FR2806940A1 - Ferritic stainless steel sheet containing aluminium and rare earth metals suitable for use in the catalyst support elements of motor vehicle exhaust systems - Google Patents

Abstract

A sheet of ferritic stainless steel containing aluminium has the following composition by weight: 10.5 - 20 % Cr, 1 - 8 % Al, 0.003 - 0.06 % C, 0.003 - 0.04 % N, 0 - 0.6 % Si, 0 - 0.6 % Mn, 0 - 0.002 % S and 0.001 - 1.0 % rare earth elements with the balance being Fe and residual elements inherent to production and in which the Cr and Al contents satisfy the relationship %Cr/%Al is less than 5. An Independent claim is included for the method of producing this stainless steel sheet.

Description

<B> STAINLESS STEEL STAINLESS STEEL </ B> FERRITIC <B> CONTAINER </ B> <B> ALUMINUM, USED IN PARTICULAR FOR A CATALYST SUPPORT </ B> 'EXHAUST <B> OF </ B> VEHICLE < This invention relates to a sheet of ferritic stainless steel containing aluminum which can be used in particular for a motor vehicle engine exhaust catalyst support and to a motor vehicle exhaust catalyst support system. its manufacturing process. In the manufacture of catalytic converters located in a motor vehicle exhaust line, two types of materials are used to carry out the mechanical support of catalyst compounds: ceramics or metals. The choice of metals is based on base alloys containing in their composition chromium and aluminum.

The alloy must form alumina during oxidation in a temperature range between 700 C and 1200 C. In addition, the strip must contain enough aluminum to form alumina throughout the duration of use while the catalyst support is hot.

An alloy of the Fe-20% Cr-5% AI type produced by casting in a steelworks is known. The transformation of the sheets made with this alloy presents difficulties in the field of cold rolling, in particular, because of its fragile behavior.

It is also known a method of aluminum bonding with a stainless steel sheet in which a strip of stainless steel is cold-pressed between two aluminum sheets, the laminate obtained is laminated, and the laminate is then annealed in a conventional manner. to cause diffusion of the aluminum in the steel strip.

To successfully resist oxidation, stainless steels must contain a minimum of 10.5% chromium.

It is commonly taught that increasing the chromium content also increases corrosion resistance as well as resistance to oxidation at high temperature, around 950 C.

A recognized way of improving the resistance of stainless steels to oxidation for temperatures above 1100 C is to add aluminum in order to promote the formation of a durable protective layer. aluminum less than about 3%, the development of stainless steels can be done by a conventional method of production such as continuous casting followed by rolling hot and cold. For higher aluminum contents, other processes are used for bonding aluminum foil or soaking a sheet in a molten aluminum bath.

The object of the invention is to present a sheet of ferritic stainless steel containing aluminum and its manufacturing method, sheet having a surface condition favorable to its use, especially in the field of the exhaust of a motor vehicle for a support of catalyst.

The subject of the invention is a sheet of ferritic stainless steel containing aluminum which can be used in particular for a motor vehicle engine exhaust catalyst support, characterized in that it comprises, in its following composition by weight, 0.5%: g 20% aluminum <8% 003% _ carbon <0.06% 003% <nitrogen <0.04% <silicon <0.6% <manganese <0.6% <sulfur <0.002 % 001% <_ rare earths 1.0%, iron and residual elements inherent in the elaboration, and in which the contents of chromium and aluminum satisfy the relationship% Cr /% AI <5.

Other features of the invention are - the steel comprises less than 18.5% of chromium, - the contents of chromium and aluminum satisfy the relationship% Cr /% AI <3 and preferably less than 2, - the composition comprises in addition stabilizing elements, titanium, zirconium, niobium taken alone or in combination, with an overall content of less than or equal to 0. The invention also relates to a method for producing the strip, by bonding, in which one cold plate, or a stainless steel sheet and an aluminum sheet, ie a sheet of stainless steel between two aluminum sheets, characterized in that - a hot-rolled ferritic stainless steel sheet is coated with a thickness less than or equal to 1.5 mm of the following composition by weight 10.5%: 9 chromium <20% aluminum <3% 003% <carbon <0.06% 003% @ nitrogen _ <0.04% _ <silicon <0 , 6% <manganese <0.6% _ <sulfur <0.002% iron and d residual elements inherent in the production, subject to softening annealing, with at least one aluminum foil containing in its composition rare earth elements in a proportion of between 0.001% and 1.0%; the laminate obtained is laminated, the laminate is annealed so as to cause diffusion of the aluminum and elements contained in the aluminum foil.

