US20180179612A1

US20180179612A1 - Duplex stainless steel sheet having excellent corrosion resistance and method for manufacturing the same

Info

Publication number: US20180179612A1
Application number: US15/842,756
Authority: US
Inventors: Seong In Jeong; Hyung Tae CHUNG; Suk Kyun HWANG
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-12-23
Filing date: 2017-12-14
Publication date: 2018-06-28
Also published as: CN108237209A

Abstract

Provided are a duplex stainless steel sheet having high surface quality and high corrosion resistance manufactured through a twin roll strip casting process, and a method for manufacturing the duplex stainless steel sheet.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims benefit of priority to Korean Patent Application No. 10-2016-0177539 filed on Dec. 23, 2016 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field

The present disclosure relates to a duplex stainless steel sheet having high surface quality and high corrosion resistance manufactured through a twin roll strip casting process, and a method for manufacturing the duplex stainless steel sheet.

2. Description of Related Art

Stainless steel is a steel material having chromium (Cr) in an amount of 18% or greater for high corrosion resistance, and is classified, according to chemical composition or metallurgical microstructure, as one of an austenitic stainless steel, a ferritic stainless steel, a precipitation hardening stainless steel, a martensitic stainless steel, or a duplex stainless steel.
A duplex stainless steel is a stainless steel having a structure in which austenite and ferrite are mixed. Such a duplex stainless steel has the merits of austenitic stainless steel and ferritic stainless steel and a high degree of strength. To date, various kinds of duplex stainless steels have been developed, and an example thereof is disclosed in Patent Document 1.
Duplex stainless steel has high corrosion resistance in various corrosive environments and is more resistant to corrosion than austenitic stainless steels such as American Iron and Steel Institute (AISI) 304 or 316 grade stainless steels. However, since duplex stainless steel contains large amounts of relatively expensive alloying elements such as nickel (Ni), the production cost of duplex stainless steel is high, and thus duplex stainless steel has lower price competitiveness than other steels.
In recent times, there has been increasing interest in cost-effective lean duplex stainless steel to which relatively inexpensive alloying elements are added, while minimizing the addition of relatively expensive alloying elements such as nickel (Ni), and high-nitrogen duplex stainless steel containing nitrogen in an amount of 1000 ppm (maximally 3000 ppm when the solubility of nitrogen is considered) is a typical example of lean duplex stainless steel.
It is effective to use a twin roll strip casting process to manufacture such high-nitrogen stainless steels. In such a twin roll strip casting process for directly manufacturing a strip having a thickness of 10 mm or less, a key technique is to produce a strip having no cracks and a desired thickness at a high production rate by supplying molten steel, through a casting nozzle, to a gap between internal cooling type casting rolls rotating in opposite directions at high speed.
When a twin roll strip casting process is used to manufacture such a high-nitrogen stainless steel, a casting roll surface treatment technique for discharging gas is required so as to prevent the formation of cracks, and it is necessary to properly control gas discharge conditions and casting conditions.

