Patent Application
20060042727
This invention relates to aluminum alloy vehicular structural parts or members and more particularly, it relates to a method of casting aluminum alloy into sheet having good forming characteristics and to forming the sheet into vehicular structural parts or members such as dash panel, floor panel, door panel, window trim, radio bracket, reinforcements for panels, etc.
In many instances, continuous casting of molten aluminum into slab utilizing twin belt, twin roll or block casters is favored over DC casting because continuous casting can result in substantial energy savings and total conversion cost savings compared to the DC cast method. In the continuous casting process, molten metal is continuously introduced to an advancing mold and a slab is produced which may be continuously formed into a sheet product which is collected or wound into a coil. However, the continuous casting is not without problems. For example, it has been discovered that the alloy composition and the processing steps must be carefully controlled in order to have the formability level to avoid cracking during forming and yet have the requisite strength properties in the final product. That is, the alloy and the processing thereof must be carefully controlled to provide sheet having the formability suited to the fabricating steps necessary to form the final product or vehicular structural parts. If the alloy and processing steps are not controlled, then in the forming steps, fracture can occur and the formed parts have to be scrapped. Thus, there is a great need for selection of an aluminum alloy, continuous casting thereof, and thermal mechanical processing methods which provide a sheet product having forming characteristics and strength properties which permit forming operations such as bending, stamping, deep drawing, stretching or crimping to hold fasteners during production of vehicular structural parts or members while avoiding problems of fracturing or cracking, for example.
The continuous casting of molten aluminum and rolling slab produced therefrom into a sheet product is disclosed in various patents. For example, U.S. Pat. No. 5,976,279 discloses a process for continuously casting aluminum alloys and improved aluminum alloy compositions. The process includes the steps of continuously annealing the cold rolled strip in an intermediate anneal using an induction heater and/or continuously annealing the hot rolled strip in an induction heater. The alloy composition has mechanical properties that can be varied selectively by varying the time and temperature of a stabilizing anneal.
U.S. Pat. No. 6,264,765 discloses a method and apparatus for casting, hot rolling and annealing non-heat treatment aluminum alloys. The method and apparatus comprises continuous casting, hot rolling and in-line inductively heating the aluminum sheet to obtain the mechanical properties within the specification tolerance of the hot rolled product.
U.S. Pat. No. 5,985,058 discloses a process for continuously casting aluminum alloys and improved aluminum alloy compositions. The process includes the step of heating the cast strip before, during or after hot rolling to a temperature in excess of the output temperature of the cast strip from the chill blocks. The alloy composition has a relatively low magnesium content yet possesses superior strength properties.
U.S. Pat. No. 5,993,573 discloses a process for continuously casting aluminum alloys and improved aluminum alloy compositions. The process includes the steps of (a) heating the cast strip before, during or after hot rolling to a temperature in excess of the output temperature of the cast strip from the chill blocks and (b) stabilization or back annealing in an induction heater of cold rolled strip produced from the cast strip.
U.S. Pat. No. 5,833,775 discloses an aluminum alloy sheet and a method for producing an aluminum alloy sheet. The aluminum alloy sheet is useful for forming into drawn and ironed container bodies. The sheet preferably has an after-bake yield strength of at least about 37 ksi and an elongation of at least about 2 percent. Preferably the sheet also has earing of less than about 2 percent.
U.S. Pat. No. 6,086,690 discloses a process of producing an aluminum alloy sheet article of high yield strength and ductility suitable, in particular, for use in manufacturing automotive panels. The process comprises casting a non heat-treatable aluminum alloy to form a cast slab, and subjecting said cast slab to a series of rolling steps to produce a sheet article of final gauge, preferably followed by annealing to cause recrystallization. The rolling steps involve hot and warm rolling the slab to form an intermediate sheet article of intermediate gauge, cooling the intermediate sheet article, and then warm and cold rolling the cooled intermediate sheet to final gauge at a temperature in the range of ambient temperature to 340° C. to form said sheet article. The series of rolling steps is carried out continuously without intermediate coiling or full annealing of the intermediate sheet article. The invention also relates to the alloy sheet article produced by the process.
U.S. Pat. No. 5,244,516 discloses an aluminum alloy plate for discs superior in Ni—P platability and adhesionability of plated layer and having a high surface smoothness with a minimum of nodules and micropits, said aluminum alloy plate comprising an aluminum alloy containing as essential elements Mg in an amount more than 3% and equal to or less than 6%, Cu in an amount equal to or more than 0.03% and less than 0.3%, and Zn in an amount equal to or more than 0.03% and equal to or less than 0.4%, and as impurities Fe in an amount equal to or less than 0.07% and Si in an amount equal to or less than 0.06% in the case of semi-continuous casting, or Fe in an amount equal to or less than 0.1% and Si in an amount equal to or less than 0.1% in the case of strip casting, and also containing Al—Fe phase intermetallic compounds, with the maximum size being smaller than 10 μm and the number of particles larger than 5 μm being less than 5 per 0.2 mm2, and Mg—Si phase intermetallic compounds, with the maximum size being smaller than 8 μm and the number of particles larger than 5 μm being less than 5 per 0.2 mm2.
