TECHNICAL FIELD

The present invention relates to a method and system for obtaining metal strips characterized by having at least two different thicknesses along its width. The method combines manufacturing processes such as stretch forming, rolling, flattening and localized heating in order to vary the metal strips thickness at specific areas. Particularly, the present method refers to efficiently and effectively achieve a thickness reduction in particular areas of a metal strip .

BACKGROUND OF THE INVENTION

Lighter structural components have become increasingly important in the transportation industry. Higher standards and regulations have demanded structural components manufacturers to increase their competencies in the use of alternative methods and materials which can achieve significant weight reduction without sacrificing safety and performance. Specifically in the manufacture of chassis frames components, such as side rails, steel has proven to be the material with better saving performance based on its cost implications.

Some of the most innovative and competitive solutions in the industry have been related to the thickness variation of steel. Thickness variation allows the structural component manufacturing companies to design components tailored according to the load and strength needs, making easier to comply with weight and cost industry requirements .

One current solution in the prior art to obtain strips with varying thicknesses is through tailor-welded blanks. Tailor-welded blanks have been manufactured by first creating two separate blanks of the sheet metal materials having different characteristics, and then butting the edges of the two sheets together and welding the adjoining edges of the sheets to create a permanent attachment. In this way, a blank assembly is created that is comprised of two different sheet materials having different characteristics.

The main disadvantage that these blanks for some later manufacturing processes is that subsequent bending imparts concentrated stress zones over the welds that could affect the final component characteristics. Additionally, the welding precision required for the attachment of tailor- welded blanks have cost implications, since an inaccurate welding operation could have severe performance implications for the final component.

Furthermore, another manufacturing process found in the prior art to achieve variable thickness profiles refers to a strip profile rolling process that achieves a variable thickness profile in a steel strip by shifting material from the center of the strip to its edges. This shifting is accomplished arranging rolling elements in a diagonal progressive arrangement, generating a pressure front from the center to the edges of the material. A disadvantage of this process is that each rolling step can only reduce thickness in a small percentage of the total thickness, therefore, significant number of rollers are required to achieve considerably high thickness reductions, as the ones needed in the commercial vehicle industry.

Some other attempts have been made to achieve variable thickness in steel materials, but none has been found suitable the amount of weight reduction required the manufacturing of steel side rails for the transportation industry.

Therefore, there is an identified need for a strip comprising a gage variation along its width through the entire length of a commercial vehicle side rail component. Subsequently, there is a need for a strip forming system which is able to reduce efficiently the thickness of the strips in the least amount of rolling steps.

OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide a method for processing metallic strips to form a variable thickness profile which could be used in the manufacturing of commercial vehicle's side rails.

The present invention is also directed to provide a system which enables cost-effective manufacturing of variable thickness steel strips compared to the methods found in the prior art.

The embodiments of the invention comprise a combination of features and advantages that substantially improve the rolling methods and systems for reducing the thickness of a metal strip. These and various other features and advantages of the invention will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of the invention, reference will be made to the attached drawings in which:

Figure 1 shows a block diagram of the process for varying the gage of the metal strips .

Figure 2 shows the forming of the strip at each of the steps of the method of Figure 1.

Figure 3 shows a system for varying the gage of a metal strip. Figure 4 shows a set of stretch forming rolls for the process of the present invention.

Figure 5 is a isometric simplified view of the stretch forming process of the present invention.

DE TAILED DESCRI PTION OF THE PREFERRED EMBODIMENT

Various embodiments of the invention are described with reference to the attached drawings wherein like reference numerals are used for like parts throughout the various views. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form. Likewise, some details of conventional elements may not be shown for the sake of clarity and conciseness.

The preferred embodiments of the invention refer to a method and system for varying the gage of a strip. The invention is susceptible to embodiments of different forms. Specific embodiments of the invention are shown in the drawings, and will be further described, with the understanding that the specification will be considered as an exemplification of the principles of the invention and it is not intended to limit the invention to that as illustrated and described herein. It will be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination so as to produce the desired results.

Figure 1 shows a block diagram of the process for varying the gage of metal strips according to the present invention wherein a metal strip is presented (101), preferably made out of steel, and wherein said strip is heated to a formable temperature through heating means (102) . After said heating step, the strip passes through a pair of stretch forming rolls, wherein its central section is formed according to the rolls shape in a stretch forming step (104) . Said forming rolls, as shown in Figure 4, are comprised by a stretching roller (9) and a backing roller (10), wherein the stretching roller (9) has a central knob (9c) which aligns into the adjacent backing roller recess (10b) . At the moment of the rolling process through the stretching rolls, the steel strip is driven by being in contact with the sections (9b), (10c), (9a) and (10a) as described in Figure 4. The central section (9c) deforms the material but the material does not necessarily is in contact with the section (10b) . Therefore, the section (9c) does not necessarily fits perfectly into section (10b) . Thus, to guarantee that the material is sufficiently flattened before a first set of forming rolls, there could be a prior step of flattening (103) with flattening rolls, as optionally shown in Figure 1. As better seen in Figure 3, the element (11) is used to give the optional dimensional flattening (103) that the process requires after being heated (102) . In case that the steel strip is already within the desired flat tolerances, this prior optional step of flattening (103) would not be required, as depicted in dotted lines. Then, as depicted in Figure 1, the strip still heated in its central section, follow into a flattening step by flattening rolls (105) . After the first stretch forming step (104) plus the first flattening step (105), the process can be repeated until the desired thickness reduction is achieved in the heated section of the strip, as depicted in dash-dot lines.

