Superaustenitic Material

Abstract

A superaustenitic material is provided for use in chemical plant construction, maritime conditions, oilfield or gas field technology. The material resists corrosion, in particular corrosion in mediums with high chloride concentrations or in sulfuric acid conditions.

Description

FIELD OF THE INVENTION

The invention relates to a superaustenitic material and a method for producing the same.

BACKGROUND OF THE INVENTION

Materials of this kind are used, for example, in chemical plant construction, under maritime conditions, or in oilfield or gas field technology.

One requirement of materials of this kind is that they must also resist corrosion, in particular corrosion in mediums with high chloride concentrations or in sulfuric acid conditions.

Materials of this kind are known, for example, from CN 107876562 A, CN 104195446 A, or DE 43 42 188.

EP 1 069 202 A1 has disclosed a paramagnetic, corrosion-resistant austenitic steel with a high yield strength, strength, and ductility, which should be corrosion-resistant particularly in mediums with a high chloride concentration; this steel should contain 0.6% by weight to 1.4% by weight nitrogen, and 17 to 24% by weight chromium, as well as manganese and nitrogen.

WO 02/02837 A1 has disclosed a corrosion-resistant material for use in mediums with a high chloride concentration in oilfield technology. In this case, it is a chromium-nickel-molybdenum superaustenite, which is embodied with comparatively low nitrogen concentrations, but very high chromium concentrations and very high nickel concentrations.

By comparison to the previously mentioned chromium-manganese-nitrogen steels, these chromium-nickel-molybdenum steels usually have an even better corrosion behavior. By and large, chromium-manganese-nitrogen steels constitute a rather inexpensive alloy composition, which nevertheless offers an outstanding combination of strength, toughness, and corrosion resistance. The above-mentioned chromium-nickel-molybdenum steels achieve significantly higher corrosion resistances than chromium-manganese-nitrogen steels, but entail significantly higher costs because of the very high nickel content.

Characteristic values for the corrosion resistance include among others the so-called PREN₁₆ value; it is also customary to define the so-called pitting equivalent number by means of MARC; a superaustenite is identified as having a PREN₁₆ of α>42, where PREN=% Cr+3.3×% Mo+16×% N.

The known MARC formula for describing the pitting resistance for steels of this kind is the following: MARC=% Cr+3.3×% Mo+20×% N+20×% C−0.25×% Ni−0.5×% Mn.

Comparable steel grades are also known for use as shipbuilding steels for submarines; in this case, these are chromium-nickel-manganese-nitrogen steels, which are also alloyed with niobium in order to stabilize the carbon, but this diminishes the notched-bar toughness. Basically, these steels contain less manganese and as a result, have a relatively good corrosion resistance, but they do not yet achieve the strength of pure high nitrogen-alloyed CrMnN steels.

Known superaustenites usually have molybdenum concentrations >4% in order to achieve the high corrosion resistance. But molybdenum increases the segregation tendency and thus produces an increased susceptibility to precipitation (particularly of sigma or chi phases), which results in the fact that these alloys require a homogenization annealing and at values above 6% molybdenum, a remelting is required in order to reduce the segregation.

SUMMARY OF THE INVENTION

The object of the invention is to produce a superaustenitic, high-strength, and tough material, which can be produced in a comparatively simple and inexpensive way and is particularly suitable for a corrosive, sulfuric acid environment.

The object is attained with a material having the features described and claimed herein. Advantageous modifications are also described and claimed herein.

Another object of the invention is to create a method for producing the material.

The object is attained with the features described and claimed herein. Advantageous modifications are also described and claimed herein.

When % values are given below, they are always expressed in wt % (percentage by weight).

According to the invention, the material is intended for use in shipbuilding and in chemical plant construction or in the combination of the two, in this case particularly in flue-gas desulfurization systems of seagoing vessels. It can also be used in all other areas in which corrosion particularly due to sulfuric acid or acid gas is expected. In this connection, the material has a fully austenitic structure even after an optional cold forming. After the strain hardening, the yield strength should be R_p0.2>1000 MPa.

