Patent Application
20060130940
The present invention relates to structural automotive components and the like, and in particular to a method for making heat treated structural automotive components with reduced surface oxides and/or scale.
Structural automotive components and parts, such as door pillars or beams, frame rails, frame components, bumpers, bumper beams, and related bumper components, side impact beams, instrument panel reinforcements, and the like, are well known in the art, and improve vehicle safety and performance in impact situations. Many such structural components have a specially contoured shape, and are heat treated to meet exacting safety specifications.
Some types of structural automotive components are manufactured using a hot press method, such as that disclosed in U.S. Pat. No. 5,972,134, which is assigned to Benteler AG, related company to the assignee of the present application. In the method disclosed in the Benteler AG U.S. Pat. No. 5,972,134, plates of high strength steel are cut from a coil, heated in a furnace or electromagnetic heat inductor, positioned in a pressing tool, pressed to shape in the pressing tool and cooled in the pressing tool to heat treat the formed component.
Exemplary problems experienced with such prior art processes include the formation of surface oxides and/or scale (collectively referred to herein as “scale”) on the steel during heating, and decarburization of the steel. When heating steel in an open atmosphere, the carbon content in the furnace atmosphere and/or surrounding ambient air is lower than in the steel. As a result, carbon may be extracted via diffusion from the body of the steel, which is a phenomenon known as “decarburization”. Further, the iron in the steel will react with the atmosphere to produce iron compounds, or scale on the surfaces of the steel. In some applications, protective gases can be used as a protective atmosphere to prevent these reactions from occurring. In many applications, however, a protective atmosphere is impractical due to cost or technical constraints. The result of atmospheric interaction is a blackened or gray surface with “heat scale” that may be attached or loose, but that is generally in a layer on the material surface. The depth of the scale can be measured by microscope, and generally covers the entire surfaces of the component evenly to a given depth. This layer of scale is generally less desirable from a mechanical strength aspect, and also creates difficulties for secondary processing operations, such as welding and painting. There is currently no known practical advantage in the presence of the scale. When scale must be prevented on an uncoated base steel during heating, the furnace atmosphere must contain no water vapor, oxygen or carbon dioxide, and complete prevention can only be completed in a fully enclosed furnace that is quite costly, and may not be economical for most high volume vehicle part processing. Further, oil coating on incoming parts to alleviate oxide formation can add undesirable elements to the furnace atmosphere, and result in environmental concerns. In addition, reduced scale processing typically requires a furnace that does not allow the combustion gases to mix with the heating atmosphere, which again is more costly than an open air furnace. Furthermore, since hot press forming techniques require transporting the heated blank to a pressing tool, exposure to ambient air during transport can still result in some amount of scale formation, even when an enclosed furnace is used.
The scaling of vehicle components manufactured using prior art technologies cause several serious problems. One such problem is the deposition of scale in the hot forming tools, and related part handling equipment. The handling of the heated blanks and subsequent pressing to shape and removal of the formed parts causes scale to detach from the surfaces of the blank. The loosened scale then becomes lodged in the handling and/or pressing portions of the tooling. The scale thus deposited eventually causes the parts to be formed out of tolerances, such that the scale must be removed on a regular basis. Because the scale is relatively abrasive, galling and other tool wear results. While the die can be cleaned and polished to temporarily correct such problems, the wear caused by scale abrasion is sufficiently severe that the tool must eventually be replaced. This regular cleaning and replacement of the pressing and handling portions of the tooling adds substantial cost to the process, and results in expensive machine downtime.
As noted above, another serious problem resulting from scale formed on the components relates to secondary operations on the formed vehicle part, such as welding, painting, rust-proofing and the like. In order to obtain a suitable weld when performing post-form operations, such as the attaching of mounting brackets, or the like, as well as the final installation of the component in a vehicle, most of the scale must be eliminated. The existence of scale adversely impacts the quality of spot welds, and also shortens the life of the welder tips. Similarly, scale adversely impacts the adherence of paint and/or rust-proofing, such that most scale must be eliminated from the components prior to these post-form operations as well.
One common method to remove scale from the finished component is to abrasive blast the parts with steel, glass, ceramic or cast iron shot particles. While abrasive blasting is generally effective, it adds additional cost to the manufacturing process. Special abrasive blasting equipment and booths must be provided, and a portion of the shot is consumed during the blasting operation, and must be replaced regularly. Further, since abrasive blasting processes are normally conducted in a separate building from fabrication or stamping, due to noise and cleanliness concerns, additional material handling and/or shipping costs are incurred. Even when sophisticated abrasive blasting processes are used to control dimensional characteristics of the vehicle components by accurately monitoring the flow and pressure rates of the cleaning particles and accurate placement of the blasting nozzles, any slight deviations from process parameters may result in a component that is out of specification dimensionally, resulting in possible warping or distortion. On automotive components made from thinner steel materials at approximately 3.0 millimeters thickness or less, the warping or distortion becomes more of a concern, and special tooling may be required to hold the component during abrasive blast processing.