The other characteristics of the invention are: the laminate is annealed in a static manner or in a defile under an atmosphere containing hydrogen and having a dew point of less than -30 ° C., the laminate is subjected to cold rolling in two steps may comprise an intermediate annealing, the second step being carried out with a higher rolling ratio or equal to 20% of the final thickness envisaged for strip, the thickness of the laminate is between 0.15 mm and 0.4 mm the composition of the web forming the core of the laminate comprises nitrogen carbon contents which preferably satisfy the relationship: (C + N) <0.1%, - the composition of the web forming the core of the laminated contains less than 0, aluminum, - the composition of the steel strip forming the laminate core optionally comprises less than 1.5% copper, less than 1% nickel, minus 5% molybdenum . The following description will make the invention clear.

Figure 1 shows different ratios of% Cr /% AI and preferred chromium and aluminum content areas of the invention.

Figure 2 shows the variation of the hardness of three strips according to the invention as a function of the chromium content of their composition.

FIG. 3 shows, for three strips according to the invention, the influence of the chromium level on the oxidation resistance at 1100 C for durations greater than 400 hours, the resistance to oxidation being materialized by the mass gain of samples tested.

Figure 4 shows a series of curves, characteristics of resistance to air oxidation at 1050 C for different strips showing the influence of% Cr /% AI ratio on the corrosion resistance.

Figure 5 shows the hardness of two steels respectively containing 15% chromium and 1% chromium and having the same ratio Cr / Al.

The process according to the invention deals with the production of a sheet of ferritic stainless steel containing aluminum which can be used in particular for a motor vehicle exhaust catalyst support in which a strip of rolled stainless steel of a lower thickness or equal to 1.5 mm of the following composition 10.5%: 9 chromiums 20% aluminum <3% 0.003% <_ carbon <0.06% 0.003% <_ nitrogen <0.04% 0% silicon <_ 0.6% 0% 5 manganese <0.6% 0% <_ sulfur <0.002% of the iron and residual elements inherent to the manufacture, is cold-pressed for example between two aluminum foils, at least of which one contains, in its composition, rare earth elements in a proportion of between 0.001% and 1.0%. Rare earth means the elements: Ce; The ; Y; Pr; Nd; Gd; michmétal. In an exemplary application of the invention, the ferritic steel strip is hot rolled and cold rolled to a thickness of less than or equal to 1.5 mm, and subjected to softening annealing at a temperature between 600 C and 1200 C, a plating between two sheets of aluminum whose sum of thicknesses is between 0.05 times and 0.32 times the thickness of the sheet steel strip to obtain a laminate, - laminating of the laminate to a thickness of between 0.05 mm 0.25 mm to form a strip, - static diffusion annealing of the strip under a controlled atmosphere of hydrogen with a dew point of less than -30 C a finishing rolling of the strip with a total reduction ratio of greater than 20% ensuring a final roughness Ra of less than 0.25 μm; the strip may be subjected, in addition, to a continuous continuous softening annealing; at a temperature between 600 C and 1200 C.

In one form of the invention, the laminate, the thickness of which is between 0.15 mm and 0.4 mm, is cold-rolled, for example, in two stages, the second stage being carried out with a rolling rate. greater than or equal to 20% of the final thickness envisaged for the strip. After rolling, the thickness of the strip obtained is between 30 and 50 μm. Diffusion annealing may optionally be performed between the two cold rolling steps.

According to the invention, at least one aluminum foil contains in its composition by weight rare earth elements in a proportion of between 0.001% and 0.5%.

After cold rolling, the laminate is annealed so as to cause diffusion of the aluminum and the elements contained in the aluminum, under a substantially pure hydrogen atmosphere and having a dew point below -30 C in a static manner or at the parade. The laminate can also be vacuum annealed.

The sheet of ferritic steel sheet forming the core of the laminate comprises a chromium content of less than 20% and preferably of between 10 and 18.5%. In this form, the processing of the sheet metal is greatly improved, compared to the processing of a steel sheet containing about 20% chromium or more.

Currently, the transformation of a sheet metal strip containing no stabilizer of the titanium, zirconium or niobium type with a chromium content greater than about 1 is rendered difficult by the brittleness due to the chromium carbonitrides which are formed in the steel of the band.

To improve the transformation of the sheet metal strip used for the manufacture of the strip according to the invention, it is possible to introduce, in the composition of said sheet, stabilizing elements, titanium, zirconium and niobium, taken alone or in combination, with a content overall less than or equal to 0.6%.