RELATED ART DOCUMENT

(Patent Document 1) U.S. Pat. No. 5,624,504

SUMMARY

Aspects of the present disclosure may provide a duplex stainless steel sheet having high corrosion resistance and reduced surface cracks, manufactured by a strip casting method using casting rolls having an improved surface shape, and a method of manufacturing the duplex stainless steel sheet.
The scope of the present disclosure is not limited to the above-mentioned aspects. Other aspects of the present disclosure are stated in the following description, and the aspects of the present disclosure will be clearly understood by those having ordinary skill in the art through the following description.
According to an aspect of the present disclosure, a method for manufacturing a duplex stainless steel sheet having high corrosion resistance may include: supplying molten steel having a stainless steel composition including, by wt %, carbon (C): 0.1% or less (excluding 0%), silicon (Si): 0.2% to 1.0%, manganese (Mn): 1.0% to 4.0%, chromium (Cr): 19.0% to 23.0%, nickel (Ni): 1.0% to 3.0%, nitrogen (N): 0.1% to 0.3%, molybdenum (Mo): 0.5% to 3.0%, and a balance of iron (Fe) and inevitable impurities; and casting the supplied molten steel as a strip by discharging the molten steel through a gap between a pair of rotating casting rolls while solidifying the molten steel, wherein each of the casting rolls may include fine grooves in a surface thereof, and the fine grooves may have a sum of a ridge width and a furrow width within a range of 600 μm to 1200 μm, a depth of 100 μm to 200 μm, and a ridge area fraction of 10% to 40% in a widthwise center region and 40% to 70% in a widthwise edge region.
According to another aspect of the present disclosure, a duplex stainless steel sheet having high corrosion resistance may include, by wt %, carbon (C): 0.1% or less (excluding 0%), silicon (Si): 0.2% to 1.0%, manganese (Mn): 1.0% to 4.0%, chromium (Cr): 19.0% to 23.0%, nickel (Ni): 1.0% to 3.0%, nitrogen (N): 0.1% to 0.3%, molybdenum (Mo): 0.5% to 3.0%, and a balance of iron (Fe) and inevitable impurities, wherein the duplex stainless steel sheet may have a microstructure including ferrite and austenite, wherein the duplex stainless steel sheet may have a pitting potential of 600 mV to 720 mV.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features, and advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view illustrating a twin roll strip casting method;

FIGS. 2A and 2B are views illustrating an example in which fine grooves are formed in a casting roll surface;

FIG. 3 is an image showing a crack formed in a strip surface of a comparative material in an example of the present disclosure;

FIG. 4 is an enlarged view of the crack shown in FIG. 3;

FIG. 5 is a graph illustrating a ridge area fraction in a surface of a comparative casting roll in the example of the present disclosure;

FIG. 6 is an image illustrating a surface of a strip being a comparative material in the example of the present disclosure;

FIG. 7 is a graph illustrating a ridge area fraction in a surface of an inventive casting roll in the example of the present disclosure;

FIG. 8 is an image illustrating a surface of a strip of an inventive material of an example of the present disclosure;

FIG. 9 is an image illustrating results of evaluation of corrosion resistance of an inventive material and a related art material in an example of the present disclosure; and

