DE60205269T2 - FERRITIC STAINLESS STEEL AND ITS USE IN MANUFACTURING HIGH-TEMPERATURE-RESISTANT PRODUCTS - Google Patents

Abstract

Ferritic stainless steel, comprising the following Chemical elements expressed in percentage by weight: -Cr 14.0-20.0 -Al 0.50-1.50 -Zr 0.10-0.50 -Si 0.30-1.50 -Ti 0.10-0.35 -Nb 0.25-0.55 -C<0.035 -N<0.035 provided that the content of Ti, Nb, C and N satisfy the following relation: %Ti+%Nb/1.94>9 (%C+%N) and substantially iron q. s. to 100. This steel may also comprise Yttrium and/or rare earth elements in a percentage by weight comprised in the range 0.10-0.30. The invention also encompasses products manufacturable with the steel of the invention, in particular vehicle exhaust manifold systems. Figure (I) shows a cyclic oxidation test carried out at 1000° C. on samples having the composition of an embodiment of the steel according to the invention.

Description

The The present invention relates to the field of ferritic stainless steels, which is used for the production of high temperature resistant products can be.

The The object of the present invention is to provide a ferritic to create stainless steel, which due to its composition and due to suitable thermomechanical treatments for production can be used by products which provide high performance levels show high temperatures, and which in particular for the production Can be used by vehicle exhaust systems.

at The production of vehicle exhaust system components will be the ferritic stainless steels due to their lower thermal expansion coefficient across from austenitic stainless steels prefers. In the development of the technology field in question will be ferritic stainless steel steels used for the production of vehicle exhaust system components, which at temperatures up to 750 ° C operate. For the production of an exhaust pipe, which temperatures higher than 800 ° C exposed was, for a long time cast iron compared to ferritic stainless toughen prefers. However, the application of this earlier material proved as unsatisfactory, due to the appearance of disadvantages based on an increase in weight, thereby causing fuel consumption, and related to design problems because of the large thickness the components produced in this way.

efforts Cast iron for replace this particular application with ferritic stainless steels provided useful in reducing the weight of the product Results. However, to this day the limitations are due to insufficient performance of advanced materials, in the face of a burden by High-temperature exhaust gases and repeated heat shocks under oxidation conditions, in particular with regard to spalling, not mastered.

Therefore there is a demand for a ferritic in this particular area stainless steel, which is suitable for the production of pipes, and performance values free from the disadvantages of the previously used Has materials.

• – Cr 14,0–20,0;
• – Al 0,50–1,50;
• – Zr 1,10–0,50;
• – Si 0,30–1,50;
• – Ti 0,10–0,35;
• – Nb 0,25–0,55;
• – C < 0,035;
• – N < 0,035;

unter der Vorraussetzung, dass die Anteile an Ti, Nb, C und N die nachfolgende Beziehung erfüllen: %Ti + %Nb/1,94 > 9 (%C + %N),und mit optionalen Komponenten, wie unten genannt, und einen Restbetrag an Eisen und an Verunreinigungen.In fact, the object of the present invention is to provide a ferritic stainless steel comprising the following chemical elements expressed in weight percent:

• Cr 14.0-20.0;
• Al 0.50-1.50;
• Zr 1.10-0.50;
• Si 0.30-1.50;
• Ti 0.10-0.35;
• Nb 0.25-0.55;
• - C <0.035;
• - N <0.035;

provided that the proportions of Ti, Nb, C and N satisfy the relationship: % Ti +% Nb / 1.94> 9 (% C +% N), and with optional components, as mentioned below, and a residual amount of iron and impurities.

Of the Ferritic according to the invention stainless steel can as well Yttrium and / or rare earth elements in a weight percentage which moves in a range of 0.1-0.3.

Further For example, the present invention includes strip steels or steel sheets which from a steel according to the invention are made, and which are weldable, deformable and highly resistant to oxidation and creeping are.

• – Heiß- und/oder Kaltwalzen;
• – Spannungsfreiglühen nach Heiß- und/oder Kaltwalzen bei einer Temperatur, welche von 900°C bis 1200°C reicht, und für Zeiträume, welche kürzer als 3600 Sekunden sind;
• – Optionales Beizen nach Heiß- und/oder Kaltwalzen.

A further object of the present invention is to provide a method for producing strip steel or steel sheets, wherein the steel according to the invention is subjected to the following treatment steps:

• - hot and / or cold rolling;
• Tension annealing after hot and / or cold rolling at a temperature ranging from 900 ° C to 1200 ° C and for periods shorter than 3600 seconds;
• - Optional pickling after hot and / or cold rolling.

The The present invention includes the use of the ferritic invention stainless steel for the production of products in the form of a flat Blockes, a steel strip, a billet, a casting, one forging or semi-finished products, as well as flat blocks, strip steels, ingots, castings, forgings and Semi-finished products, which are made of it.

Further The present invention relates to the use of a Strip steel according to the invention or steel sheet for the production of pipes or of products which can be derived from a pipe.

The pipes produced in this way may be weld-free pipes, or ge Welded tubes, evenly welded longitudinally tubes. The tubes and pipes derived products, which are made of said strip steel or steel sheets, are also part of the task of the present invention.

