KR20140127843A

KR20140127843A - Duplex steel with improved notch-impact strength and machinability

Info

Publication number: KR20140127843A
Application number: KR20147024454A
Authority: KR
Inventors: 프랑크 비쉬노브스키
Original assignee: 클라우스 쿤 에델스탈기에세레이 게엠베하
Priority date: 2012-02-03
Filing date: 2013-01-30
Publication date: 2014-11-04
Also published as: DE102012100908A1; CN104254627A; ES2581524T3; EP2809818A1; JP2015511272A; JP6322145B2; EP2809818B1; WO2013113718A1

Abstract

The present invention relates to a Lean-Duplex steel having improved notch impact strength and processability, wherein the duplex steel comprises C <0.070 wt%, S <1.5 wt%, Mn <1.0 wt%, Cr 21.0-23.0 1.0 to 3.0% by weight of Ni, 1.0 to 3.0% by weight of Cu, 0.10 to 0.30% by weight of N, 0.5% by weight of Mo and 0.01% by weight of C and the balance of Fe and impurities. Duplex steel according to the present invention is characterized by good weldability, good processability, good strength, good notch impact strength at low temperatures (e.g. -40 캜) without heat treatment, and is particularly suitable for pressure vessels.

Description

[0001] DUPLEX STEEL WITH IMPROVED NOTCH-IMPACT STRENGTH AND MACHINABILITY WITH IMPROVED NOTCH IMPACT STRUCTURE AND PROCESSING [0002]

The present invention relates to new duplex steels, particularly Lean-Duplex steels with improved notch impact strength and processability.

Stainless steel (Stainless steel) The steel, which is particularly important for the market, is austenitic stainless steel. These stainless steels are increasingly being replaced by duplex steels. Four main types of duplex steels are known to date. Standard-duplex, super-duplex, hyper-duplex and lean-duplex steels are known. The differences between these duplex steels are chemical composition and various mechanical and corrosion properties. Duplex steel is based on a two-phase structure consisting of approximately equal amounts of ferrite (α iron) phase and austenite (γ iron) phase. Duplex steels are characterized by a combination of properties in which the ferrite phase essentially provides high strength and resistance to SCC (Stress Corrosion Cracking) and the austenite phase is the cause of ductility and general corrosion resistance Loses. Duplex steel, one of the corrosion-resistant and acid-resistant steels, has existed for over 70 years.

Recently, alloy prices, especially nickel and molybdenum, have increased significantly. In particular, the high price of nickel was a starting point for promoting development to provide alternative alloys with the properties of stainless steel providing the same high strength properties and substantially the same corrosion properties as the significant reduction ratios of the high cost alloys of nickel and molybdenum.

One result of this development is lean duplex stainless steel (steel). Until recent years, the production of this corrosion resistant duplex steel with a low content of nickel and molybdenum alloys is too cumbersome and expensive. Due to the success of the new production method, it became possible to manufacture lean duplex stainless steel for the manufacturing industry. The resistance of lean duplex steels to stress cracking and formulations (lochkorrosion, pitting corrosion) is higher than comparable austenitic stainless steels. The steel expands less at the same thermal stress and the same thermal conductivity. In addition, a material made up of substantially the same parts (the same parts by weight) of ferrite and austenite has a basic strength twice that of austenitic steel in a welded state. This characteristic is used in the structural stream line formation of fixed members in construction technology. For example, the number of fixation points (Befestigungspunkten) is reduced, so that not only the installation is simplified but also the thermal bridges W

rmerbr

cken, thermal bridge). The toughness of lean duplex steel (Z

higkeit) and ductility.

Many forging alloys and cast alloys are disclosed in the ferritic-austenitic duplex steel field. Some suggestions in the prior art are described in detail below.

U.S. Patent No. 4,798,635 discloses a ferrite-austenitic steel alloy having high corrosion resistance and good weldability, the steel alloy consisting essentially of the following elements:

0.06% by weight or less C

1.5% by weight or less Si

2.0 wt% or less Mn

21.0 ~ 24.5 wt% Cr

2.0 to 5.5 wt% Ni

0.01 to 1.0% by weight Cu

0.05 to 0.3% by weight N

And the balance iron and normal impurities.

Where the elemental content is such that the ferrite content is between 35% and 65%.

These alloys are particularly suitable for environments where the temperature is greater than or equal to 60 占 폚 and the amount of chloride is simultaneously exposed up to 1,000 ppm and the austenite phase is stable in cold processing in the range of 10% to 30%.

