparing step according to a general steel making process, and a decarburization step is performed to remove carbon from the molten steel. This decarburization step is performed through an AOD (Argon Oxygen Decarburization) or VOD (Vacuum Oxygen Decarburization) facility. In the present invention, a decarburization method using an AOD facility is used.

In the subsequent desulfurization step, the molten steel of AOD is introduced into the ladle, and at the same time, Ca is added to promote the lowering of the melting point of the inclusions through the formation of CaO. The principle that the inclusions are lowered in melting point by CaO is not clear, but as shown in Table 1, it can be seen that the melting point becomes lower as the ratio of CaO and Al ₂ O ₃ is closer to 1: 1. As will be described later, Al is an element required for the deoxidation process and does not limit the content to molten steel but remains in the steel as Al ₂ O ₃ inclusions.

CaO SiO2 Al2O3 MgO Melting point (캜) High melting point
Inclusion 14.2 0 85.9 0 1838 11.7 0 84.6 3.7 1809 9.0 0 90.9 0 1945 1.7 1.1 96.1 1.1 2023 16.5 0 76.7 6.7 1780 10.1 0 82.0 7.9 1866 8.8 0 91.20 0 1949 4.4 0 81.6 14.1 1988 9.0 0 89.9 1.1 1925 8.8 0 84.5 6.7 1871 Low melting point
Inclusion 46.4 2.7 48.1 2.8 1362 47.0 0 48.4 4.6 1335 50.9 1.5 46.3 1.3 1349 48.3 1.1 49.4 1.2 1368

Therefore, forming the ratio of CaO and Al ₂ O ₃ to 1: 0.8 to 1.2 (At%) is effective for lowering the melting point of inclusions. The molten steel thus produced is completed as a finished product through a continuous casting step in which slabs are manufactured in accordance with a general steelmaking process and a hot rolling step in which the slab is formed as a heavy plate.

The molten steel preparing step is a step of preparing a molten steel having a W content of 0.03% or less (excluding 0), a Si content of 0.8% or less (excluding 0), a Mn content of 1.2% It is preferable to produce a molten steel having a composition containing 8% of Mo, 3 to 5% of Mo, 0.24 to 0.32% of N, 0.3 to 1.2% of Cu, 0.3 to 1.2% of W, and the balance Fe and unavoidable impurities. The reason for limiting the numerical value according to each component will be described below.

C (Carbon) is an austenite stabilizing element that can stabilize like nitrogen, but it can form carbide at an intermediate temperature range of 600 ~ 900 ℃ when it is cooled at high temperature, and may adversely affect corrosion resistance and toughness of base material and weld Therefore, it is limited to 0.03% or less.

Si (silicon) is a strong ferrite stabilizing element that has an effect of twice as much as chrome, but superduplex steel has a high chromium content, so that an excessive amount of content is unnecessary. Therefore, it is limited to a maximum of 0.8% or less for the purpose of maintaining stability.

 Mn (manganese) is an element having an austenite stabilizing property similar to nickel and is an element which increases the solubility of nitrogen. However, Mn (manganese) has an effect of reducing the corrosion resistance by precipitation of MnS and the like and enhancing deformation resistance in cold working.

Cr (chromium) is a typical element that enhances corrosion resistance by the formation of a chromium passive film, and it requires a lot of chromium and nickel to form a duplex microstructure. Duplex stainless steel requires at least 20% chromium, and the inventive super duplex steel requires a content in the range of 24-26%. Considering the combination with other elements, the austenite phase fraction becomes larger at less than 24%, and the ferrite phase fraction becomes larger at more than 26%. Chromium increases oxidation resistance at elevated temperatures, but it is more difficult to remove than austenitic stainless steels when oxide scales or heat stains due to heat treatment or welding occur.

Ni (nickel) is an austenite stabilizing element. Due to the characteristics of super duplex stainless steel, the nickel content is increased for the phase fraction as the chromium content is high. The content in the super-duplex steel is limited to 6 to 8%, because the ferrite phase fraction becomes larger at less than 6%, and the austenite phase fraction becomes larger at more than 8%. The addition of nickel can delay the formation of noxious alloy phases in the austenitic stainless steels. The austenite phase close to half of the microstructure of the super duplex stainless steel plays a role of increasing the toughness relative to the ferrite phase.

