WO2013178629A1 - Heat-resistant fe-al-cr steel - Google Patents

Description

 Heat-resistant Fe-Al-Cr steel

From practice, the demand for a corrosion-resistant steel has resulted, which is characterized by high strengths both at room temperature and at higher operating temperatures.

To meet these requirements, the invention proposes a steel having the alloy generally specified in claim 1.

Advantageous embodiments of the invention are in the dependent

Claims are given below and will be like the general one

Invention idea explained in detail.

A steel according to the invention therefore contains besides iron and

unavoidable impurities (in% by weight)

AI: 3 - 7%,

Cr: 3 - 1 1%,

C: up to 0.15%, C

Mn: up to 1%,

Si: up to 2%,

P: up to 0.1%,

S: up to 0.03%,

Mo: up to 2%,

optionally at least one element of the group "Zr, V, W, Nb, Ti" in amounts of up to 1% each,

Co: up to 1%, Ni: up to 2%,

B: up to 0.1%,

Cu: up to 3%,

Ca: up to 0.015%,

Rare earths: up to 0.2%,

N: up to 0.1%.

Surprisingly, it has been shown that a steel according to the invention results from its comparatively high contents of aluminum and chromium

- A high strength at room temperature by solid solution hardening, high hot and creep resistance by conversion to the

Solidification temperature T _SO i and precipitation formation,

a high resistance to oxidation and corrosion,

good hot and cold workability,

- good weldability and

- Has a reduced specific gravity.

In this case, the steel according to the invention has a ferritic microstructure.

In the case of a steel according to the invention, the density is reduced by the Al content of 3-7% by weight, in particular 3-6% by weight. At the same time, the corrosion and oxidation resistance and the tensile strength at room temperature are markedly improved by Al contents of this order of magnitude. In addition, AI contributes to the stated strengths to the heat resistance. At Al contents> 7 wt .-%, however, the cold workability is difficult, the weldability deteriorates (AI forms stable welding slag of elevated electrical Welding resistance) and there are formed with N and C embrittling phases (Al-nitrides, kappa-carbides). In order to safely use the favorable effects of Al, the Al content can be limited to a maximum of 6% by weight, in particular 2.5-5.5% by weight, with optimized properties of the steel according to the invention being established with great certainty when the Al content is 4.0-5.5 wt%.

The C content of a steel according to the invention is limited to up to 0.15% by weight, because C contents of more than 0.15% by weight cause a strong tendency to form brittle kappa carbides at grain boundaries. These carbides can not be adequately bound by micro-alloying elements, with the result that the hot and cold workability is reduced. High C contents also reduce weldability. The C content is therefore preferably at least 0.001 wt .-% and is

in particular limited to a maximum of 0.01 wt .-%.

The provided in a steel according to the invention Cr content of 3 - 1 1 wt .-% improves the corrosion and oxidation resistance significantly. At higher Cr contents, Cr carbides can form detrimental Cr carbides with high C contents for corrosion resistance.

Mn contents of up to 1% improve hot workability and weldability in steel alloyed according to the invention. Mn is also added for deoxidation. It increases the strength, but reduces the

Corrosion resistance. In order to be able to use the positive effects of Mn, Mn contents of at least 0.2% by weight may be present in the steel according to the invention.

Si contents of up to 2 wt .-% increase the strength and the

Corrosion resistance of a steel according to the invention. At levels greater than 2% by weight, the ductility and weldability of the steel decreases. Si is also added for deoxidation. To the positive effects of Si, Si contents of at least 0.2% by weight, in particular at least 0.25% by weight or at least 0.3% by weight, may be added.

The P content of a steel according to the invention is up to 0.1 wt .-%. The presence of higher P contents increases the tendency for segregations that are difficult to balance. P also deteriorates the cold workability, weldability and oxidation resistance. Therefore, the P contents are to be set low, in particular to <0.03 wt .-%, in particular less than 0.03 wt .-%, limit. Typical operationally achievable P contents are 0.02-0.03 wt%.

The S content is limited to a maximum of 0.03 wt .-%, since sulfur, for example, the hot workability during hot rolling and the

Can deteriorate corrosion resistance.

