Sheet metal with an aluminum-containing coating having low emissivity

A coating layer for sheet metal is provided that is comprised of an aluminum-silicon alloy having low emissivity. The coated sheet metal may be used as heat shield material, particularly for heat sources having temperatures greater than 500.degree. C., which sources may be, e.g., the hotter parts of the conduits of automotive exhaust systems. The sheet metal may be sheet steel coated on at least one of its principal surfaces with a layer of a coating comprised of an alloy of silicon in the amount of 7-11 wt. % and aluminum in the amount of 87-93 wt. %. The coated surface of the sheet has a monochromatic emissivity less than 0.15 for all wavelengths in the range of 1.5-15 microns.

BACKGROUND OF THE INVENTION
 The invention relates to the area of technology of sheet metal having an
 aluminum-containing coating. In particular, it relates to sheet metal
 having an aluminum-containing coating which coating comprises an
 aluminum-silicon alloy. Such coated sheet metal is used, e.g. to produce
 heat shields for the exhaust system conduits of automobiles.
 The object of a heat shield is to insulate pieces disposed behind it from a
 source of heat disposed in front of it. Thus, a heat shield should have
 the minimum possible energy absorptivity; stated otherwise, it should have
 maximum repellence for incident energy. Such behavior is characterized by
 low emissivity of the constituent material; in other words, high
 reflectance. Thus, desirable heat shields are comprised of materials which
 have satisfactory mechanical properties, have good formability for
 fabrication purposes, and good resistance to corrosion, and further have
 low emissivity.
 It is known to produce heat shields from sheet metal having an
 aluminum-containing coating which coating comprises an aluminum-silicon
 alloy. An example of such coated sheet metal is low carbon steel coated on
 its two principal surfaces with an aluminum-silicon alloy which is applied
 by passing the sheet into a bath of the fused alloy. During the passage of
 the sheet into the coating bath comprising the aluminum-silicon alloy, a
 layer of an iron-aluminum-silicon alloy develops. Accordingly, the
 metallographic cross section of the coating reveals the following
 structure:
 a surface layer having a composition close to that of the bath, and
 a subsurface layer comprising a ternary alloy, of composition Fe.sub.3
 Si.sub.2 Al.sub.12. Such sheet metal with an aluminum-containing coating
 has a low overall emissivity, less than 0.2, and thus a high reflectivity,
 greater than 80%. This characteristic is maintained at temperatures up to
 450.degree. C. The material is thereby of substantial engineering
 interest, for use for interior walls of industrial or household furnaces,
 heat reflectors for all manner of household appliances, or heat shields
 for conduits in automotive exhaust systems, but not for the relatively
 hotter such conduits.
 A method of improving the properties of such materials is known wherein the
 material is passed through a roll stand or roll housing, known as a
 "skin-pass" stand, where it is cold-worked by means of smooth rolls. This
 process is capable of slightly reducing the emissivity of the material,
 but at the cost of degradation of desirable high-temperature properties.
 The object of the present invention is to solve the above-described problem
 by devising a sheet metal having an aluminum-containing coating which
 coating comprises an aluminum-silicon alloy, wherewith said coated sheet
 metal has low emissivity and can be used as a heat shield for heat sources
 having temperatures above 500.degree. C., e.g. conduits of which
 automotive exhaust systems are formed, including the hottest such
 conduits.
 SUMMARY OF THE INVENTION
 More particularly the invention relates to steel sheet coated on at least
 one of its principal surfaces with a layer of a coating comprised of an
 aluminum-based alloy comprised of aluminum and silicon, including silicon
 less than 11 wt. %, particularly comprising 7-11 wt. % silicon and 87-93
 wt. % aluminum; characterized in that the coated surface has a
 monochromatic emissivity less than 0.15 for all wavelengths in the range
 of 1.5-15 microns. According to another feature, the coated surface has a
 monochromatic emissivity less than 0.10 for all wavelengths in the range
 of 5-15 microns, and a monochromatic emissivity in the range 0.10-0.15 for
 all wavelengths in the range of 1.5-5 microns.