Mo (molybdenum), like chromium, improves the resistance of stainless steels. By adding molybdenum when the chromium content is typically greater than 18%, resistance to formulations and crevice corrosion can be greatly increased in chloride environments. This resistive synergistic effect is about three times higher than that of chrome, but the content is limited to 3 to 5% because it increases the tendency to produce a harmful alloy phase due to the ferrite stabilizing effect of molybdenum. When the molybdenum content is less than 3%, the increase of the corrosion resistance is insignificant. When the molybdenum content exceeds 5%, a harmful alloy phase is formed and the ferrite phase fraction becomes large.

N (Nitrogen) improves the formula and crevice corrosion resistance of duplex stainless steels as an inexpensive solid solution strengthening element. If the content is less than 0.24%, the effect of suppressing the formation of intermetallic phases due to molybdenum is deteriorated and the toughness is lowered. If it exceeds 0.32%, pores due to nitrogen bubbles may occur.

The addition of Cu improves the corrosion resistance of sulfuric acid in super duplex stainless steel. It is added in the range of 0.3 ~ 1.2%. If it is less than 0.3%, there is no great effect on the sulfuric acid corrosion resistance. If it exceeds 1.2%, the ferrite increases and the austenite decreases.

W (tungsten) is a ferrite stabilizing element such as molybdenum and is limited to 0.3 to 1.2% in super duplex stainless steel. Below 0.3%, the improvement effect of the formula is insignificant. If it exceeds 1.2%, the harmful alloy phase and ferrite generation are increased.

The decarburization step includes a first deoxidation process for introducing Si of 1 to 2 wt% of the total molten steel amount, a step of introducing Si of 0.2 to 1 wt% of the total molten steel amount and Al of 1 wt% or less (excluding 0) 2 < / RTI > deacidification process.

Silicon is widely used as deoxidizer because of its excellent oxidation characteristics. Firstly, deoxidation treatment with silicon is performed, and then silicon and aluminum are added to perform complex deoxidation. In this process, inclusions of Al ₂ O ₃ are formed.

Nonmetallic inclusions are inevitably generated due to the introduction of deoxidizing agents and the introduction of refractory materials such as sulfides or oxides generated in the process of melting and deoxidizing steel. When oxygen is taken in the refining process, most nonmetallic inclusions form a slag layer and float, but it is not possible to remove 100% of nonmetallic inclusions. Although these are inevitably generated inclusions, they affect the quality of the material depending on their size, volume and composition.

In general, nonmetallic inclusions are classified as high melting point inclusions that contain MgO or Al ₂ O ₃ , which are generated at a temperature higher than 1500 ° C. based on the solidification point. High melting point inclusions are high in hardness and brittleness. When the high melting point inclusions are rolled, the length tends to increase as they are broken in the direction in which the force is applied. As the inclusions break, the width increases and the length becomes longer. On the other hand, inclusions generated below 1500 ° C are classified as low-melting inclusions and tend to increase in length without significant change in the matrix during rolling because they are relatively soft.

Fig. 1 and Fig. 2 are photographs showing changes in the rolling behavior of the high melting point inclusions and the low melting point inclusions. The high melting point inclusions 10 broken in the base material 100 and the shape of the elongated low melting point inclusions 20 can be confirmed.

When formed with high melting point inclusions, the inclusions are broken due to rolling, and accordingly, they are scattered in an irregular shape to occupy a wide volume in the length and width direction. These types of inclusions cause deterioration of impact toughness and sour corrosion resistance. As shown in FIG. 6, when a high melting point inclusion is mainly generated, it can be seen that many hydrogen induced cracking (HIC) occur. Crack length ratio, crack resistance ratio (CTR), and crack resistance ratio (CSR) are indicators of crack length, crack thickness, and crack occurrence relative to the total area of the specimen, respectively. (B) shows some puffing phenomenon. Therefore, the addition of calcium to increase the low melting point inclusions, and thereby the CaO inclusions, should be produced at an appropriate ratio with the Al ₂ O ₃ inclusions.

Comparative Examples in which high melting point inclusions are formed in Table 2 and comparison data of impact toughness and whether HIC is generated in Examples in which low melting point inclusions are formed are described. It can be seen that the contents of Al and Ca are significantly influenced by the contents of C, Si, Cr and Mo, which are the main elements.

