CA2071189A1 - Aluminized stainless steel and method for producing same - Google Patents
Aluminized stainless steel and method for producing sameInfo
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- CA2071189A1 CA2071189A1 CA 2071189 CA2071189A CA2071189A1 CA 2071189 A1 CA2071189 A1 CA 2071189A1 CA 2071189 CA2071189 CA 2071189 CA 2071189 A CA2071189 A CA 2071189A CA 2071189 A1 CA2071189 A1 CA 2071189A1
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Abstract
"ALUMINIZED STAINLESS STEEL
AND METHOD FOR PRODUCING SAME"
ABSTRACT
A method for coating a strip of stainless steel with aluminum, without the need for pre-plating the stainless steel substrate before coating with aluminum. A
non-preplated stainless steel substrate is passed through a heating furnace wherein (a) the dew point of the furnace is controlled to substantially reduce the formation of chromium oxides on the surface of the strip and (b) the temperature of the furnace is controlled to substantially reduce the growth of any chromium oxides that may form on the surface of the strip. The aluminum coating is then applied to the substrate by a hot dip procedure. The coated metal product is substantially free of uncoated areas.
AND METHOD FOR PRODUCING SAME"
ABSTRACT
A method for coating a strip of stainless steel with aluminum, without the need for pre-plating the stainless steel substrate before coating with aluminum. A
non-preplated stainless steel substrate is passed through a heating furnace wherein (a) the dew point of the furnace is controlled to substantially reduce the formation of chromium oxides on the surface of the strip and (b) the temperature of the furnace is controlled to substantially reduce the growth of any chromium oxides that may form on the surface of the strip. The aluminum coating is then applied to the substrate by a hot dip procedure. The coated metal product is substantially free of uncoated areas.
Description
2~7~89 "ALUMINIZED STAINLESS STEEL
AND METHOD F~R PRODUCING SAME"
BACKGROUND OF THE INVENTION
This invention relates generally to chromium-containing steels with aluminum coatings and more particularly to an aluminized, AISI type 409, stainless steel.
Chromium-containing steel, particularly stainless steel, is used extensively in many industrial, residential, automotive, and high temperature applications due to its excellent ability to withstand corrosive and high temperature oxidizing environments. One widespread use of this steel is in automobile exhaust components such as mufflers, tailpipes and catalytic converter shells. These components must be able to withstand internal corrosion due to high temperature gases and external corrosion due to road salt.
The corrosion-resistant properties of stainless steel are achieved by adding chromium to steel, optimally about 10 wt.% or greater, although a lesser percentage may be used. Chromium at this level forms, on the surface of the steel, a passive film that is responsible for corrosion resistance. At higher temperatures, chromium forms an impervious oxide layer which slows down the oxidation process.
It may conceivably appear to be desirable to add aluminum to steel because, at higher temperatures, a steel containing aluminum may be more oxidation-resistant than a steel containing corresponding amounts of chromium. The oxidation resistance of aluminum-containing steel is due to the formation of aluminum-rich oxides at the surface of the steel. Unfortunately, the addition to steel of aluminum in 20~1189 sufficient quantity to form an aluminum-rich oxide layer causes manufacturing and mechanical problems.
Coating a chromium-containing steel substrate with aluminum would be desirable in order to overcome these S problems and would result in both (a) high temperature oxidation resistance due to the aluminum coating and (b) the beneficial passive film formed by the chromium contained in the substrate. The superior corrosion resistance of aluminum coated, AISI type 409 stainless steel is described by Ando et al. of Nisshin Steel Co., NACE Annual Conference and Corrosion Show Paper Number 384 (March 1991).
There is another significant reason for coating stainless steel with aluminum. After being exposed to the atmosphere, especially a humid atmosphere, AISI type 409 stainless steel and similar stainless steels become discolored and appear to have surface corrosion.
Therefore, although the steel is still protected from oxidation by the tenacious chromium oxide layer, the steel does not appear "stainless." As a result, coating the stainless steel with aluminum is desired for aesthetic purposes, especially when producing visible automotive exhaust components, because many consumers would be dissatisfied with a discolored product.
However, using a hot-dip process to coat stainless steel with aluminum presents a problem. The problem arises because stainless steel forms a tenacious, chromium oxide, outer layer during an annealing step which normally precedes hot dipping, and the chromium oxide outer layer is not wettable to molten aluminum. Therefore, the process usually results in uncoated spots and poor adherence of the aluminum coating to the stainless steel substrate. Poor adherence is reflected by flaking or cracking of the coating during bending of the strip. U~
Pat. No. 4,675,214 to Kilbane et al. discloses a method for 207~189 enhancing the wettability of a chromium-containing steel by a pure aluminum coatin~; the method involves controlling the coating conditions.
Pre-plating the surface of the stainless steel substrate with copper, nickel or iron-boron improves the substrate's coatability with molten aluminum to a degree which is commercially acceptable. One such process is disclosed in U.S. Pat. No. 4,913,785 to Uchida et al.
However, this solution is undesirable due to the added time and cost of performing such an intermediate coating or pre-plating step.
SUMMARY OF THE INVENTION
The present invention is directed to a method ~or coating a strip of chromium-containing steel, e.g.
stainless steel, with aluminum without the need for pre-plating the steel substrate before coating the substrate with aluminum, and to the product produced thereby.
In accordance with the present invention, a chromium-containing, preannealed steel substrate in the form of a strip (e.g., stainless steel strip) may be continuously coated with aluminum by preferably first subjecting the strip to a pre-cleaning step in which the substrate is passed through a heated alkaline cleaning bath, followed by rinsing and drying. The substrate is then passed through a furnace or enclosed heating zone wherein the strip is subjected to heating without flame impingement, e.g. radiant heating. The dew point inside the heating zone is controlled, preferably by passing hydrogen and nitrogen gas through the heating zone and by preventing the entry of oxygen gas into the heating zone.