 The invention further relates to a method of fabricating steel sheet of the
 described type; characterized by the following steps:
 production of steel sheet coated on at least one of its principal surfaces
 by a layer of a coating in the solid state, which coating is comprised of
 an aluminum-based alloy comprised of aluminum and silicon, said alloy
 including silicon less than 11 wt. %, comprising 7-11 wt. % silicon and
 87-93 wt. % aluminum;
 heating the coating layer to a temperature T1 which is greater than the
 fusion temperature T2 of said coating;
 maintaining the coating layer at the said temperature level T1 greater than
 the fusion temperature T2 of the coating, for a duration between 0 and 100
 sec, preferably between 0 and 10 sec;
 cooling the sheet to a temperature at least equal to the limiting alloying
 temperature of alloying between the coating and the steel, and preferably
 cooling the sheet to the ambient temperature (room temperature).
 According to other features:
 The temperature T1 to which the coating layer is heated is between the
 fusion temperature T2 of the coating layer and 650.degree. C.
 The temperature T1 is 10-15.degree. C. above the fusion temperature T2 of
 the coating layer.
 The rate of heating of the coating layer is in the range 20-100.degree. C.
 per second.
 The cooling of the coated sheet metal is natural cooling in the open air,
 or "forced" radiative cooling.
 The cooling of the coated sheet metal is forced-air cooling.
 The cooling of the sheet metal is carried out in at least two stages, as
 follows:
 natural cooling to the fusion temperature T2 of the coating; followed by
 forced-air cooling to the limiting temperature of alloying between the
 coating and the steel.
 The sheet steel coated on at least one of its principal surfaces by a layer
 of a coating in the solid state, which coating is comprised of an
 aluminum-based alloy of a type comprised of aluminum and silicon, said
 alloy comprising silicon less than 11 wt. %, is fabricated by dip-coating
 a steel substrate in a fused bath comprising silicon 9-10 wt. %, iron c. 3
 wt. %, and the remainder aluminum, and cooling the coated substrate to a
 temperature less than the fusion temperature T2 of the coating.
 Finally, the invention relates to a heat shield comprised of a described
 coated sheet metal.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT
 As seen from FIG. 1, the principal characteristic of the inventive coated
 metal sheet coated on at least one of its principal surfaces with an
 aluminum-containing coating comprised of an alloy of a type comprised of
 aluminum and silicon, said alloy comprising silicon less than 11 wt.%, is
 that the coated surface has a monochromatic emissivity less than 0.15 for
 all wavelengths in the range of 1.5-15 microns.
 More precisely, the coated surface has a monochromatic emissivity less than
 0.10 for all wavelengths in the range of 5-15 microns, and a monochromatic
 emissivity in the range of 0.10-0.15 for all wavelengths in the range
 1.5-15 microns.
 The term "monochromatic emissivity" is understood to mean the ratio of the
 luminance of the material at a given wavelength to the luminance of a
 theoretical black body at the same wavelength and temperature.
 Such an inventive steel sheet having an aluminum-containing coating is
 fabricated in several stages.
 In a first stage, a steel sheet is produced which is coated on at least one
 of its principal surfaces with a layer of a coating, in the solid state,
 said coating being comprised of an aluminum-based alloy formed from
 aluminum and silicon, comprising silicon less than 11 wt. %, particularly
 comprising 7-11 wt. % silicon and 87-93 wt. % aluminum.
 In a second stage, the coating layer is heated to a temperature T1 which is
 greater than the fusion temperature T2 of said coating.
 The "fusion temperature T2" is understood to mean the temperature of the
 onset of fusion of the coating. In practice, an aluminum-based coating
 such as described hereinabove is in the form of dendrites of aluminum with
 an inter-dendritic phase and a dendritic phase. The inter-dendritic phase
 fuses at a temperature lower than the temperature at which the dendritic
 phase fuses; wherewith the temperature T2 of interest is the fusion
 temperature of the said inter-dendritic phase.
 In a third stage of the fabrication, the coating layer is maintained at the
 aforesaid temperature T1, or in any event at a temperature greater than
 T2, for a duration between 0 and 100 sec, preferably between 0 and 10 sec.
 In a final stage, the coated metal sheet is cooled to a temperature at
 least equal to the limiting alloying temperature of alloying between the
 coating and the steel, and preferably the sheet is cooled to the ambient
 temperature (room temperature).
