KR101554771B1 - Super ductile lean duplex stainless steel - Google Patents

Abstract

High-ductility lineless duplex stainless steel which can secure corrosion resistance and ductility equal to or higher than those of austenitic stainless steels is introduced.
The high ductility duplex stainless steel according to the present invention comprises 0.08% or less of C, 0.2 to 3.0% or less of Si, 2 to 4% of Mn, 19 to 23% of Cr, 0.3 to 2.5% of Ni, 0.2 to 0.3% of N, 0.5 to 2.5% of Cu, the balance of Fe and other unavoidable impurities, and the Si partition coefficient is in the range of 1.25 to 1.32.

Description

High ductile lineless duplex stainless steel. {SUPER DUCTILE LEAN DUPLEX STAINLESS STEEL}

The present invention relates to a high ductility duplex stainless steel, and more particularly, to a high ductility duplex stainless steel having an improved elongation rate by controlling the Si partition coefficient and controlling the work hardening rate with respect to the true strain rate.

In general, austenitic stainless steels having excellent processability and corrosion resistance are made of iron (Fe) as a base metal and contain chromium (Cr) and nickel (Ni) as main raw materials. Molybdenum (Mo) and copper And is being developed into a variety of steel types to meet various applications.

304 and 316 stainless steels, which are excellent in corrosion resistance and workability, contain expensive raw materials such as Ni and Mo, and 200 and 400 stainless steels have been discussed as alternatives. However, 200 and 400 stainless steels There is a problem that the formability and the corrosion resistance of the steel can not reach the stainless steel of the 300 series.

On the other hand, duplex stainless steels in which austenite phase and ferrite phase are mixed have all the advantages of austenitic and ferritic systems, and various types of duplex stainless steels have been developed to date.

United States Patent No. 5624504 (Apr. 29, 1997) discloses "high strength, high ductility duplex stainless steel and its manufacturing method ".

The present invention relates to a duplex stainless steel having a volume fraction of martensite-based structure having a particle average diameter of 10 占 퐉 of 20 to 95% and the remainder being a ferrite-based structure, Si: not more than 2.0%, Mn: not more than 4.0%, P: not more than 0.040%, S: not more than 0.010%, Ni: not more than 4.0%, Cr: not more than 10.0 to 20.0% And a balance of not more than 0.02% of O and not more than 4.0% of Cu and further not more than 0.20% of Al, not more than 3.0% of Mo, not more than 0.20% of REM, not more than 0.20% of Y, And other inevitable impurities.

United States Patent 6096441 (Aug. 1, 2000) discloses "ferrite-austenitic stainless steels with low nickel elongation and tensile elongation ".

It is preferable that iron is used as a base metal and the content of C is not more than 0.04%, that of Si is 0.4 to 1.2%, that of Mn is not more than 2 to 4.%, the content of Ni is 0.1 to 1.0%, the content of Cr is 18 to 22% , S: not more than 0.03%, P: not more than 0.1%, N: 0.1 to 0.3%, Mo: not more than 3.0%, and other unavoidable impurities, wherein austenite and ferrite are two phases, (%) + 30C (%) + 30N (%) + Cr (%) + Mo (%) + 1.5Si (%), Nieq = Ni (%) - 8.52Mi (%) - 12.51Cr (%) - IMq = 551-805 (C + N) ) - 36Ni (%) - 34.5Cu (%) - 14Mo (%) IM is in the range of 40 to 115.

On the other hand, one of the most widely used duplex stainless steels used in high corrosion environments is AL2205 (UNS S31803 or S32205) with 22% Cr, 5.5% Ni, 3% Mo and 0.16% N components.

These steels provide superior corrosion resistance in a variety of corrosive environments and have better corrosion resistance than the austenitic grades AISI 304, 316 and the like.

However, since such duplex stainless steel contains expensive elements such as Ni and Mo, not only the manufacturing cost is increased but also Ni and Mo are consumed, so that there is a disadvantage that price competitiveness with other steel types is damped.

In recent years, in order to solve such a problem, a duplex stainless steel has been proposed in which a high-priced alloy element such as Ni and Mo is excluded and a low-cost alloy element is added in place of the element, duplex stainless steels are growing in popularity.