FIG. 10 is an image illustrating results of evaluation of pitting potentials of an inventive material and a related art material in the example of the present disclosure.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described in detail. In general, strip casting methods are classified into a single roll type casting method and a twin roll type casting method, but these types of casting method are not of interest in the present disclosure. However, a twin roll strip casting method in which molten steel supplied through a gap between two casting rolls is cast by rotation of the two casting rolls may be useful in the present disclosure.
The twin roll strip casting method will now be described with reference to FIG. 1. As illustrated in FIG. 1, molten steel is supplied to a ladle 1, and the molten steel flows to a tundish 2 through a nozzle and then to casting rolls 5 in a gap between edge dams 6 provided on both end portions of the casting rolls 5. That is, the molten steel is supplied between the casting rolls 5 through a molten steel supply nozzle 3 and starts to solidify. At this time, oxidation of the molten steel in a gap between the casting rolls 5 is prevented by protecting the surface of the molten steel with a meniscus shield 7 and supplying an appropriate gas to forma protective atmosphere. As the molten steel is discharged through a roll nip at which the two casting rolls 5 meet each other, a strip is produced, and after the strip is rolled by a rolling mill 8 while being drawn, the strip is cooled by a cooling device 9 and coiled by a coiling device 10.
First, the composition of a duplex stainless steel sheet of the present disclosure will be described in detail.
The duplex stainless steel sheet of the present disclosure may include, by wt %, carbon (C): 0.1% or less (excluding 0%), silicon (Si): 0.2% to 1.0%, manganese (Mn): 1.0% to 4.0%, chromium (Cr): 19.0% to 23.0%, nickel (Ni): 1.0% to 3.0%, nitrogen (N): 0.1% to 0.3%, molybdenum (Mo): 0.5% to 3.0%, and the balance of iron (Fe) and inevitable impurities.
Carbon (C): 0.1% or less (excluding 0%)
Carbon (C), being an austenite-forming element, is effective in increasing the strength of a material by solid-solution strengthening. If the content of carbon (C) is greater than 0.1%, carbide-forming elements such as chromium (Cr), effective in improving corrosion resistance, may easily combine with carbon (C) along austenite-ferrite boundaries, and thus the content of chromium (Cr) may be decreased along grain boundaries to cause a decrease in corrosion resistance. Therefore, preferably, the content of carbon (C) may be 0.1% or less (excluding 0%).
Silicon (Si): 0.2% to 1.0%
Silicon (Si) is added to obtain a deoxidizing effect to some degree and acts as a ferrite-forming element which concentrates in ferrite during an annealing process. Therefore, silicon (Si) may be added in an amount of 0.2% or greater to obtain a proper ferrite faction. However, if the content of silicon (Si) is greater than 1.0%, the hardness of ferrite sharply increases to cause a decrease in elongation, and thus, it may be difficult to form austenite having an effect on guaranteeing elongation. In addition, if silicon (Si) is added excessively, the fluidity of slag is low in a steel making process, and since silicon (Si) forms inclusions by combining with oxygen, corrosion resistance decreases. Therefore, preferably, the content of silicon (Si) may be adjusted to be within the range of 0.2% to 1.0%
Mn (Manganese): 1.0% to 4.0%
Manganese (Mn) is an element increasing the amount of a deoxidizer and the solid solubility of nitrogen (N), and is added as an austenite-forming element to replace relatively expensive nickel (Ni). If the content of manganese (Mn) is greater than 4.0%, it is difficult to obtain corrosion resistance as high as that of 304 steel, and manganese (Mn) combines with sulfur (S) in steel to form MnS which decreases corrosion resistance. Conversely, if the content of manganese (Mn) is less than 1.0%, it is difficult to guarantee a proper austenite fraction even though austenite-forming elements such as nickel (Ni), copper (Cu), or nitrogen (N) are adjusted, and the solid solubility of nitrogen (N) is too low to sufficiently dissolve nitrogen (N) at atmospheric pressure. Therefore, it may be preferable that the content of manganese (Mn) be within the range of 1.0% to 4.0%.
Chromium (Cr): 19.0% to 23.0%
Chromium (Cr), being an element stabilizing ferrite, together with silicon (Si), guarantees the corrosion resistance of duplex stainless steel in addition to playing a major role in forming ferrite in duplex stainless steel. If the content of chromium (Cr) increases, corrosion resistance increases. In this case, however, it is necessary to increase the content of relatively expensive nickel (Ni) or the contents of other austenite-forming elements to maintain phase fractions. Therefore, preferably, the content of chromium (Cr) may be adjusted to be within the range of 19.0% to 23.0% to obtain corrosion resistance equal to or higher than that of 304 steel while maintaining phase fractions of duplex stainless steel.