The aluminum content of the strip, between 8% and 8%, is easily obtained by coiling by introducing mischmetal elements into aluminum foils rather than into the sheet-forming steel bath.

In addition, alloying elements, such as for example molybdenum, form with oxygen a volatile MoO 3 type oxide at temperatures of the order of 1000 C, which affects the cohesion of the oxide layer. strip surface. In the temperature range below 1000 C, molybdenum can be introduced in contents of less than 5%.

The aluminum contained in the sheet metal steel traps the nitrogen during the diffusion annealing operation. Indeed, in the case of a steel sheet not containing aluminum in its composition, it has been observed that the nitrogen of said steel diffuses towards the interface of the laminate and combines with the aluminum of the sheets destined to the diffusion of aluminum in steel. It then forms an interface, a layer of aluminum nitride which is a source of fragility.

The steel strip forming the core of the laminate preferably contains aluminum in its weight composition. Said strip may contain few such as, for example, less than 0.5% associated with nitrogen, the nitrogen content being less than 0.04%.

The hydrogen atmosphere, with a dew point of less than 30 ° C., makes it possible to obtain a non-oxidized metal and makes it possible to laminate the strip. During diffusion annealing, the roughness Ra of the strip is brought to a value of the order of one micrometer.

The strip may be finished laminated to provide a final roughness Ra of less than 0 μm and preferably less than or equal to 0.1 μm, the finishing lamination being followed, preferably, by a continuous final anneal.

The smooth surface state, favorable to the properties of use of the catalyst supports in motor vehicle exhausts, is obtained by cold rolling of the strip after diffusion annealing, the reduction rate of the cold rolling being greater than 20%. using for the last two rolling passes polished rolling mill rolls.

The final annealing carried out between 700 ° C. and 1200 ° C. and preferably between 800 ° C. and 1000 ° C. is continuous annealing followed by rapid cooling with a cooling rate greater than 25 ° C. per second. This annealing makes it possible to eliminate the fragility of the metal created during diffusion annealing.

The condition of the obtained strip, smoothed during the last finishing rolling passes and having a suitable roughness, preferably less than 0.1 μm, makes it possible to obtain an elongation strength during use.

In an exemplary embodiment of the invention, the sheet metal strip, containing in its carbon weight composition = 0.03%, chromium = 16 aluminum = 0.4 nitrogen = 0.02 manganese = 0.4 silicon = 0, Sulfur = 0.0005 titanium + zirconium + niobium = 0.3% and satisfying the relation% Ti +% Nb +% Cr <0.6%, is hot rolled and cold rolled to a thickness of 0.5 mm. After softening annealing, two sheets of aluminum 50, um thick, containing in their composition rare earth elements, cerium and lanthanum, in a proportion of 0.75%, are plated on the stainless steel sheet. it is rolled again to a thickness of 0.2 mm. The obtained strip is then subjected to diffusion annealing at 900 ° C. for 15 hours, in a closed vessel under a pure hydrogen atmosphere with a dew point of less than -30 ° C. The strip is then rolled to a final thickness of 50. um under a reduction rate of 75% ensuring a surface state whose roughness has a final Ra of 0.08 / im. The rolling is then followed by a final annealing continuous defile at 950 C for 40 seconds in a hydrogen atmosphere.

The strip according to the invention has an in-service thermal stress elongation characteristic as a function of the improved heat-up time compared to a 20% Cr-5% AI type strip elongation characteristic of a steel made directly steel mill.

As shown in FIG. 1, the chromium content of between 10.5% and 16%, preferably with a content of less than 15%, defines a zone 1 which unexpectedly gives improved corrosion resistance characteristics.

Zone 1 can be extended up to 18.5% according to the invention.

The range of chromium contents, combined with a chromium to aluminum ratio of less than 5, ensures oxidation resistance of the coaminated sheet at temperatures below 1100 C in use on catalyst supports.

FIG. 2 shows the variation of the hardness of the strips according to the invention as a function of the chromium content of their composition. It can be seen the interest of the use of strips containing in their composition a reduced chromium content.

In an oxidation test for strips containing less than 20% chromium as shown in FIG. 3, it has been noted, for a duration of about 400 hours at 1100 C, that the oxidation resistance is identical for different grades. in chromium, the aluminum content being constant and equal to 5%. Contrary to the idea that it is necessary to increase the chromium content above 20% to improve the corrosion resistance of the strips, it is noted that for a ratio% Cr /% AI less than 5, the resistance Oxidation is ensured for chromium contents of less than 20%. A lower chromium content in the steel composition is advantageous from the point of view of steel making, mechanical properties, and economics.