Lin Duplex Stainless Steel is an austenitic stainless steel which has corrosion resistance comparable to 304 and 316 steels. It is economical because it has low Ni content and easy to secure high strength. It is used for industrial equipment such as desalination equipment, pulp, paper, And is attracting attention as a steel material.

Such a lean duplex steel is disclosed in Japanese Patent Laid-Open Nos. 1986-0562637 (1986.3.20.), WO1996-018751 (1996.6.20.), And WO2002-027056 (Apr. 4, 2002).

Of these, lean duplex stainless steels disclosed in Japanese Patent Laid-Open No. 1986-0562637 (March 20, 1986) correspond to S32304 (representative component 23Cr-4Ni-0.13N) standardized in ASTMA240, and WO2002-027056 (Apr. ) Corresponds to S32101 (representative component 21Cr-1.5Ni-5Mn-0.22N) which is standardized in ASTMA240.

The S32205 duplex stainless steel, which is one of the representative types of duplex stainless steels having a mixed structure of austenite phase and ferrite phase at room temperature, contains a large amount of Cr, Mo and N components in order to secure high corrosion resistance. In order to secure the phase fraction, And contains at least 5% Ni component.

In the case of the S81921 steel standardized by ASTM A240 disclosed in Korean Patent Laid-Open Publication No. 2006-0074400, the content of Ni and Mo contains 2.5 and 2.4% by weight of alloying elements, respectively, which are expensive.

These duplex stainless steels are suitable for cold workability, that is, for corrosion resistance rather than for formability. The duplex stainless steel satisfies not only the superior corrosion resistance required in a specific application but also the resistance to stress corrosion cracking (SCC) , It is possible to provide a technical solution, but the ductility, which is a process-related factor, has a limit to be applied to an industrial field requiring molding and bending, in order to heat it compared with an austenitic stainless steel.

Therefore, while eliminating these expensive elements, it is possible to reduce the manufacturing costs and secure corrosion resistance equal to or higher than that of 304, 304L and 316 steels. Especially, the processability and ductility are secured at the same level as that of 304 steel. There is a need to develop a duplex stainless steel excellent in molding processability.

On the other hand, an austenitic stainless steel having excellent formability, that is, an elongation rate, contains not less than 4% of expensive Ni and, in particular, about 2% of Mo in the case of the 316 series, There is a problem of rising manufacturing costs.

Accordingly, a two-phase structure steel in which a ferrite phase and an austenite phase coexist has been developed as a method of ensuring an elongation and corrosion resistance equivalent to those of an austenitic system while reducing the content of Ni and Mo, and Japanese Unexamined Patent Publication No. 1999-071643 (March 16, 1999) limits the Ni content to 0.1 to 1% and controls the stability index of austenite present in the two-phase structure steel to be in the range of 40 to 115, thereby obtaining an austenitic ferritic stainless steel having excellent elongation And the like.

It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention and is not to be construed as adhering to the prior art already known to those skilled in the art.

United States Patent No. 5624504 (April 29, 1997) U.S. Patent No. 6096441 (Aug. 1, 2000) Japanese Laid-Open Patent No. 1986-0562637 (March 20, 1986) WO1996-018751 (June 20, 1996) WO 2002-027056 (Apr. 4, 2002) Korean Patent Publication No. 2006-0074400 (July 30, 2006)

In order to solve such conventional problems, the present invention has been made to solve the above-mentioned problems of the prior art by providing a highly ductile duplex stainless steel The purpose is to provide.

In order to achieve the above object, the present invention provides a high ductility duplex stainless steel comprising, by weight%, 0.08% or less of C (excluding 0%), 0.2-3.0% of Si, 2-4% of Mn, To 23%, Ni: 0.3% to 2.5%, N: 0.2 to 0.3%, Cu: 0.5 to 2.5%, the balance Fe and other unavoidable impurities, and the Si partition coefficient is in the range of 1.25 to 1.32.

The Si partition coefficient is characterized by being represented by a temperature condition by the following procedure.

Si partition coefficient = 1.58-2.655 x 10 ^(-4) x T ( ^o C)
Here, T is the cold rolling annealing temperature.

And controlling the work hardening rate with respect to the true strain to control the production rate of the fired organic martensite to form a mechanical twin.

The high ductility duplex stainless steel of the present invention is characterized by an elongation of 55% or more.

The work hardening speed WR is characterized by satisfying the following expressions (1), (2) and (3).