Nickel (Ni): 1.0% to 3.0%
Nickel (Ni) functions as an austenite-stabilizing element together with manganese (Mn), copper (Cu), and nitrogen (N) and plays a major role in guaranteeing the formation of austenite in duplex stainless steel. The content of relatively expensive nickel (Ni) may be maximally reduced for cost reduction, and in this case, the contents of manganese (Mn) and nitrogen (N) having a function of forming austenite may be increased to maintain balance between phase fractions in spite of a decrease in the content of nickel (Ni). However, the content of nickel (Ni) may be adjusted to be 1.0% or greater so as to suppress the formation of strain-induced martensite and sufficiently guarantee the stability of austenite. If nickel (Ni) is added in large amounts, it is difficult to maintain a proper austenite fraction because the fraction of austenite increases, and particularly, production costs increase due to relatively expensive nickel (Ni), making it difficult to guarantee competitiveness to 304 steel. Therefore, preferably, the content of nickel (Ni) may be within the range of 1.0% to 3.0%.
Nitrogen (N): 0.1% to 0.3%
Together with nickel (Ni), nitrogen (N) considerably contributes to stabilizing austenite in duplex stainless steel, and is an element concentrating in austenite during an annealing process. Therefore, if the content of nitrogen (N) is increased, corrosion resistance and strength may be concomitantly improved. However, since the solid solubility of nitrogen (N) may vary according to the content of manganese (Mn), it is necessary to adjust the content of nitrogen (N). If the content of nitrogen (N) is greater than 0.3% when the content of manganese (Mn) is within the range proposed in the present disclosure, the content of nitrogen (N) exceeds the solid solubility of nitrogen (N), and thus, surface defects may be caused because of the formation of blow holes and pin holes during a casting process. Nitrogen (N) may be added in an amount of 0.1% or greater to obtain corrosion resistance as high as that of 304 steel, and if the content of nitrogen (N) is excessively low, it is difficult to maintain proper phase fractions. Therefore, it may be preferable that the content of nitrogen (N) be within the range of 0.1% to 0.3%.
Molybdenum (Mo): 0.5% to 3.0%
Like chromium (Cr), molybdenum (Mo) stabilizes ferrite and has a strong effect on improving corrosion resistance. However, if the content of molybdenum (Mo) is excessive, a sigma phase may easily be formed during a heat treatment process, thereby decreasing corrosion resistance and impact resistance. In the present disclosure, molybdenum (Mo) helps chromium (Cr) to maintain a ferrite fraction and guarantees proper corrosion resistance, and to this end, the content of molybdenum (Mo) may preferably be adjusted to be within the range of 0.5% to 3.0%.
The other component of the duplex stainless steel sheet of the present disclosure is iron (Fe). However, impurities of raw materials or manufacturing environments may be inevitably included in the duplex stainless steel sheet, and such impurities may not be removed from the duplex stainless steel sheet. Such impurities are well-known to those of ordinary skill in manufacturing industries, and thus specific descriptions of the impurities will not be given in the present disclosure. Phosphorus (P) and sulfur (S) are typical of such impurities.
In the present disclosure, it may be preferably that the content of phosphorus (P) be 0.03% or less and the content of sulfur (S) be 0.003% or less. Phosphorus (P) may segregate along grain boundaries or phase boundaries and may thus lower corrosion resistance and toughness, and thus the content of phosphorus (P) is adjusted to be as low as possible. However, the content of phosphorus (P) may be adjusted to be 0.03% or less for the efficiency of a refining process. Sulfur (S) has a negative effect on hot workability or lowers corrosion resistance by forming MnS. Thus, the content of sulfur (S) is adjusted to be as low as possible, for example, to be 0.003% or less.
The microstructure of the duplex stainless steel sheet of the present disclosure includes ferrite and austenite, preferably, ferrite in an area fraction of 40% to 60% and austenite in an area fraction of 40% to 60%.
The duplex stainless steel sheet of the present disclosure may have a pitting potential of 600 mV to 720 mV at 30° C. and may not have surface defects caused by pin holes, thereby having high surface quality and corrosion resistance.
Next, a method of manufacturing the duplex stainless steel sheet of the present disclosure will be described in detail.
