EP2809818A1

EP2809818A1 - Duplex steel with improved notch-impact strength and machinability

Info

Publication number: EP2809818A1
Application number: EP13701640.8A
Authority: EP
Inventors: Frank WISCHNOWSKI
Original assignee: Klaus Kuhn Edelstahlgiesserei GmbH
Current assignee: Klaus Kuhn Edelstahlgiesserei GmbH
Priority date: 2012-02-03
Filing date: 2013-01-30
Publication date: 2014-12-10
Anticipated expiration: 2033-01-30
Also published as: EP2809818B1; JP2015511272A; DE102012100908A1; KR20140127843A; CN104254627A; ES2581524T3; JP6322145B2; WO2013113718A1

Abstract

The invention relates to a duplex steel having improved notch-impact strength and machinability, wherein the duplex steel has the following chemical composition or consists of the following: C < 0.070 wt%, Si < 1.5 wt%, Mn < 1.0 wt%, Cr 21.0 to 23.0 wt%, Ni 1.0 to 3.0 wt%, Cu 1.0 to 3.0 wt%, N 0.10 to 0.30 wt%, Mo < 0.5 wt% and the remainder consisting of iron and impurities. The duplex steel according to the invention is characterised by very good weldability without the need for heat treatment, good machinability, high strength, good notch-impact strength at low temperatures (e.g. - 40°C) and is particularly suitable for pressure tanks.

Description

Duplex steel with improved notched impact strength and machinability Description

The present invention relates to a new duplex steel, in particular lean duplex steel, with improved notched impact strength and machinability.

Austenitic stainless steels are of particular importance on the stainless steel market. These are increasingly displaced by duplex steels. Four main types of duplex steel are known today: standard duplex, super duplex, hyperduplex and lean duplex. The differences between these are the chemical composition as well as the different mechanical and corrosion properties. Duplex steels are based on a two-phase structure composed of approximately equal proportions of a ferrite (a-iron) phase and an austenite (y-iron) phase. The duplex steels are characterized by their combination of properties, the ferritic phase providing substantially high strength and stress corrosion cracking (SCC) resistance and the austenitic phase being responsible for ductility and general corrosion resistance. Duplex steels, which are among the stainless and acid resistant steels, have existed for more than 70 years.

In recent years, prices for alloying elements, especially nickel and molybdenum, have increased significantly. In particular, the high price of nickel has been the starting point for advancing development to provide replacement alloys with stainless steel properties which provide the same high strength properties as well as virtually the same corrosion properties with a significantly reduced alloy content of expensive nickel and molybdenum.

One result of this development is the lean duplex steel. Until a few years ago, the manufacture of this low doses corrosion-resistant duplex steel with nickel and molybdenum was too cumbersome and expensive. Due to new production processes, it was possible to enable the production of lean duplex steel for industrial production. The durability of lean duplex steel against stress cracking and pitting corrosion is higher than comparable austenitic stainless steels. At the same thermal load and thermal conductivity of the steel expands less. In addition, the material, which consists essentially of equal parts of ferrite and austenite, has a twice as high basic strength even in the welded state compared to austenitic steels. These properties can be used for the structural streamlining of fasteners in building technology. For example, it comes with less attachment points, which results in a simplification of the assembly, as well as a reduction in the number of thermal bridges in facade construction. A reduction in the carbon content during production led to better toughness and better ductility properties in lean duplex steel.

State of the art

In the field of ferritic-austenitic duplex steels, a large number of forged alloys or casting alloys have been described. In the following some suggestions from the state of the art will be explained in detail:

50, U.S. Patent 4,798,635 describes a ferritic-austenitic steel alloy having high corrosion resistance and good weldability, the steel alloy consisting essentially of the following elements:

C not more than 0.06 wt%

51 not more than 1.5% by weight

Mn not more than 2.0% by weight

Cr 21.0 to 24.5% by weight

Ni 2.0 to 5.5% by weight

Cu 0.01 to 1.0% by weight

N 0.05 to 0.3% by weight

and the balance of this composition is iron and the usual impurities. The contents of the elements are matched to one another in such a way that the ferrite content α is between 35 and 65%. The alloy is particularly suitable for environments where the alloy has temperatures above 60 ° C as well as chlorides in amounts up to 1,000 ppm is exposed simultaneously, the austenite phase is resistant to cold deformation in the range between 10 and 30%.