The alloy according to the invention comprises the following elements in particular (all values expressed in % by weight):

Elements	Preferred	More preferred
Carbon (C)	0.01-0.50	0.01-0.30	0.01-0.10
Silicon (Si)	<0.5	<0.5	<0.5
Manganese (Mn)	0.1-5.0	0.5-4.0	1.0-4.0
Phosphorus (P)	<0.05	<0.05	<0.05
Sulfur (S)	<0.005	<0.005	<0.005
Iron (Fe)	residual	residual	residual
Chromium (Cr)	23.0-33.0	24.0-30.0	26.0-29.0
Molybdenum (Mo)	2.0-5.0	3.0-5.0	3.5-4.5
Nickel (Ni)	10.0-20.0	14.0-19.0	15.0-18.0
Vanadium (V)	<0.5	<0.3	below detection level
Tungsten (W)	<0.5	<0.1	below detection level
Copper (Cu)	0.5-5.0	0.75-3.5	1.0-2.0
Cobalt (Co)	<5.0	<0.5	below detection level
Titanium (Ti)	<0.1	<0.05	below detection level
Aluminum (Al)	<0.2	<0.1	<0.1
Niobium (Nb)	<0.1	<0.025	below detection level
Boron (B)	<0.01	<0.005	<0.005
Nitrogen (N)	0.40-0.90	0.40-0.70	0.45-0.60

With such an alloy, the positive properties of different known steel grades are combined in a synergistic and surprising way.

Basically, the steel according to the invention should exist in a precipitation-free state since precipitation has a negative effect on the toughness and the corrosion resistance. In the alloy according to the invention, the carbon content is particularly limited to 0.50%. At the same time, the copper content is intentionally added to the alloy.

With the alloy according to the invention, it is entirely surprising that very high nitrogen values can be established, which is extremely good for the strength; these nitrogen values are surprisingly higher than those that are indicated as possible in the technical literature. According to empirical methods, the high nitrogen concentrations of the alloy according to the invention could not be added to the alloy at all without PESR, see FIG. 4.

The respective elements are described in detail below, in combination with the other alloy components where appropriate. All indications relating to the alloy composition are expressed in percentage by weight (wt %). Upper and lower limits of the individual alloy elements can be freely combined with each other within the limits of the claims.

Carbon can be present in a steel alloy according to the invention at concentrations of up to 0.50%. Carbon is an austenite promoter and has a beneficial effect with regard to high mechanical characteristic values. With regard to avoiding carbide precipitation, the carbon content should be set between 0.01 and 0.25%, preferably between 0.01 and 0.10%.

Silicon is provided in concentrations of up to 0.5% and mainly serves to deoxidize the steel. The indicated upper limit reliably avoids the formation of intermetallic phases. Since silicon is also a ferrite promoter, in this regard as well, the upper limit is selected with a safety range. In particular, silicon can be provided in concentrations of 0.1-0.4%.

Manganese is present in concentrations of 0.1-5%. In comparison to materials according to the prior art, this is an extremely low value. Up to this point, it has been assumed that manganese concentrations of greater than 19%, preferably greater than 20%, are required for a high nitrogen solubility. With the present alloy, it has surprisingly turned out that even with the very low manganese concentrations according to the invention, a nitrogen solubility is achieved that is greater than what is possible according to the prevailing consensus among experts. In addition, it has been assumed up to this point that a good corrosion resistance is accompanied by very high manganese concentrations, but according to the invention, it has turned out that due to unexplained synergistic effects, this is clearly not necessary with the present alloy. The lower limit for manganese can be selected as 0.1, 0.5, 1.0, 2.0, or 2.5%. The upper limit for manganese can be selected as 3.0, 3.5, 4.0, 4.5, or 5.0%.

In concentrations of 17% or more, chromium turns out to be necessary for a higher corrosion resistance. According to the invention, a concentration of at least 23% and at most 33% chromium is present. Up to this point, it has been assumed that concentrations higher than 23% have a disadvantageous effect on the magnetic permeability because chromium is one of the ferrite-stabilizing elements. By contrast, in the alloy according to the invention, it has been determined that even very high chromium concentrations above 23% do not negatively influence the magnetic permeability in the present alloy but instead—as is known—influence the resistance to pitting and stress crack corrosion in an optimal way. The lower limit for chromium can be selected as 23, 24, 25, or 26%. The upper limit for chromium can be selected as 28, 29, 30, 31, or 32%.

Molybdenum is an element that contributes significantly to corrosion resistance in general and to pitting corrosion resistance in particular; the effect of molybdenum is intensified by nickel. According to the invention, 2.0 to 5.0% molybdenum is added. It has also turned out that Mo concentrations of >5% and particularly >6% result in powerful segregation behavior, which increases the susceptibility to precipitation of the sigma phase, which in turn would reduce the corrosion resistance. The lower limit for molybdenum can be selected as 2.0, 2.2, 2.3, 2.4, 2.5, 3.0, 3.2, 3.3, 3.4, or 3.5%. The upper limit for molybdenum can be selected as 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, or 5.0%.