Figure 1 shows exemplary second stretch forming (106) and flattening steps (107) . Preferably, the heating process might be alternate during the process in order to guarantee that the central section of the strip has the temperature needed for its deformation.

Figure 2 shows the forming of the strip at each of the steps of the method of Figure 1. As it has been mentioned, the metal strip is heated through heating means, wherein said heating means may be, but are not limited to, an induction heating device. The strip is passed through the optional straightening rolls for its flattening (step 103 in Figure 1) to obtain a substantially planar strip (21b) . In a subsequent step, the strip (21) is passed through the first forming rolls wherein a stretch forming process (step 104 of Figure 1) is performed, from which a reduced thickness profile strip is obtained, which comprises a stretched zone (21c) in the heated section of the metal strip. The width of the strip increases naturally as the material reduced is stretched widthwise. Following the first stretch forming step, the strip passes to the second straightening rolls in order to be flattened (step 105 of figure 1) wherein a first flat strip with reduced thickness is then obtained. In a subsequent step, the strip is passed through the second forming rolls undergoing a stretching process (step 106 of Figure 1), wherein a reduced thickness profile strip is obtained which comprises a stretched zone (21e) in the heated section; again, the metal strip edges (21i) have a total width dimension greater than the strip flattened by the second straightening rolls. In a following step, the strip which at this point has already presents a thickness reduction, passes to the third straightening rolls for flattening (step 107 of figure 1) wherein a second reduction of the strip gage (21f) is then obtained in the heated section and the ends of the strip ( 21j ) comprise a length higher than the strip that comes out from the second stretch forming rolls. As mentioned above, the process can continue as needed into additional forming and flattening steps. The final form of the strip is therefore characterized by a variation in gage of the metal strip along its width. It is understood that lines (120) are reference lines that illustrate how the material starts to increase along its width as the thickness decreases. Figure 3 shows a system for varying the gage of a metal strip according to the present invention, wherein heating means (19) heat the strip before its forming process and wherein the heated strip is then flattened through an optional pair of straightening rolls (11, 12) for flattening the pre-existent material deformations. After the optional straightening rolls (11, 12), the system comprises the first forming rolls (9, 10) wherein said rolls are comprised by a stretching roller (9) and a backing roller (10), as the strip goes through said set of rolls (9, 10) it is formed in a slightly convex curvature in the heated section, and the strip edges increase its dimensions proportionally to the central deformation of the strip. After the first stretch forming rolls (9, 10), there are a second pair of straightening rolls (13, 14), wherein one is located sufficiently adjacent to the other in order to exert pressure to the metal strip reducing the gage of the preheating area of the strip. After the second straightening rolls (13, 14), a second stretch forming rolls (15, 16) create a slightly convex shape in the strip by exerting pressure to the heated area. A third pair of straightening rolls (17, 18) located adjacently to each other flatten the steel strip again.

The process may repeat until the desired thickness in the desired section of the strip is achieved. Additionally, heating means may be placed repeatedly through the process in order to guarantee that the section to be deformed complies with the required temperature to achieve such deformation .

Figure 4 shows a set of forming rolls as herein proposed. Figure 4 shows a stretching roll (9) and a backer roll (10); the stretching roll (9) is characterized by a knob-like shape which is similar to a cylindrical protrusion (9c) . The backer roll (10) has a cylindrical shape recess (10b) which coincides with knob-like shape of the stretching roll (9c) .

Finally, as shown in detail in Figure 5 a steel strip is stretched by means of a traction force applied by a specifically shaped stretching roller and kept in place by a backing roller to produce a thickness variation along its width.

While the preferred embodiments have been shown and described, someone skilled in the art may make modifications thereto without departing from the scope or teachings herein. The embodiments described herein are merely exemplary and are not limiting. There are many possible variations and modifications of the systems and methods which are within the scope of the invention. For example, the number of stretch forming and straightening rolls, pressures and temperatures, among other parameters, may be varied. Consequently, the scope of protection is not limited to the embodiments herein described, but is only limited by the following claims, the scope of which shall include all equivalents of the subject matter of the claims.