KR101830526B1 - Duplex stainless steel having supper corrosion resistance and excellent surface property and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a method for manufacturing duplex stainless steel having super corrosion resistance, and duplex stainless steel having super corrosion resistance manufactured thereby. According to the present invention, with a twin roll type thin film manufacturing process, compound extraction in metal is restricted to prevent a crack and a fracture from being generated.

Description

TECHNICAL FIELD [0001] The present invention relates to a super-corrosion-resistant duplex stainless steel having excellent surface quality and a method of manufacturing the same. [0002]

The present invention relates to a super-corrosion resistant duplex stainless steel excellent in surface quality and a method of manufacturing the same.

Stainless steel is a steel with excellent corrosion resistance due to containing more than 18% of chromium (Cr). It is classified into austenitic system, ferritic system, precipitation strengthen system, martensitic system and duplex system depending on chemical composition and metallurgical structure.

Among them, duplex stainless steel is a steel in which the two phases of austenite and ferrite are metallurgically structured in a volume fraction of approximately 50:50. Duplex stainless steels generally have excellent tensile yield strengths of at least two times that of austenitic steels.

On the other hand, the grade of stainless steel can be determined by the corrosion resistance. As the content of elements such as chromium, molybdenum, tungsten, and nitrogen increases, the corrosion resistance of stainless steel improves, and accordingly, it is classified into higher grade.

The corrosion resistance according to the component concentration of the stainless steel, especially the resistance in the chlorine atmosphere, can be expressed in terms of the internal equation of equivalence (= Cr + 3.3 * (Mo + 0.5 * W) + 30 * N). Stainless steels are classified into general grade when the internal equivalent index is between 30 and 40, super grade when the grade is between 40 and 50, and super corrosion resistance grade when the grade is more than 50. Representative super corrosion resistant stainless steels include an austenitic UNS S32050 (22Cr-20Ni-6Mo-0.2N) alloy with an internal equilibrium index of 52. However, UNS S32050 is vulnerable to stress corrosion cracking due to its low strength properties.

In order to overcome such disadvantages, Patent Document 1 discloses a super-resistant duplex steel excellent in stress corrosion cracking due to a high yield strength of 200 MPa or more as compared with UNS S32050 and having a formula equivalent index of 51 (27Cr-7Ni-3.3W-2.5Mo) .

On the other hand, the greatest difficulty in the process of manufacturing a high-alloy super-corrosion resistant stainless steel is the cracking and fracture of the surface due to precipitation of an intermetallic compound during high-temperature operation. The intermetallic compound is a phase containing a large amount of chromium, molybdenum, tungsten, or the like, which is an alloy element for enhancing corrosion resistance. As the content of the alloy of the stainless steel increases, intermetallic compound precipitation becomes easy. Such an intermetallic compound causes a crack and a fracture of the material because a 1% phase fraction is precipitated when the area fraction is measured, the impact toughness is reduced by 50% as compared with a good state.

In order to solve such a problem, Patent Document 1 proposes a cooling method immediately after producing a slab having a thickness of 200 mm. In Patent Document 2, a rare earth element such as lanthanum (Ra), cerium (Ce) / RTI >

However, when the cooling method is controlled in the process of producing the slab as in Patent Document 1, although the slab has the shape, a large amount of the intermetallic compound is precipitated, so that cracks on the surface can not be prevented. That is, the slab breaking can be prevented by controlling the cooling method, but cracks on the surface portion can not be prevented. If there is a crack on the surface, it is necessary to remove all cracks from the slab since it will cause fracture in the subsequent rolling process. For this purpose, a slab polishing method is applied until a point where cracks are not observed appears from the surface where cracks are observed. However, polishing slabs with uncertain depth of cracks is economically costly.

In addition, as in Patent Document 2, the addition of a rare earth element has an advantage of suppressing the formation of an intermetallic compound to some extent. However, due to the strong oxygen affinity of the rare earth element, it reacts with oxygen to form a rare earth oxide, The molten steel supplying means is clogged when the molten steel is supplied in the process, which makes it difficult to operate the molten steel. In addition, when the rare earth element is added at a constant value or more, the effect thereof is limited because the excess amount dissolved in the solid forms an inclusion.