Attempts have been made to reduce decarburization and scale formation in making certain types of steel parts and/or components. One such technique is disclosed in European Patent No. EP1013785, which teaches coating a strip of sheet steel with aluminum or its alloy. However, such processes are relatively slow, and not particularly effective in making structural components for vehicles, and other similar applications, which use a high strength steel that must be heated relatively quickly to very high temperatures in the range of 800° C. to 1000° C., hot formed and then quenched to heat treat the part. The aluminum coating tends to soften and/or melt under such high temperatures, and can become detached from the steel, particularly during handling, thereby fouling the furnace and/or oven, transport devices, forming presses, and other such equipment. Also, the aluminum coating is not effective when pre-form notches, apertures or other such formations are desired in the part, since the coating tends to crack during the forming or pressing operations. Aluminum coatings also tend to adversely impact the weldability of the finished component, which renders post-form operations and vehicle assembly more expensive.
In view of the foregoing, it would clearly be advantageous to develop a method for making structural automotive components and the like with greatly reduced scale and decarburization to overcome those problems experienced with prior art processes, while retaining and/or improving upon the important performance characteristics of the parts, such as mechanical strength, formability, surface hardness, ductility and the like. Furthermore, the ability to achieve such results, while reducing overall manufacturing and/or assembly costs, would be particularly desirable.
One aspect of the present invention is a method for making contoured structural door beams for vehicles and the like, comprising providing an elongate strip of high strength steel having a thickness in the range of 0.5-5.0 millimeters. The steel strip is electroplated with a metal comprising primarily nickel to form at least on the opposite faces of the steel strip a nickel coating having a thickness in the range of 1.0-5.0 microns. The nickel coated steel strip is cut to length to form a flat, plate-shaped blank. The plate-shaped blank is heated in a generally open atmosphere comprising primarily ambient air to a temperature in the range of 800° C. to 1000° C. within less than ten minutes, thereby diffusing at least a portion of the nickel coating a predetermined distance into the opposite faces of the steel strip portion of the plate-shaped blank to alleviate scale formation, and simultaneously raising the temperature of the plate-shaped blank for hot forming the same. The heated plate-shaped blank is transported into a pressing tool and then formed within the pressing tool into a predetermined contoured shape to define a selected structural door beam. The formed structural door beam is then cooled in the pressing tool to heat treat the same through microstructure phase change without substantial scale formation. Finally, the heat treated structural door beam is removed from the pressing tool.
Another aspect of the present invention is a method for making a vehicle having at least one contoured structural door beam comprising providing an elongate strip of high strength steel having a thickness in the range of 0.5-5.0 millimeters. The steel strip is electroplated with a metal comprising primarily nickel to form on at least the opposite faces of the steel strip a nickel coating having a thickness in the range of 1.0-5.0 microns. The nickel coated steel strip is cut to length to form a flat, plate-shaped blank. The plate-shaped blank is heated in a generally open atmosphere comprising primarily ambient air to a temperature in the range of 800° C. to 1000° C. within less than ten minutes, thereby diffusing at least a portion of the nickel coating a predetermined distance into the opposite faces of the steel strip portion of the plate-shaped blank to alleviate scale formation, and simultaneously raising the temperature of the plate-shaped blank for hot forming the same. The heated plate-shaped blank is transported into a pressing tool, and formed in the pressing tool into a predetermined contoured shape to define a selected structural door beam. The formed structural door beam is then cooled in the pressing tool to heat treat the same through microstructure phase change without substantial scale formation. The heat treated structural door beam is removed from the pressing tool, and welded in the vehicle without cleaning the same between the removing step and the welding step.
Yet another aspect of the present invention is a method for making contoured structural parts for vehicles and the like, comprising providing a strip of steel having a thickness greater than 0.5 millimeters. The steel strip is coated with a metal comprising primarily nickel to form on at least the opposite faces of the steel strip a nickel coating having a thickness in the range of 1.0-5.0 microns. The nickel coated steel strip is cut to form a flat, plate-shaped blank. The plate-shaped blank is heated to a temperature in the range of 800° C. to 1000° C. within less than ten minutes, thereby diffusing at least a portion of the nickel coating a predetermined distance into the opposite faces of the steel strip portion of the plate-shaped blank, and simultaneously raising the temperature of the plate-shaped blank for hot forming the same. The heated plate-shaped blank is transported into a pressing tool, and formed in the pressing tool into a predetermined contoured shape to define a selected structural vehicle part. The formed structural vehicle part is then cooled in the pressing tool to heat treat the formed structural vehicle part. Finally, the heat treated structural vehicle part is removed from the pressing tool.