This alloy was developed in the forging industry to reduce the cost of the alloy. Alloy element By saving nickel and molybdenum, duplex steel has reduced corrosion resistance but with equivalent strength. The alloy is also suitable as a cast alloy.

WO 02/27056 A1 (EP 1327008 A1) also discloses a ferrite-austenitic stainless steel having a microstructure consisting essentially of 35 to 65% by volume of ferrite and 35 to 65% by volume of austenite, Stainless steel has the following weight percentages of chemical composition:

0.005 to 0.07% by volume C,

0.1 to 2.0 vol% Si,

3 to 8% by volume Mn,

19 to 23 vol% Cr,

0.5 to 1.7 volume% Ni,

(Mo + W / 2) of at most 1.0% by volume of Mo and / or W,

Optionally up to 1.0% by volume of Cu,

0.15 to 0.30 N,

The remainder are iron and normal impurities.

Further, for each of ferrite and austenite, the following conditions apply to chromium equivalents (Cr _eq ) and nickel equivalents (Ni _eq ).

20 <Cr _eq <24.5

10 <Ni _eq ,

Where Cr _eq = Cr + 1, 5 Si + Mo + 2 Ti + 0.5 Nb and

Ni _eq = Ni + 0.5 Mn + 30 (C + N) + 0.5 (Cu + Co).

To further reduce the alloy cost of the steel, the chromium content is reduced and manganese is partially replaced instead of partially expensive nickel.

The same chemical composition as WO 02/27056 A1 for stainless steel is described in WO 2009/138570 Al (EP 2 279 276 A1), especially for cast alloys. The high manganese content described and the larger grain size (Korngr

beta e, grain size), the transition temperature is changed in the alloy and the material becomes brittle at a low use temperature.

As prior art alloys of the following composition according to EP 1867748 A1 are known:

C < 0.05 wt%

21 wt% < Cr < 25 wt%

1% by weight < Ni < 2.95% by weight,

0.16% by weight < N < 0.28% by weight,

Mn < 2.0 wt%

Mo + W / 2 < 0.5 wt%

Mo <0.45% by weight,

W < 0.15 wt%

Si < 1.4 wt%

AI < 0.05 wt%

0.11% by weight < Cu < 0.50% by weight,

S < 0.010% by weight,

P < 0.040% by weight,

B < 0.0005% by weight,

Co <0.5% by weight,

REM < 0.1% by weight,

V < 0.5 wt%

Ti < 0.1 wt%

Nb < 0.3 wt%

Mg < 0.1 wt%

And the balance Fe and impurities.

In this case, the alloy is a forged alloy containing up to 2% manganese but not containing copper.

In addition, the new alloy with material number 1.4669 was presented by Ugitech at the 8th Conference on Duplex Stainless Steel in Beaune, France, from October 13-15, 2010. However, this alloy has a manganese content of 1 to 3% by weight and thus differs from the alloy of the present invention.

It is therefore an object of the present invention to provide a ferrite material having a low content of expensive alloying elements of conventional commercially available duplex stainless steels but still having good properties, especially high strength, good corrosion resistance, processability and good casting Austenitic stainless steel. In particular, the nickel and molybdenum content in the alloy should be reduced, but at the same time the desired good properties for duplex steel have to be achieved.

According to the present invention, this object is achieved by duplex steels having improved notch impact strength and processability with or consisting of the following chemical composition:

C < 0.070 wt%

S < 1.5 wt%

Mn < 1.0 wt.%,

Cr 21.0 to 23.0 wt%

1.0 to 3.0% by weight of Ni,

1.0 to 3.0% by weight of Cu,

N 0.10 to 0.30 wt%

Mo < 0.5 wt%

C <

And the balance Fe and impurities.

Accordingly, ferrite-austenitic stainless steels, especially linseed duplex stainless steels, preferably lean duplex cast alloys, are provided, which have improved notch impact strength and processability. By selecting the alloy composition according to the present invention, in addition to high strength, an alloy having a good notch impact strength and a notch impact strength even at a low temperature (for example, -40 DEG C) is provided.

The steel alloy according to the present invention exhibits good weldability. The need for post-weld heat treatment and the method of heat treatment appears as a function of the chemical composition of the material and weld filler, the shape of the components, the wall thickness, the welding conditions, the strength characteristics, the degree of nondestructive inspection and the degree necessary to adhere to the additional conditions.