Therefore, development of a super-corrosion resistant duplex stainless steel excellent in surface quality that does not cause fracture and cracking, and a method for manufacturing the same are required.

Korean Patent Publication No. 10-2015-0074691 Korean Patent Publication No. 10-2003-0077239

One aspect of the present invention is to provide a super-corrosion resistant duplex stainless steel excellent in surface quality free from fractures and cracks and a method of manufacturing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

One aspect of the present invention relates to a steel sheet comprising, by weight%, C: not more than 0.05% (excluding 0%), Si: not more than 1.0% (excluding 0%), Mn: not more than 2.0% (Excluding 0%) of Cr, 24.5 to 32.5% of Cr, 2.0 to 4.0% of Mo, 2.5 to 4.5% of W, 6.0 to 10.0% of Ni, 0.25 to 0.45% of N and Fe and unavoidable impurities and,

The microstructure relates to a super-corrosion-resistant duplex stainless steel having an excellent surface quality including an area percentage of 35 to 70% of austenite, 30 to 65% of ferrite and 5% or less of an intermetallic compound.

According to another aspect of the present invention, there is provided a method of manufacturing a steel strip, comprising the steps of: injecting molten steel into a pair of casting rolls rotating in opposite directions to obtain a strip of at least 1000 캜;

Hot rolling the strip to obtain a thermal laminate;

Cooling the thermal expansion material so that the cooling rate in the temperature range of 1000 to 700 占 폚 is 9 占 폚 / s or more; And

And winding the cooled thermal expansion material on the surface of the super-corrosion-resistant duplex stainless steel.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a super-corrosion resistant duplex stainless steel excellent in surface quality free from fracture and cracking and a method of manufacturing the same.

1 is a schematic view showing a twin roll thin plate manufacturing process.
2 is a photograph of a surface crack of a conventional example manufactured by a continuous casting process.
3 is a photograph of a microstructure of a surface crack portion of a conventional example manufactured by a continuous casting process.
4 is a photograph of the microstructure of Comparative Example 1. Fig.
5 is a photograph of microstructure of Inventive Example 1. FIG.
6 is a photograph of microstructure of Inventive Example 2. FIG.
7 is a photograph of a microstructure of Inventive Example 3. FIG.
8 is a photograph of the microstructure of Inventive Example 4. FIG.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

The inventors of the present invention have recognized that there is a problem of fracture and cracking in the process of manufacturing a superalloy superalloy stainless steel, and have studied to solve this problem.

As a result, it is possible to suppress the intermetallic compound by using the twin roll thin plate manufacturing process and precisely controlling the manufacturing method, thereby solving the problem of rupture and cracking, thereby obtaining super corrosion resistant stainless steel excellent in surface quality And the present invention has been accomplished.

Superior surface quality Candle resistance Duplex  Stainless steel

Hereinafter, a super corrosion resistant duplex stainless steel having excellent surface quality according to one aspect of the present invention will be described in detail.

A super-corrosion resistant duplex stainless steel having an excellent surface quality according to one aspect of the present invention comprises, by weight%, C: not more than 0.05% (excluding 0%), Si: not more than 1.0% (excluding 0%), Mn: not more than 2.0% (Excluding 0%), Cu: not more than 2.0% (excluding 0%), Cr: 24.5 to 32.5%, Mo: 2.0 to 4.0%, W: 2.5 to 4.5%, Ni: 6.0 to 10.0% To 0.45%, the balance Fe and unavoidable impurities, and the microstructure contains 35 to 70% of austenite, 30 to 65% of ferrite and 5% or less of intermetallic compound in an area fraction.

First, the alloy composition of the present invention will be described in detail. Hereinafter, the unit of each element content is expressed by weight% unless otherwise specified.

C: 0.05% or less (excluding 0%)

Carbon (C) is an effective element for increasing the strength of a material by solid solution strengthening. When the C content is more than 0.05%, it is possible to easily combine with the carbide forming element such as Cr effective for corrosion resistance at the austenite-ferrite phase boundary, thereby reducing the Cr content around the grain boundary and reducing the corrosion resistance. Therefore, the C content is preferably 0.05% or less (excluding 0%). , More preferably 0.01 to 0.05%, and even more preferably 0.03 to 0.04%.