This alloy was developed in the forging sector to reduce alloying costs. By saving the alloying elements nickel and molybdenum, a duplex steel with comparable strength but reduced corrosion resistance was produced. The alloy is also suitable as a casting alloy.

Furthermore, WO 02/27056 A1 (EP 1 327 008 A1) deals with a ferritic-austenitic stainless steel with a microstructure consisting essentially of 35 to 65% by volume of ferrite and 35 to 65% by volume of austenite, and has a chemical composition containing by weight:

0.005 to 0.07 C,

0.1 to 2.0 Si,

3 to 8 Mn,

19 to 23 Cr,

0.5 to 1.7 Ni,

optionally Mo and / or W in a total amount of not more than 1.0 (Mo + W / 2),

optionally Cu up to a maximum of 1.0,

0.15 to 0.30 N,

the rest iron and impurities.

Furthermore, the following conditions should apply to the ferrite and austenite formers, i. for the chromium and nickel equivalents, apply:

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 alloying costs, the chromium content in this steel was further reduced and the expensive nickel partly replaced by manganese.

An identical to WO 02/27056 AI chemical composition for stainless steel has been especially as cast alloy in WO 2009/138570 AI (EP 2 279 276 A1). The described high manganese content and the larger grain size compared to the forging alloy result in this alloy in a shift of the transition temperature and embrittle the material at low application temperatures.

The prior art according to EP 1 867 748 A1 also discloses an alloy having the following composition:

C <0.05% by weight

21% by weight <Cr <25% by weight,

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% by weight,

Mo <0.45 wt.%,

W <0.15 wt.%,

Si <1.4 wt.%,

Al <0.05% by weight,

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

S <0.010 wt%,

P <0.040 wt.%,

B <0.0005% by weight,

Co <0.5 wt.%,

REM <0.1% by weight,

V <0.5% by weight,

Ti <0.1 wt.%,

Nb <0.3 wt%,

Mg <0.1 wt.%,

and the rest of iron and impurities.

This is therefore a forging alloy containing up to 2% manganese but no copper.

Furthermore, at the 8th Conference on stainless duplex steels in Beaune, France, from October 13 to 15, 2010, a new alloy with the material number 1.4669 from Ugitech was presented. However, this one has Alloy to a manganese content of 1-3% by weight and therefore also differs from the alloy according to the invention.

DESCRIPTION OF THE INVENTION: Problem, Solution, Advantages

It is therefore an object of the present invention to provide a ferritic-austenitic stainless steel which avoids the disadvantages of the prior art which has a lower proportion of costly alloying elements than conventional commercially available duplex stainless steels but still provides good properties, particularly high Strength and good corrosion resistance, good castability and processability. 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 should be achieved.

The above-described object is achieved according to the invention by a duplex steel having improved notch impact strength and machinability, the duplex steel having the following chemical composition or consisting of:

C <0.070 wt.%,

Si <1.5% by weight,

Mn <1.0 wt%,

Cr 21.0 to 23.0 wt.%,

Ni 1.0 to 3.0% by weight,

Cu 1.0 to 3.0% by weight,

N 0.10 to 0.30 wt%

Mo <0.5% by weight

and the rest of iron and impurities.

Accordingly, a ferritic-austenitic stainless steel, particularly a lean duplex steel, preferably a lean-duplex casting alloy, is provided which has improved impact strength and machinability. By the choice of the alloy composition, an alloy was made available according to the invention which, in addition to high strength, has a good notched impact strength even at low temperatures (for example -40 ° C.). Also, the steel alloy according to the invention exhibits good weldability. The necessity and type of heat treatment after welding will depend on the chemical composition of the materials and consumables, the shape of the component, the wall thickness, the welding conditions, the strength properties, the extent of non-destructive testing and, if necessary, compliance with additional conditions.