 The described fabrication method enables a remelting of the
 aluminum-containing coating.
 The production of:
 the steel sheet coated on at least one of its principal surfaces with a
 layer of a coating, in the solid state, said coating being comprised of an
 aluminum-based alloy comprised of aluminum and silicon, comprising silicon
 less than 11 wt. %, particularly comprising 7-11 wt. % silicon and 87-93
 wt. % aluminum, which production corresponds to the first stage of the
 inventive method, may be carried out by dip-coating a steel substrate in a
 fused bath comprising silicon 9-10 wt. %, iron c. 3 wt. %, and the
 remainder aluminum, and cooling the coated substrate to a temperature less
 than the fusion temperature of the coating.
 It is very important that the steel sheet having an aluminum-containing
 coating, which coated steel sheet is produced in the first stage of the
 fabrication method, has a coating layer in the solid state, i.e. that said
 sheet has been cooled to a temperature less than the fusion temperature of
 the coating.
 A less important factor for obtaining the emissivity characteristics of the
 inventive metal sheet is for the temperature to which is cooled to be:
 several degrees below the fusion temperature of the coating, e.g.
 5-10.degree. C. below said fusion temperature; or, e.g.,--ambient (room)
 temperature.
 The temperature T1 which the sheet reaches during the heating carried out
 in the second stage of the method must mandatorily be greater than the
 fusion temperature T2 of the coating, in order to ensure re-melting of the
 coating layer so as to obtain the emissivity characteristics of the coated
 sheet metal according to the invention.
 Preferably, the temperature T1 is between the fusion temperature T2 of the
 coating layer and 650.degree. C.
 The limit of 650.degree. C. allows the cost of the second stage to be
 limited, and further has the benefit of limiting the phenomenon of
 alloying between the coating and the steel.
 To ensure that every part of the coating layer is re-melted, it is
 preferable to heat the coated metal sheet to a temperature T1 which is
 between a temperature 10.degree. C. above the fusion temperature T2 of the
 coating layer and a temperature 15.degree. C. above said fusion
 temperature T2 of the coating layer.
 This feature enables one to avoid the effect of possible minor temperature
 nonuniformities due, e.g., to nonuniformity of thickness of the coating
 layer, or due to peculiarities of the heating system. It is important that
 the temperature T1 be reached rapidly, so as to limit the phenomena of
 alloying between the coating and the steel of the substrate.
 Advantageously, the rate of such heating is in the range 20-100.degree.
 C./sec.
 In a case where the temperature of the coating layer on the metal sheet,
 which layer is produced during the first stage, is close to the fusion
 temperature T2 of the coating, one may select a heating rate in the range
 20-30.degree. C./sec, because in this case the temperature of the coated
 sheet only needs to be raised by an amount on the order of 20-50.degree.
 C.
 On the other hand, in a case where the temperature of the coating layer on
 the metal sheet, which layer is produced during the first stage, is close
 to ambient temperature, one would choose a heating rate in the range
 90-100.degree.C./sec, because in this case the temperature of the coated
 sheet needs to be raised by an amount on the order of 500 -600.degree. C.
 In the third stage of the method, the coating layer is maintained at the
 said temperature T1 for a duration between 0 and 100 sec, preferably
 between 0 and 10 sec.
 In the final stage of the method, it is possible to begin cooling the metal
 sheet immediately after all parts of the coating layer reach a temperature
 T1 greater than the fusion temperature of the coating.
 E.g., in a case where the temperature T1 reached by the coating layer
 during the heating stage (second stage of the method) is 10-15.degree. C.
 above the fusion temperature of the coating layer, it is possible to
 eliminate the period of maintenance of the coated sheet at said
 temperature T1. In any event, according to the invention, such a period of
 maintenance of the coated sheet at temperature T1 will not be detrimental
 in a major way provided that it is not longer than 100 sec.
 The Applicant has found that if this temperature T1 is maintained for a
 duration greater than 100 sec, the emissivity of the coating layer will be
 excessively increased, in the case of standard steel or a type "IF"
 titanium steel, since the emissivity begins increasing after 10 sec. In
 the case of nitride case-hardened steels, the presence of the nitrogen
 retards the alloying phenomenon, and the emissivity is not appreciably
 increased, but the surface becomes oxidized, wherewith the metal sheet
 having an aluminum-containing coating turns whitish and eventually
 yellowish.
 This phenomenon, viz., the sharp increase in emissivity after the coated
 sheet is held more than 100 sec at T1, is quite apparent in FIG. 2, which
 presents a plot of total emissivity of the coating layer as a function of
 temperature and of duration of heating.
 The plot in FIG. 2 was prepared from an experiment with a metal sheet
 having an aluminum-containing coating, which sheet comprised a substrate
 comprised of "IF" titanium steel 0.3 mm thick, coated with a coating 20
 micron thick comprised of silicon 9.5 wt. %, iron 3 wt. %, and the
 remainder aluminum.
 This coated steel sheet was heated from ambient temperature until the
 temperature of the coating layer reached T1=600.degree. C., which was
 greater than the fusion temperature (T2) of the coating, which temperature
 T2 was 480.degree. C. in this example; and the coated steel sheet was held
 at said 600.degree. C. for 450 sec. Throughout the execution of the
 heating phase and the phase of maintaining the coated sheet at 600.degree.
 C., the total emissivity of the coating layer was measured in real time,
 for wavelengths in the range of 1.5-14.5 micron, using a
 spectroradiometer.
 The plot of emissivity vs. time (FIG. 2) shows clearly that, after the
 fusion temperature (T2) is reached, the emissivity of the coating
 decreases; however, when the coating layer is maintained at 600.degree. C.
 for approximately 10 sec, the emissivity begins increasing again, slowly
 at first, then more rapidly after the coating layer has been maintained at
 600.degree. C. for 100 sec.
 The Applicant has also found that the described progressive increase in the
 emissivity is a function only of the duration of maintenance of the
 coating layer at the temperature T1.
 As seen from FIG. 2 (dashed lines), the increase in emissivity can be
 stopped by cooling the coating layer according to the invention.
 The plot represented in FIG. 3 demonstrates the effect of nitrogen on the
 phenomenon of alloying of the coating, which effect is per se known in its
 generalized aspects.
 The plot in FIG. 3 was prepared from an experiment with a metal sheet
 having an aluminum-containing coating, which sheet comprised a substrate
 comprised of nitride-case-hardened ("re-nitrided") steel with a nitrogen
 content greater than that of the "IF" titanium steel described supra. The
 coating layer and the heat treatment were the same as in the preceding
 experiment.
 It is seen clearly from the plot in FIG. 3, in comparison to that in FIG.
 2., that the emissivity does not begin to increase in this case until the
 coating has been held at temperature T1 for 120 sec.
 Accordingly, in the final stage of the process the coated metal sheet is
 cooled to a temperature at least equal to the limiting temperature of
 alloying between the coating and the steel, and preferably the sheet is
 cooled to the ambient temperature (room temperature).
 This cooling may be natural cooling in the open air, or so-called "forced
 radiative cooling", or forced-air cooling.
 Preferably the cooling of the coated metal sheet is carried out in at least
 two stages, as follows:
 natural cooling from the temperature T1 to the fusion temperature T2 of the
 coating; followed by
 forced-air cooling from said fusion temperature to the limiting temperature
 of alloying between the coating and the steel.
 In practice, it is preferable to carry out an initial stage of cooling
 without contact with the coating layer which is still in a fused state, in
 order to avoid degradation of the emissivity properties of the coating
 layer which might result from such contact.
 Suitable cooling means for this initial cooling stage are:
 natural cooling in air and
 "forced radiative cooling" by passing the coating layer close to a
 refrigerated wall.
 The application of forced means of cooling, e.g. with forced air, at least
 between the fusion temperature of the coating and the limiting temperature
 of alloying between the coating and the steel, enables any such alloying
 to be minimized.
 The shorter the duration of the thermal cycle (heating, maintaining the
 temperature, and re-cooling) the better the quality of the inventive metal
 sheet having an aluminum-containing coating, because thereby in particular
 the time said coated sheet spends at temperatures above the limiting
 temperature of alloying between the coating and the steel substrate is
 shorter, so that the amount of ternary alloy which is developed between
 the substrate and the coating is less.