A reducing atmosphere is thereby produced which will substantially reduce the formation of chromium oxides and other impurities on the surface of the chromium-containing steel strip while the strip is in the heating zone. The heating zone is kept at a controlled temperature, preferably less than 800C, so as to substantially reduce the growth of any chromium oxides which may possibly have formed on the surface of the substrate.
After the substrate passes through the heating zone, it is coated with aluminum by a hot-dip procedure, wherein the strip is dipped in a molten aluminum bath. In a preferred embodiment, the aluminum coating is an alloy which contains about 10 wt.% silicon. After the strip exits the bath, the thickness of the coating on the strip may be controlled by a jet stream of inert gas such as nitrogen. This step is preferably followed by steam cooling which solidifies the coating and prevents its transfer onto the rollers which direct the passage of the strip downstream of the bath.
The method of the present invention produces an aluminized, chromium-containing steel strip with a uniform coating and acceptable adherence. There is a substantial absence, on the coating, of spots reflecting an unwetted substrate at the interface between the substrate and the coating; no pre-plating step employing a pre-plating metal wettable by molten aluminum is necessary. Heating the steel to a low temperature in a controlled, reducing atmosphere obviates the need for such pre-plating.
Other features and advantages are inherent in the method and product claimed and disclosed or will become apparent to those skilled in the art from the following detailed description taken in conjunction with the appended drawing and claims.
BRIEF DESCRIPTION OF THE DRAWING
The sole figure is a schematic diagram illustrating a method in accordance with an embodiment of the present invention.
2 ~ 8 ~
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawing, a continuous, chromium-containing, preannealed steel strip or substrate 1 is fed from a coil 16 over a guide roller 4 into a pre-cleaning bath 2. Following pre-cleaning, substrate 1 is subjected to rinsing and drying in a rinsing and drying chamber 3.
The cleaned substrate l is fed over guide roller 19 and then passed through an enclosed furnace or heating zone ~, across supporting rollers 27 through 30. Furnace 6 has a controlled temperature and dew point. After exiting furnace 6 through a downstream furnace portion 7 or snout, substrate 1 is coated with aluminum in a hot-dip coating bath 10. The thickness of the coating is controlled by nitrogen jet knives 11 and 11'. Finally, the aluminum-coated strip is passed through a steam cooling zone 13,over a guide roller 14, and rewound into a coil 15.
The present method is applicable both to AISI
type 40g stainless steel substrates and to chromium-containing steel substrates with other compositions. The minimum chromium content of substrate 1 is, however, at least 1 wt.%, preferably at least 2 wt.%, and most preferably at least 10 wt.~. Greater than 10 wt.% chromium achieves optimal corrosion resistance.
Substrate l is preannealed before it is subjected to the invented method. Preannealing, wherein the steel is heated to a suitable temperature, held for some period of time, and then cooled at a suitable rate, gives the steel the same properties as annealing done on-line. Annealing softens the steel and can greatly increase the elongation characteristics of the steel, from less than 3% for non-annealed steel to possibly greater than 30% for annealed steel. Annealing can decrease tensile strength from about .~ .
2~71189 150 ksi for non-annealed steel to about 60 ksi for annealed steel.
The chromium-containing steel substrate may also contain any or all of the following: at least 0.25 wt.%
titanium, about 0.30 to about 1.0 wt.~ manganese, about 0.30 to about 1.0 wt.% silicon, and about 0.02 to about 0.08 wt.% carbon. The weight percentage of titanium is generally at least six times the weight percentage of carbon. Other elements that may be present in the substrate include sulfur and phosphorus. The content of sulfur, phosphorus and other residual elements should be within the acceptable ranges given by AI5I designation.
Examples of useful chromium-containing steel substrates are tabulated below:
15 Ingredient A B
chromium 11.0 wt.% 10.5 wt.% 12.5 wt.%
manganese 0.30 wt.% 1.00 wt.% 0.45 wt.%
carbon 0.04 wt.% 0.08 wt.% 0.05 wt.%
titanium 0.28 wt.% 0.48 wt.% 0.40 wt.%
phosphorus ~.03 wt.% .045 wt.% .007 wt.%
sulfur .003 wt.% .045 wt.% 0.02 wt.%
silicon 0.30 wt.% 1.00 wt.% 1.00 wt.%
iron essentially essentially essentially the balance the balance the balance In a preferred embodiment of the invention, preannealed substrate 1 is not subjected to a pre-plating step, employing a pre-plating metal wettable by aluminum, upstream of bath 10.
Substrate 1 is subjected to pre-cleaning in order to remove dirt, oil, and steel particles or fines which may be present on the surface of substrate 1. Pre-cleaning bath 2, contained in a container 21 having a guide roller 5 and either with or without scrubber brushes, may be an alkaline composition containing about 2.5 to 3.5 wt.~ NaOH.
Bath 2 typically has a temperature of about 120F to about 2~71~89 180F (48C to 82C). Rinsing and drying zone 3 employs a hot water rinse followed by hot air drying.
Furnace or enclosed heating zone 6 typically employs radiant heating under controlled conditions. There is, therefore, no flame impingement in furnace 6.
The dew point in heating zone 6 is controlled in order to minimize the formation of ~a) chromium oxides and (b) reaction products other than chromium oxides, on the surface of the chromium-containing steel strip or substrate 1, while the strip is in heating zone 6. This is accomplished by maintaining, in heating zone 6, a reducing atmosphere which is substantially devoid of water vapor and consists essentially of hydrogen and nitrogen gases, e.g., about 25 vol.% to about 50 vol.% hydrogen gas, with the balance of the atmosphere being substantially nitrogen gas.