In another oxidation test, for a period greater than 400 hours, and as shown in FIG. 3, the strip containing 10.5% chromium and 5% aluminum is, surprisingly, more resistant to oxidation at 1100 C as the strip containing 20% chromium and 5% aluminum.

In summary, to obtain a good resistance to oxidation at <B> 11 </ B> 00 C for a duration of more than 400 hours the following relations should preferably be respected: 450 hours: Cr / AI <3 550 hours: Cr / Al <2, the ratio Cr / Al <2 being preferable for longer times and higher temperatures.

To obtain a strip having a good resistance to oxidation for an operating temperature of less than 1050 C for 400 hours, according to the invention, it is not necessary to have a high aluminum content, but it is important to respect the relationship% Cr /% AI <5. An embodiment of the strip according to the invention, shown in Figure 1 in zone 2, is a strip containing in its composition from 2% to 3.5% aluminum from 10.5% to 17.5% chromium and preferably from 10.5% to 16% chromium. This strip can be produced by bonding according to the invention and also by a technique of coating the aluminum by hot dipping before diffusion or, in a conventional manner, by a technique of continuous casting of the steel followed by hot rolling of slab and cold rolling of the hot-formed sheet.

Figure 4 summarizes the influence of the ratio% Cr /% AI in the field of oxidation where the weight gain due to air oxidation as a function of time for a temperature of 1050 C, decreases with the ratio % Cr / AI%.

Strips 1 to 6 according to the invention were tested and strips 8 to 10 for comparison, the compositions of which are shown in Table 1 below.

Table <SEP> 1
<tb> Feuil. <SEP> Cr / AI <SEP> Cr <SEP> if <SEP> Mn <SEP> AI <SEP> Ti <SEP> p <SEP> Nb <SEP> C <SEP> N <SEP> S <SEP> ppm <SEP> Lands
<tb> rare
<tb> 1 <SEP> 4,4 <SEP> 15,0 <SEP> 0,27 <SEP> 0,29 <SEP> 3,4 <SEP> - <SEP> 0,013 <SEP> 0,16 <SEP > 0.01 <SEP> 0.006 <SEP> 5 <SEP> 0.033
<tb> 2 <SEP> 2.6 <SEP> 15.0 <SEP> 0.26 <SEP> 0.29 <SEP> 5.8 <SEP> - <SEP> 0.013 <SEP> 0.14 <SEP > 0.013 <SEP> 0.007 <SEP> 3 <SEP> 0.043
<tb> 3 <SEP> 2.8 <SEP> 15.03 <SEP> 0.50 <SEP> 0.27 <SEP> 5.3 <SEP> - <SEP> 0.012 <SEP> 0.29 <SEP > 0.017 <SEP> 0.006 <SEP> 1 <SEP> 0.037
<tb> 4 <SEP> 1.9 <SEP> 10.5 <SEP><B> 0 </ B>, 50 <SEP> 0.27 <SEP> 5.5 <SEP> - <SEP> 0.012 <MS> 0.28 <SEP> 0.014 <SEP> 0.005 <SEP> 1 <SEP> 0.266
<tb> 5 <SEP> 4.1 <SEP> 20.0 <SEP> 0.50 <SEP> 0.26 <SEP> 4.9 <SEP> - <SEP> 0.013 <SEP> 0.25 <SEP > 0.013 <SEP> 0.005 <SEP> 1 <SEP> 0.468
<tb> 6 <SEP> 4.26 <SEP> 18.3 <SEP> 0.28 <SEP> 0.27 <SEP> 4.4 <SEP> - <SEP> 0.012 <SEP> 0.153 <SEP> 0.010 <SEP> 0.006 <SEP> 4 <SEP> 0.032
<tb> 8 <SEP> 9 <SEP> 18.0 <SEP> 0.30 <SEP> 0.26 <SEP> 2.0 <SEP> - <SEP> 0.012 <SEP> 0.157 <SEP> 0.013 <SEP > 0.007 <SEP> 5 <SEP> 0.032
<tb> 9 <SEP> 5.3 <SEP> 18.1 <SEP> 0.30 <SEP> 0.27 <SEP> 3.4 <SEP> - <SEP> 0.12 <SEP> 0.157 <SEP > 0.013 <SEP> 0.006 <SEP> 5 <SEP> 0.032
<tb> 10 <SEP> 5.1 <SEP> 15.3 <SEP> 0.30 <SEP> 0.34 <SEP> 3.0 <SEP> 0.06 <SEP> 0.014 <SEP> - <SEP > 0.011 <SEP> 0.008 <SEP> 6 <SEP> 0.034 Table 2 presents the result of the oxidation test under air for steels presented in Table 1, the X meaning a good resistance to oxidation, 0 meaning a poor resistance to oxidation.

