Method for making structural automotive components and the like

Abstract

A method for making structural automotive components and the like provides a strip of high strength steel having a selected thickness. A predetermined thickness of a metal coating, such as nickel, is applied to the opposite faces of the steel strip. The coated steel strip is cut to form a blank. The blank is heated in a generally open atmosphere 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 metal coating a predetermined distance into the faces of the steel strip portion of the blank to alleviate scale formation, and simultaneously raise the temperature of the blank for hot forming the same. The heated blank is hot formed in a pressing tool, and cooled therein to heat treat the formed component through microstructure phase change, without substantial scale formation, such that the component need not be descaled prior to post-form processing and/or assembly in a vehicle.

Description

BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram or flow chart of a method embodying the present invention.

FIG. 2 is a perspective view of a door beam manufactured in accordance with the present invention.

FIG. 3 is a lateral cross-sectional view of the door beam taken along the line III-III, FIG. 2.

FIG. 4 is a top plan view of the door beam.

FIG. 5 is a side elevational view of the door beam.

FIG. 6 is a bottom plan view of the door beam.

FIG. 7 is a diagrammatic view of a method embodying the present invention.

FIG. 8 is a photomicrograph of a portion of the door beam, having a scan line superimposed thereon.

FIG. 9 is a graph showing the nickel composition of the door beam as a function of distance along the scan line shown in FIG. 8.

FIG. 10 is a graph showing the iron composition of the door beam as a function of distance along the scan line of FIG. 8.

FIG. 11 is another photomicrograph of another portion of the door beam.

FIG. 12 is another photomicrograph of another portion of the door beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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 FIG. 2. However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following written specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.

The reference numeral 1 (FIG. 2) generally designates a door beam or pillar manufactured in accordance with the present invention. The illustrated door beam 1 has a rigid construction, and a relatively complex shape specifically designed for a particular vehicle application. In the example illustrated in FIGS. 2-6, door beam 1 has a generally W-shaped lateral cross-sectional configuration, comprising a base or web portion 2 with opposite sidewalls 3 and 4 extending along and protruding outwardly from base portion 2. Flange portions 5 and 6 extend along the outer portions of sidewalls 3 and 4, respectively, and have a generally arcuate shape designed to add rigidity to door beam 1. Base portion 2 also includes a pair of generally arcuate channel portions 7 and 8 extending longitudinally along door beam 1 on opposite sides of an interior, arcuately-shaped rib 9. One end of door beam 1 includes a flattened portion 10, while the opposite end includes an L-shaped attachment bracket portion 11. The illustrated door beam 1 also includes mounting apertures 12, 13 and 14, as well as edge slots or notches 15 and 16, which adapt door beam 1 for a particular installation.

In one aspect of the present invention, door beam 1 is manufactured in accordance with the process or method illustrated in FIGS. 1 and 7. More specifically, door beam 1 is made by providing an elongate strip 30 (FIG. 7) of high strength steel having a thickness in the range of 0.5-5.0 millimeters. The thickness of the steel strip is selected in accordance with, and is substantially commensurate with, the desired finished thickness of the door beam 1, which will vary from one application to the next. In the illustrated example, the steel strip 30 is coated with a metal comprising primarily nickel to form on at least the opposite faces 31 and 32 of steel strip 30 a nickel coating 33 (FIGS. 8-12) having a thickness in the range of 1.0-5.0 microns. The precise thickness of nickel coating 33 will vary in accordance with the shape, size and other similar characteristics of door beam 1, but is preferably of sufficient thickness to cover at least substantial portions of the opposite faces 31 and 32 of steel strip 30 during heating and hot pressing, as described in greater detail below. A length of the nickel coated steel strip 34 is then cut to length to form a flat, plate-shaped blank 35. The plate-shaped blank 35 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 33 (FIGS. 8-12) a predetermined distance into the opposite faces 31 and 32 of steel strip 30 to alleviate scale formation, and simultaneously raise the temperature of plate-shaped blank 35 for hot forming the same. Heated plate-shaped blank 35 is then transported to and placed in a pressing tool 36. The heated plate-shaped blank 35 is formed or pressed in pressing tool 36 into the predetermined contoured shape shown in FIGS. 2-6 to define door beam 1. The formed structural door beam 1 is then cooled in pressing tool 36 to heat treat the same through microstructure phase change, without substantial scale formation. Finally, the heat treated structural door beam 1 is removed from the pressing tool and transported to post-form operations and/or vehicle assembly.