According to the invention, tungsten is present in concentrations of less than 0.5% and contributes to increasing the corrosion resistance. The upper limit for tungsten can be selected as 0.5, 0.4, 0.3, 0.2, 0.1%, or below the detection level (i.e. without any intentional addition to the alloy).

According to the invention, nickel is present in concentrations of 10 to 20%, which achieves a high stress crack corrosion resistance in mediums containing chloride. The lower limit for nickel can be selected as 10, 11, 12, 13, 14, or 15%. The upper limit for nickel can be selected as 17, 18, or 19%.

It is generally known that adding Cu >0.5% to the alloy results in an increase in the sulfuric acid resistance of austenitic stainless steel products. At the same time, the literature also mentions that primarily in high nitrogen-alloyed steels, Cu increases the susceptibility to precipitation of unwanted Cr₂N precipitation, which massively diminishes corrosion properties. According to the invention, a Cr₂N-free structure can be produced despite Cu concentrations >0.5, preferably >1.0 and high N concentrations of >0.40%. This effect, however, reaches saturation after a certain quantity. According to the invention, the upper limit for copper was selected to be <5%, preferably <3% or <2.5%, in particular <2%. The lower limit for copper can be selected to be 0.6, 0.7, 0.8, 0.1, 1, or 1.1%. One application field in particular is flue-gas scrubbing, particularly in seagoing vessels, for example. With these concentrations, on the one hand, a good resistance to sulfuric acids and also acid gas corrosion can be achieved and on the other hand, it is possible by means of the overall alloy to by and large prevent the precipitation of chromium nitrides as mentioned above.

Cobalt can be present in concentrations of up to 5%, particularly in order to substitute for nickel. The upper limit for cobalt can be selected as 5, 3, 1, 0.5, 0.4, 0.3, 0.2, 0.1%, or below the detection level (i.e. without any intentional addition to the alloy).

Nitrogen in concentrations of 0.40 to 0.90% is included in order to ensure a high strength. Nitrogen also contributes to the corrosion resistance and is a powerful austenite promoter, which is why concentrations of greater than 0.40% are beneficial. In order to avoid nitrogen-containing precipitations, in particular chromium nitride, the upper limit of nitrogen is set to 0.90%; it has turned out that despite the very low manganese content, by contrast with known alloys, these high nitrogen concentrations in the alloy can be achieved. Because of the good nitrogen solubility on the one hand and the disadvantages that result from higher nitrogen concentrations, in particular ones above 0.90%, a pressure-induced nitrogen content increase as part of a PESR route is in fact out of the question. This route is also unnecessary thanks to the low molybdenum content according to the invention that is compensated for by means of chromium and nitrogen. It is particularly advantageous if the ratio of nitrogen to carbon is greater than 15. The lower limit for nitrogen can be selected as 0.40 or 0.45%. The upper limit for nitrogen can be selected as 0.90, 0.80, 0.70, 0.65, or 0.60%.

According to the general prior art (V. G. Gavriljuk and H. Berns; “High Nitrogen Steels,” p. 264, 1999), CrNiMn(Mo) austenitic steels that are melted at atmospheric pressure like the present ones achieve nitrogen concentrations of 0.2 to 0.5%. Only chromium-manganese-molybdenum austenites achieve nitrogen concentrations of 0.5 to 1%.

According to the invention, it is advantageous that contrary to all expectations, high nitrogen concentrations are achieved without requiring a pressure-induced nitrogen content increase, which would usually be required in order to achieve such concentrations

As a result, the method according to the invention is also inexpensive since the costly pressure-induced nitrogen content increase is not necessary, which also makes it possible to eliminate the remelting process connected therewith.

Moreover, boron, aluminum, and sulfur can be contained as additional alloy components, but they are only optional. The present steel alloy does not necessarily contain the alloy components vanadium and titanium. Although these elements do make a positive contribution to the solubility of nitrogen, the high nitrogen solubility according to the invention can be provided even in their absence.

The alloy according to the invention should not contain niobium since it reduces the toughness and historically, was used only for bonding the carbon, which is not necessary with the alloy according to the invention. Concentrations of up to 0.1% niobium are still tolerable, but should not exceed the concentration of inevitable impurities.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained by way of example based on the drawing and in the Tables below. In the drawing:

FIG. 1: shows a very schematic depiction of the production route and its alternatives.

DETAILED DESCRIPTION OF THE INVENTION

Table 1 shows the alloy components and percentage ranges for the alloy of the invention.