Finally, refers the invention also relates to high temperature exhaust system components, in particular on vehicle exhaust pipes.

at high temperatures shows the inventive ferritic stainless steel the desired Characteristics, in particular a resistance to the cyclic hot oxidation, because of an appropriate synergy of chemical elements such as chromium, aluminum, zirconium, silicon, titanium, niobium, carbon, nitrogen and optionally yttrium and / or rare earth metal elements. This at high temperatures desired License plates are reinforced, by hot and / or cold rolling and then by stress relieving the Steel sheets and / or strip steel, which are made in this way, meanwhile a zirconium carbonitride microstructure excretion occurs, and optionally followed by a pickling treatment.

The inventors estimate the functional contributions of the main chemical elements constituting the alloy of the present invention as follows:
Chromium significantly increases its resistance to oxidation in the weight percentages given without promoting the formation of brittle phases.
Silicone and aluminum stabilize the ferritic matrix and are responsible for increasing the resistance to hot oxidation. Use of these elements above the upper limits set has a detrimental effect on steel quality, intermetallic phase formation, and increased manufacturing problems.
Zirconium forms stable carbonitrides, the presence of which affects the microstructural evolution during stress relief annealing of the cold rolled parts.
Titanium and niobium form stable carbides and nitrides, and prevent the microstructural precipitation of chromium nitrides and chromium carbides at the grain boundaries, and prevent the resulting dechroming (sensitization) of the matrix in the immediate vicinity of the grain boundaries, and further favor the formation of a fully ferritic structure at all Temperatures by reducing the amount of carbon and nitrogen dissolved.
Yttrium and / or rare earth metal elements increase the resistance to hot oxidation in view of thermal and mechanical shocks due to increased adhesion of the oxide to the metal substrate.

To Herein, the present invention has been generally described. With support The illustrations and examples below are more detailed Description of specific embodiments of the invention, with the aim of carrying out the tasks, the marks, to make clear the advantages and the modes of application of the invention.

1 Figure 1 shows a weight variation per unit surface area made on austenitic and ferritic steel samples to identify a prior art steel which can demonstrate the best characteristics of cyclic hot oxidation resistance by, for this purpose, a cyclic oxidation test is performed according to a number of cycles at a temperature of 1000 ° C.

2 Figure 4 shows a weight variation per unit surface area on a prior art AISI 441 steel sample identified as being capable of detecting the best characteristics for cyclic hot oxidation by, for this purpose, a cyclic oxidation test corresponding to a number of cycles at a temperature of 1000 ° C C was executed.

3 shows the weight variation per surface unit in a steel sample according to the invention, for which purpose a cyclic oxidation test was carried out corresponding to a number of cycles at a temperature of 1000 ° C.

EXAMPLE 1

Around the resilience across from a thermal cycling under critical operating conditions to evaluate steel elements, which in a hot area are positioned by vehicle exhaust systems, these elements were cyclic Subjected to oxidation resistance tests.

Latter were in known steels carried out at different temperatures to the known steel type to determine which meets the requirements of thermal cycling resistance better enough.

It a standard test procedure was used, with 25 minutes of heating / oven residence time and 5 minutes of spontaneous air cooling cycles.

The ferritic stainless steels 441, 436, 429, and the austenitic steels 321, 309, 310, 4828 were tested at an operating temperature of 1000 ° C.

1 Figure 1 shows the weight variation per unit surface of different types of steel according to a number of cycles.

Obviously show the austenitic steels a worse material behavior, with a fast weight loss due to chipping. In particular, the steel 321 has a high instability on, and decomposes over a very short period of time.

in the In contrast, the results with respect to the ferritic steels are more satisfactory, as the weight during all oxidation cycles remains virtually unchanged.

Especially learns the steel 441 a slight one Weight gain compared to steels 436 and 429, which instead a minor one Show weight loss after the test. As a result, the steel became 441, which show the best performance under the known steels can, as comparison steel to the advanced steel according to the present Invention used.

This Conventional steel AISI 441 was made according to the following, in weight percent expressed Composition produced: C 0.016; Cr 18,18; Si 0.60; Al 0.09; Nb 0.42; Ti 0.18; Mn 0.18; Cu 0.08; N 0,012, and a balance to iron.

Corresponding This composition was a flat product in the form of a flat Block produced, which was heated to 1150 ° C, and which after subsequent Hot rolling strip steel from Thickness of 5 mm, which was then stress-free for 1 minute at 1050 ° C were annealed.

To The pickling became the hot ones steel Strips for cold rolling to 1.5 mm thickness, and then 40 seconds long annealed at a temperature of 1080 ° C stress-free.

The resistance to cyclic oxidation of the strip steel produced in this way was tested at 1000 ° C and at 1050 ° C in a 1000 cycle test, each test cycle consisting of 25 minutes heating / oven residence time and 5 minutes air cooling. The test results are in 2 shown.

the same hot rolled and then cold Rolled strip steel was subjected to SAG testing at a temperature for 100 hours from 1000 ° C subjected to evaluation of its creep resistance. The SAG Test result receives one, by the permanent deflection of the sample ends after a Oven load of the samples is measured.