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 FIGS. 1 and 7, steel strip 30 is provided in the form of a coil 40 having a predetermined width to form the related structural component, such as door beam 1. As previously noted, the thickness of the illustrated steel strip 30 is within a range of 0.5-5.0 millimeters, but will vary in accordance with the particular application. The steel from which strip 30 is made is preferably a high strength steel, which can be heat treated to the specific specifications desired for a particular formed component, such as door beam 1. Both hot rolled and cold rolled steel may be used. One example of a suitable steel is disclosed in the Benteler AG U.S. Pat. No. 5,972,134, which is hereby incorporated herein by reference. In one 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:

• • carbon (C) 0.20% to 0.27%,
  • silicon (Si) 0.15% to 0.50%,
  • manganese (Mn) 1.0% to 1.40%,
  • phosphorus (P) 0.0% to 0.03%,
  • chromium (Cr) 0.0% to 0.35%,
  • molybdenum (Mo) 0.0% to 0.35%,
  • sulfur (S) at most 0.0% to 0.01%,
  • titanium (Ti) 0.0% to 0.05%,
  • boron (B) 0.0005% to 0.0040%,
  • aluminum (Al) 0.0% to 0.06%, and
  • copper (Cu) 0.0 to 0.10%,
    where the remainder is iron, including impurities brought about as a result of smelting.

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:

• • carbon (C) 0.18% to 0.3%,
  • silicon (Si) 0.1% to 0.7%,
  • manganese (Mn) 1.0% to 2.50%,
  • phosphorus (P) at most 0.025%,
  • chromium (Cr) 0.1% to 0.8%,
  • molybdenum (Mo) 0.1% to 0.5%,
  • sulfur (S) at most 0.01%,
  • titanium (Ti) 0.02% to 0.05%,
  • boron (B) 0.002% to 0.005%, and
  • aluminum (Al) 0.01% to 0.06%,
    where the remainder is iron, including impurities brought about as a result of smelting.

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:

• • carbon (C) 0.23% to 0.27%,
  • silicon (Si) 0.15% to 0.50%,
  • manganese (Mn) 1.10% to 1.400%,
  • phosphorus (P) at most 0.025%,
  • chromium (Cr) 0.15% to 0.35%,
  • molybdenum (Mo) 0.10% to 0.35%,
  • sulfur (S) at most 0.01%,
  • titanium (Ti) 0.03% to 0.05%,
  • boron (B) 0.002% to 0.004%,
  • aluminum (Al) 0.02% to 0.06%, and
  • copper (Cu) at most 0.10%,
    where the remainder is iron, including impurities brought about as a result of smelting.

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:

• • carbon (C) 0.20% to 0.26%,
  • silicon (Si) 0.15% to 0.3%,
  • manganese (Mn) 1.0% to 1.40%,
  • phosphorus (P) at most 0.03% maximum,
  • chromium (Cr) no applicable,
  • molybdenum (Mo) not applicable,
  • sulfur (S) at most 0.01%,
  • titanium (Ti) not applicable,
  • boron (B) 0.0005% to 0.0030%, and
  • aluminum (Al) 0.06% maximum,
    where the remainder is iron, including impurities brought about as a result of smelting.

With reference to FIG. 1, coil 40 is first uncoiled at an uncoiling station 41, which feeds lengths of steel strip 30 to the process at a predetermined rate. The uncoiled steel strip 30 is then straightened at a straightening station 42, using conventional techniques. In the event the finished structural component requires apertures, notches or other similar formations, such as apertures 12-14 and/or edge notches 15 and 16 in door beam 1, a punching station 43 is provided after straightening station 42 to form such features. The punched steel strip 30 is then cleaned at a cleaning station 44. The specific type of cleaning required will vary in accordance with the electroplating process, or other coating method used.

