KR20230089731A - Duplex stainless steel and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a duplex stainless steel and a manufacturing method thereof, which provide improved hole expandability. To this end, the present invention comprises, based on wt%: 0.01%-0.03% of C; 0.3%-0.5% of Si; 0.3%-1.0% of Mn; 23.0%-25.0% of Cr; 5.0%-8.0% of Ni; 3.0%-4.0% of Mo; 0.1%-1.0% of Cu; 0.01%-0.3% of W; 0.01%-0.3% of N; and the balance being Fe and other inevitable impurities. The present invention satisfies the following formula (1) and formula (2). Formula (1): -0.072 × Cr + 0.133 × Ni - 0.100 × Mo - 0.047 × W - 0.335 × N +1.320 > 0.1 Formula (2): -3 < 0.006 × Cr - 1.241 × Ni + 0.305 × Mo - 0.968 × W + 133.295 × N - 13.348 < 4 Here, C, Si, Mn, Cr, Ni, Mo, Cu, W, and N signify the content of each element (wt%).

Description

Duplex stainless steel and its manufacturing method {DUPLEX STAINLESS STEEL AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a duplex stainless steel with improved hole expansion and a method for manufacturing the same, and more particularly, to reduce the hardness difference between austenite and ferrite phases and increase the austenite fraction at the same time to prevent crack generation during hole expansion molding. It relates to suppressed duplex stainless steel and a manufacturing method thereof.

In order to supply purified water from the waterworks to each household, it is transported through waterworks pipes and stored for a long time in water purification ponds, reservoirs, and water tanks. In the past, concrete with the inside coated with epoxy was used, but there is a high possibility that the coated epoxy will come off and contaminate the water. For this reason, the concrete structure, which was widely used in reservoirs in the past, has recently been replaced by a stainless steel material with excellent corrosion resistance and hygiene, revealing problems such as water quality, leakage, and maintenance.

Duplex stainless steel, which is a mixture of ferrite phase and austenite phase in a similar ratio, is used as a material for water tanks that has replaced existing concrete materials due to its excellent corrosion resistance, high strength, and ease of manufacture. On the other hand, due to the different physical properties of the ferrite phase and the austenite phase of the duplex stainless steel material, cracks occur at the interface between strains, resulting in poor formability. Since hole expansion formability is essential for forming a water tank inlet for water tank application, hole expansion formability must be improved in order to apply a duplex stainless steel water tank.

Therefore, in order to improve the formability of the duplex stainless steel material, it is necessary to develop a new steel type that reduces the difference in mechanical properties between the ferrite phase and the austenite phase and at the same time increases the fraction of the austenite phase with relatively excellent formability.

Embodiments of the present invention reduce the difference in hardness between the phases of annealed ferrite and austenite after cold rolling and annealing at 1060 ° C, and at the same time increase the fraction of the austenite phase to improve hole expansion formability, thereby requiring corrosion resistance and high strength while inlet molding is required. It is an object of the present invention to provide a duplex stainless steel used for water tanks and the like.

Stainless steel with improved hole expansion formability according to an embodiment of the present invention, in weight %, C: 0.01% to 0.03%, Si: 0.3% to 0.5%, Mn: 0.3% to 1.0%, Cr: 23.0 % to 25.0%, Ni: 5.0% to 8.0%, Mo: 3.0% to 4.0%, Cu: 0.1% to 1.0%, W: 0.01% to 0.3%, N: 0.01% to 0.3%, and the rest It contains Fe and unavoidable impurities, and satisfies the following formulas (1) and (2).

Equation (1): -0.072×Cr + 0.133×Ni - 0.100×Mo - 0.047×W - 0.335×N +1.320 > 0.1

Equation (2): -3 < 0.006 × Cr - 1.241 × Ni + 0.305 × Mo - 0.968 × W + 133.295 × N - 13.348 < 4

Here, C, Si, Mn, Cr, Ni, Mo, Cu, W, and N mean the content (wt%) of each element.