Furthermore, the steel provided according to the invention has good corrosion resistance. The Equivalent to Pitting Resistance (abbreviated as PRE: Eitting resistance equivalent), also referred to as the "effective sum", is used to estimate the corrosion resistance of a nickel-containing alloy against pitting or crevice corrosion Pitting corrosion generally refers to small appearing or punctiform corrosion sites in surfaces of Crevices are a locally accelerated corrosion and cause corrosion deposits in the area of gaps (eg joint gaps) The ability of the steel to protect against this form of corrosion hangs of different contents of the alloying elements The pitting sum is calculated according to the following formula:

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

wherein the percentage of elements chromium, molybdenum and nitrogen, by weight, is included in the formula. The higher the effective sum, the more resistant the material is to pitting or crevice corrosion.

The chemical composition of the steel, in particular the lean

Duplex cast alloy of the present invention now has a PRE value of over 26 defined by the following formula:

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

Furthermore, the duplex steel according to the invention has particularly good mechanical properties.

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

_Yield point: R _p0 , ₂ > 400 MPa Tensile strength: R _m > 600 MPa

Elongation: A> 30%

Impact work: Av> 80 y

Av (-40 ° C)> 27 years

The steel according to the invention can preferably be used where the duplex steel is advantageous due to its properties. These are, for example, areas where high strength, good weldability, good machinability, good notched impact strength, in particular also at low temperatures play a role. Only examples are: drum coats in centrifuges or decanter construction, pressure vessels, also in the form of welded construction, rolls for the chemical industry and the paper industry.

The individual alloying elements of the lean duplex steel according to the invention are explained in detail below with regard to their properties, significance and interactions in the steel:

In the case of the alloying elements it is fundamentally to be distinguished whether they are carbide, austenite or ferrite formers, ie. H. for what purpose they are added to the steel. Each alloy element gives the steel specific properties depending on its content. Multiple alloying elements may enhance the effect, but may have opposite effects and influence each other accordingly, resulting in a complex overall effect that is not readily predictable. The presence of certain alloying elements in the steel only creates the prerequisite for a desired property, but only the processing and heat treatment shows the actual characteristics achieved.

Carbon (mp 3974 ° C):

In the steel alloy of the present invention, carbon is an optional ingredient. It is an element for stabilizing the austenite phase. Carbon lowers the melting point as an alloying element in iron, and as an interstitial dissolved alloying element it increases its strength. As the carbon content increases, the formation of M ₂₃ C ₆ carbides increases, reducing ductility, toughness and corrosion resistance. Therefore, according to the invention, less than 0.070% by weight of carbon is used. less than 0.050 wt%, more preferably less than 0.030 wt%, to improve corrosion resistance.

Silicon (melting point 1410 ° C):

Silicon, which is also only an optional component of the steel alloy of the present invention, is a ferrite stabilizer and serves as a deoxidizer. It has the disadvantageous effect of accelerating the formation of brittle intermetallic phases (sigma and similar phases) at higher contents, thereby reducing the ductility of the steel. Silicon increases strength and wear resistance, increases the fluidity of molten steel, and thereby reduces surface defects in casting. At high levels of silicon, the additive increases scale resistance, acid resistance and corrosion resistance. Silicon is therefore used according to the invention in a content <1.5% by weight, preferably <1.0% by weight, more preferably less than 0.50% by weight, in order to improve the toughness.