Si: 1.0% or less (excluding 0%)

Si is an element that acts as a ferrite stabilizing element, but it promotes the precipitation of an intermetallic compound when it is excessive. Therefore, therefore, the Si content is preferably 1.0% or less (excluding 0%). , More preferably 0.01 to 1.0%, and still more preferably 0.3 to 0.7%.

Mn: 2.0% or less (excluding 0%)

Mn acts as an austenite stabilizing element and is added to replace the expensive Ni to secure the phase fraction of the duplex stainless steel. However, if the Mn content is excessive, inclusions such as MnS are formed to lower the corrosion resistance, so that the Mn content is preferably 2.0% or less (excluding 0%). , More preferably 0.01 to 2.0%, and still more preferably 0.8 to 1.4%.

Cu: 2.0% or less (excluding 0%)

Cu, like Mn, is an austenite stabilizing element and improves the corrosion resistance in a sulfuric acid atmosphere. However, it is preferable that the Cu content is 2.0% or less (excluding 0%) because it decreases the formal resistance in the chlorine atmosphere and lowers the hot workability. , More preferably 0.01 to 2.0%, and still more preferably 0.8 to 1.4%.

Cr : 24.5 to 32.5%, Mo : 2.0 to 4.0% and W: 2.5 to 4.5%

Chromium (Cr), molybdenum (Mo), and tungsten (W) are essential elements for securing high corrosion resistance as well as for securing the ferrite phase of duplex stainless steel as a ferrite phase stabilizing element. However, when the content is increased, the corrosion resistance is increased. However, if the content of the austenite phase stabilizing elements is increased together with the austenite-ferrite phase fraction, the cost increases and the hot workability deteriorates. Therefore, Likewise, the content range is limited.

Chromium (Cr) is the most element among the elements for improving corrosion resistance of duplex stainless steel and is a basic element, but has a relatively small influence on the deterioration of hot workability. Therefore, the content range is limited to 24.5 ~ 32.5% in order to secure corrosion resistance and control the precipitate for plate production in consideration of Mo and W content.

Molybdenum (Mo) and tungsten (W) are more resistant to corrosion than Cr. W, which is twice as heavy as Mo, is known to exhibit the same effect as Mo in the improvement of corrosion resistance when calculated by atomic ratio. However, since it shows a synergistic effect when it is contained in combination with Mo rather than that contained alone, it maximizes the effect of improving corrosion resistance Complex addition is essential. The Mo content is limited to 2.0 to 4.0% and the W content is limited to 2.5 to 4.5% in consideration of the official equivalence index.

Ni : 6.0 to 10.0% and N: 0.25 to 0.45%

Nickel (Ni) and nitrogen (N) play a major role in securing the austenite phase of the duplex stainless steel as an austenite phase stabilizing element and can improve the corrosion resistance or deteriorate workability and corrosion resistance depending on the alloy composition, Considering the addition amount of Cr, Mo and W based on securing the corrosion resistance, the content range is defined as follows for each element so that the volume fraction of ferrite phase is 30 to 65%.

Nickel (Ni) is the most powerful element among the austenitic stabilizing elements, and has the advantage of not only having a main role in balancing the phase fraction of duplex stainless steel, but also improving the frontal corrosion resistance. The increase in Ni content is directly related to the increase in raw material price, so it needs to be minimized. Therefore, the range of Ni content is limited to 6.0 ~ 10.0% within the range satisfying the formula of 50 or more.

Nitrogen (N) is a useful element that can enhance not only the corrosion resistance in the chlorine atmosphere but also the strength of duplex stainless steel. However, if the N content is too high, the hot workability is reduced and the yield rate is lowered. Therefore, it is desirable to limit the N content to 0.25 ~ 0.45% considering Cr, Mo and W contents for securing the official equivalent index.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

At this time, the formula-equivalent index defined by the following formula 1 may be 50 or more.

Relative expression 1: My official equivalent index = Cr + 3.3 * (Mo + 0.5 * W) + 30 * N

(In the above-mentioned relational expression 1, the symbol of each element represents the content of each element in weight%.)