The hydrogen and nitrogen gases are substantially devoid of water vapor and oxygen gas and are thus "dry" gases as they enter heating zone 6. These dry gases may flow continuously through heating zone 6, and this procedure will reduce the relative amount of water vapor in heating zone 6. The dew point is also controlled by preventin~
entry into heating zone 6 of oxygen gas, for example by avoiding leaks.
In a preferred embodiment of the invention, heatiny zone 6 has an upstream portion 17 (the main body of the heating zone~ having an entry 24, and there is a downstream furnace portion 7 (known as the furnace "snout"~
having an exit end 25. In this embodiment, the dew point in upstream portion 17 is maintained at a temperature in 30 the range of about -30F to about -10F (-35C to -23C).
At entry 24 into upstream furnace portion 17l the dew point is maintained in the range of about -20F to -10F (-29C
to -23C). The dew point in downstream furnace portion 7 is maintained at a temp~rature in the range of about -50F
35 to about -45F (-46C to -42C). The dew point is 20~1189 maintained at about -60F (-51C) at exit end 25 of downstream portion 7.
The temperature in heating zone 6 is controlled so as to substantially reduce the growth o~ any chromium oxides which may possibly form on the surface of substrate 1. The temperature at entry 24 of upstream furnace portion 17 is maintained in the range of about 200F to about 300F
(93C to 149C). The maximum temperature in heating zone 6 is no greater than about 1570F to about 1670~F (854C to 910C).
Substrate 1 is heated in heating zone 6 to a temperature in the range of about 1350F to about 1400F
(733C to 760C) and maintained at that temperature in heating zone 6. The maximum temperature of substrate 1 in heating zone 6 is no greater than about 1470F (800C).
The temperature of substrate 1 cools down to about 1250F
to about 1350F (677C to 733C) by the time it exits downstream furnace portion 7.
The hot dip coating procedure of the present invention may employ a coating metal consisting essentially of aluminum. This is known as "Type 2" aluminum.
Alternatively, the adherent coating may be an alloy of aluminum containing about 10 wt.% silicon, known as "Type 1" aluminum.
Coating bath 10, contained in container 22 and employing guide roller 2~, commonly referred to as a sinker roll, has a temperature which is maintained in the range of about 1200F to about 1250F (648C to 677C). Substrate l is immersed in coating bath 10 for a time of about 3 seconds to about 10 seconds.
Jet knives 11 and 11', which control the thickness of the coating, direct nitrogen or other gas at the coated strip. The thickness of the aluminum coating is controlled to be in the range of about 10 microns to about 25 microns.
20711~9 The cooling of coated strip 18 is performed by directing steam against coated strip 18 in steam cooling zone 13 which is contained in housing 23. This cools coated strip 18 to a temperature in the range of about 700F to about 900F (370C to 482C) before it reaches roller 1~. Cooling solidifies the coating and prevents transfer of the coating onto a roller 14 which directs the passage of coated strip 18 downstream of housing 23.
Coated strip 18 is then wound into coil 15, for easy transportation and storage.
~ metal product made from coil 15 includes a chromium-containing steel substrate and an adherent, hot-dip coating on the substrate. The coating consists essentially of aluminum or an alloy of aluminum. The metal product is characterized by the substantial absence, on the coating, of spots reflecting an unwetted substrate at the interface between the substrate and the coating. Such spots are absent despite the absence of (a) a pre-plating metal, wettable by molten aluminum, between the substrate and the coating, and (b) any diffusion product of a pre-plating metal wettable by molten aluminum, within the metal product.
Pre-plating metals include at least one ingredient such as copper, nickel or iron-boron, which are conventionally used to provide a surface wettable by molten aluminum. The diffusion product of the pre-plating metal could be generated during heating in furnace 6 and would include at least one of the following: (a) the pre-plating metal, (b) an ingredient of the pre plating metal, (c) an alloy of the pre-plating metal ingredient and aluminum, and ~d) an alloy of the pre-plating metal ingredient and an ingredient of the steel substrate; none of these are present in the metal product.
207~189 The metal product is further characterized by uniformity of coating and acceptable adherence of the coating to the steel substrate.
The adherence of the coating to the steel substrate is at least equal to the adherence exhibited by the same aluminum coating where there is (a) a pre-plating metal between the coating and the substrate or (b) a diffusion product of a pre-plating metal within the metal product.
10The metal product also exhibits corrosion resistance at least equal to that of (a) the same metal product having a pre-plating metal between the aluminum coating and the steel substrate or of (b) the same metal product having, within that product, a diffusion product of the pre-plating metal.
The metal product may be in the form of an automobile exhaust component, such as a muffler, a tailpipe, a catalytic converter shell and other visible automobile components.
20The foregoing detailed description has been given for clearness of understanding only~ and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.
AND METHOD F~R PRODUCING SAME"
BACKGROUND OF THE INVENTION
This invention relates generally to chromium-containing steels with aluminum coatings and more particularly to an aluminized, AISI type 409, stainless steel.
Chromium-containing steel, particularly stainless steel, is used extensively in many industrial, residential, automotive, and high temperature applications due to its excellent ability to withstand corrosive and high temperature oxidizing environments. One widespread use of this steel is in automobile exhaust components such as mufflers, tailpipes and catalytic converter shells. These components must be able to withstand internal corrosion due to high temperature gases and external corrosion due to road salt.
The corrosion-resistant properties of stainless steel are achieved by adding chromium to steel, optimally about 10 wt.% or greater, although a lesser percentage may be used. Chromium at this level forms, on the surface of the steel, a passive film that is responsible for corrosion resistance. At higher temperatures, chromium forms an impervious oxide layer which slows down the oxidation process.