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 FIGS. 1 and 8, nickel coating 33 is applied to all exterior surfaces of steel strip 30 by electroplating, wherein the cleaned steel strip is run through an electroplating bath or series of baths, which dip plate steel strip 30 with nickel coating 33. As discussed above, nickel coating 33 is relatively thin to minimize cost. As shown in FIGS. 8, 11 and 12, nickel coating 33 need not be sufficiently thick or of sufficient quality to form a “Class A” or “Class 1” surface of the type required for finished, polished vehicle surfaces. Rather, nickel coating 33 can be so thin that surface coverage is occasionally incomplete, as shown in FIG. 12. So long as a majority of the faces 31 and 32 of steel strip 30 are covered with nickel coating 33, scale formation is greatly diminished to the extent that the finished part need not be abrasive blasted, or otherwise cleaned or descaled prior to post-form operations 64 and/or vehicle assembly 65. The elimination of such cleaning and/or descaling operations, in combination with reduced tool wear and maintenance, results in savings far in excess of the additional cost associated with applying nickel coating 33. Furthermore, nickel coating 33 also reduces decarburization of the steel strip 30 during processing.

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 FIGS. 8, 11 and 12, nickel coating 33 has a thickness of around 3.0 microns, and has also been found effective at a thickness of around 1.5 microns.

In the example illustrated in FIGS. 1 and 7, the nickel coated strip 34 then proceeds to a cutting station 46, where nickel coated strip 34 is cut by stamping or the like to form flat, plate-shaped blanks 35. Preferably, the cutting step is performed at substantially ambient temperature, and cuts the blank 35 so that its edges correspond to the developed configuration of door beam 1. The cutting step may be performed before the metal coating step, if desired for a particular application.

In the example illustrated in FIGS. 1 and 7, the individual, flat, plate-shaped blanks 35 are then transported to a heating station 47, such as the elongate furnace 48 illustrated in FIG. 7. In the noted example, blanks 35 are transported through furnace 48 on a plurality of rollers 49. The illustrated furnace 48 has a generally open atmosphere comprising primarily ambient air, and is capable of heating the blanks 35 to a temperature in the range of 800° C. to 1000° C. within less than ten minutes. This heating step diffuses at least a portion of the nickel coating 33 a predetermined distance into the opposite faces 31 and 32 of the steel strip portion 30 of blanks 35 to alleviate scale formation, and simultaneously raises the temperature of the blank 35 for hot forming the same. Nickel coating 33 also alleviates decarburization of the blanks 35 during the heating step.

In the embodiment of the present invention illustrated in FIGS. 1 and 7, the heating step comprises heating blanks 35 to a temperature in the range of 850° C. to 950° C. within a range of three to seven minutes to retain a thin, substantially undiffused exterior layer 55 of nickel over at least a substantial portion of the opposite faces 31 and 32 of steel strip 30 to alleviate scale formation as the temperature of blank 35 is raised for hot forming the same, such that the heat treated structural door beam 1 need not be descaled prior to a post-form processing and/or assembly in a vehicle. FIG. 8 is a photomicrograph of a portion of the finished door beam 1, with a scan line 70 superimposed thereon extending generally perpendicularly inwardly from the exteriormost surface of the door beam 1. In the illustrated example, scan line 70 is 12.3 micrometers in length. As best illustrated in FIGS. 7-12, in the door beam shown in FIG. 8, the nickel coating 33 diffuses into the opposite faces 31 and 32 of steel strip 30 a distance in the range of 2 to 8 microns. The nickel coating 33 preferably has a melting temperature above 1000° C., and is selected to match the characteristics of the steel strip 30, such that the heating required to properly heat treat the steel in door beam 1 through microstructure phase change, corresponds to a predetermined diffusion rate of the nickel coating 33 into the faces 31 and 32 of steel strip 30, so as to provide the desired degree of diffusion penetration shown in FIGS. 8-12, while maintaining the thin substantially undiffused layer 55.

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.

FIGS. 8, 11 and 12 are photomicrographs portions of working embodiments of door beam 1 according to the present invention. The associated specimens were prepared in a generally conventional fashion, using a saw to obtain sectioned samples, and mounting the same metallographically, such that the cross sectioned areas are visible in the as polished condition. The mounts were polished using standard metallographic techniques in accordance with ASTM standard E3-01. The mounts were final polished using colloidal silica media with a 0.05 micrometer particle size. The martensitic microstructures of the specimens were photographed at 1000 power magnification, after etching with two percent nital, as shown in FIGS. 11 and 12.

In the process illustrated in FIGS. 1 and 7, each heated blank 35 is then transported to a pressing or forming station 60, and placed into the hot forming or pressing tool 36. The opposite halves of pressing tool 36 close over the heated blank 35, and press the same into the shape illustrated in FIGS. 2-6.