U.S. Pat. No. 5,514,228 discloses a method for manufacturing aluminum sheet stock which includes hot rolling an aluminum alloy sheet stock, annealing and solution heat treating it without substantial intermediate cooling and rapid quenching.
In spite of these disclosures, there is a great need for selection of aluminum alloy and method for producing vehicular parts or members utilizing a continuous caster, optimized thermal mechanical processing, to provide good strength and levels of formability which permit ease of forming into intricate parts without cracking.
The term “formability” when used herein is used to describe the ease with which a sheet of metal can be shaped through plastic deformation. Formability of a metal can be evaluated by measuring strength, ductility, and the amount of deformation to cause failure.
The term “aluminum” when used herein is meant to include aluminum and its alloys.
The term “automotive” as used herein is meant to include automobile and other vehicular parts or members as described herein and other transport parts or members having similar construction.
It is an object of the invention to provide an improved, low cost process including continuous casting and rolling to continuously produce aluminum sheet product having consistent levels of formability.
It is another object of the invention to provide a process including continuously casting a slab and rolling the slab into a sheet product suitable for use in producing vehicular parts.
It is still another object of the invention to provide a process employing continuous casting of molten aluminum into slab and rolling the slab into sheet product for meeting the forming requirements, such as bending, stamping, stretching or deep drawing of vehicle structural parts or members.
And yet it is another object of the invention to provide an improved process for producing aluminum sheet product employing a continuous caster to produce slab, continuously rolling the slab to produce a sheet product and annealing the sheet product for forming into vehicular structural parts or panel members having fasteners such as threaded fasteners attached thereto by crimping the sheet product around the fastener.
It is yet another object of the invention to provide a process for producing vehicular members such as shallow or deep formed panel members which includes continuously casting an aluminum alloy into a slab, rolling the slab to a sheet product and annealing the sheet product having good levels of formability, forming the sheet product into a panel having threaded fasteners attached thereto by crimping to provide a formed vehicular member for mechanically fastening to support members, for example.
And yet it is another object of the invention to provide a process for casting a molten alloy comprising 2.7 to 3.6 wt. % Mg, 0.1 to 0.4 wt. % Mn, 0.02 to 0.2 wt. % Si, 0.05 to 0.30 wt. % Fe, 0.1 wt. % max. Cu, 0.25 wt. % max. Cr, 0.2 wt. % max. Zn, 0.15 wt. % max. Ti, the remainder aluminum, incidental elements and impurities, casting the alloy into a slab which is hot rolled and annealed to provide a sheet product suitable for forming into a vehicular structural part or frame member where good formability is necessary.
In accordance with these objects, there is provided a process for producing aluminum vehicular structural parts or members from molten aluminum alloy using a continuous caster to cast the alloy into a slab. The method comprises providing a molten aluminum alloy consisting essentially of 2.7 to 3.6 wt. % Mg, 0.1 to 0.4 wt. % Mn, 0.02 to 0.2 wt. % Si, 0.05 to 0.25 wt. % Fe, 0.1 wt. % max. Cu, 0.25 wt. % max. Cr, 0.2 wt. % max. Zn, 0.15 wt. % max. Ti, the remainder aluminum, incidental elements and impurities and providing a continuous caster such as a belt caster for continuously casting the molten aluminum alloy. The molten aluminum alloy is cast into a slab having Al—Fe, Al—Fe—Mn or Mg2Si containing intermetallic particles. The slab is rolled into a sheet product which is then annealed to provide a sheet product having a substantially uniform distribution or less striations of intermetallic particles for improved formability. Thereafter, the sheet product is formed into a vehicular structural part or member such as a panel member for a door or hood, for example, having fasteners crimped thereto.
Alternatively, the hot rolled sheet may be cold rolled after hot rolling, and then annealed prior to the forming steps. In yet another embodiment, the hot rolled sheet may be annealed or even homogenized and then cold rolled to a cold rolled sheet product. The cold rolled product can be annealed to provide a product suited to the various forming steps.
These and other objects will become apparent from a reading of the specification and claims appended hereto.
The vehicular structural parts or members, for example, of the invention are comprised of an aluminum base 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. Al—Fe, Al—Fe—Mn or Mg2Si intermetallic particles form during solidification. That is, the distribution, size and amount of such intermetallic particles after rolling of continuous cast slab can drastically influence the formability of the sheet material.
The Al—Fe, Al—Fe—Mn or Mg2Si containing intermetallic particles form during solidification. The distribution of such intermetallic particles after rolling of continuous belt cast aluminum slab can be severely striated or lined causing forming problems. By comparison, direct chill (D.C.) ingot cast material has a more uniform distribution of intermetallic particles providing good formability. Striations of intermetallic particle structure causes stress concentration during plastic deformation which deteriorate formability of the sheet product. Thus, it is desired that the rolled sheet of the invention has a substantially uniform distribution or less striations of intermetallic particles to provide for improved formability.