It may conceivably appear to be desirable to add aluminum to steel because, at higher temperatures, a steel containing aluminum may be more oxidation-resistant than a steel containing corresponding amounts of chromium. The oxidation resistance of aluminum-containing steel is due to the formation of aluminum-rich oxides at the surface of the steel. Unfortunately, the addition to steel of aluminum in 20~1189 sufficient quantity to form an aluminum-rich oxide layer causes manufacturing and mechanical problems.
Coating a chromium-containing steel substrate with aluminum would be desirable in order to overcome these S problems and would result in both (a) high temperature oxidation resistance due to the aluminum coating and (b) the beneficial passive film formed by the chromium contained in the substrate. The superior corrosion resistance of aluminum coated, AISI type 409 stainless steel is described by Ando et al. of Nisshin Steel Co., NACE Annual Conference and Corrosion Show Paper Number 384 (March 1991).
There is another significant reason for coating stainless steel with aluminum. After being exposed to the atmosphere, especially a humid atmosphere, AISI type 409 stainless steel and similar stainless steels become discolored and appear to have surface corrosion.
Therefore, although the steel is still protected from oxidation by the tenacious chromium oxide layer, the steel does not appear "stainless." As a result, coating the stainless steel with aluminum is desired for aesthetic purposes, especially when producing visible automotive exhaust components, because many consumers would be dissatisfied with a discolored product.
However, using a hot-dip process to coat stainless steel with aluminum presents a problem. The problem arises because stainless steel forms a tenacious, chromium oxide, outer layer during an annealing step which normally precedes hot dipping, and the chromium oxide outer layer is not wettable to molten aluminum. Therefore, the process usually results in uncoated spots and poor adherence of the aluminum coating to the stainless steel substrate. Poor adherence is reflected by flaking or cracking of the coating during bending of the strip. U~
Pat. No. 4,675,214 to Kilbane et al. discloses a method for 207~189 enhancing the wettability of a chromium-containing steel by a pure aluminum coatin~; the method involves controlling the coating conditions.
Pre-plating the surface of the stainless steel substrate with copper, nickel or iron-boron improves the substrate's coatability with molten aluminum to a degree which is commercially acceptable. One such process is disclosed in U.S. Pat. No. 4,913,785 to Uchida et al.
However, this solution is undesirable due to the added time and cost of performing such an intermediate coating or pre-plating step.
SUMMARY OF THE INVENTION
The present invention is directed to a method ~or coating a strip of chromium-containing steel, e.g.
stainless steel, with aluminum without the need for pre-plating the steel substrate before coating the substrate with aluminum, and to the product produced thereby.
In accordance with the present invention, a chromium-containing, preannealed steel substrate in the form of a strip (e.g., stainless steel strip) may be continuously coated with aluminum by preferably first subjecting the strip to a pre-cleaning step in which the substrate is passed through a heated alkaline cleaning bath, followed by rinsing and drying. The substrate is then passed through a furnace or enclosed heating zone wherein the strip is subjected to heating without flame impingement, e.g. radiant heating. The dew point inside the heating zone is controlled, preferably by passing hydrogen and nitrogen gas through the heating zone and by preventing the entry of oxygen gas into the heating zone.
A reducing atmosphere is thereby produced which will substantially reduce the formation of chromium oxides and other impurities on the surface of the chromium-containing steel strip while the strip is in the heating zone. The heating zone is kept at a controlled temperature, preferably less than 800C, so as to substantially reduce the growth of any chromium oxides which may possibly have formed on the surface of the substrate.
After the substrate passes through the heating zone, it is coated with aluminum by a hot-dip procedure, wherein the strip is dipped in a molten aluminum bath. In a preferred embodiment, the aluminum coating is an alloy which contains about 10 wt.% silicon. After the strip exits the bath, the thickness of the coating on the strip may be controlled by a jet stream of inert gas such as nitrogen. This step is preferably followed by steam cooling which solidifies the coating and prevents its transfer onto the rollers which direct the passage of the strip downstream of the bath.
The method of the present invention produces an aluminized, chromium-containing steel strip with a uniform coating and acceptable adherence. There is a substantial absence, on the coating, of spots reflecting an unwetted substrate at the interface between the substrate and the coating; no pre-plating step employing a pre-plating metal wettable by molten aluminum is necessary. Heating the steel to a low temperature in a controlled, reducing atmosphere obviates the need for such pre-plating.
Other features and advantages are inherent in the method and product claimed and disclosed or will become apparent to those skilled in the art from the following detailed description taken in conjunction with the appended drawing and claims.
BRIEF DESCRIPTION OF THE DRAWING
The sole figure is a schematic diagram illustrating a method in accordance with an embodiment of the present invention.
2 ~ 8 ~
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawing, a continuous, chromium-containing, preannealed steel strip or substrate 1 is fed from a coil 16 over a guide roller 4 into a pre-cleaning bath 2. Following pre-cleaning, substrate 1 is subjected to rinsing and drying in a rinsing and drying chamber 3.
The cleaned substrate l is fed over guide roller 19 and then passed through an enclosed furnace or heating zone ~, across supporting rollers 27 through 30. Furnace 6 has a controlled temperature and dew point. After exiting furnace 6 through a downstream furnace portion 7 or snout, substrate 1 is coated with aluminum in a hot-dip coating bath 10. The thickness of the coating is controlled by nitrogen jet knives 11 and 11'. Finally, the aluminum-coated strip is passed through a steam cooling zone 13,over a guide roller 14, and rewound into a coil 15.
The present method is applicable both to AISI
type 40g stainless steel substrates and to chromium-containing steel substrates with other compositions. The minimum chromium content of substrate 1 is, however, at least 1 wt.%, preferably at least 2 wt.%, and most preferably at least 10 wt.~. Greater than 10 wt.% chromium achieves optimal corrosion resistance.