In the example illustrated in FIGS. 1 and 7, pressing tool 36 is equipped with cooling and/or quenching devices, which heat treat door beam 1 within pressing tool 36. The cooling and/or quenching may be selective, so as to harden different parts of door beam 1 in different manners to achieve the desire strength and ductility. In the present example, the rapid heating of high strength steel strip 30 produces a phase change to the Austenite phase, and subsequent selective cooling and/or quenching change the microstructure of all or a majority of the material into the Martensite phase, as shown in FIGS. 11 and 12.

In the example illustrated in FIGS. 1 and 7, the heat treated formed structural door beam 1 is then removed from pressing tool 36, and transported to one or more post-form processing stations 64 at which additional operations may be performed prior to assembly in a vehicle. In the illustrated example, post-form station 64 includes welding mounting brackets to door beam 1, and painting and/or rust-proofing the assembled part, which is in turn transported to an assembly station 65, where door beam 1 is assembled in an associated vehicle. Typically, door beam 1 is assembled into an associated vehicle by welding, such as spot welding and/or MIG welding.

While the examples illustrated in FIGS. 1 and 7 apply a nickel coating to steel strip 30, it will be understood by those having ordinary skill in the art, strip 30 may be coated with other metals capable of forming a suitable scale and/or decarburization barrier. For example, copper may be used to coat steel strips 30. Copper has a high degree of similarity to nickel, and has almost complete solubility in nickel. Further, a duplex coating with a nickel substrate and copper surface layer can also be used to impart corrosion resistance to the part. It may also be feasible to use chromium and/or other metals for creating a resistance to scale, decarburization and/or corrosion.

In one working embodiment of the present invention, a nickel bath with the following properties was determined suitable.

• • 1. 99.9% nickel
  • 2. boiling point 2730° C.
  • 3. specific gravity (H20=1)=8.9
  • 4. vapor pressure (mm Hg.): 1 m at 1810° C.
  • 5. solubility in water=non-soluble
    The properties of the nickel as an element or as a protective coating to the base steel offer characteristics such as resistance to corrosion, erosion, scaling, fretting wear and abrasion. The nickel exhibits good performance concerning galling effects with steel. The melting temperature of nickel is very close to the melting temperature of iron, improving weldability. The lubricity of nickel at elevated temperatures improves formability. The lubricity and corrosion resistance of nickel can reduce tool wear. Also, nickel is more corrosion resistant than iron, providing some corrosion protection to the steel parts.

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.

Claims

1. 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;

electroplating the steel strip 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;

cutting a length of the nickel coated steel strip to form a flat, plate-shaped blank;

heating the plate-shaped blank 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;

transporting the heated plate-shaped blank into a pressing tool;

forming the heated plate-shaped blank in the pressing tool into a predetermined contoured shape to define a selected structural door beam;

cooling the formed structural door beam in the pressing tool to heat treat the formed structural door beam through microstructure phase change without substantial scale formation; and

removing the heat treated structural door beam from the pressing tool.

2. A method as set forth in claim 1, wherein:

said heating step includes diffusing the nickel coating into the opposite faces of the steel strip a distance in the range of 2 to 8 microns.

3. A method as set forth in claim 2, wherein:

said heating step comprises heating the plate-shaped blank to a temperature in the range of 850° C.-950° C. within a range of 20 to 25 seconds to retain a thin undiffused exterior layer of nickel over at least a substantial portion of the opposite faces of the steel strip to alleviate scale formation as the temperature of the plate-shaped blank is raised for hot forming the same, such that the heat treated structural door beam need not be descaled prior to assembly in a vehicle.

4. A method as set forth in claim 3, wherein:

said heating step comprises forming a thin oxide layer over the undiffused exterior layer of nickel for enhanced surface protection.

5. A method as set forth in claim 3, wherein:

said heating step includes providing an elongate furnace; and including

conveying the plate-shaped blank through the elongate furnace prior to said transporting step; and

performing said heating step within a range of three to seven minutes.

6. A method as set forth in claim 3, wherein:

said heating step includes providing an induction heater; and including performing said heating step in less than 45 seconds.

7. A method as set forth in claim 6, wherein:

said heating step is performed within a range of 20 to 25 seconds.

8. A method as set forth in claim 5, including:

cleaning the steel strip prior to said electroplating step.