Mo contents of up to 2% increase the tensile strength at room temperature and the heat resistance in the steel according to the invention. Mo forms with C fine carbides, which contribute to a fine structure. However, higher contents of Mo deteriorate hot and cold workability. In order to be able to use the positive effects of Mo, Mo contents of at least 0.004% by weight, in particular at least 0.1% by weight, may be added.

In a steel according to the invention, at least one element of the group "Zr, V, W, Nb, Ti" is present in amounts of up to 1% by weight each.

Zr optionally present in the alloy according to the invention forms strength-enhancing Zr carbides, but hinders recrystallization at high levels and reduces cold workability. At the same time, Zr can improve the oxidation resistance. Therefore, the Zr content of a steel according to the invention is preferably in the range of 0.002-1 wt%, especially 0.05-1 wt%, with the positive effects of Zr then give particularly safe if the content of Zr is at least 0.1% by weight or at least 0.2 wt .-%. To exclude possible negative influences particularly safe, the upper limit of the Zr content range to 0.9 wt .-%, in particular 0.85 wt .-% or

0.8% by weight.

V forms strength-enhancing carbides. In order to make optimum use of this effect, a steel according to the invention can optionally contain 0.005-1% by weight, in particular 0.05-1% by weight, V, the positive effects of V then being set up particularly reliably when the content V is at least 0, 1 wt .-% or at least 0.2 wt .-%. In order to exclude possible negative influences particularly safe, the upper limit of the

V content range to 0.9 wt .-%, in particular 0.85 wt .-% or

0.8% by weight.

W also forms strength-enhancing carbides. In order to make optimum use of this effect, the optional W content of a steel according to the invention can be adjusted to 0.002-1% by weight, in particular 0.05-1% by weight, with the positive effects of W then being particularly reliable when the content of W is at least 0.1% by weight or at least 0.2% by weight. In order to exclude possible negative influences particularly reliably, the upper limit of the W content range can be set to 0.9% by weight, in particular 0.85% by weight or 0.8% by weight.

In the steel according to the invention, Ti and Nb are optionally present in the following content ranges. With simultaneous presence of Ti and Nb Laves phases can be formed, which are high

Warm strength of the steel of the invention can be expected.

Ti in amounts of up to 1% by weight forms strength-enhancing Ti carbides and can improve the oxidation resistance as well as the heat resistance. To compensate for the positive effects enabled by the optional presence of Ti use, the Ti content is preferably at least 0.1 wt .-%. In excessively high levels, Ti degrades cold formability and welding properties and forms undesirable Ti nitrides. To avoid this, the

Upper limit of the Ti content to 0.7 wt .-% are set. Ti can also form with Nb Laves phases. With the simultaneous presence of Nb, titanium contents of 0.3-0.6% by weight have proven to be particularly advantageous with regard to the formation of high volume fractions of Lavesphase.

Nb forms strengthening Nb carbides, improves heat resistance and can form in simultaneous presence with Ti Laves phases. In order to utilize these effects, the optionally present Nb content is therefore preferably at least 0.1% by weight, with Nb contents of at most 0.7% by weight being found to be particularly advantageous when negative influences of the presence be excluded from Nb particularly safe. With the simultaneous presence of Ti, niobium contents of 0.3-0.6% by weight have also been found to be particularly advantageous with regard to the formation of high volume fractions of Lavesphase.

The Co content should be limited to a maximum of 1% by weight in a steel according to the invention. Co increases the recrystallization temperature, is expensive and does not contribute to a property improvement at higher levels.

Ni in amounts of up to 2% by weight increases the strength and toughness of a steel according to the invention and improves its corrosion resistance. In order to be able to use the positive effects of Ni, Ni contents of at least 0.02% by weight, in particular 0.1% by weight, may be added.

B can be present in a steel according to the invention in amounts of up to 0.1% by weight. Its presence causes a fine texture, but at high levels reduces cold workability and oxidation resistance. To make use of this, the B content is preferably set to 0.0005-0.1% by weight, in particular> 0.001% by weight. Cu contents of up to 3% by weight improve the corrosion resistance. However, higher contents of Cu deteriorate hot workability and weldability. In order to be able to use the positive effects of Cu, it is possible to add Cu contents of at least 0.01% by weight, in particular at least 0.05% by weight or at least 0.1% by weight.