In addition, steel (steel) according to the present invention provides good corrosion resistance. The resistance equivalent (equivalent to PRE, Pitting Resistance Equivalent) to the formula, also known as "Wirksumme", is a measure of the corrosion resistance of a nickel-containing alloy to a formula or Lochfraße Spaltkorrosion It is used to evaluate. Formulas (pitting corrosion) generally refer to small areas of the surface of the metal or point-shaped corrosion sites (spots), which can extend significantly below the surface. Crevice corrosion is a localized accelerated corrosion, and is defined as the crevice area (for example, joint crack, F

gespalten) to the deposition of corrosion (deposition). To protect this form of corrosion, the ability of steel (steel) to follow various alloying elements. The PRE (Pitting Resistance Equivalent) is calculated according to the following formula:

PRE = [wt%] Cr + 3.3 [wt%] Mo + 16 [wt%] N,

Wherein the alloying element chromium, molybdenum and nitrogen are substituted into the above formula based on weight. The higher the effective amount, the greater the resistance to formal and crevice corrosion.

The chemical composition of steel (steel), in particular the chemical composition of the duplex cast alloy of the present invention, is defined by the following formula: PRE value greater than 26:

PRE = [wt%] Cr + 3.3 [wt%] Mo + 16 [wt%] N> 26

Further, the duplex stainless steel (steel) of the present invention has particularly excellent mechanical properties.

The minimum requirements for the material in the RT according to the invention are as follows:

Yield stress Dehngrenze: Rp _0.2 > 400 MPa

Tensile stress Zugfestigkeit: R _m> 600 MPa

Elongation Dehnung: A> 30%

Notch impact strength Kerbschlagarbeit: Av> 80 J

Av (-40 < 0 > C) > 27 J.

The steel according to the present invention is preferably used where duplex steel is advantageous based on its characteristics. Examples thereof are areas which can exhibit high strength, good weldability, good workability and good notch impact strength, particularly at low temperatures. By way of example only, the following are mentioned: a drum shell of a centrifugal or decanter construction (Dekanterbau, Trommelm

ntel), pressure vessels in the form of welded structures, rollers for the chemical industry and paper industry.

Each alloy element of the lean duplex steel according to the present invention is described in terms of its characteristics, significance and interaction in steel as follows:

The alloying elements are basically explained to distinguish them from carbide (carbide), austenite or ferrite, ie for what purpose they are used as alloying elements in steel. Each alloy element provides special properties to steel (steel) depending on its content. Several alloying elements can increase their effectiveness as needed, but they can have opposite effects and can have mutual influences, resulting in complex and unpredictable effects. The presence of certain alloying elements in steel (steel) only creates conditions for the desired properties, but the treatment and heat treatment actually represent the properties achieved.

Carbon C (melting point 3974 DEG C):

In steel (steel) alloys according to the present invention, carbon is an optional component. This is an element for stabilizing the austenite phase. As an alloying element, carbon lowers the melting point of steel (steel) and increases the strength of the alloy as a dissolved interstitial element. As the carbon content increases, the risk of forming M23C6 carbide (carbide) increases, resulting in ductile

t), toughness (Z

higkeit) and corrosion resistance. Thus, according to the present invention less than 0.070 wt% carbon, preferably less than 0.050 wt% carbon, more preferably less than 0.030 wt% carbon is used to improve corrosion resistance.

Silicon (silicon) Si (melting point 1410 캜):

Further, the silicon constituting the only selected component of the alloy steel of the steel alloy of the present invention is a ferrite stabilizer and acts as a deoxidizer. This is because at relatively high concentrations the brittle intermetallic phase (spr

den intermetallischen Phasen (a phase similar to the sigma phase) is promoted, which reduces the ductility of steel. Silicon increases strength and abrasion resistance and increases the flowability of molten steel, thereby reducing surface defects during cast iron manufacturing (Guéherstellung). Corrosion resistance (Zunderbest

ndigkeit), increases acid resistance and corrosion resistance. Therefore, according to the present invention, silicon is used in an amount of less than 1.5% by weight, preferably less than 1.0% by weight, more preferably less than 0.50% by weight, in order to improve toughness.