Classification C
(%) Si
(%) S
( ppm ) Cr
(%) Mo
(%) Al
(%) Ca
( ppm ) HIC Impact toughness
-46 ° C (J) Comparative Example 1 0.024 0.3 5 24.6 3.8 0.032 14 Not occurring 105 Comparative Example 2 0.017 0.3 3 24.6 3.7 0.045 15 Not occurring 110 Comparative Example 3 0.018 0.4 4 24.8 3.6 0.039 12 Not occurring 116 Comparative Example 4 0.013 0.2 5 24.3 3.7 0.028 3 Occur 91 Comparative Example 5 0.025 0.3 37 25.2 3.2 0.035 8 Occur 115 Comparative Example 6 0.021 0.3 33 25.5 3.8 0.029 14 Not occurring 99 Comparative Example 7 0.015 0.3 29 25.2 3.8 0.031 16 Occur 108 Comparative Example 8 0.018 0.4 35 25.1 3.8 0.032 15 Occur 109 Example 1 0.01 0.3 35 25.5 3.9 0.0079 34 Not occurring 169 Example 2 0.016 0.3 22 25.6 3.9 0.0071 25 Not occurring 181 Example 3 0.015 0.3 32 25.6 3.9 0.0076 26 Not occurring 174 Example 4 0.011 0.3 37 25.6 3.9 0.0074 25 Not occurring 209 Example 5 0.013 0.3 32 25.6 3.9 0.0072 23 Not occurring 184 Example 6 0.016 0.3 29 25.1 4.0 0.0074 28 Not occurring 195

The super duplex stainless steel according to any one of claims 1 to 3, wherein Wt% is C: 0.03% or less (excluding 0), 24 to 26% of Cr, 6 to 8% of Ni, 3-5% of Mo, 0.8% or less (excluding 0), Mn: 1.2% or less (excluding 0), Cu: 0.3 ~ 1.2 %, W: 0.3 ~ 1.2%, the balance Fe and including unavoidable impurities, and, CaO and Al ₂ O ₃ inclusions therein (At%) of 1: 0.8 to 1.2 is preferable.

The detailed description such as the numerical limitation on the composition is replaced with the explanation of the method invention described above.

It is preferable that Ca contains 0.002 to 0.004%, and the impact toughness at -46 캜 is 150 J or more.

Calcium forms deoxidation and CaO inclusions and is removed by slag, but some of them remain in the base material as nonmetallic inclusions. The content of calcium should be in the range of 0.002 to 0.004% mentioned above. If the content is less than 0.002%, the effect on the formation of low melting point inclusions due to CaO formation and the low melting point of the formed high melting point inclusions is insufficient. If the content exceeds 0.004%, the content of calcium element in the base material is excessive, I can do it.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

10: High melting point inclusion
20: low melting point inclusion
30: HIC defect
100: Stainless steel base material

Claims (6)

A method of manufacturing a super duplex stainless steel having improved corrosion resistance and impact toughness,
Molten steel preparing step;
A first deoxidation process in which Si of 1 to 2 wt% of the total molten steel amount is injected and a second deoxidation process of injecting Al of 0.2 to 1 wt% of the total molten steel amount and Si of 1 wt% or less (excluding 0) A decarburization step of removing carbon contained in the molten steel;
A desulfurization step of adding Ca to the molten steel to form CaO, and controlling the amount of Ca to be added so that the ratio (At%) of CaO to Al ₂ O ₃ produced during the second deoxidation is 1: 0.8 to 1.2;
A continuous casting step of making the molten steel as a slab; And
A hot rolling step of making the slab as a thick plate; ≪ / RTI >
The method according to claim 1,
Wherein the molten steel preparation step comprises the steps of: C: 0.03% or less (excluding 0), Si: 0.8% or less (excluding 0), Mn: 1.2% or less (excluding 0), Cr: 24 to 26% To 8%, Mo: 3 to 5%, N: 0.24 to 0.32%, Cu: 0.3 to 1.2%, W: 0.3 to 1.2%, the balance being Fe and unavoidable impurities , A method of manufacturing super duplex stainless steel.
delete (Excluding 0), Cr: 24 to 26%, Ni: 6 to 8%, Mo: 3 to 5%, N: 0.24 to 0.32%, Si: not more than 0.8% ), Mn: 1.2% or less (excluding 0), Cu: 0.3 ~ 1.2 %, W: 0.3 ~ 1.2%, Ca: 0.002 ~ 0.004%, the balance Fe and unavoidable within the base material including the impurities of CaO and Al ₂ O ₃ Wherein the inclusions are formed at a ratio (At%) of 1: 0.8 to 1.2.
delete The method of claim 4,
And has an impact toughness of 150 J or more at -46 캜.

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