Manganese (melting point 1221 ° C):

Manganese is an austenitic stabilizer. It serves, for example, to increase the solubility of nitrogen. Manganese binds sulfur as manganese sulfides and thereby reduces the adverse influence of iron sulfide, has a deoxidizing effect during the melting of duplex stainless steels and serves to improve the hot workability of the steels. Manganese therefore has a favorable effect on forgeability and weldability. The yield strength, the strength and the wear resistance are increased by a manganese addition. Manganese increases the tensile strength and thus the load capacity. However, a large amount of manganese impairs corrosion resistance and facilitates the formation of the brittle intermetallic phases which are undesirable. Accordingly, according to the present invention, the manganese content is limited to <1.0% by weight, more preferably less than 0.50% by weight, to improve the toughness. Manganese may also be completely absent as an optional ingredient in the steel of the present invention.

Chrome (melting point 1920 ° C): In the steel according to the invention, chromium is an essential element, in particular with regard to the maintenance of the corrosion resistance and for the adjustment of the ferrite-austenite ratio. Chromium has a ferrite-stabilizing effect. If the chromium content is too high, there is an increased formation of intermetallic compounds such as the sigma phase, which results in embrittlement of the material. Chromium is therefore used in the duplex steel of the present invention in the range of 21.0 to 23.0 weight percent.

Nickel (mp 1455 ° C):

Nickel is a cubic face centered element, and therefore acts in the

Range of the solution annealing temperature austenite-stabilizing. It has a favorable effect on the toughness of the steel as it increases the stacking fault energy of the austenite. With increasing stacking fault energy, the mechanical and / or thermal transformation of austenite into martensite is made more difficult, thereby increasing the toughness of the steel. Excessively high nickel contents at specified chromium and molybdenum contents increase the austenite content and thus reduce the strength. The raw material price of nickel is relatively high compared to the other alloying elements and varies greatly, so that according to the invention other alloying elements are used as far as possible to replace nickel. According to the invention, therefore, a nickel content of 1.0 to 3.0 wt .-%, preferably 2.0 to 3.0 wt .-% is used.

Copper (melting point 1083 ° C):

Copper is also a stabilizer of the austenite phase and also has a favorable influence on the corrosion resistance, especially in acidic media. Since the solubility of copper in the ferritic phase of the duplex steel decreases rapidly at low temperatures, a copper-rich phase precipitates in the ferrite. This increases the yield strength ratio. Furthermore, copper can reduce pitting corrosion resistance. According to the invention therefore a copper content of 1.0 to 3.0 wt .-%, preferably 1.5 to 2.5 wt .-% is used. Furthermore, copper such as nickel has a positive effect on the low temperature toughness.

Nitrogen: Nitrogen is an austenite former, ie it stabilizes the austenitic structural constituent. Nitrogen is usually interstitially dissolved in duplex steel, with 95% of the nitrogen being enriched in austenite. This leads to a strong lattice strain of the austenite and thus to a hardness increase of the austenitic phase and to an increase in strength of the duplex steel as a whole. This lattice strain of austenite leads to a reduction of toughness with decreasing temperature. With increasing contents of dissolved nitrogen, the resistance to perforation and crevice corrosion is also increased.

However, undissolved nitrogen reduces the toughness by forming nitrides in the ferritic phase. Therefore, the nitrogen content according to the invention is 0.10 to 0.30 wt .-%, preferably 0.15 to 0.25 wt .-%.

Molybdenum (m.p. 2622 ° C):

Molybdenum is an optional ingredient in the duplex steel alloy of the present invention. Molybdenum serves to stabilize the ferritic phase. Molybdenum is a very large atom compared to iron. As a dissolved substitution atom, it therefore causes the yield strength and tensile strength to increase. The addition of molybdenum also improves corrosion resistance, especially in media containing chloride. Excessive levels of molybdenum lead to embrittlement of the steel during its production. Since the raw material prices for molybdenum are very high and volatile, only a low Mo content of <0.5 wt .-% is used.

In addition to the above-mentioned elements, the steel according to the invention preferably has substantially no further added constituents but only iron and unavoidable impurities. Unavoidable impurities are, for example, sulfur, phosphorus and the like.