The corrosion resistance according to the component concentration of the stainless steel, in particular, the resistance to chlorine in the atmosphere, can be quantified by the above-mentioned internal equivalent index, and the internal equivalent index of 50 or more is classified into the super corrosion resistance grade.

Hereinafter, the microstructure of a super corrosion resistant duplex stainless steel having excellent surface quality according to one aspect of the present invention will be described in detail.

The microstructure of the stainless steel of the present invention contains 35 to 70% of austenite, 30 to 65% of ferrite and 5% or less of an intermetallic compound in an area fraction.

The numerical value range of the austenite and ferrite fraction corresponds to the range of general duplex stainless steel, and the microstructure of the present invention is characterized by an intermetallic compound of 5% or less.

In the present invention, the intermetallic compound means a phase containing a large amount of chromium, molybdenum, tungsten, or the like, which is an alloy element for enhancing corrosion resistance. The intermetallic compound may cause cracking and fracture of the material because a 1% phase fraction is precipitated when the area fraction is measured, the impact toughness is reduced by 50% as compared with a good state. Therefore, it is preferable that the intermetallic compound is controlled to 5% or less. On the other hand, the smaller the intermetallic compound is, the more advantageous it is to suppress the crack and the breakage, and therefore the lower limit is not particularly limited, but includes 0%.

On the other hand, the stainless steel of the present invention may have a bending repetition number of three times or more. As described above, since the intermetallic compound is suppressed to 5% or less, the number of bending cycles can be secured at least three times. When the actual bending conditions are considered, cracking and fracture are more likely to occur at less than three cycles of repeated bending.

Superior surface quality Candle resistance Duplex  Manufacturing method of stainless steel

Hereinafter, a method for manufacturing a super-corrosion resistant duplex stainless steel having excellent surface quality, which is another aspect of the present invention, will be described in detail.

A method of manufacturing a super-corrosion resistant duplex stainless steel having an excellent surface quality, which is another aspect of the present invention, includes the steps of: injecting molten steel into a pair of casting rolls rotating in opposite directions to obtain a strip of at least 1000 ° C; Hot rolling the strip to obtain a thermal laminate; Cooling the thermal expansion material so that the cooling rate in the temperature range of 1000 to 700 占 폚 is 9 占 폚 / s or more; And winding the cooled heat spreader.

The intermetallic compound differs depending on the alloy concentration of the material, but is precipitated at about 700 to 1000 ° C. Generally, stainless steel is produced by slab production by continuous casting or ingot manufacturing method, and it can not prevent precipitation of intermetallic compound because the time to stay in the 700 to 1000 ° C range is long during the cooling process. As a result, precipitation of intermetallic compounds causes cracks in the slab and the ingot.

When intermetallic compounds are precipitated, they can be removed by a heat treatment at a temperature of 1100 ° C or higher. However, considering that the slab and the ingot have a thickness of 200 mm or more, the time required to reach 1100 ° C and the long- It is difficult to prevent precipitation of intermetallic compounds because the time to stay in the 700-1000 ° C range is long in the cooling process after the heat treatment. In addition, in order to produce the strip, the slab material or the ingot material is rolled at a high temperature. Since the temperature interval during which the process proceeds and the temperature range during which the intermetallic compound precipitates are similar, there is a risk of plate breakage during operation. In addition, when the size of the slab, the ingot or the strip is large, a temperature deviation occurs in the width direction. At the temperature of 1100 ° C or more, the width central portion is sufficiently heat- As the temperature of the negative portion drops to a relatively low temperature of 1000 占 폚 or less, an intermetallic compound precipitates and cracks occur.

Therefore, in order to produce a good strip without cracking and plate breakage caused by an intermetallic compound, the inventors of the present invention have to maximize the cooling effect in order to limit the precipitation amount of the intermetallic compound to a critical value or less. For this purpose, It is desirable to utilize the process of the present invention.

Hereinafter, the production method of the present invention will be described in detail for each step.

Steps to Obtain Strip

Molten steel is injected between a pair of casting rolls rotating in the opposite direction to obtain a strip of at least 1000 ° C.