WR? 10 ^{(2.715 - 0.50974 log (?)) -} (1)

WR? 10 ^{(3.85 + 0.85 x log (?)) -} (2)

WR? 10 ^{(3.28 + 0.4 x log (?)) -} (3)
In this case, the following formulas are satisfied at 0.12 ≤ true strain (ε) ≤ 0.25, and (2) and (3) at true strains ε 0.25.

And the true strain (?) Is controlled to be 0.12 to 0.25.

The present invention can reduce the manufacturing cost as well as reduce the manufacturing cost by controlling the alloy component content and the N component of the high-priced elements Ni, Si, Cu, and Mo due to the technical structure described above. There is an advantage that a corrosion resistance equal to or higher than the equivalent level and an elongation of 55% or more can be secured.

1 is a view showing a change in Si partition coefficient depending on an annealing temperature,
Figure 2 shows the relationship between typical nominal stress-nominal strains,
3 is a graph showing a relationship curve between the work hardening speed and the true strain rate
4 is a transmission electron microscope photograph showing a transformed structure of austenite phase during cold working.

Hereinafter, a highly ductile linseed duplex steel according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

The high ductility duplex steel of the present invention relates to a duplex stainless steel having two phases of an austenite phase and a ferrite phase and is a high alloyed steel such as nickel (Ni), molybdenum (Mo), silicon (Si) and copper The present invention relates to a lean duplex stainless steel in which the content of N is decreased and the content of N is increased to secure corrosion resistance.

The highly ductile linseed duplex steel of the present invention can maintain corrosion resistance equal to or higher than that of 304 steel, which is austenitic stainless steel, due to a high nitrogen content.

In addition, it is possible to secure an elongation more than that of an austenitic stainless steel, and an elongation ratio equal to 304 steel can be secured.

These highly ductile duplex steels have excellent elongation and can be used in alternative applications to 304 steel. They can be used in general products for molding as well as in corrosive environments. They can be used in products such as strips, bars, plates, sheets, pipes and tubes Processed and used.

The high ductility duplex steel according to the present invention comprises, by weight, 0.08% or less of C, 0.2 to 3.0% of Si, 2.0 to 4.0% of Mn, 19.0 to 23.0% of Cr, 0.3 to 2.5% of Ni, 0.2 to 0.3% of N, 0.5 to 2.5% of Cu, and the balance of Fe and other unavoidable impurities.

C is an element for forming an austenite phase, and is an effective element for increasing the strength of a material by solid solution strengthening.

However, in the case of excessive addition, it is easily bonded to a carbide forming element such as Cr effective for corrosion resistance at the ferrite-austenite phase boundary to reduce the corrosion resistance by lowering the Cr content around the grain boundary. Therefore, in order to maximize the corrosion resistance, .

Si is partially added for the deoxidizing effect, and is an element which is concentrated into ferrite upon annealing with an element for forming a ferrite phase.

Therefore, 0.2% or more is added in order to ensure proper ferrite phase fraction.

However, when 3.0% or more is added, the hardness of the ferrite phase is rapidly increased to lower the elongation, which makes it difficult to secure the austenite phase, which affects the elongation.

In addition, when the steel is over-added, the slag fluidity is lowered in the steelmaking process, and the steel is combined with oxygen to form inclusions and reduce corrosion resistance.

Therefore, the Si content is preferably limited to 0.2 to 3.0%.

N is an element contributing greatly to the stabilization of the austenite phase together with Ni in duplex stainless steel and is one of the elements which is concentrated in the austenite phase during annealing heat treatment.

Therefore, by increasing the N content, the corrosion resistance can be improved and the strength can be increased. However, since the solubility of N may vary depending on the content of Mn added, it is necessary to control the content thereof.

If the N content exceeds 0.3% in the Mn range of the present invention, blowholes and pinholes are generated during casting due to excess nitrogen solubility, thereby causing surface defects of the product have.

In order to secure corrosion resistance at the level of 304, N should be added in an amount of at least 0.2%, and if the N content is too low, it becomes difficult to secure an appropriate phase fraction.

Therefore, it is preferable to limit the N content to 0.20 to 0.30%.

Mn is an element that increases deoxidizing agent and nitrogen solubility and is added in place of expensive Ni as an austenite forming element.