Ca in amounts of up to 0.015 wt .-% binds existing in the steel

Sulfur, which can reduce corrosion resistance. In order to be able to use the positive effects of Ca, Ca contents of at least 0.001% by weight, in particular at least 0.003% by weight or at least 0.005% by weight, may be added.

Rare earths ("REM") in amounts of up to 0.2 wt

Improve oxidation resistance. In order to be able to use the positive effects of REM, REM contents of at least 0.002% by weight, in particular at least 0.01% by weight or at least 0.05% by weight, can be provided.

Steel according to the invention contains as low as possible amounts of N in order to keep the formation of disadvantageous Al nitrides low. Al-Nitride can be the

deteriorate mechanical properties and ductility. Therefore, the N content is limited to at most 0.1% by weight, preferably at most 0.015% by weight or at most 0.01% by weight. Typical operating values are in the range of 0.005-0.015% by weight.

Heavy plate, hot strip or cold strip can be processed from Fe-Al-Cr steels according to the invention. The procedure may be as follows: a) Melting

Basically, due to the ability to liquefy high levels of alloy for on-site melt production, one appears high alloy steel grade according to the invention, the electric furnace route,

in particular via the arc furnace route, more suitable than the classic blast furnace converter route of an integrated steelworks. When produced via a standard electric furnace route, however, the accompanying elements (Cr, Ni, Mo, P, Cu and others) "introduced" into the steel may prove problematic since these are the mechanical ones

Properties and the performance properties as well as the processability of the material can adversely affect.

Low accompanying element contents or low contents of certain

Elements (eg, C, N, O) in the pre-melt are beneficial to the properties of the steel. You can use today's high-performance secondary metallurgy (including vacuum equipment, but also ladle furnace,

Flushing) available measures are achieved.

The melting of the steel according to the invention can be carried out in a conventional manner by melting the pre-melt, adding

Alloy elements (Si, Mn) and larger amounts of AI take place.

Subsequently, the further addition of AI takes place in up to three steps until a sufficient amount for the respective target content is added. b) casting

Continuous casting seems to be problematic because of the high Al contents.

Lower casting widths tend to be advantageous in continuous slab casting, but this can not completely eliminate the existing essential restrictions. Preblock casting in thicker formats can be an alternative compared to traditional continuous casting. The near-net strip casting process removes the restrictions of other processes and lends itself to the production of Fe-Al-Cr steels precursors, which are then further processed into hot or cold rolled strips. If the Vorblockgießen is used, the following can

Vorblockgießparameter be used:

The casting takes place with a waiting time of about 15 minutes. after the last alloy addition. The casting temperature is specific to the analysis 1550 - 1600 ° C. The cooling takes place under vacuum. c) hot rolling

The steel according to the invention can be hot rolled in a conventional manner in terms of temperature control, rolling forces and achievable degree of deformation. The rolling forces are comparatively low, but increase sharply at rolling temperatures below 950 ° C. Below 850 ° C is also expected to be a strong drop in ductility.

Typical recommended operational hot strip production parameters for the production of hot strip over conventional slab production:

Slab heating: 1200 - 1300 ° C for 120 - 240 min;

Hot rolling: rolling end temperature WET> 850 ° C;

Coiling temperature HT: from room temperature to 650 ° C, preferably HT is around 500 ° C.

After hot rolling, a hot strip annealing can optionally be performed. This is used in an optional optional subsequent cold rolling lowering the cold rolling resistance and increasing the maximum achievable degree of cold rolling. At the same time the annealing promotes a texture selection, which together with a high degree of cold deformation, the formation of a suitable ribbon texture with the desired

Promotes property profile. For the hot strip annealing is a

Dome annealing process with peak temperature above 650 ° C suitable. d) pickling

Bepflbarkeit with common media may be difficult due to surface coverage with stable Al oxide, but is basically possible with all pickling media. By adjusting the pickling time, the respective desired

Beiz result can be achieved. e) cold rolling

A high degree of cold rolling promotes the formation of a suitable ribbon texture. f) final annealing

The recrystallizing final annealing in a continuous annealing process or in a bell annealing sets the desired recrystallized microstructure and texture. Suitable are continuous annealing processes with temperatures above 780 ° C and bell annealing processes above 650 ° C. g) coating

Common corrosion protection coatings based on Zn or Al can be applied using standard process parameters. h) component molding

In the case that the cold formability due to unrecrystallized

Bandkernbereiche or heavy kaltumformbarer alloy layers (eg, with a high Cr content) is reduced, a steel flat product according to the invention can be formed by hot forming.