9. A method as set forth in claim 8, wherein:

said cutting step includes stamping the steel strip at substantially ambient temperature to the final configuration of the plate-shaped blank.

10. A method as set forth in claim 9, wherein:

said heating step includes heating the plate-shaped blank to a temperature of about 900° C.

11. A method as set forth in claim 10, wherein:

said electroplating step includes selecting a nickel based metal material which has a melting temperature above 1000° C.

12. A method as set forth in claim 11, wherein:

said strip providing step comprises selecting the strip from a cold rolled steel alloy.

13. A method as set forth in claim 12, wherein:

said strip providing step includes selecting a coil of steel, uncoiling the coil and straightening the coil to define the strip.

14. A method as set forth in claim 13, wherein:

said electroplating step comprises dip plating.

15. A method as set forth in claim 14, wherein:

said strip providing step comprises selecting the strip from a steel alloy comprising a predetermined percent by weight of carbon (C), manganese (Mn) and boron (B).

16. A method as set forth in claim 15, including:

forming at least one aperture in said steel strip prior to said electroplating step.

17. A method as set forth in claim 16, wherein:

said strip providing step includes selecting the strip from a steel alloy comprising, in percent by weight,

carbon (C) 0.20% to 0.27%,

silicon (Si) 0.15% to 0.50%,

manganese (Mn) 1.0% to 1.40%,

phosphorus (P) 0.0% to 0.03%,

chromium (Cr) 0.0% to 0.35%,

molybdenum (Mo) 0.0% to 0.35%,

sulfur (S) 0.0% to 0.01%,

titanium (Ti) 0.0% to 0.05%,

boron (B) 0.002% to 0.0040%,

aluminum (Al) 0.0% to 0.06%, and

copper (Cu) 0.0% to 0.10%,

where the remainder is iron, including impurities brought about as a result of smelting.

18. A method as set forth in claim 1, wherein:

said heating step comprises heating the plate-shaped blank to a temperature in the range of 850° C.-950° C. to retain a thin undiffused exterior layer of nickel over at least a substantial portion of the opposite faces of the steel strip to alleviate scale formation as the temperature of the plate-shaped blank is raised for hot forming the same, such that the heat treated structural door beam need not be descaled prior to assembly in a vehicle.

19. A method as set forth in claim 1, wherein:

said heating step comprises forming a thin oxide layer over an undiffused exterior layer of the nickel coating for enhanced surface protection.

20. A method as set forth in claim 1, wherein:

said heating step includes providing an elongate furnace; and including conveying the plate-shaped blank through the elongate furnace prior to said transporting step; and

performing said heating step within a range of three to seven minutes.

21. A method as set forth in claim 1, wherein:

said heating step includes providing an induction heater; and including performing said heating step in less than 45 seconds.

22. A method as set forth in claim 21, wherein:

said heating step is performed within a range of 20 to 25 seconds.

23. A method as set forth in claim 1, including:

cleaning the steel strip prior to said electroplating step.

24. A method as set forth in claim 1, wherein:

said cutting step includes stamping the steel strip at substantially ambient temperature to the final configuration of the plate-shaped blank.

25. A method as set forth in claim 1, wherein:

said heating step includes heating the plate-shaped blank to a temperature of around 900° C.

26. A method as set forth in claim 1, wherein:

said electroplating step includes selecting a nickel based metal material which has a melting temperature above 1000° C.

27. A method as set forth in claim 1, wherein:

said strip providing step comprises selecting the strip from a cold rolled steel alloy.

28. A method as set forth in claim 1, wherein:

said strip providing step includes selecting a coil of steel, uncoiling the coil and straightening the coil to define the strip.

29. A method as set forth in claim 1, wherein:

said electroplating step comprises dip plating.

30. A method as set forth in claim 1, wherein:

said strip providing step comprises selecting the strip from a steel alloy comprising a predetermined percent by weight of carbon (C), manganese (Mn) and boron (B).

31. A method as set forth in claim 1, including:

forming at least one aperture in said steel strip prior to said electroplating step.