1, the molten steel received in the ladle 1 flows into the tundish 2 along the nozzle, and the molten steel introduced into the tundish 2 flows into the tundish 2 through the edge dam (6), that is, between the casting rolls (5) through the molten steel injection nozzle (3) and solidification starts. In the molten metal between the two rolls, the molten metal surface is protected with a meniscus shield (7) to prevent oxidation and an appropriate gas is injected to appropriately control the atmosphere. The strip is produced as it exits the roll nip where both rolls meet.

In the case of using a general continuous casting method, it is not easy to control the temperature within the full thickness range because the thickness of the slab is thicker than 200 mm. In the continuous casting method, although the intermetallic compound is not precipitated in the surface layer by controlling the cooling, a large amount of intermetallic compound precipitates directly under the surface layer, thereby generating a crack. As a result, the subsequent process can not proceed further, and the continuous casting method has a limitation in manufacturing a high alloy super-corrosion resistant duplex stainless steel.

On the other hand, in the case of using the twin roll type thin plate manufacturing process, it is possible to produce a thin strip, thereby facilitating temperature control within the entire thickness range, and cooling the 700-1000 ° C section can do.

At this time, it is preferable to control the thickness of the strip to be 5 mm or less.

The temperature immediately after the molten steel passes between the casting rolls to form the strip is related to the thickness of the strip, and the thicker the produced strip, the lower the temperature just below the casting roll. If the thickness of the strip exceeds 5 mm, the temperature of the strip may fall below 1000 캜 and intermetallic compounds may precipitate.

Further, the thickness of the strip is 5 mm or less so that the temperature control over the entire thickness range is easy, and the cooling rate can be rapidly cooled in the range of 700 to 1000 占 폚 while uniformly cooling the thickness in the entire thickness range.

At this time, the molten steel may have an alloy composition of the above-described stainless steel.

Heat series  Steps to Obtain

The strip is hot rolled to obtain a thermal laminate. The hot rolling may be carried out according to ordinary conditions and is not particularly limited. For example, it can be performed within 3 seconds with a reduction rate of 20 to 40%.

At this time, it may further include heating or heating the strip so that the temperature of the strip is maintained at 1000 ° C or more before the hot rolling. The temperature control may be insufficient only by controlling the thickness of the strip, or the temperature may be lowered to less than 1000 캜 before the hot rolling after the strip is produced.

Cooling and Winding  step

The thermal laminate is cooled to a cooling rate of 9 ° C / s or higher in a temperature range of 1000 to 700 ° C, and then wound. This is because the intermetallic compound precipitates at 1000 to 700 ° C so that cracks and fractures do not occur by avoiding the intermetallic compound precipitation section.

If the cooling rate in the temperature range of 1000 to 700 占 폚 is less than 9 占 폚 / s, excessive intermetallic compounds may precipitate and cause cracking and fracture.

At this time, the cooling rate can be controlled by the thickness of the heat spreader, the application of the cooling water, and the amount of the cooling water when the cooling water is applied. Even with the same coolant injection conditions, if the thickness of the heat spreader is large, the cooling rate is slowed. Conversely, if the thickness of the heat spreader is thin, the cooling rate is increased.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

Molten steel having the composition shown in the following Table 1 was manufactured using the twin roll type thin plate manufacturing process shown in FIG. 1 under the conditions shown in Table 2 below. However, in the case of the conventional example, the stainless steel was manufactured using the continuous casting process.

The ferrite and intermetallic compound fractions, the number of repetitive bending cycles and the occurrence of cracks in the stainless steels were measured and are shown in Table 2 below.

In Table 2, the starting temperature of the cooling section is measured in FIG. 1 A, and the ending temperature of the cooling section is the value measured in FIG.

The number of repetitive bending times was 20 mm and 120 mm. The specimens were sandwiched between test machines with a curvature of 3.0 mm, and how many times they could withstand when bending in the left and right directions were measured. And the average value was described. Considering the actual operating conditions, the probability of occurrence of cracks and fractures is high when the number of repeated bending is less than three.