Manganese Mn (melting point: 1221 DEG C):

Manganese is an austenitic stabilizer. Manganese serves to increase the solubility of nitrogen, for example. Manganese is a manganese sulfide that binds sulfur and reduces the adverse effects of iron sulfide. It has deoxidizing effect in the melting of duplex stainless steel (steel) and improves the hot workability of steel. Therefore, manganese has an advantageous effect on forging and weldability. The yield strength, strength and abrasion resistance are increased by the addition of manganese. Manganese increases tensile strength and hence stress. However, large amounts of manganese impair the corrosion resistance and facilitate the formation of undesirable brittle intermetallic phases. Therefore, according to the present invention, in order to improve the toughness, the manganese content is set to less than 1.0 wt%, preferably less than 1.0 wt%, more preferably less than 0.50 wt%. Manganese may not appear completely as an optional component in the steel according to the invention.

Cr Cr (melting point 1920 ° C):

In the steel according to the present invention, chromium retains corrosion resistance and has a ferrite-austenite ratio (Ferrit-

) Is a particularly important factor in adjusting. Chromium acts as a ferrite stabilizer. At an excessively high chromium content, the formation of an intermetallic compound such as a sigma phase is increased thereby to be associated with the brittleness of the material. Therefore, in the steel according to the present invention, chromium is used in a range of 21.0 wt% to 23.0 wt%.

Nickel Ni (melting point 1455 DEG C):

Nickel is the face-centered cubic element (kubisch fl

chenzentriertes Element) and thus serves as a stabilizer (stabilizer) for the austenite in the solution treatment temperature range. Since this austenite has an increased stacking fault energy (Stapelfehler energie), it has a beneficial effect on the toughness of steel (steel). Increasing the stacking defect energy makes it difficult to mechanically and / or thermally convert the austenite into martensite, thereby improving the toughness of the steel. Higher nickel content in fixing the chromium content and molybdenum content causes an increase in austenite, thus reducing strength. Since the raw material cost of nickel is relatively high and greatly fluctuates compared to other alloying elements, other alloying elements according to the present invention are used as much as possible to replace nickel. Thus, according to the present invention, a nickel content of 1.0 wt.% To 3.0 wt.%, Preferably a nickel content of 2.0 wt.% To 3.0 wt.%, Is used.

Copper Cu (melting point 1083 캜):

Copper is also a stabilizer of the austenitic phase and has an especially beneficial effect on the corrosion resistance especially in acidic media. The solubility of copper rapidly decreases at low temperatures on the ferrite of duplex steel, so that the copper-iron phase (kupfereiche phase) is eliminated on the ferrite. As a result, the yield strength-strength ratio (Dehngrenzen-Festigkeitsverh

ltnis increases. In addition, copper fitting resistance or corrosion resistance can be reduced. Therefore, according to the present invention, a copper content of 1.0 to 3.0 wt%, preferably 1.5 to 2.5 wt%, is used. Also, copper, like nickel, has a beneficial effect on low temperature toughness.

Nitrogen N:

Nitrogen is an austenite-forming body, that is, stabilizes the austenitic structural component. Nitrogen is usually interstitial in duplex steels, and nitrogen is abundant in austenite up to 95%. This causes strong austenite lattice strain, thereby increasing the hardness of the austenitic phase and increasing the strength of the entire duplex steel. The lattice strain of the austenite leads to a decrease in toughness due to temperature reduction. The increased concentration of dissolved nitrogen increases the resistance to corrosion of fittings and crevice corrosion.

However, the undissolved nitrogen lowers the toughness by forming nitrides on the ferrite. Therefore, according to the present invention, a nitrogen content of 0.10 wt.% To 0.30 wt.%, Preferably a nitrogen content of 0.15 wt.% To 0.25 wt.%, Is used.

Molybdenum Mo (melting point: 2622 DEG C):

Molybdenum is an optional element of a duplex stainless steel alloy according to the present invention. Molybdenum serves as a stabilizer for the ferrite phase. Molybdenum is a very large atom compared to iron. As a result, the yield strength and the tensile strength are increased. With the addition of molybdenum, corrosion resistance is improved, especially in chlorine-containing media. If the molybdenum content is high, steel (steel) becomes brittle during steel making. Since molybdenum raw material costs are very high and fluctuating, only less than 0.5% by weight of molybdenum is used.

In addition to the above-mentioned elements, the steel according to the present invention preferably contains substantially no other additive components but only iron and unavoidable impurities. The inevitable impurities are sulfur (S), phosphorus (P), and the like.

Therefore, the duplex stainless steel according to the present invention is a cost effective alternative to austenitic steel, particularly in the form of a lean duplex alloy, preferably a lean duplex cast alloy, particularly at low temperatures (e.g., -40 DEG C) Strength, good processability, and good weldability without heat treatment. The duplex stainless steel according to the invention is particularly advantageous in various applications, especially in the form of cast alloys, especially where the steel according to the invention is particularly suited to the required profile.

