DE102007060133A1 - Conduit made of nickel-free steel for an exhaust system - Google Patents

Abstract

Proposed is the use of low-nickel austenitic stainless steel with a nickel content of less than 8 wt .-% for the production of cold-formed pipe parts (1) for an exhaust system, in particular a motor vehicle, in particular bellows, pipe, hose or braiding elements.

Description

The The present invention relates to a cold-formed conduit part for an exhaust system, in particular of a motor vehicle, in particular bellows, pipe, hose or braiding element, from an austenitic stainless steel.

Farther The invention relates to the use of low-nickel austenitic Stainless steel for the production of cold-formed piping parts of the type mentioned.

For the production of cold-formed conduit parts for exhaust systems, in particular of motor vehicles, stainless austenitic or even ferritic steels are usually used today. With regard to the use of stainless ferritic steels is exemplified in DE 697 23 595 T2 directed.

austenitic Stainless steels stand out during processing (Forming) by a good processability or ductility out. The latter is not only smooth, d. H. tubular bent pipes (eg in an exhaust gas recirculation) but especially with regard to flexible, in particular bellows-type pipe parts (decoupling elements) of importance. The mentioned conduit parts are greatly transformed during manufacture, which is generally of great value to the consumer Forming quantitatively expressed, for example, specifically in a small value for the ratio of forming radius and line diameter or in a large value for the ratio is Balgaußendurchmesser and Balginnendurchmesser. Above all, bellows-like pipe parts due to their Flexibility have a high degree of formation. decisive is in this context the so-called A80 value for the elongation at break, being austenitic stainless steels Achieve an elongation of at least 40% with Zug. Compared ferritic stainless steels reach only one here maximum elongation of about 25% or less.

One decisive disadvantage of nickel-rich steels, so-called Standard austenite, that is austenitic stainless Steels with a nickel content of more than 8 wt .-% is in the explosive development of the nickel price in recent years to see the collection cost significantly noticeable, steadily increasing and also longer term incalculable alloy surcharges (LZ) required power.

Of the Invention is based on the object, a cold-formed conduit part to create the type mentioned, the cheaper can be produced as a line parts, using nickel-rich Standard austenites are created, and the possibility of doing so offers the good workmanship and durability properties of to maintain said conventional pipe parts.

The The present invention solves this problem by means of a cold-formed conduit part with the characteristics of the patent claim 1 and by using low-nickel austenitic Stainless steel according to the claim 10th

advantageous Further developments of the invention are the subject of dependent claims, the text of which is hereby incorporated by express reference in the description is added to unnecessary text repeats to avoid.

According to the invention a cold-formed conduit part for an exhaust system in particular a motor vehicle, in particular bellows, tube, hose or Braid element made of austenitic stainless steel characterized in that the steel has a nickel content of less has 8 wt .-%.

Farther the invention proposes the use of austenitic low-nickel stainless steel containing less than 8% by weight of nickel for the production of cold formed conduit parts for an exhaust system, in particular a motor vehicle, in particular Bellows, pipe, hose or braiding elements, before.

One The fundamental aspect of the present invention is that for the first time the use of low-nickel austenitic stainless Steels having a nickel content of less than 8% by weight and preferably only 4 to 5 wt .-% for the preparation of the above mentioned conduit parts is proposed, whereas in professional circles currently due to misperceptions and Prejudice is a clear dislike. The applicant is However, due to intensive studies managed to prove that nickel-free austenitic stainless steels for the above-mentioned use, that is for the production cold-formed conduit parts for an exhaust system are suitable.

The decisive advantage here is that the alloy surcharge in so-called low-nickel austenites with significantly less than 8 wt .-% nickel content, in particular about 4 wt .-%, is significantly smaller than the standard austenite 1.4301 or 1.4541, on whose Composition below based on the 2 will be discussed in more detail.

The applicant has in this together For the first time it has been demonstrated that low-nickel austenites can be used as a substitute for the abovementioned standard austenites in the production of the line parts mentioned at the outset, in particular also for vehicle decoupling elements in exhaust systems.

Next the temperature resistance and the fatigue strength is the corrosion resistance, in the case of line parts or decoupling elements for exhaust systems, in particular the Resistance to pitting, intergranular Corrosion and high temperature oxidation, crucial for the suitability of a material for use in exhaust systems. In this In particular, the appended subclaims propose a relationship preferred developments of the present invention, in which due to selected alloy compositions of the steel used, the risks for the types of corrosion mentioned even with low-nickel austenites not appreciable of standard austenites differ.

As already mentioned, is in the context of the present invention decisive question, to what extent low-nickel austenite in comparison to standard austenites correspondingly good corrosion resistance exhibit.