TABLE 1
Alloy Components, % by weight
Alloying	Composition		More
element	range	Preferred	preferred
Carbon (C)	0.01-0.50	0.01-0.30	0.01-0.10
Silicon (Si)	<0.5	<0.5	<0.5
Manganese (Mn)	0.1-5.0	0.5-4.0	1.0-4.0
Phosphorus (P)	<0.05	<0.05	<0.05
Sulfur (S)	<0.005	<0.005	<0.005
Iron (Fe)	residual	residual	residual
Chromium (Cr)	23.0-33.0	24.0-30.0	26.0-29.0
Molybdenum (Mo)	2.0-5.0	3.0-5.0	3.5-4.5
Nickel (Ni)	10.0-20.0	14.0-19.0	15.0-18.0
Vanadium (V)	<0.5	<0.3	below detection level
Tungsten (W)	<0.5	<0.1	below detection level
Copper (Cu)	0.5-5.0	0.75-3.5	1.0-2.0
Cobalt (Co)	<5.0	<0.5	below detection level
Titanium (Ti)	<0.1	<0.05	below detection level
Aluminum (Al)	<0.2	<0.1	<0.1
Niobium (Nb)	<0.1	<0.025	below detection level
Boron (B)	<0.01	<0.005	<0.005
Nitrogen (N)	0.40-0.90	0.40-0.70	0.45-0.60

Table 2 is a table with three different alloys within the concept according to the invention and the resulting actual values of the nitrogen content compared to the theoretical nitrogen solubility of such alloys according to the prevailing school of thought.

TABLE 2
Examples of the Invention
	Chemical composition (percentage by weight)/residual Fe	Pressure
Example	C	1.00	Mn	Cr	Mo	Ni	V	W*	Cu	Co*	Ti*	Al*	Nb*	N	[MPa]
A	0.01	1.00	5.0	23.01	3.1	15.98	0.05	0	0.15	0	0	0	0	0.51	1.00
B	0.01	1.00	5.0	27	3.1	14	0.05	0	0.10	0	0	0	0	0.7	1.00
C	0.01	0.4	5.0	24	3.1	14	0.05	0	0.10	0	0	0	0	0.55	1.00
	N solubility [% N]*
	Medovar at temperature:
		Stein	Satir	Kowanda	1550° C.	1525° C.	1500° C.	1450° C.
	A	0.36	030	0.34	0.34	0.35	0.36	0.39
	B	0.61	0.41	0.65	0.47	0.49	0.51	0.56
	C	0.44	0.34	0.45	0.38	0.40	0.41	0.45
	*Values are below detectable level
	**Calculated values for N according to different methods (Source: on Restricting Aspects in the Production of Nonmagnetic Cr—Mn—N—Alloyed Steels, Saller, 2005)

Table 3 shows the mechanical properties (strengths) of the Examples in Table 2 before a possible strain hardening.

TABLE 3
Mechanical Properties
Examples according	Strength [MPa]
to the invention	Rm	Rp0.2
A	905	505
B	960	560
C	915	515

The components are melted under atmospheric conditions and then undergo secondary metallurgical processing. Then, blocks are cast, which are hot forged immediately afterward. In the context of the invention, “immediately afterward” means that no additional remelting process such as electroslag remelting (ESR) or pressure electroslag remelting (PESR) is carried out.

According to the invention, it is advantageous if the following relation applies:

MARC_opt: 40<% Cr+3.3×% Mo+20×% C+20×% N−0.5×% Mn

The MARC formula is optimized to such an effect that it has been discovered that the otherwise usual removal of nickel does not apply to the system according to the invention and the limit of 40 is required.

Then cold forming steps are carried out as needed in which a strain hardening takes place, followed by the mechanical processing, which in particular can be a turning, milling, or grinding.

FIG. 1 shows examples of the possible processing routes for the production of the alloy composition according to the invention. One possible route will be described below by way of example. In the vacuum induction melting unit (VID), molten metal simultaneously undergoes melting and secondary metallurgical processing. Then the molten metal is poured into ingot molds and in them, solidifies into blocks. These are then hot formed in multiple steps. For example, they are pre-forged in the rotary forging machine and are brought into their final dimensions in the multiline rolling mill or are rolled into sheet form in two-high rolling stands. Depending on the requirements, a heat treatment step can also be performed.

In order to further increase the strength, a cold forming step can also be performed.

A superaustenitic material according to the invention can be produced not only by means of the production routes described (and in particular shown in FIG. 1), the advantageous properties of the alloy according to the invention can also be achieved by means of a production route using powder metallurgy.