According to the present disclosure, the stainless steel sheet manufacturing method includes a process of supplying molten steel having the above-described composition between a pair of rotating casting rolls (strip casting rolls) and a process of casting the molten steel as a strip by discharging the molten steel through a gap between the pair of rotating casting rolls while solidifying the molten steel.
In this case, the casting rolls may have fine grooves formed in surfaces thereof so as to prevent the formation of cracks in the duplex stainless steel sheet of the present disclosure.
As illustrated in FIGS. 2A and 2B, in general, the fine grooves of the casting rolls may be formed by linearly continuously forming ridges and furrows in a circumferential direction. The fine grooves function as discharge paths such that gas layers formed between the molten steel and the casting rolls may be easily discharged.
Preferably, the fine grooves of the casting rolls for manufacturing the duplex stainless steel sheet of the present disclosure may have a sum of a ridge width and a furrow width within the range of 600 μm to 1200 μm and a depth within the range of 100 μm to 200 μm. If the sum of the ridge width and the furrow width of the fine grooves is less than 600 μm, the ridge width and the furrow width are too small to obtain a gas discharging effect, and thus, surface cracks and depressions may be formed. Conversely, if the sum of the ridge with and the furrow width is greater than 1200 μm, the furrow width may be excessively large, and thus surface cracks may be formed.
In addition, if the depth of the fine grooves is less than 100 μm, nitrogen gas may not be sufficiently discharged, and thus depressions may be formed. Conversely, if the depth of the fine grooves is greater than 200 μm, cracks may be formed because solidification ability largely varies between the ridges and furrows.
According to the present disclosure, preferably, a ridge area fraction of the fine grooves of each of the casting rolls may be within the range of 10% to 40% in a widthwise center region (the ridge area fraction is constant in the widthwise center region, and the widthwise center region refers to a region not including a widthwise edge region), and may be within the range of 40% to 70% in an edge region (in which the ridge area fraction varies and the solidification ability is low, and in general the edge region is a region located within 200 mm of an end of the casting roll). In addition, the ridge area fraction may continuously vary from the widthwise center region to the edge region, and in this case, the width of the transition region may preferably be 50 mm to 200 mm.
If the ridge area fraction in the center region is less than 10%, the casting rolls and solidified shells may stick together, and thus it may be difficult to continue the casting process. Conversely, if the ridge area fraction in the center region is greater than 40%, the solidification ability may not considerably vary between the center region and the edge region, and thus edge solidification delays may not be prevented.
The transition region, in which the ridge area fraction varies toward the edge region, is a region in which edge solidification delays occur, and when the ridge area fraction is 40% or greater in the edge region, the solidification ability may not be relatively low in the edge region. However, if the ridge area fraction is greater than 70% in the edge region, gas may not be sufficiently discharged, and thus depression defects may be formed.
The fine grooves may be formed in the casting rolls by various methods such as a laser machining method or an etching method, and detailed shapes and conditions of the fine grooves may be determined as described above.
After the casting process, a hot rolling process may be performed (using the rolling mill 8 illustrated in FIG. 1). The hot rolling process may be performed using inline rollers arranged successively from the casting rolls. In this case, preferably, the hot rolling process may be performed at a reduction ratio of 20% to 50%. If the reduction ratio is less than 20%, pores may be formed in a central segregation region, resulting in poor quality. Conversely, if the reduction ratio is greater than 50%, it may be practically difficult to perform the hot rolling process because of limitations of rolling equipment.
After the hot rolling process, a cooling process and a coiling process may be performed (using the cooling device 9 and the coiling device 10 illustrated in FIG. 1). After the hot rolling process, the cooling process, and the coiling process, a cold rolling process and an annealing process may be performed. Preferably, the annealing process may be performed at a temperature of 1000° C. to 1250° C.
Hereinafter, the present disclosure will be described more specifically through examples. However, the following examples should be considered in a descriptive sense only, rather than for purposes of limitation. The scope of the present invention is defined by the appended claims, and modifications and variations reasonably made therefrom.