Steel flat products produced in accordance with the invention are suitable as materials for highly heat-stressed automobile components (eg exhaust systems) and can also include casting solutions (eg turbocharger housings and other turbocharger parts). replace. Also, the steel according to the invention is suitable for the production of pistons for internal combustion engines.

For the field of building construction can be made of steel according to the invention components that are to have a high fire resistance. Similarly, from steel according to the invention make parts for facades and roofs, which should have a high corrosion resistance.

In the field of plant construction, steels according to the invention can be used for components which, for example, in petrochemical plants of a

exposed to aggressive environment and must have a correspondingly high corrosion resistance. Likewise are suitable

Steels of the invention due to their high heat resistance to

Production of forging dies. Also, flat steel products according to the invention, in particular in the form of heavy plates, are suitable for steam boiler, container and pipeline construction.

Because of their range of properties, components made of steels according to the invention, such as gratings, rollers, tubes, steam conductors, heat exchangers, are suitable for use in incineration plants and power plants.

Also, due to its high corrosion resistance, cutlery, dishes, appliances and coverings can be produced for use in the home.

In the field of extraction of raw materials and energy can be used from steel according to the invention, for example, pipes and other components for drilling for gas, oil, hot water production and components for fermentation in biogas plants as a substitute made of stainless steel or concrete components.

Also, steel according to the invention is suitable for the production of

High-temperature energy storage, high-temperature fuel cell stacks, High-temperature battery systems (housing and piping), heat storage (eg salt storage for solar systems).

Likewise applications in the low-temperature range with demands on the corrosion behavior are conceivable, for example for brake disks and components for vehicle underbody or constructions that were previously not executable in steel (eg biogas plants).

Examples of compositions of the inventive

Alloy specification falling steels 1-14 are given in Table 1.

To 3 mm thick hot strips produced from the steels 1 - 13 at a hot rolling end temperature of 950 ° C and a coiler temperature of 500 ° C, each of the longitudinal direction "L" and transverse direction "Q" are detected

Tensile strength Rm, yield strength Rp0.2, uniform elongation Ag and elongation A50 are listed in Table 2.

In the cupping experiment, it was demonstrated that the products obtained and obtained in the above-mentioned manner

Hot strip samples at a limit ratio ß of 1, 6 deformed into wells. In Table 3, the yield strength Rp0.2 and the

Tensile strength Rm, which are obtained from the steels 1 - 14 hot strips, when they have a test temperature of 600 ° C.

are exposed (test direction longitudinal). The samples produced in each case from steel containing about 0.5% by weight of Ti and Nb show superior properties here.

Further samples of the hot strips produced from steels 2, 6, 9 and 10 were subjected to hot strip annealing at 850 ° C. The deformation behavior of the thus annealed samples has also been investigated in cup experiments. It was found that each of the samples could be formed at boundary draw ratios β of 1, 6 and 2.0 wells. In Table 4, for the samples produced from Steels 2, 6, 9 and 10, the respective ones in Longitudinal direction "L" and transverse direction "Q" detected tensile strength Rm and

Yield point Rp0.2 recorded after the hot strip annealing.

Table 5 summarizes the results of tests performed on steel 14 samples. In these experiments, the hot rolling end temperature WET and the coiling temperature HT have been varied. In addition, in some experiments, a bell annealing was performed at a temperature TG, while such annealing was omitted in other samples. This is also indicated in Table 5 for the samples produced from the steel 14, as in each case in

Transverse direction yield strength Rp0,2, tensile strength Rm,

Equal expansion Ag, elongation A and the modulus of elasticity.

It can be deduced from the tests which were carried out on the basis of the steel strip 14 according to the parameters given in Table 5, that in the case of steels of the type in question, in particular if they are within the invention

provided ranges Ti and Nb in combination, a reduction of the hot rolling end temperature WET from 950 ° C to 870 ° C to higher

Yield strengths, tensile strengths and elongation values. If a buffing is performed at room temperature, it results in lower elongation values than at a reel temperature which is in the range of 500 ° C. Highest ductilities can be achieved with a hot rolling end temperature WET of 850 ° C, a coiler temperature HT of 500 ° C in combination with a hot strip annealing.