The Final test measurement revealed a permanent deflection of over 40 mm, far beyond the 18 mm threshold, which is generally due to the current technology is applied.

EXAMPLE 2

It becomes a steel according to the invention with the following composition expressed in weight percent prepared: Cr 17.68; Al 0.94; Zr 0.15; Si 1.16; Ti 0.21; Nb 0.40; C 0.022; N 0.013, and essentially a residual amount of iron.

Of the Steel is made by a casting process in flat blocks cast, with a small thickness, which is typically 50 mm to 90 mm, and in this example is equal to 60 mm. This flat block made according to the one described above Composition, was heated to 1150 ° C, and by a subsequent Hot rolling this Blocks were strip steel produced with a thickness of 5 mm.

The Band steels produced in this manner were allowed to stand for 1 minute a temperature of 1050 ° C Annealed stress-free.

To The pickling became the hot ones steel Strips conveyed to a cold rolling up to 1.5 mm thickness and then de-energized for 40 seconds at a temperature of 1080 ° C annealed.

The resistance of the strip steel thus produced to cyclic oxidation was tested in a 1000 cycle test at 1000 ° C and 1050 ° C, each cycle consisting of 25 minutes heating / oven residence time and 5 minutes air cooling. The test results are in 3 shown.

the same hot and cold rolled strip steel was SAG tested for 100 hours at one Temperature of 1000 ° C subjected to evaluation of its creep resistance.

The SAG test result regarding a measurement of the permanent deflection the sample ends after furnace loading of the strip steel samples a value of 11 mm.

Consequently obviously the steel according to the invention remains below the 18 mm deflection threshold, which is generally applied by the current technology becomes.

EXAMPLE 3

It becomes a steel according to the invention prepared with the following composition expressed in weight percent: Cr 18; Al 0.94; Zr 0.15; Si 0.95; Ti 0.18; Nb 0.44; C 0.015; N 0.013, continue comprising yttrium and 0.20% b / w rare earth elements, and im Essentially a residual amount of iron.

Of the Steel was poured into flat blocks by a traditional casting process, which first to a temperature of 1150 ° C were heated. Then 5 mm thick strip steel was produced by subsequent hot rolling.

The Band steels produced in this manner were allowed to stand for 1 minute a temperature of 1050 ° C Annealed stress-free.

To The pickling became the hot ones steel Strips for cold rolling to 1.5 mm thickness, and then 40 seconds long annealed at a temperature of 1080 ° C stress-free.

The resistance of the strip steel thus produced to cyclic oxidation was tested in a 1000 cycle test at a temperature of 1000 ° C and 1050 ° C, each cycle composed of 25 minutes heating / oven residence time and 5 minutes air cooling. The test results are essentially equivalent to those found in 3 are shown.

the same hot rolled and then cold Rolled strip steel was heated at a temperature of 100 ° C for 100 hours 1000 ° C SAG Subjected tests to evaluate its creep resistance. The after The test carried out showed a permanent deflection of approximately 13 mm, which is below the 18 mm deflection threshold, which is in the Generally applied by the current technology.

Claims (10)

Ferritic stainless steel, characterized in that it comprises the following chemical elements, expressed as a percentage by weight: Cr 14.0-20.0; Al 0.50-1.50; Zr 0.10-0.50; Si 0.30-1.50; Ti 0.10-0.35; Nb 0.25-0.55; - C <0.035; - N <0.035; and optionally yttrium and / or rare earth metal elements, in a percentage by weight which includes the range 0.10 - 0.30; provided that the proportions of Ti, Nb, C and N satisfy the relationship: % Ti +% Nb / 1.94> 9 (% C +% N); and a residual amount of iron and impurities. Strip steel or sheet steel, characterized that they are made of steel according to claim 1, and in that they are weldable, deformable and highly resistant to oxidation and creeping are. Method for producing strip steel or steel sheet according to claim 2, characterized in that the steel according to claim 1 the following Treatment steps is subjected to: - hot and / or cold rolling; - Stress relieving after Hot- and / or cold rolling at a temperature ranging from 900 ° C to 1200 ° C, and for periods, which shorter than 3600 s are; - optional Pickling by hot and / or cold rolling. Use of the ferritic stainless steel according to claim 1 in the production of products in the form of a flat pre-rolled Blockes, a steel strip, a billet, a casting, one forging or a semi-finished product. Flat pre-rolled blocks, strip steels, ingots, castings, forgings and Semi-finished products, characterized in that they are made of steel according to claim 1 are made. Use of steel strip or steel sheet according to claim 2 for the Production of pipes or products from a pipe can be derived. Use of steel strip or steel sheet according to claim 6 for the Production of weld-free Pipes or welded pipes, evenly in the longitudinal direction welded Pipes. Pipes and products derived from a pipe can be characterized in that they are made of strip steel or steel sheet according to claim 2 are made. Product derived from a pipe can, according to claim 8, which is suitable as part of a high-temperature exhaust system is. Product according to claim 9, which is suitable as an exhaust pipe of a vehicle exhaust system.