Substrate l is preannealed before it is subjected to the invented method. Preannealing, wherein the steel is heated to a suitable temperature, held for some period of time, and then cooled at a suitable rate, gives the steel the same properties as annealing done on-line. Annealing softens the steel and can greatly increase the elongation characteristics of the steel, from less than 3% for non-annealed steel to possibly greater than 30% for annealed steel. Annealing can decrease tensile strength from about .~ .
2~71189 150 ksi for non-annealed steel to about 60 ksi for annealed steel.
The chromium-containing steel substrate may also contain any or all of the following: at least 0.25 wt.%
titanium, about 0.30 to about 1.0 wt.~ manganese, about 0.30 to about 1.0 wt.% silicon, and about 0.02 to about 0.08 wt.% carbon. The weight percentage of titanium is generally at least six times the weight percentage of carbon. Other elements that may be present in the substrate include sulfur and phosphorus. The content of sulfur, phosphorus and other residual elements should be within the acceptable ranges given by AI5I designation.
Examples of useful chromium-containing steel substrates are tabulated below:
15 Ingredient A B
chromium 11.0 wt.% 10.5 wt.% 12.5 wt.%
manganese 0.30 wt.% 1.00 wt.% 0.45 wt.%
carbon 0.04 wt.% 0.08 wt.% 0.05 wt.%
titanium 0.28 wt.% 0.48 wt.% 0.40 wt.%
phosphorus ~.03 wt.% .045 wt.% .007 wt.%
sulfur .003 wt.% .045 wt.% 0.02 wt.%
silicon 0.30 wt.% 1.00 wt.% 1.00 wt.%
iron essentially essentially essentially the balance the balance the balance In a preferred embodiment of the invention, preannealed substrate 1 is not subjected to a pre-plating step, employing a pre-plating metal wettable by aluminum, upstream of bath 10.
Substrate 1 is subjected to pre-cleaning in order to remove dirt, oil, and steel particles or fines which may be present on the surface of substrate 1. Pre-cleaning bath 2, contained in a container 21 having a guide roller 5 and either with or without scrubber brushes, may be an alkaline composition containing about 2.5 to 3.5 wt.~ NaOH.
Bath 2 typically has a temperature of about 120F to about 2~71~89 180F (48C to 82C). Rinsing and drying zone 3 employs a hot water rinse followed by hot air drying.
Furnace or enclosed heating zone 6 typically employs radiant heating under controlled conditions. There is, therefore, no flame impingement in furnace 6.
The dew point in heating zone 6 is controlled in order to minimize the formation of ~a) chromium oxides and (b) reaction products other than chromium oxides, on the surface of the chromium-containing steel strip or substrate 1, while the strip is in heating zone 6. This is accomplished by maintaining, in heating zone 6, a reducing atmosphere which is substantially devoid of water vapor and consists essentially of hydrogen and nitrogen gases, e.g., about 25 vol.% to about 50 vol.% hydrogen gas, with the balance of the atmosphere being substantially nitrogen gas.
The hydrogen and nitrogen gases are substantially devoid of water vapor and oxygen gas and are thus "dry" gases as they enter heating zone 6. These dry gases may flow continuously through heating zone 6, and this procedure will reduce the relative amount of water vapor in heating zone 6. The dew point is also controlled by preventin~
entry into heating zone 6 of oxygen gas, for example by avoiding leaks.
In a preferred embodiment of the invention, heatiny zone 6 has an upstream portion 17 (the main body of the heating zone~ having an entry 24, and there is a downstream furnace portion 7 (known as the furnace "snout"~
having an exit end 25. In this embodiment, the dew point in upstream portion 17 is maintained at a temperature in 30 the range of about -30F to about -10F (-35C to -23C).
At entry 24 into upstream furnace portion 17l the dew point is maintained in the range of about -20F to -10F (-29C
to -23C). The dew point in downstream furnace portion 7 is maintained at a temp~rature in the range of about -50F
35 to about -45F (-46C to -42C). The dew point is 20~1189 maintained at about -60F (-51C) at exit end 25 of downstream portion 7.
The temperature in heating zone 6 is controlled so as to substantially reduce the growth o~ any chromium oxides which may possibly form on the surface of substrate 1. The temperature at entry 24 of upstream furnace portion 17 is maintained in the range of about 200F to about 300F
(93C to 149C). The maximum temperature in heating zone 6 is no greater than about 1570F to about 1670~F (854C to 910C).
Substrate 1 is heated in heating zone 6 to a temperature in the range of about 1350F to about 1400F
(733C to 760C) and maintained at that temperature in heating zone 6. The maximum temperature of substrate 1 in heating zone 6 is no greater than about 1470F (800C).
The temperature of substrate 1 cools down to about 1250F
to about 1350F (677C to 733C) by the time it exits downstream furnace portion 7.
The hot dip coating procedure of the present invention may employ a coating metal consisting essentially of aluminum. This is known as "Type 2" aluminum.
Alternatively, the adherent coating may be an alloy of aluminum containing about 10 wt.% silicon, known as "Type 1" aluminum.
Coating bath 10, contained in container 22 and employing guide roller 2~, commonly referred to as a sinker roll, has a temperature which is maintained in the range of about 1200F to about 1250F (648C to 677C). Substrate l is immersed in coating bath 10 for a time of about 3 seconds to about 10 seconds.
Jet knives 11 and 11', which control the thickness of the coating, direct nitrogen or other gas at the coated strip. The thickness of the aluminum coating is controlled to be in the range of about 10 microns to about 25 microns.
20711~9 The cooling of coated strip 18 is performed by directing steam against coated strip 18 in steam cooling zone 13 which is contained in housing 23. This cools coated strip 18 to a temperature in the range of about 700F to about 900F (370C to 482C) before it reaches roller 1~. Cooling solidifies the coating and prevents transfer of the coating onto a roller 14 which directs the passage of coated strip 18 downstream of housing 23.