For this suggests the present invention in this context suggest that the steel used has a chromium content of at least 16 wt .-%, preferably 17 wt .-%.

Around specifically to further limit the pitting corrosion risk the present invention in another development also suggest that the steel used as small as possible having sulfur, the maximum sulfur content being 0.030% by weight, preferably 0.015 wt .-% is.

nitrogen improves resistance to pitting corrosion. Another development of the invention Line parts therefore provides that the steel used has a nitrogen content of more than 0.1 wt .-%, preferably 0.2 to 0.3 wt .-%.

As With standard austenites, even with low-nickel austenites, the Risk of intercrystalline corrosion (IK risk) by low Carbon content can be reduced. Therefore, sees another development of the line part according to the invention that the steel used has the lowest possible amount of carbon wherein the maximum carbon content is preferably 0.03 Wt .-% is.

Regarding their mechanical strength has low-austenite austenite in usually a similar or even slightly higher Strength than the standard austenite, in particular, the nitrogen alloyed variants by a significantly increased strength distinguished.

The Ductility of low nickel austenite is one with comparable to standard austenites, taking at qualities with less than 4 wt.% nickel content of increasing cold cracking sensitivity during forming or deep drawing is expected. See accordingly Further developments of the conduit part according to the invention suggest that the steel used to make it has a nickel content of at least 4 wt .-%, in particular 4 to 5 wt .-%, such as already mentioned above.

On the increased strength of low nickel austenite was also already noted above. If not needed and the ductility in forming is in the foreground, can Nickel in the context of the present invention in principle partially be substituted by copper. However, this is too consider that for copper alloyed 1% Ni austenites from a clearly deteriorated repassivation behavior and a speed accelerated by a factor of 10 to 100 compared with 1.4301.

alternative is therefore provided in the context of the present invention, that the copper content of the steel used as low as possible is kept, in particular less than 1 wt .-%.

One in the context of the present invention basic nickel substituent is manganese, which acts as Austenitbildner and thus in particular is also capable of the ferrite forming properties of Compensate for chromium. A corresponding development of the line part according to the invention therefore, provides that the steel used has a manganese content between 5 and 10 wt .-%, preferably 6 and 9 wt .-% and most preferably Has 6.4 to 7.5 wt .-%.

The Admixing manganese has the further effect that by a increased manganese content of the steel also increased Addition of nitrogen is possible. This in turn has Effects on the wear resistance of the austenite, which is increased by the addition of nitrogen. As already mentioned, therefore, the steel used preferably a nitrogen content of more than 0.1% by weight, preferably 0.2 to 0.3% by weight.

It has also been found that in low-nickel materials, in contrast to the material 1.4301, the number of cycles does not decrease with increasing grain size of the material. Here is another advantage of the use of low-nickel austenitic in the context of the present invention, since accordingly less stringent requirements must be placed on the grain size, which has a positive effect on the price of the material.

On In this way, the present invention encounters a whole series of prejudice against low-nickel austenites and beats in a novel way their use for the production of cold formed Piping for exhaust systems ago. The mentioned Prejudices are based inter alia on the fact that low-cost manufacturers in the past very bad qualities of low nickel austenite with strong impurities put on the market, so it as a result, when used to serious corrosion damage came. In addition, in low-nickel austenites makes the Lowering of the nickel content to some to 1 wt .-% and a Lowering the chromium content to up to 13 wt .-% required, what generally to a deteriorated corrosion resistance leads. In addition, label fraud also leads the wrong use of the low-nickel austenite and thus corrosion damage. Users trusted here because of the likewise unmangetic properties of low nickel austenite insisting that this with the material 1.4301 identical.

• – ausreichender Chromgehalt von mindestens 16 Gew.-%, vorzugsweise 17 Gew.-%, um eine ausreichende Korrosionsbeständigkeit zu erreichen;
• – ausreichender Nickelgehalt von über 4 Gew.-% zur Vermeidung von Kaltrissgefahr;
• – zulegierter Stickstoff (etwa 0,2 bis 0,3 Gew.-%) zur Erhöhung der Nasskorrosionsbeständigkeit;
• – möglichst geringer Schwefelgehalt (≤ 0,015 Gew.-%), ebenfalls zur Verbesserung der Nasskorrosionsbeständigkeit;
• – geringer Kohlenstoffgehalt (≤ 0,03 Gew.-%) zur Verbesserung der IK-Beständigkeit; und
• – geringer Kupfergehalt (≤ 1 Gew.-%) zur Vermeidung eines potentiell erhöhten Korrosionsrisikos.