32. A method as set forth in claim 1, wherein:

said strip providing step includes selecting the strip from a steel alloy comprising, in percent by weight,

carbon (C) 0.20% to 0.27%,

silicon (Si) 0.15% to 0.50%,

manganese (Mn) 1.0% to 1.40%,

phosphorus (P) 0.0% to 0.03%,

chromium (Cr) 0.0% to 0.35%,

molybdenum (Mo) 0.0% to 0.35%,

sulfur (S) 0.0% to 0.01%,

titanium (Ti) 0.0% to 0.05%,

boron (B) 0.002% to 0.0040%,

aluminum (Al) 0.0% to 0.06%, and

copper (Cu) 0.0% to 0.10%,

where the remainder is iron, including impurities brought about as a result of smelting.

33. A method as set forth in claim 1, wherein:

said strip providing step includes selecting the strip from a steel alloy comprising, in percent by weight,

carbon (C) 0.23% to 0.27%,

silicon (Si) 0.15% to 0.50%,

manganese (Mn) 1.10% to 1.400%,

phosphorus (P) at most 0.025%,

chromium (Cr) 0.15% to 0.35%,

molybdenum (Mo) 0.10% to 0.35%,

sulfur (S) at most 0.01%,

titanium (Ti) 0.03% to 0.05%,

boron (B) 0.002% to 0.004%,

aluminum (Al) 0.02% to 0.06%, and

copper (Cu) at most 0.10%,

where the remainder is iron, including impurities brought about as a result of smelting.

34. In a method for making a vehicle having at least one contoured structural door beam, the improvement, comprising:

providing an elongate strip of high strength steel having a thickness in the range of 0.5-5.0 millimeters;

electroplating the steel strip 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;

cutting a length of the nickel coated steel strip to form a flat, plate-shaped blank;

heating the plate-shaped blank 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;

transporting the heated plate-shaped blank into a pressing tool;

forming the heated plate-shaped blank in the pressing tool into a predetermined contoured shape to define a selected structural door beam;

cooling the formed structural door beam in the pressing tool to heat treat the formed structural door beam through microstructure phase change without substantial scale formation;

removing the heat treated structural door beam from the pressing tool; and

welding the heat treated structural door beam in the vehicle without cleaning the same between said removing step and said welding step.

35. A method as set forth in claim 34, wherein:

said heating step includes diffusing the nickel coating into the opposite faces of the steel strip a distance in the range of 2 to 8 microns.

36. A method as set forth in claim 35, wherein:

said heating step includes providing an elongate furnace; and including

conveying the plate-shaped blank through the elongate furnace prior to said transporting step; and

performing said heating step within a range of three to seven minutes.

37. A method as set forth in claim 35, wherein:

said heating step comprises providing an induction heater, and heating the plate-shaped blank to a temperature in the range of 850° C.-950° C. within a range of 20 to 25 seconds to retain a thin undiffused exterior layer of nickel over at least a substantial portion of the opposite faces of the steel strip to alleviate scale formation as the temperature of the plate-shaped blank is raised for hot forming the same, such that the heat treated structural door beam need not be descaled prior to assembly in a vehicle.

38. A method as set forth in claim 36, wherein:

said heating step comprises forming a thin oxide layer over the undiffused exterior layer of nickel for enhanced surface protection.

39. A method as set forth in claim 38, including:

forming at least one aperture in said steel strip prior to said electroplating step; and

wherein

said strip providing step includes selecting the strip from a steel alloy comprising, in percent by weight,

carbon (C) 0.20% to 0.27%,

silicon (Si) 0.15% to 0.50%,

manganese (Mn) 1.0% to 1.40%,

phosphorus (P) 0.0% to 0.03%,

chromium (Cr) 0.0% to 0.35%,

molybdenum (Mo) 0.0% to 0.35%,

sulfur (S) 0.0% to 0.01%,

titanium (Ti) 0.0% to 0.05%,

boron (B) 0.002% to 0.0040%,

aluminum (Al) 0.0% to 0.06%, and

copper (Cu) 0.0% to 0.10%,

where the remainder is iron, including impurities brought about as a result of smelting.

40. 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;

coating the steel strip 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;

cutting the nickel coated steel strip to form a flat, plate-shaped blank;

heating the plate-shaped blank 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;

transporting the heated plate-shaped blank into a pressing tool;

forming the heated plate-shaped blank in the pressing tool into a predetermined contoured shape to define a selected structural vehicle part;

cooling the formed structural vehicle part in the pressing tool to heat treat the formed structural vehicle part; and

removing the heat treated structural vehicle part from the pressing tool.

41. A method as set forth in claim 40, wherein:

said heating step includes diffusing the nickel coating into the opposite faces of the steel strip a distance in the range of 2 to 8 microns.