If the Mn content is more than 4%, it becomes difficult to obtain the corrosion resistance at the level of 304. When Mn is added more than the above, it is effective to improve the nitrogen solubility, but MnS is formed by bonding with S in the steel, .

When the content of Mn is less than 2%, it is difficult to secure a proper austenite phase fraction even when Ni, Cu, N, etc. as the austenite forming elements are controlled, and the solubility of N added is low, Can not be obtained.

Therefore, it is preferable to limit the content of Mn to 2% to 4%.

Cr is a ferrite stabilizing element together with Si, which plays a major role in securing ferrite phase of two-phase stainless steel and is an essential element for securing corrosion resistance.

Increasing the Cr content increases the corrosion resistance, but there is a disadvantage that the content of expensive Ni and other austenite forming elements must be increased to maintain the phase fraction.

Therefore, the content of Cr is limited to 19 to 23% in order to maintain the phase fraction of the two-phase stainless steel and ensure corrosion resistance higher than 304 steel.

Ni is an austenite stabilizing element together with Mn, Cu and N, and plays a major role in securing the austenite phase of the duplex stainless steel.

In order to reduce the cost, instead of reducing the Ni content which is high in price, it is possible to increase the Mn and N, which are the other austenite phase forming elements, to sufficiently maintain the phase fraction balance by the reduction of Ni.

However, in order to secure sufficient austenite phase stability by suppressing the formation of fired organic martensite which occurs during cold working, it is required to add at least 0.3%.

When a large amount of Ni is added, the austenite fraction increases and it is difficult to secure a proper austenite fraction.

Therefore, it is preferable to limit the Ni content to 0.3% to 2.5%.

In order to reduce the cost, it is desirable to reduce the Cu content to a minimum. In order to secure sufficient austenite phase stability by suppressing the formation of fired organic martensite during cold working, it should be added at least 0.5%.

On the other hand, if the Cu content is 2.5% or more, the product is difficult to process due to hot brittleness, and the Cu content is preferably adjusted to 0.5 to 2.5%.

On the other hand, the Si partition coefficient means the concentration of Si concentrated in the ferrite with respect to the Si concentration concentrated in the austenite, and it is preferable to control the Si concentration in the range of 1.25 to 1.32.

When the Si partition coefficient is less than 1.25, securing of the ferrite phase fraction is difficult. When the Si partition coefficient is more than 1.32, the hardness of the ferrite phase is rapidly increased to lower the elongation, making it difficult to secure the austenite phase which affects the elongation.

That is, Si is a ferrite forming element, which is concentrated on the ferrite, and when it is solidified in the austenite phase, it affects the stability of the austenite phase and the stacking fault energy. As the Si content increases in the austenite phase, the amount of deformation to form fired organic martensite or twinning increases.

When the Si distribution coefficient exceeds 1.32 and the annealing temperature does not reach 1000 캜, sufficient diffusion of Si does not occur and the amount of Si concentrated in the austenite phase is insufficient, so that fired organic martensite is easily formed even at low strain, Resulting in an increase in the hardening index.

If the Si partition coefficient is less than 1.25 and the annealing temperature exceeds 1250 deg. C, diffusion of Si becomes active at high temperature, excessive Si concentration occurs, and stabilization of the austenite phase occurs. As a result, formation of fired organic martensite is suppressed A work hardening index capable of maintaining stress in the section of deformation can not be obtained.

The Si partition coefficient can be expressed by various factors such as temperature and time. The present inventors have expressed the Si partition coefficient as a temperature condition.

As shown in FIG. 1, when the alloying element distribution behavior of the highly ductile duplex steel of the present invention was investigated precisely, Ni, Cu, and Mn, which are known as austenite forming elements, It was found that the distribution coefficient changes with the cold annealing temperature.

The definition of the Si partition coefficient as a function of temperature is as follows.

1.25? Si Partition coefficient = 1.58 - 2.655 x 10 ^(-4) x T (占 폚)? 1.32

Therefore, from the relationship between the range of the Si partition coefficient and the annealing condition, that is, the temperature, the annealing conditions can be derived, and the annealing condition is 1000 to 1250 占 폚.

When such an annealing condition is satisfied, a cold-rolled annealed material having an elongation of 55% or more can be obtained as shown in the curve of the nominal stress-nominal strain shown in Fig. 2. (type 3)

On the other hand, the relationship between the work hardening speed (WR) and the true strain rate (ε) after cold-annealing the high-softening linseed duplex steel having the above-described alloy composition was examined.