In addition, according to one embodiment of the present invention, after cold rolling and annealing at 1060 ℃ hole expansion formability may be 55% or more.

According to an embodiment of the present invention, it is possible to provide duplex stainless steel with improved hole expansion formability by reducing the hardness difference between ferrite/austenite phases and increasing the fraction of the austenite phase after cold rolling and annealing.

Figure 1 shows the ranges of formulas (1) and formulas (2) and examples and comparative examples for each invention component range.

Preferred embodiments of the present invention are described below. However, the embodiments of the present invention can be modified in many different forms, and the technical spirit of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Terms used in this application are only used to describe specific examples. Therefore, for example, an expression in the singular number includes a plural expression unless the context clearly requires it to be singular. In addition, the terms "include" or "have" used in this application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, but other features It should be noted that it is not intended to be used to preliminarily exclude the presence of any steps, functions, components, or combinations thereof.

Meanwhile, unless defined otherwise, all terms used in this specification should be regarded as having the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Accordingly, certain terms should not be construed in an overly idealistic or formal sense unless explicitly defined herein.

In addition, "about", "substantially", etc. in this specification are used at or in the sense of a value or close to that value when manufacturing and material tolerances inherent in the stated meaning are presented, and are accurate to aid in understanding the present invention. or absolute numbers are used to prevent unfair use by unscrupulous infringers of the stated disclosure.

(ingredient)

The content of carbon (C) is 0.01% to 0.03%.

Since carbon (C) is an element that is unavoidably added during manufacturing, it may be added in an amount of 0.01% or more. However, if the content is excessive, the upper limit is limited to 0.03% because Cr carbide is formed between welds to deteriorate corrosion resistance.

The content of silicon (Si) is 0.3% to 0.5%.

Si is an element added for deoxidation in molten steel during the steel manufacturing process, and may be added in an amount of 0.3% or more. However, if the content is excessive, the quality of the material may be deteriorated after surface pickling, and edge cracks may occur during manufacturing due to an increase in inclusions, resulting in poor quality of the material. Considering this, the upper limit of the Si content may be limited to 0.5%.

The content of manganese (Mn) is 0.3% to 1.0%.

Mn is an element that improves hot workability by combining with sulfur (S) in molten steel to form MnS, and can be added in an amount of 0.3% or more. However, if the addition amount is excessive and grows coarsely with MnS, corrosion resistance is lowered, so the upper limit can be limited to 1.0%.

The content of chromium (Cr) is 23.0% to 25.0%.

Cr is an element that must be added in order to improve corrosion resistance, and 23% or more of Cr must be added in stainless steel requiring high corrosion resistance as in the present invention. However, since Cr is a strong ferrite stabilizing element, if the added amount is excessive, the ferrite/austenite phase fraction, which should be maintained at a similar level, may be broken or excessively segregated in the ferrite phase, resulting in corrosion resistance deterioration due to corrosion resistance imbalance between the ferrite/austenite phases. So, the upper limit is limited to 25.0%.

The content of nickel (Ni) is 5.0% to 8.0%.

Nickel (Ni) is a strong austenite stabilizing element and must be added in an amount of 5.0% or more to maintain the ferrite/austenite phase fraction of duplex stainless steel. However, as an expensive element, it causes an increase in raw material costs as the amount added increases, so the upper limit is limited to 8.0%.

The content of molybdenum (Mo) is 3.0% to 4.0%.

Mo is an element that is essentially added to maintain high corrosion resistance of stainless steel. In addition, since it is segregated in the ferrite phase and hardens the ferrite phase, which is relatively soft compared to the austenite phase, the hardness difference between phases can be reduced, so it can be added in an amount of 3.0% or more. However, if the addition amount is excessive, ferrite phase segregation may break the ferrite/austenite phase fraction or cause a difference in corrosion resistance between phases, so the upper limit is limited to 4.0%.