Coated strip 18 is then wound into coil 15, for easy transportation and storage.
~ metal product made from coil 15 includes a chromium-containing steel substrate and an adherent, hot-dip coating on the substrate. The coating consists essentially of aluminum or an alloy of aluminum. The metal product is characterized by the substantial absence, on the coating, of spots reflecting an unwetted substrate at the interface between the substrate and the coating. Such spots are absent despite the absence of (a) a pre-plating metal, wettable by molten aluminum, between the substrate and the coating, and (b) any diffusion product of a pre-plating metal wettable by molten aluminum, within the metal product.
Pre-plating metals include at least one ingredient such as copper, nickel or iron-boron, which are conventionally used to provide a surface wettable by molten aluminum. The diffusion product of the pre-plating metal could be generated during heating in furnace 6 and would include at least one of the following: (a) the pre-plating metal, (b) an ingredient of the pre plating metal, (c) an alloy of the pre-plating metal ingredient and aluminum, and ~d) an alloy of the pre-plating metal ingredient and an ingredient of the steel substrate; none of these are present in the metal product.
207~189 The metal product is further characterized by uniformity of coating and acceptable adherence of the coating to the steel substrate.
The adherence of the coating to the steel substrate is at least equal to the adherence exhibited by the same aluminum coating where there is (a) a pre-plating metal between the coating and the substrate or (b) a diffusion product of a pre-plating metal within the metal product.
10The metal product also exhibits corrosion resistance at least equal to that of (a) the same metal product having a pre-plating metal between the aluminum coating and the steel substrate or of (b) the same metal product having, within that product, a diffusion product of the pre-plating metal.
The metal product may be in the form of an automobile exhaust component, such as a muffler, a tailpipe, a catalytic converter shell and other visible automobile components.
20The foregoing detailed description has been given for clearness of understanding only~ and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.
Claims (50)
1. A method for producing a coated strip substantially free of uncoated areas and comprising a chromium-containing steel substrate and a coating composed of aluminum or an alloy thereof, said method comprising the steps of:
passing a chromium-containing steel substrate through an enclosed heating zone;
controlling the dew point of the atmosphere within said heating zone to substantially reduce the formation of chromium oxides on the surface of said strip while the strip is in said heating zone;
controlling the temperature within said heating zone to substantially reduce the growth of any chromium oxides which may form on said surface;
and then applying to said substrate, by a hot dip procedure, a coating composed of aluminum or an alloy thereof.
passing a chromium-containing steel substrate through an enclosed heating zone;
controlling the dew point of the atmosphere within said heating zone to substantially reduce the formation of chromium oxides on the surface of said strip while the strip is in said heating zone;
controlling the temperature within said heating zone to substantially reduce the growth of any chromium oxides which may form on said surface;
and then applying to said substrate, by a hot dip procedure, a coating composed of aluminum or an alloy thereof.
2. A method as recited in claim 1 wherein:
said method is performed without previously subjecting said steel substrate to a pre-plating step employing a pre-plating metal wettable by aluminum.
said method is performed without previously subjecting said steel substrate to a pre-plating step employing a pre-plating metal wettable by aluminum.
3. A method as recited in claim 1 and further comprising:
subjecting said steel substrate to a pre-cleaning step.
subjecting said steel substrate to a pre-cleaning step.
4. A method as recited in claim 3 wherein:
said pre-cleaning step comprises passing said substrate through an alkaline cleaning bath.
said pre-cleaning step comprises passing said substrate through an alkaline cleaning bath.
5. A method as recited in claim 4 wherein:
said alkaline cleaning bath contains about 2.5 to 3.5 wt.% NaOH and has a temperature of about 120°F to about 180°F (48°C to 82°C).
said alkaline cleaning bath contains about 2.5 to 3.5 wt.% NaOH and has a temperature of about 120°F to about 180°F (48°C to 82°C).
6. A method as recited in claims 3 or 4 and further comprising: rinsing and drying said substrate after said pre-cleaning step.
7. A method as recited in claim 1 and comprising:
controlling said atmosphere within said heating zone additionally to prevent formation on said steel substrate of reaction products other than chromium oxides.
controlling said atmosphere within said heating zone additionally to prevent formation on said steel substrate of reaction products other than chromium oxides.
8. A method as recited in claims 1 or 7 and further comprising:
preventing entry into said heating zone of oxygen gas.
preventing entry into said heating zone of oxygen gas.
9. A method as recited in claims 1 or 7 wherein said step of controlling the dew point comprises:
maintaining in the heating zone an atmosphere consisting essentially of hydrogen and nitrogen gases and which is substantially devoid of water vapor.
maintaining in the heating zone an atmosphere consisting essentially of hydrogen and nitrogen gases and which is substantially devoid of water vapor.
10. A method as recited in claim 1 wherein said temperature controlling step comprises:
maintaining said heating zone at an elevated temperature no greater than about 1670°F (910°C).
maintaining said heating zone at an elevated temperature no greater than about 1670°F (910°C).
11. A method as recited in claim 1 wherein:
said atmosphere in said heating zone contains from about 25 vol.% to about 50 vol.% hydrogen.
said atmosphere in said heating zone contains from about 25 vol.% to about 50 vol.% hydrogen.
12. A method as recited in claim 11 wherein:
the balance of said atmosphere is substantially nitrogen gas.
the balance of said atmosphere is substantially nitrogen gas.
13. A method as recited in claims 11 or 12 wherein:
said hydrogen and nitrogen gases are substantially devoid of water vapor and oxygen gas.
said hydrogen and nitrogen gases are substantially devoid of water vapor and oxygen gas.
14. A method as recited in claims 11 or 12 wherein:
said hydrogen and nitrogen gases continuously flow through said heating zone.
said hydrogen and nitrogen gases continuously flow through said heating zone.