In the context of a particular embodiment of the present invention, this thus provides preferably a cold-formed conduit part of the type mentioned, in particular for use as a decoupling element in the exhaust system of a motor vehicle, wherein the steel used in addition to a reduced nickel content of less than 8 wt .-% by the characterized by the following properties:

• - sufficient chromium content of at least 16 wt .-%, preferably 17 wt .-%, in order to achieve a sufficient corrosion resistance;
• - sufficient nickel content of over 4 wt .-% to avoid cold cracking hazard;
• - alloyed nitrogen (about 0.2 to 0.3 wt%) for increasing wet corrosion resistance;
• - the lowest possible sulfur content (≤ 0.015 wt .-%), also to improve the wet corrosion resistance;
• Low carbon content (≤ 0.03 wt%) to improve the IK resistance; and
• - Low copper content (≤ 1 wt .-%) to avoid a potentially increased risk of corrosion.

Further Features and advantages of the present invention will become apparent from the following description of exemplary embodiments the drawing. It shows:

1 a simplified schematic longitudinal sectional view of a pipe part according to the invention in the form of a ring-waved bellows element; and

2 a table comparing the chemical composition of standard and low nickel austenites.

1 schematically shows a longitudinal section through a cold-formed according to the invention line part for an exhaust system, in particular a motor vehicle in the form of a ring-waved bellows element 1 , It should be noted, however, that the present invention is not limited to bellows-like, ie ring or helically corrugated conduit parts, but in particular also includes smooth, tubular conduit parts.

The bellows waves with wave crests 2 and troughs 3 are shown for reasons of clarity only in the right part of the figure partially. The pipe part 1 consists of a stainless austenitic steel with a wall thickness W between 0.1 and 1.0 mm. The pipe part 1 has connection ends 4 . 5 on, with which it is ver bindable with other components of the exhaust system, which is not shown here. The illustrated line part 1 was relatively strong (cold) formed during its production, which is in the case of the illustrated bellows element 1 expedient manner by the ratio Da / Di of outer diameter Da and inner diameter Di as a measure of the made (cold) deformation can quantitatively characterize, said ratio also only slightly greater than the absolute diameter of the line part 1 depends. The more that the ratio Da / Di deviates from the value 1 for a smooth, ie non-waved, line part, the greater the extent of the deformation and the more important are the observations made in the context of the present invention and the characteristic features or specifications derived therefrom concerning a material to be used. Preferred values are in the range Da / Di> 1.30, preferably Da / Di>1.35; also oval cross sections are possible.

As an alternative to the above explanations, in the case of a not shown smooth, ie, non-corrugated pipe, the ratio of a relative to the longitudinal axis L of the pipe part 1 measured pipe bending radius r and a diameter D (in 1 each not shown) of the line part 1 be turned off. This ratio depends in practice on the wall thickness W and according to the invention is in particular less than 2, preferably less than 1.5, that is to say r / D <2 or r / D <1.5.

Accordingly, as the person skilled in the art will appreciate, the present invention is not limited to bellows elements, in particular not to ring-waved bellows elements. Rather, it can be in the cold-formed conduit part 1 also be a pipe, hose or braid element, if this has corresponding bends, transformations or the like.

The peculiarity of the pipe element 1 according to 1 consists now in that the austenitic stainless steel from which the conduit part 1 prepared, has a nickel content which is less than 8 wt .-%. In this way can be in the production of the line part 1 realize a significant cost savings in terms of material costs, since the use of nickel currently requires the collection of a constantly increasing alloy surcharge (LZ). To the line part 1 However, to suspend no increased risk of cold cracking, the nickel content is preferably 4 wt .-%.

Other compositional properties of austenitic stainless steels used in the manufacture of the pipe part 1 Use can be found in the table according to 2 ,

2 shows a comparison of the chemical composition of standard and low-nickel austenite, wherein the amounts of the individual components in wt .-% of the melt analysis are given. The first column of the table indicates the type of steel, with the two upper blocks containing materials according to the norm DIN EN 10088 and in the lower block materials of the 200 series of the American AISI standard are given. The other table columns - with the exception of the last column of the table - indicate wt .-% ranges or maximum wt .-% for various chemical elements, namely (from left to right) carbon (C), silicon (Si), manganese (Mn ), Sulfur (S), nitrogen (N), chromium (Cr), nickel (Ni) and other elements such as copper (Cu), titanium (Ti) or molybdenum (Mo).

The last column of the table provides a parameter for determining the resistance to pitting, which is calculated according to the empirical formula PRE = Cr + 3.3 Mo + 30 N (Alloy elements each in wt .-%) is determined. PRE stands for Pitting Resistance Equivalent and indicates resistance to pitting corrosion. In other words, the greater the value of the last column of the table, the less sensitive the material is to pitting corrosion.