42. A method as set forth in claim 41, wherein:

said heating step comprises heating the plate-shaped blank to a temperature in the range of 850° C.-950° C. to retain a thin undiffused exterior layer of nickel over at least a substantial portion of the opposite faces of the steel strip to alleviate scale formation as the temperature of the plate-shaped blank is raised for hot forming the same, such that the heat treated structural vehicle part need not be descaled prior to assembly in a vehicle.

43. A method as set forth in claim 42, wherein:

said heating step comprises forming a thin oxide layer over the undiffused exterior layer of nickel for enhanced surface protection.

44. A method as set forth in claim 43, wherein:

said heating step includes providing an elongate furnace; and including

conveying the plate-shaped blank through the elongate furnace prior to said transporting step; and

performing said heating step within a range of three to seven minutes.

45. A method as set forth in claim 43, wherein:

said heating step includes providing an induction heater; and including

performing said heating step in less than 45 seconds.

46. A method as set forth in claim 44, including:

forming at least one aperture in said steel strip prior to said electroplating step.

47. A method as set forth in claim 46, wherein:

said strip providing step includes selecting the strip from a steel alloy comprising, in percent by weight,

carbon (C) 0.20% to 0.27%,

silicon (Si) 0.15% to 0.50%,

manganese (Mn) 1.0% to 1.40%,

phosphorus (P) 0.0% to 0.03%,

chromium (Cr) 0.0% to 0.35%,

molybdenum (Mo) 0.0% to 0.35%,

sulfur (S) 0.0% to 0.01%,

titanium (Ti) 0.0% to 0.05%,

boron (B) 0.002% to 0.0040%,

aluminum (Al) 0.0% to 0.06%, and

copper (Cu) 0.0% to 0.10%,

where the remainder is iron, including impurities brought about as a result of smelting.

48. A method for making contoured structural parts, comprising:

providing a blank made from steel having a thickness greater than 0.5 millimeters;

coating the steel blank on at least the opposite faces thereof with a metal selected from the group consisting essentially of: a) nickel, b) copper, and c) chromium;

heating the coated blank 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;

transporting the heated coated blank into a pressing tool;

forming the heated coated blank in the pressing tool into a predetermined contoured shape to define a selected structural part;

cooling the formed structural part in the pressing tool to heat treat the same; and

removing the heat treated structural part from the pressing tool.

49. A method as set forth in claim 48, wherein:

said heating step includes diffusing the metal into the opposite faces of the steel strip a distance in the range of 2 to 8 microns.

50. A method as set forth in claim 49, wherein:

said heating step comprises heating the blank to a temperature in the range of 850° C.-950° C. to retain a thin undiffused exterior layer of metal over at least a substantial portion of the opposite faces of the steel to alleviate scale formation as the temperature of the blank is raised for hot forming the same, such that the heat treated structural part need not be descaled prior to assembly.

51. A method as set forth in claim 50, wherein:

said heating step comprises forming a thin oxide layer over the undiffused exterior layer of metal for enhanced surface protection.

52. A method as set forth in claim 51, wherein:

said coating step comprises electroplating.

53. A method as set forth in claim 52, wherein:

said heating step includes providing an elongate furnace; and including conveying the blank through the elongate furnace prior to said transporting step; and

performing said heating step within a range of three to seven minutes.

54. A method as set forth in claim 52, wherein:

said heating step includes providing an induction heater; and including

performing said heating step in less than 45 seconds.

55. A method as set forth in claim 53, including:

forming at least one aperture in said steel strip prior to said electroplating step.

56. A method as set forth in claim 55, wherein:

said blank providing step includes selecting the blank from a steel alloy comprising, in percent by weight,

carbon (C) 0.20% to 0.27%,

silicon (Si) 0.15% to 0.50%,

manganese (Mn) 1.0% to 1.40%,

phosphorus (P) 0.0% to 0.03%,

chromium (Cr) 0.0% to 0.35%,

molybdenum (Mo) 0.0% to 0.35%,

sulfur (S) 0.0% to 0.01%,

titanium (Ti) 0.0% to 0.05%,

boron (B) 0.002% to 0.0040%,

aluminum (Al) 0.0% to 0.06%, and

copper (Cu) 0.0% to 0.10%,

where the remainder is iron, including impurities brought about as a result of smelting.