By way of example, samples of the hot strips produced from steels 2, 6, 9 and 10 were rolled to cold strip. Table 6 shows in each case final-annealed cold-rolled strip samples produced in longitudinal direction "L" and "Tglüh" produced from steel 2 at the annealing temperatures indicated in Table 6

Transverse direction "Q" recorded tensile strength Rm, yield strength Rp0.2,

Equal expansion Ag and elongation A50 indicated. Unless explicitly stated otherwise, the contents of certain alloying elements are given in wt .-% in the present text.

Steel C Al Cr Ti Mo Mo Nb Zr B

1 0.003 6.8 6 0.04

 2 0.007 3.0 3 0.05

 3 0.007 5.4 3 0.05

 4 0.007 5.4 6 0.05

 5 0.007 5.2 9 0.05

 6 0.007 5.4 1 1 0.05

 7 0.007 5.3 6 0.04 0.05

8 0.007 5.4 6 0.04 1

 9 0.06 5.3 6 0.04 0.5

 10 0.06 5.5 6 0.04 0.5

 11 0.1 5.4 6 0.5

 12 0.12 5.4 6.4 0.04 1 0.5 0.5 0.05

13 0.12 5.4 6 0.49 0.52

 14 0.02 5.5 5.9 0.47 0.48

 Residual Fe and unavoidable impurities, contents in wt

Table 1

Table 2

Table 3

Table 4 WET HT Rp0.2 Rm modulus

TG Ag A

 [° C] [° C] [MPa] [MPa] [%] [%] [GPa]

500 554 677 13.3 18.3 227

950

 RT no 486 581 15.9 17.0 256

500 annealing 617 721 1 1, 8 18,8 255

870

 RT 535 638 12.3 13,8 238

500 520 680 15.5 20.3 221

950 850 ° C

 RT 514 678 14.3 20.4 214 hood

500 grams 453 61 1 14.1 20.4 210

870

 RT 510 657 10.4 17.9 205

Table 5

Table 6

Claims

PATENT CLAIMS 1. Heat-resistant Fe-Al-Cr steel, which, in addition to iron and unavoidable impurities (in wt.%) AI: 3 - 7%, Cr: 3 - 11%, C: up to 0.15%, Mn: up to 1%, Si: up to 2%, P: up to 0.1%, S: up to 0.03%, Mon: up to 2%, optionally at least one element from the group “Zr, V, W, Nb, Ti” in contents of up to 1% each, Co: up to 1%, Ni: up to 2%, B: up to 0.1%, Cu: up to 3%, Approx: up to 0.015%, REM: up to 0.2%, N: contains up to 0.1%. 2. Steel according to claim 1, characterized in that its Al content is 3 - 6% by weight. 3. Steel according to one of the preceding claims, characterized marked that its AI content is 4.0 - 5.5% by weight. 4. Steel according to one of the preceding claims, characterized characterized in that its C content is at least 0.001% by weight. 5. Steel according to one of the preceding claims, thereby marked that its C content is a maximum of 0.01% by weight. 6. Steel according to one of the preceding claims, thereby characterized in that its Mn content is at least 0.2% by weight. 7. Steel according to one of the preceding claims, thereby marked that its Si content is at least 0.2% by weight. 8. Steel according to one of the preceding claims, thereby marked that its Mo content is at least 0.1% by weight. 9. Steel according to one of the preceding claims, characterized in that the elements of the group "Zr, V, W, Nb, Ti", if present, are each present in amounts of 0.05 - 1% by weight. 10. Steel according to claim 9, characterized in that, if Nb or Ti are present, the Nb content or the Ti content is each at least 0.1% by weight. . Steel according to claim 10, characterized in that, if Nb or Ti is present, the Nb content or the Ti content is each 0.3 - 0.6% by weight. 12. Steel according to one of the preceding claims, characterized characterized in that its Ni content is at least 0.1% by weight. 13. Steel according to one of the preceding claims, characterized characterized in that its B content is 0.0005 - 0.1% by weight. 14. Steel according to one of the preceding claims, characterized characterized in that its Cu content is at least 0.01% by weight. 15. Steel according to one of the preceding claims, characterized characterized in that its REM content is at least 0.01% by weight.