The present invention also provides the use of duplex steel according to the present invention in areas where pressure and / or temperatures below 0 DEG C are important. Particularly preferred uses are as follows:

- centrifuges and decanter constructions, especially drum shells,

- All kinds of pressure vessels,

- Rollers in chemical industry and paper industry.

Hereinafter, the present invention will be described by way of examples illustrating the teaching of the invention, but the present invention is not limited thereto.

Preparation A melt according to the invention having the chemical composition according to the duplex stainless steel set forth in Table 1 below was prepared:

Melt C Si Mn P S Cr Ni Mo Cu N C 39895 0.030 0.40 0.27 0.017 0.0060 22.13 2.87 0.23 1.33 0.233 B 40674 0.027 0.26 0.36 0.023 0.0078 22.22 2.43 0.19 1.57 0.202 D 24640 0.033 0.34 0.46 0.018 0.0058 22.11 2.36 0.15 1.43 0.223

For the melt shown in Table 1, the mechanical properties shown in Table 2 below were measured at room temperature.

Melt Rp0.2
[MPa] RP1.0
[MPa] Rm
[MPa] A
[%] Z
[%] Av1
[J] Av2
[J] Av3
[J] Av
media[%] C 39895 438 497 663 45 59 169 170 183 174 B 40674 432 486 659 42 42 222 234 200 219 D 24640 442 494 634 42 48 158 152 145 152

The characteristic values given in Table 3 below were determined for the impact energy at low temperatures:

take
(Charge) Av (RT) Av (0) Av (-20) Av (-40) C 39895 181 161 105 56 B 40674 218 91 67 38 D 24640 152 57 31 30

The properties determined in Tables 2 and 3 confirm the advantageous characteristics of the duplex stainless steel according to the present invention.

Claims

Duplex steel characterized by having improved impact strength and processability, having or consisting of the following chemical composition:
C < 0.070 wt%
S < 1.5 wt%
Mn < 1.0 wt.%,
Cr 21.0 to 23.0 wt%
1.0 to 3.0% by weight of Ni,
1.0 to 3.0% by weight of Cu,
N 0.10 to 0.30 wt%
Mo < 0.5 wt%
C <
And the balance Fe and impurities.

The method according to claim 1,
Wherein the duplex steel comprises less than 0.050 wt% carbon.

The method according to claim 1,
Wherein the duplex steel comprises less than 0.030 wt% carbon.

10. A method according to at least one of the preceding claims,
Wherein the duplex steel comprises less than 1.0 wt% silicon.

10. A method according to at least one of the preceding claims,
Wherein the duplex steel comprises less than 0.50 wt% silicon.

10. A method according to at least one of the preceding claims,
Wherein the duplex steel comprises less than 1.0 wt% manganese.

10. A method according to at least one of the preceding claims,
Wherein the duplex steel comprises less than 0.50 wt% manganese.

10. A method according to at least one of the preceding claims,
Wherein the duplex steel comprises 21.5 wt% to 22.5 wt% chromium.

10. A method according to at least one of the preceding claims,
Wherein the duplex steel comprises 2.0 wt% to 3.0 wt% nickel.

10. A method according to at least one of the preceding claims,
Wherein the duplex steel comprises 1.5 wt% to 2.5 wt% copper.

10. A method according to at least one of the preceding claims,
Wherein the volume percentage of the ferrite phase is in the range of 35 volume% to 65 volume%, and the volume percentage of the austenite phase is in the range of 35 volume% to 65 volume%.

10. A method according to at least one of the preceding claims,
The formula effective Lochfraβ-Wirksumme PRE is defined by the following equation and is greater than 26: Duplex steel:
PRE = [wt%] Cr + 3.3 [wt%] Mo + 16 [wt%] N

10. A method according to at least one of the preceding claims,
Wherein the notch impact strength Av (room temperature) is greater than 80J.

10. A method according to at least one of the preceding claims,
Wherein the notch impact strength Av (-40 DEG C) is greater than 27J.

In the use of a duplex steel according to at least one of the preceding claims,
Lt; RTI ID = 0.0 > 0 C < / RTI > is important.

Use of a duplex steel, characterized in that, in the use of a duplex steel according to at least one of the preceding claims, it is used in the following.
- centrifuges and decanter constructions, especially drum shells,
- All kinds of pressure vessels,
- Rollers in chemical industry and paper industry.