Table <SEP> 2
<tb> strips <SEP> 1050 C, <SEP> 400h <SEP> 11000C, 400h <SEP> 1100 <B> 0 </ B><SEP> C, 450h
<tb> 1 <SEP> X <SEP> 0 <SEP> 0
<tb> 2 <SEP> X <SEP> X <SEP> X
<tb> 3 <SEP> X <SEP> X <SEP> X
<tb> 4 <SEP> X <SEP> X <SEP> X
<tb> 5 <SEP> X <SEP> X <SEP> 0
<tb> 6 <SEP> X <SEP> X <SEP> 0
<tb> 8 <SEP> 0 <SEP> 0 <SEP> 0
<tb> 9 <SEP> 0 <SEP> 0 <SEP> 0
<tb> 10 <SEP> 0 <SEP> O <SEP> 0 Comparing the strips 1 to 6 according to the invention and the strips 8 to comparison, we find that the strip 1 with a chromium content of 15% present a better oxidation resistance at 1050 C than the strip 8 with a chromium content of 18%. Note that in the strip composition the ratio% Cr /% AI <5 is not respected.

strips 1 and 6 according to the invention have the same ratio% Cr /% AI of about 4.3 and also the same oxidation behavior to 1 as shown in Figure 4. However, as shown in Figure hardness these steels is very different, the hardness of the strip containing 1 chromium being lower than the hardness of the strip containing 18% of chromium.

The strip according to the invention can be used, on the one hand, for the catalyst supports but also as electrical resistance, for example, catalyst preheating.

Claims (10)

<B> <U> CLAIMS </ U> </ B> 1. Sheet ferritic stainless steel containing aluminum used in particular for a vehicle engine exhaust catalyst support characterized in that it comprises in its composition by weight 10, _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ aluminum <_ 8% 0.003% 5 carbon <0.06% 0.003% <_ nitrogen <0.04% 0% _ <silicon <0.6% 0% <_ manganese <_ 0.6% 0% <_ 0.002% 0.001% rare earth <1.0%, and residual elements inherent in the production, and in which the chromium and aluminum contents satisfy the ratio% Cr /% AI <5. 2. The strip of claim 1 characterized in that it comprises less than 18.5% chromium. 3. Sheet according to claim 1 characterized in that the chromium and aluminum contents satisfy the relationship% Cr /% AI <3 and preferably less than 2. 4. A strip according to claim 1 characterized in that the composition further comprises stabilizing elements, titanium, zirconium, niobium taken alone or in combination, with an overall content of less than or equal to 0.6%. 5. A method of producing the strip according to claims 1 to 4, by bonding, in which is cold plate, either a stainless steel sheet and an aluminum sheet, or a stainless steel sheet between two sheets of stainless steel. aluminum, characterized in that - a hot-rolled ferritic stainless steel sheet having a thickness of less than or equal to 1.5 mm of the following weight composition is chromed 10 <chromium 20% aluminum <3% 0.003% <carbon <_ 0.06% 0.003%: 9 Nitrogen <0.04% <B> 0%: G </ B> Silicon <0.6% 0%: G Manganese: 9 0.6% <B> 0% </ B <<0.002% sulfur of the iron and residual elements inherent in the preparation, subject to softening annealing, with at least one aluminum foil containing in its composition rare earth elements in a proportion of between 0.001% and 1.0%, the laminate obtained is laminated, the laminate is annealed so as to cause diffusion. aluminum and elements contained in the aluminum foil. 6. Method according to claim 5 characterized in that the laminate is annealed statically or parade in an atmosphere containing hydrogen and having a dew point less than -30 C. 7. Process according to claim 5, characterized in that the laminate is subjected to a cold rolling in two stages which may comprise an intermediate annealing, the second stage being carried out with an upper rolling ratio equal to 20% of the final thickness. considered for the strip. 8. The method of claim 5 characterized in that the thickness of the laminate between 0.15 mm and 0.4 mm. 9. The method of claim 1 characterized in that the composition of the web forming the laminate core comprises carbon and nitrogen content which preferably satisfy the relationship: (C + N) <0.1%. 10. The method of claim 1 characterized in that in the composition of the strip forming the core of the laminate contains less than 0.5% aluminum. <B> 11. </ B> Process according to claim 1, characterized in that the composition of the steel strip forming the core of the laminate optionally comprises less than 1.5% copper, less than 1% of nickel, less molybdenum.