Accordingly, the aluminum base alloy consists essentially of 2.7 to 3.6 wt. % Mg, 0.1 to 0.4 wt. % Mn, 0.02 to 0.2 wt. % Si, 0.05 to 0.3 wt. % Fe, 0.1 wt. % max. Cu, 0.25 wt. % max. Cr, 0.2 wt. % max. Zn, 0.15 wt. % max. Ti, the remainder aluminum, incidental elements and impurities. Preferably, magnesium is maintained in the range of 2.8 to 3.3 or 3.5 wt. % and manganese is preferably maintained in the range of 0.1 to 0.25 or 0.35 wt. %. Further, preferably iron is maintained in the range of 0.05 or 0.10 to 0.25 wt. %, typically 0.05 to 0.2 wt. % and silicon is maintained in the range of 0.05 to 0.15 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 in the process of the invention to form automotive members having the requisite properties requires careful control of the alloying 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 member fabricated from the sheet must be prepared in accordance with specific method steps in order to produce sheet and automotive structural parts or members therefrom having the desirable characteristics. That is, the process must be controlled in order to produce product having near formability properties of DC ingot fabricated material without the cost penalties of the DC ingot process.
Thus, referring now to
In
Another casting apparatus that may be used in the present invention is a block caster wherein the blocks are connected to form belts and is included herein as a belt caster. As described with respect to belt caster 2, a tundish and nozzle are provided to transfer molten metal to the block belts 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 1220° to 1320° F., typically 1250° to 1285° F., and exits the caster at a temperature in the range of 750° to 1150° F., typically 860° to 950° F. In addition, typically the continuous slab exiting the belt caster has a thickness in the range of 0.2 to 2 inches, for example, 0.2 to 1 inch. A typical slab thickness for the belt caster is about 0.6 to 0.875 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 obtain microstructures with less striations or lines of intermetallics such as Al—Fe, Al—Fe—Mn or Mg2Si for purposes of formability and corrosion resistance. It should be noted that DC cast material normally has good or substantially uniform distribution of intermetallic particles. But, as noted earlier, DC cast material has the penalty of higher conversion costs than the subject continuous cast slab. Thus, the present invention provides continuous cast slab for forming into sheet material with near DC cast properties to obtain the cost savings and yet retain 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 stands or rolls are illustrated in
Hot mill 30 can reduce the thickness of the slab about 60 to 95% of its original thickness, with 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 400° 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 parts 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. Hot rolled sheet in the fully annealed condition can have a tensile strength in the range of 28 to 35 ksi, a yield strength in the range of 12 or 13 to 17.5 ksi and an elongation greater than 19%.
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 O-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
As an example of the desirable microstructures which have good forming characteristics of continuously cast (CC) aluminum sheet, reference is made to
Referring now to
Formed panels can include doors, hoods, trunk lids, fenders, floors, wheels and bumper backup bars and can be formed from flat sheets of aluminum alloy formed between mating dies to provide a three-dimensional structure. The dual or plural structure as depicted employs peripheral seaming or hemming to provide the vehicular structural member; however, other means of joining can include welding, riveting, adhesive bonding and thus the inner and outer panels can be joined by any of these methods and such is contemplated. The seaming or hemming referred to is shown in
In some instances, the structural member may include a combination of steel and aluminum alloy, but such structure would not provide the same weight savings.
The alloy of the invention is required to have good formability for yet another reason. That is, hinges 106 and struts 116, for example, are preferred to be joined to steel threaded fasteners. Thus, at 118 where strut 116 is connected to lift gate 100, it is preferred to use a metal fastener such as a steel fastener. Accordingly, a threaded fastener 130 is crimped into the sheet metal of the inner panel as shown in
Thus, aluminum alloy vehicular parts or members produced in accordance with the foregoing practices provide material having the strength and formability for use as vehicular or automotive sheet which can be formed into many different automotive structural members.
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.
The following example is further illustrative of the invention.
An aluminum base alloy containing 3.267 wt. % Mg, 0.201 wt. % of Mn, 0.080 wt. % Si, 0.164 wt. % Fe, 0.020 wt. % Cu, 0.004 wt. % Cr and 0.024 wt. % Zn, was fed to a twin belt caster at a temperature of 1260° F. and solidified to produce a 0.875 inch thick slab existing the caster at a temperature of 900° F. The slab was directly fed into a three stand hot rolling mills and rolled to final gauge of 0.100 inch. The temperature of introducing the slab to the hot rolling mill was at about 820° F. and the temperature of exiting the mill was at about 520° F. The hot rolled sheet was wound into a coil. The coil was annealed in an anneal furnace at a temperature of 730° F. for 4 hours. The annealed coil was tension leveled and slit into the required width and then the coil was given a surface pretreatment and lubricated. The material had properties in the rolling direction before forming into automotive parts of: ultimate tensile strength of 32.8 ksi, yield strength of 15.5 ksi, elongation of 21.4%. All these properties met the requirement identified by Aluminum for Automotive Body Sheet Panels, published by The Aluminum Association. The material was formed into inner structural panels, and threaded fasteners were crimped into the sheet with satisfied quality inspection. Thus, the alloy can be cast in a twin belt caster, rolled into a sheet product, stamped or shaped into an automotive structural part or member 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 members.
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.