Example 1

Preparation of Casting Rolls
As illustrated in FIG. 5, casting rolls (comparative casting rolls) having fine grooves were prepared, the fine grooves having a constant ridge area faction of about 15% in widthwise center and edge regions. In addition, as illustrated in FIG. 7, casting rolls (inventive casting rolls) having fine grooves were prepared, wherein the fine grooves had a ridge area fraction of about 15% in a widthwise center region and up to 65% in a widthwise edge region, and a transition region had a width of about 100 mm. The sum of a ridge width and a furrow width of the fine grooves was 1000 μm, and the depth of the fine grooves was 150 μm.
Strip Production and Evaluation
Molten high-nitrogen duplex stainless steel was prepared, the molten steel including, by wt %, carbon (C): 0.04%, manganese (Mn): 3%, chromium (Cr): 20%, nitrogen (N): 0.25%, nickel (Ni): 2.0%, molybdenum (Mo): 1.5%, silicon (Si): 0.5%, and the balance of iron (Fe) and inevitable impurities. In this case, the contents of phosphorus (P) and sulfur (S) were adjusted to be as low as possible to prevent segregation.
A twin roll strip casting process was performed on the molten steel using the comparative casting rolls and the inventive casting rolls. In this case, 90 tons of molten steel was cast under the following conditions: a casting width of 1300 mm, and a casting thickness of 4.0 mm. Strips produced using the comparative and inventive casting rolls are respectively referred to as a comparative material and an inventive material.
The surfaces of the comparative material and the inventive material were observed, and results thereof are respectively illustrated in FIGS. 6 and 8. As illustrated in FIG. 6, the comparative material had an edge portion solidification delay. It could be observed that since edge portions of solidified shells came off from the casting rolls, the casting rolls had a low cooling ability in edge portions to cause a high temperature in edge regions of the strip and even solidification failure.
In particular, as illustrated in FIGS. 3 and 4, it could be confirmed that the comparative material had a surface crack (please refer to FIG. 3), and when the comparative material was magnified using a microscope, the crack existed in connection with a segregate (refer to FIG. 4).
This means that a crack was formed in a solidification delay portion, and the reason for this may be a solidification ability difference between ridges and furrows of the fine grooves.
However, the inventive material illustrated in FIG. 8 had a uniform widthwise temperature and no depression. That is, the inventive material was normal.

Example 2

The corrosion resistance and pitting potential of the inventive material produced according to Example 1, and the corrosion resistance and pitting potential of a type 316L commercial cold-rolled stainless strip (hereinafter referred to as a related art material) were evaluated, and results thereof are shown in FIGS. 9 and 10.
In the above, corrosion resistance was evaluated by checking the degree of surface rust after immersion in salt water. In addition, the pitting potentials for evaluating corrosion resistance were measured by applying a current density of 10 μA/cm²at 30° C. and 35° C. and measuring potential values at which corrosion occurs. The higher pitting potential, the higher corrosion resistance.
As illustrated in FIG. 9, the inventive material had less rust than the related art material. Referring to the results shown in FIG. 10, the pitting potential of the inventive material was on the level of 600 mV to 720 mV at 30° C., but the pitting potential of the related-art material, STS 316L, was on the level of 400 mV. Therefore, it can be understood that the duplex stainless steel sheet of the present disclosure has a high pitting potential and thus high corrosion resistance.

Example 3

Duplex stainless steel sheets were produced using molten steels having the compositions shown in Table 1 below (wt %, the balance is iron (Fe) and inevitable impurities). In the case of Comparative Steel 1, a general continuous casting process was performed to produce a slab, and the slab was rolled. In the case of Comparative Steels 2 and 3, and Inventive Steels 1 to 7, a twin roll strip casting process was performed using the inventive casting rolls of Example 1 so as to produce steel sheets.
The corrosion resistance of the produced steel sheets, and the influence of nitrogen added to molten steels in amounts greater than the solid solubility of nitrogen on strips were evaluated, and results thereof are shown in Table 1.

TABLE 1

	Composition	Corrosion	Nitrogen	Pin

NO.	C	Si	Mn	Cr	Ni	Mo	N	PREN	resistance	discharge	holes

CS 1	0.05	0.55	2.8	20.3	2.5	1.55	0.23	32.3	X	X	◯
CS 2	0.047	0.54	2.83	20.4	2.6	1.56	0.33	35.4	X	X	◯
CS 3	0.047	0.54	2.83	20.4	2.6	0.46	0.09	24.6	X	◯	X
IS 1	0.045	0.58	3.02	19.63	2.55	1.6	0.272	33.1	◯	◯	X
IS 2	0.071	0.53	3.81	19.69	2.4	1.49	0.24	31.8	◯	◯	X
IS 3	0.051	0.58	3.07	20.02	2.44	1.52	0.24	32.2	◯	◯	X
IS 4	0.051	0.57	3.09	20.41	2.41	1.5	0.25	32.9	◯	◯	X
IS 5	0.02	0.51	2.63	20.53	2.52	1.52	0.22	32.1	◯	◯	X
IS 6	0.05	0.51	2.79	20.3	2.6	1.53	0.15	29.8	◯	◯	X
IS 7	0.052	0.52	2.83	20.5	2.51	1.55	0.1	28.6	◯	◯	X

CS: Comparative Steel,
IS: Inventive Steel

Although Comparative Steel 1 had a nitrogen content of 0.23 wt %, a continuous casting method of the related art was used, and thus pin holes were formed because gas was not easily discharged. Comparative Steel 2 had a high nitrogen content, and thus pin holes were formed even though a twin roll strip casting method was used. In addition, since Comparative Steel 3 had a low molybdenum (Mo) content, the corrosion resistance of Comparative Steel 3 was poor.
However, Inventive Steels 1 to 7 satisfying the conditions of the present disclosure did not have pin holes because nitrogen was smoothly discharged, and had an PREN index (corrosion resistance index, =Cr+3.3Mo+30N, where each element symbol refers to the content of each element in wt %) within the range of 25 or greater, that is, satisfactory corrosion resistance.

Example 4

Molten steel having the composition stated in Example 1 was prepared (molten high-nitrogen duplex stainless steel including, wt %, carbon (C): 0.04%, manganese (Mn): 3%, chromium (Cr): 20%, nitrogen (N): 0.25%, nickel (Ni): 2.0%, molybdenum (Mo): 1.5%, silicon (Si): 0.5%, and the balance of iron (Fe) and inevitable impurities), and casting rolls having surface characteristics in center and edge regions thereof as shown in Table 2 below were prepared. Then, a twin roll strip casting process was performed under the conditions of Example 1 so as to produce strips.
The surfaces of the produced strips were observed, and results thereof are shown in Table 2.

TABLE 2

	Center Region	Edge Region

Fw + Rw

Fw

Rw

D

RAF

Fw

Rw

D

RAF

Fw + Rw

Edge

No.

(μm)

(%)

(μm)

(%)

(μm)

Depression

Crack

Sticking

Bulging

CE4-1	750	600	150	80	20.0	450	400	80	50.0	800	◯	X	X	X
IE4-1	750	600	150	100	20.0	450	400	100	50.0	800	X	X	X	X
IE4-2	750	600	150	150	20.0	450	400	150	50.0	800	X	X	X	X
IE4-3	750	600	150	200	20.0	450	400	200	50.0	800	X	X	X	X
CE4-2	750	600	150	220	20.0	450	400	220	50.0	800	X	◯	X	X
CE4-3	550	400	150	150	27.3	450	400	150	50.0	800	◯	◯	x	X
IE4-4	600	450	150	150	25.0	450	400	150	50.0	800	X	X	X	X
IE4-5	833	500	333	150	40.0	450	400	150	50.0	800	X	X	X	X
IE4-6	1100	600	500	150	45.5	450	400	150	50.0	800	X	X	X	◯
IE4-7	1000	700	300	150	30.0	450	400	150	50.0	800	X	X	X	X
IE4-8	1000	800	200	150	20.0	450	400	150	50.0	800	X	X	X	X
IE4-9	1060	950	110	150	10.4	450	400	150	50.0	800	X	X	X	X
CE4-4	1050	950	100	150	9.5	450	400	150	50.0	800	X	X	◯	X
IE4-10	1200	800	400	150	33.3	450	400	150	50.0	800	X	X	X	X
CE4-5	1300	900	400	150	30.8	450	400	150	50.0	800	X	◯	X	X
CE4-6	750	600	150	150	20.0	250	300	150	54.5	550	◯	◯	X	X
IE4-11	750	600	150	150	20.0	400	400	150	50.0	800	X	X	X	X
IE4-12	750	600	150	150	20.0	500	500	150	50.0	1000	X	X	X	X
IE4-13	750	600	150	150	20.0	600	600	150	50.0	1200	X	X	X	X
CE4-7	750	600	150	150	20.0	650	600	150	48.0	1250	X	◯	x	X
IE4-14	750	600	150	150	20.0	600	400	150	40.0	1000	X	X	X	X
CE4-8	750	600	150	150	20.0	650	400	150	38.1	1050	X	X	X	X
IE4-15	750	600	150	150	20.0	400	700	150	63.6	1100	X	X	X	X
CE4-9	750	600	150	150	20.0	300	800	150	72.7	1100	◯	X	X	X
IE4-16	750	600	150	150	20.0	400	350	150	46.7	750	X	X	X	X
IE4-17	750	600	150	150	20.0	400	500	150	55.6	900	X	X	X	X
IE4-18	750	600	150	150	20.0	300	700	150	70.0	1000	X	X	X	X