The content of copper (Cu) is 0.1% to 1.0%.

Cu is an element that is essentially added in a stainless steel manufacturing process using scrap and may be added in an amount of 0.1% or more. However, the addition amount is limited to 1.0% or less because excessive amounts can generate low-temperature liquid phase and cause edge defects during hot rolling.

The content of tungsten (W) is 0.01% to 0.3%.

W is an element that is essentially added to improve the corrosion resistance of duplex stainless steel and may be added in an amount of 0.01% or more to reduce the hardness difference between phases because it is segregated on the ferrite phase and hardens the relatively soft ferrite phase. However, when the addition amount is excessive, the ferrite/austenite phase fraction may be broken or a difference in corrosion resistance between phases may occur, so the upper limit is limited to 0.3%.

The content of nitrogen (N) is 0.01% to 0.3%.

Nitrogen (N) is a strong austenite stabilizing element and an element that can improve corrosion resistance. In order to maintain the phase fraction of duplex stainless steel, it must be added in an amount of 0.01% or more. When the added amount is excessive beyond the known solubility, internal bubbles are generated to cause defects, and the load on equipment increases by increasing the hot deformation resistance, so the upper limit is limited to 0.3%.

The content of phosphorus (P) is 0.035% or less.

Phosphorus (P) is an impurity unavoidably contained in steel, and since it is an element that causes intergranular corrosion or inhibits hot workability, it is desirable to control its content as low as possible. In the present invention, the upper limit of the P content is managed to 0.035% or less.

The content of sulfur (S) is 0.01% or less.

Sulfur (S) is an impurity inevitably contained in steel, and since it is an element that segregates at grain boundaries and is a major cause of impeding hot workability, it is desirable to control its content as low as possible. In the present invention, the upper limit of the S content is managed to 0.01% or less.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

In order to improve the hole expansion formability of duplex steel, the austenite phase fraction should be increased and the difference in hardness between the ferrite phase and the austenite phase should be reduced. Equation (1) was used as an equation representing the phase fraction difference between the austenite phase and ferrite phase, and Equation (2) was used as an equation representing the phase difference between the austenite and ferrite phases. Equation (1) must satisfy 0.1 or more to increase the austenite phase fraction, and equation (2) representing the difference in hardness between phases must satisfy the range of -3 < Equation (2) < 4 to reduce the difference in hardness.

Equation (1): -0.072×Cr + 0.133×Ni - 0.100×Mo - 0.047×W - 0.335×N +1.320 > 0.1

Equation (2): -3 < 0.006 × Cr - 1.241 × Ni + 0.305 × Mo - 0.968 × W + 133.295 × N - 13.348 < 4

It is possible to manufacture duplex stainless steel with improved hole expansion formability after cold rolling and annealing only for compositions satisfying Equations (1) and (2).

Next, a method for manufacturing a duplex stainless steel with improved hole expansion formability according to another aspect of the present invention will be described.

The present invention relates to a duplex stainless steel with improved hole expandability and a method for manufacturing the same, in which, in weight %, C: 0.01% to 0.03%, Si: 0.3% to 0.5%, Mn: 0.3% to 1.0%, Cr: 23.0 % to 25.0%, Ni: 5.0% to 8.0%, Mo: 3.0% to 4.0%, Cu: 0.1% to 1.0%, W: 0.01% to 0.3%, N: 0.01% to 0.3%, and the rest Preparing a slab containing Fe and unavoidable impurities and satisfying the following formulas (1) and (2); hot rolling the slab to a thickness of 3.5 mm; annealing the hot-rolled steel sheet at 1090° C. for 180 seconds; Cold rolling the material between annealing to 1.5 mm; and annealing the cold-rolled steel sheet at 1060° C. for 180 seconds, followed by water cooling.

Equation (1): -0.072×Cr + 0.133×Ni - 0.100×Mo - 0.047×W - 0.335×N +1.320 > 0.1

Equation (2): -3 < 0.006 × Cr - 1.241 × Ni + 0.305 × Mo - 0.968 × W + 133.295 × N - 13.348 < 4

Here, C, Si, Mn, Cr, Ni, Mo, Cu, W, and N mean the content (wt%) of each element.

The reason for the numerical limitation of the alloying element content is as described above.

The stainless steel having the above composition may be manufactured into a cast steel by continuous casting or ingot casting, and subjected to a series of hot rolling, hot rolling annealing, cold rolling, and cold rolling annealing to form a final product.

Conventionally, it was necessary to control the annealing temperature in order to control the austenite phase fraction after hot rolling during manufacturing. In the present invention, manufacturing a slab that does not cause a load in manufacturing and distribution even under existing manufacturing process conditions; hot rolling the slab; Hot-rolling and annealing the hot-rolled steel sheet; Cold rolling the annealed material; And it provides a duplex-based stainless steel with improved hole expansion even when performing the step of annealing the cold-rolled steel sheet.

The slab may be hot-rolled at a temperature of 1,100 to 1,300 ° C, which is a normal rolling temperature, and the hot-rolled steel sheet may be hot-rolled and annealed even at a temperature of 1090 ° C. At this time, the hot rolling annealing may proceed for 180 seconds. The annealed hot-rolled steel sheet may be annealed at 1060° C. for 180 seconds after cold rolling to 1.5 mm.

In this way, if the alloy components are controlled, it is possible to provide duplex stainless steel with improved hole expandability even under normal process conditions and normal hot rolling and annealing conditions without load.

The duplex stainless steel with improved hole expansion according to the present invention can be applied to areas requiring corrosion resistance strength and hole expansion formability, such as water tanks.

Hereinafter, the present invention will be described in more detail through examples.

(Example)

For the alloy example component ranges shown in Table 1 below, a slab was prepared by melting an ingot, heated at 1,250 ° C for 2 hours, and then hot rolled. After hot rolling, annealing was performed at 1,090 ° C for 180 seconds did The annealed hot-rolled steel sheet was cold-rolled to 1.5 mm, and then annealed at 1,060 ° C for 180 seconds.

The alloy composition (wt%) and the values of Equations (1) and Equations (2) for each test steel type are shown in Table 1 below.

steel grade Ingredients (% by weight) Equation (1) Equation (2) Austenitic phase fraction Ferrite phase fraction C Si Mn Cr Ni Cu Mo W N Example 1 0.026 0.45 0.82 24.5 7.06 0.18 3.06 0.09 0.18 0.12 2.88 0.56 0.44 Example 2 0.023 0.45 0.82 24.04 7.13 0.16 3.02 0.14 0.159 0.18 0.07 0.59 0.41 Example 3 0.021 0.45 0.82 24.12 7.13 0.15 3.02 0.19 0.146 0.17 1.85 0.59 0.41 Example 4 0.026 0.45 0.82 24.04 6.77 0.18 3.25 0.02 0.157 0.11 0.29 0.56 0.44 Comparative Example 1 0.015 0.496 0.741 25.66 6.88 0.211 2.99 0.025 0.157 0.03 0.08 0.52 0.48 Comparative Example 2 0.018 0.35 0.7 25 6.22 0.143 3.2 0.14 0.17 0.04 2.58 0.52 0.48 Comparative Example 3 0.02 0.45 0.08 25.92 5.51 0.15 3.2 0.05 0.161 0.19 2.36 0.60 0.40 Comparative Example 4 0.026 0.45 0.82 24.56 7.03 0.18 3.0 0.27 0.19 0.07 4.08 0.54 0.46

For the cold-rolled material prepared as in the above composition, annealing was performed at 1060 ° C. for 180 seconds. For the annealed specimen, a 130 mm × 130 mm hole expansion specimen and a 90 mm × 90 mm Ericsian specimen were prepared to perform a hole expansion test and an Erics test, and the results are shown in Table 2 below. Ericsson evaluation was determined by whether or not 10 mm or more was satisfied.

steel grade Expansion rate (%) Ericsson (mm) Example 1 59.7 11.0 Example 2 64.0 11.0 Example 3 56.8 10.8 Example 4 57.2 10.5 Comparative Example 1 53.2 10.4 Comparative Example 2 50.3 10.7 Comparative Example 3 49.8 11.0 Comparative Example 4 53.8 10.3

Referring to Table 2, Examples 1 to 4 satisfying both Equations (1) and (2) as well as component ranges satisfied not only Erics evaluation but also hole expansion evaluation of 55% or more, while Equation (1) or Comparative Examples 1 to 4 not satisfying Equation (2) confirmed that the hole expansion evaluation was less than 55%.

1 is a graph showing the correlation between Equations (1) and Equations (2) for component ranges. Examples in Group 1 and Comparative Examples in Group 2 were selected that satisfy Equations (1) and (2) in the component range, and it was confirmed that the Examples satisfied not only the Erics evaluation but also the hole expansion formability of 55% or more. .

In this way, according to the disclosed embodiment, by controlling the alloy components and the relational expression, it was possible to manufacture duplex stainless steel having a hole expansion formability of 55% or more.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those skilled in the art are within the scope of the concept and scope of the claims described below. It will be appreciated that various changes and variations are possible in

Claims (4)

In weight percent, C: 0.01% to 0.03%, Si: 0.3% to 0.5%, Mn: 0.3% to 1.0%, Cr: 23.0% to 25.0%, Ni: 5.0% to 8.0%, Mo: 3.0% to 4.0 %, Cu: 0.1% to 1.0%, W: 0.01% to 0.3%, N: 0.01% to 0.3%, the remainder including Fe and unavoidable impurities, and the following formulas (1) and (2) Satisfying duplex stainless steels:
Equation (1): -0.072×Cr + 0.133×Ni - 0.100×Mo - 0.047×W - 0.335×N +1.320 > 0.1
Equation (2): -3 < 0.006 × Cr - 1.241 × Ni + 0.305 × Mo - 0.968 × W + 133.295 × N - 13.348 < 4
Here, C, Si, Mn, Cr, Ni, Mo, Cu, W, and N mean the content (wt%) of each element. According to claim 1,
A duplex stainless steel with an austenite phase fraction higher than the ferritic phase fraction by 10% or more. According to claim 1,
Duplex stainless steel with an expansion rate of 55% or more in hole expansion evaluation (HER). C: 0.01% to 0.03%, Si: 0.3% to 0.5%, Mn: 0.3% to 1.0%, Cr: 23.0% to 25.0%, Ni: 5.0% to 8.0%, Mo: 3.0% to 4.0%, by weight % , Cu: 0.1% to 1.0%, W: 0.01% to 0.3%, N: 0.01% to 0.3%, the remainder including Fe and unavoidable impurities, satisfying the following formulas (1) and (2) Preparing a slab to;
hot rolling the slab to a thickness of 3.5 mm;
annealing the hot-rolled steel sheet at 1090° C. for 180 seconds; Cold rolling the material between annealing to 1.5 mm; and
Method for producing duplex stainless steel, comprising the step of annealing the cold-rolled steel sheet at 1060 ° C. for 180 seconds and then water-cooling:
Equation (1): -0.072×Cr + 0.133×Ni - 0.100×Mo - 0.047×W - 0.335×N +1.320 > 0.1
Equation (2): -3 < 0.006 × Cr - 1.241 × Ni + 0.305 × Mo - 0.968 × W + 133.295 × N - 13.348 < 4
Here, C, Si, Mn, Cr, Ni, Mo, Cu, W, and N mean the content (wt%) of each element.

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