15. A method as recited in claim 1 wherein:
said heating zone has an upstream portion and a downstream portion.
said heating zone has an upstream portion and a downstream portion.
16. A method as recited in claim 15 wherein:
the dew point in said upstream portion of said heating zone is maintained at a temperature in the range of about -30°F to about -10°F (-35°C to -23°C).
the dew point in said upstream portion of said heating zone is maintained at a temperature in the range of about -30°F to about -10°F (-35°C to -23°C).
17. A method as recited in claim 15 wherein:
the dew point in said downstream portion of said heating zone is maintained at a temperature in the range of about -50°F to about -45°F (-46°C to -42°C).
the dew point in said downstream portion of said heating zone is maintained at a temperature in the range of about -50°F to about -45°F (-46°C to -42°C).
18. A method as recited in claims 1 or 15 wherein:
said upstream portion of said heating zone has an entry;
and the dew point at said entry is maintained at a temperature in the range of about -20°F to about -10°F (-29°C to -23°C).
said upstream portion of said heating zone has an entry;
and the dew point at said entry is maintained at a temperature in the range of about -20°F to about -10°F (-29°C to -23°C).
19. A method as recited in claims 1 or 15 wherein:
said downstream portion of said heating zone has an exit;
and the dew point at said exit is maintained at a temperature of about -60°F (-51°C).
said downstream portion of said heating zone has an exit;
and the dew point at said exit is maintained at a temperature of about -60°F (-51°C).
20. A method as recited in claim 15 wherein said upstream portion of said heating zone has an entry and said temperature controlling step comprises:
maintaining said entry at a temperature in the range of about 200°F to about 300°F (93°C to 149°C).
maintaining said entry at a temperature in the range of about 200°F to about 300°F (93°C to 149°C).
21. A method as recited in claim 1 wherein:
said hot dip procedure employs a hot dip coating bath having a temperature which is maintained in the range of about 1200°F to about 1250°F (648°C to 677°C).
said hot dip procedure employs a hot dip coating bath having a temperature which is maintained in the range of about 1200°F to about 1250°F (648°C to 677°C).
22. A method as recited in claims 1 or 21 wherein:
said hot dip procedure employs a hot dip coating bath;
and said steel substrate is immersed in said hot dip coating bath for about 3 seconds to about 10 seconds.
said hot dip procedure employs a hot dip coating bath;
and said steel substrate is immersed in said hot dip coating bath for about 3 seconds to about 10 seconds.
23. A method as recited in claim 1 wherein:
said coated strip is cooled following said hot dip procedure.
said coated strip is cooled following said hot dip procedure.
24. A method as recited in claim 23 wherein:
said cooling step is performed by directing steam against said coated strip.
said cooling step is performed by directing steam against said coated strip.
25. A method as recited in claims 23 or 24 wherein:
said coated strip is cooled to a temperature in the range of about 700°F to about 900°F (370°C to 482°C).
said coated strip is cooled to a temperature in the range of about 700°F to about 900°F (370°C to 482°C).
26. A method as recited in claim 1 wherein:
said hot dip procedure employs, as the coating metal, an aluminum alloy which contains about 10 wt.% silicon.
said hot dip procedure employs, as the coating metal, an aluminum alloy which contains about 10 wt.% silicon.
27. A method as recited in claim 1 wherein:
said steel substrate is heated in said heating zone to a temperature of about 1350°F to about 1400°F (733°C to 760°C) and maintained at that temperature before said hot dip procedure.
said steel substrate is heated in said heating zone to a temperature of about 1350°F to about 1400°F (733°C to 760°C) and maintained at that temperature before said hot dip procedure.
28. A method as recited in claim 1 wherein:
said chromium-containing steel substrate is cooled to a temperature in the range of about 1250°F to about 1350°F
(677°C to 733°C) before said hot dip procedure.
said chromium-containing steel substrate is cooled to a temperature in the range of about 1250°F to about 1350°F
(677°C to 733°C) before said hot dip procedure.
29. A method as recited in claim 1 wherein:
said chromium-containing steel substrate contains at least 1 wt.% chromium.
said chromium-containing steel substrate contains at least 1 wt.% chromium.
30. A method as recited in claim 1 wherein:
said chromium-containing steel substrate contains at least 2 wt.% chromium.
said chromium-containing steel substrate contains at least 2 wt.% chromium.
31. A method as recited in claim 1 wherein:
said chromium-containing steel substrate contains at least 10 wt.% chromium.
said chromium-containing steel substrate contains at least 10 wt.% chromium.
32. A method as recited in claim 1 and further comprising:
controlling the thickness of said aluminum coating on said substrate with one or more streams of gas directed at said substrate.
controlling the thickness of said aluminum coating on said substrate with one or more streams of gas directed at said substrate.
33. A method as recited in claim 1 and further comprising:
subjecting said chromium-containing steel substrate to a preannealing step.
subjecting said chromium-containing steel substrate to a preannealing step.
34. A method as recited in claim 1 wherein:
said chromium-containing steel substrate contains at least 1 wt.% chromium, at least 0.25 wt.% titanium, about 0.30 to about 1.0 wt.% manganese, about 0.30 to about 1.0 wt.% silicon, and about 0.02 to about 0.08 wt.% carbon.
said chromium-containing steel substrate contains at least 1 wt.% chromium, at least 0.25 wt.% titanium, about 0.30 to about 1.0 wt.% manganese, about 0.30 to about 1.0 wt.% silicon, and about 0.02 to about 0.08 wt.% carbon.