Table 2 (above) shows three different variants within the alloy compositions according to the invention, with the respectively measured nitrogen values, which have been produced with the method according to the invention in connection with the alloys according to the invention. These very high nitrogen concentrations contrast with the nitrogen solubility indicated in the subsequent columns according to Stein, Satir, Kowandar, and Medovar from “On restricting aspects in the production of non-magnetic Cr—Mn—N-alloy steels, SaIler, 2005.” In Medovar, different temperatures are indicated. It is clear, however, that the high nitrogen values far exceed the theoretically expected values.

This is even more astonishing since with the alloy according to the invention, a route was taken that does not in fact justify the expectation of a high nitrogen solubility, particularly because the manganese content, which has a very positive influence on the nitrogen solubility, is sharply reduced compared to known corresponding alloys.

The invention therefore has the advantage that an austenitic, high-strength material with an increased corrosion resistance and low nickel content is produced, which simultaneously exhibits high strength and paramagnetic behavior. Even after the cold forming, a fully austenitic structure is present so that it has been possible to successfully combine the positive properties of an inexpensive CrMnN steel with the outstanding corrosion-related properties of a CrNiMo steel.

One special feature of the invention is that because of the high nitrogen content, the strain hardening rate is higher than in other superaustenites in order to thus be able to achieve tensile strengths (R_m) of 2000 MPa. It is thus possible as a last production step to achieve a high strain hardening by means of cold rolling or other cold forming processes with high deformation rates.

Typical application fields of the materials according to the invention are shipbuilding and chemical plant construction or the combination of the two, in this case particularly in flue-gas desulfurization systems of seagoing vessels, but also in all other areas in which sulfuric acid corrosion is particularly expected.

Especially in applications in which very high strengths are required, the strength can be increased even more by means of cold deformation, as described above.

Claims

1. A superaustenitic material comprising an alloy with the following alloy elements in % by weight:

2. The superaustenitic material according to claim 1, wherein the alloy comprises the following elements in % by weight:

3. The superaustenitic material according to claim 1, wherein the alloy comprises the following elements in % by weight:

4. The superaustenitic material according to claim 1, whereinthe material is produced by a method comprising secondary metallurgical processing of the molten metal, casting into blocks, hot forming, optional cold forming, and optional further mechanical processing.

5. The superaustenitic material according to claim 1, whereinthe material has a yield strength Rp0.2 in excess of 500 MPA.

6. The superaustenitic material according to claim 1, whereinthe material has a notched bar impact work at room temperature in the longitudinal direction Av in excess of 300 J.

7. The superaustenitic material according to claim 1, whereinafter the cold deformation, the material is fully austenitic.

8. The superaustenitic material according to claim 1, wherein themanganese is present at about 0.5% to about 4.0% by weight of the alloy.

9. The superaustenitic material according to claim 1, wherein thechromium is present at about 24% to about 29.8% by weight of the alloy.

10. The superaustenitic material according to claim 1, wherein themolybdenum is present at about 2.5% to about 4.5% by weight of the alloy.

11. The superaustenitic material according to claim 1, wherein thenickel is present at about 12% to about 18% by weight of the alloy.

12. The superaustenitic material according to claim 1, wherein thenitrogen is present at about 0.50% to about 0.85% by weight of the alloy.

13. The superaustenitic material according to claim 1, wherein thecobalt is present at less than about 1% by weight of the alloy.

14. The superaustenitic material according to claim 1, wherein thecopper is present at about 1% to about 4% by weight of the alloy.

15. The superaustenitic material according to claim 1, wherein thetungsten is present at less than 0.3% by weight of the alloy.

16. A method for producing a superaustenitic material, comprising the steps of:providing an alloy comprising the following elements in % by weight:

17. The method for producing a superaustenitic material according to claim 16, wherein the alloy comprises the following elements in % by weight:

18. The method for producing a superaustenitic material according to claim 16, wherein the alloy comprises the following elements in % by weight:

19. The method for producing a superaustenitic material according to claim 16, wherein the hot forming comprises a plurality of sub-steps.

20. The method for producing a superaustenitic material according to claim 19, further comprising the steps of: re-heating the block between the sub-steps and after the last sub-step, and optionally solution annealing after the last sub-step.

21. The method for producing a superaustenitic material according to claim 20, wherein after the last sub-step and the optional solution annealing, sufficient cold forming is performed to achieve a tensile strength Rm>1000 MPa.

22. A use of a superaustenitic material according to claim 1 for systems and system components that are exposed to a sulfuric acid corrosion.

23. A use of a superaustenitic material formed according to the method of claim 16 for systems and system components that are exposed to a sulfuric acid corrosion.

24. A superaustenitic material comprising an alloy with the following alloy elements in % by weight: