Patent Application
20080041501
The automotive heat shields of the invention are comprised of an aluminum base alloy derived from aluminum scrap, the alloy containing controlled amounts of magnesium, iron, silicon and manganese for the required strength and formability in the sheet product produced by the casting and thermomechanical process. The total amounts of the alloying elements are required to be controlled to meet the strength requirement without causing casting difficulty in the process. Further, the amount of alloying elements also is required to be controlled to meet the formability requirements, especially the amount of iron, manganese and silicon.
Accordingly, the aluminum base alloy consists essentially of 0.1 to 0.7 wt. % Si, 0.2 to 0.9 wt. % Fe, 0.05 to 0.5 wt. % Cu, 0.05 to 1.3, wt. % Mn, 0.2 to 2.8, wt. % Mg, 0.3 wt. % max. Cr, 0.3 wt. % max. Zn, 0.2 wt. % max. Ti, the remainder aluminum, incidental elements and impurities. Preferably, magnesium is maintained in the range of 0.3 to 2.5 wt. % and manganese is preferably maintained in the range of 0.07 to 0.8 or 1.2 wt. %. Further, preferably iron is maintained in the range of 0.25 or 0.3 to 0.85 wt. %, typically 0.3 to 0.8 wt. % and silicon is maintained in the range of 0.15 to 0.65 wt. %. Impurities are preferably limited to not more than 0.05 wt. % each and the combination of impurities should not be greater than 0.15 wt. % total.
Thus, it will be understood that to use an alloy of the above composition derived from aluminum scrap in the process of the invention to form automotive heat shields having the requisite properties requires careful control of the elements in the alloy and the casting thereof to avoid forming intermetallic particle structures adverse to the forming operation. That is, it will be appreciated that in the present process, there is great difficulty in balancing all the constituents in the alloy for strength and procedural steps necessary to forming a sheet product having desirable properties for forming into the final product while avoiding undesirable properties which leads to fracture or cracking, for example, during the forming process.
Not only is it important to have alloying elements and impurities in the controlled amounts as herein described, but the slab produced by continuous casting, the sheet formed from the slab and automotive heat shields fabricated from the sheet must be prepared in accordance with specific method steps in order to produce sheet and automotive heat shields therefrom having the desirable characteristics.
Thus, referring now to
In
From
Another casting apparatus that may be used in the present invention is a block caster wherein the blocks are connected to function as belts and is included herein as a belt caster. As described with respect to belt caster 3, a tundish and nozzle are provided to transfer molten metal to the blocks of the block caster wherein solidification occurs to provide a solidified slab 15 and the blocks are chilled to aid in solidification of the molten metal.
Yet another apparatus that may be utilized to cast a continuous strip or slab 15 is a roll caster which includes two rolls which rotate to provide the continuously advancing mold. As in the belt caster, a tundish and nozzle are used to transfer molten aluminum to the mold defined by the two rolls. Again, the rolls are normally chilled to aid in solidification of the molten metal into a strip or slab. The different casters are described in U.S. Pat. No. 5,452,827. By the use of the term “continuous caster” is meant to include all these casters.
Molten aluminum alloy of the invention is introduced to the caster in a temperature range of about 1230° to 1350° F., typically 1250° to 1300° F., and exits the caster at a temperature in the range of 750° to 1150° F., typically 860° to 1050° F. In addition, typically the continuous slab exiting the continuous caster has a thickness in the range of 0.2 to 2 inches, for example, 0.25 to 1 inch. A typical slab thickness for the belt caster is about 0.5 to 1 inch. Belt casting speed can range from 10 to 40 ft/min, depending on the thickness of the slab. It is important to adhere to these casting conditions in order to achieve the volume and quality required for the end product. Thus, the present invention provides continuous cast slab for forming into sheet material with high cost savings and yet retains the desirable properties such as formability.
After exiting the caster, the slab 15 is directed to rolling mill 30 where it is rolled to form a rolled strip or flat product 34 using preferably a hot mill. Hot mill 30 is comprised of one or more pairs of oppositely opposed rolls 32 which reduces the thickness of the slab a controlled amount as it passes between each stand of rolls. Three sets of hot mill stands or rolls are illustrated in
Hot mill 30 can reduce the thickness of the slab about 60 to 98% of its original thickness, with a typical reduction being 75 to 95%. Depending on the end use of the sheet product, heat may be applied to the strip or slab between hot stands in addition to or instead of heating prior to the hot mill.
The temperature of the aluminum alloy sheet exiting the hot mill can be in the range of about 350° to 825° F., depending on whether there was heat input before or during hot rolling.
After hot rolling, hot rolled strip 34 can have a deformation texture and deformed grain structure. The hot rolled strip can have a partially or fully recrystallized grain structure with an optimum texture depending on previous heat input and rolling reduction. If the structure remains deformed and a recrystallized grain structure is necessary for the end product, then annealing of the hot rolled strip 34 can be applied to promote recrystallization of the deformed structures. For example, it is important for automotive application using the aluminum alloy of the invention to have a fine, fully recrystallized grain structure with random texture for the purpose of forming automotive heat shields in accordance with the invention. Thus, in the present invention, it is preferred that the hot rolled sheet be fully annealed to O-temper in annealer 40. If the hot rolled strip is already recrystallized with an optimum texture, then annealing is not required. Hot rolled sheet in the fully annealed condition can have a tensile strength in the range of 12 to 35 ksi, a yield strength in the range of 5 to 20 ksi and an elongation greater than 15%.
Referring to
Instead of continuous annealing, the hot rolled sheet may be batch annealed. That is, hot rolled sheet 42 is wound into coils 48 or 49. These coils are then placed in a furnace and soaked in a temperature range of 600° to 1000° F. for 2 to 10 hours to provide the rolled sheet in a fully annealed or 0-temper condition. If the slab has been hot rolled to a gauge suitable for forming, then no further thermal mechanical processing is necessary and the sheet is in condition for the forming steps. If the slab has been hot rolled to an intermediate gauge, then after annealing, the annealed material is subjected to cold rolling followed by further annealing to provide sheet in the O-temper for forming operations.
After hot rolling, the hot rolled sheet or flat product may be allowed to cool prior to other operations. For example, after hot rolling, with or without annealing and cooling, the resulting strip 42 may be cold rolled (not shown in
After cold rolling to final gauge, the sheet product is subject to further anneal to ensure the required crystallographic texture and grain structure necessary for forming into the final automotive product.
After hot rolling or annealing sheet 42 may be subject to a continuous rapid quenching such as cold water quench 50 prior to further operations. Quench 50, if used and shown after anneal, can be located at different locations in the process.
Referring to
All ranges provided herein are meant to include all the numbers within the range as if specifically set forth, e.g., 1 to 5 would include 1.1, 1.2, 1.3, etc., or e.g., 2, 3, 4.
Another shield that is sometimes employed is located on the firewall and is identified by the reference number 58. This is useful in directing heat from engine 56 away from the inside of the automobile.
And yet another use is shield 60 which is used to direct heat from catalytic converter 62 downwards and away from the automobile body. This use or shield is particularly important because the catalytic converter can become very hot as it burns pollutants in the exhaust stream.
Next in the exhaust system there is shown heat shield 64 which is used to direct downwardly the exhaust heat emanating from pipe 66. Also, exhaust muffler 68 is provided with a shield 70 to direct heat away from the automobile bottom and gas tank. Thus, this shield is important from a safety standpoint.
The following example is further illustrative of the invention.
Aluminum scrap was melted to provide an aluminum base alloy containing 0.23 wt. % Si, 0.54 wt. % Fe, 0.16 wt. % Cu, 1.0 wt. % of Mn, 0.91 wt. % Mg, 0.03 wt. % Cr, 0.05 wt. % Zn, 0.013 wt. % Ti, and incidental elements and impurities. The melt was fed to a twin belt caster at a temperature of 1280° F. and solidified to produce a 0.875 inch thick slab existing the caster at a temperature of 1020° F. The slab was directly fed into a three stand hot rolling mills and rolled to a gauge of 0.90 inch. The temperature of introducing the slab to the hot rolling mill was at about 950° F. and the temperature of exiting the mill was at about 475° F. The hot rolled sheet was wound into a coil. The coil was cold rolled to a final gauge of 0.035 inch and annealed in an anneal furnace at 850° F. for four hours. The annealed coil was tension leveled and slit into the required width. The material had properties in the rolling direction before forming into automotive heat shields of: ultimate tensile strength of 28.7 ksi, yield strength of 13.2 ksi, elongation of 16.3%. The material was formed into heat shields. Thus, the scrap based alloy can be cast in a twin belt caster, rolled into a sheet product, stamped or shaped into an automotive heat shields with sufficient strength and formability.
It will be seen that the continuous caster can be used to produce a slab which can be thermomechanically treated to form a sheet product having the properties for forming into vehicular parts or heat shields.
Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.