Yet another aspect of the present invention is a method for making contoured structural parts comprising providing a blank made from steel having a thickness greater than 0.5 millimeters. The steel blank is coated on at least the opposite faces thereof with a metal barrier coating having a melting point greater than 800° C., preferably greater than 1000° C. Examples of suitable metal coatings are selected from the group consisting of nickel, copper and chromium. The metal coating may be applied to surfaces of the steel blank using any of several well known techniques, such as electroplating, sputtering, etc. The coated blank is heated to a temperature in the range of 800° C. to 1000° C., thereby diffusing at least a portion of the metal coating a predetermined distance into the opposite faces of the steel strip portion of the coated blank, and simultaneously raising the temperature of the coated blank for hot forming the same. The heated coated blank is transported into a pressing tool, and formed in the pressing tool into a predetermined contoured shape to define a selected structural part. The formed structural part is then cooled in the pressing tool to heat treat the same, and then removed from the pressing tool.
Yet another aspect of the present invention is a method for making structural automotive components and the like with greatly reduced scale and decarburization to overcome those problems experienced with prior art processes, while retaining and/or improving upon the important performance characteristics of the parts, such as mechanical strength, formability, surface hardness, ductility, weldability and the like. Furthermore, the method has the ability to achieve such results, while reducing overall manufacturing and/or assembly costs. The method is efficient in use and particularly well adapted for the proposed use.
These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings.
For purposes of description herein, the terms “upper”, “lower”, “right”, “left”, “rear”, “front”, “vertical”, “horizontal” and derivatives thereof shall relate to the invention as oriented in
The reference numeral 1 (
In one aspect of the present invention, door beam 1 is manufactured in accordance with the process or method illustrated in
As will be readily apparent to those having skill in the art, while the above-identified process has been described in conjunction with making a specific door beam 1, the various methods disclosed herein can also be used in conjunction with the manufacture of other structural components, particularly those for vehicles and the like.
In the example of the present invention illustrated in
In another working example of the present invention, steel strip 30 is hot rolled, and selected from a steel alloy comprising, in percent by weight, the following:
In yet another working example of the present invention, steel strip 30 is hot rolled, and selected from a steel alloy comprising, in percent by weight, the following:
In yet another working example of the present invention, steel strip 30 is hot rolled, and selected from a steel alloy comprising, in percent by weight, the following:
With reference to
Nickel coating 33 is then applied to at least the opposite faces 31 and 32 of cleaned steel strip 30. In the example of the present invention illustrated in
Nickel coating 33 also assists in the formability of blank 35 into door beam 1. Because nickel has good lubricity at high temperatures, nickel coating 33 improves the hot forming or pressing operation 60, and reduces tool wear. After forming, nickel coating 33 also provides some corrosion protection, although, as noted above, nickel coating 33 need not be of a conventional corrosion resistant grade to achieve those advantages outlined above. For example, in the examples illustrated in
In the example illustrated in
In the example illustrated in
In the embodiment of the present invention illustrated in
In an alternative embodiment of the present invention, furnace 48 is in the form of an induction heater, which heats the blanks 35 to a temperature in the range of 800° C. to 1000° C. within less than 45 seconds. Preferably, blanks 35 are heated by the induction heater to a temperature in the range of 850° C. to 950° C. within a range of 20 to 25 seconds to retain the thin, substantially undiffused exterior layer 55 of nickel, as noted above. The induction heated blanks 35 are then processed in the same manner described above with respect to oven or furnace heating.
In the process illustrated in
In the example illustrated in
In the example illustrated in
While the examples illustrated in
In one working embodiment of the present invention, a nickel bath with the following properties was determined suitable.
Other alternate embodiments of the present invention will be apparent to those skilled in the art. For example, a stationary or dynamic electromagnetic heat inductor may be used to heat blank 35 instead of furnace 48. Also, some level of inert atmosphere may be added to furnace 48, although higher processing costs would likely be experienced. Blank 35 can be zone heat treated to retain ductility in some portions of door beam 1. Also, blanks 35 and/or partially pre-formed blanks can be cut to size, notched, etc., then individually coated with nickel coating 33 prior to heating and pressing.
Other methods may also be used to plate nickel or other similar metals to steel strip 30. For example, an electroless or autocatalytic chemical process may be used, although the same is currently more expensive than electrodeposition. Also, powder deposition, either electrostatic or dipped, may also be possible. Thermal spray may also be used to coat steel strip 30 with nickel or another suitable metal.
The precise thickness of nickel coating 33 may be selected to properly balance between the requirements for a scale-free surface and a predetermined hardness, without adversely impacting the formability and/or ductility of the blank or the hardenability of the same to achieve exacting safety specifications. Nickel coating 33 not only greatly improves tool life by alleviating scale, but also adds lubricity to the part which prevents galling and other wear problems in the tooling.
In the foregoing description, it will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims by their language expressly state otherwise.