The microstructures were polished sequentially using # 200, # 400 and # 2000 abrasives and polished with diamond particles of 9 μm and 1 μm to prepare specimens and then photographs of the specimens measured by scanning electron microscope backscattering electron images were analyzed Respectively. In Fig. 6, the light gray is ferrite, the deep gray is austenite, and the phase appearing in white is an intermetallic compound. In Table 2, the phase other than the ferrite and the intermetallic compound is an austenite.

Steel grade C Si Mn Cr Ni Cu N Mo W Relationship 1 One 0.035 0.5 1.07 27.5 6.80 1.19 0.45 2.47 3.25 53 2 0.030 0.57 1.06 27.5 6.93 1.00 0.4 2.50 3.28 54

In Table 1, the unit of each element content is% by weight. Relation 1 represents the formula equivalent exponent, Cr + 3.3 * (Mo + 0.5 * W) + 30 * N.

division Steel grade thickness
(mm) Cooling
speed
(° C / s) Cooling section (℃) Microstructure (area%) Repeated bending
Number of times crack start End Bera
It Intermetallic
compound Conventional example One 200 0.08 - - - 30 - O Comparative Example 1 One 4 8.4 1100 700 32% 5.5 One O Inventory 1 One 4 16 1100 650 39% 0.1 16.3 X Inventory 2 One 3 15 1050 680 37% 2 5.3 X Inventory 3 2 3 17 1100 620 45% 0.5 20 X Honorable 4 2 3 19 1100 530 48% 0 37 X

In the case of the conventional example, a large amount of intermetallic compound was precipitated using a general continuous casting process, and a large crack occurred on the surface as shown in FIG.

3 is a photograph of a section of a crack-generating portion in the conventional example. When the precipitation of the intermetallic compound is completed, the ferrite is transformed into an intermetallic compound while the austenite remains intact. Since the intermetallic compound is weak, cracks are likely to be generated even by a small impact. As a result, it can be confirmed that the cracks are generated along with the intermetallic compound.

In the case of Comparative Example 1, the cooling rate was less than 9 ° C / s. As shown in FIG. 4, a large amount of intermetallic compound precipitated and cracks occurred on the surface, and the number of repetitive bending cycles was less than 3 times.

On the other hand, Inventive Examples 1 to 4, which satisfy all of the conditions described in the present invention, can suppress the intermetallic compound to 5% or less as shown in FIGS. 5 to 8, have a repetitive bending number of 3 or more, And it can be seen that it did not occur.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

1: ladle 2: turn-off 3: immersion nozzle
4: Sump 5: Casting roll 6: Edge dam
7: Meniscus shield 8: Inline hot rolling 9: Cooling device
10: Winder A: Position of the temperature probe before cooling
B: Position of pre-winding temperature sensor after cooling

Claims (8)

delete delete delete Injecting molten steel into a pair of casting rolls rotating in opposite directions to obtain a strip of at least 1000 캜;
Hot rolling the strip to obtain a thermal laminate;
Cooling the thermal expansion material so that the cooling rate in the temperature range of 1000 to 700 占 폚 is 9 占 폚 / s or more; And
(Excluding 0%), Si: not more than 1.0% (excluding 0%), Mn: not more than 2.0%, and the like. (Excluding 0%), Cu: not more than 2.0% (excluding 0%), Cr: 24.5 to 32.5%, Mo: 2.0 to 4.0%, W: 2.5 to 4.5%, Ni: 6.0 to 10.0% ~ 0.45%, balance Fe and unavoidable impurities. ≪ RTI ID = 0.0 > 21. < / RTI >
5. The method of claim 4,
The step of obtaining the strip is performed such that the thickness of the strip is 5 mm or less
A method of manufacturing a super corrosion resistant duplex stainless steel having excellent surface quality.
5. The method of claim 4,
Heating or stripping the strip to maintain the temperature of the strip at 1000 ° C or higher before the hot rolling.
delete 5. The method of claim 4,
Wherein the molten steel has an internal formula weight index of 50 or more, which is defined by the following relational expression (1): " (1) "
Relative expression 1: My official equivalent index = Cr + 3.3 * (Mo + 0.5 * W) + 30 * N
(In the above-mentioned relational expression 1, the symbol of each element represents the content of each element in weight%.)

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