 The Applicant has found that the metal sheet having an aluminum-containing
 coating obtained according to the described method not only has a total
 emissivity lower than that of a comparable coated sheet of the customary
 type, such as the coated sheet exiting the first stage of the described
 process, but also the inventive coated sheet has a monochromatic
 emissivity which is substantially uniform over the wavelength range of
 1.5-15 micron.
 This characteristic may be seen from FIG. 1, showing the spectral
 emissivity of:
 a metal sheet B having an aluminum-containing coating according to the
 invention, and
 a second metal sheet A having an aluminum-containing coating according to
 the state of the art.
 The spectral plot of the emissivity of the coated sheet A according to the
 state of the art was prepared from a coated sheet comprising a substrate
 comprised of "IF " titanium steel 0.3 mm thick, coated with a coating 20
 microns thick comprised of silicon 9.5 wt. %, iron 3 wt. %, and the
 remainder aluminum.
 The emissivity of this coated sheet A was measured over the range of
 wavelengths of 1.3-15 microns, which wavelengths are characteristic of the
 infrared band.
 As may be seen, the monochromatic emissivity of the coated sheet A is
 greater than 0.35 for wavelengths between 2 microns and 3.6 microns, is
 below 0.15 only at wavelengths above 7.5 microns, and is everywhere above
 0.07.
 Accordingly, a heat shield produced from such a coated sheet will be
 suitable to insulate against sources having their radiative emissions of
 energy principally at wavelengths above 7.5 micron, corresponding to
 temperatures below 500.degree. C. in the case of a gray body which may be
 deemed similar to automotive exhaust conduits.
 On the other hand, the heat shielding will be less effective in the case of
 sources having appreciable emissions at wavelengths below 7.5 microns,
 corresponding to automotive exhaust conduits operating at temperatures
 above 500.degree. C., viz. the hottest parts of exhaust systems, e.g. the
 catalytic unit.
 The second spectral plot, a spectral plot of the emissivity of a coated
 metal sheet B according to the invention, was prepared from a coated sheet
 comprising a substrate comprising a sheet comprised of "IF" titanium steel
 0.3 mm thick, coated with a coating 20 microns thick comprised of silicon
 9.5 wt. %, iron 3 wt. %, and the remainder aluminum. After being cooled to
 ambient (room) temperature, this coated sheet was reheated to 600.degree.
 C., maintained at 600.degree. C. for 5 sec, and then cooled by natural air
 cooling back to ambient temperature. The emissivity of this coated sheet B
 was also measured over the wavelength range 1.3-15 micron.
 As may be seen, the monochromatic emissivity of coated sheet B according to
 the invention was lower than 0.15 over the entire wavelength range 1.5-15
 micron; in particular said emissivity was in the range 0.10-0.15 for
 wavelengths of 1.5-4.5 microns, was in the range 0.07-0.10 for wavelengths
 of 4.5-6.5 microns, and was below 0.7 for wavelengths of greater than 6.5
 microns.
 Accordingly, a heat shield produced from such a coated sheet will be well
 suited to insulate against sources having their principal radiative
 emissions of energy in the entire wavelength range of 1.5-15 microns, i.e.
 over the entire infrared band.
 Such a coated sheet according to the invention is thus suitable for
 producing heat shields regardless of the temperature attained by the
 thermal source to be insulated against; e.g. in the case of an automotive
 exhaust conduit system the sheet is suitable for insulating with respect
 to any part of such system, even the hottest parts.
 The inventive coated sheet metal has emissivities which are only slightly
 higher than those of aluminum, namely higher by on the order of 0.02-0.03
 for wavelengths in the range of 5.5-15 microns, and higher by on the order
 of 0.03-0.05 for wavelengths in the range of 1.5-5.5 microns.
 KEY to FIG. 1:
 Ordinate: Emissivite=Emissivity.
 Abscissa: Wavelength (micron). Tole aluminiee A=Metal sheet A having an
 aluminum-containing coating.
 KEY to FIGS. 2 and 3:
 Emissivite=Emissivity.
 Abscissa: Time of heating (seconds).