Thus, the duplex stainless steel of the invention is a cost effective alternative to austenitic steels, especially in the form of a lean duplex alloy, preferably lean duplex cast alloy, which has particularly good properties, such as improved impact strength, especially at low temperatures (eg, -40 ° C) C), good machinability, high strength and good weldability without the need for post heat treatment. The duplex stainless steel, particularly in the form of a cast alloy, of the present invention is particularly useful in various applications Applications advantageous where a requirement profile is present, for which the erfindungsgennäße steel is particularly suitable.

The invention also relates to the use of the duplex steel according to the invention in areas in which pressure and / or temperatures below 0 ° C are of importance. Particularly preferred uses are:

- in centrifuge and decanter construction, especially for drum shells,

- for pressure vessels of all kinds,

- for rollers in the chemical industry and the paper industry.

Hereinafter, the present invention will be explained by way of examples, which illustrate the teaching of the invention, but not to limit.

Examples:

The following melts indicated in Table 1, which have a chemical composition according to the duplex steel according to the invention, were prepared:

Table 1

For the melts given in Table 1, the following mechanical properties given in Table 2 were determined at room temperature:

Table 2

Melt Rp0.2 RP1.0 Rm A Z Avl Av2 Av3 Av

[MPa] [MPa] [MPa] {%] [%] [J] [J] [%] Medium

[96]

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 For the impact work at lower temperatures, the following characteristic values given in Table 3 were determined:

Table 3

The determined and indicated above values in Tables 2 and 3 confirm the advantageous properties of the duplex steel according to the invention.

Claims

1. Duplex steel having improved impact strength and machinability, the duplex steel having or consisting of the following chemical composition:

C <0.070 wt.%,

Si <1.5% by weight,

Mn <1.0 wt%,

Cr 21.0 to 23.0 wt.%,

Ni 1.0 to 3.0% by weight,

Cu 1.0 to 3.0% by weight,

N 0.10 to 0.30 wt%

Mo <0.5% by weight

and the rest of iron and impurities.

2. duplex steel according to claim 1, characterized in that it contains up to 0.050 wt .-% carbon.

3. Duplex steel according to claim 1, characterized in that it contains up to 0.030 wt% carbon.

4. Duplex steel according to at least one of the preceding claims, characterized in that it contains <1.0 wt .-% silicon.

5. duplex steel according to at least one of the preceding claims, characterized in that it contains <0.50 wt .-% silicon

6. duplex steel according to at least one of the preceding claims, characterized in that it contains <1.0 wt .-% manganese.

7. duplex steel according to at least one of the preceding claims, characterized in that it contains <0.50 wt .-% manganese

8. Duplex steel according to at least one of the preceding claims, characterized in that it contains 21.5 to 22.5 wt .-% chromium.

9. Duplex steel according to at least one of the preceding claims, characterized in that it contains 2.0 to 3.0 wt .-% nickel.

10. Duplex steel according to at least one of the preceding claims, characterized in that it contains 1.5 to 2.5 wt .-% copper.

11. duplex steel according to at least one of the preceding claims, characterized in that it contains 0.15 to 0.25 wt .-% nitrogen.

12. duplex steel according to at least one of the preceding claims, characterized in that the volume fraction of the ferritic phase is in the range of 35 to 65%, and that the volume fraction of the austenitic phase is in the range of 35 to 65%.

13. duplex steel according to at least one of the preceding claims, characterized in that the pitting sum (PRE), defined by the following formula:

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

greater than 26.

14. duplex steel according to at least one of the preceding claims, characterized in that the notch impact work Av (room temperature)> 80 J.

15. duplex steel according to at least one of the preceding claims, characterized in that the impact energy Av (-40 ° C)> 27 J is.

16. Use of the duplex steel according to at least one of the preceding claims in areas in which pressure and / or temperatures below 0 ° C are important.

17. Use of the duplex steel according to at least one of the preceding claims

- in centrifuge and decanter construction, especially for drum shells,

- for pressure vessels of all kinds,

- for rollers in the chemical industry and the paper industry.