In the table according to 2 For example, three austenitic stainless austenitic steels are characterized by relatively low nickel content (*, **), which are particularly suitable for use in the present invention because of their properties, such as ductility during processing, corrosion resistance (even at high temperatures within an exhaust system) or strength. Preferably, this is the austenite AISI 201LN (marked with **) with a nickel content between 4 and 5 wt .-%, a chromium content of 16 to 17.5 wt .-% to increase the corrosion resistance, a manganese content between 6, 4 and 7.5 wt .-%, a nitrogen content of 0.1 to 0.25 wt .-% to achieve a better wet corrosion resistance and only small amounts of carbon, sulfur and copper, to generally ensure increased corrosion resistance.

Furthermore, the austenite AISI 216L can be used preferably in the course of the present invention. Compared with the austenite AISI 201LN, the austenite AISI 216L has a higher nickel content and a higher chromium content (5 to 7 wt% and 17.5 to 22 wt%, respectively). Accordingly, the manganese content is increased (7.5 to 9.0 wt .-%), so that nitrogen can be added to a greater extent (0.25 to 0.5 wt .-%). In addition, the austenite 216L still contains molybdenum in a proportion of 2 to 3 wt .-%. In particular, owing to the molybdenum content and the relatively high nitrogen content, the austenite 216L has a particularly high PRE value according to the active principle formula explained above, as can be seen directly from the table in FIG 2 results.

alternative can also be the standard austenite 1.4371 in the context of the present Find invention use. The composition corresponds here similar to that of the AISI 201LN.

It should be noted at this point that the material selection in the context of the present invention by no means to the marked materials in the table according to 2 are limited. Rather, in particular, all low-nickel austenite AISI 200 series (lower block of the table in 2 ) may alternatively be used as materials for producing the Kaltumformleitungsteils invention.

Furthermore, in the upper block of the table according to 2 still some standard austenite with "normal" nickel content in the range of 8 to 13.5 wt .-%, but which are disadvantageous by an increased alloying addition due to the relatively high nickel content and thus have a corresponding cost disadvantage, by the present invention is avoided.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 69723595 T2 [0003]

Cited non-patent literature

• - DIN EN 10088 [0038]

Claims (10)

Cold-formed conduit part ( 1 ) for an exhaust system, in particular a motor vehicle, in particular bellows, pipe, hose or braiding element, made of an austenitic stainless steel, characterized in that the steel has a nickel content of less than 8 wt .-%. Line part ( 1 ) according to claim 1, characterized in that the nickel content is 1.5 to 7 wt .-%, preferably 4 to 7 wt .-%, most preferably 4 to 5 wt .-%. Line part ( 1 ) according to claim 1 or 2, characterized in that the steel has a chromium content of at least 16 wt .-%, preferably 17 wt .-%. Line part ( 1 ) according to at least one of claims 1 to 3, characterized in that the steel has a manganese content of 5 to 10 wt .-%, preferably 6 to 9 wt .-%, most preferably 6.4 to 7.5 wt .-% , Line part ( 1 ) according to at least one of claims 1 to 4, characterized in that the steel has a nitrogen content of more than 0.1 wt .-%, preferably 0.2 to 0.3 wt .-%. Line part ( 1 ) according to at least one of claims 1 to 5, characterized in that the steel has as small amounts of sulfur, carbon and / or phosphorus, wherein the maximum sulfur content of 0.030 wt .-%, preferably 0.015 wt .-%, and the maximum carbon content 0.03 wt .-% is. Line part ( 1 ) according to at least one of claims 1 to 6, characterized in that a wall thickness (W) of the line part ( 1 ) is between 0.1 and 1.0 mm. Line part ( 1 ) according to at least one of claims 1 to 7, characterized in that the line part ( 1 ) is at least partially formed as a ring-waved or helically corrugated bellows and that a ratio, Da / Di, Balgaußendurchmesser, Da, and Balginnendurchmesser, Di, greater than 1, preferably greater than 1.30 and most preferably greater than 1.35. Line part ( 1 ) according to at least one of claims 1 to 8, characterized in that the line part ( 1 ) is formed at least in sections as a smooth, non-corrugated pipe and that a ratio r / D, from bending radius, r, and diameter, D, the pipe is less than 2, preferably less than 1.5. Use of low-nickel austenitic stainless steel having a nickel content of less than 8% by weight for the production of cold-formed conduit parts ( 1 ) for an exhaust system, in particular a motor vehicle, in particular bellows, pipe, hose or braiding elements.