 As shown in Fig. 3, the elongation of the highly ductile lean duplex steel in the hatched region is 55% or more.

The hatched region expressed by the correlation of the work hardening speed WR with respect to the true strain? Can be expressed by the following equation.

WR = 10 ^{(2.715 - 0.50974 log (?))}

WR ≥ 10 ^{(2.715 - 0.50974 log (ε))} ---------- 2

WR? 10 ^{(3.85 + 0.85 x log (?)) -}

WR? 10 ^{(3.28 + 0.4 x log (?)) -}

In the early stage of deformation, follow the formula (1) and satisfy the equations (2) and (3) at 0.12 ≤ true strain (ε) ≤ 0.25 and satisfy the equations (3) and (4) at the true strain (ε) ≥ 0.25.

As the deformation increases, the work hardening rate decreases monotonically at the initial stage of deformation. When the true deformation rate is 0.12, the work hardening rate increases monotonically according to Eq.

It can be interpreted that the work hardening rate is increased by forming a high hardness organosite martensite on austenite during cold working.

However, when firing organic martensite is formed (type 1) in the region where the true strain (epsilon) is smaller than 0.12, the fired organic martensite having a high hardness is formed at the beginning of the deformation, Elongation decreases.

Therefore, the fired organic martensite should be produced at a true strain of at least 0.12.

On the other hand, the deformation mode of the austenite phase present in the two-phase steel is influenced by the Si partition coefficient depending on the temperature, and the work hardening rate can be changed from the negative slope to the positive slope when the true strain? .

This is because the austenitic deformation mechanism existing in the two-phase steels changes to a tendency to form mechanical twins in the fired organic martensite as the stability of the austenite increases. The critical true strain (ε) at which such a mechanical twin is formed is 0.25, and at this time, the type 3 can secure an elongation of 55% or more.

When the true strain rate (ε) is 0.25 or more, no mechanical twinning is formed in the austenite phase, and deformation due to the austenitic phase transition occurs, the work hardening rate of the two- Continuously monotonically decreasing with inclination. In this case, the elongation rate of the type 2 rapidly decreases to 55% or less.

On the other hand, as shown in FIG. 2, when the formation of fired organic martensite in the austenite phase existing in the two-phase steel occurs at a strain of 0.12 or less (type 1), the elongation rate sharply decreases due to the initial rapid increase in strength.

In addition, it can be seen that, in the case of no mechanical twin formation at 0.25 or more strain rate (type 2), the elongation rate again decreases rapidly due to lack of work hardening

On the other hand, as shown in FIG. 4, it can be seen that the elongation is 55% or more due to the occurrence of proper work hardening by forming or mixing austenite phase with sintered organic martensite or mechanical twinning in the range of true strain of 0.12 to 0.25 (Type 3).

While the present invention has been particularly shown and described with reference to specific embodiments thereof, 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 by the following claims It will be apparent to those of ordinary skill in the art.

Claims (6)

Cr: 19 to 23%, Ni: 0.3 to 2.5%, N: 0.2 to 0.3%, C: 0.08% or less (excluding 0%), Si: 0.2 to 3.0% , Cu: 0.5 to 2.5%, the balance Fe and other unavoidable impurities,
The Si partition coefficient is in the range of 1.25 to 1.32,
The Si partition coefficient is expressed by the temperature condition by the following equation,
Si partition coefficient = 1.58-2.655 x 10 ^(-4) x T ( ^o C) (T is the cold rolling annealing temperature)
It is possible to adjust the work hardening rate WR with respect to the true strain epsilon within the range of the following formulas (1), (2) and (3) to control the production rate of fired organic martensite and form mechanical twinning Highly ductile lineless duplex stainless steel.
WR? 10 ^{(2.715 - 0.50974 log (?)) -} (1)
WR? 10 ^{(3.85 + 0.85 x log (?)) -} (2)
WR? 10 ^{(3.28 + 0.4 x log (?)) -} (3)
In this case, the following formulas are satisfied at 0.12 ≤ true strain (ε) ≤ 0.25, and (2) and (3) at true strains ε 0.25.
delete delete The method according to claim 1,
And an elongation of 55% or more.
delete The method according to claim 1,
Characterized in that the true strain (?) Is adjusted to 0.12 to 0.25.

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