35. A metal product comprising a chromium-containing steel substrate and an adherent hot-dip coating on said substrate, said adherent coating consisting essentially of aluminum or an alloy thereof, said product being characterized by:
the substantial absence on said coating of spots reflecting an unwetted substrate at the interface between the substrate and the coating;
the absence of a preplating metal wettable by molten aluminum, between said substrate and said coating;
and the absence of any diffusion product of said preplating metal within said metal product.
the substantial absence on said coating of spots reflecting an unwetted substrate at the interface between the substrate and the coating;
the absence of a preplating metal wettable by molten aluminum, between said substrate and said coating;
and the absence of any diffusion product of said preplating metal within said metal product.
36. A metal product as recited in claim 35 and characterized by:
the absence of a preplating metal comprising at least one ingredient;
and the absence of a diffusion product comprising at least one of (a) said preplating metal, (b) an ingredient of said preplating metal, (c) an alloy of a preplating metal ingredient and aluminum, and (d) an alloy of a preplating metal ingredient and an ingredient of said substrate.
the absence of a preplating metal comprising at least one ingredient;
and the absence of a diffusion product comprising at least one of (a) said preplating metal, (b) an ingredient of said preplating metal, (c) an alloy of a preplating metal ingredient and aluminum, and (d) an alloy of a preplating metal ingredient and an ingredient of said substrate.
37. A metal product as recited in claim 35 wherein:
said aluminum alloy coating contains about 10 wt.% silicon.
said aluminum alloy coating contains about 10 wt.% silicon.
38. A metal product as recited in claims 35 or 37 wherein:
said metal coating has a thickness of about 10 to 25 microns.
said metal coating has a thickness of about 10 to 25 microns.
39. A metal product as recited in claim 35 wherein:
said chromium-containing steel substrate contains at least 1 wt.% chromium.
said chromium-containing steel substrate contains at least 1 wt.% chromium.
40. A metal product as recited in claim 35 wherein:
said chromium-containing steel substrate contains at least 2 wt.% chromium.
said chromium-containing steel substrate contains at least 2 wt.% chromium.
41. A metal product as recited in claim 35 wherein:
said chromium-containing steel substrate contains at least 10 wt.% chromium.
said chromium-containing steel substrate contains at least 10 wt.% chromium.
42. A metal product as recited in any of claims 39-41 wherein:
said chromium-containing steel substrate contains at least 0.25 wt.% titanium.
said chromium-containing steel substrate contains at least 0.25 wt.% titanium.
43. A metal product as recited in any of claims 39-41 wherein:
said chromium-containing steel substrate contains about 0.30 to about 1.0 wt.% manganese.
said chromium-containing steel substrate contains about 0.30 to about 1.0 wt.% manganese.
44. A metal product as recited in any of claims 39-41 wherein:
said chromium-containing steel substrate contains about 0.30 to about 1.0 wt.% silicon.
said chromium-containing steel substrate contains about 0.30 to about 1.0 wt.% silicon.
45. A metal product as recited in any of claims 39-41 wherein:
said chromium-containing steel substrate contains about 0.04 to about 0.08 wt.% carbon.
said chromium-containing steel substrate contains about 0.04 to about 0.08 wt.% carbon.
46. A metal product as recited in any of claims 39-41 wherein:
said chromium-containing steel substrate contains at least 0.25 wt.% titanium, about 0.30 to about 1.0 wt.%
manganese, about 0.30 to about 1.0 wt.% silicon, and about 0.04 to about 0.08 wt.% carbon.
said chromium-containing steel substrate contains at least 0.25 wt.% titanium, about 0.30 to about 1.0 wt.%
manganese, about 0.30 to about 1.0 wt.% silicon, and about 0.04 to about 0.08 wt.% carbon.
47. A metal product as recited in claim 35 wherein:
said metal product exhibits corrosion resistance at least equal to that of the same metal product with (a) said preplating metal between said coating and said substrate or (b) a diffusion product of said preplating metal within said product.
said metal product exhibits corrosion resistance at least equal to that of the same metal product with (a) said preplating metal between said coating and said substrate or (b) a diffusion product of said preplating metal within said product.
48. A metal product as recited in claim 35 wherein:
said coating exhibits adherence to said substrate at least equal to the adherence exhibited by the same coating with (a) said preplating metal between said coating and said substrate or (b) a diffusion product of said preplating metal within said product.
said coating exhibits adherence to said substrate at least equal to the adherence exhibited by the same coating with (a) said preplating metal between said coating and said substrate or (b) a diffusion product of said preplating metal within said product.
49. A metal product as recited in claim 35 wherein:
said metal product is in the form of an automobile exhaust component.
said metal product is in the form of an automobile exhaust component.
50. A metal product as recited in claim 35 wherein said product is further characterized by:
the absence of said diffusion product of said preplating metal in either said substrate or said coating.
the absence of said diffusion product of said preplating metal in either said substrate or said coating.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US83990992A | 1992-02-21 | 1992-02-21 | |
| US07/839,909 | 1992-02-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2071189A1 true CA2071189A1 (en) | 1993-08-22 |
Family
ID=25280949
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2071189 Abandoned CA2071189A1 (en) | 1992-02-21 | 1992-06-12 | Aluminized stainless steel and method for producing same |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2071189A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5447754A (en) * | 1994-04-19 | 1995-09-05 | Armco Inc. | Aluminized steel alloys containing chromium and method for producing same |
-
1992
- 1992-06-12 CA CA 2071189 patent/CA2071189A1/en not_active Abandoned
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5447754A (en) * | 1994-04-19 | 1995-09-05 | Armco Inc. | Aluminized steel alloys containing chromium and method for producing same |
| US5591531A (en) * | 1994-04-19 | 1997-01-07 | Armco Inc. | Aluminized steel alloys containing chromium |
| AU687989B2 (en) * | 1994-04-19 | 1998-03-05 | Armco Inc. | Aluminized steel alloys containing chromium and method for producing same |
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