CE: Comparative Example,
IE: Inventive Example,
Fw: Furrow Width,
RW: Ridge Width,
D: Depth,
RAF: Ridge Area Fraction

As shown in Table 2, casting rolls satisfying the conditions of the present disclosure did not lead to surface defects. However, comparative casting rolls not satisfying the conditions of the present disclosure resulted in problems such as surface defects or cracks, poor surface quality, or negative effects on processes.
As set forth above, the present disclosure provides a method for manufacturing a stainless steel sheet having high corrosion resistance owing to increased nitrogen (N) and molybdenum (Mo) contents. In the method, the surface shape of strip casting rolls is improved to facilitate the discharge of gas, thereby preventing edge bulging and surface cracks and improving quality and casting stability.
While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

What is claimed is:

1. A method for manufacturing a duplex stainless steel sheet having high corrosion resistance, the method comprising:

supplying molten steel having a stainless steel composition comprising, by wt %, carbon (C): 0.1% or less (excluding 0%), silicon (Si): 0.2% to 1.0%, manganese (Mn): 1.0% to 4.0%, chromium (Cr): 19.0% to 23.0%, nickel (Ni): 1.0% to 3.0%, nitrogen (N): 0.1% to 0.3%, molybdenum (Mo): 0.5% to 3.0%, and a balance of iron (Fe) and inevitable impurities; and

casting the supplied molten steel as a strip by discharging the molten steel through a gap between a pair of rotating casting rolls while solidifying the molten steel,

wherein each of the casting rolls comprises fine grooves in a surface thereof, and the fine grooves have a sum of a ridge width and a furrow width within a range of 600 μm to 1200 μm, a depth of 100 μm to 200 μm, and a ridge area fraction of 10% to 40% in a widthwise center region and 40% to 70% in a widthwise edge region.

2. The method of claim 1, wherein the ridge area fraction gradually increases from the widthwise center region to the widthwise edge region.

3. The method of claim 1, wherein a transition region between the widthwise center region to the widthwise edge region has a width of 50 mm to 200 mm.

4. The method of claim 1, wherein after the casting of the supplied molten steel, the method further comprises hot rolling at a reduction ratio of 20% to 50%.

5. The method of claim 4, wherein after the hot rolling, the method further comprises coiling, cold rolling, and annealing.

6. A duplex stainless steel sheet having high corrosion resistance, the duplex stainless steel sheet comprising, by wt %, carbon (C): 0.1% or less (excluding 0%), silicon (Si): 0.2% to 1.0%, manganese (Mn): 1.0% to 4.0%, chromium (Cr): 19.0% to 23.0%, nickel (Ni): 1.0% to 3.0%, nitrogen (N): 0.1% to 0.3%, molybdenum (Mo): 0.5% to 3.0%, and a balance of iron (Fe) and inevitable impurities,

wherein the duplex stainless steel sheet has a microstructure comprising ferrite and austenite,

wherein the duplex stainless steel sheet has a pitting potential of 600 mV to 720 mV.

7. The duplex stainless steel sheet of claim 6, wherein the duplex stainless steel sheet comprises ferrite in an area fraction of 40% to 60% and austenite in an area fraction of 40% to 60%.

8. The duplex stainless steel sheet of claim 6, wherein the duplex stainless steel sheet has the following PREN index within a range of 25 or greater,

PREN index=Cr+3.3Mo+30N where each element symbol refers to a content of each element in wt %.

9. The duplex stainless steel sheet of claim 6, wherein the duplex stainless steel sheet is manufactured by the method of claim 1.